Every cell is separated from the world by a structure only 7–10 nanometres thick. That membrane is not a wall — it's a dynamic, constantly shifting surface that controls everything that enters and leaves.
Before reading on, make a prediction:
A cell membrane needs to do two contradictory things: keep the cell's contents in, and let essential molecules like glucose and oxygen pass through. Predict: what structural features would a membrane need to achieve both goals simultaneously?
Come back to this at the end of the lesson.
Core Content
Every living cell must maintain an internal environment radically different from its surroundings. A neuron maintains a negative internal charge. A cell in your kidney must concentrate urea while keeping glucose in. A red blood cell carries oxygen without oxidising its own machinery.
The solution is a membrane that is selectively permeable — not simply a barrier, but a gatekeeper. It must block some molecules absolutely, allow others to pass freely, and actively pump others against their concentration gradient using energy. All of this must happen continuously, simultaneously, in a structure just 7–10 nm thick.
The model has two words that both carry specific meaning the HSC will test:
Fluid — the phospholipid molecules are not fixed in place. They move laterally (sideways) within each layer, and occasionally flip between layers. The membrane is more like a liquid crystal than a solid wall. Temperature affects fluidity: warmer = more fluid; colder = more rigid. Cholesterol buffers this effect.
Mosaic — proteins are embedded throughout the bilayer in varying positions, like tiles in a mosaic. Some span the entire bilayer (integral/transmembrane proteins); others sit on the surface (peripheral proteins). This variety of proteins creates the membrane's functional diversity.
The structural foundation. Each molecule has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. In water they spontaneously arrange into a bilayer — heads facing outward toward water, tails facing inward away from it. This arrangement requires no energy and is thermodynamically stable.
Span the entire bilayer. Function as channel proteins (allow specific ions/molecules to pass through), carrier proteins (bind and transport molecules), or receptor proteins (detect signals from outside the cell). Essential for selective permeability.
Attached to the inner or outer surface of the bilayer without penetrating it. Function in cell signalling, structural support, and as enzymes. Easier to remove than integral proteins.
Interspersed between phospholipids in animal cell membranes. Acts as a fluidity buffer: at high temperatures, cholesterol restricts phospholipid movement (preventing excess fluidity); at low temperatures, it prevents phospholipids from packing too tightly (preventing rigidity). Maintains consistent membrane function across temperatures.
Proteins with carbohydrate chains attached, extending from the outer membrane surface. Function in cell recognition (e.g. ABO blood group antigens), immune response, and cell-to-cell communication. The carbohydrate layer on the cell surface is called the glycocalyx.
Phospholipids with carbohydrate chains attached on the outer surface. Also contribute to the glycocalyx. Function in cell recognition and adhesion — important for tissue formation and immune surveillance.
The membrane's selective permeability means different substances cross by different mechanisms. The key distinction is whether the process requires energy (ATP) or not.
Direction: High → low concentration (down gradient)
Energy: None (passive)
Mechanism: Small nonpolar molecules (O₂, CO₂, ethanol) dissolve through the lipid bilayer directly
Example: O₂ into respiring cells; CO₂ out
Direction: High → low concentration (down gradient)
Energy: None (passive)
Mechanism: Polar or large molecules use channel or carrier proteins to cross — they cannot dissolve through the bilayer
Example: Glucose into cells via GLUT transporters; ions via ion channels
Direction: High water potential → low water potential
Energy: None (passive)
Mechanism: Water moves through aquaporin channel proteins or directly through the bilayer, from dilute to concentrated solution
Example: Water into root hair cells from soil; water into red blood cells in hypotonic solution
Direction: Low → high concentration (against gradient)
Energy: ATP required
Mechanism: Carrier proteins (pumps) use ATP to move substances against their concentration gradient
Example: Na⁺/K⁺ pump in neurons; uptake of mineral ions into root cells
Direction: Into cell
Energy: ATP required
Mechanism: Cell membrane folds inward, engulfing large particles or fluid into a vesicle. Phagocytosis (solids), pinocytosis (liquids)
Example: White blood cells engulfing pathogens; cells taking up LDL cholesterol
Direction: Out of cell
Energy: ATP required
Mechanism: Vesicles fuse with the cell membrane and release their contents outside the cell
Example: Neurotransmitter release at synapses; insulin secretion from pancreatic beta cells; mucus secretion
Misconception: The cell membrane is a solid, rigid barrier.
The membrane is fluid — phospholipids move laterally at rates of about 2 µm per second. Proteins also move within the bilayer. The membrane is better described as a two-dimensional liquid.
