Due to the presence of a waterproof layer, gas exchange is not possible across the whole surface of plants. Gas exchange in plants occurs through stomata, which are positioned on green stems and the underside of leaves. Like gas exchange in protozoa and humans, it occurs by diffusion. However unlike in humans and protozoa, carbon dioxide diffuses in and oxygen diffuses out. In woody plants, gas exchange occurs across stomata in leaves and also in lenticles, which are loosely packed bark cells.
Although humans have large exteriors, gas exchange only occurs in the respiratory system, which is situated deep in the thorax. Gas exchange occurs in the lungs, a pair of organs encased by the rib cage. As in protozoa and plant systems, human lungs have high surface area to volume ratios to allow maximum exchange of gases (diffusion can occur across the whole surface of protozoa and plants have thin leaves, which have high surface area to volume ratio.) In humans, alveoli are small (100μm in diameter) and surrounded by very narrow capillaries, through which red blood cells are squeezed as they pass along them. This narrow passage reduces the speed at which red blood cells pass through alveoli hence allowing more oxygen gas to be absorbed in a given amount of time.
In all of the three organisms, the gas, which enters it is soon dissolved on moist surfaces so that it can be transported more easily. In plants, the carbon dioxide dissolves on the moist surfaces of cells in the spongy mesophyll layer and moist layers of root hair cells. In protozoa diffusion is passive across a plasma membrane. The oxygen dissolves in the cytoplasm of the cell. In humans, oxygen gas dissolves in the moisture of alveolar epithelium and diffuses across this and the endothelium of the capillary into the erythrocyte. The red blood cells contain haemoglobin, to which the oxygen combine forming oxyhaemoglobin. Carbon dioxide diffuses from the blood into the alveolus, so that it can leave the lungs in expired air.
Gas exchange is possible through only one route in humans but can enter plants and protozoa in more than one way. Oxygen can either diffuse into plant through the root hair cells from air spaces in the soil, through lenticels in the bark or through the stomata in the leaves. In a similar way, protozoa allow gas exchange over the entire surface of their cell. Gas exchange only occurs across alveoli in the lungs in humans.
The human respiratory system is more specialised than the systems present in the other two organisms. The lungs only allow exchange of gasses, whereas in plants the cells used for gas exchange are not. Water is also able to diffuse by osmosis and minerals and water can also diffuse across the root hair cells. In protozoa, ammonia, the waste product also leaves the cell across the plasma membrane.
The respiratory system in humans in protected, whereas those in plants and protozoa are not. The lungs protect the delicate lungs from being punctured. Unlike the respiratory systems in plants and protozoa, humans only have two lungs, whereas the other organisms have several million ways of dissolving into them.
A high surface area to volume ratio makes gas exchange efficient in protozoa in relation to their size and hence rate at which oxygen is consumed. Humans have, in average of 700 million alveoli, which cover an area of approximately 90m2, this also makes gas exchange efficient. Plants have several million root hair cells and stomata, hence also making gas exchange efficient in relation to their size.
