NIOS Class 12 Biology Photosynthesis Terminal Solutions

NIOS Biology Class 12 Chapter 11 Photosynthesis focuses on the process by which plants make food using sunlight, water, and carbon dioxide. It explains light and dark reactions, photosystems, and the significance of photosynthesis in life.

Terminal exercise solutions are included for practice and exam preparation. This chapter helps students link biochemical processes with daily life and provides a strong foundation for advanced biological concepts in NIOS Class 12 Biology studies.

NIOS Biology Class 12 Chapter 11 Photosynthesis

1. Describe briefly the process of photosynthesis.

Photosynthesis is the process by which green plants, in the presence of light, combine water and carbon dioxide to form carbohydrates. Oxygen is released as a byproduct of photosynthesis. In photosynthesis, CO₂ is fixed (or reduced) to carbohydrates (glucose C₆H₁₂O₆). Water is split in the presence of light (called photolysis of water) to release O₂.

2. Write short notes on (i) Ultrastructure of chloroplast and (ii) Pigments involved

in photosynthesis.

(i) Ultrastructure of chloroplast

• Chloroplasts are disc-shaped or lamellated.
• It is made of a double membrane with an outer and an inner membrane.
• Sac-like thylakoids placed one above the other constitute the granum.
• Stack-like grana are interconnected by lamellae.
• The inside of a chloroplast is filled with the fluid called stroma.
• Thylakoids contain photosynthetic pigments that absorb light of certain wavelengths to carry out photosynthesis.

Pigments involved in photosynthesis

• The photosynthetic pigment chlorophyll is the principal pigment involved in photosynthesis. It is a large molecule and absorbs light maximally in the violet-blue and in the red region of the visible spectrum, and reflects green light and thus leaves appear green in colour.
• Carotenoids (carotene and xanthophyll) absorb light in the regions of the spectrum not absorbed by the chlorophylls and transfer that energy to chlorophyll to be used in photosynthesis.
• Chlorophyll a (a special type of chlorophyll) is the main pigment that traps solar energy and converts it into chemical energy.
• All other pigments, such as chlorophyll b and carotenoids, are collectively called accessory pigments since they pass on the absorbed light energy to the chlorophyll a (Chl-a) molecule to be utilized for photosynthesis.

3. What are accessory pigments? Why are they called so?

Chlorophyll b and carotenoids are collectively called accessory pigments since they pass on the absorbed light energy to the chlorophyll a (Chl-a) molecule to be utilized for photosynthesis.

4. Mention the path of electrons in the light reaction of photosynthesis.

• Water is the primary source of electrons in the light reaction of photosynthesis in plants.
• When the PSII absorbs light energy and passes it to its reaction centre, P₆₈₀.
• The now excited P₆₈₀ transfers its electrons to a primary electron acceptor, Pheophytin, and comes back to its ground state.
• The P₆₈₀ is now oxidised, which causes the photolysis of water to release O₂ and H+.
• When P₆₈₀ absorbs electrons from a water molecule, and regains its electrons.
• The Pheophytin, which accepts electrons from P_680, passes the electrons to Plastoquinine and then to Cytochrome Complex.
• From the Cytoschrome complex, electrons are passed to Plastocyanin and from there to the PSII reaction centre- P₇₀₀.
• The excited P₇₀₀ passes the electrons to its primary acceptor and then to Ferredoxin. From Ferridoxin, it is passed to NADP (Nicotinamide Adenine Dinucleotide Phosphate) to form 2 molecules of NADPH, a reducing agent that reduces CO₂ to form carbohydrates.

5. What do you understand by photophosphorylation?

Photophosphorylation is the process of binding inorganic phosphate (Pi) with ADP to form ATP in the presence of light. Photophosphorylation occurs in the chloroplast.

• It can be cyclic photophosphorylation and non-cyclic photophosphorylation.
• In cyclic photophosphorylation, there is only one photosystem, PSI. Here, the electrons travel cyclically and return to the reaction centre P₇₀₀.
• In non-cyclin photophosphorylation, the electrons travel from PSII to PSI and then to NADPH and do not return to their initial reaction centre.

6. Discuss photolysis of water and its significance.

When the P680 loses its electrons to its primary acceptor, it becomes oxidized. This causes the splitting of a water molecule into H+ and O2. This light-dependent splitting of water molecules is called photolysis. The liberated oxygen molecule of photosynthesis comes from the water molecule due to photolysis. 

7. Describe the reactions occurring during the dark reaction of photosynthesis.

• Both NADPH2 and ATP produced during the light reaction are essential requirements for the synthesis of carbohydrates.
• This series of reactions, which catalyse the reduction of CO2 to carbohydrates (also called fixation of CO2), takes place in the stroma of the chloroplast.
• These reactions are independent of light, i.e., light is not necessary, but can continue in light as well if products of the light reactions are available. Thus, it is also called the dark reaction.
• The carbon fixation reactions produce sugar in the leaves of the plant, from where it is exported to other tissues of the plant as a source of both organic molecules and energy for growth and metabolism.
• There are two major pathways by which CO2 fixation (Dark reaction) takes place.

