All organisms need water. It is the medium for metabolic reactions, a reactant in photosynthesis and a solvent for transport, and in plants it keeps cells turgid for support. Water availability depends on rainfall, evaporation and drainage and, for animals, on access to drinking water or water in food.Plants. When water is short, stomata close to reduce water loss. This also reduces carbon dioxide uptake, so photosynthesis and growth slow. If water loss continues, cells lose turgor and the plant wilts. Xerophytes are plants adapted to dry conditions, such as cacti and marram grass. Their adaptations include:
• a thick waxy cuticle
• rolled leaves, sunken stomata and hairs that trap moist air, reducing the water potential gradient
• reduced leaves or spines, lowering the surface area for transpiration
• deep or widespread roots, and water storage tissue in swollen stemsAnimals. Desert animals reduce water loss by producing very concentrated urine and dry faeces, having few sweat glands and being active at night. The kangaroo rat, for example, has very long loops of Henle and gets almost all its water from its food and from respiration (metabolic water).Distribution. Water availability is a key factor deciding which biome develops: high rainfall supports forest, moderate rainfall grassland and low rainfall desert. Within a habitat, damp microhabitats such as leaf litter support organisms that lose water easily, such as woodlice and amphibians.Too much water can also be a problem, because waterlogged soil lacks air.
• a thick waxy cuticle
• rolled leaves, sunken stomata and hairs that trap moist air, reducing the water potential gradient
• reduced leaves or spines, lowering the surface area for transpiration
• deep or widespread roots, and water storage tissue in swollen stemsAnimals. Desert animals reduce water loss by producing very concentrated urine and dry faeces, having few sweat glands and being active at night. The kangaroo rat, for example, has very long loops of Henle and gets almost all its water from its food and from respiration (metabolic water).Distribution. Water availability is a key factor deciding which biome develops: high rainfall supports forest, moderate rainfall grassland and low rainfall desert. Within a habitat, damp microhabitats such as leaf litter support organisms that lose water easily, such as woodlice and amphibians.Too much water can also be a problem, because waterlogged soil lacks air.