The Cell Membrane and Transport

In short: The cell membrane is a phospholipid bilayer with proteins in it (the fluid mosaic model), and it controls what enters and leaves the cell. Diffusion and osmosis move substances from high to low concentration without energy, while active transport uses ATP to move them the other way.

Starting point: what you already know

• Particles are always moving, and they spread out from where they are crowded (Year 9 and 10 chemistry).
• A solution is a solute (like salt) dissolved in a solvent (like water).
• Cells need to take in oxygen and glucose and get rid of carbon dioxide and wastes.

The fluid mosaic model

The cell membrane is a thin double layer of phospholipids. Each phospholipid has a head that likes water (hydrophilic) and two tails that avoid water (hydrophobic). In water, they line up tail to tail.

It is called the fluid mosaic model because the phospholipids and proteins move about (fluid) and the proteins are scattered through it like tiles (mosaic). The membrane also has cholesterol (keeps it stable) and glycoproteins (sugar chains that act as ID tags).

The membrane is selectively permeable: small or fat-soluble things like oxygen and carbon dioxide pass through easily, but large or charged things like glucose and ions need help from proteins.

Passive transport (no energy needed)

Diffusion is the net movement of particles from a high concentration to a low concentration, down a concentration gradient. Oxygen enters cells and carbon dioxide leaves by diffusion.

Facilitated diffusion is diffusion through a channel or carrier protein. It is still downhill and needs no energy. Glucose enters many cells this way.

Osmosis is the diffusion of water across a selectively permeable membrane, from where water is more concentrated (a dilute solution) to where it is less concentrated (a concentrated solution).

Active transport (needs energy)

Active transport moves substances against their concentration gradient, from low to high. It needs a carrier protein (a pump) and energy from ATP. Root hair cells use it to take in mineral ions from soil that has fewer ions than the cell.

Bulk transport moves very large things in vesicles and also uses energy:

• Endocytosis: the membrane folds in and wraps something to bring it in. A white blood cell swallowing a bacterium is phagocytosis ("cell eating").
• Exocytosis: a vesicle joins the membrane and releases its contents out, such as hormones or enzymes.

Why cells are small

As a cell grows, its volume grows faster than its surface area. A big cell would not have enough membrane to supply its insides.

Quick summary

• The membrane is a phospholipid bilayer with proteins: the fluid mosaic model. It is selectively permeable.
• Diffusion and osmosis are passive: high to low concentration, no energy.
• Osmosis moves water toward the more concentrated solution.
• Active transport moves substances from low to high using protein pumps and ATP.
• Endocytosis brings large things in. Exocytosis sends them out.
• Small cells have a larger surface area to volume ratio.

Quiz