Question

In \(\mathrm{C}_{4}\) plants, Calvin cycle enzymes are present in (a) chloroplasts of mesophyll cells (b) chloroplasts of bundle sheath cells (c) cytoplasm of guard cells (d) cytoplasm of epidermal cells.

Short answer

In C4 plants, Calvin cycle enzymes are present in the chloroplasts of bundle sheath cells.

Step-by-step solution

Understanding C4 Photosynthesis

C4 photosynthesis is a two-stage process taking place in different cell types in the leaves. The initial fixation of CO2 occurs in the mesophyll cells, producing a four-carbon compound that is then shuttled to the bundle sheath cells. In the bundle sheath cells, the CO2 is released for use in the Calvin cycle.

Location of Calvin Cycle Enzymes

The Calvin cycle takes place in the chloroplasts of cells where CO2 is present. C4 plants concentrate CO2 in their bundle sheath cells by capturing it in mesophyll cells and then transferring it through plasmodesmata. Thus, the Calvin cycle enzymes that require this CO2 are located in the chloroplasts of bundle sheath cells.

Answering the Multiple Choice Question

Based on the information about C4 photosynthesis, it can be determined that the Calvin cycle enzymes are present in the chloroplasts of bundle sheath cells. Therefore, the correct answer is (b) chloroplasts of bundle sheath cells.

Key concepts

Calvin Cycle Enzymes

The Calvin cycle is a sequence of reactions that take place in the stroma of chloroplasts. Consumption of energy from ATP and NADPH, along with CO2 fixation, results in the synthesis of glucose. A crucial aspect for students to understand is the role of Calvin cycle enzymes. These enzymes, including ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), are integral for carbon fixation and subsequent steps.

Importantly, RuBisCO is one of the most abundant enzymes on earth, precisely because it plays such a critical role in the conversion of CO2 into organic compounds. It's important for students to identify that these enzymes are not just floating freely within the plant cells but are compartmentalized within the chloroplasts where they can most efficiently carry out their functions.

To assist students in visualizing the Calvin cycle and the action of these enzymes, analogies can be useful. For example, one can compare the Calvin cycle to a factory assembly line, where each enzyme is a worker specialized in a particular task, collaborating to build the final product — glucose.

Chloroplasts in Bundle Sheath Cells

Within C4 plants, the adaptation for hot and arid environments is exemplified by the presence of chloroplasts in bundle sheath cells. These specialized cells form a sheath around the vascular bundles and are critical for the plant's ability to minimize water loss while maintaining photosynthesis. For students, visualizing the structural differences in these cells compared to ordinary mesophyll cells can enhance comprehension.

The chloroplasts in these cells are the site of the Calvin cycle, having been provided with concentrated CO2 via the four-carbon compounds transported from the mesophyll cells. Key to understanding this concept is the recognition of the spatial separation of processes in C4 photosynthesis, which reduces photorespiration and increases efficiency.

An exercise improvement tip is to use cross-section diagrams of leaves to show the arrangement of mesophyll and bundle sheath cells. This helps students picture where the chloroplasts are and understand their functional significance in the specialized C4 pathway.

CO2 Fixation in Mesophyll Cells

In C4 photosynthesis, CO2 fixation begins in the mesophyll cells, which is distinct from the typical process observed in C3 plants where the Calvin cycle occurs within the same cell. The initial fixation in mesophyll cells involves the enzyme PEP carboxylase, which has a high affinity for CO2 and results in the formation of a four-carbon compound.

The concept of CO2 fixation in C4 plants includes this spatial separation of initial CO2 capture and the Calvin cycle, which can initially confuse students. Simplifying this, educators may liken this process to a two-step assembly line, where the first step takes place in the mesophyll cells that fix CO2 even at low concentrations, and the second step in the bundle sheath cells that further process the carbon compound.

Using comparative charts to contrast the C3 and C4 pathways can be an effective instructional approach, helping students recognize the advantages of the C4 mechanism, such as enhanced water use efficiency and reduced photorespiration, making these plants well-suited for hot, dry climates.