Cell Membrane Structure and Selective Permeability

The cell membrane is a thin, flexible boundary made of a phospholipid bilayer with embedded proteins, and it is selectively permeable, which means it lets some substances cross easily, others only with help and others not at all. In brief: small nonpolar molecules such as oxygen and carbon dioxide pass straight through the lipid layer, while ions and large polar molecules such as glucose need protein channels or carriers. This control keeps the inside of the cell different from the outside.

Every cell is surrounded by a membrane, and many organelles have their own. Understanding its structure explains why some substances cross quickly and others cannot. This guide describes the fluid mosaic model, the roles of lipids, proteins and cholesterol, what selective permeability means, compares the ways molecules cross, works through a prediction example and corrects common mistakes.

What is the structure of the cell membrane?

The accepted model is the fluid mosaic model, proposed by S. J. Singer and Garth Nicolson in 1972. “Fluid” means the lipids and many proteins can move sideways in the membrane, like boats on a pond. “Mosaic” refers to the patchwork of different proteins scattered through the lipid layer.

The basic structure is a phospholipid bilayer. Each phospholipid has a hydrophilic (water-loving) head containing a phosphate group and two hydrophobic (water-fearing) fatty acid tails. In water, the molecules arrange themselves in two layers with the tails pointing inward and the heads pointing outward toward the watery surroundings and the cytoplasm. The bilayer is about 7 to 10 nanometers thick.

What do the components of the membrane do?

• Phospholipids: form the barrier. The hydrophobic center blocks the passage of ions and polar molecules.
• Integral proteins: span the bilayer. Channel proteins form pores for ions and water, and carrier proteins change shape to move specific molecules across.
• Peripheral proteins: attach to the surface and can act as enzymes or link the membrane to the cytoskeleton.
• Receptor proteins: bind signaling molecules such as hormones and trigger a response in the cell.
• Cholesterol (in animal cells): sits between the phospholipids, makes the membrane less flexible at high temperatures and prevents it from becoming too rigid in the cold.
• Glycoproteins and glycolipids: have carbohydrate chains on the outer surface and are used in cell recognition. Blood groups depend on such markers.

What is selective permeability?

A membrane is permeable if substances can pass through it. A selectively (or partially) permeable membrane allows some substances to cross while it restricts others. What passes depends mainly on size, charge and solubility in lipids.

• Pass freely: small nonpolar molecules, such as oxygen, carbon dioxide and fat-soluble substances including steroid hormones. Small uncharged polar molecules, such as water and urea, cross slowly in small amounts.
• Need help: ions (sodium, potassium, chloride, calcium) and larger polar molecules, such as glucose and amino acids. Charged particles cannot easily cross the hydrophobic core.
• Cannot cross directly: very large molecules such as proteins and polysaccharides. They enter or leave by endocytosis and exocytosis.

Water is a special case. Although water molecules are polar, they cross the bilayer in small amounts, and many cells also have water channel proteins called aquaporins that greatly increase the rate. This is what makes osmosis fast in cells such as those in kidney tubules.

What crosses the membrane, and how?

Worked example: predicting how a molecule crosses

Use three questions to predict the route of a molecule: Is it small and nonpolar? Is it charged or highly polar? Is it very large?

1. Ethanol: small, uncharged and partly lipid-soluble. It diffuses directly through the bilayer, which is why alcohol enters the body so quickly.
2. Potassium ion (K⁺): charged, so it cannot pass through the hydrophobic core. It must use a channel or a pump, which are specific for this ion.
3. Glucose: uncharged but large and polar. It crosses through carrier proteins by facilitated diffusion, or with energy by active transport in some cells.
4. Albumin (a protein): too big to use channels or carriers. Cells can take it in by endocytosis in a vesicle.

A related idea is the effect of temperature. Membranes become more fluid when heated and more rigid when cooled. If a membrane is heated enough, proteins are denatured and the membrane becomes leaky. A classic demonstration uses beetroot cells: the red pigment betalain is trapped inside the vacuoles, and when beetroot pieces are put in water at higher temperatures, more pigment leaks out, showing that the membranes have been damaged.

How was the membrane model developed?

In 1925 Evert Gorter and François Grendel extracted lipids from red blood cells and found enough to cover the cells with a layer two molecules thick, which suggested a bilayer. Hugh Davson and James Danielli proposed in 1935 a “sandwich” model with proteins coating both sides. Electron microscopy in the 1950s, led by J. David Robertson, supported a three-layer appearance, and freeze-fracture studies in the 1960s showed proteins embedded within the bilayer, which led to the fluid mosaic model.

Why does membrane structure matter?

Many medicines are designed to cross or to act on membranes. Drugs that are fat-soluble pass through membranes easily, which is important for getting into the brain. Local anesthetics block sodium channels in nerve cell membranes. Faulty membrane proteins cause diseases such as cystic fibrosis, in which a chloride channel does not work properly.

Membrane-like structures are used in technology. Artificial membranes are used in kidney dialysis machines and in water purification by reverse osmosis, and liposomes made of phospholipids can deliver drugs to cells, as in some vaccines.

Which mistakes are common?

Mistake 1: Picturing the membrane as a solid barrier with holes

It is a fluid bilayer with embedded proteins. Its structure changes continuously.

Mistake 2: Saying water cannot cross the membrane

Water crosses by osmosis, through the bilayer slowly and through aquaporins quickly.

Mistake 3: Confusing selectively permeable with impermeable

Some substances cross freely. The control is selective, not total.

Mistake 4: Placing the hydrophilic heads in the center

The heads face outward toward the water, and the hydrophobic tails face inward.