3. Transpiration Flashcards
Definition of Transpiration
The loss of water vapour (by evaporation & diffusion) from the surface of leaves & stems of a plant
Where does the water vapour leave from
Plants are constantly taking water in at their roots & losing water via the stomata (in leaves)
- Around 99% of water absorbed by a plant is lost through transpiration
Transpiration is an inevitable consequence of…
Gas exchange
1. A plant needs to open its stomata to let in CO2 so that it can produce glucose (by photosynthesis)
2. But this also lets water out - theres a higher concentration of water inside the leaf than in the air outside, so water moves out of the leaf down the water concentration gradient when the stomata open
3. So transpirations really a side effect of gas exchange needed for photosynthesis
Adaptations of leaves to control rate of transpiration
- Waxy cuticle (waterproof layer)
- Guard cells (open/close stomata)
- Very few stomata on upper surface of leaf
What is the transpiration stream
The flow of water (in continuous columns), up the xylem vessels from roots to leaves
What are many of the properties of water due to
Its ability to form hydrogen bonds
How do water molecules join together
- The slight negative charge on the oxygen atom makes it attract the slightly positive hydrogen atom of another water molecule
4 main factors that affect transpiration rate
- Light
- Temperature
- Humidity
- Wind
Temperature, humidity & wind all alter the water potential gradient, but light is slightly different
4 main factors that affect transpiration rate: Light
- The lighter it is the faster the transpiration rate. This is bc the stomata open when it’s light, so CO2 can diffuse into the leaf for photosynthesis.
- When its dark, the stomata are usually closed, so there little transpiration
4 main factors that affect transpiration rate: Temperature
- The higher the temperature the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster.
- This increases the water potential gradient between the inside & outside of the leaf, making water diffuse out of the leaf faster
4 main factors that affect transpiration rate: Humidity
- The lower the humidity the faster the transpiration rate. If the air around the plant is dry, the water potential gradient between the leaf & the air is increased, which increases transpiration
4 main factors that affect transpiration rate: Wind
- The windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata.
- This increases the water potential gradient, which increases the rate of transpiration
What is a potometer
- A special piece of apparatus used to estimate transpiration rates.
- It actually measures water uptake by a plant, but its assumed that water uptake by the plant is directly related to water loss by the leaves
- You can use it to estimate how diff factors affect transpiration rate
Method of using a potometer to estimate transpiration rate
- Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the SA available for water uptake
- Assembled the potometer in water & insert the shoot underwater, so no air can enter
- Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water
- Check that the apparatus is watertight & airtight
- Dry the leaves, allow time for the shoot to acclimatise, & then shut the tap
- Remove the end of the capillary tube from the beaker of water until 1 air bubble has formed, then put the end of the tube back into the water
- Record the starting position of the air bubble
- Start a stopwatch & record the distance moved by the bubble per unit time (eg, per minute). The rate of air bubble movement is an estimate of the transpiration rate
- Remember, only change one variable (eg. temp) at a time. All other conditions (eg. light, humidity) must be kept constant
see pg92 for diagram of potometer set up
What are Xerophytes
Plants like cacti & marram grass (which grow on sand dunes).
They’re adapted to live in dry climates. Their adaptations prevent them losing too much water by transpiration
How are Xerophytes adapted to reduce water loss: Marram grass
- Marram grass has stomata that are sunk in pits, so they’re sheltered from the wind. This helps to slow transpiration down
- It has a layer of ‘hairs’ on the epidermis - this traps moist air round the stomata, which reduces the water potential gradient between the leaf & the air, slowing transpiration down
- In hot or windy conditions, marram grass plants roll their leaves - again this traps moist air, slowing down transpiration. It also reduces the exposed SA for losing water & protects the stomata from wind
How are Xerophytes adapted to reduce water loss: Cacti
- Both **marram grass* & cacti have a thick, waxy layer on the epidermis - this reduces water loss by evaporation bc the layer is waterproof
- Cacti have spines instead of leaves - this reduces the SA for water loss
- Cacti close their stomata at the hottest times of the day when transpiration rates are the highest
see pg93 for cross section dia of marram grass leaf
What are Hydrophytes
Plants like water lilies, which live in aquatic habitats.
As they grow in water, they dont need adaptations to reduce water loss (like xerophytes), but they do need adaptations to help them cope w a low oxygen level
How are Hydrophytes adapted to survive in water
- Air spaces in the tissue help the plants to float & can act as a store of oxygen for use in respiration. For eg, water lilies have large air spaces in their leaves. This allows the leaves to float on the surface of the water, increasing the amount of light they receive. Air spaces in the roots & stems allow oxygen to move from the floating leaves down to parts of the plant that are underwater
- Stomata are usually only present on the upper surface of floating leaves. This helps maximise gas exchange
- Hydrophytes often have flexible leaves & stems - these plants are supported by the water around them, so they dont need rigid stems for support. Flexibility helps to prevent damage by water currents
