Every cell is wrapped in a film only about 7 nm thick. It is made of molecules that drift around all the time, yet it decides what enters, what leaves and which chemical messages the cell responds to.
A cell surface membrane is a phospholipid bilayer: the hydrophilic phosphate heads face the watery cytoplasm and tissue fluid, while the hydrophobic fatty acid tails point inward, away from water. It is fluid because the phospholipids and many proteins move sideways within their layer, and a mosaic because proteins are scattered through it. Cholesterol regulates fluidity and stability, channel and carrier proteins transport polar substances and ions, and the carbohydrate chains of glycoproteins and glycolipids act as receptors and recognition markers. In cell signalling a cell secretes a ligand, the ligand is transported to target cells, and it binds to a complementary receptor, which triggers a response inside the cell.
Rotate the 3D membrane and pick each component in turn. Then drag the Movement slider from nearly frozen to very fluid and watch the phospholipids and proteins drift sideways.
The hydrophobic core is what makes the membrane a barrier: ions and polar molecules such as glucose cannot pass through it easily. Say "hydrophobic core" or "hydrophobic fatty acid tails" in your answer, not just "the membrane is in the way".
Release glucagon with the liver cell selected, then switch to the muscle cell and release it again. Watch which cell binds the hormone, and compare the numbers of ligands bound, cAMP molecules made and glucose molecules released.
| Stage | What happens | Glucagon example |
|---|---|---|
| 1 secretion | a cell releases a specific chemical, the ligand | alpha cells in the pancreas secrete glucagon when blood glucose is low |
| 2 transport | the ligand travels to target cells, often in the blood | glucagon reaches every cell in the body |
| 3 binding | the ligand binds to a cell surface receptor with a complementary shape | only liver cells have glucagon receptors, so only they respond |
| 4 response | the receptor changes shape and starts a chain of events inside the cell | a second messenger (cyclic AMP) activates enzymes that break glycogen down to glucose |
Each activated receptor leads to many cAMP molecules, and each of those activates many enzyme molecules, so one signal is amplified into a large response. Small lipid-soluble ligands, such as steroid hormones, are the exception: they cross the bilayer and bind to receptors inside the cell.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Why do phospholipids form a bilayer when they are placed in water?
In the 3D membrane, which component is a protein with a carbohydrate chain attached on the outer surface?
In the signalling sim, glucagon reaches the muscle cell but nothing happens inside it. Why?
In the signalling sim, a few bound hormone molecules lead to many more glucose molecules being released. What is this called?
Exam questions ask for roles with reference to stability, fluidity, permeability, transport, signalling and recognition. Learn each component against those words.
| Component | Where it is | Role |
|---|---|---|
| phospholipids | the bilayer itself | form a barrier to ions and polar molecules; allow lipid-soluble substances through; give fluidity |
| cholesterol | between phospholipid tails in both layers | regulates fluidity (less fluid when warm, stops tails packing solid when cold), adds mechanical stability, reduces permeability to ions and polar molecules |
| channel proteins | span the bilayer | water-filled pore lets specific ions cross; some are gated (open or shut) |
| carrier proteins | span the bilayer | bind a specific molecule or ion and change shape to move it across, in facilitated diffusion or active transport |
| glycoproteins | proteins with carbohydrate chains on the outer surface | cell surface receptors for signalling; antigens for cell recognition; help cells stick together |
| glycolipids | lipids with carbohydrate chains on the outer surface | cell recognition markers (antigens, such as the ABO blood groups); stability by hydrogen bonding with water |
Carbohydrate chains are found only on the outer surface of the cell surface membrane, facing the tissue fluid. A diagram with glycoproteins pointing into the cytoplasm will lose the mark.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which statement about cholesterol in a cell surface membrane is correct?
Which sequence shows the main stages of cell signalling in the correct order?
Why can sodium ions not cross a membrane through the phospholipid bilayer?
A group of cells all carry the same glycolipids and glycoproteins on their surface. What is the most likely role of these molecules?
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