Movement into and out of cells – Cambridge A-Level Biology 9700
Movement into and out of cells
Understanding how substances cross the plasma membrane is essential for explaining nutrient uptake, waste removal, and cell signalling. The following notes describe the main mechanisms involved.
1. Simple Diffusion
Simple diffusion is the net movement of molecules from an area of higher concentration to an area of lower concentration, driven solely by the concentration gradient.
Occurs directly through the phospholipid bilayer.
Only small, non‑polar or uncharged molecules (e.g., O₂, CO₂, lipid‑soluble hormones) pass efficiently.
No energy (ATP) is required.
Rate depends on temperature, surface area, membrane thickness, and the diffusion coefficient (D).
Fick’s first law describes the flux (J) of a substance:
\$J = -D \frac{dC}{dx}\$
Suggested diagram: Molecules moving down a concentration gradient across a lipid bilayer.
2. Facilitated Diffusion
Facilitated diffusion also moves substances down their concentration gradient, but uses specific membrane proteins to aid transport.
Carrier proteins undergo conformational changes to move the solute.
Channel proteins provide a hydrophilic pathway (e.g., ion channels).
Selectivity is high – only the correct substrate fits the carrier.
Still passive; no ATP is consumed.
Suggested diagram: Carrier protein alternating between outward‑ and inward‑facing states.
3. Osmosis
Osmosis is the diffusion of water across a semi‑permeable membrane toward the region of higher solute concentration.
Water moves through the lipid bilayer and/or aquaporin channels.
Direction is determined by the water potential (Ψ): water moves from higher Ψ to lower Ψ.
Pressure potential (Π) can oppose osmotic flow (e.g., turgor pressure in plant cells).
Water potential equation:
\$\Psi = \Psis + \Psip\$
Suggested diagram: Water movement into a plant cell causing turgor pressure.
4. Active Transport
Active transport moves substances against their concentration gradient and requires energy, usually from ATP hydrolysis.
Primary active transport: ATP directly powers the pump (e.g., Na⁺/K⁺‑ATPase).
Secondary active transport: Uses the electrochemical gradient created by a primary pump (e.g., Na⁺‑glucose symporter).
Often involves integral membrane proteins that change conformation during the transport cycle.
Suggested diagram: Na⁺/K⁺‑ATPase cycle showing three Na⁺ out, two K⁺ in per ATP hydrolysed.
5. Endocytosis
Endocytosis is the process by which cells internalise extracellular material by engulfing it with the plasma membrane.
Phagocytosis – “cell eating”; large particles (e.g., bacteria) are taken up in vesicles.
Pinocytosis – “cell drinking”; fluid and dissolved solutes are internalised.