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
Examiner Tip
Make sure you know how and why each system above is adapted for efficient gas exchange.