The shape of the lesson
By the end of the lesson, learners can
- Why a bilayer forms: Rotate the 3D membrane and pick each component in turn.
- Messages from outside: Release glucagon with the liver cell selected, then switch to the muscle cell and release it again.
- Every component has a job: Exam questions ask for roles with reference to stability, fluidity, permeability, transport, signalling and recognition.
- Heads hydrophilic, face water; tails hydrophobic, face inward.
- Fluid = molecules move sideways; mosaic = scattered proteins.
- Carbohydrate chains face outward: receptors and recognition.
- Signalling: secrete, transport, bind a complementary receptor, respond.
One picture every idea returns 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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, A membrane that never sits still.. Every cell is wrapped in a film only about 7 nm thick. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Why a bilayer forms | Rotate the 3D membrane and pick each component in turn. Project tab 1 of the lab (Cell membranes lab, fluid mosaic model) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Messages from outside | Release glucagon with the liver cell selected, then switch to the muscle cell and release it again. Project tab 2 of the lab (Cell membranes lab, cell signalling) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Every component has a job | Exam questions ask for roles with reference to stability, fluidity, permeability, transport, signalling and recognition. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 18 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Every component has a job, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| 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". |
| 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. |
The fluid mosaic model: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Explain what is meant by the term fluid mosaic when describing a cell surface membrane.
Explain how hydrophobic and hydrophilic interactions cause phospholipids to form a bilayer in water.
Describe the arrangement of cholesterol and of glycolipids in a cell surface membrane.
A membrane was taken from an organism that lives at low temperatures. It contained a high proportion of phospholipids with unsaturated fatty acids. Suggest and explain the advantage of this.
Glucagon is carried in the blood to all cells in the body, but only liver cells respond to it. Outline the stages of cell signalling in this example and explain why only liver cells respond.
Compare channel proteins and carrier proteins.
Answer key and marking notes
- Fluid: the phospholipid molecules (and many proteins) move about sideways within their own layer.
- Mosaic: proteins of different sizes and shapes are scattered through the bilayer.
- So the pattern of proteins in the membrane is constantly changing.
- The phosphate head is polar (hydrophilic) and is attracted to water.
- The fatty acid tails are non-polar (hydrophobic) and are repelled by water.
- So the tails point toward each other in the middle, shielded from water, while the heads face the water on both sides.
- Cholesterol: between the fatty acid tails of the phospholipids, in both layers of the bilayer.
- Glycolipids: in the outer layer only, with their carbohydrate chains projecting outward from the cell.
- Unsaturated fatty acids have kinks (C=C double bonds) in their chains.
- The kinks stop the tails packing closely together, so the membrane stays fluid at low temperature.
- A fluid membrane lets proteins move and work, and allows processes such as transport and vesicle formation to continue.
- Secretion: cells in the pancreas release glucagon (the ligand) into the blood.
- Transport: the blood carries glucagon to all cells.
- Binding: glucagon binds to receptors on the cell surface membrane that have a shape complementary to it; this triggers a response inside the cell (for example a second messenger).
- Only liver cells have glucagon receptors; other cells lack a complementary receptor, so glucagon does not bind to them.
- Similarities: both span the membrane (intrinsic proteins) and both are specific to particular ions or molecules.
- Channel proteins have a water-filled pore that ions pass through, and some are gated.
- Carrier proteins bind the molecule and change shape to move it across; they can be used in active transport as well as facilitated diffusion, while channel proteins are used only in facilitated diffusion.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which part of a phospholipid molecule is hydrophobic?
- Athe two fatty acid chains
- Bthe phosphate group
- Cthe glycerol and the phosphate group
- Dthe whole molecule, which is why it forms a barrier
The diagram shows part of a cell surface membrane. Which labelled structure regulates the fluidity of the membrane and reduces its permeability to ions?
- AQ
- BP
- CR
- DS
Which statement describes how an intrinsic (transmembrane) protein is held in the phospholipid bilayer?
- Aamino acids with hydrophobic R groups lie next to the fatty acid tails, and those with hydrophilic R groups are exposed to water
- Bamino acids with hydrophilic R groups lie next to the fatty acid tails, and those with hydrophobic R groups are exposed to water
- Cthe protein is held in place by covalent bonds to the phosphate heads of the phospholipids
- Dthe protein is fixed by cholesterol so that it cannot move within the bilayer
Steroid hormones such as oestrogen bind to receptors inside the cytoplasm of their target cells, not to receptors on the cell surface membrane. What allows this?
- Asteroids are lipid-soluble, so they can diffuse through the phospholipid bilayer
- Bsteroids are charged, so they pass through channel proteins
- Csteroids are too large to bind to cell surface receptors
- Dsteroids are taken into every cell by endocytosis
Cells were treated with an enzyme that removes carbohydrate chains from the outer surface of the cell surface membrane, without damaging the bilayer. Which function is most likely to be lost?
- Arecognition of the cells by other cells, for example as self by the immune system
- Bsimple diffusion of oxygen into the cells
- Cthe hydrophobic barrier that stops ions crossing the bilayer
- Dthe sideways movement of phospholipids within each layer
Pieces of beetroot were kept in water at different temperatures for 30 minutes, and the absorbance of the water (a measure of red pigment released from the cell vacuoles) was recorded: 20 °C 0.04; 30 °C 0.05; 40 °C 0.08; 50 °C 0.31; 60 °C 0.74; 70 °C 0.80. What is the best explanation for the large increase between 40 °C and 60 °C?
