Lipid Bilayers, Membrane Proteins, Osmosis, and How Things Get Into and Out of Cells
by Professor Stephen T. Abedon Steve Abedon, Ph.D., has taught well over 100 individual classes and ~10 individual courses (including introductory biology, microbiology, physiology, biochemistry, etc.) over his 30+ years as a college professor. He has over 150 publications, including multiple monographs, and is ranked in the top 1 or 2 percent among researchers of all disciplines worldwide. His research focuses on bacteriophage biology and phage therapy. (abedon.1@osu.edu)
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Every cell is wrapped in a membrane, and the membranes of eukaryotic cells also divide their insides into compartments. A membrane is mostly a film of oily molecules only two molecules thick, and yet it decides what gets into a cell, what stays in, and what leaves. Much of what a cell does for a living comes down to that: bringing in what it needs from outside, and getting rid of what it has made but does not want.
The lipid bilayer by itself lets only a few kinds of molecules through, mostly small ones that dissolve in oil, along with water. Nearly everything else that crosses a membrane does so through a protein: a channel, a carrier, or a pump. Some things are too big for any of those and are taken in or sent out whole, wrapped in membrane. The tabs of this laboratory take those ways of crossing one at a time.
Every activity on this page asks you to commit to an answer before it tells you anything. That is deliberate: the answer is the reward, and it is worth much more when you have had to think your way to it first.
The tabs run from easier to harder. Those marked Going further bring in some arithmetic and go beyond what an introductory biology course usually asks.
Your answers are kept as you go. In most browsers they are still here if you leave and come back on the same device, though a private window, a different device, or cleared browser data will start you fresh. When you are done, download a summary to hand in or keep. It lists your answers and, for each, whether it was right the first time.
Water molecules are polar: they carry partial charges, and they cling to one another and to anything else that is polar or charged. Oils are nonpolar. Water has nothing to grab onto in an oil, so it arranges itself to touch the oil as little as possible. Oil droplets in water therefore merge into bigger droplets, because a bigger droplet has less surface per unit of volume, and less surface means less contact with water. Hydrophobic (“water-fearing”) substances clump in water not so much because they attract one another as because water, attracted to itself, pushes them together.
Sand coated with a hydrophobic layer, sold as “magic sand,” makes the point vividly. Water beads up on it rather than wetting it, and sand pushed under water comes back out dry. A detergent works the other way round. Each detergent molecule has an oily end and a water-loving end, so it can coat a blob of grease with its oily ends and face the water with its other ends, lifting the grease into suspension.
A phospholipid is built much like a fat. A fat (a triglyceride) has three fatty acid tails attached to glycerol. A phospholipid has only two, and in place of the third is a phosphate-containing group that carries electrical charges. The result is a molecule with a hydrophilic (“water-loving”) head and two hydrophobic tails: it is amphipathic, much like a detergent. Drawn simply, it looks like a clothespin.
Put phospholipids into water and they assemble on their own, with no energy spent and no enzyme needed, into structures that hide their tails from water. Which structure forms depends on the shape of the molecule. A molecule with one tail is roughly cone-shaped, wide at the head and narrow at the tail, and cones pack into a ball: a micelle. A phospholipid, with two tails, is roughly cylindrical, and cylinders pack into a flat sheet: a bilayer. A bilayer sheet cannot leave its edges exposed to water, so it closes on itself into a hollow sphere, a vesicle (a liposome when made in the laboratory). A cell membrane is such a bilayer, closed around the cell.
The same difference explains why detergents destroy membranes. Added to a bilayer, single-tailed detergent molecules mix in among the two-tailed phospholipids and break the sheet into small mixed micelles. That is also part of why washing with soap inactivates viruses wrapped in a membrane envelope, such as influenza viruses and coronaviruses.
