Cell membrane
The cell membrane, or plasma membrane, is a phospholipid bilayer about 7–8 nm thick that separates a cell from its surroundings. Embedded proteins let it choose what enters and leaves, receive signals, and hold on to neighbors.
- 1NucleusOrganelle · double membrane
- 2MitochondrionOrganelle · double membrane
- 3RibosomeMolecular machine · no membrane
- 4Endoplasmic reticulumOrganelle · single membrane network
- 5Golgi apparatusOrganelle · single membrane stacks
- 6LysosomeOrganelle · single membrane
- 7Cell membraneBoundary · phospholipid bilayer
- 8CentriolesMicrotubule pair · organizes the spindle
Animal cell: what you are looking at
A flexible, roughly rounded cell 10–30 µm across, bounded only by its membrane. It has a pair of centrioles near the nucleus and many lysosomes, but no wall, no chloroplasts, and no large central vacuole.
Centrioles. A pair of short microtubule cylinders at right angles; they organize the spindle during cell division.
The cell membrane is outlined in coral; it occurs in animal cells, plant cells, fungi, protists, bacteria and archaea. Hover or tab through the key to light up the other structures, and click any of them to open its page.
Compare all eight organelles ↗Know its structure
| Found in | Animal cells, Plant cells, Fungi, Protists, Bacteria and archaea |
| Membranes | It is one: a bilayer with hydrophilic phosphate heads facing out and hydrophobic fatty-acid tails facing in |
| Thickness | About 7–8 nm, invisible under a light microscope |
| Number per cell | One continuous sheet; a human red blood cell has about 140 µm² of it |
| Composition | Roughly half lipid and half protein by mass in a typical animal cell, with about 50 lipid molecules for every protein, plus cholesterol and a sugar-coat (glycocalyx) |
| Model | The fluid mosaic model, proposed by S. J. Singer and Garth Nicolson in 1972 |
What it does
- Controls what gets in and out. Small nonpolar molecules such as O₂ and CO₂ diffuse straight through; ions and sugars need channel or carrier proteins
- Pumps against the gradient. The Na⁺/K⁺-ATPase moves 3 Na⁺ out and 2 K⁺ in per ATP, setting the cell’s charge and volume
- Receives signals. Receptor proteins bind hormones such as insulin and relay the message inside
- Identifies and connects. Glycoproteins act as ID tags (the ABO blood groups are membrane sugars) and junction proteins knit cells into tissues
Cell membrane: common questions
Wondering about the why? Start here.
What is the cell membrane made of?
A phospholipid bilayer about 7 to 8 nm thick, with the phosphate heads facing the water on either side and the fatty-acid tails buried in the middle, plus cholesterol and a mosaic of embedded proteins. By mass a typical animal cell membrane is roughly half lipid and half protein, about 50 lipid molecules per protein. The fluid mosaic model, proposed by Singer and Nicolson in 1972, captures how the lipids and proteins drift sideways within the sheet.
Read: cell transport and osmosisWhat can pass through the cell membrane?
Small nonpolar molecules such as O₂, CO₂, and steroid hormones dissolve through the lipid bilayer directly, and water crosses slowly on its own or quickly through aquaporin channels. Ions and sugars cannot cross the oily interior, so they need channel or carrier proteins, and the Na⁺/K⁺ pump uses one ATP to push 3 Na⁺ out and 2 K⁺ in against their gradients. Polarity matters more than size: a tiny Na⁺ ion is blocked while a much larger steroid slips through.
Calculate which way water movesWhat is the difference between the cell membrane and the cell wall?
Every cell has a membrane, a thin selective bilayer that controls what enters and leaves. A cell wall is an extra rigid layer outside the membrane found in plants (cellulose), fungi (chitin), and bacteria (peptidoglycan) but never in animal cells; it is porous to water and small molecules, so it does not choose what gets in. Instead it resists swelling, letting a plant cell build up turgor pressure of around 0.5 MPa and stay firm rather than burst.
Practice cell questions