Gas Exchange

• A high surface area to volume ratio ensures high rate of exchange such as heat loss
• Larger animals with a lower surface area to volume ratio need a system of mass transport and specialised organs for gas exchange
• Gas exchange surfaces are very thin to shorten diffusion distances and selectively permeable
• Internal gills are protected from damage and dehydration
• Single celled organisms have a high surface area to volume ratio meaning they can exchange gas through the cell surface membrane down a concentration gradient
• Birds methods of gas exchange are unidirectional making them more efficient as inhaled gas will not mix with exhaled gas which would reduce the concentration gradient
• Multicellular organisms such as insects, fish and mammals require a method of gas exchange due to their higher metabolic rate and oxygen demand
• Gas exchange in plants is entirely by diffusion in leaves due to the high surface area to volume ratio and the short diffusion pathway
• In leaves, oxygen and carbon dioxide diffuse through air spaces in the leaf providing easy access for palisade and mesophyll cells
• The stomata are mainly found on the bottom of the leaf to prevent excessive evaporation
• Guard cells on stomata become turgid when the plant has enough water, causing the stomata to open
• When the plant does not have sufficient water, the guard cells become flaccid, closing the stomata
• Stomata are closed at night to prevent unnecessary water loss as photosynthesis will not occur without light
• Stomata also tend to close partially at midday as this is the hottest time of day so water loss will increase
• Xerophytes are plants living in dry environments meaning they need to prevent water evaporation more urgently
• Xerophytes may have a thicker cuticle than a normal plant to prevent evaporation
• Xerophytes may roll their leaves, have stomata in pits or hairy stomata to decrease the water potential gradient between the atmosphere and the stomata, preventing water evaporation locally
• Xerophytes may have spines to reduce their surface area to volume ratio, reducing water evaporation
• There may be less stomata overall or on the top of the leaves to prevent evaporation
• Insects are small but have high metabolic rates, especially those who can fly
• Gas exchange in insects will occur through spiracles which are connected to the trachea and then the tracheoles which each connect to an individual cell
• At the ends of the tracheoles, there is water to allow gas exchange across the cell surface membrane of each cell
• When insects undergo high rates of activity, they will respire anaerobically, releasing lactic acid and lowering the water potential gradient inside the cell
• When the water potential inside the insect cell is lower than the tracheoles due to lactic acid buildup, water diffuses into the cell via osmosis, reducing the diffusion distance between the air and the cell
• Gas exchange in resting insects occurs through concentration gradients, oxygen in the cells is low, causing oxygen to diffuse across the membrane down a concentration gradient, carbon dioxide concentrations increase in cells, thus more carbon dioxide will diffuse into the atmosphere
• Rings of chitin line the tracheoles of insects to prevent them from collapsing
• Insects undergoing high activity will ventilate, pumping muscles in the thorax and the abdomen
• In insect inspiration, the thorax spiracles are open and the abdominal spiracles are closed, the abdomen will contract, increasing the volume in the thorax and decreasing the pressure, this causes air to move in down a pressure gradient
• In insect exhalation, the thorax spiracles will close and the abdomens spiracles will open; the abdomen will relax, increasing the abdominal cavity's volume and decreasing its pressure, air will move down a pressure gradient into the abdomen and out of the spiracles, flooding the trachea with air
• Insect ventilation is unidirectional, increasing efficiency
• Fish have specialised gills which water flows over, exchanging gases with the countercurrent blood flow
• A countercurrent blood flow ensures that there is always a concentration gradient maintained between the lamellae and the bloodstream over the entire lamellae so diffusion is constant
• The gill bar has gill rakers to separate debris from water flowing over the gills
• The gill bar has many gill filaments to increase surface area, on each gill filament, there are gill lamellae also known as gill plates with a good blood supply to maintain concentration gradients
• The gill lamellae are thin, keeping a short diffusion distance
• In parallel blood flow, dynamic equilibrium is reached between water and blood meaning blood has less oxygen association making it less efficient
• Fish inspiration begins with the mouth opening and the bottom of the mouth contracting, increasing the volume of the buccal cavity
• The increase in volume in the buccal cavity decreases the pressure, causing water to enter the buccal cavity down a pressure gradient
• Once water has entered the buccal cavity, the mouth shuts and the bottom of the mouth relaxes, increasing the pressure by decreasing the volume
• The operculum will open as the buccal cavity closes, increasing the volume and decreasing the pressure in the opercular cavity, this causes water to flow over the gills where gas is exchanged and out of the operculum down a pressure gradient