Difficulty: Introductory to Intermediate | Prerequisites: Part 1 of these notes (membrane structure and fluidity). Understanding of concentration gradients and basic energy concepts (ATP).
The membrane's selective permeability means nothing gets in or out without following rules. This section covers how substances actually cross: passively (no energy, down the gradient), actively (energy required, against the gradient), and in bulk (vesicles for large molecules). If you understand this, you understand how cells eat, drink, signal, maintain ion balance, and stay alive. This material is heavily tested and connects to nearly every later topic in the course, from cellular respiration to neuron signalling.
Small, nonpolar molecules slip through the membrane on their own; everything else needs help. Passive transport (diffusion, osmosis, facilitated diffusion) moves substances down their concentration gradient with no energy cost. Active transport uses ATP to push substances against their gradient, as in the sodium-potassium pump. Very large molecules move in or out via vesicle-based bulk transport (exocytosis and endocytosis).
Passive transport
Movement of a substance across a membrane with no energy investment, down its concentration gradient. Think of it as rolling a ball downhill: the gradient does the work.
Diffusion
The tendency for molecules to spread out evenly into the available space, moving from areas of higher concentration to areas of lower concentration. Molecules keep moving even after reaching equilibrium; there is simply no net directional movement.
Osmosis
The diffusion of water across a selectively permeable membrane. Water moves towards the side with higher solute concentration (lower free water concentration). In simple terms, water chases the solute.
Tonicity
The ability of a surrounding solution to cause a cell to gain or lose water. It depends on the concentration of solutes that cannot cross the membrane, relative to the inside of the cell.
Isotonic
A solution in which the solute concentration is the same as inside the cell. No net water movement occurs. The cell stays the same size.
Hypertonic
A solution with a greater solute concentration than inside the cell. The cell loses water and shrinks (crenation in animal cells, plasmolysis in plant cells).
Hypotonic
A solution with a lower solute concentration than inside the cell. The cell gains water and swells. Animal cells may lyse (burst); plant cells become turgid (firm), which is their normal, healthy state.
Facilitated diffusion
Passive transport aided by transport proteins (channel proteins or carrier proteins). No energy required; movement is still down the concentration gradient.
Channel protein
A membrane protein that provides a hydrophilic corridor for specific molecules or ions to pass through. Some are gated (open or close in response to a stimulus).
Aquaporin
A specialised channel protein that facilitates the rapid passage of water molecules across the membrane.
Carrier protein
A transport protein that binds a specific molecule and changes shape to shuttle it across the membrane. Used in both facilitated diffusion and active transport.
Active transport
Movement of a substance across a membrane against its concentration gradient, requiring energy (usually ATP). All proteins involved are carrier proteins.
Sodium-potassium pump (Na+/K+ pump)
An active transport carrier protein that pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell per cycle, using one ATP. Maintains the electrochemical gradient in animal cells.
Membrane potential
The voltage (charge difference) across a membrane, created by the uneven distribution of ions. The cytoplasmic side is typically negative relative to the extracellular side.
Electrochemical gradient
The combined driving force on an ion, consisting of its concentration gradient (chemical force) and the membrane potential (electrical force). Both together determine the direction an ion will move.
Electrogenic pump
A transport protein that generates voltage across a membrane. The Na+/K+ pump is the main electrogenic pump in animal cells; the proton pump serves this role in plants, fungi, and bacteria.
Proton pump
An active transport protein that pumps H+ ions out of the cell, generating differences in H+ concentration and electric charge. Found in plants, fungi, and bacteria.
Cotransport
A process in which the active transport of one solute indirectly drives the transport of a second substance. One solute moves down its gradient while pulling a second solute against its own gradient. In simple terms, one molecule hitches a ride on another's energy.
Exocytosis
Bulk export from the cell. Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents outside the cell.
Endocytosis
Bulk import into the cell. The membrane folds inward to form a vesicle containing extracellular material. Three types: phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Phagocytosis
"Cell eating." The cell engulfs large particles or other cells by wrapping pseudopods around them to form a food vacuole. Lysosomes then digest the contents.
Pinocytosis
"Cell drinking." The cell takes in small amounts of extracellular fluid and dissolved molecules by forming tiny vesicles. Non-specific.
Receptor-mediated endocytosis
A highly specific form of endocytosis in which the binding of specific solutes to membrane receptors triggers vesicle formation. Allows for a controlled level of uptake.
Pass through easily (passive transport):
Hydrophobic molecules: O2, CO2, N2, steroids.
Small uncharged polar molecules: H2O, glycerol, urea, ethanol.
Cannot pass through unaided:
Large uncharged polar molecules: glucose, sucrose.
Ions: Na+, K+, H+. These are critically important for energy processes and transport.
Diffusion is the movement of molecules from high concentration to low concentration, requiring no energy.
Speed of diffusion depends on: molecule size, temperature of the solution, and steepness of the concentration gradient.
