Transportation in Plants

8.1: Transport System LO:

State the functions of xylem and phloem

Identify the position of xylem and phloem in transverse sections of root, stems and leaves.

Transport in Plants

Plants have two different types of 'transport' tissue.

Xylem transports water and solutes from the roots to the leaves, phloem transports food from the leaves to the rest of the plant.

A group of xylem vessels and phloem tubes is called a vascular bundle.

Xylem

Xylem cells have extra reinforcement in their cell walls, and this helps to support the weight of the plant.

Stem the xylem and phloem are arranged in bundles near the edge of the stem to resist compression and bending forces.

Comparison of Xylem and Phloem Tissue Process Functions Structure

Xylem Transpiration Transport water and mineral ions up to the leaves

Columns of hollow, dead reinforced cells. Their walls are made of cellulose and lignin.

Phloem Translocation

Transport sucrose and amino acids down to the roots and up to the flowers and fruits.

Columns of living cells joined end to end. Each sieve tube element has a companion cell next to it. Companion cell have nucleus.

Xylem Vessel and Phloem Sieve Tube

Distribution of Xylem and Phloem

Distribution of xylem and phloem tissue in roots of dicotyledonous plants

ALBIO9700/2006JK

ALBIO9700/2006JK

Distribution of xylem and phloem tissue in stems of dicotyledonous plants

ALBIO9700/2006JK

Distribution of xylem and phloem tissue in leaves of dicotyledonous plants

Comparison between monocots and dicots

Learning Check!

Plenary

1. Xylem vessels transport water and mineral ions from the roots up the stem to the leaves.

2. Phloem vessels transport sucrose, made in the leaves during photosynthesis and in storage organs, to other parts of the plant.

3. Xylem and phloem are distributed in a central core inside a root. Inside a stem they are organised into vascular bundles.

8.2: Water Uptake

LO:

Identify root hairs as seen under microscope and relate their structures to water uptake.

State the pathway taken by water through root, stem and leaf

The Internal Structure of a root

Root Hairs

Adaptations:

- many root hairs

- long extension, thin

- permeable cell walls

To provide large surface area which water and mineral ions are absorbed.

water potential gradient

Root hair

Xylem vessels

Leaves air

How water is absorbed by a plant?

Roots

Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by having a large surface area to speed up osmosis.

The absorbed water is transported through the roots to the rest of the plant where it's used for different purposes:

1. It's a reactant used in photosynthesis

2. It supports leaves and shoots by keeping the cells rigid

3. It cools the leaves by evaporation

4. It transports dissolved minerals around the plant

Soil water is a dilute solutions which contains mineral ions such as nitrate and potassium.

The cell sap in the root hair cells is more concentrated solution than soil water. This is because root hair cells absorb mineral ions and contains sugar.

Therefore, water diffuses from soil into the root hair cells by osmosis.

How water is absorbed by a plant?

How water is absorbed by a plant?

Water then pass across the cortex of the root. Most of the water moves through the cell walls and in the spaces between the cells. Some moves from cells to cells. Water eventually reaches the xylem in the centre of the root.

From here water moves up the xylem through the stem and to the leaves where it enters the spongy mesophyll cells.

Much of the water enters the cell walls, evaporates to form water vapour and then diffuses through stomata to the atmosphere.

Extension:

Transpiration Pull:

Transpiration is the evaporation of water from the leaves and the loss of water vapour to the atmosphere through the stomata.

Water is pulled up the xylem in the stem from the roots to the leaves by transpiration pull.

Transpiration Pull

The mass flow of water up the xylem relies on two properties of water:

Cohesion- the water molecules tend to attract each other, sticking together.

Adhesion the water molecules tend to stick to the inside of the xylem vessel.

The movement of water up the xylem is called the transpiration stream.

Transpiration Stream

Learning Check!

Learning Check 2

Plenary

1 Water enters a root hair cell by osmosis. It then passes across the cells of the root cortex by osmosis before passing into the xylem and then up the stem to the leaves.

2 Root hairs are well adapted for the absorption of water since they have thin cell walls and have a large surface area.

3 Loss of water from the leaves 'pulls' more water up the stem from the roots in the transpiration stream.