Temperature

Temperature affects organisms mainly through its effect on enzymes. As temperature rises, molecules have more kinetic energy, so enzyme-substrate complexes form more often and reactions such as photosynthesis and respiration speed up. Above the optimum, enzymes begin to denature and the rate falls sharply. At very low temperatures, ice crystals can form inside cells and damage membranes.Effects on plants. Plant growth and the rate of photosynthesis depend on temperature, so in cold climates the growing season is short and productivity is low. Temperature also affects transpiration and the germination of seeds, and frost limits the distribution of many plant species.Effects on animals. The body temperature of ectotherms such as insects and reptiles follows that of the environment, so their activity, growth and development are faster when it is warm. Endotherms are less directly affected, but need more food to keep warm in cold conditions.Effects on distribution. Each species has a range of temperatures it can tolerate, so temperature helps determine where species are found. With climate change, the ranges of many UK species, such as some butterflies, have shifted northwards.In water, temperature changes much less than in air, but warmer water holds less dissolved oxygen.Worked example. For many enzyme-controlled processes between about 10 and 30 °C, a rise of 10 °C roughly doubles the rate (the temperature coefficient, Q₁₀, is about 2). So an insect larva that takes 20 days to develop at 15 °C may take about 10 days at 25 °C.