The student's statement is incorrect because the proton pumps in companion cells do not directly transport sucrose into the phloem sieve tubes. Instead, they create a favorable electrochemical gradient that *indirectly* facilitates sucrose transport. Here's a detailed explanation:

Incorrect Statement Breakdown: The statement implies that the proton pumps directly bind to sucrose and transport it across the membrane. This is not the case. Proton pumps are responsible for creating a proton gradient, not for directly transporting sucrose.

Actual Role of Proton Pumps: Proton pumps (H⁺-ATPases) are located in the plasma membrane of the companion cell. Their primary function is to actively transport protons (H⁺) from the cytoplasm into the apoplast (the space outside the plasma membrane). This process requires energy in the form of ATP. This creates a low pH environment in the apoplast surrounding the companion cell and the sieve tube element.

Actual Role of Co-transporter Protein: A specific co-transporter protein is embedded in the plasma membrane of the sieve tube element. This co-transporter is a symporter, meaning it transports sucrose *with* the movement of protons down their electrochemical gradient. The low pH in the apoplast (created by the proton pumps) provides the driving force for the co-transporter to move sucrose from the apoplast into the sieve tube element. The co-transporter binds to both sucrose and protons, and then releases them on the other side of the membrane. Therefore, the proton pumps create the gradient, and the co-transporter utilizes that gradient to move sucrose into the sieve tube element.

In summary, the proton pumps create the proton gradient, and the co-transporter protein uses this gradient to actively transport sucrose into the sieve tube element. The proton pumps do not directly transport sucrose; they create the conditions necessary for the co-transporter to do so. The co-transporter is the key protein responsible for the actual sucrose transport.

The hydrostatic pressure at the source and sink are inversely related, and this pressure difference is the driving force behind phloem transport. The relationship is governed by the principles of osmosis and water potential.

Source (Leaf): As described previously, the influx of sucrose into the sieve tubes at the source lowers the water potential within the sieve tubes. This creates a higher water potential within the sieve tubes compared to the surrounding mesophyll cells. Due to water moving from areas of high water potential to low water potential, water enters the sieve tubes by osmosis. This influx of water increases the turgor pressure within the sieve tubes, resulting in a higher hydrostatic pressure at the source.

Sink (Fruit): At the sink, sucrose is actively transported out of the sieve tubes into the sink cells. This increases the solute concentration within the sieve tubes, raising the water potential within the sieve tubes compared to the sink cells. Water then moves out of the sieve tubes by osmosis, driven by the water potential gradient. This efflux of water decreases the turgor pressure within the sieve tubes, resulting in a lower hydrostatic pressure at the sink.

Pressure Gradient and Water Potential: The difference in water potential between the source and sink creates a hydrostatic pressure gradient. The higher pressure at the source drives the bulk flow of phloem sap towards the lower pressure at the sink. The water potential gradient is the fundamental driver of the osmotic movement of water, and the hydrostatic pressure gradient is the result of this osmotic movement. The pressure gradient is directly related to the water potential difference; a larger water potential difference results in a larger pressure difference.

In essence, the source creates a high pressure by drawing water in, and the sink creates a low pressure by releasing water. This pressure difference, driven by the water potential gradient, is what propels the phloem sap from the source to the sink.

Answer:

Xerophytic leaves exhibit several structural adaptations to minimize water loss via transpiration. These adaptations typically involve modifications to the epidermis, mesophyll, and stomata. Below is an annotated transverse section of a typical xerophytic leaf illustrating three key adaptations:

1. Thickened Epidermis with a Cuticle: The epidermis is often significantly thicker than in mesophytic plants. This thickness is due to a prominent cuticle, a waxy layer covering the outer surface. The cuticle is composed of cutin, a hydrophobic polymer, which is impermeable to water. This reduces water evaporation from the leaf surface.

[Annotated Drawing: A transverse section of a xerophytic leaf. Clearly label the epidermis, cuticle, and mesophyll. The cuticle should be depicted as a thick layer on the outer surface of the epidermis. Arrows should indicate the direction of water movement away from the mesophyll through the cuticle.]

2. Reduced Surface Area (e.g., Spines or Scale-like Leaves): Many xerophytes have reduced leaf surface area. This can take the form of spines (modified leaves) or scale-like leaves. Reducing the surface area exposed to the air directly minimizes the area from which water can evaporate. This is a highly effective adaptation.

[Annotated Drawing: A transverse section of a leaf with spines. The spines are clearly visible and labelled. The remaining leaf tissue is shown as a smaller area.]

3. Sunken Stomata or Stomatal Crypts: Stomata are pores through which water vapor escapes. In xerophytes, stomata are often sunken within pits or crypts on the leaf surface. These crypts create a humid microclimate around the stomata, reducing the water potential gradient between the leaf and the atmosphere. This lowers the rate of transpiration.

[Annotated Drawing: A transverse section of a leaf showing sunken stomata within a crypt. The crypt is clearly labelled, and arrows indicate the direction of water vapor movement from the leaf into the crypt, reducing the rate of transpiration.]

These three adaptations, among others, work synergistically to significantly reduce water loss in xerophytic plants, allowing them to thrive in arid environments.