84Transport in plantsChapter 5Plant transport systemsPlant cells, like animal cells, need a regular supply of oxygen and nutrients. All plants are multicellular, and some of them are very large. Most plants, however, have a much more branching shape than animals, and this provides a much larger surface area : volume ratio for exchange with their environment than in an animal of the same body mass.The requirements of plants differ from those of animals in several ways, both in the nature of the nutrients and gases required and the rate at which these need to be supplied.•Carbon dioxide: Photosynthesising plant cells need a supply of carbon dioxide during daylight. They obtain this from the air. Aquatic plants get carbon dioxide from the water that surrounds them.By the end of this chapter you should be able to:a explain the uptake of ions by active transport in roots, and the role of the endodermis;b describe the entry of water into plant roots in terms of water potential;c relate the structure of xylem vessels to their functions in transport and support;d make drawings from prepared slides of xylem vessels;e outline the ascent of water in plants, including the roles of root pressure, capillarity, cohesion, adhesion and transpiration pull, and the role of stomata in transpiration;f investigate the impact of environmental factors, including light and air movements, on the rate of transpiration;g relate the structure of sieve tubes and companion cells to their function;h make drawings of sieve tubes and companion cells from prepared microscope slides;i label pertinent features in an electron micrograph of a sieve tube and companion cell;j explain how phloem loading in the leaves occurs against a concentration gradient;k discuss mass (pressure) flow as a possible method of translocation, including experimental evidence for and against this hypothesis.•Oxygen: All living plant cells need oxygen for respiration. Cells that are actively photosynthesising produce more than enough oxygen for their needs. Cells that are not photosynthesising have to take in oxygen from their environment, but they do not respire at such a high rate as mammals and so they do not need such a rapid oxygen supply.•Organic nutrients: Some plant cells make many of their own organic food materials, such as glucose, by photosynthesis. However, many plant cells do not photosynthesise and need to be supplied with organic nutrients from photosynthetic or storage cells.•Inorganic ions and water: All plant cells require a range of inorganic ions and also water. These are taken up from the soil, by roots, and are transported to all areas of the plant.
Chapter 5: Transport in plants85a higher concentration in the soil than inside the root hair cell, then it will be absorbed by facilitated diffusion. This does not require any energy input by the plant. If, however, the concentration of the ion in the soil is lower than that inside the root hair cell, then it must be absorbed by active transport(Unit 1, Chapter 3, page 70). Specific transporter