9.2 Transport in angiospermophytes Plant Science 9.2 Transport in angiospermophytes

Root system The root system of a plant must supply sufficient water and mineral ions. For this reason it has developed a large surface area due to branching. In addition, there are tiny root hairs that further increase the surface area.

Root system

Root hair cell structure

Ways in which mineral ions move into roots Diffusion – if the concentration of a mineral in the soil solution is greater than its concentration in a root hair cell, then the mineral may enter the root hair cell by diffusion. Mass flow of water – some mineral ions enter the root along with water. They may then be carried along in solution to the cytoplasm and eventually xylem vessels.

Ways in which mineral ions move into roots contd. Fungal hyphae - A mycorrhiza is a mutualistic relationship between a fungus and a plant root. The fungus functions like a root by growing into the soil and absorbing nutrients for the plant. The plant provides the fungus with products of photosynthesis (sugar). Many plants do not do well or do not grow at all without the fungi. Approximately ninety percent of all plants develop mycorrhizae.

Mineral ion transport If the concentration of a mineral in the soil solution is less than that in a root hair cell, it may be absorbed by ACTIVE TRANSPORT. Most minerals are absorbed in this way. Because active absorption requires energy, the rate depends on respiration.

Support in terrestrial plants Thickened cellulose – cellulose is made from microfibrils and polysaccharides that lend mechanical strength. Cell turgor – cells that are full of water, with the cytoplasm pushing outwards on the cell wall, are firm and lend support. Lignified xylem – the walls of xylem vessels are strengthened with lignin.

Transpiration Transpiration is the loss of water vapour from the leaves and stems of plants. Transpiration causes a flow of water from the roots, through stems to the leaves. This is called the TRANSPIRATION STREAM.

Transpiration stream When explaining how water is carried upwards in the transpiration stream, the following factors must be considered: Structure of xylem vessels Transpiration pull Cohesion Adhesion Evaporation

Structure of xylem vessels Function for water transport and support. Consist of vessels and tracheids, both dead tissue. Made of long cells joined end to end to allow water to flow in a continuous column. The end walls have broken down to give an uninterrupted flow of water. Lignified walls prevents collapse under large tension forces. Very narrow to increase capillary action.

Evaporation and Transpiration pull Water evaporates from the cells of the spongy mesophyll layer of the leaf. The water that evaporates is replaced by water from the xylem vessels in the leaf. Low pressure, or suction, is created inside xylem vessels when water is pulled out. This is called TRANSPIRATION PULL.

Cohesion and adhesion The transmission of the transpiration pull through xylem vessels depends on the cohesion of water molecules, due to H-bonding. Adhesive forces exist between the water molecules and the walls of the xylem vessels. This is known as capillarity. Transport of water in xylem is PASSIVE.

Transpiration contd.

Measuring transpiration

Guard cells Guard cells can regulate transpiration by opening and closing stomata. The hormone abscisic acid causes the closing of stomata. (eg when water is scarce)

Guard cells

Factors affecting rate of transpiration Light – guard cells close the stomata at night so transpiration is much greater during the day. Temperature – heat is required for evaporation of water from the surface of cells, so as temperature rises, so does rate of transpiration. Higher temperatures also increase rate of diffusion and reduce relative humidity of the air outside the leaf.

Factors affecting transpiration contd. Humidity – water diffuses out of the leaf when there is a concentration gradient between the air spaces within the leaf and the air outside. The lower the humidity outside the leaf, the steeper the gradient and therefore the faster the rate of transpiration.

Factors affecting transpiration contd. Wind – when air is still, pockets of saturated air form near the stomata, reducing transpiration. Wind blows away the saturated air, thus increasing transpiration rate.

Adaptations of xerophytes Plants adapted to grow in very dry habitats are called XEROPHYTES. Eg. giant cactus (Cereus giganteus). Surface area of leaves reduced to spines to limit transpiration. Very wide-spreading network of shallow roots to maximise water absorption after rains.

Adaptations of xerophytes Thick waxy cuticle covering stem. Thick stems containing water storage tissue. Vertical stems to absorb sunlight early and late in the day, but not at midday when sunlight is most intense. CAM physiology, whereby the stomata open during cooler nights instead of the intense heat of the day.

More adaptations…. Deep roots to access lower water levels. Rolled leaves to trap humid air near stomata. Reduced numbers of stomata. Stomata sunken in pits surrounded by hairs to trap saturated air thus lowering transpiration rate.

Translocation Translocation is the movement of substances around the plant in the phloem tubes. This could be sugars produced in photosynthesis, amino acids, or chemicals from pesticides. Translocation occurs through sieve tubes and is an ACTIVE process requiring ATP.

Translocation contd.