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Summary Xylem structure and function A level Biology
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Year 1 Biology AQA A level notes. Achieved an A* in my final exam using these notes. They are based on the mark scheme requirements.
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Xylem structureand function
summary notes
09 May 2023 17:45
- Water absorbed by roots by osmosis through root hair cells
- Transported through hollow, thick walled tubes- xylem vessels- transpiration is the ma
pulls water through
- Cell wall made of spirals of lignin- stronger
Movement of water out through stomata
- Water potential gradient from the air spaces through the stomata to the air- humidity
atmosphere usually less than that of air spaces next to stomata
- Water vapour diffuses out from the air spaces into surrounding air
- Water lost by diffusion from air spaces is replaced by water evaporating from cell wall
surrounding mesophyll cells
- Can control rate of transpiration by changing size of stomatal pores
Movement of water across cells of a leaf
- Water lost from mesophyll cells by evaporation from their cell walls to the air spaces o
- This is replaced by water reaching the mesophyll cells from the xylem by the cell walls
cytoplasm
For the cytoplasmic route:
- Mesophyll cells lose water to air spaces by evaporation due to heat from sun
- These cells have a lower water potential and so water enters by osmosis from neighbo
- Loss of water from these neighbouring cells lowers their water potential
- They then take in water from their neighbours by osmosis
Movement of water up the stem in the xylem - Cohesion tension theory
- Water evaporates from mesophyll cells due to heat from sun- leads to transpiration
- Water molecules form hydrogen bonds between one another- stick together- cohesion
- Water forms a continuous, unbroken column across mesophyll cells and down the xyle
- As water evaporates from mesophyll cells in the leaf into the air spaces beneath the st
more water molecules drawn up behind - due to cohesion
- Hydrogen bonds form between the polar water molecule and the lignin in the cell wall
xylem vessels by adhesion
- Column of water pulled up xylem- transpiration pull
- Transpiration pull puts xylem under tension- this stretches the water column- decrease
of the xylem vessels slightly- decreases pressure that the water column applies to xyle
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