NEET 2017 Biology solution (With NCERT Reference)

NEET 2017 Biology solution

(With NCERT Reference)

1. Which one of the following statements is correct, with reference to enzymes?

  1. Holoenzyme = Apoenzyme + Coenzyme
  2. Coenzyme = Apoenzyme + Holoenzyme
  3. Holoenzyme = Coenzyme + Co-factor
  4. Apoenzyme = Holoenzyme + Coenzyme

Ans. (1) [NCERT 11th Page 159]

2. A decrease in blood pressure / volume will not cause the release of:

(1) Atrial natriuretic factor

(2) Aldosterone

(3) ADH

(4) Renin
Ans. (1) [NCERT 11th Page 297]

3. Which cells of “Crypts of Lieberkuhn” secrete antibacterial lysozyme?

(1) Paneth cells

(2) Zymogen cells

(3) Kupffer cells

(4) Argentaffin cells
Ans. (1)

4. Which of the following are not polymeric?

(1) Proteins

(2) Polysaccharides

(3) Lipids

(4) Nucleic acids
Ans. (3) [NCERT 11th Page 146]

5. Functional megaspore in an angiosperm develops into?

(1) Endosperm

(2) Embryo sac

(3) Embryo

(4) Ovule
Ans. (2) [NCERT 12th Page 26]

6. Myelin sheath is produced by:

  1. Astrocytes and Schwann cells
  2. Oligodendrocytes and Osteoclasts
  3. Osteoclasts and Astrocytes
  4. Schwann cells and Oligodendrocytes

Ans. (4) [NCERT 11th Page 317]

7. Attractants and rewards are required for :

(1) Entomophily

(2) Hydrophily

(3) Cleistogamy

(4) Anemophily
Ans. (1) [NCERT 12th Page 30,237]

8. Receptor sites for neurotransmitters are present on :

  1. Pre-synaptic membrane
  2. Tips of axons
  3. Post-synaptic membrane
  4. Membrane of synaptic vesicles

Ans. (3) [NCERT 11th Page 320]

9. Coconut fruit is a :

(1) Berry

(2) Nut

(3) Capsule

(4) Drupe
Ans. (4) [NCERT 11th Page 76]

10. Adult human RBCs are enucleated. Which of the following statement(s) is/are most appropriate explanation for this feature?

a. They do not need to reproduce

b. They are somatic cells

c. They do not metabolize

d. All their internal space is available for oxygen transport
(1) only (a)

(2) (a), (c) and (d)

(3) (b) and (c)

(4) only (d)
Ans. (4)

11. Capacitation occurs in:

  1. Epididymis
  2. Vas deferens
  3. Female reproductive tract
  4. Rete testis

Ans. (3)

12. Which of the following are found in extreme saline conditions ?

(1) Eubacteria

(2) Cyanobacteria

(3) Mycobacteria

(4) Archaebacteria
Ans. (4) [NCERT 11th Page 19]

13. Asymptote in a logistic growth curve is obtained when :

  1. K = N
  2. K > N
  3. K < N
  4. The value of ‘r’ approaches zero

Ans. (1) [NCERT 12th Page 231]

14. Artificial selection to obtain cows yielding higher milk output represents :

  1. Directional as it pushes the mean of the character in one direction
  2. Disruptive as it splits the population into two, one yielding higher output and the other lower output
  3. Stabilizing followed by disruptive as it stabilizes the population to produce higher yielding cows
  4. Stabilizing selection as it stabilizes this character in the population

Ans. (1) [NCERT 12th Page 136]

15. Select the mismatch :

  1. Rhodospirillum – Mycorrhiza
  2. Anabaena – Nitrogen fixer
  3. Rhizobium – Alfalfa
  4. Frankia – Alnus

Ans. (1) [NCERT 11th Page 202]

16. Good vision depends on adequate intake of carotene rich food :

Select the best option from the following statements :

  1. Vitamin A derivatives are formed from carotene
  2. The photopigments are embedded in the membrane discs of the inner segment
  3. Retinal is a derivative of Vitamin A
  4. Retinal is a light absorbing part of all the visual photopigments

Options :
(1) (a), (c) and (d) (2) (a) and (c)

(3) (b), (c) and (d) (4) (a) and (b)
Ans. (1) [NCERT 11th Page 324]

17. The DNA fragments separated on an agarose gel can be visualised after staining with :

(1) Acetocarmine

(2) Aniline blue

(3) Ethidium bromide

(4) Bromophenol blue
Ans. (3) [NCERT 12th Page 198]

18. The hepatic portal vein drains blood to liver from :

(1) Stomach (2) Kidneys (3) Intestine (4) Heart
Ans. (3) [NCERT 11th Page 286]

19. The vascular cambium normally gives rise to :

(1) Primary phloem (2) Secondary xylem (3) Periderm (4) Phelloderm
Ans. (2) [NCERT 11th Page 95]

20. Thalassemia and sickle cell anemia are caused due to a problem in globin molecule synthesis. Select the correct statement :

  1. Both are due to a quantitative defect in globin chain synthesis
  2. Thalassemia is due to less synthesis of globin molecules
  3. Sickel cell anemia is due to a quantitative problem of globin molecules
  4. Both are due to a qualitative defect in globin chain synthesis

Ans. (2) [NCERT 12th Page 89]

21. The genotypes of a husband and Wife are IAIB and IAi

Among the blood types of their children, how many different genotypes and phenotypes are possible?

