Movement of phloem sap from a source to a sink a. occurs through the apoplast of sieve-tube elements. b. depends ultimately on the activity of proton pumps. c. depends on tension, or negative pressure potential. d. results mainly from diffusion.

Verified step by step guidance

1. Step 1: Understand the terms. In plants, 'source' refers to the part where sugars are produced (like leaves), and 'sink' refers to the part where these sugars are used or stored (like roots, fruits, or young leaves). Phloem sap is the sugar-rich liquid transported from sources to sinks.
2. Step 2: Understand the process. The movement of phloem sap from a source to a sink is called translocation. It's an active process, meaning it requires energy.
3. Step 3: Evaluate the options. Option a suggests that the movement occurs through the apoplast of sieve-tube elements. However, the apoplast pathway is associated with the movement of water and minerals, not sugars. So, this option is incorrect.
4. Step 4: Option b suggests that the movement depends on the activity of proton pumps. This is correct. Proton pumps create a concentration gradient that allows sucrose to be loaded into the phloem, which is a crucial step in the process of translocation.
5. Step 5: Options c and d are incorrect. The movement of phloem sap does not depend on tension or negative pressure potential (this is associated with xylem and the movement of water), and it does not result mainly from diffusion (it's an active process). So, the correct answer is b.

Key Concepts

Phloem Transport

Phloem transport refers to the movement of sap containing nutrients, primarily sugars, from source tissues (where they are produced, like leaves) to sink tissues (where they are used or stored, like roots). This process is essential for plant growth and energy distribution, and it occurs through specialized cells called sieve-tube elements.

Proton Pumps and Active Transport

Proton pumps are membrane proteins that actively transport protons (H+ ions) out of cells, creating a gradient that drives the movement of other substances. In phloem transport, the activity of these pumps is crucial as it helps establish the osmotic pressure needed for the flow of sap from sources to sinks, facilitating nutrient movement.

Pressure Flow Hypothesis

The pressure flow hypothesis explains how phloem sap moves through the plant. It posits that the loading of sugars into sieve tubes creates a high osmotic pressure, causing water to enter and generating positive pressure that pushes the sap toward sinks. This process is distinct from tension-driven movement seen in xylem transport.