biology4alevel600

1. Multicellular organisms with small surface area to volume ratios need transport systems. 2. Water and mineral salts are transported through a plant in xylem vessels. Movement of water is a  passive process in which the water moves down a water potential gradient from soil to air.  3. The energy for this process comes from the Sun, which causes evaporation of water from the wet walls of mesophyll cells in leaves. Water vapour in the air spaces of the leaf diff uses out of the leaf through stomata, in a process called transpiration. This loss of water sets up a water potential gradient throughout the plant.  4. Transpiration is an inevitable consequence of gaseous exchange in plants. Plants need stomata so that carbon dioxide and oxygen can be exchanged with the environment.  5. The rate of transpiration is affected by several environmental factors, particularly temperature, light intensity, wind speed and humidity. It is difficult to measure rate of transpiration d...

The movement of substances in phloem tissue is called translocation . The main substances that are moved are sucrose and amino acids , which are in solution in water. These substances have been made by the plant and are called assimilates . Phloem tissue Phloem tissue contains cells called sieve tube elements . Unlike xylem vessel elements, these are living cells and contain cytoplasm and a few organelles but no nucleus. Their walls are made of cellulose. A companion cell is associated with each sieve tube element. Sources and sinks  Vascular plants produce nutrients such as sucrose in their leaves. These nutrients must then be transported to the rest of the shoot or to the root tips, where growth occurs. The leaves are referred to as the source, and the shoot and root tips - sink. A source is an organ that produces more sugar than it requires. That's where assimilates enter the phloem.  A sink is an organ that consumes sugar for its own growth and storage. That's where...

Most plants secure the water and minerals they need from their roots. The path taken is: soil -> roots -> stems -> leaves. The minerals (e.g., K+, Ca2+) travel dissolved in the water. Water and minerals enter the root by separate paths which eventually converge in the stele. Transpiration Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf, or atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface.  Water from the roots is pulled up by this tension. At night, when stomata close and transpiration stops, the water is held in the stem and leaf by the cohesion of water molecules to each other as well as the adhesion of water to the cell walls of the xylem vessels and tracheids. This is called the  cohesion–tension  theory of sap ascent. How water moves from soil ...

Plants have 2 transport systems: xylem:  transports water and inorganic ions from the roots to the leaves. phloem:  transports food made in the plant (sucrose and amino acids) from the leaves to the rest of the plant.  Both of these systems are rows of cells that make continuous tubes running the full length of the plant. Plants can be very large, but they have a branching shape which helps to keep the surface area to volume ratio fairly large. Their energy needs are generally small compared with those of animals, so respiration does not take place so quickly. They can therefore rely on  diffusion  to supply their cells with O 2  and to remove CO 2 . Their leaves are very thin and have a large surface area inside them in contact with the air spaces. This means that diffusion is sufficient to supply the mesophyll cells with CO 2  for photosynthesis, and to remove O 2 . Plant transport systems therefore do not transport gase...