Cell membranes are built from a phospholipid bilayer studded with proteins, and they control what enters and leaves the cell. The fluid mosaic model describes how lipids and proteins drift within the membrane, kept fluid by cholesterol and the saturation of fatty acid tails. Molecules cross the membrane by passive transport (diffusion, osmosis, facilitated diffusion) or active transport (which costs ATP), depending on size, polarity, and concentration gradients.
Difficulty: Introductory | Prerequisites: Basic chemistry (polarity, bonds, energy).
Every living cell is wrapped in a membrane that separates its interior from the outside world. This topic sits at the heart of introductory biology because nearly everything the cell does, from importing nutrients to sending signals to neighbouring cells, depends on membrane structure and transport. You should already be comfortable with polarity, hydrogen bonding, and the basics of protein structure. If you are arriving cold, think of the membrane as a selective gatekeeper: it lets some things through freely, forces others to use dedicated doors, and blocks the rest entirely.
Fluid mosaic model
The model describing the plasma membrane as a dynamic structure in which phospholipids form a fluid bilayer and proteins float, drift, or are anchored within it.
In simple terms, picture a sea of lipids with protein icebergs bobbing around in it.
Phospholipid
An amphipathic lipid with a hydrophilic (polar) head and two hydrophobic (nonpolar) fatty acid tails. Phospholipids are the most abundant membrane lipid.
Think of it as a molecule with a water-loving head and two water-fearing tails that naturally arranges itself into a two-layered sheet.
Integral protein (integral membrane protein)
A protein permanently embedded in the lipid bilayer, penetrating into the hydrophobic interior.
In simple terms, these are proteins that sit inside the membrane rather than on its surface.
Transmembrane protein
An integral protein that spans the entire width of the membrane, with regions exposed on both the cytoplasmic and extracellular sides.
Think of it as a protein tunnel or bridge that passes all the way through the membrane.
Peripheral protein
A protein loosely bound to the membrane surface (often attached to exposed parts of integral proteins) rather than embedded in the bilayer.
Glycoprotein
A protein with short carbohydrate chains covalently bonded to it, typically on the extracellular face. Important for cell-cell recognition.
Glycolipid
A lipid with a short carbohydrate chain attached. Found on the extracellular face of the membrane.
Channel protein
A transport protein with a hydrophilic channel through which specific molecules or ions pass. Think of it as a selective tunnel.
Aquaporin
A channel protein made of four identical polypeptide subunits that facilitates the rapid passage of water molecules across the membrane (osmosis).
Carrier protein
A transport protein that binds its passenger molecule and changes shape to shuttle it across the membrane.
Diffusion
The net movement of molecules from an area of higher concentration to an area of lower concentration, driven by kinetic (thermal) energy and probability. No energy input required.
In simple terms, molecules spread out on their own until they are evenly distributed.
Osmosis
The diffusion of water across a selectively permeable membrane, from the side with lower solute concentration to the side with higher solute concentration.
Facilitated diffusion (passive transport)
Movement of molecules across a membrane through transport proteins, down the concentration gradient, without ATP.
Active transport
Movement of molecules across a membrane against the concentration gradient, requiring energy from ATP hydrolysis. Can be primary (direct ATP use) or secondary (driven by an ion gradient set up by primary active transport).
Hypertonic solution
A solution with a higher concentration of non-permeating solutes relative to the other side of the membrane. A cell in a hypertonic solution will lose water.
Hypotonic solution
A solution with a lower concentration of non-permeating solutes. A cell in a hypotonic solution will gain water.
Isotonic solution
Solutions on both sides of the membrane have equal concentrations of non-permeating solutes. No net water movement.
Electrochemical gradient
The combined force of the concentration gradient and the electrical charge difference acting on an ion across the membrane.
Membrane potential
The voltage (electrical potential) difference between the two sides of the plasma membrane.
Endocytosis
The process by which the cell membrane folds inward to form a vesicle, bringing material into the cell.
Ion channel
A transport protein that allows specific ions to diffuse through the membrane.
Ion pump
A protein that moves ions across the membrane by primary active transport, using ATP.
Cotransporter
A protein that moves two or more molecules through the membrane simultaneously, powered by secondary active transport.
