Chapter 9

TRANSPORT IN PLANTS

Chapter 9

Transport System

The vascular tissues found in plants are the xylem and the phloem

The function of the xylem is to transport water and mineral salts

The function of the phloem is to transport food materials: mainly sugars and amino acids

Chapter 9

Where Are The Xylem And Pholem?In the Leaf

phloem xylemvein

mid-rib

Chapter 9

Where Are The Xylem And Pholem?

Cross-section of stem

High magnification of a single vascular bundle

Phloem

Xylem(note thickened cross walls)

In the Stem

Chapter 9

Transport Of Water And Mineral Salts

Water is absorbed from surrounding soil by root hairs

transported up the stem by xylem vessels

is used in respiration and other cellular activities

and is lost to the surroundings by the process called transpiration

Chapter 9

Transport Of Water And Mineral Salts

across phloemabsorbed into root hair cell

into xylem vessels

across cortex cellsWater from soil

Up xylem vessels to plant

The Uptake of Water

Chapter 9

Transport Of Water And Mineral SaltsThe Uptake of Mineral Salts

Plants absorb mineral salts in the form of ions dissolved in soil water through the root hairs. This may take place via diffusion.

When there is a lower concentration of ions in the soil than the root hairs, ions do not diffuse into the root hairs.

It involves the movement of ions against a concentration gradient, and to do this root hairs need energy. This process is called active transport.

Chapter 9

Transport Of Water And Mineral SaltsAdaptations of the Root Hair Cell

Soil particles

Water and mineral salts

The root hair extends from the root cell. It is long and narrow. This provides a larger surface area to volume ratio for water and nutrients to be absorbed.

The cell sap in the root hair cell is of lower water potential than the soil solution. This allows water to enter the root hair by osmosis.

The root hair cell is living. It carries out respiration to replace energy that is required for active transport.

Chapter 9

Transport Of Water And Mineral SaltsTransport in the Xylem

There are 3 forces that move the water upwards in the xylem:

a) Root pressure - a force that pushes water up the xylem (produced by the continuous movement of water through the root cells.

b) Capillary action - a force that pushes water up the narrow xylem vessels.

c) Transpirational pull - a force that pulls water up the xylem (produced by evaporation of water from the leaves).

Chapter 9

Transpiration Pull In Plants

Transpiration is the loss of water in the form of water vapour from the leaves, especially through the stomata, as a consequence of gaseous exchange

The importance of transpiration:– Transpiration pull which is a main factor in ‘lifting’

and sustaining the flow of water and mineral salts from roots up to the rest of the plant

Chapter 9 Factors Affecting The Rate Of Transpiration

Factors affecting the rate of transpiration: Transpiration is dependent on the opening of the stomata and

the evaporation of water in the intracellular spaces of the leaf This is affected by environmental conditions:

– Humidity of the air – the more water vapour present in the surrounding air, the lower the rate of evaporation from the plant

– Temperature – which also affects the rate of evaporation– Light – which stimulates the stomata to open– Wind speed – which transports water vapour away from the

transpiring surface

Chapter 9 Factors Affecting The Rate Of Transpiration

What happens during wilting?1. Water escapes from the vacuole.2. Turgor pressure decreases.3. The cytoplasm shrinks and the cell membrane pulls away from

the cell wall.4. The cell loses its firmness and becomes soft. The plant becomes

flaccid and limp. At this stage, cell is plasmolysed.

Wilting

cytoplasm

cell surface membrane

Chapter 9

Translocation

Glucose produced by photosynthesis in leaf is converted to sugars (mainly sucrose) and translocated to different parts of the plant

To growing regions to be used as energy for growth

To storage organ (fruit)to be stored mainly as sugars

To storage organ (tubers in roots)To be stored mainly as starch

Transport in the phloem occurs in both directions up and down the plant (bidirectional movement)

If the plant requires more energy than can be produced, food stores are mobilised, converted back to sugars, and transported to wherever it is needed

The movement of sugars and amino acids via the phloem is called translocation