Transport in Plants
- Plants need to move molecules over very long distances, much more than animals do; they also do not have a circulatory system in place.
- Water taken up by the roots has to reach all parts of the plant, up to the very tip of the growing stem. The photosynthates or food synthesised by the leaves have also to be moved to all parts including the root tips embedded deep inside the soil.
- Movement across short distances, say within the cell, across the membranes and from cell to cell within the tissue has also to take place.
- In a flowering plant the substances that would need to be transported are water, mineral nutrients, organic nutrients and plant growth regulators.
- Small distance transport – by diffusion and by cytoplasmic streaming supplemented by active transport.
- Long distance Transport – through the vascular system (the xylem and the phloem) and is called translocation.
- In rooted plants, transport in xylem (of water and minerals) is essentially unidirectional, from roots to the stems.
- Organic and mineral nutrients however, undergo multidirectional transport.
- Organic compounds synthesised in the photosynthetic leaves are exported to all other parts of the plant Including storage organs. From the storage organs they are later re-exported.
- The mineral nutrients are taken up by the roots and transported upwards into the stem, leaves and the growing regions. When any plant part undergoes senescence, nutrients may be withdrawn from such regions and moved to the growing parts.
- Hormones or plant growth regulators and other chemical stimuli are also transported, though in very small amounts, sometimes in a strictly polarised or unidirectional manner from where they are synthesised to other parts.
Means of Transport
- Passive movement and may be from one part of the cell to the other, or from cell to cell, or over short distances, say, from the intercellular spaces of the leaf to the outside.
- No energy expenditure takes place.
- Molecules move in a random fashion, the net result being substances moving from regions of higher concentration to regions of lower concentration.
- Diffusion is a slow process and is not dependent on a ‘living system’.
- Diffusion is obvious in gases and liquids, but diffusion in solids rather than of solids is more likely.
- Diffusion is very important to plants since it the only means for gaseous movement within the plant body.
- Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, size of the substances, temperature and pressure.
- The diffusion of any substance across a membrane also depends on its solubility in lipids, the major constituent of the membrane.
- Substances soluble in lipids diffuse through the membrane faster.
- Substances that have a hydrophilic moiety, find it difficult to pass through the membrane; their movement has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane.
- They do not set up a concentration gradient: a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins. This process is called facilitated diffusion.
- In facilitated diffusion special proteins help move substances across membranes without expenditure of ATP energy.
- Facilitated diffusion cannot cause net transport of molecules from a low to a high concentration – this would require input of energy.
- Transport rate reaches a maximum when all of the protein transporters are being used (saturation).
- Facilitated diffusion is very specific: it allows cell to select substances for uptake.
- It is sensitive to inhibitors which react with protein side chains.
- The proteins form channels in the membrane for molecules to pass through. Some channels are always open; others can be controlled. Some are large, allowing a variety of molecules to cross.
- The porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through.
- When an extracellular molecule bound to the transport protein; the transport protein then rotates and releases the molecule inside the cell, e.g., water channels (made up of eight different types of aquaporins.)
Passive symports and antiports
- Some carrier or transport proteins allow diffusion only if two types of molecules move together.
- In a symport, both molecules cross the membrane in the same direction; in an antiport, they move in opposite directions.
- When a molecule moves across a membrane independent of other molecules, the process is called uniport.
- Active transport uses energy to pump molecules against a concentration gradient.
- Active transport is carried out by membrane-proteins. Hence different proteins in the membrane play a major role in both active as well as passive transport.
- Pumps are proteins that use energy to carry substances across the cell membrane. These pumps can transport substances from a low concentration to a high concentration (‘uphill’ transport).
- Transport rate reaches a maximum when all the protein transporters are being used or are saturated.
- Like enzymes the carrier protein is very specific in what it carries across the membrane.
- These proteins are sensitive to inhibitors that react with protein side chains.
Comparison of Different Transport Processes
- Proteins in the membrane are responsible for facilitated diffusion and active transport and hence show common characterstics of being highly selective; they are liable to saturate, respond to inhibitors and are under hormonal regulation.
- But diffusion whether facilitated or not – take place only along a gradient and do not use energy.
- Water is essential for all physiological activities of the plant and plays a very important role in all living organisms.
- It provides the medium in which most substances are dissolved. The protoplasm of the cells is nothing but water in which different molecules are dissolved and (several particles) suspended. A watermelon has over 92 per cent water; most herbaceous plants have only about 10 to 15 per cent of its fresh weight as dry matter.
- Of course, distribution of water within a plant varies – woody parts have relatively very little water, while soft parts mostly contain water. A seed may appear dry but it still has water – otherwise it would not be alive and respiring!
- Terrestrial plants take up huge amount water daily but most of it is lost to the air through evaporation from the leaves, i.e., transpiration. A mature corn plant absorbs almost three litres of water in a day, while a mustard plant absorbs water equal to its own weight in about 5 hours. Because of this high demand for water, it is not surprising that water is often the limiting factor for plant growth and productivity in both agricultural and natural environments.
- Water potential (Psi w) is a concept fundamental to understanding water movement. Solute potential (Psi s) and pressure potential (Psi p) are the two main components that determine water potential.
- Water molecules possess kinetic energy. In liquid and gaseous form they are in random motion that is both rapid and constant. The greater the concentration of water in a system, the greater is its kinetic energy or ‘water potential’. Hence, it is obvious that pure water will have the greatest water potential.
- If two systems containing water are in contact, random movement of water molecules will result in net movement of water molecules from the system with higher energy to the one with lower energy. Thus water will move from the system containing water at higher water potential to the one having low water potential.
- This process of movement of substances down a gradient of free energy is called diffusion.
- Water potential is denoted by the Greek symbol Psi and is expressed in pressure units such as pascals (Pa).
- By convention, the water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero.
- If some solute is dissolved in pure water, the solution has fewer free water and the concentration of water decreases, reducing its water potential. Hence, all solutions have a lower water potential than pure water; the magnitude of this lowering due to dissolution of a solute is called solute potential or Psi s. Psi s is always negative. The more the solute molecules, the lower (more negative) is the Psi s.
- For a solution at atmospheric pressure (water potential) Psi w = (solute potential) Psi s.
- If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. It is equivalent to pumping water from one place to another.
- Pressure can build up in a plant system when water enters a plant cell due to diffusion causing a pressure built up against the cell wall, it makes the cell turgid, this increases the pressure potential. Pressure potential is usually positive.
- Though in plants negative potential or tension in the water column in the xylem plays a major role in water transport up a stem. Pressure potential is denoted as (p.
- Water potential of a cell is affected by both solute and pressure potential. The relationship between them is as follows:
- The plant cell is surrounded by a cell membrane and a cell wall. The cell wall is freely permeable to water and substances in solution hence is not a barrier to movement.
- In plants the cells usually contain a large central vacuole, whose contents, the vacuolar sap, contribute to the solute potential of the cell.
- In plant cells, the cell membrane and the membrane of the vacuole, the tonoplast together are important determinants of movement of molecules in or out of the cell.
- Osmosis is the term used to refer specifically to the diffusion of water across a differentially- or semi-permeable membrane. Osmosis occurs spontaneously in response to a driving force.
- The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient.