- Amembrane proteins denature and the bilayer becomes more fluid, so the membranes become permeable to the pigment
- Bthe pigment is actively transported out of the vacuoles faster at higher temperatures
- Cthe cellulose cell walls are digested at high temperature, releasing the pigment
- Dthe pigment is non-polar, so it diffuses through the fatty acid tails faster when warm
In a liver cell, one activated glucagon receptor causes 100 molecules of a second messenger to be made. Each second messenger molecule leads to the activation of 10 enzyme molecules, and each enzyme molecule releases 1000 glucose molecules from glycogen. How many glucose molecules are released as a result of one glucagon molecule binding?
- A1 000 000
- B1110
- C100 000
- D10 000
A mouse cell and a human cell were fused. Their membrane proteins were tagged with two different fluorescent dyes. At 37 °C the two colours were evenly mixed over the whole surface after 40 minutes; at 15 °C they stayed on separate halves. What do these results show?
- Aproteins move sideways within a fluid bilayer, and this movement slows at low temperature
- Bproteins are fixed in position, and new proteins are made faster at 37 °C
- Cthe dyes diffuse through the cytoplasm from one cell to the other at 37 °C
- Dproteins move only by active transport, which stops at 15 °C
Quiz key
The fatty acid chains are long hydrocarbon chains with no charged or strongly polar groups, so they are hydrophobic and point into the middle of the bilayer. The phosphate group carries a charge, which makes it hydrophilic; it is the part that faces the water. Glycerol and phosphate together make up the hydrophilic head, not the hydrophobic part. A phospholipid is not hydrophobic overall: it has a hydrophilic head and hydrophobic tails, and that combination is exactly why it forms a bilayer in water.
Q is cholesterol. It lies between the fatty acid tails, restricts their movement when the membrane is warm, stops them packing solid when it is cold, and makes the bilayer less permeable to ions and polar molecules. P is a glycoprotein, a protein with a carbohydrate chain on the outer surface, which acts as a receptor or a recognition antigen. R is a channel protein, which has a water-filled pore that increases the permeability to specific ions rather than reducing it. S is a glycolipid, a carbohydrate chain attached to a lipid, used in cell recognition.
The part of the protein that passes through the hydrophobic core has non-polar (hydrophobic) R groups on its surface, which interact with the fatty acid tails; the parts that project into the cytoplasm and tissue fluid have polar (hydrophilic) R groups that interact with water. Swapping the two would place polar groups in the non-polar core, which is unstable. Proteins are held by these hydrophobic and hydrophilic interactions, not by covalent bonds to phospholipids. Many proteins can drift sideways, which is the fluid part of the fluid mosaic model; cholesterol does not lock them in place.
Steroids are small, non-polar lipids, so they dissolve in the hydrophobic core and diffuse straight through the bilayer to reach receptors in the cytoplasm. Most other ligands, such as glucagon, are polar or charged and must bind to cell surface receptors instead. Steroids are not charged, and channel proteins carry specific ions, not hormones. Size is not the reason: many large protein hormones bind to cell surface receptors. Steroids enter by diffusion, which needs no energy, not by endocytosis.
Carbohydrate chains on glycoproteins and glycolipids act as antigens and recognition markers, and some form part of receptor binding sites, so removing them affects cell recognition and some signalling. Oxygen diffuses through the phospholipid bilayer, which is undamaged. The barrier to ions is the hydrophobic core of fatty acid tails, which carries no carbohydrate. Fluidity depends on the phospholipids, their fatty acids and cholesterol, none of which has been removed.
Below about 40 °C little pigment leaks, so the membranes are intact. Above this, the extra kinetic energy makes the phospholipids move more and leave gaps, and the membrane proteins denature, leaving holes, so the pigment (which is polar and normally kept in the vacuole) leaks out. Active transport would be needed to move pigment against a gradient, and it depends on enzymes and carriers that denature at high temperature, so it would slow, not speed up. Cellulose walls are fully permeable and are not what holds the pigment in. The pigment is water-soluble and polar; if it could diffuse through the tails, it would leak readily even at 20 °C.
The signal is amplified at every stage, so the numbers multiply: 1 receptor × 100 second messenger molecules × 10 enzymes × 1000 glucose = 1 000 000 (106) glucose molecules. 1110 adds the stages (100 + 10 + 1000) instead of multiplying them, which misses the cascade. 100 000 leaves out the factor of 10 for the enzymes. 10 000 multiplies only 10 × 1000 and ignores the second messenger stage.
The tagged proteins spread over the whole surface, so they must drift sideways within the membrane; this is direct evidence for the fluid part of the fluid mosaic model. At 15 °C the phospholipids have less kinetic energy and the membrane is less fluid, so the proteins hardly move. If proteins were fixed, newly made proteins would carry no dye, because only the original proteins were tagged. The dyes are attached to membrane proteins, so the pattern reflects movement in the membrane, not in the cytoplasm. Proteins drifting in the bilayer is passive lateral movement, not active transport, which moves substances across a membrane.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.