Cells

This page explains what cells are, what their parts do, and how cells take things in, divide and die. It uses MedlinePlus Genetics from the US National Library of Medicine, a NASA classroom guide to cell organelles, and an open textbook by Gerald Bergtrom of the University of Wisconsin-Milwaukee. It explains how cells work and gives no medical advice.

What a cell is

Cells are the basic building blocks of all living things, and a human body is made of trillions of them.^[1] They give the body structure, take in nutrients from food, turn those nutrients into energy and carry out specialised jobs.^[1] Cells also hold the body’s hereditary material and can make copies of themselves.^[1]

A membrane, the plasma membrane, surrounds every cell.^[3] Inside, the cytoplasm is a jelly-like fluid called the cytosol together with the structures around the nucleus.^[1]

Two kinds of cell

Eukaryotes are living things whose cells have a nucleus, a sort of sack that holds the cell’s DNA.^[2] Animals, plants, protists and fungi are all eukaryotes.^[2] The cells of prokaryotes have no nuclear membrane; their DNA sits in a protein and nucleic-acid structure called the nucleoid.^{[2, 3]} Bacteria are all prokaryotes.^[2]

Prokaryotic cells also lack mitochondria, chloroplasts, the endoplasmic reticulum and other internal membranes.^[3] A typical eukaryotic cell has roughly 1000 times the volume of a typical bacterial cell.^[3]

A cell wall surrounds the membrane of prokaryotic, algal, fungal and plant cells, giving a rigid structure that supports the cell’s shape.^[3]

The organelles and their jobs

Organelles are specialised structures that carry out particular tasks inside the cell.^[1]

• Nucleus: the cell’s command centre, sending signals to grow, mature, divide or die; it holds the DNA behind a membrane called the nuclear envelope.^[1]
• Ribosomes: they read the cell’s genetic instructions to build proteins, floating freely or attached to the endoplasmic reticulum.^[1]
• Endoplasmic reticulum and Golgi apparatus: the endoplasmic reticulum helps process the molecules the cell makes and sends them where they are needed.^[1] The Golgi apparatus processes and packages proteins, including those to be sent out of the cell.^{[2, 1]}
• Lysosomes and peroxisomes: the cell’s recycling centre, digesting invading bacteria, removing toxic substances and recycling worn-out parts.^[1] Peroxisomes break down toxic peroxides made as a by-product of the cell’s chemistry.^[4]
• Mitochondria: they convert energy from food into a form the cell can use, and they carry their own DNA, separate from the nucleus.^{[1, 2]} Nearly all eukaryotic cells have them.^[4]
• Chloroplasts and the central vacuole: chloroplasts use chlorophyll to make glucose by photosynthesis.^[4] A large watery central vacuole fills much of the volume of many plant cells.^[4]

Mitochondria most likely evolved from a bacterium that was swallowed by an early eukaryotic cell.^[4] The plastids of plants and some algae also have their own DNA and most likely began as engulfed cyanobacteria.^[4]

The membrane and how things cross it

All cell membranes are two layers of fatty phospholipids with proteins attached or embedded, a structure called the fluid mosaic.^[7] They are selectively permeable, letting only certain substances through.^[8]

Molecules cross in three ways: passive diffusion, facilitated transport and active transport.^[9] Only a few small, mostly uncharged molecules diffuse across unaided, from higher to lower concentration; water-loving molecules need help.^[9] Active transport uses energy to build up differences in concentration.^[9] Osmosis is the diffusion of water across a membrane towards the side with more dissolved material.^[9]

Large molecules, particles and even microorganisms are brought in by endocytosis.^[10] Exocytosis usually sends large molecules out, and it also gets rid of cell waste.^[10]

The cytoskeleton

The cytoskeleton is a network of long fibres that forms the cell’s framework, shaping it, taking part in cell division and letting cells move.^[1] Its main parts are microfilaments, intermediate filaments and microtubules.^{[4, 11]}

Dividing and dying

When they are not crowded, bacteria copy their DNA throughout their lives and split in two by binary fission.^[3] An actively growing bacterium completes its cycle in roughly 30 to 60 minutes, while a typical eukaryotic cell takes roughly 16 to 24 hours, depending on cell type.^[12] In eukaryotes, DNA is copied during a stage called the S phase.^[12] Checkpoints can pause the cycle to check that division is on track.^[13]

Not every cell keeps dividing: terminally differentiated cells spend the rest of their lives doing one job and no longer cycle.^[12] Most cells live for a limited time and are removed by programmed cell death, or apoptosis, when no longer needed or when damaged.^[14] Enzymes called caspases start the cell’s self-digestion.^[14]

How cells are studied

In light microscopy, the specimen is viewed through glass lenses; in electron microscopy, the image is made by electrons passing through or bouncing off it.^[6] A transmission electron microscope can magnify up to about 10⁶ times and resolve details of 2.0 nm, far beyond any light microscope.^[6] Seeing a part did not reveal its job: that needed cell fractionation, which breaks cells open and spins the contents in a centrifuge to separate parts by mass.^[5]

Common misconceptions

“Every cell has a nucleus.” Prokaryotic cells, including all bacteria, do not.^{[2, 3]}

“Cell death means something went wrong.” Much of it is programmed and normal.^[14]

What this page does not cover

This page does not cover cell theory or its history, the size range of cells, the stages of mitosis step by step, meiosis, stem cells and specialised cell types, cell signalling in detail, or the chemistry of metabolism.

Going further

See Genetics, Microbes, Plants, Evolution and Immune System.