Transpiration. C should equal D.BUT SOMETIMES. 1. Loss in mass is greater than volume of water added.

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1 Transpiration Transpiration is the loss of water by evaporation from the leaves through the stomata. The source of water for the plants is soil water. It is taken up by root hair cells by osmosis. Once in the root hair cell, water moves by osmosis from cell to cell through the endodermis into the Xylem. The water is then pulled under pressure through the transpiration stream up the Xylem (a long hollow tube) to the leaves. The water moves by osmosis into the palisade cells and will saturate the air spaces in the leaves. Water is lost from air spaces within stomata to outside. Function of water in leaves 1. Photosynthesis 2. To maintain turgor shape. A potometer measures water loss in leaves. There are two types. Weight potometer and bubble potometer. A weight potometer measures the amount of water lost by a plant through transpiration. Experiment: 1. Place plant with roots and leaves into a conical flask. 2. Add water and mark level of water. 3. Add a layer of oil to surface of water to prevent evaporation of water. 4. Weigh - A g 5. Wait for 48 hours 6. Reweigh - B g 7. Loss in mass: A B = C g 8. Note volume of water added to restore water volume to previous level D C should equal D.BUT SOMETIMES. 1. Loss in mass is greater than volume of water added. This could mean that the plant could be dying or losing leaves. 2. Loss in mass is less than volume of water added. This could mean that the plant is growing and photosynthesising.

2 Bubble potometer Measures how fast a plant loses water through transpiration over a short period of time under different environmental conditions. Precautions while setting up! 1. Cut shoot under water, to avoid air bubbles in Xylem. 2. Make a diagonal cut in stem: longer surface area for uptake of water. \/ \/ 3. Seal the join with Vaseline to avoid air bubbles in the system 4. Dry/blot leaves to create a gradient in favour of water evaporation. 5. Reset scale to zero before starting. Factors affecting transpiration Factor Effect Explanation Light Increased Transpiration Stomata are open Dark Decreased Transpiration Stomata are closed High temperature Increased Transpiration Any given volume of air requires more water to saturate at high temperature (i.e. greater evaporation rate) Low temperature Decreased Transpiration Less evaporation High wind Increased Transpiration Water molecules moved away from stomata, which increases gradient in favour of diffusion of water from leaves. Low wind Decreased Transpiration Less gradient formed High humidity (moist air) Low humidity (dry air) Defoliation Availability of water Decreased Transpiration Increased Transpiration Decreased Transpiration Reduced water loss Moisture in air reduces gradient Greater gradient in favour of evaporation Reduced surface area/fewer stomata from which water can evaporate At times of drought, plants can close their stomata to prevent water loss.

3 Experiments to detect water loss from leaves 1. Cobalt chloride paper: sensitive to moisture. Blue (Dry) Pink (Moist) Result: Lower surface goes pink quicker as it has more stomata. Exceptions are glass and plants on water such as lily. 2. To prove that the greater part of transpiration occurs on the lower surface: Hang leaves by their stalks from a line. Ensure the stalks are covered with Vaseline to prevent water loss. A B C D Treatment A No Vaseline applied (to either surface) B Vaseline applied to upper surface only C Vaseline applied to lower surface only Result Very wilted water lost from both surfaces Wilted water lost from lower surface Some wilting water loss from upper surface not as much as in B D Vaseline applied to both surfaces No wilting no water loss leaves fresh Conclusion 1. Vaseline seals water (known from beginning) 2. Some water is lost from the upper surface 3. More water is lost from the lower surface as there are more stomata there

4 Transpiration Stream This is a force. The evaporation of water from the leaves causes a type of suction, which pulls (this is the force!) water up the stream from the roots. The water travels in the xylem vessels of the vascular bundles. Water will move out of the xylem by osmosis to ensure the cell vacuoles are full of water (turgid). In the leaf, the air spaces are saturated with water. This allows cell membranes to be moist, to allow rapid diffusion. There are several factors assisting the water from the root to the leaf: 1. Xylem vessels are very narrow 2. Cohesion water molecules stick together 3. Root pressure forces water up the xylem 4. Transpiration evaporation from leaves Importance of transpiration 1. Supplies water to leaf for photosynthesis 2. Supplies mineral ions in solution to leaf (e.g. magnesium for chlorophyll) 3. Cooling for leaf 4. For turgor shape 5. Moist cell membranes (gases only pass through solution) Adaptation of plants to reduce water loss (normally in arid conditions) 1. Rolled up leaves to reduce surface area for water loss 2. Leaves reduced to spines (e.g. cactus or conifer trees) 3. Sunken stomata to trap water as they are unexposed to wind and sun 4. Hairs around stomata (to trap moisture) 5. Waxy cuticle (e.g. holly) waterproof and reflects light 6. Swollen stems to store water

5 Stomata Stomata hole where gases (and water vapour) can diffuse in and out of Two turgid guard cells normally these are very close to each other Day Chloroplasts absorb light for photosynthesis Cell sap gets stronger (i.e. lower water potential) Night No light no photosynthesis Cell sap gets weaker (i.e. higher water potential) No water enters Guard cells are flaccid Water enters guard cell via osmosis Guard cell swells and becomes turgid This pulls the guard cells apart The stomata are open Stomata are closed