A membrane is fluid because its lipids are not covalently bonded to one another. Each phospholipid diffuses about within its own layer, swapping places with its neighbors millions of times a second, much as molecules move about in a drop of oil. A membrane is a mosaic because it is made of more than one kind of piece: phospholipids, cholesterol in animal cells, proteins, and on the outer surface, sugars attached to proteins and lipids. The two words together name the model of membrane structure proposed by S. Jonathan Singer and Garth Nicolson in 1972.
Proteins, being much bigger, move about more slowly than lipids, and some are tethered to the cytoskeleton and hardly move at all. That proteins move at all was shown in 1970 by Larry Frye and Michael Edidin. They fused a mouse cell with a human cell and labeled each cell’s surface proteins with a different color. At first each color covered its own half of the fused cell. Within about 40 minutes at body temperature the two colors were mixed over the whole surface.
The fluidity is two-dimensional. A phospholipid moves easily within its own layer but only rarely crosses from one layer to the other, a move called a “flip-flop,” because that would drag its charged head through the oily middle. (Cells do move particular lipids between layers on purpose, with enzymes called flippases.) Membrane proteins never flip. A protein inserted with one end facing outward keeps that end facing outward for as long as it exists. The two faces of a membrane can therefore differ, in which lipids they hold and in which way their proteins point, and the differences last. That asymmetry is what lets a receptor have a hormone-binding end outside the cell and a signaling end inside.
A membrane must stay fluid to work. If it cools too far, its fatty acid tails line up and pack tightly, much as butter or bacon fat sets in the refrigerator, and the membrane turns into a stiff gel. A gelled membrane leaks, and its proteins stop working properly. Three things control how fluid a membrane is:
Many organisms adjust these to their temperature. Bacteria such as Escherichia coli, grown at a lower temperature, make membranes with more unsaturated fatty acids, so that the membrane ends up about as fluid as before. Michael Sinensky called this homeoviscous adaptation when he described it in 1974.
Cholesterol is a sterol, and sterols differ among organisms. Animals use cholesterol; fungi use ergosterol, which is why several antifungal drugs target ergosterol or its synthesis; plants use sterols of their own. Most bacteria have no sterols at all, though some have sterol-like molecules called hopanoids. Archaea are different again: their membrane lipids are built from branched chains joined to glycerol by ether rather than ester bonds, and in some archaea that live in very hot places a single lipid molecule spans the whole membrane, making a monolayer rather than a bilayer.
Nobody could see a membrane’s structure directly until well into the twentieth century, so it was worked out from indirect evidence, step by step. Put these steps in the order they happened, earliest at the top.
Phospholipids make the barrier, and proteins do nearly everything else a membrane does. By weight, a typical plasma membrane is roughly half protein. Membrane proteins are sorted by how they sit in the bilayer:
Textbooks do not all draw the lines in the same place. Some count lipid-anchored proteins as integral, because, like integral proteins, they come off a membrane only when the bilayer is dissolved with detergent; peripheral proteins can be washed off with salt or a change of pH. Others group lipid-anchored proteins with the peripheral ones. This page keeps the three apart, which is the clearest way to tell them apart in a drawing.
The part of a transmembrane protein that crosses the oily middle of the bilayer is itself oily: a stretch of about 20 amino acids, most with nonpolar side chains, coiled into an alpha helix. The parts that stick out into the water on either side are polar. A transmembrane protein thus has the same arrangement as the bilayer it sits in, hydrophobic in the middle and hydrophilic on both faces. Some transmembrane proteins cross the bilayer as a barrel of beta strands instead, notably the porins of the outer membranes of Gram-negative bacteria and of mitochondria.
Each round draws a membrane with five proteins, one of each kind, in a new arrangement and a new style. Click the protein asked for. Your first click in each round is the one that counts. Every kind is asked for once in each run of five rounds.
