How Cell Membranes Work: Structure, Permeability and Transport

Every living cell is wrapped in a thin barrier called the plasma membrane. It separates the watery interior of the cell from the outside environment, yet it cannot be a solid wall, because a cell must constantly take in nutrients and release waste. The membrane solves this problem by being selectively permeable: some substances pass through easily, some pass only with help, and some are kept out almost entirely.

This article explains how that barrier is built, why it lets certain molecules through and blocks others, and how cells use energy to move material across it.

The phospholipid bilayer

The backbone of the membrane is a double layer of phospholipids. Each phospholipid has a head group that carries a phosphate and is attracted to water (hydrophilic), and two fatty acid tails that repel water (hydrophobic). In water, these molecules arrange themselves automatically into a bilayer: the heads face the watery fluid on each side, and the tails tuck together in the middle.

This arrangement is not something the cell has to maintain by hand. It follows from the chemistry of the molecules, which is why artificial lipid bilayers also form spontaneously in the laboratory.

The hydrophobic core is the main reason the membrane blocks most charged and polar molecules. Ions and sugars dissolve well in water but do not dissolve in fatty tails, so they cannot simply slip across.

The fluid mosaic model

The structure is usually described with the fluid mosaic model, proposed by S. Jonathan Singer and Garth Nicolson in 1972. The word "fluid" reflects the fact that phospholipids and many proteins drift sideways within their layer. The word "mosaic" reflects the many different proteins scattered through it, each a separate tile in the pattern.

Membranes are not rigid sheets. They flow, bend, fuse and reseal during processes such as vesicle formation and cell division, and this flexibility is essential to normal cell life.

Cholesterol and fluidity

Animal cell membranes also contain cholesterol, a rigid molecule that sits between phospholipid tails. Cholesterol acts as a buffer for fluidity. At high temperatures it restrains the tails from moving too freely, which keeps the membrane from becoming too loose. At low temperatures it stops the tails from packing tightly and freezing into a stiff, brittle state.

Plant cells usually rely on other sterols for the same job, and bacterial membranes use related molecules, so the buffering idea is broader than cholesterol alone.

Membrane proteins

Proteins make up a large share of membrane mass and do most of the specialized work. Broadly, they fall into two groups:

• Integral proteins span the bilayer, often passing through it several times, and cannot be removed without disrupting the membrane.
• Peripheral proteins sit on one face of the membrane, held by weak interactions, and are easier to detach.

Their jobs include forming channels, acting as carriers, receiving chemical signals, catalyzing reactions and marking the cell's identity. The surface markers on cells are one reason the immune system can distinguish the body's own cells from invaders.

Glycolipids and glycoproteins carry short sugar chains on the outside of the membrane. These sugar chains help cells recognize one another and adhere to neighbors in tissues.

What passes freely and what does not

Permeability depends largely on size, charge and polarity. The table below shows the general pattern for a typical plasma membrane.

Molecule type	Example	Crosses unaided?	Route
Small nonpolar gas	O2, CO2	Yes, quickly	Simple diffusion
Small uncharged polar	Water	Slowly	Diffusion, or aquaporin channels
Sugar	Glucose	No	Carrier protein (GLUT family)
Ion	Na+, K+, Ca2+, Cl-	No	Channel or pump
Large molecule	Proteins	No	Vesicle transport

The pattern follows from the bilayer core. Oxygen and carbon dioxide are small and nonpolar, so they dissolve into the fatty interior and cross in both directions. Glucose is polar and fairly large, so it needs a protein carrier. Ions carry charge, which makes them especially resistant to passing through a hydrophobic layer.

Active and passive transport

Transport across the membrane falls into two broad classes. Passive transport moves substances down their concentration gradient and uses no direct energy input. Active transport moves substances against their gradient and requires energy, usually from ATP.

The sodium-potassium pump, or Na+/K+-ATPase, is the classic example of active transport. It is an enzyme that uses ATP to move three sodium ions out of the cell and two potassium ions into it on each cycle. The cycle works like this:

1. Three sodium ions from the inside bind to the pump.
2. ATP transfers a phosphate group to the pump, changing its shape.
3. The new shape releases the sodium ions outside the cell.
4. Two potassium ions from outside bind to the pump.
5. Release of the phosphate resets the shape, and the potassium ions are released inside.

Because three positive charges leave for every two that enter, the pump creates a net positive charge outside the cell. This charge difference is a key part of the resting membrane potential, which in many neurons sits at roughly -70 millivolts. Nerve signals and muscle contraction depend on that voltage.

Secondary transport and bulk transport

Some transporters do not use ATP directly. They borrow energy from the sodium gradient that the pump built. A carrier can move glucose into an intestinal cell together with sodium, an arrangement called symport. Other carriers exchange one molecule for another, called antiport.

Very large cargo cannot pass through any protein. Cells handle it by wrapping material in membrane vesicles. Endocytosis brings material in, and exocytosis releases material out. Both processes reshape the membrane, which is one more reason its fluid nature matters.

Why the membrane matters

The cell membrane is the boundary that makes a cell a cell. It keeps ion concentrations different inside and outside, which allows nerves to fire and muscles to move. It controls nutrient uptake and waste removal. It also serves as the surface where cells sense and respond to hormones, signals and neighboring cells.

Disruption of membrane transport underlies many diseases. Cystic fibrosis, for example, results from defects in a chloride channel protein, and changes in transporter function are linked to a range of other conditions. Drugs also target membranes: many medicines work by blocking or activating a specific channel or carrier.

Key takeaways

The plasma membrane is a phospholipid bilayer whose hydrophobic core blocks most charged and polar molecules. Cholesterol keeps its fluidity stable across temperatures, and embedded proteins perform the selective transport and signaling work.

Small nonpolar molecules cross easily, while ions and sugars need proteins. Active transport such as the sodium-potassium pump spends ATP to move ions against their gradient, and this spending creates the voltage that powers nerves and muscles.