Edexcel Biology Gcse - Plant Organisation Part 4
Low humidity increases the transpiration rate because the air outside the leaf has fewer water molecules. This creates a steeper concentration gradient, causing water to diffuse out of the leaf more quickly.
Explain how the factor humidity increases the transpiration rate
Reduces the concentration of water molecules outside the leaf; diffusion of water from the leaf increases
Key Terms
Explain how the factor humidity increases the transpiration rate
Reduces the concentration of water molecules outside the leaf; diffusion of water from the leaf increases
Explain how the factor air movement increases the transpiration rate
Removes water vapour from leaf surfaces; more water diffuses from the leaf because a high concentration gradient is maintained
Explain how the factor light intensity increases the transpiration rate
Increases the rate of photosynthesis; stomata open so that water diffuses out of the leaf
What conditions will decrease the rate of transpiration?
Decrease in temperature, increase in humidity, no or reduced air movement and low light intensity.
What is a simple method for investigating water loss from plant leaves?
A simple method for investigating water loss from plant leaves is to measure their change in mass over a period of time.
Various factors that affect water loss from the leaf can be investigated using this method, for instance:
air movement - direct a fan on the leaves
temperature
obstructing the stomata, eg. with petroleum jelly
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| Term | Definition |
|---|---|
Explain how the factor humidity increases the transpiration rate | Reduces the concentration of water molecules outside the leaf; diffusion of water from the leaf increases |
Explain how the factor air movement increases the transpiration rate | Removes water vapour from leaf surfaces; more water diffuses from the leaf because a high concentration gradient is maintained |
Explain how the factor light intensity increases the transpiration rate | Increases the rate of photosynthesis; stomata open so that water diffuses out of the leaf |
What conditions will decrease the rate of transpiration? | Decrease in temperature, increase in humidity, no or reduced air movement and low light intensity. |
What is a simple method for investigating water loss from plant leaves? | A simple method for investigating water loss from plant leaves is to measure their change in mass over a period of time. |
Various factors that affect water loss from the leaf can be investigated using this method, for instance: | air movement - direct a fan on the leaves temperature obstructing the stomata, eg. with petroleum jelly |
A simple method for investigating water loss from plant leaves is to measure their change in mass over a period of time. Explain this method step by step. | 1) Remove a number of leaves from a bush or tree. 2) Find the mass of each leaf. 3) Suspend each leaf from a piece of wire or string. 4) After a set period of time, re-measure the mass. |
Water loss through the stomata | Water is lost through open stomata. Scientists sometimes count all the stomata on a leaf surface, but usually, there are too many to count. In these instances, they take a sample. This must be a representative sample - it must give a true picture of the numbers of stomata on the leaf. |
To be representative of the whole leaf, the representative sample must: | include a sufficient number of counts - not just one or two - of stomata over different parts of the slide must be random, and not select areas where there are many or few stomata |
Water loss through the stomata sample counting stomata | A number of random counts of stomata should be made with a microscope. Count the number of stomata in the field of view. Then move the slide slightly and count the number of stomata in a different field of view. Make at least five random counts, then calculate a mean. Using this method, and a calibrated eyepiece graticule, you could estimate the number of stomata per unit area (mm2). |
Investigating transpiration How to measure the uptake of water | The uptake of water can be measured using a potometer. Under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration. A simple potometer is a piece of capillary tubing to which a plant has been connected. The water uptake is measured by recording the time taken for a bubble in the tube to move a set distance. |
Measuring water uptake - Potometers Investigating transpiration step by step | 1) The potometer is filed with water 2) A shoot is cut from a plant 3) The end of the shoot is cut under water to ensure the xylem remains water-filled and prevents air locks. 4) The shoot is inserted into the rubber tubing at the end of the potometer 5) The potometer is raised so that a bubble of air is taken up. 6) The potometer is lowered into the water. The distance travelled by the air bubble is recorded over a period of time. |
Investigating factors that affect the rate of transpiration | Insufficient water affects the yields of crops. Different factors affect transpiration and therefore water uptake. They can be investigated using a potometer. |
Laboratory experiments are designed to simulate conditions a plant may encounter such as: | air movement coatings on leaves that block stomata high light intensity high temperature |
Laboratory experiments are designed to simulate conditions a plant may encounter such as air movement. Explain when this is encountered by the plant and how the condition is produced in the laboratory. | When encountered by plant: Windy conditions How the condition is produced in the laboratory: Fan |
Laboratory experiments are designed to simulate conditions a plant may encounter such as coatings on leaves that block stomata. Explain when this is encountered by the plant and how the condition is produced in the laboratory. | When encountered by plant: Airborne pollution; horticultural use on cuttings and Christmas trees How the condition is produced in the laboratory: Smear leaf surface(s) with petroleum jelly |
Laboratory experiments are designed to simulate conditions a plant may encounter such as High light intensity. Explain when this is encountered by the plant and how the condition is produced in the laboratory. | When encountered by plant: Sunny climates; artificial lighting in greenhouses How the condition is produced in the laboratory: Artificial lighting |
Laboratory experiments are designed to simulate conditions a plant may encounter such as High temperature. Explain when this is encountered by the plant and how the condition is produced in the laboratory. | When encountered by plant: Warm and hot climates How the condition is produced in the laboratory: Heater or greenhouse |
When investigating the effect of light intensity on water uptake, what are the control variables? | Temperature, no air movement or draughts. It is also important to use the same species of plant, and shoots with a similar leaf surface if different plant shoots are used. |
Under which conditions is the rate of transpiration greatest? | At high temperatures. |
Scientists use sampling and counting techniques to investigate the distribution of stomata on leaves. They count stomata to investigate: | their numbers, density and distribution on upper and lower surfaces numbers that are open and closed at any time adaptations of plants to environmental conditions, eg desert and water plants effects of changing conditions such as increased carbon dioxide concentrations from climate change |
Give 2 methods (step by step) by which stomata can be counted. | Method 1 1) Put a small drop of water on a microscope slide. 2) Hold the leaf with the surface you want to examine uppermost. 3) Tear the leaf at an angle so as to reveal part of the epidermis. 4) Place the leaf on the microscope slide and examine. Method 2 1) Paint the surface of the leaf with clear nail varnish. 2) Allow to dry. 3) Peel off the nail varnish with forceps. 4) Place on a dry microscope slide and examine |
Recording the distribution | The density of stomata on a leaf is recorded per unit area, usually the number per sq mm. A microscope is calibrated so that its field of view is known. |
Adaptations of plants to extreme environments | Cacti are well adapted for survival in the desert. They have: | Stems that can store water. Widespread or very deep root systems that can collect water from a large area or from very deep underground. Spines which are modified leaves. These minimise the surface area and so reduce water loss. The spines also protect the cacti from animals that might eat them. Very thick, waxy cuticle to reduce water loss by evaporation. Reduced number of stomata to reduce water loss by transpiration. |
Which tissue in the leaf absorbs the most light for photosynthesis? | Palisade mesophyll |
Which tissue in the leaf is adapted for efficient gas exchange? | Spongy mesophyll |
Which tissue transports water and minerals from the root to the leaves? | Xylem |
Which tissue transports sugars from the leaves to other parts of the plant including roots for storage? | Phloem |
Translocation is the movement of which substance? | Sucrose |
Which cells control the stomata? | A pair of guard cells surround each stoma and when they are turgid, the stoma will be open. |
What sort of process is translocation? | active process |
What is the process which causes guard cells to become turgid? | Osmosis |
Which of these describes the stomata of a desert plant? | Reduced in number and mainly closed |