ViewWiki
ExploreLife

The world, a little closer

Cells.

Every living thing is built from cells, tiny bags of chemistry with the same working parts. Cut one open and look at what each part does.

Explore the scene Interactive article

01 / The experiment

A city in a bag.

Cut the cell open, pick a part to look at, then switch between animal and plant.

Interactive scene

Preparing your experiment…

The explanation below is available while the scene loads.

A generalised cell. Real cells vary enormously in shape and in which parts they emphasise.Organelles are drawn as simple shapes and far larger than life. The membrane is thinner than any line could show.

02 / Three organelles

Same parts, different jobs.

Two shared by every cell. One that only plants have.

Control01

Nucleus

The instructions live here. Copies of the parts in use are sent out to the rest of the cell; the originals stay put.

Job
Holds the DNA
Found in
Animal and plant
Look for
Largest body inside
Power02

Mitochondrion

Sugar and oxygen go in; usable energy comes out. Cells that work hard, such as muscle, are packed with them.

Job
Releases energy from food
Found in
Animal and plant
Look for
Bean-shaped, often many
Sunlight03

Chloroplast

Catches sunlight and uses it to build sugar out of air and water. Everything green owes its colour to these.

Job
Makes sugar from light
Found in
Plants only
Look for
Green, lens-shaped
The big idea

Every living thing is made of cells, and every cell runs on the same parts. A leaf and a fingertip differ mostly in which parts they have more of.

1 cell ∞ jobs

The smallest unit of life

Take any living thing, a moss, a whale, a mushroom, you, and look closely enough, and it resolves into cells. Some organisms are a single cell. Your body is about thirty trillion of them. A cell is the smallest thing that does everything we mean by alive: it takes in material, gets energy from it, builds new parts, responds to its surroundings, and copies itself.

Most cells are far too small to see. A typical one is a few hundredths of a millimetre across, so a line of a thousand would fit on a fingernail.

A bag with a skin

Every cell is wrapped in a membrane, a thin flexible skin. It is not a wall. It lets some things through, like water and oxygen, blocks others, and has gates that actively pump chosen molecules in or out. The membrane is what makes a cell an inside and an outside, and controlling that border is most of what keeps a cell alive.

Inside is a watery gel called the cytoplasm, crowded with molecules and with larger structures called organelles, each with a job.

The control centre

The nucleus is usually the largest organelle. It holds the cell’s DNA, the full set of instructions for building and running the cell. The instructions never leave; instead the nucleus sends out copies of the parts that are needed, and the rest of the cell reads them.

Try it: choose Nucleus, then Ribosomes. The instructions are written in one and carried out in the other.

Power and factories

Mitochondria are the cell’s power stations. They take sugar and oxygen and release the energy stored in them, in a form the rest of the cell can spend. A busy cell, like one in a muscle, can hold thousands.

Ribosomes are the factories. They are tiny, far smaller than a mitochondrion, and there are many thousands of them. Each reads a copy of a gene and strings together a protein to match. Nearly every part of the cell, and nearly every job it does, comes down to proteins built here.

What plants add

Plant cells have everything above, plus three things animal cells lack. A stiff cell wall outside the membrane gives the cell a box-like shape and lets plants stand up without bones. Chloroplasts catch sunlight and use it to make sugar from air and water, the process called photosynthesis, which is where almost all the food on Earth begins. A large vacuole, a bag of water that fills much of the cell, presses outward on the wall and keeps the plant firm. A wilting plant is one whose vacuoles have run low.

Switch Animal to Plant to see the wall, the chloroplasts and the vacuole appear, and the nucleus pushed to one side to make room.

How we know

Cells were discovered when the first microscopes were pointed at a slice of cork, and the tiny boxes seen there were named after the small rooms of a monastery. It took two more centuries to realise that every plant and animal is made of them, and that every cell comes from an earlier cell. That last idea is the one that ties the whole of biology together.