Biology IGCSE - Transport in Plants

Xylem

transport water and dissolved minerals from the root up to leaves → helps supporting stem

Phloem

transport food nutrients (sucrose and amino acids) from the leaves and to leaves (source to sink) → called translocation

Differences between xylem and phloem

Xylem	Phloem
no cytoplasm made of dead cells cell wall has lignin (impermeable to water) flow is upward	cell wall made of cellulose only made of living cells permeable cell walls flows up and down

Vascular bundles

Group of xylem and phloem vessels found closely together

Position of xylem and phloem in roots

Position of xylem and phloem in stems

Position of xylem and phloem in leaves

Root hair cell diagram

Root hair cell functions

• Roots are covered in many root hair cells that have root hairs to increase surface area

• Root hair cells contain lots of mitochondria to provide energy for active transport

• The cell membranes of root hair cells have lots of carrier proteins for active transport

Root hair cell adaptations

1. large surface area

2. thin/permeable

Surface areas of root hair cells

large surface area of root hairs increases the uptake of water and mineral ions

Pathway taken by water through root, stem and leaf as: root hair cells, root cortex cells, xylem, mesophyll cells

Transpiration

the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata.

How water and minerals ions are moved from the roots to the leaves in the plants via transpiration

Water will travel down its concentration gradient, towards the root cortex. Excessive water loss in the leaves cause it to have lower water potential compared to the root cortex. As a result, water and mineral ions will move down its concentration gradient to the xylem. Finally, it will reach the leaves to compensate the water loss due to transpiration. This creates a transpiration stream from the xylem to the leaves.

Step 1 of transpiration

Water evaporates from the mesophyll cells (spongy + palisade) and diffuses out of the stomata of the leaves

Step 2 of transpiration

Water is pulled up the xylem vessels in the stem

Step 3 of transpiration

Water diffuses into the root hair cells from the soil by osmosis & travels into the root cortex cells to the xylem

high concentration of minerals → low water potential → water more attracted to diffuse via osmosis

How water vapour occurs

water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour

Describe the effects of variation of temperature and wind speed on transpiration rate

High temperature → increase transpiration rate

Low temperature → decrease transpiration rate

greater the wind speed → increased transpiration rate

larger leaf surface → increase transpiration rate

Explain the effects on the rate of transpiration because of temperature

increased temperature → kinetic energy of water molecules increase → move and evaporate (stomata are open) more quickly → rate of transpiration increase

Explain the effects on the rate of transpiration because of wind speed

increased wind speed → removes layers of humid air surrounding leaf → the water released is carried away faster → increase concentration gradient for water vapour diffusion → increase rate of transpiration

Explain the effects on the rate of transpiration because of humidity

increased humidity → more amount of water in the air → harder for more water to evaporate → decreases concentration gradient for water vapour diffusion from leaf to air → decreased rate of transpiration

What conditions will increase the rate of transpiration

1. warm (sunny)

2. dry (decrease humidity, less water potential outside leaf)

3. windy

4. sunny

Explain how and why wilting occurs

stomata open → leads to lots of water loss → loss of water not compensated → stomata become flaccid → wilting

How to measure rate of transpiration

Potometer

Translocation

the movement of sucrose and amino acids in phloem from sources to sinks

Sources

A part of the plant that release sucrose or amino acids (PRODUCTION)

Sink

A part of the plant that use or store sucrose or amino acids (USAGE AND STORAGE)