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Ch. 36 - Resource Acquisition and Transport in Vascular Plants
Campbell - Campbell Biology 11th Edition
Urry11th EditionCampbell BiologyISBN: 9789357423311Not the one you use?Change textbook
All textbooksUrry 11th EditionCh. 36 - Resource Acquisition and Transport in Vascular PlantsProblem 3
Chapter 36, Problem 3

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
Understand the concept of phloem sap movement: Phloem sap is transported from a source (where it is produced, like leaves) to a sink (where it is used or stored, like roots or fruits). This process is known as translocation.
Identify the mechanism of phloem transport: Phloem transport is primarily driven by pressure flow, also known as the mass flow hypothesis. This involves the active transport of sugars into the phloem, creating a high osmotic pressure that drives the flow of sap.
Consider the role of proton pumps: Proton pumps are involved in creating a proton gradient across the cell membrane, which is crucial for the active transport of sucrose into the phloem. This active transport is essential for generating the pressure gradient needed for sap movement.
Evaluate the options given: Analyze each option to determine which one correctly describes the mechanism of phloem sap movement. Consider the role of proton pumps and pressure potential in the process.
Conclude with the correct understanding: Based on the pressure flow mechanism, the movement of phloem sap depends on the activity of proton pumps to create the necessary pressure gradient for sap flow from source to sink.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Phloem Transport

Phloem transport involves the movement of organic nutrients, primarily sugars, from a source (where they are produced, like leaves) to a sink (where they are used or stored, like roots or fruits). This process is essential for distributing energy throughout the plant and is driven by pressure differences between source and sink.
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Active Transport

Proton Pumps

Proton pumps are integral membrane proteins that actively transport protons (H+) across cell membranes, creating a proton gradient. This gradient is crucial for various cellular processes, including the active transport of nutrients in plants, as it provides the energy needed for the movement of substances against their concentration gradient.
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Pressure Flow Hypothesis

The pressure flow hypothesis explains the mechanism of phloem transport, suggesting that the movement of sap is driven by a pressure gradient created by osmotic pressure. Sugars are actively loaded into the phloem at the source, increasing osmotic pressure and drawing water in, which generates a flow towards the sink where sugars are unloaded.
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Related Practice
Textbook Question

Which of the following is an adaptation that enhances the uptake of water and minerals by roots?

a. Mycorrhizae

b. Pumping through plasmodesmata

c. Active uptake by vessel elements

d. Rhythmic contractions by cells in the root cortex

Textbook Question

Which structure or compartment is part of the symplast?

a. The interior of a vessel element

b. The interior of a sieve tube

c. The cell wall of a mesophyll cell

d. An extracellular air space

Textbook Question

Photosynthesis ceases when leaves wilt, mainly because

a. The chlorophyll in wilting leaves is degraded.

b. Accumulation of CO2 in the leaf inhibits enzymes.

c. Stomata close, preventing CO2 from entering the leaf.

d. Photolysis, the water-splitting step of photosynthesis, cannot occur when there is a water deficiency.

Textbook Question

What would enhance water uptake by a plant cell?

a. Decreasing the Ψ of the surrounding solution

b. Positive pressure on the surrounding solution

c. The loss of solutes from the cell

d. Increasing the Ψ of the cytoplasm

Textbook Question

A plant cell with a ΨS of −0.65MPa maintains a constant volume when bathed in a solution that has a ΨS of −0.30MPa and is in an open container. The cell has a

a. ΨP of +0.65MPa

b. Ψ of −0.65MPa

c. ΨP of +0.35MPa

d. ΨP of 0 MPa

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