The cell: the unit everything living is built from

Cell theory makes three claims: all living things are made of cells, the cell is the basic unit of life, and every cell comes from a pre-existing cell. That last point ended centuries of belief in spontaneous generation and is the reason biology treats reproduction, not creation, as the origin of any organism.

Cells are small for a physical reason. As anything grows, volume rises faster than surface area. Since nutrients enter and waste leaves across the surface, a cell that grows too large cannot supply its own interior. Staying small, or being folded and flattened, keeps the ratio workable.

Prokaryotic cells, which include bacteria and archaea, have no nucleus. Their DNA sits loose in the cytoplasm in a region called the nucleoid, and they lack membrane-bound organelles. They are typically a few micrometres across and reproduce by binary fission.

Eukaryotic cells, which include animals, plants, fungi and protists, keep their DNA inside a membrane-bound nucleus and contain specialised organelles. They are usually ten to a hundred times larger by volume. The leading explanation for how they arose is endosymbiosis: an ancestral cell engulfed a bacterium that became the mitochondrion, which is why mitochondria still carry their own DNA and their own double membrane.

• Nucleus: stores DNA and controls which genes are transcribed.
• Mitochondrion: releases energy from glucose through aerobic respiration, producing ATP.
• Ribosome: assembles proteins from amino acids using instructions carried by messenger RNA.
• Rough endoplasmic reticulum: studded with ribosomes, folds and transports proteins.
• Smooth endoplasmic reticulum: makes lipids and handles detoxification.
• Golgi apparatus: modifies, sorts and packages proteins for delivery.
• Lysosome: contains digestive enzymes that break down waste and worn-out parts.
• Cell membrane: controls what enters and leaves.
• Chloroplast, in plants and algae only: captures light energy for photosynthesis.
• Cell wall, in plants, fungi and many bacteria: provides rigid structural support.

The cell membrane is a phospholipid bilayer. Each phospholipid has a water-attracting head and two water-repelling tails, so in water they spontaneously arrange into a double sheet with tails inward. Proteins float within this layer, which is why the structure is called the fluid mosaic model.

Small, uncharged molecules such as oxygen and carbon dioxide cross by simple diffusion, moving down their concentration gradient. Water crosses by osmosis. Larger or charged particles need protein channels, which is facilitated diffusion. Moving a substance against its gradient requires active transport, which costs ATP. Very large items enter by endocytosis, where the membrane folds inward and pinches off.

Mitosis produces two genetically identical daughter cells and underlies growth, repair and asexual reproduction. The stages are prophase, metaphase, anaphase and telophase, followed by cytokinesis, the physical splitting of the cytoplasm.

Meiosis produces four cells with half the chromosome number, the gametes used in sexual reproduction. Two features make them genetically varied: crossing over, where matching chromosomes swap segments, and independent assortment, where each pair lines up independently of the others. This variation is the raw material natural selection acts on.