At equilibrium, molecules have not stopped moving. They continue to move randomly; there is simply no net movement in either direction.
Osmosis is the diffusion of water across a selectively permeable membrane.
Water moves towards the side with higher solute concentration because that side has less free water available.
As total solute concentration increases on one side, the concentration of free water on that side decreases, driving water across.
Isotonic solution: solute concentration outside equals inside the cell. No net water movement. Animal cell: normal shape. Plant cell: flaccid.
Hypertonic solution: solute concentration outside is greater than inside. Cell loses water. Animal cell: shrivels (crenation). Plant cell: plasmolyses (membrane pulls away from wall).
Hypotonic solution: solute concentration outside is less than inside. Cell gains water. Animal cell: swells and may lyse (burst). Plant cell: becomes turgid (firm), which is healthy and normal.
Plant cells handle osmotic stress better than animal cells because of the rigid cell wall and the central vacuole.
Osmoregulation: the control of solute concentrations and water balance. Some cells use contractile vacuoles, which fill with fluid from a canal system radiating through the cytoplasm, then contract to expel excess water.
Passive transport aided by membrane proteins. Still moves substances down their concentration gradient, still requires no energy.
Channel proteins provide hydrophilic corridors for specific molecules or ions.
Some are gated channels that open or close in response to a stimulus (voltage, ligand binding).
Aquaporins are channel proteins specialised for rapid water transport.
Carrier proteins bind to a specific molecule, change shape, and shuttle it across the membrane.
Specificity matters: each carrier protein transports only certain molecules.
Requires energy (usually ATP) to move solutes against their concentration gradient (low to high).
All proteins involved in active transport are carrier proteins.
Allows cells to maintain internal concentrations that differ from their surroundings.
Maintains high K+ and low Na+ inside animal cells, relative to the extracellular fluid.
Cycle (six steps):
Three cytoplasmic Na+ ions bind to the pump. The protein shape has high affinity for Na+.
Na+ binding stimulates phosphorylation by ATP (a phosphate group is transferred from ATP to the protein).
Phosphorylation changes the protein's shape, reducing its affinity for Na+. The three Na+ ions are released outside the cell.
The new shape has high affinity for K+. Two K+ ions from the extracellular fluid bind, triggering release of the phosphate group.
Loss of the phosphate group restores the original protein shape, which has low affinity for K+.
The two K+ ions are released inside the cell. Affinity for Na+ is high again. The cycle repeats.
Net result per cycle: 3 Na+ out, 2 K+ in, 1 ATP consumed.
Membrane potential is the voltage (charge difference) across a membrane.
The cytoplasmic side is negative relative to the extracellular side.
Two forces drive ions across the membrane, collectively called the electrochemical gradient:
A chemical force: the ion's concentration gradient.
An electrical force: the effect of the membrane potential on the ion's movement.
An electrogenic pump is a transport protein that generates voltage across the membrane.
The Na+/K+ pump is the major electrogenic pump in animal cells (it moves 3 positive charges out for every 2 in, creating a net negative charge inside).
In plants, fungi, and bacteria, the main electrogenic pump is the proton pump.
The proton pump actively transports H+ out of the cell, generating differences in H+ concentration and electric charge.
This difference drives cations (e.g., K+) into the cell and, through coupled transport proteins, moves anions (e.g., Cl-) into the cell as well.
One substance's movement down its concentration gradient is coupled to the transport of a second substance against its own gradient.
Example: the H+/sucrose cotransporter in plant cells. The proton pump creates a H+ gradient; as H+ diffuses back into the cell, it brings sucrose with it against sucrose's own gradient.
In effect, ATP powers the pump, the pump creates the gradient, and the gradient powers the second transport.
Cystic fibrosis is a genetic disorder in which cells cannot properly release Cl- ions through their chloride channels.
Water is retained in the cell as a consequence, causing mucus in the airways to thicken.
Over 50% of cystic fibrosis patients carry a particular null allele with a single codon change that disrupts protein sorting, preventing the channel from reaching the membrane.
Small molecules cross via the lipid bilayer or transport proteins. Large molecules (polysaccharides, proteins) require vesicle-based bulk transport.
Bulk transport requires energy.
Exocytosis: transport vesicles migrate to the plasma membrane, fuse with it, and release contents outside the cell. Used for product export (e.g., secretion of hormones, neurotransmitters).
Endocytosis (three types):
Phagocytosis (cell eating): the cell engulfs large particles. Pseudopods wrap around the particle, forming a food vacuole. Lysosomes fuse with it and digest the contents.
Pinocytosis (cell drinking): small portions of extracellular fluid and dissolved molecules are "gulped" into tiny vesicles. Non-specific.
Receptor-mediated endocytosis: specific solutes bind to membrane receptors, triggering vesicle formation. Highly selective and controlled.
Channels: passive transport.
Carriers: passive transport (facilitated diffusion).
Pumps: active transport.
