A gradient maintained at a cost, a membrane that decides what crosses, a receptor that converts a chemical signal into a cellular one, and an energy supply that fails before anything else does. Every system module on this site assumes those four things. This module builds them, in the order they depend on each other, and it is the right place to start if any later lesson has felt like it was assuming something you had not been shown.
Orientation
What this module covers, and how to work through it
The sequence is deliberate. Lesson 1 establishes what a cell is and what it costs to keep it alive, because the sodium-potassium pump consuming a large share of that budget is the fact the next two lessons rest on. Lesson 2 is transport, which is how anything gets in or out. Lesson 3 uses transport to build the resting membrane potential and then the action potential. Lesson 4 is signalling — how a molecule outside the cell changes what happens inside it — and cell injury, which is what the whole arrangement looks like when the energy runs out.
Two of these lessons are prerequisites named explicitly by other modules. Membranes and transport is assumed by the blood-brain barrier and cerebrospinal fluid lessons; membrane potentials is assumed by both neuronal excitability and cardiac electrical activity. If those lessons have felt like they were skipping a step, this is the step.
Syllabus
MMed syllabus mapping
| Syllabus area | Lessons | What is covered |
|---|---|---|
| Cellular organisation and energy | Lesson 1 | The cell as a steady state rather than an equilibrium; organelles and their functions; ATP production, aerobic and anaerobic; and what energy failure costs |
| Membranes and transport | Lesson 2 | Membrane structure and permeability; simple and facilitated diffusion; primary and secondary active transport; vesicular transport; osmosis, osmolarity, osmolality and tonicity |
| Membrane potentials | Lesson 3 | Electrochemical gradients; the Nernst and Goldman-Hodgkin-Katz equations; the Na⁺/K⁺-ATPase; the resting membrane potential; the action potential and its conduction |
| Cell signalling and injury | Lesson 4 | Receptor families; G proteins and second messengers; calcium as an intracellular signal; cellular mechanisms of anaesthetic action; reversible and irreversible cell injury, necrosis and apoptosis |
The pathway
All 4 lessons, in order
Part I
The cell and its membrane
What a cell is, what it costs to run, and how anything gets across the boundary. Nothing in the second half works without these.
- Cellular organisation and energy
The cell as a steady state maintained at a cost, the organelles and what each contributes, aerobic and anaerobic ATP production, and the sequence that follows when that production stops.
- Membranes and transport
The fluid mosaic membrane and what determines its permeability; diffusion, carrier-mediated transport, pumps and vesicular transport; and osmosis, with osmolarity, osmolality and tonicity kept properly apart.
Part II
Excitability and signalling
What the gradients built in part I are used for: an electrical signal, a chemical one, and what happens to both when the energy supply fails.
- Membrane potentials
Electrochemical gradients, the Nernst and Goldman-Hodgkin-Katz equations derived and interpreted, the pump that maintains them, and the action potential with an interactive trace.
- Cell signalling and injury
The receptor families and the G-protein cascades they drive, calcium as the common intracellular signal, where anaesthetics act at cellular level, and the difference between reversible injury, necrosis and apoptosis.
Objectives
The full objective wording
- Organisation and energyCellular organisation, organelles, the steady state and cellular energy production
- Membranes and transportMembrane structure and permeability; diffusion, facilitated diffusion, active and vesicular transport; osmosis and tonicity
- Membrane potentialsElectrochemical gradients, the Nernst and Goldman-Hodgkin-Katz equations, the Na⁺/K⁺-ATPase, the resting membrane potential, the action potential and its conduction
- Signalling and injuryReceptor families, G proteins and second messengers, calcium as a signal, anaesthetic mechanisms at the cellular level, and reversible and irreversible cell injury