Plant Science: Cell, Physiology, Nutrition and Growth Hormones

What to remember

• Photosynthesis makes food in chloroplasts (light reaction in grana, dark reaction in stroma). Respiration releases energy in mitochondria. Transpiration pulls water up the xylem.
• Sixteen elements are essential for plants: C, H and O from air and water, and the rest from soil. Macronutrients are needed in large amounts and micronutrients in traces.
• The five main hormones are auxin, gibberellin, cytokinin (growth promoters), abscisic acid and ethylene (inhibitors). Each has a standard role that is asked often.

1. The plant cell

A plant cell has a cell wall outside the plasma membrane. The wall is made mainly of cellulose. In woody tissue, lignin is deposited in the wall and makes it hard.

Cell division. Mitosis gives two identical cells and is used for growth. Meiosis gives four cells with half the chromosome number and is used to make gametes. The stages of mitosis are prophase, metaphase, anaphase and telophase.

Plant tissues. Meristematic tissues divide. Apical meristem increases the length. Lateral meristem (cambium) increases the girth. Permanent tissues are simple (parenchyma, collenchyma, sclerenchyma) and complex (xylem and phloem). Xylem carries water and minerals upward. Phloem carries food in both directions.

2. Water relations

• Diffusion: movement of molecules from high to low concentration.
• Osmosis: movement of water through a semi-permeable membrane from lower solute concentration to higher solute concentration.
• Imbibition: swelling of dry material, such as seeds and wood, by absorbing water.
• Water potential: Ψw = Ψs + Ψp. Here Ψs is solute potential (negative) and Ψp is pressure potential. Pure water has the highest water potential, which is zero. Water moves from higher to lower water potential.
• Plasmolysis: shrinking of protoplasm away from the wall in a strong solution. Turgor is the pressure of the cell contents on the wall.

*Worked example:* a cell has Ψs = −0.8 MPa and Ψp = +0.3 MPa. Ψw = −0.8 + 0.3 = −0.5 MPa.

Ascent of sap. Roots absorb water mainly through root hairs. The most accepted theory is the cohesion–tension (transpiration pull) theory. Water molecules stick to each other (cohesion) and to the xylem walls (adhesion). Loss of water from leaves creates tension that pulls the water column up.

Transpiration is loss of water vapour from aerial parts, mainly through stomata. Guard cells control stomatal opening. Types are stomatal, cuticular and lenticular. About 90 to 95 per cent of water absorbed is lost by transpiration. It increases with high temperature, low humidity and wind, and decreases in high humidity.

3. Photosynthesis

Overall equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (light and chlorophyll).

• Light reaction (in grana): light splits water (photolysis), giving oxygen, ATP and NADPH.
• Dark reaction (Calvin cycle, in stroma): CO₂ is fixed using ATP and NADPH to make sugar.
• C3 plants use the Calvin cycle only. The first product is a 3-carbon acid, PGA. Most trees are C3.
• C4 plants (maize, sugarcane) first form a 4-carbon acid, oxaloacetic acid. They are efficient in hot, bright conditions.
• CAM plants (cactus, Agave) open stomata at night.
• Factors: light, CO₂, temperature, water and chlorophyll. The limiting factor is the one in shortest supply.
• Chlorophyll absorbs mainly blue and red light. Green is reflected. Oxygen released comes from water.

4. Respiration

Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (38 ATP is the theoretical gain per glucose).

• Glycolysis occurs in the cytoplasm. Krebs cycle and electron transport occur in mitochondria.
• Anaerobic respiration (fermentation) gives alcohol and CO₂ with a small amount of energy (2 ATP).
• Respiratory quotient (RQ) = volume of CO₂ released ÷ volume of O₂ absorbed. For carbohydrates RQ = 1. For fats it is less than 1 (about 0.7). For proteins it is about 0.9. For organic acids it is greater than 1.

*Worked example:* a seed releases 80 ml CO₂ and takes in 100 ml O₂. RQ = 80/100 = 0.8, so the substrate is likely a mixture such as protein or fat and carbohydrate.

5. Mineral nutrition

Essential elements are those without which the plant cannot complete its life cycle.

Deficiency symptoms:

• Nitrogen: general yellowing (chlorosis) of older leaves, stunted growth.
• Phosphorus: purple or dark green leaves, poor roots.
• Potassium: yellow edges and scorching of older leaves.
• Iron: yellowing between veins of young leaves.
• Magnesium: yellowing between veins of older leaves.
• Calcium: death of growing tips and young leaves.

Nitrogen cycle: nitrogen fixation (by *Rhizobium* in root nodules of legumes, *Azotobacter*, *Frankia* in actinorhizal trees, blue-green algae), ammonification, nitrification (ammonia to nitrite by *Nitrosomonas*, nitrite to nitrate by *Nitrobacter*) and denitrification. Many forest tree species such as Casuarina and Acacia fix nitrogen through root symbionts. Mycorrhiza is a fungus–root association that helps phosphorus uptake.