Membrane proteins do a great many jobs. A common way of sorting them, used in Campbell Biology, names six:
Every one of these depends on the protein facing the right way. A receptor must have its binding site outside and its signaling end inside, and a pump must take in on one side and release on the other. Because membrane proteins never flip, a cell needs to set them in the right way only once, and they then do their jobs on their own. Why the sugars on membrane proteins always end up on the outer face of the plasma membrane, and never the inner one, is taken up on the Bulk Transport tab.
Cells of different kinds, and organisms of different kinds, differ largely in which proteins they have, and membrane proteins are a big part of that. A red blood cell, a nerve cell, and a cell lining the gut share the same kind of bilayer, but each carries the channels, carriers, pumps, and receptors that suit its job.
A lipid bilayer is selectively permeable: some molecules cross it readily, some slowly, and most hardly at all. Two properties decide which. The first is how readily a molecule dissolves in oil, since crossing means passing through the bilayer’s oily middle. The second is size. Small, nonpolar molecules, such as oxygen and carbon dioxide, cross most easily, which is how oxygen gets into the cells of your lungs and carbon dioxide gets out. Many drugs whose targets lie inside cells are small and fairly hydrophobic for the same reason: they must reach their targets without help.
A small hydrophobic molecule crosses by entering the bilayer, wandering about within it, and leaving from one face or the other. Molecules enter the bilayer far more readily than they leave it, so they build up inside it, and where more arrive on one side than on the other, more leak out on the other side than come back.
Small molecules that are polar but uncharged, such as water and urea, cross more slowly. Larger polar molecules, such as glucose, and anything carrying a charge, such as ions, hardly cross at all: a charge or a cluster of polar groups is held tightly by water and has no place in the oil. Very large molecules, such as proteins and starch, do not cross a bilayer at all. Everything in those last groups that a cell needs to move across a membrane, it moves with the help of proteins or, for the largest, by wrapping it in membrane.
Place each molecule on the ladder by how readily it crosses a pure lipid bilayer, with no proteins to help. Then check.
Diffusion is the spreading out of molecules by their own random jiggling. Nothing pushes a molecule from where it is crowded to where it is not. Each one simply moves at random, and where there are more molecules, more of them happen to move away. The overall drift is toward an equilibrium in which the molecules are spread roughly evenly through the space open to them. Across a membrane, a molecule must first collide with the membrane to have a chance of crossing it. More collisions come from the crowded side, so more molecules cross from that side than come back, and the net movement is from higher concentration to lower: down the concentration gradient, until the concentrations on the two sides are about equal. No energy has to be spent. The concentration difference is a departure from equilibrium, and the system drifts back toward equilibrium on its own.
The plot counts molecules on each side over time. The crossing counts are running totals since the start.
At equilibrium the concentrations are equal, and molecules still cross in both directions, but at equal rates, so nothing changes overall. That is a dynamic equilibrium. A concentration difference held away from equilibrium is a store of energy, a little like water held behind a dam, and the later tabs show how cells tap it.
Water crosses lipid bilayers fairly readily, partly because it is small and partly because there is so very much of it on both sides. Osmosis is the net movement of water across a selectively permeable membrane, from where water is more concentrated to where it is less concentrated. A solution’s water is less concentrated where more solute is dissolved in it, so the same rule can be stated the other way round: water moves from the side with less solute to the side with more.
The reason is the same collision argument as for any diffusion. Water molecules cross the membrane in both directions. On the side with more solute, some of the molecules hitting the membrane are solute rather than water, and if the solute cannot cross, those collisions are wasted. Fewer water molecules cross from that side than come back from the other, and the difference is the net flow.
Much of the water crossing cell membranes does not go through the bilayer at all but through water channels called aquaporins, which let water through many times faster than a bare bilayer does. The first was found in red blood cell membranes by Peter Agre, work recognized with a share of the 2003 Nobel Prize in Chemistry. Aquaporins change how fast water moves, not which way: the direction is still set by the solutes.