  1. 3 genotypes ; 4 phenotypes
  2. 4 genotypes ; 3 phenotypes
  3. 4 genotypes ; 4 phenotypes
  4. 3 genotypes ; 3 phenotypes

Ans. (2) [NCERT 12th Page 77]

22. Which of the following facilitates opening of stomatal aperture ?

  1. Decrease in turgidity of guard cells
  2. Radial orientation of cellulose microfibrils in the cell wall of guard cells
  3. Longitudinal orientation of cellulose microfibrils in the cell wall of guard cells
  4. Contraction of outer wall of guard cells

Ans. (2) [NCERT 11th Page 187]

23. In Bougainvillea thorns are the modifications of :

(1) Adventitious root (2) Stem (3) Leaf (4) Stipules
Ans. (2) [NCERT 11th Page 68]

24. Which one of the following is related to Ex-situ conservation of threatened animals and plants ?

  1. Biodiversity hot spots
  2. Amazon rainforest
  3. Himalayan region
  4. Wildlife safari parks

Ans. (4) [NCERT 12th Page 267]

25. Root hairs develop from the region of :

(1) Elongation (2) root cap (3) Meristematic activity (4) Maturation
Ans. (4) [NCERT 11th Page 67]

26. A disease caused by an autosomal primary nondisjunction is :

(1) Klinefelter’s Syndrome

(2) Turner’s Syndrome

(3) Sickel Cell Anemia

(4) Down’s Syndrome
Ans. (4) [NCERT 12th Page 91]

27. The water potential of pure water is :

  1. Less than zero
  2. More than zero but less than one
  3. More than one
  4. Zero

Ans. (4) [NCERT 11th Page 179]

28. Which of the following options gives the correct sequence of events during mitosis ?

1. Condensation ( nuclear membrane disassembly ( arrangement at equator ( centromere division ( segregation ( telophase
2.Condensation ( crossing over ( nuclear membrane disassembly ( segregation ( telophase
3. Condensation ( arrangement at equator ( centromere division ( segregation ( telophase

4. Condensation ( nuclear membrane disassembly ( crossing over ( segregation ( telophase
Ans. (1) [NCERT 11th Page 164-166]

29. The process of separation and purification of expressed protein before marketing is called :

  1. Downstream processing
  2. Bioprocessing
  3. Postproduction processing
  4. Upstream processing

Ans. (1) [NCERT 12th Page 204]

30. A temporary endocrine gland in the human body is :

(1) Corpus cardiacum (2) corpus luteum (3) Corpus allatum (4) Pineal gland
Ans. (2) [NCERT 11th Page 332,337; 12th Pg 51]

31. Which of the following is made up of dead cells?

(1) Collenchyma (2) Phellem
(3) Phloem (4) Xylem parenchyma
Ans. (2) [NCERT 11th Page 86,87,96]

32. An example of colonial alga is :

(1) Volvox (2) Ulothrix (3) Spirogyra (4) Chlorella
Ans. (1) [NCERT 11th Page 30,31]

33. Match the following sexually transmitted diseases (Column-I) with their causative agent (Column-II) and select the correct option :

Column-I Column-II

(a)Gonorrhea. (i)HIV

(b) Syphilis. (ii)Neisseria

(c)Genital Warts (iii) Treponema

(d) AIDS (iv) Human papilloma-Virus

(a) (b) (c) (d)
iii iv i ii
iv ii iii i
iv iii ii i
ii iii iv i
Ans. (4) [NCERT 12th Page 63]

34. The function of copper ions in copper releasing IUD’s is :

  1. They inhibit gametogenesis
  2. They make uterus unsuitable for implantation
  3. They inhibt ovulation
  4. The suppress sperm motility and fertilising capacity of sperms

Ans. (4) [NCERT 12th Page 60]

35. Which of the following in sewage treatment removes suspended solids ?

(1) Secondary treatment (2) Primary treatment
(3) Sludge treatment (4) Tertiary treatment
Ans. (2) [NCERT 12th Page 184]

36. An important characteristic that Hemichordates share with Chordates is :

  1. Ventral tubular nerve cord
  2. Pharynx with gill slits
  3. Pharynx without gill slits
  4. Absence of notochord

Ans. (2) Gills are given but gill slits are not [NCERT 11th Page 54]

37. The final proof for DNA as the genetic material came from the experiments of :

  1. Hershey and Chase
  2. Avery, Mcleod and McCarty
  3. Hargobind Khorana
  4. Griffith

Ans. (1) [NCERT 12th Page 101]

38. Among the following characters, which one was not considered by Mendel in his experiments on pea ?

  1. Trichomes – Glandular or non-glandular
  2. Seed – Green or Yellow
  3. Pod – Inflated or Constricted
  4. Stem – Tall or Dwarf

