What you'll learn

• How the fluid mosaic model explains the structure of cell surface membranes.
• How substances cross membranes by diffusion, facilitated diffusion, osmosis and active transport.
• How membrane structure links to function, including transport proteins and cholesterol.
• How practical data from membrane permeability and osmosis investigations can be interpreted.

Why cell membranes matter

A cell surface membrane is the thin boundary around a cell, usually about 7 nm thick. It separates the cell contents from the external environment and controls which substances enter and leave.

Membranes are not just “bags”. They are active, organised structures involved in transport, cell signalling, recognition, adhesion and compartmentalisation inside eukaryotic cells.

In Topic 2, this is especially important because faulty membrane transport proteins can affect health. For example, cystic fibrosis involves a faulty chloride ion channel protein, which changes water movement by osmosis and makes mucus thicker.

The fluid mosaic model

The main structure of a membrane is a phospholipid bilayer: two layers of phospholipids arranged tail-to-tail.

A phospholipid is a lipid molecule with a hydrophilic phosphate head and two hydrophobic fatty acid tails. Hydrophilic means “water-attracting”; hydrophobic means “water-repelling”.

Because cells are surrounded by watery fluid and contain watery cytoplasm, the hydrophilic heads face outwards, while the hydrophobic tails point inwards, away from water.

Main membrane components

• Phospholipids form the bilayer and create a hydrophobic core, which restricts charged ions and most polar molecules.
• Cholesterol sits between phospholipid tails in animal cell membranes. It helps stabilise the membrane and regulates fluidity.
• Channel proteins form hydrophilic pores that allow specific ions or polar molecules through.
• Carrier proteins bind specific molecules and change shape to move them across the membrane.
• Glycoproteins are proteins with carbohydrate chains attached. They can act as receptors or cell-recognition markers.
• Glycolipids are lipids with carbohydrate chains attached. They are also involved in cell recognition and signalling.

Concentration gradients and passive transport

A concentration gradient is a difference in concentration between two regions. If a substance moves from a region of higher concentration to a region of lower concentration, it moves down its concentration gradient.

Passive transport is movement across a membrane without direct energy input from ATP.

The word net matters. Particles still move in both directions, but overall more particles move from high concentration to low concentration until equilibrium is reached.

Simple diffusion

Simple diffusion is diffusion directly through the phospholipid bilayer. It is used by small, non-polar molecules such as oxygen and carbon dioxide.

The rate of simple diffusion increases when:

• the concentration gradient is steeper
• the diffusion distance is shorter
• the membrane surface area is larger
• temperature increases, because particles have more kinetic energy
• the molecule is smaller or more lipid-soluble

Facilitated diffusion

Facilitated diffusion is passive movement down a concentration gradient through a membrane protein. It is needed for charged ions and many polar molecules, because they cannot pass easily through the hydrophobic core of the bilayer.

There are two main protein types:

• Channel proteins, which provide a hydrophilic pathway through the membrane.
• Carrier proteins, which bind a molecule and change shape to release it on the other side.

Unlike simple diffusion, facilitated diffusion can become limited by the number of available proteins. Once all carrier or channel proteins are occupied, increasing concentration has less effect on the rate.

Osmosis: water movement

Water can move across membranes through the phospholipid bilayer, but in many cells it mainly moves through protein channels called aquaporins.

Water potential is the tendency of water molecules to move. Pure water has the highest water potential, defined as 0 kPa. Adding solute lowers the water potential, making it more negative.

So, water moves:

• from dilute solution to more concentrated solution
• from higher water potential to lower water potential
• through a selectively permeable membrane

In animal cells, too much water entry can cause cells to swell and burst. In plant cells, the cellulose cell wall resists swelling, so the cell becomes turgid, meaning firm due to internal water pressure. If plant cells lose water, the cytoplasm shrinks away from the cell wall; this is plasmolysis.

Active transport

Sometimes cells need to move substances against their concentration gradient, from lower concentration to higher concentration. This cannot happen by diffusion.

ATP is hydrolysed to release energy. This energy allows a carrier protein to change shape and move the substance across the membrane.

Active transport is important in many biological processes, including:

• uptake of mineral ions by root hair cells
• maintaining ion gradients in nerve cells
• reabsorption of useful substances in kidney tubules
• glucose absorption in the ileum, using sodium ion gradients

A related process is co-transport, where the movement of one substance down its gradient is used to move another substance against its gradient. The sodium-glucose co-transporter is a common example.

Vesicular transport

Large substances cannot pass through membrane proteins. Instead, cells can use membrane-bound sacs called vesicles.

Endocytosis is bulk movement into a cell by the membrane folding inwards and forming a vesicle. Exocytosis is bulk movement out of a cell when a vesicle fuses with the cell surface membrane.

These processes require ATP because they involve movement and reshaping of membranes.

Investigating membrane permeability

A common practical investigates how temperature or ethanol concentration affects membrane permeability using beetroot tissue. Beetroot cells contain red pigment in their vacuoles. If membranes become more permeable, more pigment leaks out into the surrounding solution.

A typical method involves:

• cutting beetroot cylinders to the same size to control surface area and volume
• rinsing pieces to remove pigment released by cutting
• placing pieces in known temperatures or ethanol concentrations for a fixed time, such as 600 s
• using a colorimeter to measure absorbance of the solution
• repeating results and calculating a mean

Higher absorbance usually means more pigment has leaked out, so membrane permeability has increased.

Good controls include the volume of solution, beetroot size, exposure time, pH and the same colorimeter wavelength. Useful evaluation points include variation between beetroot samples, damage caused during cutting, and whether the tissue fully equilibrated to the set temperature before timing began.