8.3Transport in Plants
Transpiration — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions
5 min read 18 parts Lesson 6 of 9 1 quiz
Lesson summary
8.3 Transpiration — IGCSE BIOLOGY notes: Define transpiration, explain the transpiration stream and the effect of temperature, wind and humidity, and describe wilting.Transpiration = the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophy…
- Water supply
- Photosynthesis
- Evaporation site
- Surface area
- Direction — Movement in the xylem is in one direction only, from roots to leaves
- Minerals — Transports mineral ions
Section 1 of 18Transpiration
Transpiration
Transpiration 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.
Water supply
- Water travels up the xylem from the roots to replace the water lost.
Direction
- Movement in the xylem is in one direction only, from roots to leaves.
Section 2 of 18Functions of transpiration
Functions of transpiration
Minerals
- Transports mineral ions.
Support
- Provides water to keep cells turgid.
Photosynthesis
- Provides water to leaf cells.
Cooling
- Heat energy is used to change liquid water into vapour, which cools the plant.
Section 3 of 18Water vapour loss (Supplement)
Evaporation site
- Water evaporates from the surfaces of the spongy mesophyll cells.
Surface area
- The interconnecting air spaces and the stomata create a large surface area.
Result
- Evaporation is rapid when the stomata are open.
Section 4 of 18Transpiration stream (Supplement)
Cohesion
- Water molecules attract each other and form a continuous column up the plant.
Pull
- Water loss from the leaves produces tension on the water in the xylem.
Rate
- If transpiration increases, water is pulled up the xylem faster.
Section 5 of 18Investigating temperature and wind speed
1
Dry the leaves and cut the shoot under water.
2
Place the shoot in the tube and make the apparatus airtight with Vaseline.
3
Let one air bubble form in the capillary tube.
4
Set up the environmental factor and let the plant acclimatise for 5 minutes.
5
Record the starting position of the bubble and leave for a set time.
6
Record the end position; the further it moves, the faster the water uptake.
7
Change only temperature or wind speed, reset the bubble and repeat.
Section 6 of 18Effects of environmental factors (Supplement)
| Factor | Condition | Rate | Reason |
|---|---|---|---|
| Wind speed | High | Faster | Air movement removes vapour and keeps a steep gradient out of the stomata |
| Humidity | High | Slower | Smaller concentration gradient between leaf air spaces and the air |
| Temperature | High | Faster | More kinetic energy increases evaporation and diffusion |
Section 7 of 18Wilting (Supplement)
Cause
- More water evaporates from the leaves than the roots can take up from the soil.
Effect
- Cells are no longer full of water, so the cell walls cannot support the plant and it collapses.
Section 8 of 18More from the specification
Phloem
- Tissue carrying sucrose and amino acids.
Vessel
- A long tube formed from dead xylem cells.
Translocation
- Movement of sucrose and amino acids in phloem from sources to sinks.
Sucrose
- The sugar mainly transported in phloem.
Tissue
- A group of cells of similar structure that perform the same function.
Xylem vessel
- A tube of dead lignified cells carrying water upwards.
Section 9 of 18What transpiration is
Transpiration
Transpiration loss of water vapour from the aerial parts of a plant, mainly through the leaf stomata.
Mechanism
- Mechanism water evaporates from the wet surfaces of mesophyll cells into the air spaces, then diffuses through the stomata to the drier air outside.
Guard cells
- Guard cells open and close the stomata.
- Water uptake makes them turgid so they bend apart and open the pore;
- water loss makes them flaccid and the pore closes.
Timing
- Timing stomata are open mostly in the light, letting carbon dioxide in for photosynthesis.
Section 10 of 18Why transpiration is useful
1Transpiration pull
- Transpiration pull draws water and mineral ions up the xylem.
2Cooling
- Cooling evaporation of water takes heat from the leaf.
3Photosynthesis and support
- Photosynthesis and support supplies water to leaf cells and keeps cells turgid.
4Disadvantage
- Disadvantage if water loss exceeds uptake, the plant wilts.