Ans. (1) [NCERT 12th Page 71]

39. Plants which produce characteristic pneumatophores and show vivipary belong to :

(1) Halophytes (2) Psammophytes (3) Hydrophytes (4) Mesophytes
Ans. (1) viviparous germination is not given [NCERT 11th Page 67]

40. The pivot joint between atlas and axis is a type of :

(1) Cartilaginous joint (2) Synovial joint (3) Saddle joint (4) Fibrous joint
Ans. (2) [NCERT 11th Page 312]

41. With reference to factors affecting the rate of photosynthesis, which of the following statements is not correct ?

  1. Increasing atmospheric CO2 concentration up to 0.05% can enhance CO2 fixation rate
  2. C3 plants respond to higher temperatures with enhanced photosynthesis while C4 plants have much lower temperature optimum
  3. Tomato is a greenhouse crop which can be grown in CO2 – enriched atmosphere for higher yield
  4. Light saturation for CO2 fixation occurs at 10% of full sunlight

Ans. (2) All four statements are clearly mentioned. [NCERT 11th Page 223, 224]

42. DNA fragments are:

  1. Negatively charged
  2. Neutral
  3. Either positively or negatively charged depending on their size
  4. Positively charged

Ans. (1) [NCERT 12th Page 198]

43. Which of the following components provides sticky character to the bacterial cell ?

  1. Nuclear membrane
  2. Plasma membrane
  3. Glycocalyx
  4. Cell wall

Ans. (3) Exactly sticky character is not given but its very obvious [NCERT 11th Page 128]

44. Which of the following options best represents the enzyme composition of pancreatic juice?

  1. amylase, pepsin, trypsinogen, maltase
  2. peptidase, amylase, pepsin, rennin
  3. Lipase, amylase, trypsinogen, procarboxypeptidase
  4. amylase, peptidase, trypsinogen, rennin

Ans. (3) [NCERT 11th Page 262]

45. Which among these is the correct combination of aquatic mammals ?

  1. Dolphins, Seals, Trygon
  2. Whales, Dolphins, Seals
  3. Trygon, Whales, Seals
  4. Seals, Dolphins, Sharks

Ans. (2) Seals is not given [NCERT 11th Page 56,57,60]

46. Fruit and leaf drop at early stages can be prevented by the application of:

(1) Ethylene (2) Auxins (3) Gibberellic acid (4) Cytokinins
Ans. (2) [NCERT 11th Page 248]

47. Select the correct route for the passage of sperms in male frogs:

  1. Testes –> Vasa efferentia –>Kidney –>Seminal Vesicle –> Urinogenital duct –> Cloaca
  2. Testes –>Vasa efferentia –> Bidder’s canal —> Ureter –> Cloaca
  3. Testes –>Vasa efferentia –> Kidney –>Bidder’s canal –> Urinogenital duct –> Cloaca
  4. Testes –> Bidder’s canal –> Kidney –> Vasa efferentia –> Urinogenital duct –> Cloaca

Ans. (3) [NCERT 11th Page 119]

48. In case of a couple where the male is having a very low sperm count, which technique will be suitable for fertilisation?

  1. Gamete intracytoplasmic fallopian transfer
  2. Artificial Insemination
  3. Intracytoplasmic sperm injection
  4. Intrauterine transfer

Ans. (2) [NCERT 12th Page 64]

49. Which ecosystem has the maximum biomass?

  1. Grassland ecosystem
  2. Pond ecosystem
  3. Lake ecosystem
  4. Forest ecosystem

Ans. (4)

50. Lungs are made up of air-filled sacs, the alveoli. They do not collapse even after forceful expiration, because of:

  1. Inspiratory Reserve Volume
  2. Tidal Volume
  3. Expiratory Reserve Volume
  4. Residual Volume

Ans. (4) [NCERT 11th Page 272]

51. Presence of plants arranged into well defined vertical layers depending on their height can be seen best in:

(1) Tropical Rain Forest (2) Grassland
(3) Temperate Forest (4) Tropical Savannah
Ans. (1) not given in exact wordings in NCERT.

52. Which of the following statements is correct?

  1. The descending limb of loop of Henle is impermeable to water.
  2. The ascending limb of loop of Henle is permeable to water.
  3. The descending limb of loop of Henle is permeable to electrolytes.
  4. The ascending limb of loop of Henle is impermeable to water.

Ans. (4) [NCERT 11th Page 294 ]

53. Alexander Von Humbolt described for the first time:

  1. Laws of limiting factor
  2. Species area relationships
  3. Population Growth equation
  4. Ecological Biodiversity

Ans. (2) [NCERT 11th Page 262]

54. Zygotic meiosis is characteristic of;

(1) Fucus (2) Funaria (3) Chlamydomonas (4) Marchantia
Ans. (3) [NCERT 11th Page 42]

55. If there are 999 bases in an RNA that codes for a protein with 333 amino acids, and the base at position 901 is deleted such that the length of the RNA becomes 998 bases, how many codons will be altered?

