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Describe the cohesion-tension theory of water transport in the xylem. [5 marks]
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Use your knowledge of leaf structure to explain why less water is lost through the upper surface of leaves than is lost through the lower surface.
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Chapter 7 - Plant MT
A Level Biology Chapter 7 - Transpiration, Translocation
| Question | Answer |
|---|---|
| Describe the cohesion-tension theory of water transport in the xylem. [5 marks] | Water lost from leaf by evaporation of water Lowers water potential of mesophyll Water pulled up xylem creating tension Water molecules cohere ‘stick’ together by hydrogen bonds forming continuous water column Ahesion of water to walls of xylem |
| Use your knowledge of leaf structure to explain why less water is lost through the upper surface of leaves than is lost through the lower surface. | Thick waxy cuticle prevents evaporation from top of leaf. |
| Explain how the following is related to the function of xylem tissue: Xylem tissue contains hollow tubes. | Maintains transpiration stream |
| Explain how the following is related to the function of xylem tissue: Lignin is present in xylem walls | Provides support and strength and provides adhesion and doesn't absorb anything |
| The student cut a shoot and put it into the potometer under water. Explain why. | To not break the transpiration stream |
| The student wanted to calculate the rate of water uptake by the shoot in cm3 per minute. What measurements did she need to make? | Distance and Time |
| The student assumed that water uptake was equivalent to the rate of transpiration. Give two reasons why this might not be a valid assumption? | Water used in photosynthesis Water used to provide turgidity |
| The student measured the rate of water uptake three times. Suggest how the reservoir allows repeat measurements to be made. | Returns bubble to the start by opening the tap |
| Why would someone do repeats of this experiment? | Increases reliability of results and allows to see and eliminate anomalies. |
| Give two environmental conditions that the student should have kept constant during an investigation into transpiration. | Humidity Temperature Light Wind |
| Describe and explain the effects of increasing light intensity on the tension in the xylem vessels in the leaves. [5 marks] | light intensity is increased tension in the xylem increases stomata open more; increased transpiration; the water potential of leaf cells decreases; more water moves from xylem to surrounding cells; down a water potential gradient; H bonds, cohesion; |
| Student removed piece of epidermis to measure amount of stomatas. Why is it important piece of epidermis was thin? | Single layer of cells So light can pass through and cells absorb light |
| Explain how xylem tissue is adapted for its function. | long tubes with no end walls; continuous water columns; no organelles; to obstruct flow / allows easier water flow; lignin; support and withstand tension / waterproof / keeps water in cells; pits in walls; allow lateral movement; |
| Explain how sucrose gets into the phloem? | Sucrose actively transported into sieve tube by companion cells lowers WP Water moves in by osmosis Increases HP sucrose moves down sieve tube to sink by mass flow At sink sucrose exits by AT (for respiration) Osmosis = water out reduces HP at sink |
| A stem was cut from a transpiring plant. The cut end of the stem was put into a solution of picric acid, which kills plant cells. The transpiration stream continued. Suggest an explanation for this observation. | water moves in dead cells / xylem is non-living tissue; the process is passive / no energy is needed; |
| Explain humidity's effect on transpiration? | Higher humidity = decreased rate of transpiration High humidity = more water so higher water potential reduced diffusion gradient so less evaporation of water |
| Describe how water is moved through a plant according to the cohesion-tension hypothesis | Water evaporates from leaves causes water potential in cell to decrease water is drawn out of xylem this causes tension water molecules cohere together and adhere to xylem walls water is pulled up in a column. |
| Some xerophytic plants have sunken stomata. Explain the advantage of this adaptation. | Reduced water loss because more humid in sunken places. Increased water potential here so lower water potential gradient. |
| Explain why diameter of trunk is smallest when highest rate of transpiration? | adhesion / attraction of water molecules to walls of xylem; results in tension as water pulled up stem; pulling in walls; |
| explain why the diameter of the trunk is smallest at midday. [6 marks] | At midday highest light intensity and temperature Stomata open in light= more water loss; Water evaporates more when warm cohesion between water molecules; Adhesion between walls of xylem vessels; Xylem pulled in by tension; |
| Describe and explain three ways in which the leaves of xerophytic plants may be adapted to reduce water loss. | Small leaves so small SA for less water loss Stomata sunken in leaves to lower the water potential gradient so less water loss. Thick waxy cuticle to prevent evaporation of water. |
Created by:
ksatti12