2.8Cells

Mechanisms of Transport — AP BIOLOGY Revision Notes & Topical Questions

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2.8 Mechanisms of Transport — Notes — AP Biology notes: Describe the processes that allow ions and other molecules to move across membranes.Gradients — selective permeability allows solute concentration gradients to form, either because the membrane blocks a substance or because …

Passive transport
Rate of simple diffusion
Active transport
Metabolic poisons
Electrogenic pumps
Bulk transport

Section 1 of 12Introduction to membrane transport

Gradients

Gradients selective permeability allows solute concentration gradients to form, either because the membrane blocks a substance or because proteins pump it to build a high concentration on one side

Factors deciding the mechanism

The substance to be transported, whether small, large, polar or nonpolar.
The concentration gradient across the membrane.

Mechanisms

Mechanisms passive transport and active transport (including endocytosis and exocytosis)
- (diffusion, osmosis)

Section 2 of 12Passive transport

Passive transport

Passive transport the net movement of molecules from high to low concentration without direct input of metabolic energy

Types

Simple diffusion moves molecules down a concentration gradient directly across the phospholipid bilayer.
Facilitated diffusion moves molecules down a concentration gradient through specialised transport proteins.
Osmosis moves water down a water potential gradient between the phospholipids or via aquaporins.

Rate of simple diffusion

Rate of simple diffusion it increases with a steeper gradient, higher temperature, larger surface area and smaller molecules

Section 3 of 12Active transport

Active transport

Active transport the direct input of energy to move molecules from low concentration to high concentration

Energy

Energy it uses ATP from respiration and always involves carrier proteins (pumps) across a membrane

Gradients

Gradients it establishes and maintains concentration gradients, such as K⁺ high inside and Na⁺ high outside cells,
which are essential for nerve impulses and cotransport

Metabolic poisons

Metabolic poisons substances such as cyanide stop ATP production,
so primary active transport stops and secondary active transport follows as gradients dissipate,
while passive transport continues

Section 4 of 12Primary and secondary active transport

Primary and secondary active transport

Primary active transport

Primary active transport it uses ATP directly to power a pump, such as Na⁺/K⁺ ATPase or proton pumps

Secondary active transport (cotransport)

Secondary active transport (cotransport) it uses the energy stored in a gradient made by primary active transport to move another substance against its gradient

Direction

Direction in symport both substances move in the same direction and in antiport they move in opposite directions

Examples

The Na⁺–glucose cotransporter in intestinal cells moves Na⁺ down its gradient and glucose against its gradient into the cell,
- and glucose leaves into the blood by facilitated diffusion.
In plants, proton pumps create an H⁺ gradient and H⁺–sucrose cotransporters load sucrose into the phloem.

Electrogenic pumps

Electrogenic pumps pumps such as the proton pump generate voltage across membranes, storing energy for cellular work

Section 5 of 12Na⁺/K⁺ ATPase

Three sodium ions from inside the axon bind to the pump.

ATP attaches and transfers a phosphate to the pump, which changes shape and opens to the outside.

The three sodium ions are released outside the axon.

Two potassium ions from outside bind to their binding sites.

The phosphate is released, which changes the shape of the pump again.

The shape change releases the two potassium ions inside the axon.

Na⁺/K⁺ ATPase

Function

Function the pump uses ATP to maintain sodium and potassium gradients and the membrane potential of nerve cells, so it supports nerve impulses

Ratio

Ratio each cycle uses one ATP to pump 3 Na⁺ out and 2 K⁺ in

Section 6 of 12Exocytosis and endocytosis

Bulk transport

Bulk transport large molecules, large amounts of substances, particles and even whole cells are moved in membrane-bound vesicles,
which needs energy,
so both processes are active transport

Exocytosis

Exocytosis materials are transported out of the cell
The substance is packaged into internal vesicles.
The vesicles travel to the plasma membrane, fuse with it and release their contents outside the cell.
Examples are neurotransmitter release, insulin secretion, digestive enzyme secretion and plant cell wall materials.
The vesicle membrane becomes part of the plasma membrane, so exocytosis adds membrane to the cell surface.

