3.1 Organisation of the bilayer #
3.1.1 Amphipathic lipids #
Membrane lipids are amphipathic: a hydrophilic head and hydrophobic tails. In water they seal into bilayers, or into closed vesicles, because an edge would leave tails exposed to water. The bilayer is the only arrangement that is both satisfied chemically and able to enclose a compartment. Typical thickness is 5–8 nm.
The main lipids of animal membranes are glycerophospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol), sphingomyelin, and cholesterol. Glycolipids carry sugars and sit almost entirely in the outer leaflet, where they form part of the glycocalyx, the carbohydrate coat used in recognition.
The two leaflets are different. Phosphatidylserine is held on the cytosolic face by flippases that spend ATP. Its appearance on the outside is an “eat me” signal on apoptotic cells. Making the bilayer and then making it asymmetric are separate steps. Phospholipids are synthesised on the cytosolic face of the ER and scrambled across both leaflets there; the plasma membrane later imposes asymmetry.
3.1.2 Fluidity and cholesterol #
Lipids and many proteins diffuse laterally in the plane of the membrane. Frye and Edidin fused mouse and human cells and watched surface antigens mix, which is the classical demonstration. A lipid molecule can travel the length of a bacterium in about a second. Flip-flop from one leaflet to the other is rare without a protein catalyst, because the head would have to cross the hydrophobic core.
Fluid Mosaic
Fluidity depends on temperature, on the length and unsaturation of the tails, and on cholesterol. Short chains and cis double bonds (which kink the chain) keep the bilayer fluid at lower temperature. Cholesterol slots between phospholipids. It restrains movement when the membrane would otherwise be too fluid, and it prevents the tails from packing into a gel when the membrane would otherwise freeze. Organisms adjust tail composition as temperature changes. This is called homeoviscous adaptation.
“Lipid rafts” are small, transient regions enriched in sphingolipids and cholesterol, proposed to concentrate particular proteins. Caveolae, flask-shaped pits containing caveolin, are the morphologically clear version. Rafts in living cells are real but smaller and shorter-lived than the detergent-resistant fractions prepared in the cold, and those fractions should not be equated with rafts.
3.1.3 The fluid-mosaic model #
Singer and Nicolson (1972) described the membrane as a two-dimensional solution of oriented lipids and proteins. The evidence that forced proteins into the bilayer, rather than smeared on its surface, was:
- freeze-fracture replicas show particles inside the hydrophobic plane;
- integral proteins are released only by detergents that dissolve the bilayer, not by salt, which releases peripheral proteins;
- several proteins span the membrane, with parts exposed on both sides, shown by labelling and by sequencing hydrophobic transmembrane helices.
Integral membrane proteins usually cross as alpha helices of about 20 hydrophobic residues, or, in the outer membranes of bacteria, mitochondria, and chloroplasts, as beta barrels (porins). Peripheral proteins bind to lipid heads or to integral proteins and can be washed off. Many proteins are confined, despite the fluid bilayer, by attachment to the cytoskeleton or the extracellular matrix, or by tight junctions that fence off one region of a polarised cell from another. Fluidity is the default; restriction is imposed for a reason.
3.1.4 The plasma membrane as a boundary #
The plasma membrane is a selective barrier, a sensor, and a platform for attachment. It keeps macromolecules and most polar solutes in or out, carries receptors for hormones and matrix, and anchors the cell to neighbouring cells and to the extracellular matrix. In bacteria and archaea it is also the energy-transducing membrane. In eukaryotes that job has moved to the inner mitochondrial membrane and the chloroplast thylakoid, which are topologically descendants of bacterial plasma membranes.
3.2 How solutes cross #
3.2.1 A decision table #
| Route | What uses it | Protein? | Energy beyond the gradient? | Direction |
|---|---|---|---|---|
| Simple diffusion | O₂, CO₂, N₂, small uncharged lipids such as steroid hormones | No | No | Down its concentration gradient |
| Through a channel | Ions, water (aquaporins) | Yes | No | Down the electrochemical gradient |
| Facilitated diffusion via a carrier | Glucose into red blood cells (GLUT) | Yes | No | Down its gradient |
| Primary active transport | Na+, K+, Ca2+, H+ pumps | Yes | ATP or light or redox energy, directly | Against the gradient |
| Secondary active transport | Glucose and amino acids into intestinal epithelium (SGLT); many bacterial sugar uptake systems | Yes | The gradient of a second solute, usually Na+ or H+, which a pump created | The driving solute goes down; the cargo can go up |
A membrane protein does not decide “active or passive” by its shape alone. The test is whether the cargo can be moved against its electrochemical gradient, and if so, where the free energy comes from.
3.2.2 Diffusion, osmosis, and water channels #
Net diffusion continues until the concentration is equal on both sides. The rate across a bilayer is high for small nonpolar molecules, low for small polar molecules such as water and urea, and essentially zero for ions and for anything large and polar. Water does cross pure lipid bilayers slowly. In cells that must move water quickly (kidney collecting duct, red blood cells, plant roots) it goes through aquaporins, channels that pass water and exclude protons. Their discovery, by Agre, explained how tissues change water permeability in response to hormones such as vasopressin, which inserts aquaporins into the apical membrane.
Osmosis is the net diffusion of water toward the side with the higher concentration of solute that cannot cross. Cells behave as osmometers.
- In a hypotonic medium, water enters and an animal cell swells and can lyse. A plant cell swells too, but the wall stops it, and the resulting turgor pressure is what holds the plant up.
- In a hypertonic medium, water leaves. An animal cell shrivels. A plant cell undergoes plasmolysis: the plasma membrane pulls away from the wall.
- An isotonic medium produces no net water movement. Culture media and physiological saline are matched to the cytosol for this reason. Isolated organelles are kept in isotonic sucrose for the same reason.
3.2.3 Channels and carriers #
A channel forms a pore. Gating opens or closes it. Selectivity is chemical: a potassium channel dehydrates K+ and coordinates it with carbonyl oxygens spaced for K+ and not for the smaller Na+. Once open, a channel can pass millions of ions a second, which is fast enough for electrical signalling. Channels are not pumps. They dissipate gradients; they do not create them.
A carrier (transporter) binds the substrate on one side, changes conformation, and releases it on the other. It never opens a continuous pore. Rates are hundreds to thousands of molecules a second, closer to an enzyme than to a channel. The glucose transporter GLUT1 of red blood cells is the textbook passive carrier. It speeds glucose down its gradient and shows saturation and specificity, the kinetic signatures of a binding site.
3.2.4 Primary active transport: the sodium–potassium pump #
The Na+/K+ ATPase of animal plasma membranes is a P-type pump. In each cycle it hydrolyses one ATP, exports three Na+, and imports two K+. The cycle is electrogenic: one net positive charge leaves the cell per ATP. That contributes a few millivolts to the membrane potential. The larger role of the pump is to maintain the gradients that everything else spends.
Scheme Sodium Potassium Pump 2
Those gradients are expensive. In a resting animal cell the pump can account for roughly a third of ATP use, and in a nerve cell for more, because electrical activity constantly leaks the gradients back.
The mechanism, in outline, is an alternating-access cycle. Phosphorylation by ATP drives a conformational change that releases Na+ outside and then binds K+. Dephosphorylation returns the pump to the inward-facing form and releases K+ inside. Cardiac glycosides such as ouabain and digoxin bind the outward-facing pump and inhibit it. Intracellular Na+ rises, the Na+/Ca2+ exchanger slows, intracellular Ca2+ rises, and heart muscle contracts more strongly. A membrane protein is the drug target.
Other primary pumps to place beside it:
- Ca2+ ATPases in the plasma membrane and in the ER (the SERCA pump of muscle) keep cytosolic Ca2+ near 100 nM, against millimolar stores. This steep gradient is why opening a Ca2+ channel is a useful signal.
- V-type H+ ATPases acidify lysosomes, endosomes, and the plant vacuole.
- ABC transporters bind ATP and move a wide range of substrates. The cystic-fibrosis transmembrane conductance regulator is an ABC protein that functions as a chloride channel. The multidrug resistance protein P-glycoprotein exports hydrophobic drugs from cells, which is one reason chemotherapy fails.
Sodium Pump
3.2.5 Secondary active transport #
A symporter moves two solutes the same way. An antiporter exchanges them. Intestinal and kidney epithelial cells take up glucose from the lumen through SGLT, a Na+-glucose symporter, even when luminal glucose is scarce, because Na+ is flowing down the gradient the pump maintains on the basolateral membrane. Glucose leaves the cell into the blood through a passive GLUT carrier on the basolateral side. The epithelium is polarised, and the polarity of the transporters is the polarity of absorption. The same logic, with H+ instead of Na+, runs many uptake systems in bacteria and in plant and fungal plasma membranes.
3.2.6 Membrane potential #
An unequal distribution of charge across a thin membrane is a voltage. Typical resting animal cells sit between about −40 and −90 mV, inside negative. The immediate cause is selective permeability, mostly to K+. The pump has made K+ high inside and Na+ high outside. Open K+ channels let K+ leave. The departing positive charge makes the inside negative, and that voltage pulls K+ back. At equilibrium the chemical and electrical forces on K+ balance. The Nernst equation gives that voltage for one ion:
E = (RT / zF) ln([out] / [in])
which at room temperature for a monovalent cation is about 58 mV times log10([out] / [in]). For K+, with the inside concentration much higher, E_K is about −90 mV. The real resting potential is a little less negative because the membrane also leaks some Na+. The Goldman–Hodgkin–Katz equation weights each ion by its permeability. Changing which channels are open changes the voltage, which is the basis of the action potential in excitable cells: voltage-gated Na+ channels open, Na+ enters, the membrane spikes toward E_Na, then K+ channels restore the resting state. The pump does not make the spike. It maintains the gradients the spike spends.
3.3 Key terms, summary, and self-check #
Key terms. Amphipathic; fluid mosaic; leaflet asymmetry; aquaporin; channel; carrier; Na+/K+ ATPase; secondary active transport; membrane potential; Nernst potential.
Summary. The bilayer is a fluid, asymmetric barrier that polar molecules and ions do not cross unaided. Channels and passive carriers permit downhill movement. Pumps create gradients, and secondary transporters spend those gradients. The resting voltage is mostly a K+ diffusion potential standing on gradients the sodium pump maintains.
Self-check.
1. Why does phosphatidylserine on the outer leaflet mean something different from phosphatidylserine on the inner leaflet?
2. A solute crosses a membrane until its concentrations are equal, and the process saturates as concentration rises. Channel, pump, or passive carrier?
3. Why does inhibiting the Na+/K+ pump eventually stop intestinal glucose uptake, even though SGLT itself does not hydrolyse ATP?
4. Red blood cells in distilled water lyse. Plant leaf cells in distilled water become turgid and do not burst. What structure explains the difference?
5. Estimate, qualitatively, what happens to the resting potential if a cell’s K+ channels close and its Na+ permeability stays the same.
