Cells

The little creatures that rule the world.

Drifting through the warm ocean waters, the Portuguese Man-o’-War is no ordinary jellyfish. It is not a jellyfish at all. In fact, it may not even be an ‘it’, but a fusion of many different creatures living in a tight-knit colony.

Those creatures are called ‘polyps’.

Sea-anemones are also polyps, as are the corals that live in huge reefs in the sea. But these polyps are a special group of four types that have become so used to living together that they can never survive alone.

The Portuguese Man-o’-War is a group of polyps. The floaty sail on top is one polyp, and the long poisonous tentacles sticking down are other polyps. And then there are the group of polyps whose only job is to digest food and share its nutrients with the others.

Finally, the last kind of polyps have the job of generating new polyps, either to add to the community or to start a whole new one.

Does it seem unusual that so many different creatures have come together and taken on specialised jobs, to work together and act as one? Actually, it happens all the time — though on a much smaller scale.

Every living being you see is made up of billions of smaller creatures, each designed for only one specific task but still very much alive on its own. (Some living beings are made up of only one or a few of these sub-creatures, not billions. But those beings are so small that you don’t usually see them.)

One of the first humans to see these sub-creatures was the scientist Dr. Robert Hooke. Having made the most powerful microscope of his time, Dr. Hooke proceeded to train it onto many different objects — revealing a world of tiny details that he had never seen before.

One of the things Dr. Hooke saw was tiny chambers that plants seemed to be divided into. He called the “cells”, because they reminded him of the small rooms called cells that monks used to live in. But these chambers were so tiny, a small bit of plant would contain more of them than the number of all the monks in the world.

Nowadays, we know that all life on this planet is made up of cells. Not exactly the same cells Dr. Hooke saw, but cells that are specialised to do their own things. The first life-forms were single cells, just like the bacteria of today. And most cells still do work very much like that bacteria of today.

So let’s go and take a look at those bacteria.

The E. coli, or Escherichia coli in full, is a typical example of a bacterium. You can see the “cell membrane”, which is the thin layer in place of skin that separates the inside of the bacterium from the outside world. The membrane is not exactly skin, because it lets useful stuff seep into the body and not-so-useful stuff soak back out.

That’s why cells don’t need to have mouths: because they can eat through their whole body.

Outside of the cell membrane re the whiplike ‘flagella’, which allows the E. coli to beat its way through the water. In fact, the original flagellum is a Latin word that we can probably translate as “whiplet”, because it’s related to the word flagrum that means “whip”. Not all cells — or even all bacteria — have whiplets. Like polyps, some of them don’t need to move. And, some have other ways to get about.

Within the cell membrane is a watery liquid, filled with lots of complex chemicals. This is where most of the stuff happens. It could well be called the “soup of life”.

We don’t know when it’ll happen. It could be after a billion years, when we’re all dead and gone; it could also be tomorrow. But some day, Betelgeuse, the red start in constellation Orion, is going to explode into a supernova. It will then shine so bright that it’ll even compete with the full moon — in spite of being fifteen billion times as far away.

For a big star like Betelgeuse, the supernova is the last stage of death. But this explosion will also release the chemical elements into space, the things which most of the things we see— including living creatures — are made of.

There are many different kinds of chemical elements, or “atoms”, which react with one another in different ways. They may join together into “molecules”, or separate into smaller bits, or go and re-join in different ways. Those reactions are called “chemical reactions”, because they are done by chemicals.

Most of the elements in the world were created inside the fiery hearts of stars, but many of them join together to make up the materials we see today. If you look at anything with a strong enough microscope — water, paper, asteroids, or even cells — you’ll see that they are all made up of billions of atoms joined together.

Atoms that were released in a fiery explosion, when a giant star died, billions of years ago.

When a termite hill is damaged, an army of workers immediately comes in to repair it. Of course, they could all wait until grains of mud fell over the cracks, sealing them over time. But that’ll take way too long. That’s why they set to work, speeding up the process and placing mud where it actually needs to go.

If a cell were a termite-hill, then ‘enzymes’ would be the termites.

Chemical reactions happen anyway, but enzymes are what make the useful ones happen faster, and at the right time. Unlike termites, different enzymes work on different kinds of jobs. Most of the time, they just float around inside the cell until a molecule they recognise drifts by. Then, they go to that molecule and work on it.

For example, the ‘maltase’ enzyme works only on the ‘maltase’ molecule, breaking it into two pieces of energy-giving glucose. A lot of enzymes are named after the specific molecules that they work on.

