Cells

Everything alive is made of cells

Look at your hand. Look at a mango leaf, a goat, a mushroom, a fish in the market. They look completely different. Yet all of them are built from the same kind of tiny unit: the cell.

A cell is the smallest part of a living thing that is itself alive. It takes in food, uses energy, grows, gets rid of waste and can make copies of itself. Nothing smaller than a cell can do all of these things.

Most cells are far too small to see with your eyes alone. A single drop of your blood holds about five million red blood cells. Your whole body has roughly thirty trillion cells. That is 30 000 000 000 000.

Some living things are made of just one cell. Bacteria and the tiny creatures in pond water are like this. Others, like you and a mango tree, are made of many cells working together.

In this book you will learn what a cell is, how plant and animal cells differ, what the parts inside a cell do, and how scientists use microscopes to see them.

Who discovered cells?

In 1665, an English scientist named Robert Hooke built one of the first microscopes and used it to look at a thin slice of cork, the bark of a tree.

He saw that the cork was made of hundreds of tiny boxes, packed together like the rooms of a building. They reminded him of the small rooms in which monks slept, which were called "cells". So he gave the boxes the same name.

Hooke was actually looking at dead cells. Cork is dead tissue, and all that remained were the empty walls. But the name stuck.

A few years later a Dutch cloth merchant, Antonie van Leeuwenhoek, ground his own lenses and saw living cells for the first time: tiny moving creatures in a drop of pond water. He called them "animalcules".

Nearly two hundred years later, in 1839, scientists put the evidence together and stated the cell theory, which you will meet on the next page.

The cell theory

The cell theory is three simple statements that every biologist agrees with. It is one of the most important ideas in all of science.

1. All living things are made of one or more cells. A bacterium is one cell. A mosquito is many cells. A baobab tree is many millions. There is no living thing that is not made of cells.

2. The cell is the basic unit of life. The cell is the smallest thing that can be called alive. A single cell can feed itself, grow and reproduce. The parts inside a cell cannot do this on their own.

3. All cells come from other cells. New cells are made when existing cells divide in two. You began as a single cell, which divided again and again to become the many trillions of cells you have now. Cells never appear out of nothing.

Think of a cell like a brick in a wall. A building is made of bricks, the brick is the basic unit, and you cannot make a wall without bricks. But a cell is much more than a brick: it is alive.

Try this Write down five living things and five non-living things. Which of them are made of cells?

How small is a cell?

Cells are measured in micrometres. One micrometre (written µm) is one thousandth of a millimetre. The thinnest line you can draw with a sharp pencil is about 200 µm wide.

Here are some sizes to compare:

• A typical bacterium: 1 to 2 µm
• A red blood cell: about 8 µm
• A cheek cell from inside your mouth: about 60 µm
• An onion skin cell: about 250 µm (just visible as a speck)
• A human egg cell: about 100 µm, one of the largest human cells
• A chicken egg yolk: a single cell, about 3 cm across

So most cells are far below the size you can see. Your eye can only pick out something about 100 µm wide, and even then it is just a dot.

This is why the microscope matters so much. Without it, the whole world of cells stays hidden.

Try this Take a ruler and look at one millimetre. Now imagine fitting about 15 cheek cells side by side in that space, or 500 bacteria. Draw a line 1 mm long and try to mark it into ten equal parts.

One cell or many?

Living things can be sorted by how many cells they have.

Unicellular organisms are made of a single cell. That one cell does everything: feeding, moving, growing and reproducing. Examples include bacteria, the amoeba (which crawls through pond water by changing shape), and yeast (which makes bread rise and palm wine ferment).

Multicellular organisms are made of many cells. Humans, goats, fish, mango trees, maize and mushrooms are all multicellular. In these organisms, cells do not all do the same job. Some become skin, some muscle, some blood, some leaf. This is called specialisation.

Why be multicellular? A single cell cannot grow very large, because food and oxygen must reach its middle by soaking through. Many small cells together can build something huge, like an elephant or a silk-cotton tree, while each cell stays small enough to feed itself.

Try this Sort these into unicellular or multicellular: amoeba, cockroach, yeast, cassava plant, bacteria that cause cholera, you.

Specialised cells

In a multicellular organism, cells have different shapes because they have different jobs. The shape fits the job.

Red blood cells are shaped like flat discs with a dip in the middle. This gives them a large surface to pick up oxygen in the lungs and release it in the body. They have no nucleus, leaving more room for the oxygen-carrying substance, haemoglobin.

Nerve cells are very long and thin, with branches at each end. A single nerve cell can stretch from your spine to your toe, carrying messages like a wire.

