A-Level Biology: Understanding Biological Membranes

Fluid Mosaic Model

Think of cell membranes as a flexible sandwich made of phospholipids - molecules with water-loving heads and water-hating tails. These create a phospholipid bilayer that forms the basic structure of every cell membrane.

The membrane isn't just made of phospholipids though. Cholesterol acts like a temperature control system, keeping the membrane at just the right flexibility - not too rigid, not too melted. Various proteins float in this structure like icebergs, each with specific jobs like transporting molecules or recognising other cells.

Temperature and chemicals can seriously mess with membranes. Too cold and ice crystals pierce through them; too hot (above 45°C) and the whole thing starts melting. Solvents like alcohol dissolve the lipids, which is why drinking too much alcohol damages your cells.

Quick Check: Remember that polar or charged molecules can't just slip through membranes - they need special protein helpers to get across!

Cell Transport

Moving stuff across cell membranes happens in several ways, and understanding these is key to biology success. Passive diffusion is the easiest - molecules simply slide from areas where there's loads of them to areas where there's fewer, like perfume spreading across a room.

Facilitated diffusion is for molecules that need a helping hand, like glucose and amino acids. They use carrier proteins (which change shape to shuttle molecules across) or channel proteins (which act like doorways) to cross the membrane.

Active transport is the real workhorse - it pumps substances uphill against the concentration gradient using ATP energy. Your gut uses this to absorb nutrients from food, and plant roots use it to grab minerals from soil.

Osmosis is just water moving to balance things out. Put a cell in pure water (hypotonic) and it swells up; put it in salty water (hypertonic) and it shrinks. Plant cells handle this better than animal cells because their cell walls prevent bursting.

Exam Tip: Remember that active transport always needs energy (ATP) and moves against the gradient - that's what makes it "active"!