Samacheer Kalvi 9th Science Plant Physiology Exercise Solutions

Photosynthesis is the process by which green plants synthesise glucose (food) from carbon dioxide and water, using light energy absorbed by chlorophyll, with oxygen released as a by-product. It occurs in the chloroplasts of leaves.

Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of sunlight and chlorophyll)

The four raw materials required are: 1. Carbon dioxide (CO₂) — absorbed from the atmosphere through stomata. 2. Water (H₂O) — absorbed from the soil by roots and transported to leaves via xylem. 3. Sunlight — the energy source, trapped by chlorophyll. 4. Chlorophyll — the green pigment that absorbs light energy.

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata on the leaves.

Two factors affecting transpiration: 1. Temperature — Higher temperature increases the rate of transpiration because water evaporates faster. 2. Humidity — Higher humidity decreases the rate because the water vapour gradient between leaf and atmosphere is reduced.

Other factors: wind speed (increases rate), light (opens stomata, increases rate), and water availability.

Root pressure is the positive hydrostatic pressure developed in the root xylem due to active absorption of minerals from the soil by roots. This creates an osmotic gradient that pushes water upward into the xylem vessels.

It helps transport water to small heights (1–2 metres) in herbaceous plants. However, it is insufficient for tall trees, which rely on transpiration pull.

The nitrogen cycle is the continuous circulation of nitrogen between the atmosphere, soil, and living organisms through various chemical and biological processes.

Main steps: 1. Nitrogen fixation — Conversion of atmospheric N₂ into ammonia (by Rhizobium, Azotobacter, or industrial/Haber process). 2. Nitrification — Conversion of ammonia into nitrites (by Nitrosomonas) and then nitrates (by Nitrobacter). 3. Assimilation — Absorption of nitrates by plants and conversion into plant proteins; animals obtain nitrogen by eating plants. 4. Ammonification — Decomposition of dead organic matter and animal waste into ammonia by decomposer bacteria. 5. Denitrification — Conversion of nitrates back into atmospheric N₂ by bacteria like Pseudomonas.

Photosynthesis occurs in two main stages within the chloroplast:

Stage 1 — Light Reaction (in Grana):

• Chlorophyll in the thylakoid membranes absorbs light energy (photons).
• This energy splits water molecules into H⁺, electrons, and O₂ (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂.
• The oxygen is released through stomata.
• The electrons pass through the electron transport chain, generating ATP (photophosphorylation) and reducing NADP⁺ to NADPH.
• These energy-rich compounds (ATP and NADPH) are used in the dark reaction.

Stage 2 — Dark Reaction / Calvin Cycle (in Stroma):

• CO₂ enters the stroma and combines with a 5-carbon acceptor molecule RuBP (Ribulose-1,5-bisphosphate), catalysed by the enzyme RuBisCO.
• This forms an unstable 6-carbon intermediate that immediately splits into two 3-carbon molecules (3-PGA).
• Using ATP and NADPH from the light reaction, 3-PGA is reduced to G3P (glyceraldehyde-3-phosphate).
• Some G3P molecules exit the cycle to form glucose; others are recycled to regenerate RuBP.

Overall equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of sunlight and chlorophyll)

Significance of Photosynthesis: 1. Produces food for all living organisms (directly or indirectly). 2. Releases oxygen, which is essential for respiration. 3. Maintains the O₂-CO₂ balance in the atmosphere. 4. Source of fossil fuels (coal, petroleum) formed from ancient plant material.

Definition: Transpiration is the loss of water vapour from the aerial parts of a plant, primarily through stomata on leaves. A small amount occurs through lenticels (on stems) and the cuticle.

Process: 1. Water is absorbed by roots from the soil through root hairs. 2. Water moves upward through the xylem vessels by capillarity, root pressure, and mainly transpiration pull. 3. In the leaf, water evaporates from the moist cell walls of mesophyll cells into the intercellular spaces. 4. The water vapour diffuses out through stomatal pores into the atmosphere.

Significance of Transpiration: 1. Transpiration pull — Creates a suction force that pulls water and minerals upward from roots through the xylem (cohesion-tension theory). 2. Cooling effect — Evaporation of water cools the leaf surface, preventing overheating. 3. Mineral transport — Helps transport dissolved minerals from the soil to all parts of the plant. 4. Maintains turgidity — Keeps cells turgid, maintaining the shape and structure of soft plant parts. 5. Concentration of cell sap — Increases concentration of cell sap, facilitating osmotic absorption of water.

Factors Affecting Transpiration:

Anti-transpiration measures: Plants in dry habitats develop thick cuticles, sunken stomata, fewer stomata, and rolled leaves to reduce water loss.

The nitrogen cycle is the continuous circulation of nitrogen through living organisms and the environment. Atmospheric nitrogen (N₂, ~78% of air) cannot be used directly by most organisms and must be converted into usable forms.

Stages of the Nitrogen Cycle:

1. Nitrogen Fixation: Conversion of atmospheric N₂ into ammonia (NH₃) or nitrates.

• Biological fixation: Symbiotic bacteria (Rhizobium in legume root nodules) and free-living bacteria (Azotobacter, Clostridium) convert N₂ → NH₃ using the enzyme nitrogenase.
• Abiotic fixation: Lightning converts N₂ → NOₓ; industrial Haber process converts N₂ + H₂ → NH₃ under high temperature and pressure.

2. Nitrification: Conversion of ammonia into nitrites and then nitrates.

• Ammonia → Nitrites: by bacteria Nitrosomonas and Nitrosococcus (2NH₃ + 3O₂ → 2NO₂⁻ + 2H⁺ + 2H₂O).
• Nitrites → Nitrates: by bacteria Nitrobacter (2NO₂⁻ + O₂ → 2NO₃⁻).
• Nitrates are the most absorbable form of nitrogen for plants.

3. Assimilation: Uptake of nitrogen by living organisms.

• Plants absorb nitrates from the soil and convert them into plant proteins, amino acids, and nucleic acids.
• Animals obtain nitrogen by eating plants (herbivores) or other animals (carnivores).

4. Ammonification (Decomposition): Conversion of organic nitrogen back into ammonia.

• Decomposer bacteria and fungi break down dead plants, animals, and animal waste (urea, uric acid) into ammonia.
• Ammonia re-enters the soil for nitrification.

5. Denitrification: Conversion of nitrates back into atmospheric N₂.

• Bacteria such as Pseudomonas and Thiobacillus reduce nitrates to N₂ under anaerobic conditions (waterlogged soils).
• This returns nitrogen to the atmosphere, completing the cycle.

Why is the Nitrogen Cycle Important? 1. Maintains soil fertility by recycling nitrogen. 2. Ensures a continuous supply of nitrogen to plants for protein and DNA synthesis. 3. Maintains the nitrogen balance in the atmosphere. 4. Leguminous crops (pulses) enrich soil nitrogen, reducing the need for chemical fertilisers.

Magnesium is a constituent of chlorophyll and also activates enzymes. Its deficiency causes:

• Yellowing of leaves (chlorosis), starting from older leaves.
• Reduced photosynthesis due to less chlorophyll.

Leguminous plants (peas, beans, lentils) harbour Rhizobium bacteria in their root nodules, which fix atmospheric nitrogen into ammonia. This enriches the soil with nitrogen, reducing the need for chemical fertilisers for the next crop.