Transport Across Cell Membranes

AQA spec ref: 3.2.3 - Transport across cell membranes

The plasma membrane is a selectively permeable phospholipid bilayer that controls the movement of substances into and out of the cell. Different substances cross membranes by different mechanisms depending on their size, polarity, and the concentration gradient. The four mechanisms you need for AQA are: diffusion, facilitated diffusion, osmosis, and active transport. You also need to know co-transport (relevant to the ileum) and the role of vesicles in endocytosis/exocytosis.

The Fluid Mosaic Model

The membrane is described by the fluid mosaic model (Singer and Nicolson, 1972). It is:

Fluid - the phospholipid molecules are not fixed; they move laterally within the bilayer (at body temperature, the membrane is a viscous liquid rather than a solid). Cholesterol, which is embedded in the bilayer, regulates fluidity - it prevents the fatty acid tails from packing too tightly (which would solidify the membrane) but also prevents them from being too disordered.
Mosaic - various proteins are embedded in or on the surface of the bilayer, giving it a mosaic-like appearance.

Phospholipid structure: each molecule has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty acid tails. In water, they spontaneously arrange into a bilayer with the tails pointing inward, away from the aqueous environments on both sides. This is thermodynamically driven - the hydrophobic tails are shielded from water.

Membrane proteins:

Intrinsic (integral) proteins - span the full width of the bilayer (transmembrane proteins). These include channel proteins and carrier proteins.
Extrinsic (peripheral) proteins - attached to one surface only. These include enzymes, receptors, and structural proteins.
Glycoproteins - proteins with attached carbohydrate chains on the outer surface. Important in cell signalling and cell recognition (see Cell Recognition and the Immune System).
Glycolipids - lipids with carbohydrate chains. Also on the outer surface; involved in cell recognition.

Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. It is a passive process - no energy (ATP) is required. Movement occurs because of the random kinetic energy of particles.

Diffusion continues until equilibrium is reached (no net movement, though individual particles continue moving).

Rate of diffusion is affected by:

Concentration gradient - steeper gradient=faster rate
Surface area - larger surface area=faster rate (this is why gas exchange surfaces have so many folds)
Thickness of membrane/diffusion distance - thinner = faster (Fick's Law: rate ∝ surface area × concentration difference / thickness)
Temperature - higher temperature=more kinetic energy=faster diffusion
Size of molecule - smaller molecules diffuse faster
Lipid solubility - non-polar, lipid-soluble molecules (like O₂, CO₂, steroid hormones) can diffuse directly through the lipid bilayer; polar or charged molecules cannot

Facilitated Diffusion

Large or polar molecules cannot cross the lipid bilayer directly. They cross via protein channels or carrier proteins - this is facilitated diffusion. Like simple diffusion, it is passive (no ATP required) and moves substances down their concentration gradient.

Channel proteins form water-filled pores through the membrane. They are specific - each channel is shaped to allow only one type of ion or molecule through. Some are permanently open; others are gated channels that open only in response to a signal (e.g. a change in voltage, or binding of a ligand). Ion channels (for Na⁺, K⁺, Ca²⁺, Cl⁻) are all facilitated by gated channel proteins - this is critical for nerve impulse transmission (see Neurons and Synapses).

Carrier proteins bind the molecule on one side of the membrane, change shape, and release it on the other side. They are slower than channel proteins (shape change takes time) but highly specific. They also move substances down the concentration gradient without using ATP.

Osmosis

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane. It is a specific type of diffusion - only water molecules move.

Water potential (Ψ) is measured in kilopascals (kPa). Pure water has a water potential of 0 kPa, which is the maximum. Adding solutes always lowers (makes more negative) the water potential. So solutions always have negative water potential values (e.g. −500 kPa, −1000 kPa). Water moves from the less negative (higher) to the more negative (lower) water potential.

\Psi=\Psi_s+\Psi_p

Where Ψs = solute potential (always negative or zero) and Ψp = pressure potential (often positive in plant cells due to turgor; zero in animal cells and solutions in a container).

Terminology:

Isotonic - same solute concentration as the cell. No net water movement.
Hypotonic - lower solute concentration than the cell (higher water potential). Water enters the cell by osmosis.
Hypertonic - higher solute concentration than the cell (lower water potential). Water leaves the cell.

Effects on cells:

Animal cells (no cell wall):

In hypotonic solution→water enters→cell swells→lysis (bursts)
In hypertonic solution→water leaves→cell shrinks→crenation
In isotonic solution→no net movement→normal shape

Plant cells (have cell wall):

In hypotonic solution → water enters → vacuole expands → cell becomes turgid (wall resists further expansion, creating turgor pressure - this is the normal state for plant cells and is what keeps plants rigid)
In hypertonic solution → water leaves → vacuole shrinks → cytoplasm pulls away from cell wall → plasmolysis (the point where cytoplasm just detaches is the incipient plasmolysis point)
Flaccid - a plant cell with no turgor pressure but not yet plasmolysed (in equilibrium with surrounding water potential)

Active Transport

Active transport moves molecules or ions against their concentration gradient (from low to high concentration). It requires ATP and uses carrier proteins. The carrier protein binds the molecule on the low-concentration side, ATP is hydrolysed, the protein changes shape, and the molecule is released on the high-concentration side.

