Ch 1 · 4.1 Fluid mosaic membranes · AS & A Level Biology 9700
Topic 1 of 2 in Cell membranes and transport — AS & A Level Biology 9700 — 58 lessons.
Introduction
Every cell, and every membrane-bound organelle inside it, is bounded by a membrane built to the same basic plan. Singer and Nicolson's fluid mosaic model (1972) remains the working model examined at A Level: it must account for selective permeability, cell signalling and cell recognition, so precise use of its vocabulary (intrinsic protein, extrinsic protein, glycoprotein, glycolipid) is essential for full marks. (Syllabus section 4.1.)
Key ideas — The fluid mosaic model
The membrane is a phospholipid bilayer: two layers of phospholipid molecules with hydrophilic phosphate heads facing the aqueous cytoplasm and extracellular fluid, and hydrophobic fatty acid tails facing each other, forming a barrier to water-soluble substances. "Fluid" describes the constant lateral movement of phospholipids and most proteins within their own layer, giving the membrane flexibility. "Mosaic" describes the irregular, scattered pattern of proteins of different shapes and sizes embedded in the bilayer, resembling a mosaic when viewed from above.
🧑🏫 Teacher's explanation
Imagine a crowded, busy swimming pool filled with colorful beach balls. The water represents the environment, and the beach balls are the phospholipids. The balls aren't locked in place; they bob and drift around each other, which is why we call the membrane 'fluid'. Now, imagine there are also various inflatable rafts and floating toys scattered among the balls. These represent the proteins. Because these rafts are of different shapes and sizes and are scattered irregularly, they create a 'mosaic' pattern.
📖 Definitions
🔍 Worked example — Easy
A student observes a cell membrane under an electron microscope. They identify a molecule consisting of a phospholipid with a carbohydrate chain attached to its head. Identify the specific term for this molecule and state its primary function in the membrane.
🔍 Worked example — Standard
Explain why the phospholipid bilayer is described as 'semi-permeable' (or selectively permeable) based on its chemical structure.
🔍 Worked example — Exam-style
A researcher increases the temperature of a cell membrane. Predict and explain the effect on membrane fluidity and stability.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Phospholipids and cholesterol
Phospholipids are amphipathic, so they spontaneously form the bilayer in water and give the membrane its basic barrier property: small non-polar molecules such as O_2 and CO_2 cross freely, but ions and polar molecules cannot. Cholesterol molecules sit between the fatty acid tails. At high temperature cholesterol restricts the movement of phospholipids, reducing fluidity and permeability; at low temperature it prevents the tails packing closely and crystallising, maintaining fluidity. Cholesterol also increases the mechanical stability of the membrane, which is particularly important in animal cells that have no cell wall.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where everyone is wearing a heavy coat. The dancers are the phospholipids. Most of the dancers have their heads (hydrophilic heads) facing the crowd and their tails (hydrophobic tails) tucked inward to stay dry. This creates a natural barrier. Now, imagine a few people walking through the crowd who aren't dancing; these are the proteins, moving around freely within the crowd.
📖 Definitions
🔍 Worked example — Easy
A cell surface membrane contains a bilayer of phospholipids. Explain why the membrane is impermeable to ions like Na^+ but permeable to O_2.
🔍 Worked example — Standard
Compare the role of cholesterol in animal cell membranes at high versus low temperatures.
🔍 Worked example — Exam-style
Calculate the percentage increase in membrane fluidity if a temperature change causes the movement of phospholipids to increase from 2.5 × 10^-7 to 4.0 × 10^-7 units per second.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Proteins, glycoproteins and glycolipids
Intrinsic (integral) proteins span the bilayer. Channel proteins form water-filled pores through which specific ions or polar molecules diffuse; carrier proteins bind a specific molecule and change shape to move it across, in facilitated diffusion or active transport. Extrinsic (peripheral) proteins sit on one face only, often acting as enzymes or anchoring the cytoskeleton. Glycoproteins and glycolipids carry short carbohydrate chains projecting from the outer surface; they act as receptors for hormones and neurotransmitters and as antigens that allow cells to recognise one another, for example the antigens that determine ABO blood groups.
