Podcast
Questions and Answers
What is the role of H+ ions in the loading of sucrose into companion cells?
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?
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?
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?
How is sucrose typically unloaded at the sink in plants?
Which statement best describes translocation in plants?
Which statement best describes translocation in plants?
What is the primary form of sugar produced by photosynthesis that is then converted for transport?
What is the primary form of sugar produced by photosynthesis that is then converted for transport?
Which process actively transports hydrogen ions out of the companion cells?
Which process actively transports hydrogen ions out of the companion cells?
How does sucrose loading affect the water potential in the companion cell?
How does sucrose loading affect the water potential in the companion cell?
What structure allows the movement of sucrose and water from the companion cell to the sieve tube?
What structure allows the movement of sucrose and water from the companion cell to the sieve tube?
What drives the mass flow of nutrients in the phloem after sucrose loading?
What drives the mass flow of nutrients in the phloem after sucrose loading?
Which of the following statements about translocation and transpiration is correct?
Which of the following statements about translocation and transpiration is correct?
Where does the initial loading of sucrose take place?
Where does the initial loading of sucrose take place?
What happens to water after sucrose is loaded into the sieve tube element?
What happens to water after sucrose is loaded into the sieve tube element?
Flashcards
Photosynthesis
Photosynthesis
The process where plants convert light energy into chemical energy in the form of glucose.
Sucrose
Sucrose
A soluble sugar produced by plants, primarily for transport within the plant.
Sucrose Loading
Sucrose Loading
The process where sucrose is actively loaded into companion cells, using energy to move it against its concentration gradient.
Water Entry
Water Entry
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Translocation
Translocation
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Source
Source
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Sink
Sink
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Transpiration
Transpiration
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Companion Cell Loading: Step 1 (Active Transport)
Companion Cell Loading: Step 1 (Active Transport)
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Companion Cell Loading: Step 2 (Co-transport)
Companion Cell Loading: Step 2 (Co-transport)
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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.