- Water will move from its region of higher chemical potential (or concentration) to its region of lower chemical potential until equilibrium is reached. At equilibrium the two chambers should have the same water potential.
In above Fig two chambers, A and B, containing solutions are separated by a semi-permeable membrane.
- Solution of which chamber has a lower water potential? – B
- Solution of which chamber has a lower solute potential? – B
- In which direction will osmosis occur? – A(B
- Which solution has a higher solute potential?- A
- At equilibrium which chamber will have lower water potential?- Equal
- If one chamber has a psi of -2000 kPa, and the other -1000 kPa, which is the chamber that has the higher psi ?- second with -1000kP.
- Experiment – a solution of sucrose in water taken in a funnel is separated from pure water in a beaker through a semi-permeable membrane (Egg membrane – Can be obtained by removing the yolk and albumin through a small hole at one end of the egg, and placing the shell in dilute solution of hydrochloric acid for a few hours. The egg shell dissolves leaving the membrane intact). Water will move into the funnel, resulting in rise in the level of the solution in the funnel. This will continue till the equilibrium is reached. External pressure can be applied from the upper part of the funnel such that no water diffuses into the funnel through the membrane. This pressure required to prevent water from diffusing is the osmotic pressure and this is the function of the solute concentration; more the solute concentration, greater will be the pressure required to prevent water from diffusing in. Numerically osmotic pressure is equivalent to the osmotic potential, but the sign is opposite. Osmotic pressure is the positive pressure applied, while osmotic potential is negative.
- The behaviour of the plant cells (or tissues) with regard to water movement depends on the surrounding solution.
- If the external solution balances the osmotic pressure of the cytoplasm, it is said to be isotonic.
- If the external solution is more dilute than the cytoplasm, it is hypotonic and if the external solution is more concentrated, it is hypertonic.
- Cells swell in hypotonic solutions and shrink in hypertonic ones.
- Plasmolysis occurs when water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall. This occurs when the cell (or tissue) is placed in a solution that is hypertonic (has more solutes) to the protoplasm. Water moves out; it is first lost from the cytoplasm and then from the vacuole.
- The water when drawn out of the cell through diffusion into the extracellular (outside cell) fluid causes the protoplast to shrink away from the walls. The cell is said to be plasmolysed.
- The movement of water occurred across the membrane moving from an area of high water potential (i.e., the cell) to an area of lower water potential outside the cell.
- The process of plamolysis is usually reversible.
- When the cells are placed in a hypotonic solution (higher water potential or dilute solution as compared to the cytoplasm), water diffuses into the cell causing the cytoplasm to build up a pressure against the wall, that is called turgor pressure. The pressure exerted by the protoplasts due to entry of water against the rigid walls is called pressure potential. Because of the rigidity of the cell wall, the cell does not rupture.
- This turgor pressure is ultimately responsible for enlargement and extension growth of cells.
- Imbibition is a special type of diffusion when water is absorbed by solids – colloids – causing them to enormously increase in volume. e.g., absorption of water by seeds and dry wood.
- The pressure that is produced by the swelling of wood had been used by prehistoric man to split rocks and boulders.
- If it were not for the pressure due to imbibition, seedlings would not have been able to emerge out of the soil into the open.
- Imbibition is also diffusion since water movement is along a concentration gradient; the seeds and other such materials have almost no water hence they absorb water easily.
- Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition.
- In addition, for any substance to imbibe any liquid, affinity between the adsorbant and the liquid is also a pre-requisite.
Long Distance Transport of Water
- Long distance transport of substances within a plant cannot be by diffusion alone. Diffusion is a slow process. It can account for only short distance movement of molecules. For example, the movement of a molecule across a typical plant cell (about 50µm) takes approximately 2.5 s.
- In large and complex organisms, often substances have to be moved across very large distances. sometimes the sites of production or absorption and sites of storage are too far from each other; diffusion or active transport would not suffice. Special long distance transport systems become necessary so as to move substances across long distances and at a much faster rate.
- Water and minerals, and food are generally moved by a mass or bulk flow system.
- Mass flow is the movement of substances in bulk or en masse from one point to another as a result of pressure differences between the two points. It is a characteristic of mass flow that substances, whether in solution or in suspension, are swept along at the same pace, as in a flowing river.
- This is unlike diffusion where different substances move independently depending on their concentration gradients.
- Bulk flow can be achieved either through a positive hydrostatic pressure gradient (e.g., a garden hose) or a negative hydrostatic pressure gradient (e.g., suction through a straw).
- The bulk movement of substances through the conducting or vascular tissues of plants is called translocation.
- The higher plants have highly specialised vascular tissues – xylem and phloem.
- Xylem is associated with translocation of mainly water, mineral salts, some organic nitrogen and hormones, from roots to the aerial parts of the plants.
- The phloem translocates a variety of organic and inorganic solutes, mainly from the leaves to other parts of the plants.
How do Plants Absorb Water?
- The responsibility of absorption of water and minerals is of the root hairs.
- Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.
- Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion.
- once water is absorbed by the root hairs, it can move deeper into root layers by two distinct pathways: Apoplast pathway, Symplast pathway.
- Apoplast is the system of adjacent cell walls that is continuous throughout the plant, except at the casparian strips of the endodermis in the roots.
- The apoplastic movement of water occurs exclusively through the intercellular spaces and the walls of the cells.
- Movement through the apoplast does not involve crossing the cell membrane.
- This movement is dependent on the gradient.
- The apoplast does not provide any barrier to water movement and water movement is through mass flow.
- As water evaporates into the intercellular spaces or the atmosphere, tension develop in the continuous stream of water in the apoplast, hence mass flow of water occurs due to the adhesive and cohesive properties of water.
- The symplastic system is the system of interconnected protoplasts. Neighbouring cells are connected through cytoplasmic strands that extend through plasmodesmata.
- During symplastic movement, the water travels through the cells – their cytoplasm; intercellular movement is through the plasmodesmata.
- Water has to enter the cells through the cell membrane, hence the movement is relatively slower.
- Movement is again down a potential gradient.
- symplastic movement may be aided by cytoplasmic streaming.
- Most of the water flow in the roots occurs via the apoplast since the cortical cells are loosely packed, and hence offer no resistance to water movement. However, the inner boundary of the cortex, the endodermis, is impervious to water because of a band of suberised matrix called the casparian strip.
- Water molecules are unable to penetrate the layer, so they are directed to wall regions that are not suberised, into the cells proper through the membranes.
- The water then moves through the symplast and again crosses a membrane to reach the cells of the xylem.
- The movement of water through the root layers is ultimately symplastic in the endodermis. This is the only way water and other solutes can enter the vascular cylinder.
- Once inside the xylem, water is again free to move between cells as well as through them. In young roots, water enters directly into the xylem vessels and/or tracheids. These are non-living conduits and so are parts of the apoplast.
- Some plants have additional structures associated with them that help in water (and mineral) absorption. A mycorrhiza is a symbiotic association of a fungus with a root system.
- The fungal filaments form a network around the young root or they penetrate the root cells. The hyphae have a very large surface area that absorb mineral ions and water from the soil from a much larger volume of soil that perhaps a root cannot do.
- The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.
