Photosynthesis in Higher Plants

Carboxylation is the most crucial step of the Calvin cycle where $\mathrm{CO}_2$ is utilised for the carboxylation of RuBP. This reaction is catalysed by enzyme RuBP carboxylase. Since this enzyme also has an oxygenase activity, RuBisCO has higher affinity for carbon dioxide than oxygen.

A chromatographic separation of the leaf pigments shows that the colour that we see in leaves is not due to single pigment but due to four pigments.

The light reaction of photosynthesis, also known as the photochemical phase, takes place in the thylakoid membranes of the chloroplasts. This phase requires several key components to occur, which include:

Chlorophyll (D): Chlorophyll is the pigment that absorbs light energy, which is crucial for the light reaction. When chlorophyll absorbs light, it becomes excited and this energy is used to convert $\mathrm{ADP}$ and $\mathrm{P_i}$ (inorganic phosphate) into $\mathrm{ATP}$ and to reduce $\mathrm{NADP}^+$ into $\mathrm{NADPH}$.

Light (E): Light provides the energy necessary for the excitation of chlorophyll electrons. Without light, the light reactions cannot take place.

$\mathrm{H}_2 \mathrm{O}$ (C): Water molecules are split during the light reaction in a process known as photolysis. This splitting produces electrons that replace those lost by chlorophyll, and it also generates oxygen as a byproduct.

While $\mathrm{CO}_2$ (A) is essential for photosynthesis, it is specifically used in the Calvin Cycle (dark reaction) and not directly required for the light reaction. $\mathrm{O}_2$ (B) is a product of the light reaction, not a requirement.

Therefore, the correct components required for the light reaction of photosynthesis are:

Option B: C, D, and E only.

In the process of photosynthesis, several products are formed within the chloroplasts of plant cells. Photosynthesis involves the conversion of light energy into chemical energy, resulting in the production of glucose and oxygen as its primary products. Let's evaluate the options given:

Option A: ADP

ADP (Adenosine Diphosphate) is not a product of photosynthesis. Instead, it is involved in the cellular respiration process and is converted to ATP (Adenosine Triphosphate) during the light-dependent reactions of photosynthesis.

Option B: NADPH

NADPH (Nicotinamide Adenine Dinucleotide Phosphate) is a product of the light-dependent reactions of photosynthesis, but it remains within the chloroplast to be used in the Calvin cycle for the synthesis of glucose. Hence, it does not diffuse out of the chloroplast.

Option C: O$_2$

Oxygen (O$_2$) is a byproduct of the light-dependent reactions in the thylakoid membranes of the chloroplasts. This oxygen is released into the atmosphere and hence, diffuses out of the chloroplast.

Option D: ATP

ATP (Adenosine Triphosphate) is also produced during the light-dependent reactions of photosynthesis. However, like NADPH, ATP is utilized within the chloroplast for the Calvin cycle and does not diffuse out.

Therefore, the correct answer is:

Option C: O$_2$

The calvin cycle proceeds in three stages:

(A) Carboxylation, during which CO$_2$ combines with ribulose-1,5-bisphosphate.

(B) Reduction, during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH.

(C) Regeneration during which the CO$_2$ acceptor ribulose-1,5-bisphosphate is formed again so that the cycle continues.

The Calvin cycle, also known as the Calvin-Benson-Bassham cycle, is the set of chemical reactions that take place in chloroplasts during photosynthesis. The cycle is light-independent because it takes place after the energy has been captured from sunlight. The Calvin cycle uses ATP and NADPH as energy sources, and incorporates $\mathrm{CO}_2$ into organic molecules, eventually producing glucose.

The cycle consists of three main stages:

• Carbon fixation
• Reduction phase
• Regeneration of the ribulose-1,5-bisphosphate (RuBP)

In the Calvin cycle:

1. Carbon Fixation: Each $\mathrm{CO}_2$ molecule is attached to a five-carbon sugar named ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). The product of this reaction is a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction Phase: Each 3-PGA is phosphorylated by ATP (consuming 2 ATP molecules total for two 3-PGA molecules) to form 1,3-bisphosphoglycerate. This molecule is then reduced by NADPH (using 2 NADPH molecules in total) to form glyceraldehyde-3-phosphate (G3P). Out of every six G3P formed, one exits the cycle to contribute towards forming glucose, while the rest are recycled to regenerate RuBP.
3. Regeneration of RuBP: For every three CO₂ molecules fixed, five molecules of G3P are used to regenerate three molecules of RuBP, requiring further ATP input (3 more ATP molecules).

Thus, for each incorporated $\mathrm{CO}_2$ molecule, 3 ATP molecules and 2 NADPH molecules are required:

• 3 ATP molecules (2 for the reduction of two molecules of 3-PGA into 1,3-bisphosphoglycerate and then one for the regeneration phase of RuBP).
• 2 NADPH molecules are used to reduce two molecules of 1,3-bisphosphoglycerate to G3P.

Therefore, the correct answer to the question is Option D: 3 molecules of ATP and 2 molecules of NADPH are required for every molecule of $\mathrm{CO}_2$ fixed in the Calvin cycle.