Muscle cells are long and can shorten. When thousands shorten together, your arm bends.

Root hair cells on a plant root have a long thin extension that reaches between soil particles to take in water.

Palisade cells in a leaf are tall and packed with chloroplasts to catch sunlight.

Sperm cells have a tail for swimming.

Every one of these is still a cell with the same basic parts. But each has been shaped for a task, the way a hoe, a cutlass and a sickle are all tools but look different.

Two main kinds: plant and animal cells

All the cells in your body are animal cells. All the cells in a mango tree are plant cells. They share many parts, but there are important differences.

Both have:

• a cell membrane around the outside
• cytoplasm filling the inside
• a nucleus that controls the cell
• mitochondria that release energy

Only plant cells have:

• a stiff cell wall outside the membrane
• a large permanent vacuole full of watery sap
• chloroplasts that make food using sunlight

Notice the shapes. The animal cell is rounded and irregular because it has only a soft membrane. The plant cell is boxy and regular because the wall holds its shape, like the cork cells Hooke saw.

We will look at each part in turn over the next pages.

Recap: what cells are

Let us gather the first ideas before we go inside the cell.

• A cell is the smallest unit of life. Every living thing is made of cells.
• Robert Hooke named cells in 1665 after seeing boxes in cork. Van Leeuwenhoek saw the first living cells.
• The cell theory: all living things are made of cells; the cell is the basic unit of life; all cells come from other cells.
• Cells are measured in micrometres (µm). Most are far too small to see without a microscope.
• Unicellular organisms have one cell (bacteria, amoeba, yeast). Multicellular organisms have many (you, a goat, a tree).
• In multicellular organisms, cells are specialised for different jobs. Shape fits the job.
• Plant cells have a cell wall, a large vacuole and chloroplasts. Animal cells do not.

Quick check

1. Who first used the word "cell", and what was he looking at?
2. Name the three parts of the cell theory.
3. Which is bigger, a red blood cell or a bacterium?
4. Name three things found in plant cells but not animal cells.

The cell membrane

The cell membrane is the thin skin around every cell. It separates the inside of the cell from the outside world.

But it is not just a bag. The membrane is selectively permeable, which means it chooses what gets through. It lets in useful substances like water, oxygen and dissolved food. It lets out waste like carbon dioxide. It keeps out most things the cell does not want.

Think of the gate at a school. The gatekeeper lets pupils and teachers in, lets them out at closing time, and keeps strangers out. The membrane is the gatekeeper of the cell.

The membrane is extremely thin, about 10 nanometres, which is one hundred thousandth of a millimetre. You cannot see it as a separate line even under a school microscope. You see it only as the edge of the cell.

It is also flexible. An animal cell can change shape because its membrane bends and stretches. A white blood cell can wrap itself around a bacterium and swallow it whole.

Key words: membrane, selectively permeable.

Cytoplasm

Inside the membrane, the cell is filled with cytoplasm. It is a jelly-like substance, mostly water, with proteins, salts and sugars dissolved in it.

The cytoplasm has two main jobs.

1. It is where most of the cell's chemical reactions happen. Food is broken down, new proteins are built, and waste is prepared for removal, all in the cytoplasm. Think of it as the factory floor.

2. It holds the other parts in place. The organelles (the small structures with special jobs, like the nucleus and mitochondria) float in the cytoplasm the way pieces of yam float in soup.

The cytoplasm is not still. Under a good microscope you can sometimes see it slowly streaming around inside a plant cell, carrying chloroplasts with it. This movement is called cytoplasmic streaming and it helps spread nutrients through the cell.

In an animal cell, cytoplasm fills almost the whole cell. In a plant cell, the big central vacuole pushes the cytoplasm into a thin layer around the edge.

Key words: cytoplasm, organelle.

The nucleus

The nucleus is the control centre of the cell. It is usually the largest and most obvious structure inside, a round dark spot that stains easily under the microscope.

The nucleus contains the cell's genetic material, called DNA, arranged into threads called chromosomes. The DNA is a set of instructions for building and running the whole cell: which proteins to make, when to grow, when to divide.

Think of the nucleus as the headmaster's office. The instructions for the whole school come from there. Copies of the timetable (the DNA) go out to every class, but the original stays safely in the office.

The nucleus has its own membrane, the nuclear membrane, with tiny holes that let messages pass out into the cytoplasm.

When a cell divides to make two new cells, the nucleus divides first. Each new cell receives a complete copy of the DNA, so both have the full set of instructions.