- Some plants have an obligate association with the mycorrhizae. e.g., Pinus seeds cannot germinate and establish without the presence of mycorrhizae.
Water Movement up a Plant
- As various ions from the soil are actively transported into the vascular tissues of the roots, water follows (its potential gradient) and increases the pressure inside the xylem. This positive pressure is called root pressure, and can be responsible for pushing up water to small heights in the stem.
- Effects of root pressure is also observable at night and early morning when evaporation is low, and excess water collects in the form of droplets around special openings of veins near the tip of grass blades, and leaves of many herbaceous parts. Such water loss in its liquid phase is known as guttation.
- Root pressure can, at best, only provide a modest push in the overall process of water transport. They obviously do not play a major role in water movement up tall trees.
- The greatest contribution of root pressure may be to re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration.
- Root pressure does not account for the majority of water transport; most plants meet their need by transpiratory pull.
- The flow of water upward through the xylem in plants can achieve fairly high rates, up to 15 metres per hour.
- Most researchers agree that water is mainly ‘pulled’ through the plant, and that the driving force for this process is transpiration from the leaves. This is referred to as the cohesion-tension-transpiration pull model of water transport. But, what generates this transpirational pull?
- Water is transient in plants. Less than 1 per cent of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost through the stomata in the leaves. This water loss is known as transpiration.
- Transpiration is the evaporative loss of water by plants. It occurs mainly through the stomata in the leaves.
- Besides the loss of water vapour in transpiration, exchange of oxygen and carbon dioxide in the leaf also occurs through pores called stomata (sing.: stoma).
- Normally stomata are open in the day time and close during the night.
- The immediate cause of the opening or closing of the stomata is a change in the turgidity of the guard cells.
- The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape.
- The opening of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally making it easier for the stoma to open.
- When the guard cells lose turgor, due to water loss (or water stress) the elastic inner walls regain their original shape, the guard cells become flaccid and the stoma closes.
- Usually the lower surface of a dorsiventral (often dicotyledonous) leaf has a greater number of stomata while in an isobilateral (often monocotyledonous) leaf they are about equal on both surfaces.
- Transpiration is affected by several external factors: temperature, light, humidity, wind speed. Plant factors that affect transpiration include number and distribution of stomata, number of stomata open, per cent, water status of the plant, canopy structure etc.
- The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:
- Cohesion – mutual attraction between water molecules.
- Adhesion – attraction of water molecules to polar surfaces (such as the surface of tracheary elements).
- Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
- These properties give water high tensile strength, i.e., an ability to resist a pulling force, and high capillarity, i.e., the ability to rise in thin tubes. In plants capillarity is aided by the small diameter of the tracheary elements – the tracheids and vessel elements.
- The process of photosynthesis requires water. The system of xylem vessels from the root to the leaf vein can supply the needed water.
- As water evaporates through the stomata, since the thin film of water over the cells is continuous, it results in pulling of water, molecule by molecule, into the leaf from the xylem.
- Also, because of lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air. This creates a ‘pull’.
- Measurements reveal that the forces generated by transpiration can create pressures sufficient to lift a xylem sized column of water over 130 metres high.
Transpiration and Photosynthesis – a Compromise
- Transpiration has more than one purpose; it
- createstranspiration pull for absorption and transport of plants
- supplieswater for photosynthesis
- transportsminerals from the soil to all parts of the plant
- cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling
- maintainsthe shape and structure of the plants by keeping cells turgid
- An actively photosynthesising plant has an insatiable need for water. Photosynthesis is limited by available water which can be swiftly depleted by transpiration.
- The humidity of rainforests is largely due to this vast cycling of water from root to leaf to atmosphere and back to the soil.
- The evolution of the C4 photosynthetic system is probably one of the strategies for maximising the availability of CO2 while minimising water loss.
- C4 plants are twice as efficient as C3 plants in terms of fixing carbon (making sugar). However, a C4 plant loses only half as much water as a C3 plant for the same amount of CO2 fixed.
Uptake and Transport of Mineral Nutrients
- Plants obtain their carbon and most of their oxygen from CO2 in the atmosphere. However, their remaining nutritional requirements are obtained from minerals and water for hydrogen in the soil.
Uptake of Mineral Ions
- Unlike water, all minerals cannot be passively absorbed by the roots.
- Two factors account for this:
(i) minerals are present in the soil as charged particles (ions) which cannot move across cell membranes and
(ii) the concentration of minerals in the soil is usually lower than the concentration of minerals in the root.
- Therefore, most minerals must enter the root by active absorption into the cytoplasm of epidermal cells.
- This needs energy in the form of ATP.
- The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis. Some ions also move into the epidermal cells passively.
- Ions are absorbed from the soil by both passive and active transport.
- Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasms of the epidermal cells.
- Like all cells, the endodermal cells have many transport proteins embedded in their plasma membrane; they let some solutes cross the membrane, but not others.
- Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem.
- the root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.
Translocation of Mineral Ions
- After the ions have reached xylem through active or passive uptake, or a combination of the two, their further transport up the stem to all parts of the plant is through the transpiration stream.
- The chief sinks for the mineral elements are the growing regions of the plant, such as the apical and lateral meristems, young leaves, developing flowers, fruits and seeds, and the storage organs.
- Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.
- Mineral ions are frequently remobilised, particularly from older, senescing parts.
- Older dying leaves export much of their mineral content to younger leaves.
- Similarly, before leaf fall in decidous plants, minerals are removed to other parts.
- Elements most readily mobilised are phosphorus, sulphur, nitrogen and potassium.
- Some elements that are structural components like calcium are not remobilised.
- An analysis of the xylem exudates shows that though some of the nitrogen travels as inorganic ions, much of it is carried in the organic form as amino acids and related compounds.
- Similarly, small amounts of P and S are carried as organic compounds.
- In addition, small amount of exchange of materials does take place between xylem and phloem.
- Hence, it is not that we can clearly make a distinction and say categorically that xylem transports only inorganic nutrients while phloem transports only organic materials.
Phloem Transport: Flow from Source to Sink
- Food, primarily sucrose, is transported by the vascular tissue phloem from a source to a sink.
- Usually the source is understood to be that part of the plant which synthesises the food, i.e., the leaf, and sink, the part that needs or stores the food.
- But, the source and sink may be reversed depending on the season, or the plant’s needs.
- Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees, act as sink; they need energy for growth and development of the photosynthetic apparatus.
- Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e., bi-directional. This contrasts with that of the xylem where the movement is always unidirectional, i.e., upwards.
- Hence, unlike one-way flow of water in transpiration, food in phloem sap can be transported in any required direction so long as there is a source of sugar and a sink able to use, store or remove the sugar.
- Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also transported or translocated through phloem.
The Pressure Flow or Mass Flow Hypothesis
- The accepted mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis.
- As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a dissacharide). The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport.
- This process of loading at the source produces a hypertonic condition in the phloem.
- Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds up the phloem sap will move to areas of lower pressure.
- At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar – converting it into energy, starch, or cellulose.
- As sugars are removed, the osmotic pressure decreases and water moves out of the phloem.
- the movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the mass movement in the phloem.
- Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates.
- Cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments.
- As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins, and the sap moves through the phloem.