Osmosis was described in 1748 by Jean-Antoine Nollet, who saw a pig’s bladder filled with alcohol swell and burst when it was put in water. More than a century later the botanist Wilhelm Pfeffer measured the pressures involved, and from his measurements Jacobus van ’t Hoff worked out the equation for osmotic pressure given in Going further below.
A U-shaped tube with a membrane across its bottom is the classic way to show osmosis. Fill the two arms to the same height with different solutions, and see which way water moves. Choose the solutions and the membrane, predict, and then watch.
In a U-tube with a membrane that holds back the solute, the water keeps flowing toward the solute until something pushes back. Here the push comes from gravity: as one arm rises above the other, the extra height of liquid presses on the membrane and drives water back the other way. When the two flows balance, the level stops changing. The pressure that would have to be applied to the solution side to stop the net flow is the solution’s osmotic pressure.
Press harder than that, with a piston say, and water is driven the other way, out of the solution and through the membrane, leaving the solutes behind. That is reverse osmosis, the way much drinking water is purified and seawater made fresh.
The interior of a cell is a solution too, full of salts, sugars, amino acids, and proteins. Whether water moves into a cell or out of it depends on how the solution outside compares with the solution inside, and the comparison has a name, tonicity. Each term describes the solution outside, relative to the inside of the cell:
| Solution outside | Net water movement | An animal cell (no wall), such as a red blood cell | A cell with a wall: plant, fungal, or bacterial |
|---|---|---|---|
| Hypotonic | into the cell | swells and may burst: lysis (for red blood cells, hemolysis) | swells until pressed against its wall: turgid, the healthy state for most plant cells |
| Isotonic | none | keeps its shape: the normal state, since body fluids are isotonic | flaccid: limp, as in a wilting plant |
| Hypertonic | out of the cell | shrinks and puckers: crenation | the cytoplasm shrinks and pulls away from the wall: plasmolysis |
Lysis and crenation are opposites, and so are turgidity and plasmolysis. A cell wall makes the difference between the two pairs: it holds back a swelling cell the way a tire holds back an inner tube. Plants stand up because their cells are turgid, and droop when they are short of water and their cells go flaccid.
Isotonic solutions are everywhere in medicine for the same reason. Physiological saline, about 0.9% sodium chloride, is isotonic with blood, which is why it can be given intravenously without harming red blood cells. Eye drops and contact lens solution are isotonic too; pure water stings in the eye, and so does salty water.
For each cell in each solution, predict which way water moves on net, and then what happens to the cell.
Salting and sugaring are among the oldest ways of keeping food from spoiling, and both work by osmosis. Salted meat and fish, pickles in strong brine, jams, jellies, and honey all surround any bacteria or fungi that land on them with a strongly hypertonic solution. Water is drawn out of the microbes, they plasmolyze, and most cannot grow. Drying food (jerky, raisins, dried fish) works in a related way, by leaving too little free water for growth. Food scientists measure the free water as water activity: most bacteria need a water activity of about 0.9 or more to grow, while many molds and yeasts can grow at lower values, which is why mold rather than bacteria is what usually spoils jam.
Fermentation preserves food another way, with acid. Some foods use both. Sauerkraut and traditional pickles start with salt, which draws water out of the vegetables and holds back many spoilage microbes while favoring the salt-tolerant lactic acid bacteria that then sour the food. For more on fermentation, see the Doing Fermentation page.
Osmolarity counts dissolved particles, not molecules. A molecule that stays whole in solution, such as glucose, contributes one particle; a salt that splits into ions contributes one for each ion, two for NaCl (Na+ and Cl−) and three for CaCl2. The inside of a typical animal cell, and the blood around it, is about 290 milliosmoles per liter (mOsm/L). For an ideal dilute solution, van ’t Hoff’s equation gives the osmotic pressure:
where i is the number of particles per molecule, M the molar concentration, R the gas constant (0.08206 L·atm per mol·K), and T the absolute temperature. Real solutions fall somewhat short of the ideal, salts especially, so treat the results as estimates.