Almost all plant and animal cells have one nucleus. A few, like red blood cells, lose theirs. Bacteria have DNA but no nucleus around it.

Key words: nucleus, DNA, chromosome.

Mitochondria: the powerhouses

Mitochondria (one is a mitochondrion) are small sausage-shaped organelles found in both plant and animal cells. They are the cell's power stations.

Inside the mitochondria, sugar from food is combined with oxygen to release energy. This process is called respiration. The energy is stored in a chemical that the rest of the cell can use, rather like charging a phone battery.

Respiration happens in every living cell, all the time, even when you are asleep. The carbon dioxide you breathe out is the waste product of respiration in your mitochondria.

Cells that need a lot of energy have many mitochondria. A muscle cell in a footballer's leg may have thousands. A skin cell has far fewer.

Mitochondria are too small to see clearly with a school microscope, but with a powerful electron microscope you can see their folded inner membrane, which gives them a large surface for the reactions.

Interesting fact: mitochondria have their own small piece of DNA, separate from the nucleus. Scientists think they were once free-living bacteria that moved into larger cells very long ago.

Key words: mitochondrion, respiration.

Vacuoles

A vacuole is a storage space inside a cell, surrounded by its own membrane and filled with liquid.

In plant cells, there is usually one very large permanent vacuole in the middle. It takes up most of the cell's volume and pushes everything else to the edges. It is filled with cell sap: water with sugars, salts and sometimes colours dissolved in it. The red colour of hibiscus (sobolo) comes from the sap in its vacuoles.

The vacuole has an important job: it keeps the plant firm. When it is full of water, it presses outward against the cell wall, and the cell becomes stiff like a fully pumped football. Millions of stiff cells hold a plant upright. When the vacuoles lose water, the cells go soft and the plant wilts. You have seen this in a garden on a hot afternoon.

In animal cells, vacuoles are small and temporary, if present at all. They may hold food that has just been taken in, or water that is about to be pushed out.

Key words: vacuole, cell sap, wilt.

Chloroplasts: making food from light

Chloroplasts are found only in plant cells, and only in the green parts of a plant, such as leaves and young stems. There are none in roots.

Chloroplasts are green because they contain a pigment called chlorophyll. Chlorophyll catches the energy in sunlight.

Using that energy, the chloroplast joins carbon dioxide from the air and water from the soil to make sugar. It also releases oxygen. This process is photosynthesis, and it is how plants make their own food.

Every meal you have ever eaten began in a chloroplast. The starch in your kenkey, fufu and rice was made by photosynthesis. Even the meat you eat came from an animal that ate plants.

Chloroplasts are large enough to see with a school microscope as small green ovals in a leaf cell.

Key words: chloroplast, chlorophyll, photosynthesis.

The cell wall

The cell wall is a tough layer found outside the cell membrane in plant cells. Animal cells do not have one.

The wall is made mainly of cellulose, a strong fibre. Cotton is almost pure cellulose, and paper is made of cellulose from wood. So the wall is strong, like a woven basket, but it still lets water and dissolved substances pass through. It is fully permeable, unlike the selectively permeable membrane inside it.

The cell wall does two things:

1. It gives the cell a fixed shape. This is why plant cells look like boxes under the microscope and why Hooke saw neat rows of "rooms" in cork.

2. It stops the cell bursting. When the vacuole fills with water and pushes outward, the wall holds firm. An animal cell in pure water would swell and burst, because it has nothing but a soft membrane.

Together, the wall and the full vacuole act like the walls and the air in a bouncy castle: the air pushes, the walls hold, and the whole thing stands up.

Key words: cell wall, cellulose, fully permeable.

Recap: the parts of a cell

Here is a summary table to copy into your book.

Part	Found in	Job
Cell membrane	Plant and animal	Controls what enters and leaves
Cytoplasm	Plant and animal	Jelly where reactions happen; holds organelles
Nucleus	Plant and animal	Control centre; holds DNA
Mitochondria	Plant and animal	Release energy by respiration
Cell wall	Plant only	Strong outer layer; shape and support
Permanent vacuole	Plant only	Stores sap; keeps the cell firm
Chloroplasts	Plant only	Photosynthesis: make food from light

Memory helpers

• Membrane = gatekeeper
• Cytoplasm = factory floor
• Nucleus = headmaster's office
• Mitochondria = power station
• Cell wall = basket
• Vacuole = water tank
• Chloroplast = kitchen (food is made there)

Quick check

1. Which part is found in both kinds of cell and releases energy?
2. Which part is fully permeable, and which is selectively permeable?
3. Why do root cells have no chloroplasts?

