Course 1.1Lesson 2 of 3
A cell has a membrane, a nucleus and cytoplasm with small parts called organelles, and each part has a job. Plant cells also have a cell wall, chloroplasts and a large vacuole. Bacteria have no nucleus with a membrane: they are prokaryotes, while plants, animals and fungi are eukaryotes.
This lesson is free to read. Enroll free to keep your place and take the tests.
Try the code and the simulations beside the lesson.
A factory has walls, a manager, workers and machines, and each one has a job. A cell is the same. It has parts, and each part has its own job. The small parts inside a cell are called organelles. Let us go through them, from the outside to the inside.
Every cell is wrapped in a thin skin called the plasma membrane. It keeps the cell together, and it decides what goes in and what goes out. It lets some substances pass and stops others, so we say that it is selectively permeable. We use this idea again in the next course.
Plant cells have one more layer outside the membrane: the cell wall. It is stiff, and it is made mostly of cellulose. The wall gives the plant cell its shape and protects it. Animal cells have no cell wall. Bacteria and fungi have walls too, made of other substances.
The nucleus is the control room of the cell. It holds the instructions for the cell in a substance called DNA, arranged in threads called chromosomes. The nucleus has its own membrane with small holes, so that messages can pass out.
The cytoplasm is the jelly-like fluid between the nucleus and the plasma membrane. All the organelles float in it, and many of the chemical reactions of the cell happen here.
| Organelle | Its job |
|---|---|
| Ribosomes | Very small grains that make proteins. Some lie free in the cytoplasm, and some sit on the endoplasmic reticulum. |
| Endoplasmic reticulum | A network of tubes. The rough kind has ribosomes and helps make proteins. The smooth kind makes fats and removes harmful substances. |
| Golgi apparatus | It packs substances into small bags and sends them where they are needed. |
| Lysosomes | They break down waste and old cell parts. |
| Mitochondria | They release energy from food, so that the cell can work. Each has two membranes and a little DNA of its own. |
| Plastids | Found in plant cells. Chloroplasts are green and make food from light. Chromoplasts are coloured. Leucoplasts store starch, oil and proteins. |
| Vacuole | A space that stores water, food and waste. It is small in an animal cell and large in a plant cell. |
A plant cell has the same basic parts as an animal cell. But it also has a cell wall, plastids such as chloroplasts, and a large vacuole. The large vacuole is full of water. It presses the cytoplasm against the wall, and this keeps the cell firm.
You should find that a cell with all three extras looks like a leaf cell. A root cell of a plant has a wall and a large vacuole but no chloroplasts, because no light reaches it.
Bacteria are very simple cells. Their DNA is not inside a nucleus with a membrane; it lies free in the cytoplasm, in a region called the nucleoid. Bacteria also have no organelles wrapped in a membrane. A cell like this is a prokaryote. A cell with a nucleus inside a membrane is a eukaryote. Plants, animals and fungi are made of eukaryotic cells.
The plasma membrane is flexible. An amoeba uses this to eat. Its membrane folds around a small piece of food and closes, so that the food is inside the cell in a small bag. Taking things in this way is called endocytosis. Some cells send things out by the same kind of bag.
This program is a small key. It names the kind of cell from four clues.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Every lesson is open to read. Enrolling also lets you:
Log in to join the discussion.
Loading…