Gas exchange is a biological process through which different gases are transferred in opposite directions across a specialized respiratory surface. Gases are constantly required by, 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 (O
2) is required to enter cells, while waste carbon dioxide (CO
2) must be excreted; the opposite is true for photosynthesis, in which CO
2 enters plants and O
2 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.
Diffusion follows Fick’s Law. It is a passive process (no energy is required) affected by factors such as the surface area available, the distance the gas molecules must diffuse across and the concentration gradient.
Gases must first dissolve in a liquid in order to diffuse across a membrane, so all gas exchange systems require a moist environment.
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 specialized respiratory systems are required. This is the case with humans and with fish that have evolved circulatory systems: 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 oxygen does so more effectively. Carbon dioxide also dissolves in the plasma or combines with water to form bicarbonate ions (HCO−
3). This reaction is catalyzed by the carbonic anhydrase enzyme in red blood cells.
The main respiratory surface in humans is the alveoli, which 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, each ranging in diameter from 75 to 300 µm.
Inhaled oxygen is able to diffuse into the capillaries from the alveoli, while CO
2 from the blood diffuses in the opposite direction into the alveoli. The waste CO
2 can then be exhaled out of the body. Continuous blood flow in the capillaries and constant breathing maintain a steep concentration gradient.
During physical exercise, excess carbon dioxide is produced as a result of increased respiration, and muscles and cells require increased oxygen. The body responds to this change by increasing the breathing rate, maximizing the rate of possible gas exchange.
Gas exchange in plants is dominated by the roles of carbon dioxide and water vapor. CO
2 is the only carbon source for autotrophic organisms, making it essential for the conversion of light into sugar during photosynthesis. Due to the high differences in water potential in the plant versus the surrounding air, water vapor tends to evaporate from plants. Gas exchange is mediated through pores (known as stomata and located mainly on the lower side of leaves) that underlie a complex regulatory system. As the condition of the stomata unavoidably influences both the CO
2 and water vapor exchanges, plants experience a gas exchange dilemma: gaining enough CO
2 without losing too much water.
Gas exchange measurements are common tools in plant science. If the environmental conditions (humidity, CO
2 concentration, light and temperature) are fully controlled, the measurements of CO
2 uptake and water release reveal important information about the CO
2 assimilation and transpiration rates and the intercellular CO
2 concentration, which reveal important information about the photosynthetic condition of the plants.
Oxygen, 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 specialized 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 countercurrent maintains a steep concentration gradient. Oxygen is able to continually diffuse down its gradient into the blood, and the CO
2 into the water.
Summary of main systems
Insects such as crickets do not have an inner skeleton, so they exchange gases across structures known as trachea and tracheoles: tubes that run directly into the insect's body. Air enters the trachea through spiracles and diffuses into the respiring tissues.
Amphibians are able to use their skin as a respiratory surface. They also have lungs and sometimes gills can be used.
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