A labelled plant cell

Study this diagram carefully. It shows a typical plant cell with its parts labelled.

The diagram shows more parts than you need at this level. Focus on finding these seven:

1. Cell wall – the thick outer boundary
2. Cell membrane – the thin line just inside the wall
3. Cytoplasm – the pale area filling the cell
4. Nucleus – the large round body with a darker spot inside
5. Large central vacuole – the big pale space in the middle
6. Chloroplasts – the green ovals
7. Mitochondria – the small sausage shapes with folds inside

Drawing a plant cell In your book, draw a rectangle with slightly rounded corners for the wall. Draw a second line just inside it for the membrane. Add a large oval in the middle for the vacuole, a circle to one side for the nucleus, and five or six small green ovals for chloroplasts. Label each part with a ruler-straight line. Never shade a science drawing.

A labelled animal cell

Now compare the animal cell. Notice what is missing and what is the same.

Find these four parts:

1. Cell membrane – the outer boundary, irregular in shape
2. Cytoplasm – fills the whole cell
3. Nucleus – the large round body
4. Mitochondria – the sausage shapes

There is no cell wall, so the outline is soft and uneven. There are no chloroplasts, because animals do not make their own food. There is no large vacuole, so the cytoplasm fills the cell.

Drawing an animal cell Draw an irregular rounded shape, like a potato, for the membrane. Add a circle near the middle for the nucleus and three or four small sausages for mitochondria. Label with straight lines.

Try this Draw the plant cell and the animal cell side by side on one page. Under them, write three differences and three similarities.

Worked example: identify the cell

A pupil looks at a slide under the microscope and writes these notes:

"The cells are packed closely together like bricks. Each one has a clear, thick boundary. I can see a small dark circle in each cell, near the edge. There is a large empty-looking space in the middle. I cannot see any green."

Question: Is this a plant cell or an animal cell? How do you know?

Step 1. "Packed like bricks" and "thick boundary" suggest a cell wall. Animal cells have no wall and are irregular.

Step 2. "Large empty space in the middle" suggests a permanent vacuole, pushing the contents to the edge. That is why the nucleus is near the edge.

Step 3. "No green" means no chloroplasts. But that does not rule out a plant cell. Roots, onion bulbs and the inside of stems are plant tissue with no chloroplasts.

Answer: It is a plant cell, probably from a part of the plant that does not get light, such as an onion bulb. The wall and the large vacuole are the deciding clues.

Try this Write similar notes for a cheek cell, then swap with a partner and identify each other's cells.

The microscope

A microscope makes small things look bigger. The one in most school laboratories is a light microscope (also called a compound microscope). Light shines up through the specimen and two sets of lenses magnify the image.

Learn these parts:

• Eyepiece (ocular lens) – you look through this. Usually magnifies 10 times.
• Objective lenses – on the revolving nosepiece. Typically 4×, 10× and 40×.
• Stage – the platform where the slide sits, held by clips.
• Light or mirror – sends light up through the slide.
• Coarse adjustment knob – moves the stage up and down a lot, for rough focusing.
• Fine adjustment knob – moves it a little, for sharp focusing.
• Arm – hold this to carry it, with your other hand under the base.

Magnification

Magnification tells you how many times bigger the image is than the real object.

The total magnification of a microscope is found by multiplying:

Total magnification = eyepiece lens × objective lens

Worked example The eyepiece is 10× and you are using the 4× objective. Total = 10 × 4 = 40×. The cell looks 40 times bigger than it is.

Now you turn to the 40× objective. Total = 10 × 40 = 400×.

A school microscope usually goes from 40× up to 400×. That is enough to see plant cells, cheek cells, and the nucleus in each. It is not enough to see mitochondria clearly or the cell membrane as a separate line.

Worked example 2 A cheek cell appears 24 mm wide in a drawing made at 400×. What is its real size? Real size = image size ÷ magnification = 24 mm ÷ 400 = 0.06 mm = 60 µm.

Try this

1. Eyepiece 10×, objective 10×. Total?
2. Eyepiece 15×, objective 40×. Total?
3. An onion cell appears 50 mm long at 100×. Real length?

Using the microscope: step by step

Follow these steps every time and you will find your cells quickly.

1. Carry it correctly. One hand on the arm, one under the base. Put it down gently, away from the edge of the bench.

2. Start on low power. Turn the nosepiece to the smallest objective (4×). It gives the widest view, so you can find your specimen.

