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- State the functions of xylem and phloem.
The xylem transports water taken up from the soil by the roots, to the rest of the plant.
Phloem transports assimilates (substances made by the plant) from the source (i.e. areas where they are produced, like leaves) to a sink (areas where they are used or stored, like the root or the flower).
- Identify the positions of xylem and phloem tissues as seen in transverse sections of unthickened, herbaceous, dicotyledonous roots, stems and leaves.
- Identify root hair cells, as seen under the light microscope, and state their functions.
Root hair cells take up water and mineral ions from the soil.
- Relate the structure and function of root hairs to their surface area and to water and ion uptake.
The root hair in a root hair cells is essentially just a long finger-like projection from the cell. This increases the surface area of the cell. This helps speed up osmosis and the diffusion of mineral ions into the cell.
The cell sap in its vacuole has a higher salt concentration than the soil water, causing water to diffuse in down its water potential gradient. Therefore, this is a passive process. Plants have to expend energy in taking up ions against their concentration gradient, however, making it an active process.
- State the pathway taken by water through the root, stem and leaf (root hair, root cortex cells, xylem, mesophyll cells).
Water first diffuses into the root hair by osmosis, it then diffuses from cell to cell or cell wall to cell wall (or any other combination of cells and cell walls) through the root cortex, where it eventually reaches the xylem. Water is taken up through the xylem until it reaches a leaf, where it diffuses out into the surrounding mesophyll cells. Water diffuses from the mesophyll cells to the surrounding intercellular air spaces as water vapour, and finally, out of the leaf through the stomata.
- Investigate, using a suitable stain, the pathway of water through the above-ground parts of a plant.
Cut the base of a stalk of celery (the non-leafy end) under water. Place the stalk in a beaker of water that has been stained with red food dye, the base end down, and place it in room temperature conditions, under a bright light and a slight breeze. You may observe red lines travelling up the stalk, and then through the leaves. If you cut the stalk halfway, you can also observe where the xylem vessels are placed (the small areas stained red).
- Define transpiration as evaporation of water at the surfaces of the mesophyll cells followed by loss of water vapour from plant leaves, through stomata.
Pretty self-explanatory point.
- Describe how water vapour loss is related to cell surfaces, air spaces and stomata.
Water evaporates from the cell surfaces of the leaf mesophyll cells, into the air spaces between as water vapour and out through the stomata. In order to control water vapour loss, the stomata close. This happens when the humidity of the air outside is too low, conditions are too windy/ hot, etc.
- Describe the effects of variation of temperature, humidity and light intensity on transpiration rate.
Effect of temperature:
The higher the temperature, the greater the transpiration rate, because water vapour molecules will have more kinetic energy and hence move out of the leaf faster.
Effect of humidity:
The higher the humidity of the air outside the leaf relative to the air inside the leaf, the lower the transpiration rate. This is because water usually diffuses down a concentration gradient, but if the air outside is already saturated with water, there is more likely to be a net movement of water into the leaf (providing the stomata are open) rather than out.
Effect of light intensity:
The higher the light intensity, the higher the rate of transpiration. This is because the leaves will be photosynthesizing more vigorously, causing the stomata to be left wide open so that CO2 may enter the leaf.
- Explain the mechanism of water uptake and movement in terms of transpiration producing a tension (‘pull’) from above, creating a water potential gradient in the xylem, drawing cohesive water molecules up the plant.
There are some terms here, that I should explain first:
Capillary action is essentially the forces that cause water to travel up a hollow tube on its own (try sticking the end of a straw in a glass of water and watch as the water moves up a few millimetres without any help). Capillary action consists of two forces – cohesive force (the property of water molecules that make them stick to each other) and adhesive force (the property causing water molecules to stick to other things).
I think I may have already explained water potential in an earlier page of notes, but just in case, here it is again: it’s a measure of the ability of water to leave a system. Remember the definition of osmosis? How it’s only osmosis if the case concerns the movement of water across a membrane? Yeah, well, think about that – water potential is the ability of water to move across that membrane (or to leave the system it is already in). You’ll learn about this in detail in As level, but for now, simply think of it as water concentration – that helped me while I was doing my iGCSE’s.
Water moves up the xylem in much the same way as water moves all the way up a straw when you suck on the end – as water vapour evaporates from the mesophyll cell surface and leaves the leaf, a sucking force is created. This means that there is less water pressure at the top than the bottom, creating a hydrostatic pressure gradient, and hence, a water potential gradient. This draws water up the xylem.
This only works because of the cohesive and adhesive forces present:
Without cohesive forces, a gap in the column of water travelling up the xylem may be created, and as the sucking force (AKA the ‘pull’) created by transpiration can’t pass through that gap, the rest of the water can’t be drawn up.
Without the adhesive forces, the water won’t stick properly to the wall of the xylem, again creating gaps and making it difficult to draw water up the length of the xylem.
- Define translocation in terms of the movement of sucrose and amino acids in phloem; from regions of production to regions of storage or to regions of utilisation in respiration or growth.
This happens to be another self-explanatory point!
Notes submitted by Sarah.
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