AQA GCSE Biology (8461) — Biological Organisation
Every time you step into a dense forest on a hot day, the air feels noticeably cooler and more humid. This is because the trees are constantly releasing invisible moisture into the air.
Transpiration is the loss of water vapour from the leaves and stems of plants. This process is driven by evaporation from the moist surfaces of internal mesophyll cells, followed by diffusion out of the leaf.
Water vapour exits the leaf through tiny pores called stomata (singular: stoma), which are usually found on the underside of the leaf. Each stoma is surrounded by two guard cells. Guard cells have a uniquely uneven structure: a thick inner wall and a thin, flexible outer wall. When the plant has plenty of water, guard cells take in water by osmosis and become turgid (swollen). This causes them to curve outward, opening the stomatal pore. When the plant is short of water, the guard cells become flaccid (limp) and the stomata close to prevent further water loss.
You hang wet clothes outside to dry, knowing they dry fastest on a warm, dry, and windy day. Plants lose water in exactly the same way.
The rate of transpiration is affected by four main environmental factors:
How do you measure an invisible gas leaving a plant? You estimate it by measuring the liquid water going in.
A potometer is a piece of apparatus used to measure the rate of water uptake by a cut plant shoot. We use uptake as an estimate for transpiration, though it is not perfectly identical. A small amount of the uptaken water is actually used by the plant for photosynthesis and to maintain cell turgidity. As water transpires, it pulls more water up the xylem, which draws an air bubble along a capillary tube.
To set up a potometer correctly and avoid breaking the continuous water column:
To calculate the rate from a potometer experiment, you must measure the bubble movement over a set period.
Worked Example 1: Calculating Rate from Distance
Step 1: Identify the known values. An air bubble moves in .
Step 2: Substitute into the formula. .
Step 3: Calculate and state the units. .
Worked Example 2: Calculating Rate from a Graph
Experimental data is usually plotted on a line graph because time and distance are continuous data. A straight line indicates a linear relationship. The rate of transpiration is equal to the gradient of the line.
Step 1: Pick two clear points on the line. For example, at the bubble is at , and at it is at .
Step 2: Find the change in and . Change in . Change in .
Step 3: Divide to find the rate. .
Do not confuse the diffusion of water vapour with osmosis. Osmosis only moves liquid water into the guard cells or plant roots; transpiration itself is evaporation followed by diffusion.
In 6-mark 'Explain' questions about temperature, examiners specifically look for the phrase 'kinetic energy' — do not just write 'it gets hotter so water evaporates faster'.
Remember that a potometer measures water uptake, not exact transpiration. A small fraction of the water is used by the plant for photosynthesis and keeping cells turgid.
When interpreting line graphs of potometer data, the steeper the gradient, the faster the rate of transpiration.
Transpiration
The loss of water vapour from the leaves and stems of plants, mainly by evaporation from mesophyll cells and diffusion through stomata.
Water vapour
Water in its gaseous state that diffuses out of plant leaves.
Evaporation
The process where liquid water turns into a gas, occurring on the moist surfaces of mesophyll cells inside the leaf.
Mesophyll cells
Internal leaf tissue cells from which water evaporates into the leaf's internal air spaces.
Stomata
Tiny pores on the surface of a leaf that open and close to control gas exchange and water loss.
Guard cells
Specialised cells that surround each stoma and control its opening and closing by changing shape.
Turgid
Swollen and firm due to a high water content; causes guard cells to open stomatal pores.
Flaccid
Soft and limp due to a lack of water; causes guard cells to close stomatal pores.
Rate of transpiration
The speed at which water is lost from a plant, often measured in millimetres per minute (mm/min) using a potometer.
Kinetic energy
The energy possessed by particles due to their motion; increases with temperature, causing faster evaporation and diffusion.
Concentration gradient
The difference in the concentration of water vapour molecules between the inside of the leaf and the outside air.
Limiting factor
An environmental variable that restricts the rate of a process when it is in short supply.
Potometer
A piece of apparatus used to measure the rate of water uptake by a plant shoot.
Bubble movement
The physical displacement of an air bubble in a capillary tube, used as a visual indicator of water uptake in a potometer.
Linear relationship
A relationship between two continuous variables that produces a straight line when plotted on a graph.
Gradient
The steepness of a line on a graph, calculated as the change in the y-axis divided by the change in the x-axis.
Put your knowledge into practice — try past paper questions for Biology
Transpiration
The loss of water vapour from the leaves and stems of plants, mainly by evaporation from mesophyll cells and diffusion through stomata.
Water vapour
Water in its gaseous state that diffuses out of plant leaves.
Evaporation
The process where liquid water turns into a gas, occurring on the moist surfaces of mesophyll cells inside the leaf.
Mesophyll cells
Internal leaf tissue cells from which water evaporates into the leaf's internal air spaces.
Stomata
Tiny pores on the surface of a leaf that open and close to control gas exchange and water loss.
Guard cells
Specialised cells that surround each stoma and control its opening and closing by changing shape.
Turgid
Swollen and firm due to a high water content; causes guard cells to open stomatal pores.
Flaccid
Soft and limp due to a lack of water; causes guard cells to close stomatal pores.
Rate of transpiration
The speed at which water is lost from a plant, often measured in millimetres per minute (mm/min) using a potometer.
Kinetic energy
The energy possessed by particles due to their motion; increases with temperature, causing faster evaporation and diffusion.
Concentration gradient
The difference in the concentration of water vapour molecules between the inside of the leaf and the outside air.
Limiting factor
An environmental variable that restricts the rate of a process when it is in short supply.
Potometer
A piece of apparatus used to measure the rate of water uptake by a plant shoot.
Bubble movement
The physical displacement of an air bubble in a capillary tube, used as a visual indicator of water uptake in a potometer.
Linear relationship
A relationship between two continuous variables that produces a straight line when plotted on a graph.
Gradient
The steepness of a line on a graph, calculated as the change in the y-axis divided by the change in the x-axis.
