Organisms exchange substances and heat with their environment all the time, and this possibility is crucial to survival. The specific way in which this is achieved is very tightly related to the shape and structure of the specific organism, as well as its environment. For example, unicellular organisms are so small that molecules such as oxygen and water can readily diffuse in and out via the membrane, due to the short diffusion pathway. Could this be achieved by a human, or even a bee? No – they are simply too big.
Two properties are important to consider here: the volume of an organism, and the surface area of an organism. The volume is what determines the amount of substances which need exchanging, while the surface area determines the amount which can be exchanged.
Key principle: as the size of an organism increases, the surface area to volume ratio decreases.
That might seem hard to really understand. Why use a ratio in the first place? Well, the ratio shows the relationship between surface area and volume ratio, i.e. how similar or dissimilar are they?
Surface area = 12 x 6 = 6
Volume = 13 = 1
Surface area : volume = 6:1 = 6.00
Surface area = 22 x 6 = 24
Volume = 23 = 8
Surface area : volume = 24:8 = 3:1 = 3.00
3 is smaller than 6, so as the cube/organism gets larger, the surface area to volume ratio decreases.
What this basically means is that the larger an organism gets, the less surface area is available to serve its increasing needs due to its increasing volume. So what adaptations do larger organisms have to cope with the large demand for substance and heat exchange?
For one, mere diffusion directly into and out of the organism is not possible. Insects, for example, have a system of tubules which distribute air within the body so that it reaches all the different parts. Mammals have lungs and blood vessels. Fish have gills. All these are systems are specifically aimed at making it possible to exchange substances such as oxygen, carbon dioxide, nutrients, as well as heat, between an organism and its environment.
In fact, the reason behind insects’ limited size is that their tubule system can only work for those sizes. Otherwise, we might just have gigantic wasps flying around. Be thankful for surface area to volume ratio!
Having mentioned diffusion as the gas exchange method of choice in smaller organisms, let’s look into more detail at three such examples: amoeba, flatworm and earthworm. Now if you think the earthworm and flatworm are kind of the same thing, oh my god you’re so wrong.
Prettyyyyyyyyyyyyyyyyy. The flatworm lives in water or wet environments such as leaf litter, and has its flattened body in order to maximise diffusion across its body. The minimised diffusion pathway ensures it is quick enough to enable good gas exchange.
Earthworms do not live in water. Still, they carry out diffusion across their body surface area by producing a mucus which carries the diffusion gases from the environment to their tissues and back. This is why they don’t fare well if they dry out!
Finally, the amoeba is a much smaller, often microscopic organism that also does not have specialised gas exchange organs, relying on diffusion alone. It’s a single cell.
In fairness, it has a shape slightly resemblant of the flatworm.