Cell Membranes and Transport

The cell membrane, or plasma membrane, is a thin layer around every living cell. It has two jobs: it is a barrier that keeps the cell's contents in and unwanted substances out, and a gate that lets nutrients in and waste out (see Britannica).

The phospholipid bilayer. The membrane is mostly lipids and proteins. Its main lipids, phospholipids and cholesterol, each have a part that is attracted to water and a part that is not. The phospholipids form two layers, a bilayer, with their water-loving surfaces facing the watery fluid inside and outside the cell. The bilayer is about 4 to 10 nanometres thick (see Britannica).

A fluid mosaic. In 1972 S. J. Singer and Garth Nicolson described the membrane as a fluid mosaic. Proteins are set in the bilayer like tiles in a mosaic, partly buried in it, with their water-avoiding parts inside the membrane. The bilayer itself is fluid, so the whole membrane behaves like a two-dimensional solution of proteins in lipid (see Singer and Nicolson). Cholesterol helps keep the membrane stable and controls how fluid it is (see Britannica).

Membrane proteins come in two kinds. Intrinsic proteins are firmly embedded in the bilayer. Extrinsic proteins are loosely attached to its surface, or to intrinsic proteins (see Britannica).

Between them they transport substances, receive chemical signals, anchor the cell's internal skeleton, speed up chemical reactions and let cells communicate. Carbohydrates attached to proteins or lipids on the outer surface help cells recognize and stick to one another (see Britannica).

Diffusion is the net movement of a substance from where it is more concentrated to where it is less concentrated. The difference in concentration is the concentration gradient, and diffusion down it needs no outside energy (see CUNY).

Simple diffusion. Small, nonpolar molecules such as oxygen and carbon dioxide pass straight through the bilayer (see Britannica).

Facilitated diffusion. The bilayer repels large water-soluble molecules and charged ions. Glucose and many ions cross with help from membrane proteins, still down their gradient and still without using the cell's energy. Some of these proteins are open channels; others are carriers that help a solute past the lipid layer (see Britannica).

Osmosis is the diffusion of water across a membrane that lets water through but blocks dissolved substances, the solutes (see Britannica). Water moves from the side with fewer dissolved particles to the side with more, often through channel proteins called aquaporins (see Britannica).

Tonicity compares two solutions. The one with more solute is hypertonic, the one with less is hypotonic, and solutions with equal solute concentration are isotonic (see CK-12).

• Red blood cells have no wall. In a hypotonic solution they swell and burst; in a hypertonic one they lose water and shrivel. Animal cells do best in isotonic surroundings (see CK-12).
• Plant cells have a rigid wall of cellulose. In a hypotonic solution water moves in and the cell swells against the wall, which stops it bursting. This firm state is called turgid. In a hypertonic solution water leaves and the cytoplasm shrinks: plasmolysis (see CUNY). The membrane pulls away from the wall, and plant cells do best in hypotonic surroundings (see CK-12).

Sometimes a cell needs to move a substance against its gradient, from low concentration to high. That takes energy. Membrane pumps use energy, usually from ATP, to do it, and this is active transport (see Britannica).

The sodium-potassium pump. This protein spans the membrane and uses energy to pump sodium ions out of the cell and potassium ions in. It keeps potassium more concentrated inside the cell than outside, and sodium less concentrated. The nerve impulse depends on the difference it keeps up (see Britannica).

Some particles are too large to diffuse or be pumped. The membrane swallows them or pushes them out whole instead (see Britannica).

• Endocytosis. The cell engulfs fluid or large particles in membrane-bound bubbles, vesicles, and brings them inside (see Britannica).
• Exocytosis. A vesicle fuses with the membrane and releases its contents outside the cell. Cells release hormones, digestive enzymes and neurotransmitters this way (see Britannica).

Both involve membranes fusing and then re-forming as complete membranes (see Britannica).

People studied osmosis long before anyone knew what a membrane was made of.