Osmolarity is not quite tonicity. Tonicity depends only on solutes that cannot cross the membrane. Urea crosses cell membranes fairly readily, so a solution of 290 mOsm/L urea has the same osmolarity as a cell’s interior, but the urea seeps into the cell, water follows it, and the cell swells and bursts as though the urea were not there. Such a solution is isosmotic but hypotonic. The calculator above takes that into account for solutes that cross membranes.
These two exercises come from Professor Abedon’s introductory biology laboratory on osmosis and fermentation, two topics joined by food preservation: both osmosis and fermentation can keep microbes from growing, and both are used to keep food from spoiling. In the first exercise, red blood cells are mixed into five different solutions; in the second, living Elodea leaves are flooded first with pure water and then with salt. Both are done in pairs, at 400× with a compound brightfield microscope.
For each, predict before you look. Then watch the videos of the exercise being done, which are linked at the points where each result shows. A dough-ball exercise on fermentation is on the Doing Fermentation page.
Five test tubes each hold about 1 mL of a different solution, and three drops of blood are added to each and swirled to mix. (For safety, the blood comes from an animal such as a sheep, never from a human or another primate.) Each tube is first looked at by eye, then a drop from each is looked at under the microscope.
For each tube, predict three things: whether the solution is hypotonic, isotonic, or hypertonic to the red blood cells; whether the tube will look cloudy or clear; and what the cells will look like at 400×. Here is a hint for the second: you can see through a liquid only if little in it scatters light, and whole cells scatter light.
Elodea is a true plant, not an alga, that lives under water and is often grown in aquariums. Its leaves are only two cells thick, so single cells, boxed in by their cell walls and packed with green chloroplasts, show clearly under the microscope. A leaf is placed on a slide without added water and covered with a coverslip. Once a good area is in focus at 400×, a drop of liquid is placed at the edge of the coverslip, where it is drawn underneath by capillary action, while the observer watches the cells. Work near the tip of the leaf, and be quick, since leaves left on a slide dry out.
| Hypotonic outside (water in) | Hypertonic outside (water out) | |
|---|---|---|
| No cell wall (red blood cell) | Lysis: the membrane bursts | Crenation: the cell shrinks and its outline puckers into points |
| Cell wall (Elodea, bacteria, fungi) | Turgidity: the membrane presses against the wall | Plasmolysis: the membrane pulls away from the wall |
Plasmolysis is what salt and sugar do to bacteria and fungi in preserved foods. A microbe whose cytoplasm has shrunk away from its wall cannot grow, which is why salted fish, brined pickles, and jam keep.
Two questions sort out nearly every way a substance crosses a membrane. Does it need a protein to get across? And is it moving down its concentration gradient, which happens on its own, or up its gradient, which takes energy?
| Down the gradient (passive: no energy spent) | Up the gradient (active: energy spent) | |
|---|---|---|
| No protein | Simple diffusion through the bilayer: oxygen, carbon dioxide, steroid hormones, and, more slowly, water | Does not happen |
| With a protein | Facilitated diffusion, through a channel or by a carrier: ions, water through aquaporins, glucose into most body cells | Active transport, by a pump or a cotransporter: Na+ out of and K+ into animal cells, H+ out of plant and fungal cells |
To facilitate something is to make it easier (the word comes from the Latin facilis, “easy”). In facilitated diffusion, a protein makes it easier for a substance to cross the membrane, giving it a route that the bilayer alone does not. The protein does not push the substance or change which way it goes: it still moves down its gradient, only faster, and with no energy spent.
Moving something up its gradient is like pumping water back up to the top of a waterfall: it does not happen unless energy is put in. The energy comes either directly from ATP, which is primary active transport, or from another substance moving down its own gradient at the same time, which is secondary active transport.
A channel is a protein with a water-filled pore through it. The pore’s size and the chemistry of its lining let through only certain substances, such as one kind of ion or water, and they flow through it down their gradient, often millions per second. