3.1.5 Adaptations of Gas Exchange Surfaces

• Effective exchange surfaces in organisms have:
  • A large surface area
  • Short diffusion distance
  • Concentration gradient (maintained)

Across the Body Surface of a Single-celled Organism

• Chlamydomonas is a single-celled organism that is found in fresh-water ponds. It is spherical in shape and has a diameter of 20μm. Oxygen can diffuse across the cell wall and cell surface membrane of Chlamydomonas
• The maximum distance that oxygen molecules would have to diffuse to reach the centre of a Chlamydomonas is 10μm, this takes 100 milliseconds
• Diffusion is an efficient exchange mechanism for Chlamydomonas

Tracheal System of an Insect

• All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases
• Insects have evolved a breathing system that delivers oxygen directly to all the organs and tissues of their bodies
• A spiracle is an opening in the exoskeleton of an insect which has valves
  • It allows air to enter the insect and flow into the system of tracheae
  • Most of the time, the spiracle is closed to reduce water loss

• Tracheae are tubes within the insect breathing system which lead to tracheoles (narrower tubes)
  • The tracheae walls have reinforcement that keeps them open as the air pressure inside them fluctuates

• A large number of tracheoles run between cells and into the muscle fibres - the site of gas exchange
• For smaller insects, this system provides sufficient oxygen via diffusion

Image showing the structure of the tracheal system of an insect

• A concentration gradient is created as oxygen is used by respiring tissues allowing more to move in through the spiracles by diffusion
  • Carbon dioxide produced by the respiring tissues moves out through the spiracles down a concentration gradient
• Very active, flying insects need a more rapid supply/intake of oxygen. They create a mass flow of air into the tracheal system by:
  • Closing the spiracles
  • Using muscles to create a pumping movement for ventilation

• Also, during flight the production of lactate in the respiring muscles, lowers the water potential of muscle cells
  • water found at the narrow ends of the tracheoles is then drawn into the respiring muscle by osmosis
  • This allows gases to diffuse across more quickly

Gills of Fish

• Oxygen dissolves less readily in water
  • A given volume of air contains 30 times more oxygen than the same volume of water

• Fish are adapted to directly extract oxygen from water
• Structure of fish gills in bony fish:
  • Series of gills on each side of the head
  • Each gill arch is attached to two stacks of filaments
  • On the surface of each filament, there are rows of lamellae
  • The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries

• Mechanism:
  • The capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system
  • The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary
  • The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood

Image showing the structure of fish gills and the counter-current system within gills.

Leaves of Dicotyledonous Plants

• In order to carry out photosynthesis, plants must have an adequate supply of carbon dioxide
• There is only roughly 0.036% CO₂ in the atmosphere, so efficient gas exchange is necessary
• Leaves have evolved adaptations to aid the uptake of carbon dioxide
• Structure of a leaf:
  • Waterproof cuticle
  • Upper epidermis - layer of tightly packed cells
  • Palisade mesophyll layer - layer of elongated cells containing chloroplasts
  • Spongy mesophyll layer - layer of cells that contains an extensive network of air spaces
  • Stomata - pores (usually) on the underside of the leaf which allow air to enter
  • Guard cells - pairs of cells that control the opening and closing of the stomata
  • Lower epidermis - layer of tightly packed cells

• Mechanism:
  • When the guard cells are turgid (full of water) the stoma remains open allowing air to enter the leaf
  • The air spaces within the spongy mesophyll layer allows carbon dioxide to rapidly diffuse into cells
  • The carbon dioxide is quickly used up in photosynthesis by cells containing chloroplasts - maintaining the concentration gradient
  • No active ventilation is required as the thinness of the plant tissues and the presence of stomata helps to create a short diffusion pathway

Image showing the structure of a leaf from a dicotyledonous plant

Adaptations of Gas Exchange Surfaces