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B6 Transport in plants - Coggle Diagram
B6 Transport in plants
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Absorption of water
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This happens because soil water has a higher water potential than the cytoplasm of the root hair cell
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This large surface area is important as it increases the rate of the absorption of water by osmosis and mineral ions by active transport
Transport of water
Osmosis causes water to pass into the root hair cells, through the root cortex and into the xylem vessels.
Once the water gets into the xylem, it is carried up to the leaves where it enters mesophyll cells
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The pathway can be investigated by placing a plant (like celery) into a beaker of water that has had a stain added to it (food colouring will work well)
After a few hours, you can see the leaves of the celery turning the same colour as the dyed water, proving that water is being taken up by the celery
If a cross-section of the celery is cut, only certain areas of the stalk is stained the colour of the water, showing that the water is being carried in specific vessels through the stem – these are the xylem vessels
Transpiration
Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration
Transpiration is defined as the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
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Movement in xylem only takes place in one direction – from roots to leaves (unlike phloem where movement takes place in different directions)
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Transpiration stream
Water molecules are attracted to each other by cohesion – creating a continuous column of water up the plant
Water moves through the xylem vessels in a continuous transpiration stream from roots to leaves via the stem
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As water molecules are held together by cohesive forces (each individual molecule ‘pulls’ on the one below it), so water is pulled up through the plant
If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker
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Transport of food
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These are transported around the plant in the phloem tubes which are made of living cells (as opposed to xylem vessels which are made of dead cells)
The cells are joined end to end and contain holes in the end cell walls (called sieve plates) which allow easy flow of substances from one cell to the next
The transport of sucrose and amino acids in phloem, from regions of production to regions of storage or use, is called translocation
Transport in the phloem goes in many different directions depending on the stage of development of the plant or the time of year; however dissolved food is always transported from source (where it’s made) to sink (where it’s stored or used):
During winter, when many plants have no leaves, the phloem tubes may transport dissolved sucrose and amino acids from the storage organs to other parts of the plant so that respiration can continue
During a growth period (eg during the spring), the storage organs (eg roots) would be the source and the many growing areas of the plant would be the sinks
After the plant has grown (usually during the summer), the leaves are photosynthesizing and producing large quantities of sugars; so they become the source and the roots become the sinks – storing sucrose as starch until it is needed again
Xylem:
- what is moved: water and mineral ions
- process: transpiration stream
- direction of flow: one way from roots to leaves
- cells: dead
Phloem:
- what is moved: sucrose and amino acids
- process: translocation
- direction of flow: in all directions
- cells: alive