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GAS EXCHANGE IN VASCULAR PLANTS Biology ATAR Unit 2 Chapter 12 Pages 307-309 Keywords Passive diffusion Stomata Guard cells Turgid Flaccid Dissolution of gases Evaporation Cellular respiration Photosynthesis Chloroplasts Making connections Plants ‘breathe’ through small pores in their leaves. These are called stomata. Gases (O2 & CO2) diffuse in and out of the stomata This is an efficient passive process because leaves are very thin, thus maximising the surface area to volume ratio GAS EXCHANGE Gas exchange in plants occurs through passive diffusion The leaves of plants facilitate diffusion because: The leaves are flat and cells are arranged in thin layers, thus maximising the surface area to volume ratio The cells are loosely packed, allowing for rapid diffusion through intracellular spaces which are filled with air STOMATA Gas exchange occurs through tiny pores called stomata (stoma singular) Each stoma is surrounded by 2 guard cells These guard cells control the opening and closing of the stoma Gases dissolve in solution when they come into contact with the moist cell surface. GUARD CELLS Guard cells: Are joined at the ends Have a thicker cell wall on the side of the stoma Have bands of inelastic fibre around the cell wall Contain chloroplasts to make glucose for the energy needed to drive the H+/Cl- and K+/Na+ pumps GUARD CELLS Guard cells pump in K+ ions to increase the osmotic gradient across the cell membrane During the day, photosynthesis also produces a build up of glucose in the guard cells Water moves into the cell The guard cells swell lengthwise (become turgid) and the stoma opens MICRO-ENVIRONMENT The micro-environment surrounding stomata is warm and moist. This facilitates the dissolution of gases, enabling the molecules to diffuse into the cell. It also means that it is a site of water evaporation. The plant must therefore maintain a balance between this water loss and its need of CO2 for photosynthesis. OPENING AND CLOSING OF THE STOMATA DAY TIME Stomata open during the day, when the plant needs CO2 for photosynthesis The rate of photosynthesis is greater than the rate of cellular respiration CO2 and O2 are exchanged independently of one another. NIGHT TIME Stomata are closed at night to prevent unnecessary water loss through evaporation. The rate of cellular respiration is greater than the rate of photosynthesis ADAPTATIONS TO ENVIRONMENTAL CONDITIONS In terrestrial plants, stomata are mostly located on the underside of the leaf. This reduces the rate of evaporation by protecting them from the hot sun. In aquatic plants however, the stomata are found on the upper surface of the leaf. In hot, arid climates, plants have fewer and smaller stomata. This reduces the rate of water loss. Leaf size is generally also reduced. In cacti, the stomata are located on the stem of the plant. ADAPTATIONS TO ENVIRONMENTAL CONDITIONS Mangroves have specialised roots that grow upwards from waterlogged soil to obtain oxygen for the submerged root system. RATE OF GAS EXCHANGE The overall exchange of gases depends on environmental conditions which influence: 1. The rate of photosynthesis 2. The rate of cellular respiration The rate of photosynthesis and cellular respiration (or the rate of a plant’s metabolism) can be measured by its use and/or production of gases.