- Meanwhile, at the sink, incoming sugars are actively transported out of the phloem and removed as complex carbohydrates.
- The loss of solute produces a high water potential in the phloem, and water passes out, returning eventually to xylem.
- A simple experiment, called girdling, was used to identify the tissues through which food is transported.
- On the trunk of a tree a ring of bark up to a depth of the phloem layer, can be carefully removed.
- In the absence of downward movement of food, the portion of the bark above the ring on the stem becomes swollen after a few weeks.
- This simple experiment shows that phloem is the tissue responsible for translocation of food; and that transport takes place in one direction, i.e., towards the roots.
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CHAPTER 11- TRANSPORT IN PLANTS
CELL CYCLE AND CELL DIVISION
- Growth and reproduction are characteristics of living cells and organisms.
Cell Cycle –
- The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.
- Cell cycle includes three processes cell division, DNA replication and cell growth in coordinated way.
- Duration of cell cycle can vary from organism to organism and also from cell type to cell type. (e.g., in Yeast cell cycle is of 90 minutes, in human 24 hrs.)
- It is divided into 3 further phases G1, S, and G2.
G1 phase (Gap 1 Phase)
- Corresponds to the interval between mitosis and initiation of DNA replication.
- During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
S phase (synthesis phase)
- period during which DNA synthesis or replication takes place.
- During this time the amount of DNA per cell doubles. (only amount of DNA is doubled, no of chromosomes remain same.)
- In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
G2 phase (Gap 2 Phase)
- Proteins are synthesised in preparation for mitosis while cell growth continues.
- Some cells do not exhibit division like heart cells, nerve cells etc. these cells enter in an inactive phase called G0 or quiescent phase from G1 phase.
- Cells in this phase are metabolically active but they do not divide unless they are called on to do so.
Mitosis or M phase
- In animals, mitotic cell division is only seen in the diploid somatic cells while in the plants mitotic divisions can be seen in both haploid and diploid cells.
- it is also called as equational division as the number of chromosomes in the parent and progeny cells are the same.
- Mitosis is divided into the following four stages:
- It follows the S and G2 phases of interphase.
- The centrioles now begin to move towards opposite poles of the cell.
- In prophase Chromosomal material condenses to form compact mitotic chromosomes.
- Initiation of the assembly of mitotic spindle with the help of the microtubules.
- Cell organelles like Golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope disappear.
- Start of metaphase is marked by the complete disintegration of the nuclear envelope.
- The chromosomes are spread through the cytoplasm of the cell.
- condensation of chromosomes is completed and they can be observed clearly under the microscope.
- This is the stage at which morphology of chromosomes is most easily studied.
- At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere.
- centromere serve as the sites of attachment of spindle fibres to the chromosomes.
- chromosomes are moved into position at the centre of the cell.
- the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole.
- The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate or equatorial plate.
- At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids begin to move towards the two opposite poles.
- As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind
- At the beginning of telophase, the chromosomes at their respective poles decondense and form chromatin network.
- Nuclear envelope assembles around the chromatin network.
- Nucleolus, Golgi complex and ER etc cell organelles reform.
- After karyokinesis the cell itself is divided into two daughter cells by a separate process called cytokinesis.
- In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane.
- The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.
- Plant cells undergo cytokinesis by cell plate method. In cell plate method wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls.
- The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.
- At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.
- In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut). (should be coenocytic)
Significance of mitosis
- Mitosis results in the production of diploid daughter cells with identical genetic complement usually.
- The growth of multicellular organisms is due to mitosis.
- Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. Therefore, cell divide to restore the nucleo-cytoplasmic ratio.
- mitosis is important in cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.
- Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.
- The specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells called
- It is responsible for formation of haploid gametes, which during sexual reproduction form diploid zygote by fusion.
- Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.
- Interphase of meiosis is similar to interphase of mitosis.
- Prophase of the meiosis I division is typically longer and more complex than prophase of mitosis.
- It has been further subdivided into the following five phases based on chromosomal behavior.
- The bivalent chromosomes align on the equatorial plate.
- The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.
- The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
- The nuclear membrane and nucleolus reappear.
- cytokinesis follows telophase I.
- Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The stage between the two meiotic divisions is called interkinesis and is generally short lived.
- Interkinesis is followed by prophase II, a much simpler prophase than prophase I.
Meiosis II resembles a normal mitosis.
- Meiosis II is initiated immediately after cytokinesis.
- The nuclear membrane disappears by the end of prophase II.
- The chromosomes again become compact.
- At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
- splitting of the centromere of each chromosome.
- Chromosomes move toward opposite poles of the cell.
- the two groups of chromosomes once again get enclosed by a nuclear envelope.
- cytokinesis follows resulting in the formation of four haploid daughter cells).
SIGNIFICANCE OF MEIOSIS
- by meiosis conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms.
- It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.
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CHAPTER 10 – CELL CYCLE AND CELL DIVISION
CELL: THE UNIT OF LIFE
- Cell is — Basic unit of life
— Fundamental structural and functional unit of all living organisms.
- Cytology – study of cell and cellular structures.
- All unicellular organisms are capable of
- Independent existence.
- Performing the essential functions of life.
Anything less than a complete structure of a cell does not ensure independent living. Hence, cell is the fundamental structural and functional unit of all living organisms.
- Some important scientists –
|Name of scientist
|Anton von Leeuwenhoek
||first saw and described a live cell
|Schleiden (German botanist), Schwann (British Zoologist)
||Formulated Cell Theory
- Robert hooke first time describe about cell in his book ‘Micrographia’. He actually saw cell wall of dead cells not cell itself.
- Formulated by Schleiden and Schwann.
- Modified by Rudolf Virchow – he explained that new cells develop from pre existing cells by cell division (Omnis cellula-e cellula).
- Exception of cell theory – virus, viriods,
- All living organisms are composed of cells and products of cells.
- Cell is structural unit of life.
- All cells arise from pre-existing cells.
CELL SIZE AND SHAPE
- Smallest cell – mycoplasmas (PPLO – Pleuro Pneumonia Like Organisms)
- Largest cell – egg of an ostrich.
- Smallest cell in human body – Red Blood Cell.
- Largest cell in human body – Ovum.
- Longest cell in human body – Nerve Cell.
Even shape of cells may vary with the functions they perform.
- Represented by Blue Green Algae, mycoplasmas, bacteria etc.
- Cell wall
- Determine shape of cell.
- Provide strong, structural support
- Prevent bacteria from bursting or collapsing
- Plasma membrane
- Structurally similar to that of eukaryotes.
- Formed by extension of plasma membrane into cell.
- In the form of vesicles, tubules and lamella.
- Help in cell wall formation, DNA replication and distribution to daughter cells.
- Also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
- Membranous extensions into cytoplasm.
- Contain pigments.
- In cyanobacteria.
- Present in motile cells.
- Thin filamentous extensions from their cell wall.
- Composed of three parts – filament, hook and basal body.
- Pili and Fimbriae
- Pili are elongated tubular structure while fimbriae are small bristle like fibres.
- Help in attachment of bacteria.
- Associated with the plasma membrane of the cell.
- Made of two subunits – 50S and 30S units which when present together form 70S.
- Site of protein synthesis.
