The cell membrane and transport

The cell membrane is only about 5 to 10 nm thick, yet it decides what enters and leaves the cell.

A wall made of oil and protein

Drop a little oil on water and it does not mix. The cell membrane is built on the same idea. It is a phospholipid bilayer: two layers of phospholipid molecules, each with a water-loving head containing phosphate and two water-fearing fatty acid tails. The heads face the watery fluid inside and outside the cell, and the tails hide in the middle. Proteins sit in this oily layer, some reaching all the way through, and in animal cells cholesterol is mixed in too. Carbohydrates attached to lipids and proteins cover the outer surface and help the cell recognize and bind things.

Selective permeability

The membrane lets some substances through and blocks others, so it is called selectively permeable. Small molecules with no charge, such as oxygen and carbon dioxide, slip straight through the oily middle. So do fat-soluble substances like vitamins A, D, E and K. Charged particles such as sodium, potassium, calcium and chloride ions cannot cross the oily core on their own, and neither can sugars or amino acids. They need help from proteins.

Moving downhill: passive transport

Open a bottle of perfume in a room and the smell spreads until it is everywhere. That is diffusion: molecules move from where they are more concentrated to where they are less concentrated, and it costs no energy. Diffusion is faster when the difference in concentration is larger, when the temperature is higher, when the molecules are lighter and when the distance is short. Inside a cell, diffusion moves materials around the cytosol.

In facilitated diffusion, substances still move down their gradient, but through proteins. Channel proteins form a water-filled tunnel for one kind of ion or for water; the water channels are called aquaporins. Carrier proteins grab a molecule, change shape and release it on the other side. Channels are much faster: tens of millions of molecules per second, compared with about a thousand to a million for carriers.

Osmosis: water on the move

Osmosis is the diffusion of water across a membrane that blocks the dissolved substances. Water moves toward the side with more dissolved particles, because that side has less free water. If the fluid outside a cell has fewer dissolved particles than the inside, it is hypotonic, and water flows in. A red blood cell in pure water swells and can burst. If the outside has more dissolved particles, it is hypertonic, and water leaves; a red blood cell shrivels. In an isotonic fluid there is no net flow. A plant cell in fresh water does not burst, because its wall stops it. The water pushes against the wall, and this turgor pressure keeps the plant firm. Without water the membrane pulls away from the wall, which is called plasmolysis, and the plant wilts. The plant cell model below shows the vacuole that holds this water.

Moving uphill: active transport

Sometimes a cell must move a substance against its gradient, from low concentration to high. That takes energy, usually from ATP, and it is called active transport. The best-known example is the sodium-potassium pump. For each cycle, it pushes three sodium ions out of the cell and brings two potassium ions in, using one ATP. This keeps sodium high outside and potassium high inside, and it leaves the inside of the cell slightly negative. Nerve and muscle cells depend on this difference. The energy stored in the sodium gradient can then pull other substances, such as glucose and amino acids, into the cell, which is called secondary active transport.