8. Differentiate between C3 and C4 plants. 

C3 Plants	C4 Plants
Carbon dioxide fixation occurs once	Carbon dioxide fixation occurs twice, first in the mesophyll and then in bundle sheaths.
RuBP is the only one-carbon dioxide acceptor.	There are two carbon dioxide acceptors- the 3-carbon compound PEP (phosphoenol pyruvate) in the mesophyll and RuBP in the bundle sheath
RuBP carboxylase, which is not efficient when the CO2 concentration is low	PEP carboxylase, which is very efficient, even if the CO2 conc. It has low RuBP carboxylase, works efficiently because the carbon dioxideconcentration is high.
The first stable product is 3-C compound phosphoglyceric acid	The first product is the 4-C compound is oxaloacetic acid
Higher CO2 concentration promotes photosynthesis	Photosynthetic efficiency is high even if the CO2 concentration is low.
Only one type of chloroplast Kranz anatomy is absent	Two types of chloroplasts (dimorphic) or Kranz’s anatomy, i.e., two types of cells, each with its own type of chloroplasts, are present.
Photorespiration occurs. An excess of oxygen is an inhibitor of photosynthesis	Photorespiration is absent. The photosynthetic efficiency is further increased.
Less efficient photosynthesis than C4 plants. Yields are usually much lower.	More efficient photosynthesis as compared to that of the C3 plants. Yields are usually much higher.

9. Differentiate between PSI and PSII. 259

Photosystem I	Photosystem II
PS I has a reaction centre of the chlorophyll ‘a’ molecule with maximum light absorption at 700 nm wavelength. This reaction centre is referred to as P700.	PS II has a reaction centre of a chlorophyll ‘a’ molecule with maximum light absorption at 680 nm. This reaction centre is also referred to as P680.
The primary electron acceptor is an iron protein (Fe-S-protein)	Primary electron acceptor, pheophytin, is a modified chlorophyll a molecule with 2 hydrogen atoms in place of a magnesium ion.
A set of electron carriers is plastocyanin, ferredoxin, and cytochrome	A set of electron carriers is pheophytin, plastoquinone, and cytochromes.

10. What are the products of light reactions? What is the fate of these products?

The products of light reactions are ATP and NADPH₂. They move out from the thylakoid to the stroma of the chloroplast. They undergo dark reactions here, where they help reduce CO₂ to produce carbohydrates. 

11. Why is cyclic photophosphorylation called so?

Cyclic photophosphorylation occurs in photosynthetic bacteria, which lack PS-II, and it involves PSI only. During this process, electrons from PSI are not passed on to NADP. Instead, the same electrons are returned to the oxidised P700 molecule.

12. What is Kranz anatomy?

The leaves of C4 plants have vascular bundles with a sheath made of large parenchyma cells around them in the form of a wreath. Due to the presence of this wreath or Kranz, such anatomy is called Kranz anatomy. These leaves also have dimorphic chloroplasts. 

13. Name the two carboxylase enzymes in the C4 cycle.

Ribulose bisphosphate carboxylase and Phosphoenol pyruvate carboxylase.

14. What are chemosynthetic autotrophs?

Chemosynthetic autotrophs are certain bacteria that can utilise chemical energy released during the biological oxidation of certain inorganic substances to reduce carbon dioxide to carbohydrates. This process is called chemosynthesis. Ni, terrifying bacteria such as Nitrosomonas and Nitrobacter are examples. 

15. How does CO₂ concentration affect the rate of photosynthesis?

Since carbon dioxide is one of the raw materials for photosynthesis, its concentration affects the rate of photosynthesis markedly. Because of its very low concentration (0.03%) in the atmosphere, it acts as a limiting factor in natural photosynthesis. At optimum temperature and light intensity, if the carbon dioxide supply is increased, the rate of photosynthesis increases markedly until the CO₂ concentration. It is as high as 3.0%. Thus, CO₂ conc. The atmosphere is always a limiting factor for photosynthesis.

16. What is the effect of an excess of oxygen on the rate of photosynthesis?

Concentration of oxygen as an external factor is never a limiting factor for photosynthesis because it is a by-product of photosynthesis, and it easily diffuses into the atmosphere from the photosynthesising organ, the leaf. However, excesses of O₂ surrounding a green plant reduce photosynthetic rate by promoting the rate of aerobic respiration.

17. Whether light absorbed by green plants, on a global basis, is a limiting factor for photosynthesis or not! Explain

The rate of photosynthesis increases with the increase of intensity of light within physiological limits, or the rate of photosynthesis is directly proportional to light intensity. Except on a cloudy day and at night, light is never a limiting factor in photosynthesis in nature.

At a certain light intensity, the amount of CO₂ used in photosynthesis and the amount of CO₂ produced in respiration are the same. This point of light intensity is known as the compensation point.

The wavelength of light absorbed by photosynthetic pigments affects the rate of photosynthesis. Red light and, to some extent, blue light have an enhancing influence on photosynthesis.

The proportion of the total incident sunlight on Earth absorbed by green plants is generally a limiting factor. As per the estimates of the total incident light reaching the green plants, only about 1-2% is actually absorbed, because 70% is transmitted, and 28-29% is reflected into the atmosphere.

Terminal Questions Solutions