Misconception: Osmosis is just diffusion of water — no special rules apply.
Osmosis specifically refers to water movement across a selectively permeable membrane, driven by differences in water potential (solute concentration). Water moves toward the more concentrated solution (lower water potential), which is the opposite direction to solute diffusion.
Misconception: Cholesterol is always harmful to cells.
Cholesterol is an essential membrane component in animal cells — it stabilises membrane fluidity across temperature ranges. The health problems associated with cholesterol relate to excess LDL in the bloodstream leading to plaque formation, not to cholesterol's role in the membrane itself.
Activities
In your book, construct a detailed comparison table covering all six transport mechanisms from this lesson. Your table must include columns for: direction of movement relative to concentration gradient, energy requirement, mechanism (how it works structurally), and one specific biological example.
After completing the table, answer the following in two sentences: a neuron uses the Na⁺/K⁺ pump to restore its resting membrane potential after firing. Why must this process use active transport rather than facilitated diffusion?
Write your two sentences here.
A researcher places red blood cells into three different solutions:
Write your responses here or in your book.
Assessment
1. The term "fluid" in the fluid mosaic model refers to which of the following?
2. Glucose cannot cross the cell membrane by simple diffusion despite being a small molecule. The most likely reason is that glucose is:
3. A cell is placed in a hypertonic solution. Which of the following correctly describes what will happen?
4. Which of the following transport processes requires ATP?
5. Cholesterol molecules in the cell membrane function primarily to:
1. Describe the structure of the phospholipid bilayer and explain how this structure makes the membrane selectively permeable. (3 marks)
1 mark bilayer structure; 1 mark hydrophobic core explanation; 1 mark selective permeability with example
2. Compare simple diffusion and active transport. In your answer, refer to the direction of movement, energy requirements, and give one specific biological example of each. (3 marks)
1 mark direction comparison; 1 mark energy comparison; 1 mark correct example each
3. Explain how the normal membrane function of cholesterol in animal cells relates to the development of atherosclerosis when LDL cholesterol levels in the blood are elevated. (3 marks)
1 mark cholesterol's normal membrane role; 1 mark LDL uptake via endocytosis; 1 mark foam cell/plaque formation
Answers
SA1: The phospholipid bilayer consists of two layers of phospholipid molecules arranged with their hydrophilic (phosphate) heads facing outward toward the aqueous environment on each side, and their hydrophobic (fatty acid) tails pointing inward toward each other. The hydrophobic core acts as a barrier to water-soluble (polar and charged) molecules — they cannot dissolve through the fatty acid tails. This makes the membrane selectively permeable: small nonpolar molecules like O₂ and CO₂ dissolve through freely, while polar molecules like glucose and ions like Na⁺ are blocked unless specific protein channels or carriers are present.
SA2: Simple diffusion moves substances down their concentration gradient (from high to low concentration) and requires no energy (ATP). Example: O₂ diffuses from the alveoli into red blood cells along its concentration gradient. Active transport moves substances against their concentration gradient (from low to high concentration) and requires ATP. Example: the Na⁺/K⁺ pump in neurons uses ATP to pump 3 Na⁺ out and 2 K⁺ in against their respective gradients, maintaining the resting membrane potential needed for nerve signal transmission.
SA3: In animal cell membranes, cholesterol molecules are interspersed between phospholipids where they regulate fluidity — preventing membranes from becoming too rigid at low temperatures or too fluid at high temperatures. This is an essential structural function. When LDL cholesterol levels in the blood are elevated, arterial wall macrophages take up LDL particles via endocytosis — a normal cellular uptake mechanism. However, with excess LDL, macrophages become overloaded with cholesterol and transform into dysfunctional "foam cells," which accumulate within the arterial wall and form the fatty core of atherosclerotic plaques. These plaques narrow arteries and, if ruptured, can trigger blood clots leading to heart attacks or strokes — a disease process driven by the hijacking of a normal membrane transport mechanism.
You predicted what structural features a membrane would need to keep things in while selectively letting things through. How did your prediction compare?
The answer the fluid mosaic model gives: a hydrophobic phospholipid bilayer blocks most polar molecules by default, while embedded protein channels and carriers provide selective gateways for the specific molecules each cell needs. The dual nature — impermeable by default, permeable by exception — is what makes selective permeability possible.
If you predicted "some kind of gated holes" — you were essentially describing channel proteins. If you predicted "a double layer" — you were describing the bilayer. Both are right in important ways.