How Large Can a Bacteria get? Life & Size 3

In and out, in and out, staying alive is about doing things this very second. Your cells are combusting glucose molecules with oxygen to make energy available, which keeps you alive for another precious moment. To get the oxygen to your cells, you're breathing. Breathing is an answer to a very hard problem: how do you get the resources that your cells need to survive from the outside to the inside of your cells?

Every living thing has to solve this problem, and the solution is surprisingly different depending on one of the most important regulators of life: size. As we've discussed in other videos, at different scales, the physical laws of the universe have different consequences for its inhabitants. Simple things like temperature, microgravity, or surface tension might not matter to you or be a deadly danger depending on how big you are.

Living things need a lot of different materials to keep themselves going, and they somehow need to transport them from the outside to the inside. This was a huge problem for the first things on the verge of being alive because doing anything in our universe requires energy, and the first living beings on Earth did not have the abundance of tools and techniques available that life has today after billions of years of evolution.

So at the very beginning, life needed to find a way to get good stuff inside and bad stuff outside of itself without using energy. Luckily, the very first forms of life were very, very small, and because they were so small, they were able to use a free form of transport that was based on a physical law called diffusion. Diffusion is the rule of the universe that molecules, especially in liquids or gases, are constantly moving around in all directions, and because they move around and bump into each other and other molecules, they tend to spread.

For example, if you drop a sugar cube into water, then there is a lot of sugar in one place and in another place there's none. As sugar molecules dissolve in the water, they will start randomly bumping against the water molecules and other sugar molecules. Slowly, all the sugar molecules will spread out and form multiple phases of different concentrations. These random movements continue endlessly until at some point the sugar will be spread evenly in the water.

The great thing about diffusion is that life can use it for free; it doesn't require energy, and life loves free things. So all life on Earth relies on diffusion. Let's look at the smallest living being on Earth: a bacterium. Specifically, its surface cell membranes allow for diffusion of certain molecules. This specific bacterium consumes oxygen to live while carbon dioxide is produced inside as a waste product.

So inside the bacterium, there isn't a lot of oxygen but a lot of carbon dioxide. Because of diffusion, these molecules will eventually spread evenly. So the carbon dioxide diffuses out, while oxygen is constantly replenished from the outside. But this kind of breathing only works for the very small world: for bacteria, amoeba, or your cells. A few very small animals, such as insects, for example, have a fine network of trachea tunnels with a pressure gradient where air very slowly can diffuse in and exchange gases with the insect cells.

But even insects seem to be able to contract their trachea, and at least some even have specialized breathing organs like spiracles and air sacs. At certain scales, diffusion is just too slow to keep cells alive. The fundamental problem is that the exchange with the environment can only happen at the surface, and diffusion of materials can only sustain a certain amount of inside. Tiny living things have only a little bit of inside or volume and a lot of outside or surface area.

But what if we wanted to create a bacterium the size of a blue whale and had a very convenient enlargement machine? We would sadly be messed up by the square-cube law. In a nutshell, it means that if you make something 10 times larger, its outside or surface would grow by 100, but its insides or volume grow by 1,000 times.

If we compare the bacterium Pseudomonas syringae with a blue whale, we see that the bacterium has 10 million times more surface in relation to its volume than the whale. The bacterium has a lot of outsides, while the whale has a lot of insides. If we make a bacterium the size of a whale, our giant bacterium now has too much insight, and most of its inside is now very far from its surface. The oxygen our bacterium needs would never reach the inside before it would run out of oxygen. Our giant bacterium would just die.

Still, being bigger has many upsides, from making it harder to be eaten to making it easier to eat others. But the size of cells is limited by the distance oxygen and nutrients can effectively diffuse to provide the inside with enough resources. So life avoided this problem by forming multicellular structures—beings composed of many cells instead of one.

Because diffusion works better if you have many small units instead of one much bigger one, over time the cell bodies began to share work and specialize. Some cells concentrated on sensing the environment, others on digestion, and others on movement. But that still wasn't enough; the problem of diffusion, surface, and energy production remained limiting the size these first multicellular forms of life could attain.

So in order to become even bigger, life solved the diffusion problem by having holes, caves, and tunnels and by folding in on itself so diffusion could happen easily in each one of the cells. Take yourself: what you consider your outside. Your skin has a surface area of about 2 square meters, but your lungs have a surface area of about 70 square meters. They aren't like balloons; they're more like sponges, filled with many tightly packed tiny balloons surrounded by blood vessels.

When you breathe in, all these tiny balloons fill up with fresh air. Blood filled with CO2 is pumped around the balloons, and then the magic of diffusion happens. The oxygen diffuses into the blood, where it's picked up by red blood cells, and the CO2 diffuses out of the blood and into your lungs where it can be breathed out again. Your blood then carries oxygen-rich blood into the furthest corners of your body and picks up the CO2 waste.

Diffusion in the body is effective at about 1 mm, so every cell in your body is at most 1 mm away from a blood vessel. So medium-sized animals like you need a lot of blood vessels to reach every cell in the body. Your body has around 100,000 km of capillaries alone, the tiniest of your blood vessels, with a surface area of around 1,000 square meters.

This is true for all parts of you that want to exchange something with the outside world. Your body needs surfaces to take in nutrients from your food, so your gut has the surface area of half a badminton court, roughly 40 square meters. The larger you are, the more hidden surfaces you need. Take a tree; its way to stay alive is to create sugar out of thin air and sunlight, so it needs as much surface area as possible.

An orange tree with 2,000 leaves has a leaf surface area of 200 square meters, but the surface inside the leaves where diffusion actually occurs is 6,000 square meters. The same with roots, where water diffuses from the soil into countless tiny hairs that maximize the surface area. The roots of 1 square meter of grass add up to around 350 square meters of surface.

If we look at the breathtaking diversity of life on this planet, it seems like everything is pretty different, and it is. But some basic principles are the same for everybody and have not changed significantly for billions of years. If we look at the very, very small or the very, very big, waste goes out and fresh fuel comes in. Big animals just need a lot of complex plumbing to make it possible.