Question

Mesophyll chloroplast of which plant alone is capable of synthesizing starch or sucrose? (a) \(\mathrm{C}_{3}\) plant (b) \(\mathrm{C}_{4}\) plant (c) Both (a) and (b) (d) Neither \(\mathrm{C}_{3}\) nor \(\mathrm{C}_{4}\)

Short answer

The mesophyll chloroplast of a C3 plant alone is capable of synthesizing starch or sucrose.

Step-by-step solution

Clarification of Terms

Starch and Sucrose are carbohydrate end-products of photosynthesis, a process that occurs in the chloroplasts of plants. C3 and C4 plants are types of plants that follow different photosynthetic pathways. In C3 plants, carbon dioxide is first incorporated into a 3-carbon compound. In C4 plants, carbon dioxide is first incorporated into a 4-carbon compound.

Understanding the Photosynthetic Process in C3 and C4 Plants

In C3 plants, the whole process of photosynthesis, including the carbon fixation and the creation of glucose, occurs in the mesophyll cells. Hence, the chloroplasts in these cells are capable of synthesizing starch or sucrose. In contrast, in C4 plants, the process is divided between mesophyll and bundle sheath cells. The initial reactions happen in mesophyll cells and the carbon is then passed to bundle sheath cells where glucose is synthesized. The C4 plants' mesophyll chloroplasts, therefore, do not directly participate in glucose synthesis.

Determining the Answer

Since the mesophyll chloroplasts of C3 plants can synthesize starch or sucrose, while those of C4 plants do not directly participate in glucose synthesis, we can determine that (a) C3 plant is the correct answer.

Key concepts

Photosynthesis

Photosynthesis is the process where plants, algae, and some bacteria convert light energy into chemical energy. This process primarily occurs in the chloroplasts of the plant cells. The basic formula for photosynthesis is:

• Carbon dioxide + Water + Light energy → Glucose + Oxygen.

During photosynthesis, the energy from sunlight is absorbed by chlorophyll, a green pigment in the chloroplasts. This energy drives the chemical reactions that convert carbon dioxide and water into glucose, a type of sugar that plants use for energy and growth. Oxygen, which is a by-product, is released into the atmosphere.
Photosynthesis consists of two main stages: the light-dependent reactions and the Calvin cycle. The light-dependent reactions capture sunlight and transform it into chemical energy in the form of ATP and NADPH. The Calvin cycle then uses this chemical energy to produce glucose from carbon dioxide.

Carbon Fixation

Carbon fixation is an essential step in the photosynthesis process where inorganic carbon dioxide is converted into organic molecules. This process takes place in the stroma of chloroplasts during the Calvin cycle.

• In C3 plants, carbon fixation results in a 3-carbon compound called 3-phosphoglycerate (3-PGA).
• In C4 plants, carbon dioxide is initially fixed into a 4-carbon compound called oxaloacetate in the mesophyll cells.

The absorbed carbon dioxide is captured by an enzyme called ribulose bisphosphate carboxylase/oxygenase (RuBisCO) in the Calvin cycle to create sugars that the plant uses as energy. This is the first major step of photosynthesis that leads to the conversion of solar to chemical energy.
Understanding carbon fixation is crucial for grasping how plants can efficiently use the sunlight and available resources to sustain themselves and support the ecosystem.

Chloroplasts

Chloroplasts are specialized organelles in plant cells where photosynthesis occurs. They are known for their green color due to chlorophyll and possess a double-membrane structure.

• Inside chloroplasts, you can find thylakoids stacked into grana where the light-dependent reactions take place.
• The stroma, a dense fluid around the thylakoids, hosts the Calvin cycle.

Chloroplasts not only help with photosynthesis but also play a role in the synthesis of amino acids and fatty acids. These organelles have their own DNA and replicate by division, which supports the theory that they originated from symbiotic bacteria.
In plant cells, chloroplasts are abundant, especially within the green tissues like leaves. They are the powerhouses that absorb sunlight and convert it into energy, making life on Earth possible.

C4 Plants

C4 plants possess a distinctive photosynthetic process that enables them to thrive in hot, arid environments. They have a modified carbon fixation pathway that efficiently manages carbon dioxide and minimizes water loss.

• C4 plants first fix carbon into a 4-carbon compound in mesophyll cells, unlike C3 plants.
• These compounds are then moved to the bundle sheath cells, where the Calvin cycle takes place.

This specialization allows C4 plants to reduce photorespiration, a process that can inhibit plant metabolism and production. Photorespiration occurs when RuBisCO assigns oxygen instead of carbon dioxide, leading to a loss of fixed carbon and energy.
Examples of C4 plants include corn, sugarcane, and sorghum. Their ability to efficiently handle resources in challenging climates makes them vital in agriculture, particularly in areas prone to heat and drought.