Specialized Exchange Strategies (College Board AP Biology)

Specialized Exchange Strategies

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

Single-Celled Organisms

• Chlamydomonas is a single-celled organism that is found in freshwater ponds. It is spherical in shape and has a diameter of 20 μm. 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 10 μm, this takes 100 milliseconds
• Diffusion is an efficient exchange mechanism for Chlamydomonas

Insects

• 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
• 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 fibers - the site of gas exchange
• For smaller insects, this system provides sufficient oxygen via diffusion

Tracheal System of an Insect Diagram

The structure of the tracheal system of an insect

• 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 water found at the narrow ends of the tracheoles is drawn into the respiring muscle so gas diffuses across more quickly

Fish

• Oxygen is only moderately soluble in water
  • A given volume of air contains 30 times more oxygen than the same volume of water
• Fish are adapted to extract dissolved oxygen directly 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 lamella 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 countercurrent system
  • The countercurrent 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

Gas Exchange in the Gills of a Fish Diagram

The structure of fish gills and the countercurrent system within gills

Plants

• In order to carry out photosynthesis, plants must have an adequate supply of carbon dioxide
• There is only roughly 0.042% 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; a layer of tightly packed cells
  • Palisade mesophyll; a layer of elongated cells containing chloroplasts
  • Spongy mesophyll; a 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; a 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 diffuse rapidly into cells
  • The carbon dioxide is used up quickly 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

Leaf Structure Diagram

The structure of a leaf

Adaptations of Gas Exchange Surfaces