6. Plant growth and hormones

Growth is a permanent increase in size. Its phases are cell division, cell enlargement and cell maturation. A growth curve is S-shaped (sigmoid): lag, log and stationary phases.

Relative growth rate (RGR) = (ln W₂ − ln W₁) ÷ (t₂ − t₁).

*Worked example:* absolute growth rate of a seedling that grows from 20 cm to 32 cm in 6 days = (32 − 20)/6 = 2 cm per day.

A high auxin to cytokinin ratio in tissue culture favours roots. A low ratio favours shoots. Synthetic auxins used in forestry include IBA and NAA for rooting cuttings, and 2,4-D as a weed killer.

Movements: phototropism (light), geotropism (gravity), hydrotropism (water), thigmotropism (touch). Photoperiodism is the response to day length: short-day, long-day and day-neutral plants. Vernalisation is the cold treatment needed for flowering in some plants.

7. Quick formulas and standard values

• Percentage of an element in a fertiliser: kg of nutrient = fertiliser weight × (per cent nutrient ÷ 100). Urea has about 46 per cent N, so 100 kg of urea gives about 46 kg of N.
• Solution strength (g per litre) = weight of solute in g ÷ volume in litres.
• Absolute growth rate = (final size − initial size) ÷ time.
• Transpiration rate = water lost ÷ (leaf area × time).
• Water moves from high water potential to low water potential. Pure water at atmospheric pressure has Ψw = 0, so a cell sap solution always has a negative Ψs.
• Photosynthesis needs about 6 molecules of CO₂ to make 1 molecule of glucose.
• Plants in which short days promote flowering are short-day plants. Plants that flower after a critical day length longer than a threshold are long-day plants.

Forest relevance. Fast-growing species such as Eucalyptus and Casuarina have high photosynthetic and water use. Leguminous trees (Acacia, Dalbergia, Pongamia) improve soil nitrogen. Understanding hormones helps in raising cuttings and in tissue culture of teak and bamboo.

Exam traps

• 1. Photolysis occurs in the light reaction, not the dark reaction.
• 2. The oxygen released in photosynthesis comes from water, not from CO₂.
• 3. Xylem carries water upward. Phloem carries food.
• 4. Apical meristem increases length. Cambium increases girth.
• 5. Nitrifying bacteria convert ammonia to nitrate. Denitrifying bacteria convert nitrate back to nitrogen gas.
• 6. Iron deficiency shows in young leaves. Magnesium and nitrogen deficiencies show first in older leaves.
• 7. Auxin dominates the apical bud. Cytokinin breaks apical dominance.
• 8. RQ of carbohydrate is 1. RQ of fats is less than 1.

One-liners

• 1. Cellulose is the chief material of the plant cell wall.
• 2. Mitochondria are the power houses of the cell.
• 3. Chloroplasts contain chlorophyll and grana.
• 4. Calvin cycle occurs in the stroma.
• 5. Stomata are controlled by guard cells.
• 6. Cohesion–tension theory explains ascent of sap.
• 7. Rhizobium fixes nitrogen in legume root nodules.
• 8. Gibberellins break seed dormancy and elongate stems.
• 9. Ethylene is a gaseous hormone that ripens fruit.
• 10. Abscisic acid closes stomata during water stress.
• 11. Phosphorus helps root growth and energy transfer.
• 12. Lignin gives hardness to wood.

Practice questions

1. The main chemical in the plant cell wall is:

   1. Chitin
   2. Glycogen
   3. Cellulose
   4. Keratin
   Answer

   C. Cellulose

   Plant cell walls are made mainly of cellulose.

2. The site of the light reaction of photosynthesis is the:

   1. Mitochondrion
   2. Nucleus
   3. Stroma
   4. Grana
   Answer

   D. Grana

   The light reaction takes place in the grana of the chloroplast.

3. The oxygen released in photosynthesis comes from:

   1. Carbon dioxide
   2. Water
   3. Chlorophyll
   4. Glucose
   Answer

   B. Water

   Photolysis of water releases oxygen.

4. Tissue that increases the girth of a tree is:

   1. Vascular cambium
   2. Parenchyma
   3. Apical meristem
   4. Phloem fibre
   Answer

   A. Vascular cambium

   Lateral meristem or cambium adds to the thickness of stems and roots.

5. Which theory best explains ascent of sap in tall trees?

   1. Capillarity theory
   2. Cohesion-tension theory
   3. Pulsation theory
   4. Root pressure theory
   Answer

   B. Cohesion-tension theory

   Transpiration pull with cohesion of water molecules is the most accepted explanation.

