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- State the functions of xylem and phloem
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 position of xylem as seen in sections of roots, stems and leaves, limited to non-woody dicotyledonous plants
- Identify root hair cells, as seen under the light microscope, and state their functions
The cells with finger-like projections coming out of them are root hair cells. The red arrow points to a root hair cell.
- Explain that the large surface area of root hairs increases the rate of the absorption of water and ions
The root hair on root hair cells (the finger-like projection) increases the surface area of the root hair cell. Specifically, it increases the surface area of the root hair cell exposed to the soil and the soil contents. This means there is more surface area available to absorb water and mineral ions from the soil, so absorption can occur at a higher rate.
- State the pathway taken by water through the root, stem and leaf as root hair, root cortex cells, xylem and 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) underwater.
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 see where the xylem are placed (the small areas stained red).
- State that water is transported from the roots to leaves through the xylem vessels
Water is transported from the roots to leave through xylem vessels.
- Define transpiration as loss of water vapour from plant leaves by evaporation of water at the surfaces of mesophyll cells followed by diffusion of water vapour through the stomata
All you need to do is memorise that definition!
- Explain the mechanism by which water moves upwards in the xylem in terms of a transpiration pull, helping to create a water potential gradient that draws up a column of water molecules, held together by cohesion.
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).
Water potential is 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.
- Investigate and describe the effects of variation of temperature and humidity on transpiration rate
To measure the transpiration rate, you’ll need a potometer. A potometer consists of:
- A container of water
- A capillary tube
- A scale
- Rubber tubing
The rubber tubing connects the capillary tube and the plant
The scale is used to measure how far the bubble travels in the capillary tube – due to transpiration the transpiration pull created by the plant, as the water column is sucked up, the bubble moves up.
There are two measures of transpiration rate you can calculate using this method:
- Distance travelled by bubble per unit time (your unit will be m/s or m/min or cm/min, depending on the units of distance and time that you use.)
- The volume of water transpired per minute (your unit will be ml/min or l/min (litres/minute)). This is a more accurate measure of the transpiration rate than the first one.
To calculate i, simply measure the distance travelled by the bubble in a known length of time. Then divide the distance travelled by the time taken.
To calculate ii, measure the distance travelled by the bubble in a known length of time. Measure the diameter of the capillary tube, and then divide it in half to calculate the radius. Calculate the cross-sectional area of the capillary tube using , where is the radius. Multiply the cross-sectional area into the distance travelled by the bubble to get the volume transpired. Divide volume transpired by the time taken, to get the rate of transpiration. Note: some potometers come with a volume scale, so you don’t need to go through all of the described steps to calculate the volume of water transpired.
To investigate the effect of varying temperature on transpiration, conduct the experiment under different temperature conditions (cold room and warm room, or next to a heater and far away from the heater).
To investigate the effect of varying humidity on transpiration, conduct the experiment in:
- A room with a dehumidifier
- A normal room
- Spray water in a plastic bag and wrap the bag around the plant.
Effect of temperature on transpiration rate:
The higher the temperature, the greater the transpiration rate, until the plant is transpiring at its maximum rate. Then the transpiration rate remains at its maximum. Hypothetically, if you increased the temperature high enough that you damaged the plant, the transpiration rate would fall back down to zero.
Effect of humidity on transpiration rate:
The higher the humidity of the air outside the leaf relative to the air inside the leaf, the lower the transpiration rate.
- Explain the effects of variation of temperature and humidity on transpiration rate
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.
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. However, 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.
- Define translocation in terms of the movement of sucrose and amino acids in phloem:
- From regions of production (source)
- To regions of storage OR to regions where they are used in respiration or growth (sink)
Translocation is the movement of sucrose and amino acids in phloem from a source (regions of production) to a sink (region of storage or usage, where they may be used in respiration or growth).
Notes submitted by Sarah
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