What does the cell membrane do?

The cell membrane, also called the plasma membrane, surrounds every cell in every living thing, from bacteria to the cells in your brain. It is only a few billionths of a metre thick, yet it keeps the inside of the cell chemically different from the outside, which is what keeps the cell alive.

The functions of the cell membrane

Cell membrane structure: the fluid mosaic model

In 1972, S. J. Singer and Garth Nicolson described the membrane as a fluid mosaic: a fluid layer of fats with proteins dotted through it like the tiles of a mosaic. It is still the accepted picture. The main parts are:

• A phospholipid bilayer: two layers of phospholipid molecules. Each has a head that is attracted to water (hydrophilic) and two fatty tails that repel water (hydrophobic). The heads face the watery liquid inside and outside the cell, and the tails tuck into the middle, forming a barrier that water-soluble substances find hard to cross.
• Proteins: some span the whole membrane, forming channels and carriers. Others sit on the surface as receptors or enzymes.
• Cholesterol (in animal cells): sits between the phospholipids and keeps the membrane from becoming too runny or too stiff as the temperature changes.
• Carbohydrate chains: attached to proteins (glycoproteins) and lipids (glycolipids) on the outer surface. They act as identity tags for recognition.

"Fluid" means the molecules aren't fixed in place: phospholipids and many proteins drift sideways within the layer. That flexibility lets cells change shape, grow and divide, and lets the membrane seal itself after small breaks. Our Cell Biology course has an interactive model of the bilayer.

What does selectively permeable mean?

A selectively permeable (or partially permeable) membrane lets some substances through but not others. What can cross depends mainly on size and charge.

The discovery of aquaporins, water channels in the membrane, earned Peter Agre a share of the 2003 Nobel Prize in Chemistry.

How substances cross the cell membrane

Passive transport: no energy needed

• Diffusion: particles spread from where they are more concentrated to where they are less concentrated. Oxygen moves into cells this way.
• Facilitated diffusion: the same downhill movement, but through a channel or carrier protein, for substances that can't cross the bilayer.
• Osmosis: water moving through a partially permeable membrane by diffusion, from where the solution is more dilute to where it is more concentrated. Put a red blood cell in pure water and it swells and can burst. Put it in very salty water and it shrinks.

Active transport: energy needed

Sometimes a cell needs to move a substance from a low concentration to a high one, against its concentration gradient. Carrier proteins called pumps do this using energy from ATP, made in respiration. The sodium-potassium pump, found in animal cells, pushes three sodium ions out and brings two potassium ions in for each ATP it uses, as OpenStax describes. Root hair cells use active transport to take in mineral ions from soil, even when the soil holds fewer ions than the root.

Bulk transport

For very large particles, the membrane wraps around them. In endocytosis it folds inwards to bring material in, as when a white blood cell engulfs a bacterium. In exocytosis, a bubble of membrane called a vesicle fuses with the cell membrane to release its contents, which is how cells release hormones such as insulin.

Try sorting examples into the right kind of transport, then press Mark to check in the course.

Cell membrane vs cell wall

A plant cell has both. The wall is the strong outer box, and the membrane just inside it is the gatekeeper. When a plant cell takes in water by osmosis, it swells and presses against the wall, which makes the cell firm and holds the plant upright.

Worked example: why cells stay small

Everything a cell needs comes in through its membrane, so the membrane's area has to keep up with the cell's volume. Compare cube-shaped cells of different sizes:

Surface area is 6 × side², and volume is side³. As the cell gets bigger, the volume grows faster than the surface area, so each unit of volume has less membrane to supply it. Past a certain size, substances can't cross the membrane fast enough, which is one reason large organisms are made of many small cells. Cells that absorb a lot, like those lining the small intestine, fold their membranes into tiny projections called microvilli to add surface area.

Common mistakes

• Saying only animal cells have a membrane. Every cell has one. Plant cells have a wall as well.
• Calling the membrane fully permeable. It is selectively permeable: it controls what crosses.
• Mixing up diffusion and active transport. Diffusion goes down a concentration gradient with no energy. Active transport goes against it and uses ATP.
• Defining osmosis as the movement of any substance. Osmosis is specifically the movement of water across a partially permeable membrane.
• Saying the cell wall controls what enters. That is the membrane's job. The wall is mostly for support.

Read next

Biology · CellsFluid mosaic model: membrane structure explainedThe fluid mosaic model of membranes for A-level Biology: phospholipids, proteins, glycoproteins and cholesterol, why it is fluid, and a key practical.