6. The hormone known as the stress hormone is:

   1. Abscisic acid
   2. Cytokinin
   3. Auxin
   4. Gibberellin
   Answer

   A. Abscisic acid

   ABA closes stomata and induces dormancy under stress.

7. Ripening of fruits is promoted by:

   1. Cytokinin
   2. Auxin
   3. Gibberellin
   4. Ethylene
   Answer

   D. Ethylene

   Ethylene is a gaseous hormone that hastens ripening.

8. Which hormone is used to promote rooting in stem cuttings?

   1. Abscisic acid
   2. Ethylene
   3. Indole butyric acid (an auxin)
   4. Gibberellic acid
   Answer

   C. Indole butyric acid (an auxin)

   IBA and NAA are auxins used for rooting.

9. Bacteria in root nodules of legumes belong to the genus:

   1. Nitrosomonas
   2. Pseudomonas
   3. Rhizobium
   4. Nitrobacter
   Answer

   C. Rhizobium

   Rhizobium fixes atmospheric nitrogen in legume nodules.

10. Which element is part of chlorophyll?

    1. Zinc
    2. Magnesium
    3. Iron
    4. Calcium
    Answer

    B. Magnesium

    Magnesium is the central atom of the chlorophyll molecule.

11. Which element deficiency shows first as yellowing of young leaves between the veins?

    1. Phosphorus
    2. Potassium
    3. Nitrogen
    4. Iron
    Answer

    D. Iron

    Iron is not mobile in the plant, so young leaves show interveinal chlorosis.

12. Which of the following is a C4 plant?

    1. Sugarcane
    2. Rice
    3. Teak
    4. Wheat
    Answer

    A. Sugarcane

    Sugarcane and maize are C4 plants. Most trees are C3.

13. A cell has solute potential of -0.9 MPa and pressure potential of +0.4 MPa. Its water potential is:

    1. -1.3 MPa
    2. +1.3 MPa
    3. -0.5 MPa
    4. +0.5 MPa
    Answer

    C. -0.5 MPa

    Water potential = -0.9 + 0.4 = -0.5 MPa.

14. A seed takes in 50 ml O2 and releases 50 ml CO2. Its RQ is:

    1. 1.0
    2. 2.0
    3. 0.7
    4. 0.5
    Answer

    A. 1.0

    RQ = CO2 released / O2 absorbed = 50/50 = 1, typical of carbohydrate.

15. A seedling grows from 12 cm to 30 cm in 9 days. Its average growth rate per day is:

    1. 4 cm
    2. 1 cm
    3. 3 cm
    4. 2 cm
    Answer

    D. 2 cm

    (30 - 12)/9 = 18/9 = 2 cm per day.

16. Seeds respiring on fat show an RQ of about:

    1. 1.5
    2. 0.7
    3. 1.0
    4. 2.5
    Answer

    B. 0.7

    Fats need more oxygen per CO2 released, so RQ is below 1, about 0.7.

17. A leaf loses 4 g water in 2 hours from 8 square decimetres. Transpiration per square decimetre per hour is:

    1. 0.125 g
    2. 0.5 g
    3. 1.0 g
    4. 0.25 g
    Answer

    D. 0.25 g

    4 / (8 x 2) = 0.25 g per dm2 per hour.

18. Net ATP gain from anaerobic glycolysis of one glucose is:

    1. 38
    2. 8
    3. 18
    4. 2
    Answer

    D. 2

    Glycolysis and fermentation give 2 ATP per glucose.

19. If a plant weighing 4 g becomes 16 g in 10 days, its fold increase is:

    1. 2
    2. 3
    3. 4
    4. 12
    Answer

    C. 4

    16/4 = 4 times.

20. A solution is made by dissolving 5 g of a nutrient salt in water to make 500 ml. Its strength in g per litre is:

    1. 10
    2. 50
    3. 25
    4. 5
    Answer

    A. 10

    5 g in 0.5 litre = 10 g per litre.

21. If a fertiliser contains 46 per cent N, the N in 50 kg is:

    1. 18 kg
    2. 23 kg
    3. 46 kg
    4. 12 kg
    Answer

    B. 23 kg

    50 x 0.46 = 23 kg of N.

22. A culture with a high auxin to cytokinin ratio mostly produces:

    1. Roots
    2. Shoots
    3. Flowers only
    4. Fruits
    Answer

    A. Roots

    High auxin favours root formation. High cytokinin favours shoots.

23. The first stable product of the Calvin cycle in C3 plants is:

    1. Oxaloacetic acid
    2. Phosphoglyceric acid
    3. Malic acid
    4. Pyruvic acid
    Answer

    B. Phosphoglyceric acid

    C3 plants form the 3-carbon PGA first. C4 plants form oxaloacetic acid.

24. Which role is typical of gibberellins?

    1. Closing stomata
    2. Ripening fruit
    3. Breaking seed dormancy and elongating stems
    4. Causing leaf fall
    Answer

    C. Breaking seed dormancy and elongating stems

    Gibberellins break dormancy and cause stem elongation.

