Transport in Plants

Xylem and Phloem

Plants have two separate transport tissues, found together in the vascular tissue of the root, stem and leaf.

Position in the plant

In a non-woody dicotyledonous plant the two tissues are found in different places in each organ. In the root the xylem is in the centre, with the phloem between its arms. In the stem there is a ring of vascular bundles, with the xylem on the inside and the phloem on the outside. In the leaf the xylem is on the upper side of each vein and the phloem is below it.

ExtendedStructure of xylem vessels

Xylem vessels are adapted to carry water: they have thick walls containing lignin, which give support; no cell contents, so the lumen is empty; and the cells are joined end to end with no cross walls, forming a long continuous tube.

Worked example (identifying the tissue): A tissue in a root section is X-shaped and lies at the centre. It carries water. Name it. Step 1. In a root the xylem is in the centre. Step 2. It carries water and mineral ions upward. Step 3. The phloem lies between its arms, not in the middle. Answer: Xylem.

Root Hair Cells

Water and mineral ions enter the plant through its roots. The cells that do this are the root hair cells.

Structure and function

A root hair cell is an epidermal cell with a long, thin extension that grows out into the soil. It has two functions: to take up water, which enters by osmosis, and to take up mineral ions from the soil.

Worked example (explaining the adaptation): Explain why a root covered in root hairs takes up water faster than a root with none. Step 1. Each root hair is a long thin extension of an epidermal cell. Step 2. Many hairs give the root a much larger surface area. Step 3. A larger surface area increases the uptake of water and mineral ions. Answer: Larger surface area.

The Pathway of Water

Water takes a set route from the soil to the leaves. A stain in the water makes the route visible.

The pathway

Water enters a root hair cell, then passes through the root cortex cells, enters the xylem, and travels up the stem to the leaves, where it passes into the mesophyll cells.

Investigating the pathway with a stain

Stand a stem, such as celery or a white flower, in water containing a coloured stain and leave it for some hours. The colour climbs the stem. Cut across the stem, or along it, and look at the cut surface: the stain appears only in the xylem, and in the veins of the leaves.

Worked example (interpreting the result): A celery stem stood in red dye has red dots in a ring when cut across. What does this show? Step 1. The red marks where the dye has travelled. Step 2. The dots lie in the vascular bundles. Step 3. The dye is carried in the xylem, so water travels up the xylem. Answer: Water travels in the xylem.

Transpiration

Most of the water a plant takes up is lost from its leaves as vapour. This loss is called transpiration.

How the water is lost

The surfaces of the mesophyll cells are always wet. Water evaporates from them into the air spaces of the leaf. The water vapour then diffuses through the air spaces and out of the leaf through the stomata, down a concentration gradient into the drier air outside.

ExtendedWhy so much water is lost

Water vapour loss is related to the large internal surface area of the interconnecting air spaces between mesophyll cells, which allows rapid evaporation, and to the size and number of stomata: more or larger stomata mean more water vapour can escape.

Worked example (describing transpiration): Describe how water in a leaf cell reaches the air outside the leaf. Step 1. Water evaporates from the surface of a mesophyll cell into an air space. Step 2. The water vapour diffuses through the air spaces. Step 3. It leaves through a stoma as water vapour. Answer: Evaporates, then diffuses out through stomata.

Factors Affecting Transpiration

The rate of transpiration changes with the conditions around the leaf. You must be able to investigate temperature and wind speed.

Investigating the rate

Fix a leafy shoot into the potometer and note the position of the bubble. After a set time measure how far it has moved. To test temperature, repeat in places at different air temperatures; to test wind speed, put a fan at different distances. Change one factor and keep the others the same.

The Extended explanations of these effects are on page 8.

Worked example (calculating a rate): In still air a bubble moves 36 mm in 3 minutes; with a fan it moves 60 mm in 3 minutes. Calculate each rate and compare. Step 1. Still air: 36 ÷ 3 = 12 mm per minute. Step 2. With the fan: 60 ÷ 3 = 20 mm per minute. Step 3. The rate is higher with wind: wind speed increases transpiration. Answer: 12 and 20 mm per minute.

Transpiration Pull and Wilting

This page is Extended content: why conditions change the rate of transpiration, how water is drawn up a tall plant, and what happens when it cannot be replaced.

ExtendedExplaining the effects of temperature, wind speed and humidity

A higher temperature increases the rate of evaporation and the speed of diffusion. A faster wind speed blows away water vapour from near the stomata, keeping the concentration gradient steep. Higher humidity means more water vapour in the air outside, so the gradient is smaller.

ExtendedHow water moves up the xylem

As water evaporates from the mesophyll cells, it is replaced by water from the xylem. This creates a transpiration pull that draws up a column of water molecules in the xylem. The molecules are held together by forces of attraction between water molecules, so the column does not break. The plant does not use energy to lift the water.

ExtendedWhy plants wilt

If water is lost by transpiration faster than the roots can replace it, the cells lose water and lose turgor pressure. The cells become flaccid and the leaves and stems droop: the plant has wilted. When water is supplied the cells take it in, become turgid again and the plant recovers.

Worked example (explaining wilting): On a hot, windy day a plant in dry soil droops. Explain why. Step 1. Heat and wind increase the rate of transpiration. Step 2. Dry soil means the roots cannot replace the water lost. Step 3. The cells lose water and turgor, become flaccid, and the plant wilts. Answer: Water loss exceeds uptake.

Translocation

This page is Extended content: the phloem carries food from where it is made or released to where it is needed.

ExtendedSources and sinks

Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks. A source releases sucrose or amino acids, as a photosynthesising leaf does. A sink uses or stores them, as growing roots, fruits and shoot tips do.

ExtendedSource or sink at different times

Some parts of a plant are a source at one time and a sink at another. A storage organ such as a potato tuber is a sink while it is storing food in summer, and a source in spring when it releases sucrose to the growing shoots.

Worked example (source or sink?): A growing root tip uses sucrose for respiration. Is it a source or a sink? Step 1. A root tip uses sucrose that arrives in the phloem. Step 2. A part that uses or stores sucrose is a sink. Answer: A sink.