Parts of a Cell

Look closely inside any cell and you will see it is not just a blob of jelly. It is a busy factory full of tiny structures called organelles (the word means "little organs"). Each organelle has a specific job: making energy, building proteins, storing DNA, breaking down waste, holding everything in shape. The organelles work together like the departments of a factory, kept inside by a flexible outer membrane.

  • Main organellesapprox. 10In a typical animal cell
  • Cell control centreNucleusHolds the DNA
  • Energy makersMitochondriaOften called the "powerhouse of the cell"
  • Protein factoriesRibosomesSmallest organelle, around 20 nm across
  • Cell's skinCell membraneControls what goes in and out
  • Cell's fillingCytoplasmA jelly-like fluid that holds the organelles

The main parts of a cell

Every animal cell contains these key parts:

  • Cell membrane: a thin, flexible layer around the outside of the cell that controls what gets in and out. Like the skin of a balloon.
  • Cytoplasm: a thick, jelly-like fluid that fills the cell and holds everything else in place.
  • Nucleus: the control centre. Contains the cell's DNA, the instructions for making everything the cell needs.
  • Mitochondria: the energy factories. They take in glucose and oxygen and produce a chemical called ATP that powers everything the cell does.
  • Ribosomes: the protein factories. They read DNA instructions and build proteins.
  • Endoplasmic reticulum (ER): a folded network that transports things around the cell.
  • Golgi apparatus: a stack of flat sacs that packages proteins for delivery.
  • Lysosomes: small sacs of digestive juices that break down waste.

The nucleus: the control room

The nucleus is by far the most important organelle. It sits in the middle of the cell and contains all the cell's DNA, organised into chromosomes. Every instruction the cell needs (how to make every protein, how to know when to divide, what shape to grow into) is written there. The nucleus is wrapped in a double layer called the nuclear envelope, with tiny pores that let messenger molecules slip in and out.

Mitochondria: the cell's power stations

Inside almost every animal cell are hundreds or thousands of bean-shaped organelles called mitochondria. They are the cell's power stations: they take in glucose (sugar) and oxygen and combine them in a process called cellular respiration to release energy, packaged into a chemical called ATP. ATP is the universal energy currency of life: every time a cell moves, builds or signals, it spends ATP.

Cells that do a lot of work (like muscle cells) have far more mitochondria than ones that do less (like skin cells). A single heart muscle cell can have 5,000 mitochondria, taking up nearly half the cell's volume.

The cell membrane: the gatekeeper

The cell membrane is a thin double layer of fat molecules wrapping the whole cell. Despite being only a few molecules thick, the membrane is incredibly important: it decides what gets into the cell and what stays out. Small molecules like water and oxygen pass through easily. Bigger things (like proteins and ions) are escorted through by special channel and pump molecules built into the membrane.

Fact Mitochondria have their own DNA, separate from the DNA in the cell's nucleus. The mitochondrial DNA in every cell of your body was inherited entirely from your mother (it comes in through the egg, not the sperm). This means you, your mother, your maternal grandmother and your maternal great-grandmother all share identical mitochondrial DNA. Scientists have used this to trace the ancestry of all living humans back to a single woman, nicknamed Mitochondrial Eve, who lived in Africa around 150,000 years ago.

Extra parts in plant cells

Plant cells have almost everything animal cells have, plus three extras:

  • Cell wall: a tough box around the outside of the cell membrane, made of cellulose, that gives plant cells their box shape and supports the plant.
  • Chloroplasts: green organelles where photosynthesis happens, making sugar from sunlight, water and carbon dioxide.
  • Large central vacuole: a giant sac of water that takes up most of the cell's volume and gives the plant turgor (when a wilting plant droops, it is because the vacuoles in its cells have lost water).
Did you know? The ribosomes inside every cell are some of the most ancient pieces of biological machinery on Earth. They have stayed almost completely unchanged for around 3.5 billion years: the ribosomes in your cells right now are almost identical to those in a bacterium, an oak tree or a blue whale. They are so essential that natural selection has refused to let them change.
Deeper dive: how ribosomes build proteins

The most important job inside any cell is making proteins. Proteins do almost everything: they form the structural fibres of your hair and nails, they carry oxygen in your blood (haemoglobin), they speed up chemical reactions in your gut (enzymes), they form your immune defences (antibodies). Without a constant supply of new proteins, no cell could survive for very long.

Building proteins is the job of ribosomes. The process happens in two stages:

  1. Transcription: the cell makes a temporary copy of the DNA instructions for one protein, called messenger RNA (mRNA). The mRNA carries the instructions out of the nucleus.
  2. Translation: a ribosome reads the mRNA three letters at a time and joins together amino acids in the order specified, building a chain that folds up into the finished protein.

A single ribosome can add new amino acids to a growing protein at a rate of about 20 per second. There are around 10 million ribosomes in a typical animal cell, and they are working flat out around the clock. Together they make new proteins at extraordinary speed: roughly the equivalent of building several brand new copies of every protein in your body every few months.

This is also why antibiotics work. Many common antibiotics (like erythromycin and streptomycin) work by jamming the ribosomes of bacteria, which are slightly different from human ribosomes. The bacteria can no longer make proteins, so they die. Human cells, with their slightly different ribosomes, are mostly unaffected.

For the simpler picture, see what is a cell. For how animal and plant cells differ, see animal cells vs plant cells. For how cells make more of themselves, see mitosis.