Some enzymes can go outside their cells, too. Bacteria like E. coli send them out when there’s food nearby. The enzymes then “pre-digest” the food by breaking it down into smaller parts, so it can pass through the cell wall and into the bacterium. And that’s how E. coli eats!

How do all the enzymes get there in the first place? Animals get some of them from the food they eat. That’s why it’s good to eat food raw, because when you cook it, the enzymes get spoilt and you’ll have to make them all over again. Which gives us the answer of how the other enzymes get there: because they are made there.

Sitting in the middle of the termite hill is the Queen Termite, forever laying eggs for new termites to hatch out of. Floating in the middle of the cell is no Queen Enzyme, but a long, coiled-up strand of deoxyribonucleic acid.

Deoxyribonucleic acid may have a long name, but it has an even longer list of instructions. It has bit-by-bit instructions of how to make enzymes — not just one enzyme, but all the enzymes the cell will ever need to make, and more besides.

There’s no way of making the enzyme-making instructions shorter, but luckily, there is a way of making the deoxyribonucleic-acid name shorter. Most people know it as DNA.

Enzymes can be made by taking amino-acids, a special kind of molecule, and joining them together in the correct way. The order in which to join them is what’s written in the DNA, and there’s a special enzyme to walk down and read it. The Princess Enzyme (as it is not called) goes down the DNA and copies out the instructions for any single enzyme — like copying a single recipe from a recipe-book.

Then, there is the ‘ribosome’, probably the most amazing enzyme of all. Supported by an army of helpers, the ribosome walks across the copied-down instructions, reading what the correct amino-acids are and stringing them together. When it’s done, that string will be a brand-new enzyme.

So that’s how a cell works. But how do cells work? For them to be “cells”, there has to be more than one of them.

For simple cells, that’s easy. All they have to do is grow bigger, and then slowly split into two like marimo balls.

Of course, they both need to have a copy of the DNA. That’s why, like the princesses, there are also Prince DNAs called ‘DNA polymerase’. Instead of copying down a single recipe, they copy the whole book.

Nobody knows where the mitochondria first came from. It’s likely that the small bacteria either invaded or got swallowed by a larger one, and the arrangement happened to work out for them both. However it happened, this fusion turned into an entirely new kind of cell: the Eukaryote.

Eukaryotes are named for the nucleus — karyon in Greek — which they have inside to protect their DNA. But more important are their mitochondria, which provides energy for the cell to grow and sustain its structure.

A mitochondrion is a kind of cell that lives in its own little world inside other cells. Mitochondria have their own DNA and divide on their own schedule. But eukaryote cells could never live without them, because they are also powerhouses of energy. (Of course, other bacteria could produce their own energy too, but not as efficiently as the mitochondria.)

Eukaryotes liked to organise their insides into different compartments. Each compartment would focus on one specific thing, like synthesising chemicals or breaking down food. Then, suddenly, the eukaryotes began to specialise themselves as well.

The Slime Mold begins its life as a small spore sticking out of the ground, when the soil turns moist and the conditions seem just right. This spore soon grows into a single-celled eukaryotic creature, feeding on bacteria in the soil and on rotten logs. Its flexible body allows it to wrap itself round its prey, swallowing it in with its whole body.

But the interesting part happens when food runs short.

When food is low, each slime mold cell sends out an alarm signal, making all the creatures rush together…where they combine to form a single, slug-like creature! This creature can now move around faster in search of food, shifting its shape and extending arm-like pseudopodia as it gropes around.

Again, when the time is right, the cells rearrange to form a fruit. Some cells sacrifice themselves to make stiff stalks. They hold up the other cells, which become round balls, balls of hardened spores. Now, they are ready to blow in the wind and start the cycle all over again.

One of the most amazing things about the slime mold is that there’s no Queen Cell in the middle telling everyone what to do. They just organise themselves.

On second thoughts, it’s not all that amazing, because it happens in multi-celled creatures all the time.

The difference between slime molds and true multi-celled creatures is that all the cells in a slime mold are of the same type. Multi-celled creatures can have many different kinds specialised for different things: blood cells, root cells, brain cells, leaf cells, and even two different kinds of light-detecting cells in your eye. The cheetah’s sprint is powered by the contraction of its muscle-cells; water-carrying xylem cells are what allow the hibiscus to grow.

Some insects have two whole sets of cells: one for when they’re young, and one for when they turn into adults.

But the difference is not actually that great. Every cell has DNA-instructions for all the other cells in the body as well as its own. Parts of this DNA can be switched off, so the cell becomes only what it’s supposed to be and not anything else.

Though they’re the ones we see most, multi-celled creatures are still far outnumbered by the single-cell microbes. Even today, it’s the single cells that rule the world.

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