How are organisms adapted for efficient gas exchange? [25 marks]
Many organisms have different features which enable them to survive and carry out gas exchange effectively. Single-celled organisms, such as bacteria and protozoa, are in constant contact with their external environment. For them, gas exchange occurs by diffusion across their membranes. Even in simple multicellular organisms, such as green algae, their cells may be close to the environment, and gas exchange can occur easily. In larger organisms, for example animals, gas exchange follows the same general pattern as in plants. Oxygen and carbon dioxide move by diffusion across moist membranes. In simple animals, the exchange occurs directly with the environment. But with complex animals, such a mammals, the exchange occurs between the environment and the blood. The blood then carries oxygen to deeply embedded cells and transports carbon dioxide out to where it can be removed from the body. In humans the gas exchange organ system is known as respiratory or breathing system. The actual respiratory surface is on the alveoli inside the lungs. An average adult has about 600 million alveoli, giving a total surface area of about 100m², so the area is huge. The walls of the alveoli are composed of a single layer of flattened epithelial cells, as are the walls of the capillaries, so gases need to diffuse through just two thin cells. Water diffuses from the alveoli cells into the alveoli so that they are constantly moist. Oxygen dissolves in this water before diffusing through the cells into the blood, where it is taken up by haemoglobin in the red blood cells. Haemoglobin is an oxygen carrying protein which is made up of 4 ‘Haem’ groups which each bind to 1 oxygen molecule. Red blood cells are packages of haemoglobin together with a couple of enzymes that help haemoglobin load and unload oxygen effectively. ***** The steep concentration gradient across the respiratory surface is maintained in two ways:...
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