Membranes and membrane transport link bilayer structure, membrane proteins, selective permeability, gradients, and vesicle movement to cellular control; Students connect membrane structure to transport mechanisms across cellular boundaries.
A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails. In water, these interactions drive spontaneous bilayer formation.
heads face water + tails hide from water → continuous bilayer → exposed edges close → sealed membrane or vesicle
Diffusion is passive net movement from higher to lower concentration, driven by random kinetic motion. Particles continue moving both ways at equilibrium, but net movement becomes zero.
Steeper gradient + larger surface area + higher temperature → faster diffusion. Greater distance or membrane thickness → slower diffusion.
Integral proteins enter the hydrophobic bilayer and may span it; peripheral proteins attach to one membrane surface or to another protein.
Integral: channels, carriers, pumps, receptors, enzymes.
Peripheral: signalling partners, enzymes, cytoskeletal anchors and scaffolds.
Osmosis is passive net movement of water across a partially permeable membrane from higher water potential to lower water potential.
Lower solute concentration → higher water potential.
Higher solute concentration → lower water potential.
Aquaporins raise the rate but do not change the direction.
A channel is a hydrophilic pore that permits selected ions or molecules to diffuse down an electrochemical gradient without ATP.
Pore diameter + charge + chemical lining determine selectivity. Some channels remain open; gated channels change between closed and open states.
A pump binds a specific solute and couples ATP hydrolysis to a shape change that moves it against its concentration or electrochemical gradient.
bind on one side → ATP transfers energy, often by phosphorylation → conformation changes → solute released uphill → pump resets
Choose a transport route from particle chemistry and gradient direction: the bilayer sorts by size and polarity; proteins add molecular recognition; ATP permits uphill movement.
small non-polar + downhill → simple diffusion
ion/polar + downhill → channel or carrier
solute + uphill → pump or coupled transporter
very large cargo → vesicle
The fluid mosaic model describes laterally mobile lipids and many proteins arranged asymmetrically in a phospholipid bilayer.
freeze fracture → particles within membrane faces
protein extraction → integral vs peripheral attachment
fluorescent cell fusion → labelled proteins spread laterally