Topic 8: Transport in Plants

Describe xylem.

A hollow plant tissue made up of dead, empty cells joined end to end. It has no contents such as nuclei or cytoplasm and no cross walls.

What is the function of the xylem?

They transport water and mineral ions from the roots and up to the leaves.

They help support the plant through their walls of cellulose and strong lignin.

Describe the phloem.

A plant tissue made up of living cells joined end to end.

What is the function of the phloem?

They transport sucrose and amino acids from the leaves where they are made, to other parts of the plants, such as its roots and flowers.

What do the xylem and phloem compose of?

A transport system known as the Vascular Bundle.

Define the source.

The part of the plant that releases materials, to be transported to other parts of the plant.

Define the sink.

The part of the plant to which materials are being transported to, to which they are used or stored.

What is the structure of the xylem tissues/vessels?

It is composed of long continuous tubes of dead cells without cross walls.

- Thick walls with lignin
- No cell contents, hollow

Define conduction.

The upwards transport of materials in the xylem tissues.

Pathway of water through a plant

Root hair cell -> Root cortex cells -> Xylem -> Mesophyll Cells

Define transpiration.

The process by which plants lose water from the leaf. At the surface of the mesophyll cells followed by diffusion from the air spaces to the atmosphere, through the stomata.

Why is transpiration important?

- Transport of minerals
- Cooling down leaves
- Providing water for structure
- Providing water for photosynthesis

Define transpirational pull.

Water is constantly taken from the top of the xylem to supply leaves, reducing the pressure up there. This means there's a pressure difference of the xylem vessel near the leaves and the xylem vessel near the roots, causing water to flow upwards from higher pressure at the bottom to lower pressure at the top, due to water molecules sticking together due to high forces of attraction.

What are the factors that affect the rate of transpiration?

Light Intensity, Temperature, Air Movements/Wind Speed, Humidity

How does temperature affect the rate of transpiration?

The higher the temperature, the greater the kinetic energy of water molecules, meaning that water will evaporate more quickly from the surface of mesophyll cells and diffuse out of the leaf and into the air more quickly.

How does humidity affect the rate of transpiration?

An increase of humidity means less water vapor will diffuse out of the leaf because there is not much of a diffusion gradient for the water vapor between the air spaces inside the leaf and the wet air outside.

How does Air Movements/Wind Speed affect the rate of transpiration?

Increased wind speed increases the rate of transpiration. On a windy day, the air around the leaf, which has a lot of water vapor, is quickly moved away. This creates a diffusion gradient because there is less water vapor outside the leaf than inside. Thus, water vapor diffuses out of the leaf faster on windy days than on still days.

How does light intensity affect the rate of transpiration?

During the daytime, the stomata are open for gas exchange, allowing more water vapor to diffuse out. So, the higher the light intensity the higher the rate of transpiration.

Define wilting.

When plants lose water from their leaves faster than it can take up from the roots. The cells in the plants lose so much water they become flaccid and the tissues in the leaves are no longer supported by the turgid cells pushing outwards on one another, causing them to droop and be soft and floppy.

Define translocation.

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

What do root hair cells increase the rate of absorption for?

Water by osmosis & Mineral ions by active transport.

Define cohesion.

The force of attraction between similar particles.

Define adhesion.

The force of attraction between different particles.