Living in Extreme Conditions (Edexcel GCSE Biology)

• Some organisms live in environments that are very extreme, such as at very high or low temperatures, pressures, or high salt concentrations
• An example of habitats where extremophiles are found are deep-sea volcanic vents, where the conditions are extremely hot, under high pressure and there is no sunlight
• Plants are adapted to survive in extreme environments including through adaptations to their leaf size and shape, their cuticle and stomata

Xerophytic plants

• Plants that live in conditions with a plentiful supply of freshwater have leaves with a short diffusion distance through the stomata and a large surface area provided by the air spaces in the spongy mesophyll
• These factors make them vulnerable to water loss
• Plants that live in conditions where freshwater is limited have evolved very effective adaptations to conserve water, including:
  • Very few stomata
  • Sunken stomata
  • Hairs surrounding stomata
  • Needle-shaped or small leaves
  • Waxy cuticle

• Plants with these adaptations are described as xerophytic

Image showing how sunken stomata protects the escaping water vapour from the external air currents

Cacti

• Cacti are well-studied xerophytes usually found in the deserts of North and South America
• They have several characteristics adaptations
  • Their leaves have become spines that can no longer photosynthesise
  • Photosynthesis occurs in the green stem which possesses chloroplasts
  • The stem has a thick cuticle and is very large in diameter which allows it to store water
  • There are both shallow and deep penetrating roots which allow it to access all available water

Image showing the adaptations of a Cactus that enables it to survive in dry, hot environments

Marram Grass

• Sand dunes are another example of a dry environment where plants have evolved to survive
• Marram grass is commonly found on these sand dunes
• Their leaves are well adapted to minimise water loss:
  • Leaves can roll up to reduce the exposure of surfaces to the wind
  • The rolling of the leaf provides deep grooves which protect the stomata
  • The exposed surface has no stomata and a thick cuticle
  • The inner surface of the leaf possesses a large number of hairs to trap air

Image showing how the adaptations of the leaf causes water vapour to be trapped and retained near the stomata. This humid air near the stomata means the water potential gradient out of the leaf is reduced, so the rate of evaporation decreases.