NEET Biology Photosynthesis in Higher Plants Important Questions Set 02

These Class 11 NEET Biology important questions on Photosynthesis in Higher Plants give 20 questions with step-by-step answers, arranged as very short, short and long answer types, taken from a model test paper. They help NEET aspirants revise the definitions, structures, differences and key terms of the chapter.

Very Short Answer Type Questions

Question 1. What is first stable product of C₄ pathway?
Answer: Oxaloacetic acid (OAA) is the first stable product of C₄ pathway.

Question 2. Define photorespiration.
Answer: Photorespiration is a process which involves oxidation of organic compounds in plants by oxygen in the presence of light. No ATP is produced. It results in the release of CO₂ with the utilisation of ATP.

Question 3. Expand PEPcase.
Answer: Phosphoenol pyruvate carboxylase.

Question 4. Name the reaction centre in PS I and PS II.
Answer: The reaction center in PS I is P₇₀₀, the reaction center in PS II is P₆₈₀.

Question 5. What is the primary acceptor molecule of CO₂ during the Calvin cycle?
Answer: The primary acceptor molecule of CO₂ during the Calvin cycle is Ribulose Bisphosphate (RuBP).

Question 6. Give two examples of C₄ plants.
Answer: Maize, sorghum are C₄ plants.

Short Answer Type Questions

Question 7. What are accessory pigments?
Answer: Thylakoid pigments like chlorophyll b, xanthophylls, carotenoids are called accessory pigments, they enable a wider range of wavelengths of incoming light to be utilised for photosynthesis and also protect chlorophyll-a from photooxidation.

Question 8. How many turns of Calvin cycle are required for formation of sucrose molecule?
Answer: Sucrose is a 12C compound. For fixation of 12CO₂, twelve turns of Calvin cycle are required.

Question 9. Give the diagrammatic representation of the cyclic photophosphorylation.
Answer: A well-labelled diagram should show these parts: Photosystem I e– acceptor; Light; TP; A; P+iP; Electron; AD transport system; Chlorophyll P₇₀₀.

Short Answer Type Questions

Question 10. Explain Moll's half leaf experiment.

Answer:

Moll's half leaf experiment : This experiment was done to demonstrate necessity of CO₂ for photosynthesis.

A part of leaf was enclosed in a test tube containing some KOH soaked cotton (which absorbs CO₂), while the other half of leaf is exposed to air. When the two halves of leaf were tested for starch, it was found that only the exposed part of leaf tested positive for starch.

Question 11. Draw the graph showing action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll 'a'.

Answer:

A well-labelled diagram should show these parts: Rate of photosynthesis; Absorption absorbed; Light; 400; 500; 600; 700.

Wavelength of light in nanometres (nm)

on absorption spectrum of chlorophyll a

Question 12. (a) What is absorption spectrum?
(b) Which pigment forms reaction centre?

Answer:

(a) The graphic curve showing the amount of energy of different wavelengths of light absorbed by a substance is absorption spectrum.

(b) The single chlorophyll molecule forms the reaction centre.

Question 13. Where does carboxylation take place in C₃ plants? Explain the process.
Answer: Carboxylation is the first stage of Calvin cycle. It is the fixation of CO₂ into a stable organic intermediate. It occurs mainly in the mesophyll cells in C₃ plants. In this process, the CO₂ is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.

Long Answer Type Questions

Question 14. Give diagrammatic representation of the Hatch and Slack pathway.
Answer: A well-labelled diagram should show these parts: Atmospheric CO2; Mesophyll; Plasma cell membrane; Cell wall; Phosphoenol; HCO3– pyruvate; Fixation; Regeneration; C4 acid; C3 acid; Plasmodesmata; Bundle sheath cell; Transport; Calvin cycleby; CO2; Decarboxylation.

Question 15. Why productivity and yields are better in C₄ plants as compared to the C₃ plants?
Answer: In C₃ plants, a process called photorespiration occurs which leads to a 25 per cent loss of the fixed CO₂. However, in case of C₄ plants, they have evolved a mechanism to avoid occurence of photorespiration, This is because they have a mechanism that increases the concentration of CO₂ at the enzyme site (in bundle sheath cells). This takes place when the C₄ acid from the mesophyll is broken down in the bundle sheath cells to release CO₂, this results in increasing the intracellular concentration of CO₂. In turn, this ensures that the RuBisCO functions as a carboxylase minimizing the oxygenase activity. Thus, C₄ plants lack photorespiration, and yields are better.