Section 11 of 18Factors affecting the rate of transpiration
| Factor | Effect on rate | Reason |
|---|---|---|
| Light intensity | Increases | Stomata open wider in light |
| Temperature | Increases | Faster evaporation and diffusion, as water molecules have more kinetic energy |
| Humidity | Higher humidity decreases the rate | Smaller concentration gradient of water vapour |
| Wind speed | Increases | Moving air removes humid air near the leaf, keeping the gradient steep |
Section 12 of 18Transpiration stream and cohesion
Cohesion
- Cohesion water molecules are attracted to each other, forming a continuous column up the xylem.
Tension
- Tension transpiration pulls on the column and each molecule pulls on the one below.
Faster transpiration
- Faster transpiration water is pulled up the xylem faster.
Section 13 of 18Measuring transpiration with a potometer
1
Cut a leafy shoot underwater and fit it into the potometer without introducing air bubbles.
2
Dry the leaves, make the apparatus airtight (e.g. with vaseline) and introduce one air bubble into the capillary tube.
3
Allow the shoot to adjust for about 5 minutes.
4
Record the bubble position at the start and after a set time.
5
Repeat under different conditions (fan, lamp, humid bag), changing only one variable, and calculate a mean of repeats.
Section 14 of 18Interpreting potometer results
Volume of water
- Volume of water volume = πr² × distance moved by the bubble.
Worked relation
- Worked relation a tube of radius 0.5 mm with a bubble moving 20 mm gives π × 0.25 × 20 ≈ 15.7 mm3.
Highest rate
- Highest rate in the light, with a fan and in warm dry air.
Lowest rate
- Lowest rate in the dark, in still humid air or with a bag over the shoot.
Petroleum jelly
- Petroleum jelly coating the lower leaf surface reduces water loss most, because most stomata are on this side.
Section 15 of 18Water balance and wilting
Water balance and wilting
Balance
- Balance a plant is in balance when water uptake equals water loss.
Hot, dry, windy conditions
- Hot, dry, windy conditions transpiration may exceed uptake, so guard cells lose turgor and close the stomata, which also slows photosynthesis.
Wilting
- Wilting when more water is lost than taken up, cells lose turgor and the plant collapses.
Section 16 of 18Adaptations of xerophytes
| Adaptation | How it reduces water loss |
|---|---|
| Thick waxy cuticle | Waterproof barrier |
| Rolled leaves | Trap humid air next to the stomata |
| Leaf hairs and sunken stomata | Trap moist air, lowering the concentration gradient |
| Spines or needle-like leaves | Reduce surface area |
| Few stomata | Fewer pores for diffusion |
| Fleshy stems (cacti) | Store water and carry out photosynthesis |
Section 17 of 18Key terms
| Term | Meaning |
|---|---|
| Transpiration | Loss of water vapour from leaves by evaporation and diffusion through stomata. |
| Transpiration stream | Continuous movement of water up the xylem from roots to leaves. |
| Cohesion | Attraction between water molecules. |
| Stomata | Pores in the leaf through which water vapour leaves. |
| Wilting | Collapse of a plant when water loss exceeds water uptake. |
| Potometer | Apparatus measuring the rate of water uptake of a shoot. |
| Term | Meaning |
|---|---|
| Transpiration | Loss of water vapour from leaves through stomata. |
| Stoma | A pore in the epidermis for gas exchange. |
| Guard cell | A cell that controls the opening of a stoma. |
| Potometer | Apparatus for measuring water uptake by a shoot. |
| Wilting | Drooping of a plant as cells lose turgor. |
| Xerophyte | A plant adapted to dry conditions. |
Section 18 of 18Exam technique
1
Saying transpiration is only evaporation — it is evaporation followed by diffusion through the stomata.
2
Saying high humidity speeds transpiration — it slows it.
3
Not controlling other factors in the investigation — change only one factor.
4
Saying a potometer measures transpiration exactly — it measures water uptake as an estimate.
1
Saying transpiration is evaporation only from the leaf surface — it is evaporation inside the leaf followed by diffusion out.
2
Saying high humidity increases transpiration — it reduces the gradient and lowers the rate.
3
Treating a potometer as a direct measure of water loss — it measures uptake.
4
Thinking stomata open in the dark to photosynthesise — they are mostly open in light.
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