Active transport is essential for accumulating substances against their gradient - for example:

Absorption of glucose and amino acids from the ileum even when concentrations are low
Maintaining the Na⁺/K⁺ gradient across nerve cell membranes (Na⁺/K⁺-ATPase pump)
Uptake of mineral ions (e.g. nitrate, phosphate) by root hair cells against a steep gradient

Co-transport

Co-transport (secondary active transport) uses the electrochemical gradient established by active transport to power the transport of a second molecule.

The key AQA example is glucose absorption in the ileum:

The Na⁺/K⁺-ATPase pump on the basolateral surface of epithelial cells actively pumps Na⁺ out of the cell (into the blood), keeping the Na⁺ concentration inside the cell very low. This requires ATP.
Because Na⁺ is at low concentration inside the cell and high concentration in the gut lumen, Na⁺ diffuses back in through a sodium-glucose co-transporter (SGLT1) on the apical surface, carrying glucose with it. Glucose moves against its own concentration gradient, powered by the Na⁺ gradient.
Glucose accumulates inside the cell and exits into the blood via GLUT2 facilitated diffusion on the basolateral surface.

So glucose absorption ultimately depends on active transport (to maintain the Na⁺ gradient), but the glucose itself moves via co-transport without directly using ATP.

Endocytosis and Exocytosis

Some molecules are too large to cross the membrane via proteins. Instead, the membrane itself engulfs or releases them using vesicles:

Endocytosis - the cell membrane folds inward around material outside the cell, forming a vesicle that pinches off and enters the cytoplasm. Used by white blood cells to engulf pathogens (phagocytosis) and by cells to take in fluids (pinocytosis).

Exocytosis - vesicles inside the cell (e.g. secretory vesicles from the Golgi apparatus) fuse with the plasma membrane, releasing their contents outside. This is how hormones, neurotransmitters, and digestive enzymes are secreted.

Both processes require ATP for the membrane to deform and reform.

Summary

Diffusion - passive, down concentration gradient, no protein needed. Non-polar/small molecules through bilayer directly.
Facilitated diffusion - passive, down gradient, via channel or carrier proteins. For large/polar molecules.
Osmosis - passive movement of water from high to low water potential across selectively permeable membrane.
Active transport - against gradient, requires ATP, uses carrier proteins.
Co-transport - couples movement of one molecule (Na⁺ down gradient) to movement of another (glucose against gradient); ATP indirectly required.
Endocytosis - membrane engulfs material; exocytosis - vesicles fuse with membrane to release contents.
Water potential: pure water = 0 kPa; adding solutes lowers Ψ. Water moves from less negative to more negative Ψ.
Plant cells: turgid (water entered), flaccid (no pressure), plasmolysed (water left, cytoplasm detaches from wall).

AQA Exam Tips

Define osmosis precisely: "net movement of water molecules from a region of higher water potential to a region of lower water potential, across a selectively permeable membrane." Every word matters - include "net," "water potential" (not concentration), "selectively permeable membrane."
Water potential numbers: if asked to predict direction of water movement, compare values - water moves from the less negative (higher) to the more negative (lower). E.g. −400 kPa to −700 kPa.
Active vs facilitated diffusion: both use carrier proteins, but active transport moves against the gradient and requires ATP. AQA will show you a graph of transport rate vs. concentration and ask whether it's active or facilitated - look for whether rate plateaus (carrier protein saturation = facilitated) or whether rate continues even against a gradient (= active).
Co-transport AQA question: explain how glucose is absorbed in the ileum. Step 1: Na⁺/K⁺ pump lowers Na⁺ inside cell (ATP). Step 2: Na⁺ re-enters via co-transporter, carrying glucose in against its gradient. Step 3: Glucose leaves cell by facilitated diffusion into blood.
Fick's Law: rate ∝ (surface area × concentration difference) / diffusion distance. Used in any question about gas exchange efficiency.
Fluid mosaic model: phospholipid bilayer is fluid (lateral movement possible); proteins are embedded in mosaic pattern. Cholesterol regulates fluidity. Always mention selective permeability when describing membrane function.
Turgid/plasmolysed/flaccid: define precisely. Turgid = cell wall fully stretched by osmotic water entry. Plasmolysed = cytoplasm has pulled away from cell wall. Flaccid = no turgor, but not plasmolysed.