A mutant plant lacking the ability to pump protons out of leaf companion cells will be unable to do which of the following? a. initiate transpiration b. load sucrose into sieve-tube elements c. carry out photosynthesis d. transport water through the xylem

Verified step by step guidance

1. Understand the role of proton pumps in plant cells: Proton pumps in plant cells, especially in leaf companion cells, are crucial for creating a proton gradient. This gradient is essential for various transport processes across the cell membrane.
2. Relate the proton gradient to sucrose loading: The proton gradient generated by the proton pumps is used to drive the co-transport of sucrose into the sieve-tube elements of the phloem. This process is known as active transport, where sucrose is loaded against its concentration gradient.
3. Analyze the options in relation to proton pumping: Since the proton pumps are directly involved in creating the gradient needed for sucrose loading into the phloem, a lack of this function would directly impact this process.
4. Eliminate unrelated processes: The initiation of transpiration, photosynthesis, and water transport through the xylem are not directly dependent on the proton pumping in companion cells. These processes involve different mechanisms and parts of the plant.
5. Conclude the correct answer: Therefore, the correct answer is 'b. load sucrose into sieve-tube elements', as this is the process that directly requires the function of proton pumps in the companion cells.

Key Concepts

Proton Pumping

Proton pumping is a process where protons (H+ ions) are actively transported out of cells, creating a proton gradient across the cell membrane. This gradient is essential for various cellular processes, including nutrient transport. In plants, proton pumps in companion cells help load sugars like sucrose into the phloem for transport.

Sieve-Tube Elements

Sieve-tube elements are specialized cells in the phloem of plants that facilitate the transport of sugars and other organic nutrients. They are connected end-to-end, forming long tubes. The loading of sucrose into these elements is crucial for efficient nutrient distribution, and it relies on the proton gradient established by proton pumps.

Transpiration

Transpiration is the process by which water vapor is released from plant leaves into the atmosphere. It creates a negative pressure that helps draw water up from the roots through the xylem. While transpiration is vital for water transport, it is not directly dependent on the proton pumping mechanism in companion cells, which primarily affects phloem loading.