(1) 11 (2) 33 (3) 333 (4) 1
Ans. (2) Based on Frameshift mutation [NCERT 12th Page 114]

56. Flowers which have single ovule in the ovary and are packed into inflorescence are usually pollinated by:

(1) Bee (2) Wind (3) Bat (4) Water
Ans. (2) [NCERT 12th Page 29]

57. Transplantation of tissues/organs fails often due to non-acceptance by the patient’s body. Which type of immune-response is responsible for such rejections?

(1) Cell – mediated immune response
(2) Hormonal immune response
(3) Physiological immune response
(4) Autoimmune response
Ans. (1) [NCERT 12th Page 152]

58. Life cycle of Ectocarpus and Fucus respectively are:

  1. Diplontic, Haplodiplontic
  2. Haplodiplontic, Diplontic
  3. Haplodiplontic, Haplontic
  4. Haplontic, Diplontic

Ans. (2) [NCERT 11th Page 43]

59. A gene whose expression helps to identify transformed cell is known as :

(1) Vector (2) Plasmid (3) Structural gene (4) Selectable marker
Ans. (4) [NCERT 12th Page 199]

60. A dioecious flowering plant prevents both :

(1) Autogamy and geitonogamy
(2) Geitonogamy and xenogamy
(3) Cleistogamy and xenogamy
(4) Autogamy and xenogamy
Ans. (1) [NCERT 12th Page 31]

61. Which statement is wrong for Krebs’ cycle ?

  1. There is one point in the cycle where FAD+ is reduced to FADH2
  2. During conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised
  3. The cycle starts with condensation of acetyl group (acetyl CoA) with pyruvic acid to yield citric acid
  4. There are three points in the cycle where NAD+ is reduced to NADH+ H+

Ans. (3) [NCERT 11th Page 231]

62. Phosphoenol pyruvate (PEP) is the primary CO2 acceptor in:

  1. C4 plants
  2. C2 plants
  3. C3 and C4 plants
  4. C3 plants

Ans. (1) [NCERT 11th Page 219]

63. During DNA replication, Okazaki fragments are used to elongate:

  1. The lagging strand towards replication fork.
  2. The leading strand away from replication fork.
  3. The lagging strand away from the replication fork.
  4. The leading strand towards replication fork.

Ans. (3) [NCERT 12th Page 107, Fig 6.8]

64. Which of the following RNAs should be most abundant in animal cell ?

(1) t-RNA (2) m-RNA (3) mi-RNA (4) r-RNA
Ans. (4)

65. GnRH, a hypothalamic hormone, needed in reproduction, acts on:

  1. anterior pituitary gland and stimulates secretion of LH and FSH.
  2. posterior pituitary gland and stimulates secretion of oxytocin and FSH.
  3. posterior pituitary gland and stimulates secretion of LH and relaxin.
  4. anterior pituitary gland and stimulates secretion of LH and oxytocin.

Ans. (1) [NCERT 12th Page 47]

66. What is the criterion for DNA fragments movement on agarose gel during gel electrophoresis?

  1. The smaller the fragment size, the farther it moves
  2. Positively charged fragments move to farther end
  3. Negatively charged fragments do not move
  4. The larger the fragment size, the farther it moves

Ans. (1) [NCERT 12th Page 198]

67. Hypersecretion of Growth Hormone in adults does not cause further increase in height, because:

  1. Epiphyseal plates close after adolescence.
  2. Bones loose their sensitivity to Growth Hormone in adults.
  3. Muscle fibres do not grow in size after birth.
  4. Growth Hormone becomes inactive in adults.

Ans. (1)

68. DNA replication in bacteria occurs:

  1. Within nucleolus
  2. Prior to fission
  3. Just before transcription
  4. During S phase

Ans. (2)

69. Which one from those given below is the period for Mendel’s hybridization experiments ?

  1. 1840 – 1850
  2. 1857 – 1869
  3. 1870 – 1877
  4. 1856 – 1863

Ans. (4) [NCERT 12th Page 70]

70. Viroids differ from viruses in having;

  1. DNA molecules without protein coat
  2. RNA molecules with protein coat
  3. RNA molecules without protein coat
  4. DNA molecules with protein coat

Ans. (3) [NCERT 11th Page 27]

71. MALT constitutes about percent of the lymphoid tissue in human body.

(1) 20% (2) 70% (3) 10% (4) 50%
Ans. (4) [NCERT 12th Page 154]

72. Which of the following is correctly matched for the product produced by them ?

  1. Methanobacterium : Lactic acid
  2. Penicillium notatum : Acetic acid
  3. Saccharomyces cerevisiae : Ethanol
  4. Acetobacter aceti: Antibiotics

Ans. (3) [NCERT 12th Page 182]

73. Which among the following are the smallest living cells, known without a definite cell wall, pathogenic to plants as well as animals and can survive without oxygen ?

(1) Pseudomonas (2) Mycoplasma (3) Nostoc (4) Bacillus
Ans. (2) [NCERT 11th Page 20]

74. Which of the following represents order of Horse?

(1) Perissodactyla (2) Caballus (3) Ferus (4) Equidae
Ans. (1)

75. Frog’s heart when taken out of the body continues to beat for sometime.

Select the best option from the following statements.