- Ribosome of a polysome translate the mRNA into protein.
- Inclusion bodies
- For storage of reserve material in prokaryotic cells.
- These are not bounded by any membrane system and lie free in the cytoplasm.
- g., phosphate granules, cyanophycean granules and glycogen granules.
- Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.
- Include all the protists, plants, animals and fungi.
- Extensive compartmentalisation of cytoplasm through the presence of membrane bound organelles present.
- possess an organised nucleus with a nuclear envelope.
- genetic material is organised into chromosomes.
- Cell wall
- non-living, rigid structure
- forms an outer covering for the plasma membrane of fungi and plants.
- gives shape to the cell and protects the cell from mechanical damage and infection.
- it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules.
- Middle lamella
- Made up of mainly calcium pectate.
- Holds or glues the different neighbouring cell together.
- Primary wall
- Capable of growth.
- Present in young cell.
- Gradually diminishes as cell matures.
- Madeup of cellulose, hemicelluloses.
- Present in meristem, pith, cortex etc.
- Secondary wall
- Innermost layer.
- Lignified (in sclerenchyma, vesels, tracheids), suberinised (casparian strips, endodermis)
- Suberin, lignin make cell wall impermeable.
- Present in sclerenchyma, collenchyma, and vessels, tracheids.
- Cell wall and middle lamella maybe traversed by plasmodesmata which connects the cytoplasm of neighbouring cells.
- Cell membrane
- Mainly composed of bilayer phospholipids, also possess protein and carbohydrate.
- lipids are arranged within the membrane with the polar head (hydrophilic) towards the outer sides and the nonpolar tails (hydrophobic) towards the inner part.
This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment.
- The ratio of protein and lipid varies in different cell types.
( In human RBC membrane has 52% protein and 40% lipids.)
- Structure of cell membrane is explained by Fluid Mosaic Model which was given by Singer and Nicolsan.
- According to this model the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer.
- The fluid nature of the membrane is important for functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.
Fluid Mosaic Model
- Double membrane bound cell organelle.
- Mitochondria are site of aerobic respiration. They produce ATP, hence called ‘Power House Of Cell’.
- The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins. So, mitochondria also known as ‘semi autonomous organelle’.
- The mitochondria divide by fission and produce new mitochondria.
- Found in all plant cells and in euglenoides.
- They bear some specific pigments, thus imparting specific colours to the plants.
- Chloroplasts are mainly found in the mesophyll cells of the leaves.
- These are various shaped like lens, oval, spherical, discoid, ribbon.
- Double membrane bound Cell organelle. Inner is less permeable than outer.
- There are also stroma lamellae connecting the thylakoids of the different grana.
- Stroma also contains small, double-stranded circular DNA molecules and ribosomes (70S). so, it is also known ‘semi autonomous organelle’.
- Endoplasmic Reticulum
- a network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER)
- Hence, ER divides the intracellular space into two distinct compartments, i.e., luminal(inside ER) and extra luminal(cytoplasm).
- Golgi apparatus
- Discovered by Camillo Golgi.
- They consist of many flat, disc-shaped sacs or cisternae stacked parallely.
- The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face, which are interconnected.
- The golgi apparatus principally performs the function of packaging materials.
- golgi apparatus remains in close association with the endoplasmic reticulum as materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face.
- A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans
- Golgi apparatus is the important site of formation of glycoproteins and glycolipids
- These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus.
- The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH.
- These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.
- Membrane-bound space found in the cytoplasm. Membrane known as tonoplast.
- It contains water, sap, excretory product and other materials not useful for the cell.
- In plant cells the vacuoles are very large.
- In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole.
- In Amoeba the contractile vacuole is important for excretion.
- In many cells food vacuoles are formed by engulfing the food particles.
- first observed under the electron microscope by George Palade.
- They are composed of ribonucleic acid (RNA) and proteins.
- Not Bounded by any membrane.
- The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.
(‘S’ stands for the sedimentation coefficient).
- An elaborate network of filamentous proteinaceous structures present in the cytoplasm
- Functions are mechanical support, motility, maintenance of the shape of the cell.
- Cilia and Flagella
- They are hair like outgrowths of cell membrane responsible for locomotion and movement of cell.
- Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer.
- Eukaryotic cilium and flagellum are covered with plasma membrane.
- Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. (9+2)
- Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.
- Centrosome and centriole
- Centrosome is an organelle usually containing two perpendicularly lying centrioles surrounded by amorphous pericentriolar materials.
- Centriole has an organisation like the cartwheel. They are made up of nine evenly spaced triplet peripheral fibrils of tubulin.
- The central part of the centriole is also proteinaceous and called the hub, connected with peripheral tubules by radial
- The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.
- Many membrane bound minute vesicles called microbodies that contain various enzymes.
- They are present in both plant and animal cells.
- first described by Robert Brown.
- the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.
- The interphase nucleus has nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called
- the nuclear envelope is consists of two parallel membranes with a space inbetween called perinuclear space.
- The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
- At a number of places the nuclear envelope is interrupted by minute pores. These nuclear pores provide passages for movement of RNA and protein molecules.
- Normally, there is only one nucleus per cell.Some mature cells even lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants.
- The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.
- The nucleoli are spherical structures present in the nucleoplasm. It is non-membrane bound. It is a site for active ribosomal RNA synthesis.
- During cell division, chromatin network condenses into c
- Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA.
- Every chromosome essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present.
- Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite.
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CHAPTER 8 – CELL: THE UNIT OF LIFE
STRUCTURAL ORGANISATION IN ANIMALS
- A group of similar cells of common origin along with intercellular substances performing a specific function is known as tissue.
- Animal tissues are broadly classified into four types: (i) Epithelial, (ii) Connective, (iii) Muscular and (iv) Neural.
||Ecto, meso, endodermal
||Protection, absorption, secretion etc.
||To connect, support, transport etc
||Locomotion and movement
||Control and coordination
This tissue has a free surface, which faces either a body fluid or the outside environment and thus provides a covering or a lining for some part of the body.
The cells are compactly packed with little intercellular matrix.
There are two types of epithelial tissues namely simple epithelium and compound epithelium. Simple epithelium –
Connective tissues are most abundant and widely distributed in the body of complex animals.
They are named connective tissues because of their special function of linking and supporting other tissues/organs of the body.
In all connective tissues except blood, the cells secrete fibres of structural proteins called collagen or elastin which provide strength, elasticity and flexibility to the tissue.
These cells also secrete modified polysaccharides, which accumulate between cells and fibres and act as matrix (ground substance).
Connective tissues are classified into three types: (i) Loose connective tissue, (ii) Dense connective tissue and (iii) Specialised connective tissue.
- Each muscle is made of many long, cylindrical fibres arranged in parallel arrays. These fibres are composed of numerous fine fibrils, called myofibrils.
- Muscle fibres contract (shorten) in response to stimulation, then relax (lengthen) and return to their uncontracted state in a coordinated fashion.
- Their action moves the body to adjust to the changes in the environment and to maintain the positions of the various parts of the body.
- In general, muscles play an active role in all the movements of the body.
- Muscles are of three types, skeletal, smooth, and cardiac.
- Neural tissue consists of neuron and neuroglial cells.