Phospholipids are the most abundant membrane lipid
Each phospholipid has a hydrophilic polar head facing outward and two hydrophobic nonpolar tails facing inward
The bilayer is held together by hydrophobic interactions, which are weaker than covalent bonds
Phospholipids move sideways within the membrane rapidly, giving the membrane its fluid character
Cholesterol molecules sit between phospholipids and prevent them from packing too tightly
This keeps the membrane fluid at lower temperatures and prevents excessive fluidity at higher temperatures
Membranes must be fluid enough to function, but not so fluid that they lose structural integrity
Organisms adapt membrane composition to their environment:
Fish have a high proportion of unsaturated hydrocarbon tails, which maintain fluidity in cold water
Bacteria and archaea in hot environments use unusual lipids that resist excessive fluidity
The modern model of membrane structure treats the membrane as a mosaic of proteins embedded in or attached to a fluid phospholipid bilayer
Proteins are larger than lipids and move more slowly; some are held immobile by their attachment to the cytoskeleton or extracellular matrix
Integral proteins penetrate the hydrophobic interior of the bilayer
Transmembrane proteins are integral proteins that span the entire membrane, with portions exposed on both the extracellular and cytoplasmic faces
Peripheral proteins sit on the membrane surface, loosely bound to exposed parts of integral proteins
Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units, bonded to lipids (glycolipids) or, more commonly, to proteins (glycoproteins)
These carbohydrates are built by the endoplasmic reticulum (ER) and Golgi apparatus
Transport (channel and carrier proteins)
Enzymatic activity
Signal transduction
Cell-cell recognition (via glycoprotein carbohydrate markers)
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)
Selective permeability: determining what comes in and out
Protection: maintaining an interior environment different from the exterior
Permeability is necessary because complete isolation would kill the cell
Non-polar molecules (hydrocarbons, CO2, O2, small lipids) pass freely across the membrane without help
Diffusion is driven by kinetic (thermal/heat) energy and probability
Individual molecules move randomly; the net effect is movement from high to low concentration until equilibrium
At small scales diffusion is highly efficient: ATP molecules move from mitochondria to other parts of the cell by diffusion alone
Oxygen and sugar molecules diffuse from the smallest blood vessels, across vessel walls and the cell membrane, into cells
At large scales, organisms rely on active mechanisms (breathing, pumping blood) because diffusion alone is too slow
Osmosis is the diffusion of water across a selectively permeable membrane
Water moves from the hypotonic side (lower solute concentration) to the hypertonic side (higher solute concentration)
In an isotonic solution, there is no net water movement
Aquaporins are channel proteins that greatly speed up water passage
Uses transport proteins but requires no ATP
Channel proteins provide a hydrophilic tunnel for specific molecules or ions
Carrier proteins bind a molecule, change shape, and release it on the other side
Movement is always down the concentration gradient
Moves molecules against the concentration gradient
Requires energy, typically from ATP hydrolysis
Primary active transport: ATP directly powers the pump (e.g. the sodium-potassium pump)
Secondary active transport: uses the ion gradient created by primary active transport; carried out by cotransporters
Pumps 3 Na+ out and 2 K+ in per cycle of ATP hydrolysis
Maintains the resting membrane potential and the electrochemical gradient
Sodium is moved against its electrochemical gradient
The unequal exchange (3 out, 2 in) contributes to the charge difference across the membrane
The plasma membrane folds inward to form a vesicle, pulling material into the cell
Used for absorbing or ingesting large molecules or particles that cannot cross the membrane through proteins
Students often think the membrane is rigid like a wall. It is not. The lipid bilayer is fluid, with phospholipids and many proteins constantly in lateral motion.
Students confuse "integral" and "transmembrane." All transmembrane proteins are integral, but some integral proteins only extend partway into the hydrophobic interior without spanning the full membrane.
Students often think diffusion requires energy. Diffusion is a passive process driven by thermal energy and probability; no ATP is used.
Students mix up hypertonic and hypotonic by forgetting which side the label applies to. The label describes the solution, not the cell. A hypertonic solution has more solute than the cell's interior; water leaves the cell.
⚠️ Know the six functions of transmembrane proteins by name: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton/ECM.
⚠️ Be able to distinguish passive transport (diffusion, osmosis, facilitated diffusion) from active transport and explain which requires ATP.
⚠️ Predict what happens to a cell placed in hypertonic, hypotonic, or isotonic solutions.
⚠️ Understand the sodium-potassium pump ratios (3 Na+ out, 2 K+ in) and why this creates a charge gradient.
⚠️ Know what types of molecules can pass freely through the membrane (small, nonpolar) and which need protein assistance (large, polar, charged).
True or False: Cholesterol makes the membrane more rigid at all temperatures. (False: it prevents tight packing at low temperatures and prevents excessive fluidity at high temperatures.)
Fill in the blank: The sodium-potassium pump moves ___ Na+ out for every ___ K+ in. (3, 2)
True or False: Facilitated diffusion requires ATP. (False: it uses transport proteins but no energy input.)
Fill in the blank: A cell placed in a ___ solution will lose water and shrink. (hypertonic)
True or False: Peripheral proteins span the entire membrane. (False: they sit on the surface.)
Q: What are the two main components of biological membranes, and which one is more abundant?
A: Lipids and proteins. Phospholipids are the most abundant component and form the main fabric of the membrane.
Q: A cell has a higher solute concentration than the surrounding fluid. Is the cell hypertonic or hypotonic relative to the solution?
A: The cell is hypertonic to the solution (and the solution is hypotonic to the cell). Water will move into the cell by osmosis.
Q: Name two types of transport proteins and explain how they differ.
A: Channel proteins provide a hydrophilic tunnel through which specific molecules or ions diffuse. Carrier proteins bind their passenger, change shape, and release it on the other side. Both can operate in passive transport; carrier proteins can also work in active transport.
Q: Why does the sodium-potassium pump create a charge gradient across the membrane?
A: It pumps 3 positive ions (Na+) out for every 2 positive ions (K+) it brings in. The net loss of one positive charge per cycle makes the inside of the cell more negative relative to the outside.
Q: Explain why fish membranes have a high proportion of unsaturated fatty acid tails.
A: Unsaturated tails have kinks that prevent tight packing, keeping the membrane fluid at the cold temperatures where fish live. Without them, membranes would become too rigid to function.
Membrane transport connects directly to cell communication (Doc 2): receptor proteins are transmembrane proteins, and signal transduction depends on molecules binding at the membrane surface. The electrochemical gradient maintained by the sodium-potassium pump is essential for understanding neuron signalling (action potentials, resting potential), also covered in the next set of notes.
Energy and thermodynamics (Doc 3) underpin active transport: ATP hydrolysis is an exergonic reaction, and the energy it releases is coupled to the endergonic work of moving ions against their gradient.
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