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
Saturated tails are straight and pack tightly; cis-unsaturated tails contain kinks and pack loosely. More unsaturation generally raises membrane fluidity.
Cooling → organisms can increase unsaturated tails to resist stiffening. Warming → more saturated/longer tails can restrain excess fluidity. This is homeoviscous adaptation.
Cholesterol inserts between animal-membrane phospholipids: its polar –OH aligns near the heads while its hydrophobic rings lie among the tails.
Low temperature: prevents tight packing and stiffening.
High temperature: restrains phospholipid movement and excess fluidity.
Also lowers permeability to small water-soluble molecules.
Because a bilayer is fluid and self-sealing, it can curve, bud and fuse while keeping aqueous contents separated from the cytosol.
Endocytosis: membrane invaginates and pinches off inward.
Exocytosis: an internal vesicle fuses and releases contents outside while adding membrane.
Gated ion channels are selective pores whose probability of opening changes with a signal. Once open, ions move passively down their electrochemical gradients.
Ligand-gated: chemical binding changes shape.
Voltage-gated: membrane potential changes opening.
Mechanical/other gates respond to deformation or intracellular signals.
For each ATP hydrolysed, the sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in, both against their gradients.
Unequal exchange is electrogenic and helps maintain low cytosolic Na⁺, high cytosolic K⁺ and the gradients used in nerve signalling and cotransport.
In intestinal and kidney epithelia, SGLT cotransports Na⁺ downhill with glucose uphill across the apical membrane. The Na⁺ gradient supplies the immediate energy.
basolateral Na⁺/K⁺ pump uses ATP → low intracellular Na⁺ → apical SGLT imports Na⁺ + glucose → GLUT lets glucose leave downhill
Cell-adhesion molecules connect cells to one another or to extracellular matrix, linking membrane recognition with tissue strength, sealing, communication and signalling.
cadherins → cell–cell anchoring
integrins → cell–matrix attachment and signalling
tight junctions → seal
gap junctions → direct small-molecule/ion communication