🧑🏫 Teacher's explanation
Think of the cell membrane like a crowded, busy swimming pool filled with colorful beach balls. The water represents the phospholipid bilayer—it's fluid, allowing things to move around. The beach balls are the proteins. Some beach balls are huge and stuck halfway in the water (integral proteins), while others just bob on the surface (peripheral proteins).
📖 Definitions
🔍 Worked example — Easy
A student observes a cell membrane under an electron microscope. They identify a protein that spans the entire width of the phospholipid bilayer. Name the specific type of protein and state its general function in transport.
🔍 Worked example — Standard
Compare the roles of channel proteins and carrier proteins in the movement of molecules across the membrane.
🔍 Worked example — Exam-style
A hormone (ligand) is released into the bloodstream. Describe the stages of cell signalling required for this hormone to trigger a response in a target cell.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Roles of cell surface membranes
The cell surface membrane is a partially permeable barrier that controls exchange between the cell and its environment. It carries receptors for cell signalling, so that a hormone such as adrenaline can bind and trigger an intracellular response without entering the cell. Its glycoproteins allow cell-to-cell recognition and adhesion, which underlies immune recognition of self and non-self. Membranes also compartmentalise the cell, and some carry enzymes that catalyse reactions at the membrane surface.
🧑🏫 Teacher's explanation
Think of the cell membrane like a crowded, busy dance floor at a music festival. The floor itself is made of a layer of people (phospholipids) who are constantly shifting and sliding around, making it 'fluid'. Some people are holding hands to form specific shapes (proteins), and some are wearing bright, unique hats (glycolipids/glycoproteins) so you can recognize them in the crowd.
📖 Definitions
🔍 Worked example — Easy
A student observes a cell membrane under an electron microscope. They identify a molecule that is composed of a carbohydrate chain attached to a protein. Identify the specific term for this molecule and state its primary role in the membrane.
🔍 Worked example — Standard
Compare the roles of cholesterol and phospholipids in the cell surface membrane regarding membrane stability and fluidity.
🔍 Worked example — Exam-style
Outline the three main stages of cell signalling involving a hormone (ligand) and a target cell.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Cell signalling
Cell signalling is how cells communicate so that the body responds as a whole. The main stages are:
🧑🏫 Teacher's explanation
Think of cell signalling like a high-security building where a courier (the ligand) delivers a specific key to a specific door (the receptor). The courier cannot enter the building itself, but once the key is inserted into the lock, it triggers a mechanism inside the building to open a door or turn on a light.
📖 Definitions
🔍 Worked example — Easy
A cell secretes a hormone into the extracellular fluid. List the three main stages that must occur for this hormone to trigger a response in a distant target cell.
🔍 Worked example — Standard
Explain why water-soluble ligands (such as peptide hormones) require cell surface receptors rather than diffusing directly through the phospholipid bilayer.
🔍 Worked example — Exam-style
A researcher observes that a specific drug prevents a ligand from binding to its receptor. Describe the likely effect of this drug on the target cell's physiological response.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Insulin is a protein hormone secreted by β-cells in the pancreas. Liver cells and muscle cells respond to insulin, but red blood cells do not. (a) Outline the stages by which insulin reaches and affects a liver cell. [3] (b) Suggest why red blood cells do not respond to insulin. [2]
🔍 Worked example — Easy
Identify the components: A cell membrane is composed of a bilayer of phospholipids. Describe the arrangement of the hydrophilic and hydrophobic regions within this bilayer.
🔍 Worked example — Standard
A researcher observes a cell surface membrane under an electron microscope. They note several proteins embedded within the bilayer and several glycoproteins on the outer surface. Explain the role of these glycoproteins in cell recognition.
🔍 Worked example — Exam-style
Insulin is a protein hormone secreted by β-cells in the pancreas. Liver cells and muscle cells respond to insulin, but red blood cells do not. (a) Outline the stages by which insulin reaches and affects a liver cell. (b) Suggest why red blood cells do not respond to insulin.
⚠️ Common errors
Common mistakes
Practice
Answers