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Q1. What is the saturation limit for CO₂ in C₃ and C₄ plants?

Solution

The saturation values for CO₂ are C₄ plants: At about 360 μlL⁻¹ C₃ plants: Above 450 μlL⁻¹

Q2. Differentiate between C₃ and C₄ plants on the basis of anatomy.

Solution

C₃ plants C₄ plants Kranz anatomy is absent in leaves. Leaf shows the presence of Kranz anatomy, with bundle sheath cells arranged around the vascular bundle. Chloroplasts are of a single type with the presence of grana. Chloroplast is dimorphic. Chloroplasts of mesophyll cells have grana and those of bundle sheath cells do not.  

Q3. Draw a standard graph for light intensity versus the rate of photosynthesis.

Solution

 

Q4. What governs the internal factors affecting the rate of photosynthesis?

Solution

Genetic predisposition and growth of the plant determine the internal factors which affect the plant.

Q5. The membrane system of chloroplast is responsible for the synthesis of which two molecules?

Solution

ATP and NADPH

Q6. What is the anatomical structure of the leaves of C_­4­ plants known as?

Solution

Kranz anatomy

Q7. In which region of the visible light spectrum does most photosynthesis take place?

Solution

Most photosynthesis corresponds to the blue and red regions of the visible spectrum.

Q8. The Calvin cycle occurs in plants following which pathway?

Solution

The Calvin cycle occurs in all photosynthetic plants irrespective of whether they have the C₃ or C₄ pathway.

Q9. Give the equation for photolysis of water and give the fate of each of the products so formed.

Solution

2H₂O ————› 4H⁺ + O₂ + 4e⁻ The products so formed are

H⁺/protons - These are used for the reduction of NADP to NADPH.

Oxygen - It is liberated in the air and replenishes oxygen in the environment.

Electrons - These are used to replace electrons lost by PS II.

Q10. Name the site for the following in C₄ plants:

PEP regeneration

Calvin cycle

Solution

Mesophyll cells

Bundle sheath cells

Q11. Describe the experiment performed by T. W. Engelmann to determine the first action spectrum of photosynthesis.

Solution

Engelmann used a prism to split light into its spectral components, and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of oxygen evolution. He observed that bacteria mainly accumulated in the region of blue and red light of the split spectrum, thus giving the first action spectrum of photosynthesis.

Q12. Internally, the chloroplast consists of which fluid?

Solution

Stroma

Q13. Which pigment forms a bright blue or blue-green colour band on a chromatogram for pigments of green leaf?

Solution

Chlorophyll a

Q14. Factors affecting the rate of photosynthesis are grouped under which two categories?

Solution

External factors and internal factors

Q15. Differentiate between PS I and PS II.

Solution

Photosystem I Photosystem II It has absorption maxima at 700 nm. It has absorption maxima at 680 nm. It operates in both cyclic and non-cyclic photophosphorylation. It operates only in non-cyclic photophosphorylation.  

Q16. At what concentration of full sunlight does saturation of light take place for photosynthesis?

Solution

10% of full sunlight is the saturation limit for photosynthesis.

Q17. Why is photorespiration called a wasteful process?

Solution

In photorespiration, there is no synthesis of ATP or NADPH; thus, it is called a wasteful pathway.

Q18. Expand RuBP. Give its role in photosynthesis.

Solution

Ribulose bisphosphate It acts as the first acceptor molecule of CO₂ and forms the first stable compound of the biosynthetic phase.

Q19. Give a detailed account of the Hatch and Slack pathway with the help of a diagram.

Solution

• The Hatch and Slack pathway is a cyclic pathway for CO₂ fixation.
• The primary CO₂ acceptor is a 3-carbon compound phosphoenol pyruvate (PEP) which is present in mesophyll cells.
• PEP in the mesophyll cells is converted to oxaloacetic acid (OAA), which is then further converted into a 4-carbon compound such as malic acid or aspartic acid, which is then transported to the bundle sheath cells.
• In the bundle sheath cells, it is again broken down into a 3-carbon compound with the release of CO₂.
• The CO₂ released enters the Calvin cycle in the bundle sheath cells, while the 3-carbon compound is transported back to the mesophyll cells.
• In the mesophyll cells, the 3-carbon compound is converted back to PEP, thus completing the cycle.

   

Q20. Define photosynthesis.

Solution

Photosynthesis is a physicochemical process by which green plants and certain other organisms use light energy for synthesising organic compounds.

Q21. In which photophosphorylation process both ATP and NADPH are produced and in which only ATP is produced? 

Solution

In non-cyclic photophosphorylation, both ATP and NADPH are produced. In cyclic photophosphorylation, only ATP is produced.

Q22. How is oxygen produced during photosynthesis?

Solution

During non-cyclic photophosphorylation, PS I passes its electron to NADP⁺ for the production of NADPH + H⁺. This lost electron of PS I is obtained from PS II, which in turn obtains its electron from the splitting of water. When the water molecule is split to provide an electron to PS II, it produces protons as well as oxygen which is released out as a molecule of oxygen.

Q23. Give the empirical formula for photosynthesis.  

