Question

Why is the transport of phloem sap considered an active process? a. The manufacture of sucrose via photosynthesis is driven by the energy in sunlight. b. Transpiration is driven by the energy in sunlight. c. Energy is used to transport sucrose into companion cells near sources, against a concentration gradient. d. Phloem sap is able to move down through a plant with the force of gravity.

Short answer

The transport of phloem sap is considered an active process because energy is used to transport sucrose into companion cells near sources, against a concentration gradient (option C).

Step-by-step solution

Option A

Let's analyze option A: The manufacture of sucrose via photosynthesis is driven by the energy in sunlight. This statement is true, as photosynthesis uses sunlight to produce sugars, including sucrose. However, the question is about the transport of phloem sap, which is a separate process. So, option A is not the correct answer.

Option B

Now let's explore option B: Transpiration is driven by the energy in sunlight. Transpiration is the loss of water vapor from plants and is indeed driven by the energy in sunlight. But again, the question is about the transport of phloem sap, not the transpiration process. Thus, option B is not the correct answer.

Option C

Let's examine option C: Energy is used to transport sucrose into companion cells near sources, against a concentration gradient. This statement explains the use of energy during the phloem transport process, which involves moving sucrose into companion cells near sources (like leaves) against a concentration gradient. This description refers to active transport, as it requires energy to move substances against a concentration gradient. Thus, option C seems to be the correct answer.

Option D

Finally, let's check option D: Phloem sap is able to move down through a plant with the force of gravity. Although phloem sap can move down through a plant with the force of gravity, this is not the main reason why the transport of phloem sap is an active process. The active transport aspect involves moving sucrose into companion cells against a concentration gradient, which is explained by option C. Therefore, option D is not the correct answer. In conclusion, the correct answer is option C: Energy is used to transport sucrose into companion cells near sources, against a concentration gradient. This explains why the transport of phloem sap is considered an active transport process.

Key concepts

Active Transport

Active transport is a critical process in the movement of molecules across cell membranes. Unlike passive transport, which occurs without the use of energy, active transport requires energy, often in the form of ATP.
This energy is necessary to move substances like ions or organic molecules against their concentration gradient, meaning from areas of low concentration to areas of high concentration.
For instance, during phloem transport, the energy-dependent loading of sucrose into the sieve elements is a perfect example of active transport at work. By utilizing carrier proteins and ATP, plants can efficiently navigate the challenges of moving nutrients where they're needed most.

• Carrier proteins are specialized molecules within the cell membrane that aid in this process.
• Energy often comes from ATP, the cellular "currency" used to perform work.

Active transport ensures that vital nutrients can reach all parts of the plant, overcoming both gravitational forces and concentration imbalances.

Sucrose Transport

Sucrose transport is a key element in the distribution of nutrients within plants. After sucrose is produced in plant leaves through photosynthesis, it needs to be transported to different parts of the plant.
This is where phloem plays an essential role. Through the process known as phloem loading, sucrose is actively transported into the sieve elements with the help of companion cells.
Once inside the phloem, sucrose can travel to other locations, such as roots or fruits, where it is needed.

• Phloem loading involves the active transport of sucrose into the conduits of the plant.
• Companion cells aid by providing the energy required for this process.

This system allows plants to allocate resources effectively, supporting their growth and ability to adapt to environmental changes.

Concentration Gradient

Understanding concentration gradients is fundamental to grasping why certain transport systems in plants require energy. A concentration gradient occurs when there's a difference in the concentration of a substance between two regions.
In the context of phloem transport, sucrose is often moved from areas of low concentration (such as companion cells) to areas of high concentration (such as the phloem channels). This movement is energetically unfavourable, hence the need for active transport.
To move against this gradient, energy is applied, allowing sucrose to accumulate where it is less naturally abundant.

• Gradients are essential for driving many biological processes.
• Without these gradients, passive diffusion would dominate, limiting control over substance distribution.

Plants exploit these gradients as part of their intricate system to achieve efficient nutrient distribution.

Companion Cells

Companion cells are integral to the process of sucrose transport in plants. These specialized cells are located next to the sieve tube elements in the phloem. They perform numerous vital functions to support the sieve elements, including providing energy and regulating the phloem’s loading and unloading processes.
Companion cells ensure that sucrose can be actively transported into the phloem by supplying ATP for this energy-demanding process.

• They remain metabolically active, unlike the sieve elements, which lack a nucleus.
• Their intimate connection to sieve elements allows them to serve as a critical support system.

Through their specialized roles, companion cells facilitate the efficient movement of nutrients, thereby playing an indispensable role in plant nutrition and health.