- Neural tissue exerts the greatest control over the body’s responsiveness to changing conditions.
- Neuron, an excitable cell is the unit of neural system.
- The neuroglial cells which constitute the rest of the neural system protect and support neurons.
- Neuroglia make up more than one half the volume of neural tissue in our body.
- When a neuron is suitably stimulated, an electrical disturbance is generated which swiftly travels along its plasma membrane.
- Arrival of the disturbance at the neuron’s endings, or output zone, triggers events that may cause stimulation or inhibition of adjacent neurons and other cells
ORGAN AND ORGAN SYSTEM
- Tissues organise to form organs which in turn associate to form organ systems in the multicellular organisms, this results in more efficient and coordinated system of cells.
- Each organ is made of one or more type of tissues.
- Complexity in organ and organ systems displays certain evolutionary trend.
Habits and habitat –
- Earthworm is a reddish brown terrestrial invertebrate that inhabits the upper layer of the moist soil.
- During day time, they live in burrows made by boring and swallowing the soil. In the gardens, they can be traced by their faecal deposits known as worm castings.
- The common Indian earthworms are Pheretima and
- Long cylindrical body.
- Body is divided into many short segments which are similar (metameres about 100-120).
Body surfaces –
- The dorsal surface of the body is marked by a dark median mid dorsal line (dorsal blood vessel) along the longitudinal axis of the body.
- The ventral surface is distinguished by the presence of genital openings (pores).
- Anterior end consists of the mouth and the prostomium, a lobe which serves as a covering for the mouth and as a wedge to force open cracks in the soil into which the earthworm may crawl. The prostomium is sensory in function.
Segments and their related structures –
- The first body segment is called the peristomium (buccal segment) which contains the mouth.
- In a mature worm, 14th, 15th, 16th segments are covered by a prominent dark band of glandular tissue called clitellum. Thus the body is divisible into three prominent regions – preclitellar, clitellar and postclitellar segments.
- Four pairs of spermathecal apertures are situated on the ventro-lateral sides of the intersegmental grooves, i.e., 5th -9th
- A single female genital pore is present in the mid-ventral line of 14th
- A pair of male genital pores are present on the ventro-lateral sides of the 18th
- Numerous minute pores called nephridiopores open on the surface of the body.
- In each body segment, except the first, last and clitellum, there are rows of S-shaped setae, embedded in the epidermal pits in the middle of each segment. Setae can be extended or retracted. Their principal role is in locomotion.
- Layers are outermost thin non-cellular cuticle, epidermis, two muscle layers (circular and longitudinal) and an innermost coelomic epithelium
- The epidermis is made up of a single layer of columnar epithelial cells which contain secretory gland cells.
- It is a straight tube and runs between first to last segment of the body.
- It consists of a terminal mouth, buccal cavity (1-3 segments), muscular pharynx, oesophagus (5-7 segments), muscular gizzard (8-9 segments), stomach (9-14 segments), intestine (15th to last segment), anus.
- Gizzard helps in grinding the soil particles and decaying leaves.
- Calciferous glands, present in the stomach, neutralise the humic acid present in humus.
- A pair of short and conical intestinal caecae project from the intestine on the 26th segment.
- In intestine between 26-35 segments, an internal median fold of dorsal wall called typhlosole is present. It increases the effective area of absorption in the intestine.
- Pheretima exhibits a closed type of blood vascular system, consisting of blood vessels, capillaries and heart.
- Blood is confined to the heart and blood vessels. Contractions keep blood circulating in one direction. Smaller blood vessels supply the gut, nerve cord, and the body wall.
- Blood glands are present on the 4th, 5th and 6th segments. They produce blood cells and haemoglobin which is dissolved in blood plasma.
- Blood cells are phagocytic in nature.
- Earthworms lack specialised breathing devices.
- Respiratory exchange occurs through moist body surface into their blood stream.
- The excretory organs occur as segmentally arranged coiled tubules called nephridia.
- They are of three types (similar in structure) :
- septal nephridia – Present on both the sides of intersegmental septa of segment 15 to the last that open into intestine.
- integumentary nephridia – attached to lining of the body wall of segment 3 to the last that open on the body surface
- pharyngeal nephridia – Present as three paired tufts in the 4th, 5th and 6th segments.
- Nephridia regulate the volume and composition of the body fluids. (osmotic regulation).
- A nephridium starts out as a funnel that collects excess fluid from coelomic chamber. The funnel connects with a tubular part of the nephridium which delivers the wastes through a pore to the surface in the body wall into the digestive tube.
- It is basically represented by ganglia arranged segmentwise on the ventral paired nerve cord.
- The nerve cord in the anterior region (3rd and 4th segments) bifurcates, laterally encircling the pharynx and joins the cerebral ganglia dorsally to form a nerve ring.
- The cerebral ganglia alongwith other nerves in the ring integrate sensory input as well as command muscular responses of the body.
- eyes are absent but does possess light and touch sensitive organs.
- Worms have specialised chemoreceptors (taste receptors) which react to chemical stimuli.
- These sense organs are located on the anterior part of the worm.
- Earthworm is hermaphrodite (bisexual), i.e., testes and ovaries are present in the same individual.
- Male –
- two pairs of testes (10th, 11th segments).
- Their vasa deferentia run up to the 18th segment where they join the prostatic duct.
- Two pairs of accessory glands are present (in the 17th, 19th segments).
- The common prostrate and spermatic duct (vary differential) opens to the exterior by a pair of male genital pores on the ventro-lateral side of the 18th
- Female –
- Four pairs of spermathecae are located in 6th-9th segments (one pair in each segment). They receive and store spermatozoa during copulation.
- One pair of ovaries is attached at the inter-segmental septum of the 12th and 13th
- Ovarian funnels are present beneath the ovaries which continue into oviduct, join together and open on the ventral side as a single median female genital pore on the 14th segment.
- Fertilization –
- It is a protandrous animal with crossfertilisation.
- A mutual exchange of sperm occurs between two worms during mating. One worm has to find another worm and they mate juxtaposing opposite gonadal openings exchanging packets of sperms called spermatophores.
- Mature sperm and egg cells and nutritive fluid are deposited in cocoons produced by the gland cells of clitellum.
- Fertilisation and development occur within the cocoons which are deposited in soil.
- The ova (eggs) are fertilised by the sperm cells within the cocoon which then slips off the worm and is deposited in or on the soil.
- The cocoon holds the worm embryos.
- After about 3 weeks, each cocoon produces two to twenty baby worms with an average of four.
- Earthworms development is direct, i.e., there is no larva formed.
Economical uses –
- Earthworms are known as ‘friends of farmers’ because they make burrows in the soil and make it porous which helps in respiration and penetration of the developing plant roots. The process of increasing fertility of soil by the earthworms is called vermicomposting.
- They are also used as bait in game fishing.
- Brown or black bodied animals.
- Included in class Insecta of Phylum Arthropoda.
- Bright yellow, red and green coloured cockroaches have also been reported in tropical regions.
- Size ranges from ¼ inches to 3 inches (0.6-7.6 cm) and have long antenna, legs and flat extension of the upper body wall that conceals head.
- Nocturnal, Omnivores that live in damp places throughout the world.