3. Place the slide. Put it on the stage with the specimen over the hole. Fix it with the clips.

4. Light. Switch on the lamp or angle the mirror until the view is bright. Never point the mirror at the sun.

5. Focus with the coarse knob. Looking from the side, lower the stage (or raise the objective) until it is close to the slide. Then look through the eyepiece and slowly move it away until the image appears.

6. Sharpen with the fine knob.

7. Move to higher power. Centre what you want to see, then turn to the 10× and then 40× objective. Use only the fine knob on high power, or you may crash the lens into the slide.

8. Draw what you see and write the magnification under the drawing.

Making a slide: onion skin

Onion skin is the classic first slide because the cells are large and easy to see.

You need: an onion, a knife, forceps, a glass slide, a cover slip, a dropper, water, and iodine solution (a stain).

Method

1. Cut a small piece from a layer of the onion. On the inside of each layer is a thin, see-through skin, the epidermis.
2. Peel off a small piece of this skin with the forceps. It should be as thin as cling film.
3. Put a drop of water on the slide and lay the skin flat in it. Try not to let it fold.
4. Add one drop of iodine. Iodine stains the nucleus brown so you can see it.
5. Lower a cover slip onto the drop at an angle, slowly, to push the air bubbles out.
6. Dab away extra liquid with tissue.

What you will see: rows of rectangular cells, like bricks, each with a cell wall and a small round nucleus. No chloroplasts, because the bulb grows underground.

Making a slide: cheek cells

Now look at some of your own cells. These are animal cells from the lining of your mouth.

You need: a clean cotton bud, a slide, a cover slip, methylene blue stain (or iodine), water.

Method

1. Gently rub the inside of your cheek with the cotton bud. You will not feel anything come off, but thousands of cells have.
2. Smear the bud onto the middle of the slide.
3. Add one drop of stain. Methylene blue turns the nucleus dark blue.
4. Lower the cover slip at an angle.
5. Put the used cotton bud in the bin the teacher provides. Never share cotton buds.

What you will see: scattered cells, each with a soft, irregular outline, pale cytoplasm and a dark nucleus. They are not packed together like onion cells, and there is no wall.

Compare: Draw one onion cell and one cheek cell side by side at the same magnification.

Recap: the microscope

• A light microscope uses light and two lenses to make small things look bigger.
• Total magnification = eyepiece × objective. A school microscope reaches about 400×.
• Parts to know: eyepiece, objective lenses, nosepiece, stage, clips, light or mirror, coarse knob, fine knob, arm, base.
• Always start on low power, focus, then move up.
• On high power use only the fine knob.
• A slide is made by placing a thin specimen in a drop of water, adding a stain to show up the parts, and lowering a cover slip at an angle.
• Iodine stains nuclei brown. Methylene blue stains them blue.
• Onion skin shows plant cells (wall, nucleus, no chloroplasts). Cheek cells show animal cells (membrane, nucleus, irregular).

Quick check

1. Why do we start on the lowest objective?
2. Why lower the cover slip at an angle?
3. A microscope has a 10× eyepiece. Which objective gives 100×?
4. Why can onion cells and cheek cells both be seen clearly but mitochondria cannot?

Drawing what you see

A scientific drawing is a record, not a work of art. Follow these rules and your drawings will earn full marks.

Use a sharp pencil, never a pen. Mistakes must be rubbed out cleanly.

Draw only what you see. If you see three cells clearly, draw three cells. Do not add chloroplasts because you know they "should" be there.

Use clear, single lines. No sketchy, feathery strokes. No shading or colouring.

Make it large. Fill at least half the page. Small drawings hide detail.

Label with straight lines drawn with a ruler. The lines should touch the part they name and never cross each other. Write labels horizontally on the right.

Give it a title saying what it is, for example "Onion epidermis cells".

Write the magnification you used, for example "×100".

Worked example Kofi looked at onion skin at 100× and saw a row of six rectangular cells, each with a nucleus. His drawing shows six neat rectangles, each with a small circle, labelled "cell wall", "nucleus" and "cytoplasm". Title: "Onion epidermis cells". Below: "×100". Full marks.

Try this Look back at a drawing you did earlier and check it against each rule.

Other microscopes

The light microscope has a limit. It cannot magnify clearly beyond about 1 500 times, because light waves themselves are too big to show smaller detail.

To see smaller things, scientists use an electron microscope. It fires a beam of tiny particles called electrons instead of light. It can magnify over a million times. With it, we can see the inside of a mitochondrion, the ribosomes that make proteins, and even viruses, which are far smaller than any cell.

Electron microscopes are enormous, cost a great deal and need a special room. Universities in Ghana, such as the University of Ghana and KNUST, have them for research.

