Pick one before you read on. It is fine to be wrong.
By the end you can
Explain the job of the nucleus, membrane and cell wall
Explain why only green plant cells have chloroplasts
A cell is like a tiny workshop. Each part has a job. When every part does its job, the cell stays alive.
Start at the edge. The cell membrane is a thin skin. It is like a door with a guard. It lets in food and water and lets out waste. It keeps out things the cell does not need.
Draw it first
Draw what the cell membrane does
1 Draw a cell and point to its membrane. It is the skin round the cell.
2 Draw an arrow going in through the membrane. Write food in.
3 Draw an arrow going out through the membrane. Write waste out.
Next, the nucleus. It holds the cell's instructions. These tell the cell how to grow and what to make. The nucleus is the control centre.
The nucleus sends instructions to the rest of the cell.
The cytoplasm is the jelly that fills the cell. Most of the cell's work happens in it.
Now the plant cell. Its cell wall is stiff. It gives the cell a firm shape. Water and food pass through the wall easily. The thin membrane inside it decides what enters the cell.
Water in the vacuole pushes on the wall. The wall holds firm.
A plant cell full of water pushes hard against its wall, like a ball full of air. The wall holds firm and so the plant stands up. A plant with too little water goes limp.
Last, the chloroplast. It catches light. It uses the light, with water and carbon dioxide gas, to make food for the plant. This is called photosynthesis.
A chloroplast uses light to make food.
A chloroplast can only work in the light. A leaf is in the light, so its cells have many. A root is under the soil, in the dark, so its cells have none.
See it three ways
With things
Look at a whole plant. The leaves are green and they are in the light. The roots are pale and they are in the dark.
As a picture
In words
Chloroplasts need light to make food. So only cells in the light, in the green parts of a plant, have them.
Try it
Wake a limp stalk
You need: 2 limp stalks or leaves of a green vegetable, a cup, water, a clock.
Be safe: Wash your hands when you finish. Do not eat the stalks.
Choose two stalks that are limp. Bend one gently and feel how floppy it is.
Stand one stalk in a cup of water. Leave the other in the open air, in the same room.
After two hours, bend each one gently. Which feels firmer?
What you will see: The stalk that stood in water is firmer. Its cells filled with water again and pushed on their walls.
If you cannot try it, this is what happens.
A wrong idea many people have
The cell wall decides what goes in and out of a plant cell.
Why it feels right: A wall sounds like a gate, and it is at the edge of the cell.
The test that breaks it: Water flows through the cell wall easily. The wall is made of fibres with small gaps in them. It is the thin membrane just inside that decides what enters.
What is true: The membrane decides what goes in and out. The cell wall holds the shape and lets water through.
Key words
nucleus
the part that controls the cell and holds its instructions.
cell membrane
the thin skin that decides what goes in and out of a cell.
cell wall
the stiff layer that gives a plant cell its shape.
chloroplast
the part that uses light to make food. It is found in the green parts of plants.
photosynthesis
making food from light, water and carbon dioxide.
Back to your guess
The answer is B: The nucleus controls the cell, and the membrane decides what goes in and out.
The nucleus is the control centre. The thin membrane lets in what the cell needs and keeps out what it does not. The cytoplasm is full of jelly where the work is done.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What does the nucleus do?
See the answer
It controls the cell and holds its instructions.
The nucleus tells the cell how to grow and what to make. It is the control centre.
If you chose B: That is the job of the chloroplast. The nucleus is the control centre. Go to S3.1.4.
If you chose C: Shape is the job of the cell wall in a plant cell. The nucleus holds the instructions. Go to S3.1.4.
Which part decides what goes in and out of a cell?
See the answer
The cell membrane
The thin membrane lets in what the cell needs, such as food and water, and lets out waste.
If you chose A: The wall lets water through easily, and animal cells have none. The membrane is the part that decides. Go to S3.1.4.
If you chose B: The cytoplasm is the jelly inside the cell. It does not guard the edge. Go to S3.1.4.
What is the main job of the cell wall?
See the answer
To give a plant cell a firm shape.
A stiff wall, with water pushing on it from inside, helps a plant stand up.
If you chose A: The nucleus controls the cell. The wall is a stiff layer round it. Go to S3.1.4.
If you chose B: That is the chloroplast. The wall only holds the shape. Go to S3.1.4.
Why do root cells have no chloroplasts?
See the answer
Roots are in the dark soil, and chloroplasts need light.
A chloroplast uses light to make food. Underground there is no light, so root cells have none.
If you chose B: Roots need food, like every living part. They get it from the leaves. Go to S3.1.4.
If you chose C: Size is not the reason. Root cells are about as big as leaf cells. A chloroplast needs light, and a root has none. Go to S3.1.4.
A limp plant stands up again after watering. What helps it stand?
See the answer
The cells fill with water and push on their walls.
The water in the vacuole pushes on the stiff wall. A cell full of water is firm, and so is the plant.
If you chose A: The nucleus does not change with water. The water fills the vacuole and pushes on the wall. Go to S3.1.4.
If you chose B: Chloroplasts make food. It is the vacuole that fills with water. Go to S3.1.4.
From an earlier chapter
A microscope has an eyepiece of 10 times and an objective lens of 20 times. How many times does it magnify?
See the answer
200 times
Multiply the two magnifications. 10 × 20 = 200.
If you chose A: You added. The lenses work one after the other, so you multiply. Go to S3.1.2.
If you chose C: You divided. Multiply the two numbers. Go to S3.1.2.
Keep it
Copy this into your exercise book.
The nucleus controls the cell. The membrane decides what goes in and out. The cell wall gives a plant cell its shape. Chloroplasts use light to make food, so only green parts have them.
Draw: Draw a plant cell. Label the nucleus, membrane, wall and chloroplast, and write each part's job.
