Gas exchange is a biological process through which different gases are transferred in opposite directions across a specialised respiratory surface. Gases are constantly required and produced as a by-product of cellular and metabolic reactions so an efficient system for their exchange is extremely important. It is linked with respiration in animals, and both respiration and photosynthesis in plants.
In respiration, oxygen (O2) is required to enter cells whilst waste carbon dioxide (CO2) must be removed – the opposite is true for photosynthesis, where CO2 enters plants and O2 is released. The exchange of gases essentially occurs as a result of diffusion down a concentration gradient – gas molecules moving from an area of high concentration to low concentration.
- The surface area available
- The distance the gas molecules must diffuse across
- The concentration gradient
In single-celled organisms diffusion can occur straight across the cell membrane; as organisms increase in size so does the distance gases must travel across. Their surface-area-to-volume ratio also decreases. Diffusion alone is not efficient enough and specialised respiratory systems are required. This is the case with humans and fish where circulatory systems have evolved: These are able to transport the gases to and from the respiratory surface and maintain a continuous concentration gradient.
Both oxygen and carbon dioxide are transported around the body in the blood – through arteries, veins and capillaries. They bind to haemoglobin in red blood cells although this is more effective with oxygen. Carbon dioxide also dissolves in the plasma or combines with water to form bicarbonate ions (HCO−
3). This reaction is catalysed by the carbonic anhydrase enzyme in red blood cells:
The main respiratory surface in humans are the alveoli. Alveoli are small air sacs branching off from the bronchioles in the lungs. They are one-cell thick and provide a moist and extremely large surface area for gas exchange to occur. Capillaries carrying deoxygenated blood from the pulmonary artery run across the alveoli - they are also extremely thin so the total distance gases must diffuse across is only around 2-cells thick. An adult male has about 300 million alveoli which range in size from 75 to 300 microns in diameter.
Inhaled oxygen is able to diffuse into the capillaries from the alveoli, while carbon dioxide from the blood diffuses in the opposite direction into the alveoli. The waste carbon dioxide can then be exhaled out of the body. Continuous blood flow in the capillaries as well as constant breathing maintains a steep concentration gradient.
During physical exercise excess carbon dioxide is produced as a result of increased respiration: This must be removed and muscles and cells require increased oxygen. The body responds to this change by increasing the breathing rate, therefore maximizing the rate of possible gas exchange.
Gas exchange in plants is dominated by the role of two gases: carbon dioxide and water vapor. CO2 is the only carbon source for autotrophic organisms and therefor essential for the conversion of energy to sugar during photosynthesis. Due to the high differences in water potential within plant versus surrounding air, water vapor tends to evaporate from plants. Gas exchange is mediated through pores mainly on the lower side of leaves known as stomata which underlie a complex regulatory system. As the stomatal condition unavoidably influences both, the CO2 and water vapor exchange, plants exhibit a gas exchange dilemma: gaining enough CO2 without losing too much water.
Gas exchange measurements are common tools in plant science. Under full environmental control (humidity, CO2 concentration, light and temperature) the measurements of CO2 uptake and water release reveal important information about the CO2 assimilation rate, transpiration rate, the intercellular CO2 concentration et cetera. Thus important information about the photosynthetic condition of the plants.
O2 essential for respiration during the night plays a minor role in plants gas exchange as it is always present in sufficient amounts.
Fish must extract oxygen dissolved in water, not air, which has led to the evolution of gills and opercula. Gills are specialised organs containing filaments and lamellae – the lamellae contain capillaries and provide a large surface area and short diffusion distance as they are extremely thin.
Water is drawn in through the mouth and passes over the gills in one direction while blood flows through the lamellae in the opposite direction – this counter current maintains a steep concentration gradient. O2 is able to continually diffuse down its gradient into the blood and CO2 into the water.
Summary of main systems
|Large surface area||Short diffusion distance||Maintained concentration gradient|
|Human||Total alveoli = 70-100m2 ||Alveolus + Capillary = 2-cells||Constant blood flow in capillaries; breathing|
|Fish||Many lamellae and filaments per gill||Usually 1-cell||Counter-current flow|
|Plant||High density of stomata; air spaces within leaf||1-cell||Constant air flow|
Insects such as crickets do not have an inner skeleton so exchange gases across structures known as trachea and tracheoles: These are tubes that run directly into the body of the insect. Air enters the trachea through valves known as spiracles and diffusion can then occur straight into the respiring tissues.
Amphibians are able to use their skin as a respiratory surface, as well as having lungs and sometimes gills.
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