International A Level Biology / Membranes and transport
Active transport, endocytosis and exocytosis revision notes
Edexcel International A Level Biology (YBI11), chapter 4: Membranes and transport, specification point 2.5. Free lesson.
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Cells use active transport to move substances against their concentration gradient, and use endocytosis and exocytosis to move large materials in vesicles. These processes need ATP, so they are active transport processes.
How active transport moves substances uphill
The cell surface membrane does not let charged ions pass through its bilayer on their own. Active transport moves a substance from a lower to a higher concentration, against its concentration gradient, using energy from ATP. For example, a cell can move sodium ions out even when there are fewer inside than outside. The direction matters: state that the substance moves from lower to higher concentration. Active transport uses carrier proteins. Channel proteins are water-filled pores and only allow ions to move down their gradient by facilitated diffusion.
How a carrier protein uses ATP
A carrier moves an ion uphill by changing shape. ATP is the immediate energy source: respiration makes ATP, which transfers energy to the carrier where it is needed. Each trip needs ATP. ATP is hydrolysed, meaning it is split using water. This forms ADP and inorganic phosphate, written as ATP → ADP + Pᵢ. The phosphate binds to the carrier and helps it change shape. The phosphate later leaves, allowing the carrier to reset.
How endocytosis takes material into a cell
Endocytosis moves large particles into a cell in bulk. A protein carrier cannot carry something as large as a bacterium, so the cell surface membrane folds around the particle and forms a vesicle, a small sac of membrane. The process uses ATP, but does not use a carrier protein.
How exocytosis releases material
Exocytosis is bulk transport out of the cell. It releases substances the cell has made, such as an enzyme carried in a vesicle. Like endocytosis, it uses ATP.
How the transport routes compare
The first three routes are passive, so they do not use ATP. Active transport, endocytosis and exocytosis need ATP. Check both the direction and the way a substance crosses the membrane. Diffusion: small or non-polar molecules cross the bilayer, down their concentration gradient. Facilitated diffusion: charged or polar substances move down their concentration gradient through channel or carrier proteins. Osmosis: free water molecules move down a water potential gradient across a partially permeable membrane. Active transport: substances move against their concentration gradient…
Applying active transport to root hair cells
Root hair cells absorb nitrate ions from soil water even when the nitrate concentration is lower in the soil than inside the cells. This is active transport because the ions move against their concentration gradient. Mitochondria make ATP by aerobic respiration, supplying energy for the carrier proteins.