There are also simpler tools:

• A hand lens (magnifying glass) magnifies 5 to 10 times. Good for insects and seeds.
• A stereo microscope magnifies 10 to 40 times and shows a 3D view of whole objects, like a flower or a beetle.
• A phone microscope clips a small lens onto a phone camera and can reach about 100×.

Try this If your school has a hand lens, look at the underside of a leaf, a grain of sand and a strand of your hair. Draw each and note how many times bigger it looks.

How cells divide

The third part of the cell theory says that all cells come from other cells. This happens by cell division: one cell splits into two.

Before it divides, the cell makes a complete copy of its DNA. Then the nucleus divides, so each half gets one full copy. Finally the cytoplasm pinches in the middle (in animal cells) or a new wall grows across (in plant cells), and two daughter cells are formed. Each is a little smaller than the parent, and each grows until it is ready to divide again.

Cell division is how:

• a baby grows into an adult
• a seedling becomes a tree
• a cut on your finger heals (new skin cells fill the gap)
• a lizard regrows its tail
• a single yeast cell becomes millions in a bowl of dough overnight

Some of your cells divide very often. The lining of your gut is replaced every few days. Others, like most nerve cells, almost never divide after childhood, which is why nerve damage is so serious.

Try this Start with one cell. If it divides every hour, how many cells are there after 5 hours? After 10?

Moving in and out of cells

Cells need to take things in (water, oxygen, food) and send things out (carbon dioxide, waste). How do these cross the membrane?

The main way is diffusion. Particles move from where there are many of them to where there are few, spreading out until they are evenly mixed. You have seen this when the smell of frying fish spreads through a house, or when a drop of ink spreads through a glass of water.

Oxygen is plentiful in your lungs and scarce inside your blood cells, so it diffuses in. Carbon dioxide is plentiful in the cells and scarce in the air, so it diffuses out. No energy is needed; the particles simply spread.

Osmosis is diffusion of water across a membrane. Water moves towards the side with more dissolved substances. This is why:

• a plant wilts in dry soil (water leaves its cells)
• a wilted vegetable goes crisp again in a bowl of water (water enters its cells)
• salted fish keeps: water is drawn out of any bacteria that land on it

Try this Put a slice of raw yam in salty water and another in plain water for an hour. Feel them. Which is softer? Explain using osmosis.

Bacteria: cells without a nucleus

So far we have looked at plant and animal cells, which both have a nucleus. Bacteria are different. They are cells too, but much simpler and much smaller.

A bacterium has:

• a cell wall (but not made of cellulose)
• a cell membrane
• cytoplasm
• DNA, but no nucleus. The DNA floats free in the cytoplasm as a single loop.
• no mitochondria and no chloroplasts

Cells without a nucleus are called prokaryotic. Cells with a nucleus (plant, animal, fungus) are eukaryotic. Prokaryotic cells were the first life on Earth, and they are still the most numerous. There are more bacteria in a handful of garden soil than there are people on the planet.

Bacteria are not all harmful. Most are helpful or harmless:

• bacteria in your gut help digest food
• bacteria sour kenkey dough and make yoghurt
• bacteria in the soil turn dead leaves into food for plants

A few cause disease, such as cholera, typhoid and tuberculosis. Antibiotics are medicines that kill bacterial cells without harming yours, because the two kinds of cell are built differently.

Try this: build a cell model

Making a model is the best way to remember the parts of a cell. You can do this at home with things from the kitchen or the market.

For a plant cell

• A rectangular plastic container or a shoebox lid = the cell wall
• A clear polythene bag lining the inside = the cell membrane
• Clear jelly, or cooked rice water set with gelatine = the cytoplasm
• A large water-filled balloon or a small polythene bag of water in the middle = the vacuole
• A boiled egg yolk or a round stone = the nucleus
• Green beans or pieces of green pepper = chloroplasts
• Groundnuts in their shells = mitochondria

For an animal cell Use a round bowl or a polythene bag (no stiff wall), fill with jelly, add the nucleus and mitochondria, and leave out the vacuole and chloroplasts.

Rules

1. Label every part with a small flag on a toothpick.
2. Be ready to explain the job of each part, not just its name.
3. Present it to your class or your family in two minutes.

Afterwards, write one paragraph: which part was hardest to represent, and why?

Cells, tissues, organs, systems

In a multicellular organism, cells are organised into levels, like pupils into classes, classes into a school, and schools into a district.

Cells are the first level. A muscle cell, a nerve cell.

Tissues are groups of similar cells doing the same job together. Muscle tissue is many muscle cells. The onion epidermis you looked at is a tissue.

