Problem 4 - Campbell Biology

Question

Photosynthesis ceases when leaves wilt, mainly because \begin{equation}\begin{array}{l}{\text { (A) the chlorophyll in wilting leaves is degraded. }} \\ {\text { (B) accumulation of } \mathrm{CO}_{2} \text { in the leaf inhibits enzymes. }} \\ {\text { (C) stomata close, preventing } \mathrm{CO}_{2} \text { from entering the leaf. }} \\ {\text { (D) photolysis, the water-splitting step of photosynthesis, }} \\ {\text { cannot occur when there is a water deficiency. }}\end{array}\end{equation}

Step-by-Step Solution

Answer

Photosynthesis stops because stomata close, preventing CO2 from entering the leaf.

Step 1: Understand the Question

The problem is asking why photosynthesis stops when leaves wilt. Options include degradation of chlorophyll, accumulation of CO2 inhibiting enzymes, stomatal closure preventing CO2 entry, or the inability of photolysis to occur due to water deficiency.

Step 2: Review Each Option

Consider each given choice carefully and understand the effects of wilting on photosynthesis: 1. Chlorophyll degradation (Option A). 2. CO2 accumulation inhibiting enzymes (Option B). 3. Stomatal closure preventing CO2 entry (Option C). 4. Inability of photolysis to occur due to water deficiency (Option D).

Step 3: Analyze the Impact of Wilting

Wilting primarily affects the stomata of the leaves. The stomata are tiny openings that allow CO2 to enter the leaf. When leaves wilt, the stomata close to conserve water. This prevents CO2 from entering the leaf, which is essential for photosynthesis.

Step 4: Eliminate Incorrect Options

Eliminate options that do not make sense: - Option A (chlorophyll degradation) does not happen immediately due to wilting. - Option B (CO2 accumulation inhibiting enzymes) is less likely to affect photosynthesis immediately. - Option D (inability of photolysis due to water deficiency) is a valid point but less direct as the immediate impact of wilting is on stomata.

Step 5: Select the Best Option

The most immediate effect of wilting is on the closing of the stomata which prevents CO2 from entering the leaf. Hence, the best option is: Option C: Stomata close, preventing CO2 from entering the leaf.

Key Concepts

chlorophyll

Chlorophyll is a vital pigment in plants, responsible for the green color seen in leaves. It plays a key role in the process of photosynthesis by capturing light energy from the sun. This energy is then used to convert carbon dioxide (CO2) and water (H2O) into glucose (a type of sugar) and oxygen (O2). There are different types of chlorophyll, with chlorophyll a and chlorophyll b being the most common in plants. Though degradation of chlorophyll (as suggested in Option A) is a factor in photosynthesis cessation, it is not the immediate cause when leaves wilt. The direct effect comes from stomatal closure which inhibits CO2 intake.

CO2 inhibition

CO2 (carbon dioxide) is an essential component for photosynthesis. During this process, CO2 enters the plant leaves through tiny openings called stomata. Inside the leaf, CO2 is fixed into organic molecules, which later form glucose. Accumulating CO2 in the leaf can inhibit enzymes involved in photosynthesis, hence affecting the process. However, in the context of wilting leaves, CO2 inhibition is not the primary concern. The stomatal closure that occurs to preserve water restricts CO2 entry, thereby directly stopping photosynthesis. This limitation is a more precise cause than CO2 accumulation.

stomatal closure

Stomata are tiny pores on leaf surfaces that control gas exchange. They allow CO2 to enter for photosynthesis and oxygen (O2) to exit. However, when leaves start to wilt due to water deficiency, the stomata close to reduce water loss through evaporation. This closure effectively stops CO2 from entering the leaf, and without CO2, the photosynthesis process cannot proceed. This is why when leaves wilt, the immediate cessation of photosynthesis is primarily due to stomatal closure. Thus, among the provided options, Option C correctly identifies this mechanism.

photolysis

Photolysis is a critical step in photosynthesis where water molecules are split using light energy. This process generates the electrons required for creating the energy-rich molecules ATP and NADPH, and it also produces O2 as a byproduct. Photolysis occurs in the thylakoid membranes of the chloroplasts. While a water deficiency can hinder photolysis (as mentioned in Option D), during wilting, the more immediate and direct effect is the closure of stomata. This leads to a lack of CO2 intake, which halts photosynthesis. Therefore, photolysis is not the first level cause for the stoppage of photosynthesis when leaves wilt.