How does active transport build and use electrochemical gradients, and how does bulk transport move large material?
Topic 2.9 Mechanisms of Transport: explain how active transport and bulk transport move ions and large molecules across membranes and establish electrochemical gradients.
A focused answer to AP Biology Topic 2.9, covering active transport, the sodium-potassium pump, electrochemical gradients, secondary active transport, and bulk transport by endocytosis and exocytosis.
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What this topic is asking
The College Board (Topic 2.9) wants you to explain the mechanisms by which cells move ions and large molecules: active transport (including the sodium-potassium pump and the electrochemical gradients it builds) and bulk transport (endocytosis and exocytosis). Energy use is central.
Active transport and pumps
The sodium-potassium pump is the standard example. Per cycle it hydrolyzes one ATP to pump three sodium ions out and two potassium ions in, both against their gradients. This keeps sodium high outside and potassium high inside the cell.
Electrochemical gradients
The electrochemical gradient powers other processes:
- Nerve impulses (action potentials) depend on sodium and potassium gradients.
- Secondary active transport uses the energy stored in one ion's gradient (for example sodium flowing back in) to drag another substance (such as glucose) against its gradient, without directly using ATP.
Bulk transport
Large molecules and particles move in vesicles:
- Endocytosis: the membrane engulfs material into a vesicle (phagocytosis of solids, pinocytosis of fluids, receptor-mediated endocytosis for specific molecules such as cholesterol).
- Exocytosis: a vesicle fuses with the plasma membrane to release its contents (secreting neurotransmitters, hormones or enzymes).
Both require energy and connect to the endomembrane system.
Try this
Q1. Identify how many sodium and potassium ions the sodium-potassium pump moves per ATP, and in which directions. [2 points]
- Cue. Three sodium ions out of the cell and two potassium ions into the cell per ATP, both against their gradients.
Q2. Explain why exocytosis is classified as a form of active transport. [2 points]
- Cue. Exocytosis moves large material out of the cell using vesicles and requires energy (ATP) to transport and fuse the vesicle with the membrane.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 20194 marksSection II (short FRQ). The sodium-potassium pump uses ATP to maintain ion gradients across the membrane. (a) Describe the movement of sodium and potassium ions by the pump. (b) Explain how this establishes an electrochemical gradient. (c) Identify one process that depends on this gradient.Show worked answer →
A 4-point concept-explanation FRQ.
(a) Describe (1 point): the pump moves three sodium ions out of the cell and two potassium ions in, both against their concentration gradients, using ATP.
(b) Explain (1 point): because more positive ions are pumped out than in, the inside becomes more negative and ion concentrations differ across the membrane, creating an electrochemical (charge plus concentration) gradient. (1 point) for noting both the charge and concentration components.
(c) Identify (1 point): the gradient drives nerve impulse transmission (action potentials), or secondary active transport such as glucose uptake.
Markers reward the 3 Na+ out / 2 K+ in detail, recognizing both the electrical and chemical parts of the gradient, and a valid dependent process.
AP 20233 marksSection I-style data question rewritten as a short FRQ. A cell uses 30% of its ATP on the sodium-potassium pump. The cell hydrolyzes 1.0 x 10^9 ATP per second. (a) Calculate the number of ATP per second used by the pump. (b) Explain why a resting nerve cell devotes so much energy to this pump.Show worked answer →
A 3-point quantitative and concept FRQ.
(a) Calculate (1 point): ATP per second.
(b) Explain (1 point): the pump continuously moves ions against their gradients to maintain the resting membrane potential; (1 point) ions leak back across the membrane, so the pump must run constantly, which is energetically costly but essential for nerve signalling.
Markers reward the correct value ( ATP/s) and an explanation tied to maintaining the gradient against constant ion leakage.
Related dot points
- Topic 2.6 Membrane Transport: describe the mechanisms that organisms use to transport large and small molecules across the membrane and the energy requirements of passive and active transport.
A focused answer to AP Biology Topic 2.6, covering passive transport (diffusion and osmosis) versus active transport, the role of concentration gradients and ATP, and bulk transport by endocytosis and exocytosis.
- Topic 2.7 Facilitated Diffusion: explain how the structure of channel and carrier proteins allows the facilitated diffusion of polar molecules and ions across a membrane.
A focused answer to AP Biology Topic 2.7, covering facilitated diffusion through channel and carrier proteins, aquaporins, why it is passive, and how it differs from simple diffusion and active transport.
- Topic 2.5 Membrane Permeability: explain how the structure of biological membranes influences selective permeability.
A focused answer to AP Biology Topic 2.5, covering selective permeability, why the phospholipid bilayer blocks polar and charged substances, the factors affecting permeability, and the role of transport proteins.
- Topic 2.8 Tonicity and Osmoregulation: explain how concentration gradients of water and solutes affect the movement of water into and out of cells, and how organisms regulate their water balance.
A focused answer to AP Biology Topic 2.8, covering hypotonic, hypertonic and isotonic solutions, osmosis, water potential, and how cells and organisms osmoregulate, with full worked water-potential calculations.
- Topic 2.1 Cell Structure: Subcellular Components: describe the structures and functions of the subcellular components and organelles of prokaryotic and eukaryotic cells.
A focused answer to AP Biology Topic 2.1, covering the organelles of eukaryotic cells (nucleus, ribosomes, ER, Golgi, mitochondria, chloroplasts, lysosomes, vacuoles) and the endomembrane system, with structure-to-function reasoning.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)