Question 1

Water potential is generally most negative in which of the following parts of a plant?

Accepted Answer

Water potential is generally most negative in the mesophyll cells of the leaf, which helps in the movement of water upwards from the roots through the xylem to the leaves due to the gradient in water potential.

Question 2

Active transport would be least important in the normal functioning of which of the following plant tissue types?

Accepted Answer

Tracheary elements in stems, which include tracheids and vessel elements, are involved in the passive transport of water and minerals in plants. Active transport is less critical in these cells compared to others listed, as their primary function is facilitated by the passive movement of water following a gradient, a process central to the cohesion-tension theory explaining water movement in plants.

Question 3

Which of the following has the lowest (most negative) water potential?

Accepted Answer

Leaf air spaces have the lowest (most negative) water potential because water potential is lowest in areas where water is most likely to evaporate into the air, such as the air spaces in leaves. This drives the movement of water from the roots through the plant to the leaves.

Question 4

What is the driving force for the movement of solutes in the phloem of plants?

Accepted Answer

The driving force for the movement of solutes in the phloem of plants is a difference in water potential (Ψ) between the source and the sink. This creates a pressure gradient that moves solutes from an area of high pressure (the source, where solutes are actively loaded into the phloem) to an area of low pressure (the sink, where solutes are unloaded from the phloem and used for growth or storage). Gravity, root pressure, transpiration of water through the stomata, and adhesion of water to phloem sieve tubes may all play a role in plant physiology, but they are not the primary driving force for phloem transport.

Question 5

If isolated plant cells with a water potential averaging -0.5 MPa are placed into a solution with a water potential of -0.3 MPa, which of the following would be the most likely outcome?

Accepted Answer

Water moves from areas of higher water potential to areas of lower water potential. Since the water potential outside the cells (-0.3 MPa) is higher than inside the cells (-0.5 MPa), water will move into the cells. This influx of water increases the pressure potential inside the cells as they swell, but the cell walls prevent them from bursting.

Question 6

Which of the following statements is false concerning the xylem?

Accepted Answer

Movement of materials in the xylem is by passive transport, not mass flow. Solute movement occurs through diffusion, but the main driving force for water movement is transpiration and the cohesion-tension theory. The other statements are all true.

Question 7

Phloem transport of sucrose is often described as going from source to sink. Which of the following would not normally function as a sink?

Accepted Answer

Mature leaves do not require high amounts of sucrose for growth, so they would not normally function as a sink. Instead, they are often considered sources of sucrose as they can produce it through photosynthesis. The other choices (A, B, C, and E) can all function as sinks as they require sucrose for growth, development, or storage.

Question 8

Phloem transport is described as being from source to sink. Which of the following would most accurately complete this statement about phloem transport as applied to most plants in the late spring? Phloem transports ________ from the ________ source to the ________ sink.

Accepted Answer

In most plants in the late spring, phloem transports sugars from the leaf source to the apical meristem sink. The apical meristem is where new growth occurs and therefore requires a constant supply of sugars for energy.

Question 9

Guard cells do which of the following?

Accepted Answer

Guard cells help balance the photosynthesis-transpiration compromise by regulating the opening and closing of stomata, thus controlling gas exchange and water loss.

Question 10

In which plant cell or tissue would the pressure component of water potential most often be negative?

Accepted Answer

The pressure component of water potential is typically negative in the xylem of plants due to the negative pressure created by transpiration. As water is pulled up through the xylem, the negative pressure (also known as tension) can cause the water to boil and form gas bubbles, which can lead to a blockage known as cavitation.

Question 11

Which of the following does not involve active transport across membranes?

Accepted Answer

The movement of sugar from one sieve-tube element to the next occurs through the phloem and is driven by pressure flow, which is a passive process, not requiring active transport.

Question 12

Which of the following is not an important component of the long-distance transport process in plants?

Accepted Answer

The transport process in plants involves the movement of water and solutes from the source to the sink. The cohesion of water molecules and a negative water potential are important for the movement of water through the xylem. The active transport of solutes is important for the movement of nutrients through the phloem. Bulk flow is the overall movement of water and solutes from the source (site of production) to the sink (site of utilization or storage). However, root parenchyma does not play a significant role in the long-distance transport process. Root parenchyma is primarily involved in storage and radial transport of nutrients from the soil to the root.

Question 13

What is the role of proton pumps in root hair cells?

Accepted Answer

Proton pumps in root hair cells maintain the H+ gradient, which is essential for the active uptake of water and minerals from the soil. This process is critical for plant growth and development.

Question 14

The following factors may sometimes play a role in the movement of sap through xylem. Which one depends on the direct expenditure of ATP by the plant?

Accepted Answer

The movement of ions in the symplast (the continuum of cytoplasm connected by plasmodesmata between cells) often requires active transport, which directly uses ATP. This can create a concentration gradient that drives the osmotic movement of water into the xylem, facilitating sap movement. The other options (A, B, C, E) involve physical processes that do not directly require the expenditure of ATP by the plant.

Question 15

Water flows into the source end of a sieve tube because

Accepted Answer

Sucrose is actively transported into the sieve tube elements from the photosynthesizing cells, which increases the osmotic pressure inside the sieve tube, making it hypertonic relative to the surrounding cells. This osmotic gradient causes water to flow into the sieve tube from adjacent cells and xylem, facilitating the movement of the sucrose solution through the phloem to various parts of the plant.

Question 16

One would expect to find the highest density of aquaporins in which of the following?

Accepted Answer

Aquaporins are water channel proteins that facilitate the transport of water across cell membranes. Guard cells control the opening and closing of stomata in plants, which requires the uptake and release of water. Therefore, one would expect to find the highest density of aquaporins in the plasma membrane of guard cells.

Question 17

The opening of stomata is thought to involve

Accepted Answer

The opening of stomata involves an increase in the solute concentration of the guard cells, which causes water to enter the cells by osmosis, increasing turgor pressure and opening the stomata.

Question 18

Which of the following is a correct statement about sugar movement in phloem?

Accepted Answer

Sugar movement in phloem occurs both upward and downward in the plant. This is known as bidirectional transport.

Question 19

Which of the following essential nutrients plays an essential role in the opening and closing of the stomatal aperture?

Accepted Answer

Potassium (K) plays an essential role in the opening and closing of the stomatal aperture. It regulates the osmotic pressure of guard cells, which are responsible for opening and closing stomata.

Question 20

Photosynthesis begins to decline when leaves wilt because

Accepted Answer

When leaves wilt, stomata close in order to conserve water. However, this also restricts the entry of carbon dioxide into the leaf, which is required for photosynthesis. As a result, photosynthesis begins to decline.

Quiz 29: Resource Acquisition, Nutrition, and Transport in Vascular Plants