B1 N) Exchanging Substances – Part 1
However, multicellular organisms, like humans, have a very low surface area to volume ratio. This would mean that if left to diffusion, less than sufficient quantities of these substances would diffuse across membranes into or out of cells to allow these organisms to survive. Therefore, multicellular organisms have transport systems that go around the organism to take substances to where they are needed (an example of a transport system is the bloodstream). Multicellular organisms also have exchange surfaces where substances are exchanged from one place to another. These exchange surfaces are designed for efficient diffusion and have the following characteristics:
- A Large surface area so that lots of diffusion can take place at the same time
- A thin membrane/ short distance, which provides a short diffusion pathway, thus increasing the rate of diffusion
- Animals have efficient blood supply with lots of blood vessels so substances can move into and out of the bloodstream very quickly
The alveoli are found in the lungs, and they are responsible for gas exchange. There are two different gas exchanges taking place in the alveoli; one is that oxygen diffuses from the alveoli into the bloodstream, and the other is that carbon dioxide diffuses from the bloodstream into the alveoli. There are millions of alveoli in the lungs. There are two diagrams below of what the alveoli look like.
The alveoli have the following characteristics to ensure that sufficient gas exchange takes place:
> An extremely high surface area – each of the alveoli have a shape that has a very high surface area. Also, there are millions of alveoli in each of our lungs. The very high surface area allows for very fast diffusion as lots of gases can diffuse at once. The surface area for the alveoli in humans is somewhere between 60 m2 to 100 m2
> A short distance between the alveoli and the capillary (bloodstream) – the distance is usually only 2 cells (one cell for the outside of the alveoli and the other for the capillaries). This short distance increases the rate of diffusion.
> A good blood supply – a network of capillaries goes very close to all of the alveoli which means that there is plenty of surfaces for the gases to exchange across. Also, the blood moves through the capillaries quickly, which ensures that the concentrations of the gases between the alveoli and the capillaries are different
> The alveoli also have a moist lining – this moist lining dissolves gases, which increases the rate of gas exchange/ diffusion in the alveoli
We take in soluble products of digestion into the bloodstream in the small intestine. The soluble food molecules are then taken around the body through the bloodstream. The walls of the small intestine are covered in loads of villi that are covered in loads of microvilli. Below is a diagram of the villi and microvilli.
The villi are highly adapted so that they can absorb as many food molecules as possible. One of their main adaptions is that they have a very high surface area. Each villus is around 1-2 mm long and it is estimated that the surface area of all of the villi in the small intestine is around 300 m2.
There is a network of capillaries in each of the villus. There is a single layer of surface cells in between the capillaries and the small intestine. This short distance means that diffusion of food molecules into the bloodstream happens at a fast rate.
There is also a good blood supply through these capillaries that helps to ensure the quick absorption of food molecules from the small intestine into the bloodstream.
Sometimes there will be a greater quantity of food substances in the bloodstream compared with the villi. This would mean that if left to diffusion, food molecules would diffuse from the bloodstream into the small intestine. The villi ensures that this does not happen by engaging in active transport. Active transport is the net movement of particles against a concentration gradient (low to high). Energy is required in order for active transport to take place. This energy is obtained by respiration reactions. Mitochondria are responsible for respiration reactions, and we can see from the close up of a villi cell that villi cells contain lots of mitochondria. Click here for more information about active transport and what happens in the small intestine.