Exchange and transport

3.1 Photosynthesis

3.3 Plant responses

Exchange and transport

  1. Gas exchange

    1. Leaves have a large surface area to maximise photosynthesising potential, and a thin, flattened shape so that gases have less distance to diffuse
    2. At the top is a waxy cuticle, which prevents water loss by transpiration
    3. Below that is the epidermis, a layer of epidermal tissue; both are thin and transparent to allow maximum light to pass through
    4. Below that is the palisade layer, a layer of palisade mesophyll tissue packed tightly with tall, thin palisade cells containing many chloroplasts, and therefore chlorophyll, to maximise photosynthesising potential
    5. Below that is the spongy layer, a layer of spongy mesophyll, which has more spread out cells and air spaces to maximise surface area for gas exchange
    6. Below that, the epidermis contains stomata, pores that are controlled by guard cells, opening to obtain carbon dioxide from the atmosphere and remove oxygen produced in photosynthesis

  2. Water exchange

    1. Most of the water and mineral ions are absorbed by roots
    2. The surface area of roots is increased by root hair cells
    3. Plants mainly lose water vapour from leaves, especially through stomata
    4. Evaporation is more rapid in hot, dry and windy conditions
    5. If plants lose water faster than it is replaced, stomata con close to prevent wilting

  3. Transport

    1. Flowering plants have two transport tissues: xylem & phloem
    2. Xylem tissue transports water with dissolved nutrients & mineral ions from the roots to the stems & leaves
    3. The movement of water from the roots through the xylem and out of the leaves is known as the transpiration stream
    4. Phloem tissue transports dissolved sugars and other substances formed by photosynthesis from the leaves to the rest of the plant, including the growing regions and the storage organs, in a process called translocation
    5. Xylem are mainly composed of dead cells; phloem are composed of living cells
    6. In the leaf, the vascular tissue is located in the network of veins
    7. In the stem, the vascular tissue is located in bundles near the edge to resist compression and bending forces
    8. In the roots, the vascular tissue is located in the centre to withstand stretching forces
    9. Xylem are generally located nearer the interior than the phloem