Organs are made of several tissues working together to do a bigger job. Your heart is an organ made of muscle tissue, nerve tissue and connective tissue. A leaf is a plant organ.

Organ systems are groups of organs that work together. The digestive system is your mouth, stomach, intestines and more, all working on food.

Organism is the whole living thing: you, a goat, a mango tree.

So: cell → tissue → organ → organ system → organism.

Worked example Where does a palisade cell fit? It is a cell. Many together form palisade tissue. That tissue is part of the leaf, which is an organ. The leaf belongs to the shoot system. The whole plant is the organism.

Cells and health

Understanding cells helps us understand disease.

Malaria is caused by a single-celled organism called Plasmodium. A mosquito injects it into your blood. It enters your red blood cells, multiplies inside them, and bursts them open. This is why malaria causes fever and tiredness: your blood cells are being destroyed. Sleeping under a treated net stops the mosquito, which stops the parasite.

Cholera is caused by bacteria, each a single cell, that live in dirty water. They attack the cells lining your intestine and make them pour out water, causing severe diarrhoea. Clean water and hand washing keep the bacteria out.

Sickle cell disease is a condition in which red blood cells form a curved sickle shape instead of a disc. The sickle shape gets stuck in small blood vessels and causes pain. It is common in Ghana, and knowing your sickle cell status is important.

Cancer happens when cells divide without stopping, ignoring the normal controls from the nucleus.

Every one of these is a story about cells. The more we know about cells, the better we can protect them.

Cells and food

Think about what you ate today in terms of cells.

Plant foods are mostly plant cells. The crunch of a raw carrot is thousands of cell walls breaking. When you cook yam or cassava, heat softens the cell walls and the starch stored inside the cells swells, which is why cooked yam is soft. The strings in a mango are bundles of long fibre cells.

Animal foods are animal cells. Meat is muscle tissue, made of long muscle cells. Egg yolk is a single enormous cell. Milk is not cells but a liquid made by cells in the udder.

Fermented foods depend on single-celled organisms. Yeast cells turn sugar into gas and alcohol: that is what makes bread rise and palm wine fizz. Bacteria sour the maize dough for kenkey and banku, giving them their taste.

Preserving food often means stopping cells. Drying fish removes the water that bacterial cells need. Salting draws water out of them. Refrigeration slows their reactions down.

Try this List everything in one meal you ate this week. Beside each item, write whether it is plant cells, animal cells, or made by single-celled organisms.

Recap: cells in the wider world

• Cells are organised into tissues, organs, organ systems and finally the organism.
• A tissue is many similar cells; an organ is several tissues working together.
• Many diseases are stories about cells: malaria (a single-celled parasite in red blood cells), cholera (bacteria attacking gut cells), sickle cell (misshapen red cells), cancer (cells dividing without control).
• Food is cells: plant cells in vegetables, animal cells in meat, and the work of single-celled yeast and bacteria in bread, kenkey and palm wine.
• Preserving food means stopping the cells of bacteria and moulds from growing.
• Light microscopes reach about 400× in school. Electron microscopes reach over a million times and show the inside of organelles.

Quick check

1. Put in order from smallest to largest: organ, cell, organism, tissue, organ system.
2. What kind of organism causes malaria, and which of your cells does it attack?
3. Why does drying fish stop it going bad?
4. Name one thing an electron microscope can show that a light microscope cannot.

Worked example: planning an investigation

Abena wants to find out whether cells from different parts of a plant look different. Here is how she plans it.

Question: Do onion bulb cells and pondweed leaf cells look different under the microscope?

Prediction: The leaf cells will have chloroplasts and the bulb cells will not, because the bulb grows in the dark.

Method:

1. Make an onion skin slide with iodine stain.
2. Make a pondweed leaf slide. (Pondweed leaves are only one or two cells thick, so no slicing is needed. Use water, no stain, so the green shows.)
3. Look at both at 100×, then 400×.
4. Draw two or three cells from each.
5. Record: shape, presence of wall, nucleus, chloroplasts, vacuole.

Fair test: Use the same microscope and the same magnification for both.

Results table:

Feature	Onion	Pondweed
Cell wall	Yes	Yes
Nucleus	Yes (stained)	Hard to see
Chloroplasts	No	Yes, many green discs
Shape	Rectangular	Rectangular

Conclusion: Both are plant cells, but only the leaf cells have chloroplasts, as predicted.

Common mistakes

"Plant cells have mitochondria? I thought they had chloroplasts instead." Plant cells have both. Chloroplasts make sugar in the light. Mitochondria break sugar down for energy, day and night. A plant needs both.

