What cells are and how their parts work together

Cell theory: the three big ideas

Cells were first described in the 1600s, when early microscopes showed tiny compartments in cork and swimming organisms in pond water. By the mid 1800s, scientists had pulled the evidence together into cell theory, which is still the foundation of biology.

Cell theory has three parts:

• All living things are made of one or more cells.
• The cell is the basic unit of structure and function in living things.
• All cells come from existing cells, by cell division.

Prokaryotic and eukaryotic cells

There are two basic kinds of cell. Prokaryotic cells, found in bacteria and archaea, have no nucleus: their DNA sits in a region of the cytoplasm called the nucleoid. They are usually small, often 1 to 5 micrometres long, and have no membrane-bound organelles, though they do have ribosomes.

Eukaryotic cells, found in animals, plants, fungi and protists, keep their DNA inside a nucleus and have many membrane-bound organelles. They are usually 10 to 100 micrometres across, roughly ten times wider than a typical bacterium.

A useful memory hook: "pro" sounds like "no", so prokaryotes have no nucleus. "Eu" sounds like "do", so eukaryotes do.

The parts of a cell and their jobs

Think of a cell as a small factory. Each organelle has a job, and the cell only works because they work together. These are the parts students are most often asked about:

• Cell membrane: a thin, flexible layer of lipids and proteins around every cell. It is selectively permeable, letting some substances through and keeping others out.
• Cytoplasm: the jelly-like fluid that fills the cell and holds the organelles. Many chemical reactions happen here.
• Nucleus: holds the cell's DNA, the instructions for making proteins, and controls the cell's activities.
• Ribosomes: tiny structures that read instructions copied from DNA and build proteins. They are found in every cell, prokaryotic or eukaryotic.
• Mitochondria: carry out cellular respiration, releasing energy from glucose and storing it in ATP, the molecule cells use to power their work.
• Endoplasmic reticulum: a network of membranes. Rough ER is studded with ribosomes and helps make proteins; smooth ER makes lipids.
• Golgi apparatus: sorts, modifies and packages proteins and sends them to where they are needed.
• Lysosomes: sacs of digestive enzymes that break down waste and worn-out parts, found mainly in animal cells.
• Vacuoles: storage sacs for water, food and waste.

Plant cells versus animal cells

Plant and animal cells share the nucleus, membrane, cytoplasm, mitochondria, ribosomes, ER and Golgi. Three structures set plant cells apart:

• Cell wall: a rigid layer of cellulose outside the membrane that gives plant cells a fixed, often boxy shape and stops them bursting when they take in water.
• Chloroplasts: green organelles containing chlorophyll, where photosynthesis turns light energy into chemical energy stored in sugar.
• Large central vacuole: a single big sac of water that can fill most of the cell. When it is full, it presses outward against the wall and keeps the plant firm; when it empties, the plant wilts.

Worked example: how big is the cell under the microscope?

A light microscope magnifies twice, once in the eyepiece and once in the objective lens, so the total magnification is the two multiplied together. To find a cell's real size, divide the size of its image by the magnification. Converting millimetres to micrometres at the end gives a number that is easy to compare: 1 mm is 1000 micrometres.

Why cells are so small

Everything a cell needs, like oxygen and glucose, enters through its surface, and every part of the cell's volume needs supplying. As a cell grows, its volume grows faster than its surface area, so each bit of surface has to serve more and more of the inside. Past a certain size, materials cannot move in and wastes cannot move out fast enough, which is why big organisms are made of many small cells rather than one giant one.

You can see this with cubes standing in for cells.

From cells to organisms

In a many-celled organism, cells specialise. Muscle cells are long and packed with mitochondria; nerve cells have long branches that carry signals; red blood cells in mammals lose their nucleus to make room for haemoglobin, which carries oxygen.

Similar cells working together form a tissue, such as muscle tissue. Tissues form organs, such as the heart, and organs work together in organ systems, such as the circulatory system. The levels run cell, tissue, organ, organ system, organism.

Common mistakes about cells

These mix-ups come up again and again on tests:

• Saying plant cells have no mitochondria. They do: plants need cellular respiration too, day and night, alongside photosynthesis.
• Saying animal cells have a cell wall. Only plants, fungi, most bacteria and some protists have walls; animal cells have only a membrane.
• Calling the nucleus the cell's brain and stopping there. It stores the DNA instructions, but ribosomes do the actual building of proteins.
• Thinking bacteria have no ribosomes or DNA. They have both; they lack a nucleus and membrane-bound organelles.
• Mixing up the cell membrane and the cell wall. Every cell has a membrane; the wall, where there is one, sits outside it.
• Forgetting the units when sizing cells. Cells are measured in micrometres, and 1 mm is 1000 micrometres.

How to study and check your work

Draw a plant cell and an animal cell side by side from memory, label every part, then check against your textbook. Mark the three parts only plants have in a different colour.

For each organelle, write its job in one short line and an analogy that makes sense to you: the membrane as a security gate, mitochondria as a power plant, the Golgi as a post office. Then test yourself the other way round: cover the names and recall them from the jobs.

In a microscope or size question, check your answer is sensible. Most animal and plant cells are 10 to 100 micrometres across; if you get 5 cm or 0.001 micrometres, look for a slip in the magnification or the unit conversion.