Question 16. Briefly explain the photochemical phase of photosynthesis. Where does it take place?

Answer:

Light reaction or the photochemical phase of photosynthesis includes light absorption, water splitting, oxygen release and the formation of high energy chemical intermediates, ATP and NADPH.

It takes place in the thylakoids of chloroplast in green parts of the plant.

Question 17. Explain the various external factors affecting photosynthesis.

Answer:

External factors affecting photosynthesis

(a) Light : It is an essential factor for photosynthesis.

(i) Light intensity : There is a linear relationship between incident light and CO₂ fixation at low light intensities. At higher light intensities, gradually the rate does not show further increase as other factors become limiting. The light saturation occurs at 10 percent of the total sunlight available to plants. Increase in incident light beyond a point causes the breakdown of chlorophyll and thus resulting in decrease in photosynthesis.

(ii) Light quality : Light between 400–700 nm wavelength constitute the photosynthetically active radiation (PAR). Maximum photosynthesis takes place in red and blue light of the visible spectrum and minimum photosynthesis takes place in green light.

(iii) Duration of light : Light duration does not affect the rate of photosynthesis, but it affects the overall photosynthesis.

(b) Carbon dioxide concentration : It is a major limiting factor influencing the rate of photosynthesis.

The C₃ and C₄ plants respond differently to CO₂ concentration. At low light intensities neither type responds to high CO₂ concentration. At high light intensities, both C₃ and C₄ plants show increase in the rate of photosynthesis. The C₄ plants show saturation at about 360 mlL–1 (ppm), while C₃ plants show saturation only beyond 450 mlL–1 (ppm), thus, the current concentration of CO₂ is limiting for C₃ plants.

(c) Temperature : Photosynthesis can take place over a wide range of temperatures. The light reactions are temperature senstive but they are affected to a much lesser extent. The dark reaction being enzymatic are temperature controlled. Again, the temperature optimum for photosynthesis of different plants also depends on the habitat that they are adapted to. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

The C₄ plants respond to higher temperatures and they show higher rate of photosynthesis, while C₃ plants have much lower temperature optimum. Optimum temperature in C₃ plant in 20–25°C and for C₄ plant is 30–45°C.

(d) Water : Water is one of the raw materials utilized for the process of photosynthesis. Photosynthetic process utilizes less than 1% of the water absorbed by a plant, hence it is rarely a limiting factor in photosynthesis. Water stress causes the stomata to close, hence reducing the CO₂ availability as gaseous exchange could not occur. Also, water stress makes leaves wilt, thus reducing the surface area of the leaves and the metabolic activity reduces as well.

Question 18. Explain the Z-scheme of light reaction with diagram only.
Answer: A well-labelled diagram should show these parts: Photosystem II; Photosystem I; NADPH e– acceptor; Light e– acceptor; ATP; NADP⁺; ADP +iP; Electron transport system; LHC; H₂O → 2e– + 2H⁺ + [O].

Question 19. Briefly describe the process of photorespiration.

Answer:

Photorespiration : Photorespiration is a process which involves oxidation of organic compounds in plants by oxygen in the presence of light. It is initiated in chloroplasts. This process does not produce ATP or NADPH and is a wasteful process.

Photorespiration occurs usually when there is high concentration of oxygen. Under such circumstances, RuBisCO, the enzyme that catalyses the carboxylation of RuBP during the first step of Calvin cycle, functions as an oxygenase. In C₃ plants, as both light and dark reactions occur in mesophyll cells, it becomes essential for RuBisCO to catalyse both oxygenation and carboxylation reactions of RuBP, simultaneously. In C₃ plants, some O₂ (released from photolysis of water), does bind to RuBisCO and hence CO₂ fixation is decreased. The RuBP binds with O₂ to form one molecule of PGA and phosphoglycolate in the pathway of photorespiration. There is neither the synthesis of sugar, nor of ATP. Rather, it results in the release of CO₂ with the utilisation of ATP. It leads to a 25 percent loss of the fixed CO₂.

Question 20. (a) Differentiate between C₃ & C₄ plants w.r.t. energy requirement for one glucose molecule formation
(b) Choose the correct labelling for given below figures
A A B absorbed Photon B Light C 400 500 600 700 Wavelength of light in nanometres (nm)
(i) (ii)

Answer:

(a) C₃ Plant C₄Plant

18 ATP 30 ATP 12 NADPH 12 NADPH
(b) (i) A – Rate of photosynthesis

B – Absorption spectrum

(ii) A – Primary acceptor

B – Reaction centre

C – Accessory pigments