Based on Figure 7.18, which of these experimental treatments would increase the rate of sucrose transport into a plant cell?
a. Decreasing extracellular sucrose concentration
b. Decreasing extracellular pH
c. Decreasing cytoplasmic pH
d. Adding a substance that makes the membrane more permeable to hydrogen ions

Verified step by step guidance

1. Understand the mechanism of sucrose transport in plant cells, which often involves a symport system where sucrose is transported along with hydrogen ions (H⁺) across the cell membrane.
2. Consider how the concentration gradient of hydrogen ions affects the transport rate. A higher concentration of H⁺ outside the cell can drive the symport process more effectively.
3. Evaluate the effect of extracellular pH on hydrogen ion concentration. Lowering the extracellular pH increases the concentration of H⁺ ions outside the cell, potentially enhancing the symport of sucrose.
4. Analyze the impact of cytoplasmic pH. Decreasing cytoplasmic pH would increase H⁺ concentration inside the cell, potentially disrupting the gradient needed for effective symport.
5. Consider the role of membrane permeability to hydrogen ions. Increasing permeability could facilitate the movement of H⁺ ions, thereby enhancing the sucrose transport rate if the gradient is favorable.

Key Concepts

Sucrose Transport Mechanism

Sucrose transport into plant cells typically occurs via active transport mechanisms, often involving sucrose-proton symporters. These transporters use the proton gradient across the membrane to facilitate the movement of sucrose into the cell against its concentration gradient. Understanding how these symporters function is crucial for predicting how changes in environmental conditions might affect sucrose transport.

Proton Gradient and pH

The proton gradient across the cell membrane is essential for many transport processes, including sucrose transport. The gradient is influenced by the pH levels inside and outside the cell; a lower extracellular pH can increase the proton concentration outside the cell, enhancing the gradient. This gradient is a driving force for symporters that couple sucrose transport with proton movement, making pH manipulation a key factor in transport rate changes.

Membrane Permeability

Membrane permeability refers to the ability of substances to pass through the cell membrane. Altering the permeability to specific ions, such as hydrogen ions, can significantly impact the proton gradient and thus affect transport processes. Increasing membrane permeability to protons can disrupt the gradient, potentially enhancing or inhibiting the transport of molecules like sucrose, depending on the direction and magnitude of the change.