"All plant cells are green." No. Only cells in the light have chloroplasts. Root cells, onion bulb cells and the cells inside a stem are plant cells without any green.

"The cell wall is the same as the cell membrane." No. The wall is a thick, fully permeable layer of cellulose on the outside, found only in plants. The membrane is a thin, selectively permeable layer on the inside, found in all cells.

"The nucleus is the brain of the cell." It is better to say the control centre. It holds the instructions, but it does not think.

"You can see mitochondria with a school microscope." You cannot see them clearly. You can see the nucleus, the wall and chloroplasts.

"Bacteria are tiny animals." Bacteria are neither plants nor animals. They are single cells with no nucleus, in a group of their own.

"Magnification 10 + 40 = 50×." Multiply, do not add. 10 × 40 = 400×.

Key words

Cell – the smallest unit of life. Cell theory – all living things are made of cells; the cell is the basic unit of life; all cells come from other cells. Unicellular / multicellular – made of one cell / made of many cells. Specialised cell – a cell shaped for a particular job. Cell membrane – the selectively permeable skin around a cell. Cytoplasm – the jelly inside a cell where reactions happen. Nucleus – the control centre, containing DNA. Mitochondrion – organelle where respiration releases energy. Cell wall – strong cellulose layer outside plant cells. Vacuole – a sap-filled space; large and permanent in plant cells. Chloroplast – green organelle where photosynthesis makes food. Organelle – any small structure inside a cell with a special job. Magnification – how many times bigger the image is than the object. Eyepiece × objective. Stain – a dye that makes cell parts show up (iodine, methylene blue). Tissue / organ / organ system – levels of organisation above the cell. Micrometre (µm) – one thousandth of a millimetre.

Self-test

Answer without looking back. Answers on the next page.

1. Who gave cells their name, and in what year?
2. State the three parts of the cell theory.
3. Name two unicellular organisms.
4. Which part of the cell controls what enters and leaves?
5. Which part contains DNA?
6. Where in the cell does respiration happen?
7. Name three structures found in plant cells but not animal cells.
8. What is the cell wall made of?
9. What does the large vacuole do for a plant?
10. What is the green pigment in chloroplasts called?
11. Why do onion bulb cells have no chloroplasts?
12. Eyepiece 10×, objective 40×. Total magnification?
13. Which knob do you use on high power?
14. Why do we use a stain?
15. Which stain turns nuclei blue?
16. Describe the shape of a cheek cell.
17. Put in order: organ, cell, tissue, organism, organ system.
18. What single-celled organism causes malaria?
19. Why can a light microscope not show mitochondria clearly?
20. A cell appears 30 mm wide at 400×. What is its real width in micrometres?

Self-test answers

1. Robert Hooke, 1665.
2. All living things are made of cells; the cell is the basic unit of life; all cells come from existing cells.
3. Any two of: bacteria, amoeba, yeast, Plasmodium.
4. The cell membrane.
5. The nucleus.
6. In the mitochondria.
7. Cell wall, large permanent vacuole, chloroplasts.
8. Cellulose.
9. Stores sap and keeps the cell firm, so the plant stays upright.
10. Chlorophyll.
11. The bulb grows underground in the dark, so there is no light for photosynthesis.
12. 400×.
13. The fine adjustment knob only.
14. To make parts like the nucleus show up clearly.
15. Methylene blue.
16. Irregular, rounded, soft outline, with a central nucleus.
17. Cell, tissue, organ, organ system, organism.
18. Plasmodium.
19. They are too small; light waves cannot show detail that fine.
20. 30 mm ÷ 400 = 0.075 mm = 75 µm.

Score: 18–20 excellent · 13–17 good, revisit the pages you missed · below 13, reread the two recap pages on cell parts and the microscope, then try again.

Learn more

• Khan Academy: Structure of a cell — videos and practice on cell parts and what they do
• Science Learning Hub: Light microscopes — how a light microscope works, with clear diagrams
• Britannica: Cell biology — a reference article on cells and their discovery
• Wikipedia: Cell (biology) — a detailed overview with many labelled pictures

Watch more

Think about it

1. Hooke saw only the walls of dead cork cells. Why do you think he still called them "cells", and was he right to?
2. Plant cells have a wall and animal cells do not. What can animals do that this difference makes possible?
3. If every cell in your body has the same DNA, how can a skin cell and a nerve cell be so different?
4. A single cell cannot grow very large. Why not? What would go wrong in the middle?
5. Treated bed nets, clean water and hand washing all protect your cells. Which one do you think saves the most lives in Ghana, and why?