Solution

Q24. Name the different stages of the Calvin cycle.

Solution

The Calvin cycle comprises three stages:

Carboxylation

Reduction

Regeneration

Q25. Name the factors important for photosynthesis.

Solution

Sunlight, carbon dioxide and chlorophyll

Q26. Which pigments constitute the accessory pigments in the thylakoid?

Solution

Accessory pigments include

Chlorophyll b

Xanthophylls

Carotenoids

Q27. Give a diagrammatic representation for the C₄ pathway.

Solution

 

Q28. State the effect of CO₂ concentration on C₃ and C₄ plants under the following conditions:

Low light and high CO₂ concentration

High light intensities

Saturation CO₂ concentration

Solution

Both C₃ and C₄ plants do not respond to low light and high CO₂ concentration.

Both C₃ and C₄ plants respond to high light and high CO₂ concentration by an increase in the rate of photosynthesis.

Saturation concentration of CO₂ for

C₃ plants: Above 450 μlL⁻¹ C₄ plants: At 360 μlL⁻¹

Q29. Give a diagrammatic representation of cyclic and non-cyclic photophosphorylation.

Solution

       

Q30. What is the biosynthetic phase also known as? Why is it called so? What products does it require and what is the end-product of the process?

Solution

The biosynthetic phase is also called the dark reaction. It is called so because it does not require the presence of light for its commencement; that is, it is light-independent. The products necessary for the biosynthetic phase are ATP and NADPH, which are obtained from the light reactions. The final product of the biosynthetic phase is sugar (glucose).

Q31. What do ATP and NADPH stand for?

Solution

ATP - Adenosine triphosphate NADPH - Nicotinamide adenine dinucleotide phosphate

Q32. For the production of one molecule of glucose by Calvin cycle, how many molecules of CO₂ need to be fixed?

Solution

Six molecules of CO₂ are fixed for the production of one molecule of glucose by the Calvin cycle.

Q33. How are molecules of the C₄ cycle exchanged between the mesophyll and bundle sheath cells?

Solution

The molecules of the C₄ pathway are exchanged through the plasmodesmata, which are pore-like channels that connect the mesophyll and bundle sheath cells.

Q34. Julius von Sachs provided evidence for which facts based on his experiment on green plants?

Solution

He found that the green parts of the plant make glucose, and glucose is usually stored in the form of starch.

Q35. List the internal and external factors which affect the rate of photosynthesis, and give details on three external factors.

Solution

Factors affecting the rate of photosynthesis are classified under two categories:

Internal factors: Leaf size, number of leaves, shape of leaves, amount of chloroplast, mesophyll cell content, chlorophyll and internal concentration of CO₂

External factors: CO₂ concentration, sunlight, temperature and availability of water

External factors which affect the rate of photosynthesis:

Light:

• At low light intensities, there is a linear relationship between incident light and CO₂ fixation rates.
• At high light intensities, the rate of photosynthesis does not increase but other factors become limiting.
• Light saturation occurs at 10% of full sunlight.
• Light is usually not a limiting factor, but when light intensity increases beyond a point, it causes break down of chlorophyll and decrease in photosynthesis.

Temperature:

• Temperature mainly affects the dark reaction of photosynthesis.
• C₄ plants respond to higher temperatures and show higher rates of photosynthesis.
• C₃ plants, on the other hand, have a much lower temperature optimum.
• The temperature dependency of plants for the rate of photosynthesis also depends on the habitat which the plant is living and the group of plants to which it belongs.

Water:

• Water affects the rate of photosynthesis indirectly.
• Water stress causes the stomata to close in order to reduce transpiration. Closed stomata lead to decrease concentration of CO₂ in the cells, thus resulting in decrease in photosynthesis.
• Water stress also causes the leaves to wilt, thus reducing the surface area for photosynthesis.

Q36. How does water affect the rate of photosynthesis?

Solution

Water affects the rate of photosynthesis indirectly. Water stress causes the stomata to close, thus reducing the availability of CO₂. It also causes the leaves to wilt, reducing the surface area for photosynthesis.

Q37. How did Melvin Calvin find the first stable compound formed in the biosynthetic phase?

Solution

Calvin used ¹⁴C, a radioactive isotope of carbon, in CO₂ which he supplied to the green photosynthetic algae which he used for his studies. The algae used up the CO₂ for fixation in photosynthesis. He observed that the first compound formed after some time with the presence of the radioactive ¹⁴C was 3-phosphoglycerate, suggesting it as the first stable compound of CO₂ fixation.

Q38. Define photolysis of water.

Solution

It is the process by which water is split into H⁺, oxygen and electrons in the presence of light by photosystem II.

Q39. How is the yield of greenhouse crops such as tomato and bell pepper increased through photosynthesis?

Solution

The yield in greenhouse crops of tomato and bell pepper is increased by growing them in a carbon dioxide-rich environment.

Q40. The splitting of water occurs in which part of the chloroplast? What is its significance?

Solution

Splitting of water takes place near PS II, located in the inner side of the thylakoid membrane. Splitting of water releases oxygen in the atmosphere and generates a proton gradient across the thylakoid membrane.