- They have become residents of human homes and thus are serious pests and vectors of several diseases.
- Scientific name of the common species of cockroach, Periplaneta Americana.
- They are about 34-53 mm long with wings that extend beyond the tip of the abdomen in males.
- The body of the cockroach is segmented and divisible into three distinct regions – head, thorax and abdomen.
- The entire body is covered by a hard chitinous exoskeleton (brown in colour).
- In each segment, exoskeleton has hardened plates called sclerites (tergites dorsally and sternites ventrally) that are joined to each other by a thin and flexible articular membrane (arthrodial membrane).
- Head is triangular in shape and lies anteriorly at right angles to the longitudinal body axis.
- It is formed by the fusion of six segments and shows great mobility in all directions due to flexible neck.
- The head capsule bears a pair of compound eyes, a pair of thread like antennae which arise from membranous sockets lying in front of eyes. Antennae have sensory receptors that help in monitoring the environment.
- At anterior end of the head, appendages forming biting and chewing type of mouth parts are present. The mouthparts consisting of a labrum (upper lip), a pair of mandibles, a pair of maxillae and a labium (lower lip).
- A median flexible lobe, acting as tongue (hypopharynx), lies within the cavity enclosed by the mouthparts
- It consists of three parts – prothorax, mesothorax and metathorax.
- The head is connected with thorax by a short extension of the prothorax known as the neck.
- Each thoracic segment bears a pair of walking legs.
- The first pair of wings arises from mesothorax and the second pair from metathorax. Forewings (mesothoracic) called tegmina are opaque dark and leathery and cover the hind wings when at rest. The hind wings are transparent, membranous and are used in flight.
- The abdomen in both males and females consists of 10 segments.
- In females, the 7th sternum is boat shaped and together with the 8th and 9th sterna form a brood or genital pouch whose anterior part contains female gonopore, spermathecal pores and collateral glands.
- In males, genital pouch or chamber lies at the hind end of abdomen bounded dorsally by 9th and 10th terga and ventrally by the 9th It contains dorsal anus, ventral male genital pore and gonapophysis.
- Males bear a pair of short, threadlike anal styles which are absent in females.
- In both sexes, the 10th segment bears a pair of jointed filamentous structures called anal cerci.
Digestive system –
- The alimentary canal is divided into three regions: foregut, midgut and hindgut.
- Fore gut –
- Consist of mouth, pharynx, oesophagus, crop, gizzard (Proventriculus).
- Crop is sac like structure for storing of food.
- Gizzard has an outer layer of thick circular muscles and thick inner cuticle forming six highly chitinous plate called teeth. Gizzard helps in grinding the food particles.
- The entire foregut is lined by cuticle.
- Mid gut –
- A ring of 6-8 blind tubules called hepatic or gastric caecae is present at the junction of foregut and midgut, which secrete digestive juice.
- Hind gut –
- At the junction of midgut and hindgut 100-150 yellow coloured thin filamentous Malphigian tubules are present. They help in removal of excretory products from haemolymph.
- The hindgut is broader than midgut and is differentiated into ileum, colon and rectum.
- The rectum opens out through anus.
- Blood vascular system –
- Open type circulatory system.
- Blood vessels are poorly developed and open into space (haemocoel).
- Visceral organs located in the haemocoel are bathed in blood (haemolymph).
- The haemolymph is composed of colourless plasma and haemocytes.
- Heart of cockroach consists of elongated muscular tube lying along mid dorsal line of thorax and abdomen.
- It is differentiated into funnel shaped chambers with ostia on either side.
- Blood from sinuses enter heart through ostia and is pumped anteriorly to sinuses again.
- Respiratory system –
- consists of a network of trachea, that open through 10 pairs of small holes called spiracles present on the lateral side of the body.
- Thin branching tubes (tracheal tubes subdivided into tracheoles) carry oxygen from the air to all the parts.
- The opening of the spiracles is regulated by the sphincters.
- Exchange of gases take place at the tracheoles by diffusion.
- Excretory system –
- Excretion is performed by Malpighian tubules.
- Each tubule is lined by glandular and ciliated cells.
- They absorb nitrogenous waste products and convert them into uric acid which is excreted out through the hindgut. Therefore, this insect is called uricotelic.
- In addition, the fat body, nephrocytes and urecose glands also help in excretion.
- Nervous system –
- It consists of a series of fused, segmentally arranged ganglia joined by paired longitudinal connectives on the ventral side. Three ganglia lie in the thorax, and six in the abdomen.
- The nervous system of cockroach is spread throughout the body.
- The head holds a bit of a nervous system while the rest is situated along the ventral (belly-side) part of its body. So, if the head of a cockroach is cut off, it will still live for as long as one week.
- In the head region, the brain is represented by supra-oesophageal ganglion which supplies nerves to antennae and compound eyes.
- Sense organs –
- In cockroach, the sense organs are antennae, eyes, maxillary palps, labial palps, anal cerci, etc.
- The compound eyes are situated at the dorsal surface of the head. Each eye consists of about 2000 hexagonal ommatidia. With the help of several ommatidia, a cockroach can receive several images of an object. This kind of vision is known as mosaic vision with more sensitivity but less resolution, being common during night (hence called nocturnal vision).
- Reproductive system –
- Cockroaches are dioecious and both sexes have well developed reproductive organs.
- Male reproductive system –
- It consists of a pair of testes (in the 4th -6th abdominal segments), vas deferens, ejaculatory duct, seminal vesicle.
- The ejaculatory duct opens into male gonopore situated ventral to anus.
- A characteristic mushroom shaped gland is present in the 6th-7th abdominal segments which functions as an accessory reproductive gland.
- The external genitalia are represented by male gonapophysis or phallomere (chitinous asymmetrical structures, surrounding the male gonopore).
- The sperms are stored in the seminal vesicles and are glued together in the form of bundles called spermatophores which are discharged during copulation.
- Female reproductive system –
- It consists of two large ovaries (2nd – 6th abdominal segments), oviducts, vagina, genital chamber, spermathecal.
- Each ovary is formed of a group of eight ovarian tubules or ovarioles, containing a chain of developing ova.
- A pair of spermatheca is present in the 6th segment which opens into the genital chamber.
- Sperms are transferred through spermatophores.
- Fertilization and development –
- Fertilization internal.
- Fertilized eggs are encased in capsules called oothecae. Ootheca is a dark reddish to blackish brown capsule, about 3/8″ (8 mm) long.
- They are dropped or glued to a suitable surface, usually in a crack or crevice of high relative humidity near a food source.
- On an average, females produce 9-10 oothecae, each containing 14-16 eggs.
- The development of americana is paurometabolous, meaning there is development through nymphal stage. The nymphs look very much like adults. The nymph grows by moulting about 13 times to reach the adult form.
- The next to last nymphal stage has wing pads but only adult cockroaches have wings.
Habits and habitat
- Frogs can live both on land and in freshwater and belong to class Amphibia of phylum Chordata.
- Most common species of frog found in India is Rana tigrina.
- They do not have constant body temperature i.e.; their body temperature varies with the temperature of the environment. Such animals are called cold blooded or poikilotherms.
- They have the ability to change the colour to hide them from their enemies (camouflage). This protective coloration is called mimicry.
