Question

The similarity between \(\mathrm{C}_{3}\) and \(\mathrm{C}_{4}\) pathway is: (a) Both are equally efficient. (b) Organic acid is formed as the first product of \(\mathrm{CO}_{2}\) fixation. (c) Both requires one type of cell to occur. (d) Both takes place in all the plants.

Short answer

All of the options provided (a, b, c, d) are incorrect as they do not accurately represent similarities between C3 and C4 photosynthetic pathways.

Step-by-step solution

Analyze the options

Assess each statement and determine whether each is a true or false comparison between C3 and C4 pathways.

Option (a): Efficiency of both pathways

The efficiency of C3 and C4 pathways is not equal. C4 photosynthesis is more efficient than C3 photosynthesis under conditions of drought and high temperature. So, (a) is not correct.

Option (b): First product of CO2 fixation

In C3 plants, the first stable compound formed in the light-independent reactions of photosynthesis is a three-carbon compound called 3-PGA. In contrast, in C4 plants, the first product of CO2 fixation is a four-carbon compound, oxaloacetic acid (OAA). Therefore, it is not an organic acid that is formed as the first product of CO2 fixation in both pathways. So, (b) is not correct.

Option (c): Cell type requirement

C3 photosynthesis occurs in mesophyll cells, whereas C4 photosynthesis takes place in two types of cells, mesophyll and bundle sheath cells. Thus, it is not true that both pathways require only one type of cell to occur. So, (c) is not correct.

Option (d): Occurrence in plants

Not all plants undergo both C3 and C4 photosynthesis. Most plants undergo C3 photosynthesis, but some, such as maize and sugarcane, undergo C4 photosynthesis. Therefore, it is not true that both pathways take place in all plants. So, (d) is not correct.

Key concepts

Photosynthesis process

Photosynthesis is the remarkable biological process by which plants, some bacteria, and algae convert light energy into chemical energy. It's a complex procedure involving multiple steps; however, at its core, photosynthesis can be separated into two main phases: the light-dependent reactions and the light-independent reactions (Calvin cycle).

In the light-dependent reactions, which take place on the thylakoid membranes of chloroplasts, sunlight is absorbed by chlorophyll and other pigments. This captured energy is then used to split water molecules (H₂O) into oxygen, protons, and electrons. The electrons move through a series of proteins (electron transport chain) generating ATP and NADPH as energy carriers.

The light-independent reactions, occurring in the stroma of chloroplasts, do not require direct sunlight but use the ATP and NADPH from the light-dependent stage. Here, carbon dioxide (CO₂) from the atmosphere is 'fixed' into an organic molecule (glyceraldehyde-3-phosphate) through a series of reactions known as the Calvin cycle. The cycle consists of carbon fixation, reduction phase, carbohydrate formation and regeneration of the starting molecule, ribulose bisphosphate (RuBP). Enzymes play crucial roles in these steps, notably RuBisCO, notorious for fixing CO₂ in this cycle.

CO2 fixation in plants

CO₂ fixation in plants is at the heart of the Calvin cycle and is a key step in the photosynthetic process. It involves incorporating inorganic carbon dioxide from the atmosphere into organic molecules within the chloroplast. The enzyme RuBisCO facilitates the attachment of CO₂ to a five-carbon sugar, RuBP, resulting in a six-carbon compound that quickly splits into two molecules of 3-PGA, a three-carbon compound.

However, not all plants fix carbon in the same manner. While C3 photosynthesis is the most common form and involves the direct fixation of CO₂ into 3-PGA, some plants have adopted variations to optimize CO₂ fixation in different environmental conditions. For example, the C4 pathway has an additional set of reactions that effectively concentrate CO₂ in specific cells to overcome the inefficiency of RuBisCO in hot and dry climates. This adaptation minimizes photorespiration, a wasteful process where RuBisCO fixes oxygen instead of CO₂, leading to loss of energy and reduced carbohydrate production.

C3 and C4 pathway efficiency

The efficiency of the photosynthetic pathways, C3 and C4, varies according to environmental conditions. The C3 pathway, named so because the first stable product is a 3-carbon molecule, is used by the majority of plant species. It's most effective under moderate light conditions, cool nights, and when water is relatively abundant.

In contrast, C4 photosynthesis is named after the 4-carbon compound, oxaloacetate, formed as the first product in this pathway. It's an evolutionary adaptation that allows plants to flourish in high light intensities, high temperatures, and low CO₂ concentrations - conditions where C3 plants would not perform well. C4 plants utilize a unique mechanism by spatially separating CO₂ fixation and the Calvin cycle, utilizing mesophyll and bundle sheath cells to increase CO₂ concentration around RuBisCO, thereby reducing photorespiration and boosting efficiency.

When comparing C3 and C4 pathway efficiency, we must consider the environment. While C4 plants exhibit higher water-use efficiency and better growth at high temperatures, C3 plants will thrive in cooler, shadier, and more humid climates. As such, plants have evolved different mechanisms to maximize their photosynthetic capabilities according to their habitat's specific challenges.