  1. Frog is a poikilotherm.
  2. Frog does not have any coronary circulation.
  3. Heart is “myogenic” in nature.
  4. Heart is auto excitable

(1) Only(d) (2) (a) and (b) (3) (c)and(d) (4) Only(c)
Ans. (3) [NCERT 11th Page 287]

76. Homozygous pure lines in cattle can be obtained by:

  1. mating of unrelated individuals of same breed.
  2. mating of individuals of different breed.
  3. mating of individuals of different species.
  4. mating of related individuals of same breed.

Ans. (4) [NCERT 12th Page 167]

77. Identify the wrong statement in context of heartwood:

  1. It is highly durable
  2. It conducts water and minerals efficiently
  3. It comprises dead elements with highly lignified walls
  4. Organic compounds are deposited in it

Ans. (2) [NCERT 11th Page 96]

78. Anaphase Promoting Complex (APC) is a protein degradation machinery necessary for proper mitosis of animal cells. If APC is defective in a human cell, which of the following is expected to occur?

  1. Chromosomes will be fragmented
  2. Chromosomes will not segregate
  3. Recombination of chromosome arms will occur
  4. Chromosomes will not condense

Ans. (2)

79. Which of the following cell organelles is responsible for extracting energy from carbohydrates to form ATP ?

(1) Ribosome (2) Chloroplast (3) Mitochondrion (4) Lysosome
Ans. (3) [NCERT 11th Page 135]

80. Mycorrhizae are the example of:

(1) Amensalism (2) Antibiosis (3) Mutualism (4) Fungistasis
Ans. (3) [NCERT 12th Page 237]

81. Out of ‘X’ pairs of ribs in humans only Y pairs are true ribs. Select the option that correctly represents values of X and Y and provides their explanation:

  1. X = 12, Y = 5 True ribs are attached dorsally to vertebral column and sternum on the two ends.
  2. X = 24, Y = 7 True ribs are dorsally attached to vertebral column but are free on ventral side.
  3. X = 24, Y = 12 True ribs are dorsally attached to vertebral column but are free on ventral side.
  4. X = 12, Y = 7 True ribs are attached dorsally to vertebral column and ventrally to the sternum.

Ans. (4) [NCERT 11th Page 310]

82. In case of poriferans, the spongocoel is lined with flagellated cells called:

  1. oscula
  2. choanocytes
  3. mesenchymal cells
  4. ostia

Ans. (2) [NCERT 11th Page 49]

83. Which one of the following statements is not valid for aerosols?

  1. They alter rainfall and monsoon patterns
  2. They cause increased agricultural productivity
  3. They have negative impact on agricultural land
  4. They are harmful to human health

Ans. (2)

84. A baby boy aged two years is admitted to play school and passes through a dental check – up. The dentist observed that the boy had twenty teeth. Which teeth were absent?

  1. Canines
  2. Pre-molars
  3. Molars
  4. Incisors

Ans. (2)

85. Select the mismatch

  1. Cycas – Dioecious
  2. Salvinia – Heterosporous
  3. Equisetum – Homosporous
  4. Pinus – Dioecious

Ans. (4) [NCERT 11th Page 39]

86. The morphological nature of the edible part of coconut is:

  1. Cotyledon
  2. Endosperm
  3. Pericarp
  4. Perisperm

Ans. (2) [NCERT 12th Page 35]

87. Double fertilization is exhibited by :

  1. Algae
  2. Fungi
  3. Angiosperms
  4. Gymnosperms

Ans. (3) [NCERT 12th Page 34]

88. Spliceosomes are not found in cells of;

(1) Fungi (2) Animals (3) Bacteria (4) Plants
Ans. (3) [NCERT 12th Page 111]

89. The association of histone HI with a nucleosome indicates:

  1. DNA replication is occurring.
  2. The DNA is condensed into a Chromatin Fibre.
  3. The DNA double helix is exposed.
  4. Transcription is occurring.

Ans. (2)

90. The region of Biosphere Reserve which is legally protected and where no human activity is allowed is known as:

  1. Buffer zone
  2. Transition zone
  3. Restoration zone
  4. Core zone

Ans. (4)

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Neet 2017 biology solution





  • All living organisms require macromolecules, such as carbohydrates, proteins and fats, and water and minerals for their growth and development.

Methods to Study the Mineral Requirements of Plants

  • In 1860, Julius von Sachs, a prominent German botanist, demonstrated, for the first time, that plants could be grown to maturity in a defined nutrient solution in complete absence of soil. This technique of growing plants in a nutrient solution is known as hydroponics.
  • Culture of plants in a soil-free mineral solution require purified water and mineral nutrient salts.
  • After a series of experiments in which the roots of the plants were immersed in nutrient solutions and wherein an element was added / removed or given in varied concentration, a mineral solution suitable for the plant growth was obtained. By this method, essential elements were identified and their deficiency symptoms discovered.
  • Hydroponics has been successfully employed as a technique for the commercial production of vegetables such as tomato, seedless cucumber and lettuce.
  • It must be emphasised that the nutrient solutions must be adequately aerated to obtain the optimum growth.