- They take shelter in deep burrows to protect them from extreme heat and cold. This is called as summer sleep (aestivation) and winter sleep (hibernation).
- The skin is smooth and slippery due to the presence of mucus. The skin is always maintained in a moist condition.
- The colour of dorsal side of body is generally olive green with dark irregular spots. On the ventral side the skin is uniformly pale yellow.
- The frog never drinks water but absorb it through the skin.
- Body of a frog is divisible into head and trunk. A neck and tail are absent.
- Above the mouth, a pair of nostrils is present.
- Eyes are bulged and covered by a nictitating membrane that protects them while in water.
- On either side of eyes, a membranous tympanum (ear) receives sound signals.
- The forelimbs and hind limbs help in swimming, walking, leaping and burrowing. The hind limbs end in five digits and they are larger and muscular than fore limbs that end in four digits.
- Feet have webbed digits that help in swimming.
- Frogs exhibit sexual dimorphism. Male frogs can be distinguished by the presence of sound producing vocal sacs and also a copulatory pad on the first digit of the fore limbs which are absent in female frogs.
Digestive System –
- It consists of alimentary canal and digestive glands.
- The alimentary canal is short because frogs are carnivores and hence the length of intestine is reduced.
- Alimentary canal consists of mouth, buccal cavity, pharynx, oesophagus, stomach, intestine, rectum and cloaca.
- Food is captured by the bilobed tongue.
- Digestion of food takes place by the action of HCl and gastric juices secreted from the walls of the stomach.
- Partially digested food called chyme is passed from stomach to the first part of the intestine, the duodenum.
- Liver secretes bile that is stored in the gall bladder.
- Pancreas produces pancreatic juice containing digestive enzymes.
- The duodenum receives bile from gall bladder and pancreatic juices from the pancreas through a common bile duct.
- Bile emulsifies fat and pancreatic juices digest carbohydrates and proteins.
- Final digestion takes place in the intestine.
- Digested food is absorbed by the numerous finger-like folds in the inner wall of intestine called villi and microvilli.
- The undigested solid waste moves into the rectum and passes out through cloaca.
Respiratory system –
- Frogs respire on land and in the water by two different methods.
- In water, skin acts as aquatic respiratory organ (cutaneous respiration). Dissolved oxygen in the water is exchanged through the skin by diffusion.
- On land, the buccal cavity, skin and lungs act as the respiratory organs.
- The respiration by lungs is called pulmonary respiration. The lungs are a pair of elongated, pink coloured sac-like structures present in the upper part of the trunk region (thorax). Air enters through the nostrils into the buccal cavity and then to lungs.
- During aestivation and hibernation gaseous exchange takes place through skin.
Circulatory system –
- The vascular system of frog is well-developed closed type.
- Frogs have a lymphatic system also.
- The blood vascular system involves heart, blood vessels and blood.
- The lymphatic system consists of lymph, lymph channels and lymph nodes.
- Heart is a muscular structure situated in the upper part of the body cavity.
- It has three chambers, two atria and one ventricle and is covered by a membrane called pericardium.
- A triangular structure called sinus venosus joins the right atrium. It receives blood through the major veins called vena cava.
- The ventricle opens into a saclike conus arteriosus on the ventral side of the heart.
- The blood from the heart is carried to all parts of the body by the arteries (arterial system).
- The veins collect blood from different parts of body to the heart and form the venous system.
- Special venous connection between liver and intestine as well as the kidney and lower parts of the body are present in frogs. The former is called hepatic portal system and the latter is called renal portal system.
- The blood is composed of plasma and cells.
- The blood cells are RBC (red blood cells) or erythrocytes, WBC (white blood cells) or leucocytes and platelets.
- RBC’s are nucleated and contain red coloured pigment namely haemoglobin.
- The lymph is different from blood.
- It lacks few proteins and RBCs.
- The blood carries nutrients, gases and water to the respective sites during the circulation.
- The circulation of blood is achieved by the pumping action of the muscular heart.
Excretory system –
- The elimination of nitrogenous wastes is carried out by a well-developed excretory system.
- The excretory system consists of a pair of kidneys, ureters, cloaca and urinary bladder.
- Kidneys are compact, dark red and bean like structures situated a little posteriorly in the body cavity on both sides of vertebral column.
- Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons.
- Two ureters emerge from the kidneys in the male frogs. The ureters act as urinogenital duct which opens into the cloaca.
- In females the ureters and oviduct open seperately in the cloaca.
- The thin-walled urinary bladder is present ventral to the rectum which also opens in the cloaca.
- The frog excretes urea and thus is a ureotelic
- Excretory wastes are carried by blood into the kidney where it is separated and excreted.
- The chemical coordination of various organs of the body is achieved by hormones which are secreted by the endocrine glands.
- The prominent endocrine glands found in frog are pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.
Nervous system –
- The nervous system is organised into a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves) and an autonomic nervous system (sympathetic and parasympathetic).
- There are ten pairs of cranial nerves arising from the brain.
- Brain is enclosed in a bony structure called brain box (cranium).
- The brain is divided into fore-brain, mid-brain and hind-brain.
- Forebrain includes olfactory lobes, paired cerebral hemispheres and unpaired diencephalon.
- The midbrain is characterised by a pair of optic lobes.
- Hind-brain consists of cerebellum and medulla oblongata.
- The medulla oblongata passes out through the foramen magnum and continues into spinal cord, which is enclosed in the vertebral column.
Sense organs –
- Frog has different types of sense organs, namely organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with internal ears).
- Eyes and internal ears are well-organised structures and the rest are cellular aggregations around nerve endings.
- Eyes in a frog are a pair of spherical structures situated in the orbit in skull. These are simple eyes (possessing only one unit).
- External ear is absent in frogs and only tympanum can be seen externally. The ear is an organ of hearing as well as balancing (equilibrium).
Reproductive system –
- Frogs have well organised male and female reproductive systems.
- Male reproductive system –
- It consists of a pair of yellowish ovoid testes, which are found adhered to the upper part of kidneys by a double fold of peritoneum called mesorchium.
- Vasa efferentia are 10-12 in number that arise from testes.
- They enter the kidneys on their side and open into Bidder’s canal.
- Finally, it communicates with the urinogenital duct that comes out of the kidneys and opens into the cloaca.
- The cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms to the exterior.
- Female reproductive system –
- It includes a pair of ovaries. The ovaries are situated near kidneys and there is no functional connection with kidneys.
- A pair of oviduct arising from the ovaries opens into the cloaca separately.
- A mature female can lay 2500 to 3000 ova at a time.
- Fertilisation and development –
- Fertilization is external and takes place in water.
- Development involves a larval stage called tadpole.
- Tadpole undergoes metamorphosis to form the adult.
Economic importance –
- Frogs are beneficial for mankind because they eat insects and protect the crop.
- Frogs maintain ecological balance because these serve as an important link of food chain and food web in the ecosystem.
- In some countries the muscular legs of frog are used as food by man.
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CHAPTER 7: STRUCTURAL ORGANISATION IN ANIMALS
This is the Question series where we will put new questions daily based on NEET Syllabus.
DQS – 01
DQS – 02
DQS – 03
DQS – 04
DQS – 05
DQS – 06