Lesson 2: Transport in Plants: Xylem

The xylem is involved in the transport of water. Water is absorbed in the roots, travels up the stem in the xylem vessels in vascular bundles to the leaves. The loss of water vapour from the leaves and stems of plants is known as transpiration. Transpiration also carries minerals throughout the plants and can cool the plant in hot climates. Water is lost due to evaporation through the stomata, which allow for gas exchange in the leaf.

Most plants live in areas where the humidity is lower in the environment than in the leaves. During the day water vapour leaves the air spaces in the mesophyll through open stomata in the lower epidermis of the leaf and stem. The evaporating water is drawn from the xylem in the vascular bundles. The vascular bundles are continuous with the roots so a column of water is formed connecting the roots, stems, leaves and air spaces. This is known as a transpiration stream.

Environmental factors can affect the rate of transpiration in plants. Light – controls the opening and closing of the stomata. As light increases, stomata open, speeding up transpiration.

Temperature – as the temperature rises, the rate of transpiration also increases as water evaporates from the air spaces in the spongy mesophyll and diffuses out the stomata

• Atmospheric Humidity – increases in atmospheric humidity causes a decrease in transpiration due to the change in concentration gradient between the airspaces in the spongy mesophyll and the atmosphere.

• Wind Current – increase in wind currents causes an increase in transpiration due to the removal of the water saturated air near the stomata.

Transpiration is controlled by the pair of guard cells surrounding each stomata. The guard cells change shape to open and close the stomata. Guard cells open when they gain potassium ions (K+) and water from the surrounding cells. The cells actively transport potassium in and water follows by osmosis. The guard cells have unevenly thick cell walls. The cell walls closer to the stomata are thicker and the cellulose molecules are oriented such that when the cell swells (becomes turgid), it bows away from the stomata creating a larger gap.

Stomata are generally open during the day and closed at night. During the day, when light is present, CO2 can be taken in through the stomata to continue photosynthesis. At night there is no photosynthesis, the stomata close to preserve water. Potassium ion movement can be controlled by a hormone called abscisic acid produced in the roots during drought. When abscisic acid is present, potassium ions leave the guard cells, water follows by osmosis and the stomata close.

The movement of water through the xylem can be explained through the cohesion – tension theory. Loss of water vapour in the leaves results in negative pressure (tension) in the vascular bundles. Water vapour enters the air spaces in the mesophyll (parenchyma) to replace the water lost through the stomata. Water is drawn up the xylem continuously due to cohesive forces between the water molecules and adhesive forces between the water molecule and the xylem vessel walls. Water is drawn in from the cortex in the roots to replace water that is lost in transpiration The tension caused by transpiration causes water to be drawn into the roots from the soil.

The roots of a plant are responsible for absorbing water and mineral ions from the soil. Surface area is increased through extensive branching of the root system and the development of root hairs.

Minerals are found in the soil as salts. Salts dissolve in the soil water and can move into root cells. Plants require minerals to make a variety of substances: Calcium – required for cell walls Magnesium – used to make chlorophyll Iron – a cofactor in enzymes

Passive Transport Mass Flow – dissolved minerals move into the root along with the water Facilitated Diffusion – from the soil into the root hairs, down their concentration gradient. Active Transport When concentration is lower in the soil than the plant. Potassium, nitrate and phosphate are commonly moved into root hairs by active transport. Mutualistic Relationships Close association with a fungus (mycorrhiza), minerals can pass directly from the fungal hyphae to the root cells.