Photosynthesis and Sucrose Transport

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Questions and Answers

What is the role of H+ ions in the loading of sucrose into companion cells?

H+ ions actively transport sucrose out of the companion cells.

H+ ions are simply a byproduct of sucrose metabolism.

H+ ions create a gradient that allows sucrose to be co-transported with them. (correct)

H+ ions facilitate the osmosis of sucrose into the companion cell.

What effect does the accumulation of sucrose in the sieve tube have on water potential?

It has no effect on water potential, allowing fluid equilibrium.

It lowers water potential, leading to water entering by osmosis. (correct)

It increases water potential, causing water to move out of the sieve tube.

It converts sucrose into glucose, which then raises water potential.

What drives the mass flow of sucrose and water in the sieve tube from source to sink?

Active transport mechanisms moving water and sucrose independently.

Gravity causing sedimentation of sucrose.

Pressure differences created by water entry. (correct)

Diffusion of sucrose and water into surrounding tissues.

How is sucrose typically unloaded at the sink in plants?

By passive diffusion through membranes. Signup and view all the answers

Which statement best describes translocation in plants?

Translocation can occur in both directions, unlike transpiration. Signup and view all the answers

What is the primary form of sugar produced by photosynthesis that is then converted for transport?

Glucose Signup and view all the answers

Which process actively transports hydrogen ions out of the companion cells?

Active Transport Signup and view all the answers

How does sucrose loading affect the water potential in the companion cell?

Decreases water potential Signup and view all the answers

What structure allows the movement of sucrose and water from the companion cell to the sieve tube?

Plasmodesmata Signup and view all the answers

What drives the mass flow of nutrients in the phloem after sucrose loading?

Pressure gradient Signup and view all the answers

Which of the following statements about translocation and transpiration is correct?

Translocation is driven by pressure differences Signup and view all the answers

Where does the initial loading of sucrose take place?

Companion cells Signup and view all the answers

What happens to water after sucrose is loaded into the sieve tube element?

It flows into the sieve tube due to low water potential. Signup and view all the answers

Study Notes

Photosynthesis and Sucrose Transport

Photosynthesis produces glucose in leaves (source)
Glucose is converted to sucrose for efficient transport
Sucrose is a soluble sugar, ideal for translocation

Active Transport of Hydrogen Ions (H+)

H+ ions are actively transported out of companion cells
This creates a concentration gradient

Co-transport of Sucrose

H+ ions diffuse back into companion cells via co-transporter proteins, carrying sucrose with them
This happens at the companion cell's plasma membrane
Sucrose is initially loaded into companion cells, NOT directly into sieve tubes

Movement to Sieve Tube Elements

Sucrose moves from companion cells to sieve tube elements through plasmodesmata
Plasmodesmata are channels connecting the two cell types

Water Potential and Osmosis

Sucrose entering the sieve tube lowers water potential
Water moves into the sieve tube element by osmosis
This increased water influx raises the hydrostatic pressure inside the sieve tube

Mass Flow in Phloem

Pressure differences drive mass flow of sucrose and water in the sieve tubes from source (leaves) to sink (roots, fruits, etc.)
Mass flow happens within the sieve tubes

Translocation vs. Transpiration

Translocation is an active process fueled by the active transport of hydrogen ions
Transpiration is a largely passive process driven by water potential differences

Unloading at the Sink

Sucrose is unloaded at the sink (e.g., root, fruit) often by facilitated diffusion
Water follows by osmosis
Translocation can be bidirectional (unlike transpiration, which is unidirectional)

Key Points

Active transport of H+ is essential for loading sucrose into companion cells
Sucrose loading occurs in companion cells, then moves into sieve tube elements via plasmodesmata.
Water movement and pressure gradients drive the mass flow of sucrose in the phloem.

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Description

Explore the fascinating process of photosynthesis and how glucose is converted into sucrose for efficient transport within plants. This quiz covers active transport mechanisms, co-transport of sucrose, and the role of plasmodesmata in moving sucrose to sieve tube elements, as well as the importance of water potential and osmosis in this process.