The dark reactions of photosynthesis, also known as the Calvin cycle, do not directly require light to proceed and hence are termed "dark reactions." The reactions occur in the stroma of chloroplasts and primarily involve the synthesis of organic molecules from carbon dioxide ($\mathrm{CO}_2$). Here's an analysis of the options provided:

• A. Light: Not required for the dark reactions themselves; these reactions are light-independent.
• B. Chlorophyll: Chlorophyll is essential for the light-dependent reactions of photosynthesis, where it absorbs light energy and helps convert it into chemical energy. It is not used directly in the dark reactions.
• C. $\mathrm{CO}_2$: Carbon dioxide is a critical substrate in the dark reactions. It is fixed into organic sugars through a series of enzymatic steps in the Calvin cycle.
• D. ATP: Adenosine triphosphate (ATP) is a vital energy carrier generated in the light reactions. It provides the energy required for the synthesis of glucose from $\mathrm{CO}_2$ in the dark reactions.
• E. NADPH: Nicotinamide adenine dinucleotide phosphate (NADPH) is another product of the light reactions and acts as a reducing agent, donating electrons during the reduction of $\mathrm{CO}_2$ to form glucose in the Calvin cycle.

Given this understanding, the components required for the dark reaction are $\mathrm{CO}_2$ for carbon fixation, ATP for energy, and NADPH for reducing power. Therefore, the parts necessary from the given list are C, D, and E. So, the correct answer to this question is:

Option C: C, D, and E only

In $\mathrm{C}_3$ plant, some $\mathrm{O}_2$ bind to RuBisCO, and hence $\mathrm{CO}_2$ fixation is decreased. Statement II is incorrect, photorespiration does not occur in $\mathrm{C}_4$ plants as they lack RuBisCO in mesophyll. Hence statement I is the only correct option.

Option C : Both Statement I and Statement II are correct

Explanation :

Statement I : As mentioned, RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is indeed the most abundant enzyme in the world and it plays a crucial role in the process of photosynthesis. It catalyzes the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants into energy-rich molecules such as glucose.

Statement II : In C₄ plants, the RuBisCO enzyme is physically separated from the oxygen in the leaf air spaces by concentrating it in bundle sheath cells. This significantly reduces its oxygenase activity and thus the rate of photorespiration, effectively making photorespiration minimal to non-existent under normal conditions. However, under certain conditions, such as extreme drought or high temperatures, photorespiration can occur in C₄ plants as well. But in a standard environment, it's largely inhibited, making this statement correct.

Photosystems I and II are involved in the light-dependent reactions of photosynthesis, occurring in the thylakoid membranes of chloroplasts.

Option A : Grana lamellae (the stacks of thylakoid membranes in chloroplasts) do indeed have both Photosystem I (PS I) and Photosystem II (PS II).

Option C : During non-cyclic photophosphorylation, both ATP (energy currency of the cell) and NADPH (which carries electrons for the light-independent reactions, or the Calvin cycle) are produced. This statement is correct.

Option D : Stroma lamellae (parts of the thylakoid system that interconnect grana) lack PS II and NADP reductase, which are instead located in the grana lamellae. This statement is also correct.

Option B : Cyclic photophosphorylation involves only PS I, not PS II. This is because the primary purpose of cyclic photophosphorylation is to produce additional ATP without producing NADPH. Therefore, this statement is incorrect.

For every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH₂ are required. To make one molecule of glucose, 6 turns of the cycle are required. Thus, ATP and NADPH₂ molecules required for synthesis of one molecule of glucose during Calvin cycle will be

In PS-I, the reaction centre chlorophyll a has an absorption peak at 700 nm, while in PS-II, reaction centre has an absorption maxima at 680 nm.

The reduction step of Calvin cycle involves utilisation of two molecules of ATP for phosphorylation and two molecules of NADPH for reduction per molecule of CO₂ fixed.

$\bullet$ Primary acceptor of electron transfers its electron not to an electron carrier but to an H carrier.

$\bullet$ Protons increase in number in lumen of the thylakoid not in stroma.

The primary CO₂ acceptor is a 3-carbon molecule, phosphoenol pyruvate (PEP) and is present in the mesophyll cells.

Mesophyll cells of C₄ plants lack RuBisCO enzyme.

Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATP synthase. Energy is used to pump protons across a membrane to create a gradient or a high concentration of protons within the thylakoid lumen.

The NADP reductase enzyme is located on the stroma side of the membrane. Along with the electrons that come from the acceptor of electrons of PS I, protons are necessary for reduction of NADP⁺ to NADPH + H⁺.

The process does not involve breaking of electron gradient.

The oxygenation activity of RuBisCo (Ribulose-1,5-bisphosphate carboxylase/oxygenase) in photorespiration leads to the formation of:

Option A

This implies the production of 1 molecule of a 3-carbon compound (3-phosphoglycerate) and 1 molecule of a 2-carbon compound (2-phosphoglycolate).

In detail, RuBisCo facilitates the reaction where ribulose-1,5-bisphosphate reacts with O$_2$ instead of CO$_2$. This process yields one 3-phosphoglycerate (3-PGA) and one 2-phosphoglycolate, which is a distinctive feature of the oxygenation process in photorespiration as opposed to the carboxylation process in the Calvin cycle which produces two molecules of 3-PGA.

Thus, in the context of photorespiration:

$ \text{Ribulose-1,5-bisphosphate} + \text{O}_2 \rightarrow \text{3-phosphoglycerate} + \text{2-phosphoglycolate} $

This results in 1 molecule of a 3-carbon compound and 1 molecule of a 2-carbon compound.

In photorespiration, the correct sequence of cell organelles involved is as follows:

• Chloroplast: This is where the initial steps of photorespiration begin. Here, the enzyme Rubisco oxygenates RuBP, leading to the production of a 2-carbon compound.




• Peroxisome: The 2-carbon compound is then transferred to the peroxisome. Here, it is converted into a 3-carbon compound while hydrogen peroxide is produced and subsequently broken down.




• Mitochondria: Finally, the 3-carbon compound is moved to the mitochondria where it is converted back into a 3-carbon phosphate molecule that can re-enter the Calvin cycle in the chloroplast.

So, the correct option that reflects this sequence is :

Option C : Chloroplast, Peroxisome, Mitochondria.