Endocytosis

Endocytosis the cell takes in large molecules and particles
The plasma membrane folds in on itself and engulfs the material in a new vesicle.
The vesicle carries the material into the cell.

Membrane balance

Membrane balance membrane added by exocytosis is removed by endocytosis, so cell surface area stays roughly constant

Food vesicles

Food vesicles they fuse with lysosomes, whose enzymes digest the contents and the products enter the cytoplasm

Recycling

Recycling vesicles carrying receptors may be recycled back to the surface

Section 7 of 12Types of endocytosis

Type	Description	Example
Phagocytosis (cell eating)	The cell extends pseudopodia round a large particle and engulfs it in a vacuole that fuses with a lysosome	White blood cells engulfing bacteria; Amoeba feeding
Pinocytosis (cell drinking)	The cell takes in droplets of extracellular fluid and dissolved substances nonspecifically in small vesicles	Absorption by many cells, such as kidney cells
Receptor-mediated endocytosis	Specific molecules bind to receptors concentrated in coated pits, which pinch into vesicles	Uptake of cholesterol-carrying LDL by liver cells

Types of endocytosis

Selectivity

Selectivity in receptor-mediated endocytosis only molecules that bind the receptors enter, so substances in low concentration can be taken up

Disease

Disease in familial hypercholesterolaemia the LDL receptors are faulty, so cholesterol is not taken up and blood levels rise

Section 8 of 12Vesicle traffic and the endomembrane system

Vesicle traffic and the endomembrane system

Endomembrane system

Endomembrane system the movement of membrane by vesicles is the basis of the system, and cells with rapid secretion have extensive ER and Golgi

Directed traffic

Directed traffic each vesicle carries identifying proteins and docking proteins on the target membrane recognise it,
- and botulinum toxin cuts fusion proteins and causes paralysis

Pulse-chase

Pulse-chase radioactive amino acids given briefly showed the label in rough ER first, then the Golgi, then secretory vesicles and outside the cell

Section 9 of 12Examples of vesicle transport

Examples of vesicle transport

Nerve terminal

Nerve terminal calcium entry triggers neurotransmitter vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the cleft,
- and the membrane is retrieved by endocytosis

Digestive and immune systems

Digestive and immune systems intestinal cells take up fat droplets, the pancreas releases enzymes by exocytosis, macrophages engulf pathogens and plasma cells export antibodies

Plants

Golgi vesicles carry cell wall materials and membrane to the surface and to the growing cell plate

Protists

Protists food vacuoles form by phagocytosis and combine with lysosomes

Pathogens

Pathogens viruses bind receptors and enter by receptor-mediated endocytosis, so blocking the receptor can prevent infection

Section 10 of 12More from the specification

Hydrophobic

Water-hating; non-polar and unable to dissolve in water.

Tonicity

The ability of a solution to change the water content of a cell.

Hydrophilic

Water-loving; polar or charged and able to dissolve in water.

Pressure potential

Physical pressure on a solution.

Section 11 of 12Key terms

Term	Meaning
Exocytosis	Release of material by vesicle fusion.
Endocytosis	Uptake by membrane infolding.
Phagocytosis	Cell eating; uptake of large particles.
Pinocytosis	Cell drinking; uptake of fluid.
Receptor-mediated endocytosis	Specific uptake via receptors, such as LDL.
Cotransport	Secondary active transport driven by an ion gradient.
Symport	Two substances move in the same direction.
Antiport	Two substances move in opposite directions.
Vesicle	A membrane-bound sac that carries materials.

Section 12 of 12Exam technique

Saying bulk transport needs no energy — it uses ATP.

Confusing symport (same direction) with antiport (opposite directions).

Thinking cotransport uses ATP directly — it uses a gradient made by a pump.

Saying a metabolic poison stops passive transport — it only stops transport that needs ATP.

Thinking vesicles pass through the membrane — they fuse with it or form from it.

Last updated: 18 October 2026