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Since gas exchange occurs by diffusion alone, single-celled organisms such as bacteria do not need any specialised structures to achieve it for them. This is because, being so small, diffusion occurs readily as their surface area to volume ratio is high.
The distance between an oxygen molecule which must be taken in, and the place it must get to within a bacterium is short enough for diffusion to be a viable way of exchanging substances with the environment, without the need for additional structures. In mammals, for example, oxygen cannot simply diffuse into our bodies. We are too large, have a low surface area to volume ratio, hence the diffusion pathway is too long. The only way we can achieve gas exchange is through our lungs which provide a large surface area and alveoli with short diffusion pathways.
In fact, the difficulty of gas exchange as single-celled organisms get larger is a factor which leads to larger organisms being multicellular.
Before this goes any further, a few clarifications:
1. Gas exchange is central to life. Oxygen is needed in respiration* which generates usable energy without which life wouldn’t exist. Removing the resulting carbon dioxide is crucial too.
2. Water can be a gas too, in the form of water vapour. This may in certain organisms escape with the air, so water preservation versus gas exchange is always an important thing to bear in mind. This is especially important when talking about insects and plants.
Insects have a tracheal system made up of many tracheae which branch into smaller tracheoles. All tracheae connect to the exoskeleton of the insect, so that air diffuses in and out through the spiracles.
The technical terms highlighted above are important in describing what really is just a bunch of holes and tubes. Here’s a video that describes what happens. Don’t worry about the overly detailed labels. Just enjoy the smooth ride of a video!
In order to balance the opposing needs for conserving water and obtaining oxygen, insects are able to close their spiracles, as well as contract their abdomens. The former prevents water loss, while the latter enhances ventilation so that more oxygen gets inside their body.
Fish extract dissolved oxygen molecules from the surrounding water. The oxygen content of water is much lower compared to air, so fish have special adaptations which enable them to make the most of the available oxygen. These adaptations are gills.
1. Gill filaments have lamellae which increase the surface area available for diffusion, while keeping the diffusion pathway short.
2. The water flow through the fish’s mouth as well as the blood in gill capillaries follow the countercurrent principle. As seen in the above diagram, water and blood flow against each other, rather than along each other. This ensures that oxygen diffusion can take place along the whole length of the flow, not just for half of it – before the concentration of oxygen in the blood and in the water becomes equal.
This is easily exemplified (and an acceptable form of explanation in an exam) by a number table. The upper row is the oxygen concentration in the blood, while the lower is the one in the water. Along the flow, oxygen enters the bloodstream from the water, so that the concentration in blood increases, while the concentration in water decreases.
Here’s a video which explains nicely how fish carry out gas exchange:
Like insects, plants must meet the opposing demands of water retention and gas exchange. The site of photosynthesis in plants, as well as the gas exchange site, is the leaf. This is what a section through a leaf looks like:
1. The mesophyll cells are surrounded by quite a lot of empty space for air to mingle around, providing plenty of surface area for gas exchange by diffusion.
2. Air with its carbon dioxide (necessary for photosynthesis) enters the leaf through the stomata. Stomata are holes on the leaf surface, made by the guard cells. They can open and close depending on environmental factors such as humidity, temperature and wind. This controls the amount of water loss. Oxygen, the byproduct of photosynthesis, also leaves the leaf through the stomata.
*Biology is basically an endless sequence of asterisks. Sigh. Oxygen is needed in aerobic respiration, as opposed to anaerobic respiration. Biology is a beautiful, flexible and volatile thing with endless possibilities. On Earth, by and large, respiration requires oxygen. But is it possible to have respiration without oxygen? Certainly. Now imagine what there is out there, in the universe.