Unit 1 · Cells
Cell structure
Every living thing is made of cells. Before we can understand how life works, we have to know what a cell is made of — and what each part does.
Quick vocabulary
Here are three words you will see a lot in this unit. Hover over any underlined word on this page to see a quick definition.
- mitosisCell division — one cell becomes two identical cells. — what cells do when they split in two.
- cell-membraneThe flexible outer boundary of every cell — controls what enters and leaves. — the outside wall of every cell.
- diffusionThe spreading of particles from where there are many to where there are few. — how small things spread out on their own.
Short definition: cell membrane
If someone asks you what a cell membrane is, you can say: The outside wall of a cell. It decides what goes in and out.
Longer explanation: mitosis
A full explanation of how cells make new cells:
Mitosis is how your body makes new cells. A cell splits into two new cells. Both new cells are exactly the same as the starting cell. They have the same DNA.
Your body uses mitosis all the time. Your skin makes new skin cells. Your stomach makes new stomach cells. If you cut your finger, mitosis makes new cells to fix it.
Mitosis has four main steps: prophase, metaphase, anaphase, and telophase. In each step, the cell's DNA moves into the right place. At the end, one cell has become two.
Reading
Cell structure — the basics
What is a cell?
Every living thing is made of cells. You are made of cells. A tree is made of cells. A fish is made of cells. Even tiny things, like bacteria, are made of just one cell.
A cell is the smallest thing that can still be called alive. A cell can eat. A cell can grow. A cell can make new cells. This is why scientists call the cell "the basic unit of life."
The outside of the cell
Every cell has a cell-membraneThe flexible outer boundary of every cell — controls what enters and leaves.. The cell membrane is like the skin of the cell. It is the outside boundary. It keeps the cell together.
The cell membrane has a very important job. It decides what comes into the cell and what leaves the cell. Food and oxygen come in. Waste goes out.
One way things move across the cell membrane is diffusionThe spreading of particles from where there are many to where there are few.. Particles spread out from places where there are many of them to places where there are few of them.
Making more cells
Cells do not live forever. Old cells get damaged. New cells have to take their place. Your body is making new cells right now — in your skin, in your stomach, in your bones.
Most of the cells in your body make new cells by Cell division. One cell becomes two cells. Both cells have the same DNA.
This is why a cut on your finger heals. New cells grow in to replace the damaged ones. Without mitosis, your body could not grow or heal.
The phases of mitosis
Mitosis happens in four steps. In each step, something different happens to the cell's DNA.