xylem

structure of xylem vessels

1. long with no organelles or cytoplasm, made up of non - living tissue

2. no end walls - to allow continuous flow of water in columns

3. thick, strong, impermeable, lignified cell wall - prevents root from crushing as it pushes through the soil

4. walls of the vessel contain pits/pores - allows water to pass through sideways to other cells of the plant (lateral flow)

how does lignin protect the structure?

normally in the form of rings, spirals, or unlignified “bordered pits”

function of xylem

carried water and mineral ions from roots up through stem to leaves (transpiration)

structure of root hair cell

1. long thin root hairs- increases SA to speed up water uptake

2. no cuticle - to allow water permeability

3. thin cell walls - reduce resistance to osmosis

4. many mitochondria - for ATP for active transport of ions

water uptake in root hair cells

1. Uptake by root hair cells: Water enters by osmosis down a water potential gradient.

2. Transport to xylem: Water moves from the epidermis into the xylem tissue.

3. Movement in xylem: Water travels through xylem vessels from root → stem → leaves.

4. Transpiration: Water crosses leaf tissues and exits through stomata via transpiration.

5. Loss of water vapor: Water evaporates from mesophyll cell surfaces into air spaces.

6. Diffusion out of the leaf: Water vapor diffuses out of the stomata.

7. Transpiration stream: Continuous movement of water due to cohesion and adhesion in the xylem creates a transpiration pull.

what is transpiration?

evaporation of water from stomata, down a water potential gradient.

factors affecting transpiration:

1. light intensity → higher = faster rate, as more stomata will open larger SA for evaporation

2. temperature → higher = faster rate, adding heat gives H2O Ek, moved faster

3. humidity → more = slower rate, more water vapour in air gives it a higher water potential outside compared to inside leaf, so reduces gradient for water to evaporate out

4. wind → more = faster rate, carries away water vapour in air surrounding leaf, maintains a gradient so more evaporation

What is root pressure?

pressure generated in the roots due to the active transport of mineral ions into the xylem, which lowers the water potential in the xylem.

how does root pressure contribute to water movement in plants?

• Water moves into the xylem by osmosis, increasing hydrostatic pressure.

• This pushes water up the stem to a small extent.

• Evidence includes exudation of sap and guttation (water droplets at leaf edges).

• Root pressure alone cannot move water to the top of tall trees.

• Inhibited by respiratory inhibitors, low temperature, or lack of oxygen (which stops active transport).

Explain the cohesion-tension theory for the movement of water in the xylem.

1. Water evaporates from mesophyll cells into air spaces and out through the stomata (transpiration).

2. This lowers water potential in mesophyll cells.

3. Water moves from adjacent cells by osmosis.

4. Water potential gradient extends to the xylem, pulling water in a continuous stream.

5. Tension is created in the xylem, pulling water up.

6. Water forms a continuous column due to:

   • Cohesion (between water molecules via hydrogen bonding).

   • Adhesion (between water molecules and xylem walls).

7. This column is pulled up from roots to leaves as water evaporates.

What is capillarity?

• Capillarity (capillary action) is the ability of water to move up narrow tubes (like xylem vessels) due to cohesion and adhesion.

what role does capillarity play in water transport in plants?

• Cohesion: Water molecules stick together.

• Adhesion: Water molecules stick to xylem walls.

• Helps initiate movement of water up small plants or short distances in xylem.

• More significant when transpiration is low, such as at night or in small herbaceous plants.

• Works alongside root pressure and transpiration pull.

explain movement of water up the xylem.

1. water evaporates out of stomata on leaves, lowering the hydrostatic pressure

2. more water pulled upto xylem to replace water loss

3. due to H bonds, they’re cohesive creating a column of water

4. water also adheres (sticks) to the walls of the xylem to help pull the water column upwards

5. this creates tension, pulling xylem in to become narrower