Essential Mineral Elements

  • Most of the minerals present in soil can enter plants through roots.
  • In fact, more than sixty elements of the 105 discovered so far are found in different plants.
  • Some plant species accumulate selenium, some others gold, while some plants growing near nuclear test sites take up radioactive strontium.
  • There are techniques that are able to detect the minerals even at a very low concentration (10-8 g/ mL).

Criteria for Essentiality

The criteria for essentiality of an element are given below:

  1. The element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element the plants do not complete their life cycle or set the seeds.
  2. The requirement of the element must be specific and not replaceable by another element. In other words, deficiency of any one element cannot be met by supplying some other element.
  3. The element must be directly involved in the metabolism of the plant.

Based upon the above criteria only a few elements have been found to be absolutely essential for plant growth and metabolism.

These elements are further divided into two broad categories based on their quantitative requirements.

Macronutrients, and

  • Macronutrients are generally present in plant tissues in large amounts (in excess of 10 mmole Kg -1 of dry matter).
    • The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium.
    • Of these, carbon, hydrogen and oxygen are mainly obtained from CO2 and H2O, while the others are absorbed from the soil as mineral nutrition.
  • Micronutrients or trace elements, are needed in very small amounts (less than 10 mmole Kg 1 of dry matter).
    • These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.
    • In addition to the 17 essential elements named above, there are some beneficial elements such as sodium, silicon, cobalt and selenium. They are required by higher plants.

Essential elements can also be grouped into four broad categories on the basis of their diverse functions. These categories are:

  1. Essential elements as components of biomolecules and hence structural elements of cells (e.g., carbon, hydrogen, oxygen and nitrogen).
  2. Essential elements that are components of energy-related chemical compounds in plants (e.g., magnesium in chlorophyll and phosphorous in ATP).
  3. Essential elements that activate or inhibit enzymes, for example Mg2+ is an activator for both ribulose bisphosphate carboxylase- oxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation; Zn2+ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism. For this, you will need to recollect some of the biochemical pathways you have studied earlier.
  4. Some essential elements can alter the osmotic potential of a cell. Potassium plays an important role in the opening and closing of stomata. You may recall the role of minerals as solutes in determining the water potential of a cell.

Role of Macro- and Micro-nutrients

Nitrogen :

  • This is the mineral element required by plants in the greatest amount.
  • absorbed mainly as – NO3 though some are also taken up as NO2 or NH4+.
  • Nitrogen is required by all parts of a plant, particularly the meristematic tissues and the metabolically active cells.
  • Nitrogen is one of the major constituents of proteins, nucleic acids, vitamins and hormones.


  • Phosphorus is absorbed by the plants from soil in the form of phosphate ions (either as H2PO4 or HPO2-).
  • Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides, and is required for all phosphorylation reactions.


  • It is absorbed as potassium ion (K+).
  • In plants, this is required in more abundant quantities in the meristematic tissues, buds, leaves and root tips.
  • Potassium helps to maintain an anion-cation balance in cells and is involved in protein synthesis, opening and closing of stomata, activation of enzymes and in the maintenance of the turgidity of cells.


  • Plant absorbs calcium from the soil in the form of calcium ions (Ca2+).
  • Calcium is required by meristematic and differentiating tissues.
  • During cell division it is used in the synthesis of cell wall, particularly as calcium pectate in the middle lamella.
  • It is also needed during the formation of mitotic spindle.
  • It accumulates in older leaves.
  • It is involved in the normal functioning of the cell membranes. It activates certain enzymes and plays an important role in regulating metabolic activities.


  • It is absorbed by plants in the form of divalent Mg2+.
  • It activates the enzymes of respiration, photosynthesis and are involved in the synthesis of DNA and RNA.
  • Magnesium is a constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure.


  • Plants obtain sulphur in the form of sulphate (SO2-).
  • Sulphur is present in two amino acids – cysteine and methionine and is the main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and ferredoxin.


  • Plants obtain iron in the form of ferric ions (Fe3+).
  • It is required in larger amounts in comparison to other micronutrients.
  • It is an important constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes.
  • It is reversibly oxidised from Fe2+ to Fe3+ during electron transfer.
  • It activates catalase enzyme, and is essential for the formation of chlorophyll.


  • It is absorbed in the form of manganous ions (Mn2+).
  • It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism.
  • The best defined function of manganese is in the splitting of water to liberate oxygen during photosynthesis.


  • Plants obtain zinc as Zn2+ ions.
  • It activates various enzymes, especially carboxylases.
  • It is also needed in the synthesis of auxin.


  • It is absorbed as cupric ions (Cu2+).
  • It is essential for the overall metabolism in plants.
  • Like iron, it is associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu+ to Cu2+.


  • It is absorbed as BO|- or B4O2-.
  • Boron is required for uptake and utilisation of Ca2+, membrane functioning, pollen germination, cell elongation, cell differentiation and carbohydrate translocation.


