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44 · Cell Membranes and Transport

· Cell Membranes and Transport — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 1 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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4 · Cell Membranes and Transport — Fluid Mosaic Membranes — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the fluid mosaic model, the components of membranes and their roles, factors affecting permeability and cell signalling.Membranes — surround cells and …

  • Selective permeability
  • Phospholipids
  • Cholesterol
  • Glycolipids and glycoproteins
  • Intrinsic proteins
  • Extrinsic proteins

Part 1 of 7The fluid mosaic model

Membranes
  • Membranes surround cells and organelles, compartmentalise the cell, control exchange and act as a communication interface.
Selective permeability
  • Selective permeability substances cross by diffusion, osmosis and active transport.
Bilayer
  • Bilayer about 7 nm wide, with hydrophilic heads outwards and hydrophobic tails inwards.
Fluid
  • Fluid phospholipids and proteins move by diffusion, mainly sideways within their own layer.
Mosaic
  • Mosaic scattered proteins look like a mosaic from above.

Part 2 of 7Components and their roles

Components and their roles
Phospholipids
  • Phospholipids form the bilayer and are a barrier to most water-soluble substances.
Cholesterol
  • Cholesterol sits between phospholipid tails in eukaryotic membranes and regulates fluidity.
Glycolipids and glycoproteins
  • Glycolipids and glycoproteins have carbohydrate chains on the outer surface and act as receptors and cell markers.
Intrinsic proteins
  • Intrinsic proteins span the bilayer, e.g. channel and carrier proteins.
Extrinsic proteins
  • Extrinsic proteins lie on one surface and may be enzymes or receptors.

Part 3 of 7Factors affecting membrane permeability

Temperature
  • Temperature more kinetic energy increases phospholipid movement, and above about 40–50 °C proteins denature and leave gaps.
Unsaturation
  • Unsaturation unsaturated tails increase fluidity, and saturated tails reduce it.
Cholesterol
  • Cholesterol keeps fluidity steady across temperatures and reduces leakage of ions and polar molecules.
Beetroot investigation
  • Beetroot investigation higher absorbance of leaked pigment shows greater permeability.

Part 4 of 7Cell signalling

1Stages
  • Stages a signal is received by a receptor, converted by transduction, passed to a target and produces a response.
2Ligands
  • Ligands signalling molecules bind to specific receptors made of glycoproteins or glycolipids.
3Specificity
  • Specificity only cells with the matching receptor respond.

Part 5 of 7Movement across membranes

Simple diffusion
  • Simple diffusion small non-polar molecules pass through the bilayer.
Facilitated diffusion
  • Facilitated diffusion ions and polar molecules pass through channel and carrier proteins.
Osmosis
  • Osmosis water moves across a partially permeable membrane.
Active transport
  • Active transport uses ATP and carrier proteins to move substances against a gradient.

Part 6 of 7More from the specification

Phospholipid Bilayer Structure
  • The basic membrane structure consists of a double layer of phospholipid molecules, each having a hydrophilic phosphate head and two hydrophobic fatty acid tails.
Phospholipid Monolayer vs Bilayer
  • In an aqueous environment, phospholipids self-assemble into bilayers or liposomes,
  • whereas at an air-water boundary they form a phospholipid monolayer with heads submerged.
Fluid Mosaic Model Proteins
  • Membranes contain diverse types of proteins,
  • including intrinsic carrier/channel structures and extrinsic glycoproteins called peripheral proteins.
Membrane Fluidity and Cholesterol
  • Cholesterol molecules intercalated between fatty acid tails regulate membrane fluidity, preventing rigidity at low temperatures and excessive fluidity at high temperatures.
Phospholipid Bilayer Structure
  • The basic membrane structure consists of a double layer of phospholipid molecules, each having a hydrophilic phosphate head and two hydrophobic fatty acid tails.
Phospholipid Monolayer vs Bilayer
  • In an aqueous environment, phospholipids self-assemble into bilayers or liposomes,
  • whereas at an air-water boundary they form a phospholipid monolayer with heads submerged.
Fluid Mosaic Model Proteins
  • Membranes contain diverse types of proteins,
  • including intrinsic carrier/channel structures and extrinsic glycoproteins called peripheral proteins.
Membrane Fluidity and Cholesterol
  • Cholesterol molecules intercalated between fatty acid tails regulate membrane fluidity, preventing rigidity at low temperatures and excessive fluidity at high temperatures.
Monolayer
  • A single layer of phospholipids at a water surface.

Part 7 of 7More on this topic

  • Specificity
    • only cells with the matching receptor respond, so not every cell responds to a ligand.

Key terms

TermMeaning
Fluid mosaic modelThe model of a membrane as a fluid phospholipid bilayer with scattered proteins.
CholesterolA lipid in eukaryotic membranes that regulates fluidity.
GlycoproteinA protein with attached carbohydrate chains.
Intrinsic proteinA protein embedded in the bilayer.
Extrinsic proteinA protein on the surface of the bilayer.
LigandA signalling molecule that binds to a receptor.
Selectively permeableAllowing some substances to cross but not others.

Avoid these mistakes

1
Saying membranes are fixed — they are fluid.
2
Saying phospholipid heads face inward — heads face the watery surroundings.
3
Forgetting that channel and carrier proteins are specific — each carries particular molecules.
4
Saying cholesterol only makes membranes fluid — it also stabilises them at high temperature.

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