Translocation
- Created by: Eleanor Izzard
- Created on: 02-01-13 14:33
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- Translocation
- Entering the Phloem
- sucrose is actively loaded into the phloem
- ATP is used by the companion cells to pump H+ out of their cytoplasm and into the surrounding tissue
- setting up a diffusion gradient, so the hydrogen ions diffuse back into the companion cells
- through co-transporter proteins (allows movement of 1 molecule linked to the movement of another in the same direction by active transport)
- hydrogen brings sucrose molecules into the companion cells
- as the concentration of sucrose builds up, they diffuse into the sieve tube elements, through plasmo-desmata
- hydrogen brings sucrose molecules into the companion cells
- through co-transporter proteins (allows movement of 1 molecule linked to the movement of another in the same direction by active transport)
- setting up a diffusion gradient, so the hydrogen ions diffuse back into the companion cells
- Along the Phloem
- At the Sink
- sucrose may be converted into starch for storage, or used in a metabolic process
- this reduces the sucrose concentration in the cells that require sucrose
- sucrose molecules diffuse or ATP is used for them to go from the sieve tube element to the cells
- this increases the water potential of the sieve tube element, so water molecules move into the surrounding cells by osmosis
- this reduces the hydrostatic pressure in the phloem at the sink
- this increases the water potential of the sieve tube element, so water molecules move into the surrounding cells by osmosis
- sucrose molecules diffuse or ATP is used for them to go from the sieve tube element to the cells
- this reduces the sucrose concentration in the cells that require sucrose
- sucrose may be converted into starch for storage, or used in a metabolic process
- Along the Phloem
- water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving the phloem at the sink, produces a flow of water along the phloem
- this carries sucrose and other assimilates along the phloem
- MASS FLOW
- up or down the plant, depending on where sugars are needed, sometimes in the same phloem tube at different times
- this carries sucrose and other assimilates along the phloem
- water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving the phloem at the sink, produces a flow of water along the phloem
- At the Source
- sucrose entering the sieve tube element reduces the water potential, so water moves into the sieve tube elements via osmosis from surrounding tissues
- this increases the hydrostatic pressure in the sieve tube at the source
- pressure created by fluids pushing against the sides of a container
- this increases the hydrostatic pressure in the sieve tube at the source
- sucrose entering the sieve tube element reduces the water potential, so water moves into the sieve tube elements via osmosis from surrounding tissues
- At the Sink
- the transport of assimilates (sugars and other chemicals made by plant cells), in the phloem tissue
- transported in the phloem in the form of sucrose
- part of the plant that releases sucrose into the phloem is called a source
- .E.G. Leaves
- sugars made during photo-synthesis are converted to sucrose and loaded into the phloem
- .E.G. Leaves
- part of the plant that removes sucrose from the phloem is called a sink
- .E.G. Areas of growth, roots
- Evidence
- Needs ATP
- companion cells have many mitochondria
- translocation can be stopped when a metabolic poison that stops ATP formation is introduced
- rate of flow of sugars in the phloem is o high that energy must be needed (up to 10000 quicker than diffusion)
- Mechanism
- pH of companion cells is higher than that in surrounding cells
- concentration of sucrose is higher in source than sink
- Phloem
- if plant is given radioactive CO2, it soon appears in the phloem
- ringing a tree to remove the phloem results in sugars collecting above the ring
- an aphid on a plant stem can be used to show that the mouthparts are taking food from the phloem
- Negative Evidence
- not all solutes in phloem sap move at same rate
- role of sieve plates is unclear
- sucrose is moved to all parts of the plant at the same rate, rather than going more quickly to parts with a low concentration
- Needs ATP
- Entering the Phloem
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