  • Plants obtain it in the form of molybdate ions (MoO2+).
  • It is a component of several enzymes, including nitrogenase and nitrate reductase both of which participate in nitrogen metabolism.


  • It is absorbed in the form of chloride anion (Cl).
  • Along with Na+ and K+, it helps in determining the solute concentration and the anion- cation balance in cells.
  • It is essential for the water-splitting reaction in photosynthesis, a reaction that leads to oxygen evolution.

Deficiency Symptoms of Essential Elements

  • Whenever the supply of an essential element becomes limited, plant growth is retarded. The concentration of the essential element below which plant growth is retarded is termed as Critical concentration. The element is said to be deficient when present below the critical concentration.
  • Since each element has one or more specific structural or functional role in plants, in the absence of any particular element, plants show certain morphological changes. These morphological changes are indicative of certain element deficiencies and are called deficiency symptoms.
  • The deficiency symptoms vary from element to element and they disappear when the deficient mineral nutrient is provided to the plant. However, if deprivation continues, it may eventually lead to the death of the plant.
  • The parts of the plants that show the deficiency symptoms also depend on the mobility of the element in the plant. For elements that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the older tissues. For example, the deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves. In the older leaves, biomolecules containing these elements are broken down, making these elements available for mobilising to younger leaves.
  • The deficiency symptoms tend to appear first in the young tissues whenever the elements are relatively immobile and are not transported out of the mature organs, for example, elements like sulphur and calcium are a part of the structural component of the cell and hence are not easily released.
  • The kind of deficiency symptoms shown in plants include chlorosis, necrosis, stunted plant growth, premature fall of leaves and buds, and inhibition of cell division.
  • Chlorosis – loss of chlorophyll leading to yellowing in leaves.
  • caused by the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo.
  • Necrosis – death of tissue, particularly leaf tissue.
  • Caused by the deficiency of Ca, Mg, Cu, K.
  • Inhibition of cell division – Lack or low level of N, K, S, Mo.
  • Delayed flowering – N, S, Mo.

Toxicity of Micronutrients

  • The requirement of micronutrients is always in low amounts while their moderate decrease causes the deficiency symptoms and a moderate increase causes toxicity.
  • There is a narrow range of concentration at which the elements are optimum.
  • Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10 per cent is considered toxic.
  • Such critical concentrations vary widely among different micronutrients.
  • The toxicity symptoms are difficult to identify.
  • Toxicity levels for any element also vary for different plants.
  • Many a times, excess of an element may inhibit the uptake of another element.
  • Manganese toxicity – the appearance of brown spots surrounded by chlorotic veins.
  • Manganese competes with iron and magnesium for uptake
  • Manganese competes with magnesium for binding with enzymes.
  • Manganese also inhibit calcium translocation in shoot apex.
  • Therefore, excess of manganese may, in fact, induce deficiencies of iron, magnesium and calcium.
  • Thus, what appears as symptoms of manganese toxicity may actually be the deficiency symptoms of iron, magnesium and calcium.

Mechanism of Absorption of Elements

  • The process of absorption can be demarcated into two main phases.
  • In the first phase, an initial rapid uptake of ions into the ‘free space’ or ‘outer space’ of cells – the apoplast, is passive.
  • In the second phase of uptake, the ions are taken in slowly into the ‘inner space’ – the symplast of the cells is active.
  • The passive movement of ions into the apoplast usually occurs through ion-channels, the trans-membrane proteins that function as selective pores. On the other hand, the entry or exit of ions to and from the symplast requires the expenditure of metabolic energy, which is an active process.
  • The movement of ions is usually called the flux.
  • inward movement into the cells is influx and the outward movement, efflux.

Translocation of Solutes

  • Mineral salts are translocated through xylem along with the ascending stream of water, which is pulled up through the plant by transpirational pull.
  • Analysis of xylem sap shows the presence of mineral salts in it.
  • Use of radioisotopes of mineral elements also substantiate the view that they are transported through the xylem.

Soil as Reservoir of Essential Elements

  • Majority of the nutrients that are essential for the growth and development of plants become available to the roots due to weathering and breakdown of rocks.
  • These processes enrich the soil with dissolved ions and inorganic salts.
  • Since they are derived from the rock minerals, their role in plant nutrition is referred to as mineral nutrition.
  • Soil consists of a wide variety of substances. Soil not only supplies minerals but also harbours nitrogen-fixing bacteria, other microbes, holds water, supplies air to the roots and acts as a matrix that stabilises the plant.
  • Since deficiency of essential minerals affect the crop-yield, there is often a need for supplying them through fertilisers.
  • Both macro-nutrients (N, P, K, S, etc.) and micro-nutrients (Cu, Zn, Fe, Mn, etc.) form components of fertilisers and are applied as per need.

Metabolism of Nitrogen

Nitrogen Cycle

  • Apart from carbon, hydrogen and oxygen, nitrogen is the most prevalent element in living organisms.
  • Nitrogen is a constituent of amino acids, proteins, hormones, chlorophylls and many of the vitamins.
  • Plants compete with microbes for the limited nitrogen that is available in soil. Thus, nitrogen is a limiting nutrient for both natural and agricultural eco-systems.
  • Nitrogen exists as two nitrogen atoms joined by a very strong triple covalent bond (N = N). The process of conversion of nitrogen (N2) to ammonia is termed as Nitrogen Fixation.
  • in nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO2, N2O).
  • Industrial combustions, forest fires, automobile exhausts and power-generating stations are also sources of atmospheric nitrogen oxides.
  • Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.