25. Cytokinin is mainly associated with:

    1. Cell division and delay in leaf ageing
    2. Stomatal closure
    3. Fruit ripening
    4. Seed dormancy
    Answer

    A. Cell division and delay in leaf ageing

    Cytokinins promote cell division and delay senescence.

26. Casuarina can grow on poor soil mainly because it:

    1. Has deep taproots
    2. Fixes nitrogen with Frankia in root nodules
    3. Uses less water only
    4. Is a C4 plant
    Answer

    B. Fixes nitrogen with Frankia in root nodules

    Casuarina has actinorhizal nodules with Frankia that fix nitrogen.

27. Consider these statements. 1. Xylem carries water upward. 2. Phloem carries food in both directions. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

28. Consider these statements. 1. The Calvin cycle needs ATP and NADPH. 2. The Calvin cycle occurs in the grana. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The Calvin cycle uses light-reaction products, but occurs in the stroma.

29. Consider these statements. 1. Meiosis halves the chromosome number. 2. Mitosis gives four cells. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Mitosis gives two identical cells. Meiosis gives four with half the number.

30. Consider these statements. 1. Transpiration rises with higher temperature. 2. Transpiration rises with higher humidity. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    High humidity slows transpiration.

31. Consider these statements. 1. Nitrifying bacteria change ammonia to nitrate. 2. Denitrifying bacteria change nitrate to nitrogen gas. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both describe the standard roles of these bacteria.

32. Consider these statements. 1. Phosphorus is a micronutrient. 2. Zinc is a macronutrient. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Phosphorus is a macronutrient and zinc is a micronutrient.

33. Consider these statements. 1. Auxin maintains apical dominance. 2. Cytokinin helps break apical dominance. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Auxin from the apical bud inhibits side buds. Cytokinin promotes their growth.

34. Consider these statements. 1. Chloroplast contains chlorophyll. 2. Mitochondria store starch and oil. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Leucoplasts store starch and oil. Mitochondria are sites of respiration.

35. Consider these statements. 1. CAM plants open stomata at night. 2. C4 plants are efficient in hot bright conditions. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct features of CAM and C4 plants.

36. Which pair is correctly matched?

    1. Ribosome - respiration
    2. Chlorophyll - starch store
    3. Cellulose - toxic pigment
    4. Lignin - hardness of wood
    Answer

    D. Lignin - hardness of wood

    Lignin hardens the cell wall of woody tissue.

37. Which synthetic auxin is widely used as a weed killer for broad-leaved weeds?

    1. Ethephon
    2. 2,4-D
    3. IBA
    4. Kinetin
    Answer

    B. 2,4-D

    2,4-D is a synthetic auxin used as a selective herbicide.

38. Plants that need a cold treatment before flowering undergo:

    1. Imbibition
    2. Photolysis
    3. Plasmolysis
    4. Vernalisation
    Answer

    D. Vernalisation

    Vernalisation is the cold treatment that promotes flowering.

39. Shrinking of the protoplast from the cell wall in a strong solution is:

    1. Osmosis
    2. Plasmolysis
    3. Turgor
    4. Imbibition
    Answer

    B. Plasmolysis

    This is called plasmolysis.

40. A fungus-root association that helps phosphorus uptake is:

    1. Galls
    2. Mycorrhiza
    3. Lichen
    4. Nodule
    Answer

    B. Mycorrhiza

    Mycorrhiza improves uptake of phosphorus and water.

41. Swelling of dry seeds on absorbing water is called:

    1. Osmosis
    2. Diffusion
    3. Transpiration
    4. Imbibition
    Answer

    D. Imbibition

    Imbibition is absorption of water by dry colloids such as seeds.

42. Which tissue is dead at maturity and gives mechanical strength?

    1. Parenchyma
    2. Sclerenchyma
    3. Phloem sieve tube
    4. Meristem
    Answer

    B. Sclerenchyma

    Sclerenchyma cells are dead with thick lignified walls.

43. Chlorophyll absorbs light chiefly in the:

    1. Ultraviolet region
    2. Infrared region
    3. Green and yellow region
    4. Blue and red region
    Answer

    D. Blue and red region

    Blue and red light are absorbed. Green is reflected.

44. Which nutrient helps in cell wall formation and its lack kills growing tips?

    1. Potassium
    2. Sulphur
    3. Calcium
    4. Chlorine
    Answer

    C. Calcium

    Calcium is part of the cell wall (calcium pectate). Lack kills growing tips.

45. Opening and closing of stomata is controlled by:

    1. Subsidiary cells only
    2. Guard cells
    3. Epidermal hairs
    4. Xylem vessels
    Answer

    B. Guard cells

    Guard cells change shape with turgor and control the stomatal opening.