The sodium-potassium pump is what allows your neurons to fire. Every nerve impulse depends on the ion gradients this pump maintains. Kidney dialysis works on the same principles as osmosis and selective permeability: blood is passed over a membrane that lets waste through while retaining blood cells and proteins. Cystic fibrosis treatments target the faulty chloride channel, and newer therapies (CFTR modulators) work by helping the misfolded protein reach the membrane surface and function properly.
Students often think molecules stop moving once equilibrium is reached. They do not. At equilibrium, molecules continue to move randomly in all directions; there is simply no net movement in one direction.
Students sometimes confuse "hyper" and "hypo" by thinking about water rather than solute. Remember: the terms describe solute concentration relative to the cell. Hypertonic means more solute outside, so water leaves the cell.
Students frequently assume facilitated diffusion requires energy because proteins are involved. It does not. The concentration gradient provides the driving force; the protein simply provides a passage.
Students often mix up the sodium-potassium pump's stoichiometry. It moves 3 Na+ out and 2 K+ in per ATP, not equal numbers of each.
⚠️ You must know the definitions of isotonic, hypertonic, and hypotonic, and predict what happens to both animal and plant cells in each. This is marked "NEED TO KNOW" in the lecture slides.
⚠️ Be able to walk through all six steps of the sodium-potassium pump cycle, including which ion binds when, the role of phosphorylation, and the shape changes.
⚠️ Know which molecules can and cannot pass through the lipid bilayer unaided, and why.
⚠️ Distinguish passive transport, facilitated diffusion, and active transport by energy requirement and direction of movement relative to the gradient.
⚠️ Know all three types of endocytosis and what distinguishes them (phagocytosis, pinocytosis, receptor-mediated).
⚠️ Understand cotransport: how the proton pump gradient drives sucrose uptake in plant cells.
True or False: Facilitated diffusion requires ATP. (False: it is passive transport, no energy required.)
Fill in the blank: In a hypertonic solution, an animal cell will _____. (Shrink / shrivel / crenate.)
True or False: The sodium-potassium pump moves equal numbers of Na+ and K+ ions. (False: it moves 3 Na+ out and 2 K+ in.)
Fill in the blank: The three types of endocytosis are _____, _____, and _____. (Phagocytosis, pinocytosis, receptor-mediated endocytosis.)
True or False: Water moves from areas of high solute concentration to areas of low solute concentration during osmosis. (False: water moves towards high solute concentration, i.e. from low solute to high solute.)
Q: A red blood cell is placed in a solution and begins to swell. Is the solution isotonic, hypertonic, or hypotonic relative to the cell? Explain.
A: The solution is hypotonic. It has a lower solute concentration than the cell interior, so water moves into the cell by osmosis, causing it to swell.
Q: Explain why the sodium-potassium pump is considered an electrogenic pump.
A: It moves 3 Na+ ions out for every 2 K+ ions in per cycle. Because it moves more positive charges out than in, it generates a net negative charge on the cytoplasmic side, contributing to the membrane potential.
Q: A molecule of glucose cannot pass through the lipid bilayer on its own. How does it enter the cell?
A: Glucose enters by facilitated diffusion through a specific carrier protein. The carrier binds glucose and changes shape to transport it across the membrane, moving it down its concentration gradient without energy input.
Q: How does cotransport work in plant cells? Use the H+/sucrose cotransporter as an example.
A: The proton pump uses ATP to actively pump H+ out of the cell, creating a H+ concentration gradient. As H+ diffuses back into the cell down its gradient through the cotransporter, sucrose is carried along against its own concentration gradient. The energy of ATP is used indirectly.
Q: Compare phagocytosis, pinocytosis, and receptor-mediated endocytosis.
A: Phagocytosis (cell eating) engulfs large particles using pseudopods to form a food vacuole. Pinocytosis (cell drinking) takes in small amounts of fluid and dissolved molecules non-specifically via tiny vesicles. Receptor-mediated endocytosis is highly specific: particular solutes bind to surface receptors, triggering vesicle formation and allowing controlled uptake.
Q: Why does a plant cell in a hypotonic solution not burst, while an animal cell does?
A: The plant cell has a rigid cell wall that resists further expansion once the cell becomes turgid. The animal cell lacks a cell wall and has no structural resistance to swelling, so it can lyse.
Q: In cystic fibrosis, what transport defect causes the thickened mucus?
A: The CFTR chloride channel is non-functional or absent from the membrane. Cl- ions cannot leave the cell, so water is retained inside (water follows solute), and the mucus lining the airways becomes abnormally thick.
Membrane transport is the foundation for cellular respiration (Chapters 9-10), where the proton gradient across the inner mitochondrial membrane drives ATP synthesis via chemiosmosis. Neuron signalling (Chapter 48) depends entirely on the Na+/K+ pump and voltage-gated ion channels covered here. The endocytosis and exocytosis material connects to the endomembrane system (Chapter 6) and to immune cell function (phagocytosis by white blood cells).
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