2NH3 + 3O2 ——–>2NO2 + 2H+ + 2H2O
…. (i)

2NO2 + O2 ———>2NO3
…. (ii)

  • Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria in the following steps:
  • Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus. The nitrite is further oxidised to nitrate with the help of the bacterium Nitrobacter. These steps are called Nitrification.
  • The nitrifying bacteria are Chemoautotrophs.
  • The nitrate thus formed is absorbed by plants and is transported to the leaves.
  • In leaves, it is reduced to form ammonia that finally forms the amine group of amino acids.
  • Nitrate present in the soil is also reduced to nitrogen is by the process of denitrification.
  • Denitrification is carried by bacteria Pseuodomonas and Thiobacillus.

Biological Nitrogen Fixation

  • Reduction of nitrogen to ammonia by living organism is called biological nitrogen fixation.
  • The enzyme Nitrogenase which is capable of nitrogen reduction is present exclusively in prokaryotes.
  • Such microbes are called nitrogen fixers.
  • The nitrogen-fixing microbes could be free-living or symbiotic.
  • Free-living nitrogen-fixing aerobic microbes – Azotobacter and Beijerinckia.
  • Free-living nitrogen-fixing aerobic microbes – Rhodospirilhim and Bacillus.
  • A number of cyanobacteria such as Anabaena and Nostoc are also free living nitrogen-fixers.

Symbiotic biological nitrogen fixation

  • Rod-shaped Rhizobium – Symbiosis with the root nodules of several legumes such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc.
  • The microbe. Frankia, also produces nitrogen-fixing nodules on the roots of non-leguminous plants like Alnus.
  • Both Rhizobium and Frankia are free-living in soil but as symbionts, can fix atmospheric nitrogen.
  • Red or pink colour of the nodules is due to the presence of leguminous haemoglobin or leg-haemoglobin.

Nodule formation

  • Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant.
  • Principal stages in the nodule formation are summarised as follows:
  • Rhizobia multiply and colonise the surroundings of roots and get attached to epidermal and root hair cells.
  • The root-hairs curl and the bacteria invade the root-hair.
  • An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root.
  • Then the bacteria are released from the thread into the cells which leads to the differentiation of specialised nitrogen fixing cells.
  • The nodule thus formed, establishes a direct vascular connection with the host for exchange of nutrients.

  • The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin.
  • The enzyme nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia, the first stable product of nitrogen fixation.

N2 + 8H+ + 16ATP ———–> 2NH3 + H2 + 16ADP + 16Pi

  • The enzyme nitrogenase is highly sensitive to the molecular oxygen; it requires anaerobic conditions.
  • The nodules have adaptations that ensure that the enzyme is protected from oxygen.
  • To protect these enzymes, the nodule contains an oxygen scavenger called leg-haemoglobin.
  • It is interesting to note that these microbes live as aerobes under free-living conditions (where nitrogenase is not operational), but during nitrogen-fixing events, they become anaerobic (thus protecting the nitrogenase enzyme).
  • Ammonia synthesis by nitrogenease requires a very high input of energy (8 ATP for each NH3 produced). The energy required, thus, is obtained from the respiration of the host cells.

Fate of ammonia

  • At physiological pH, the ammonia is protonated to form NH4+ (ammonium) ion.
  • While most of the plants can assimilate nitrate as well as ammonium ions, the latter is quite toxic to plants and hence cannot accumulate in them.
  • NH4+ is used to synthesise amino acids in plants.
  • There are two main ways in which this can take place:
  • Reductive amination: In these processes, ammonia reacts with α- ketoglutaric acid and forms glutamic acid.

α-ketogluatric acid + NH4+ + NADPH ——–> glutamate + H2O + NADP

  • Transamination: It involves the transfer of amino group from one amino acid to the keto group of a keto acid.
  • Glutamic acid is the main amino acid from which the transfer of NH2, the amino group takes place and other amino acids are formed through transamination.
  • The enzyme transaminase catalyses all such reactions. For example,

  • The two most important amides – asparagine and glutamine, found in plants are a structural part of proteins.
  • They are formed from two amino acids, namely aspartic acid and glutamic acid, respectively, by addition of another amino group to each.
  • The hydroxyl part of the acid is replaced by another NH2- radicle.
  • Since amides contain more nitrogen than the amino acids, they are transported to other parts of the plant via xylem vessels.
  • In addition, along with the transpiration stream the nodules of some plants (e.g., soyabean) export the fixed nitrogen as ureides.
  • These compounds also have a particularly high nitrogen to carbon ratio.


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Chapter 11

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.

Facilitated Diffusion

  • 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

  • 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.

Plant-Water Relations

  • 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

  • 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

Root Pressure

  • 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.

Transpiration 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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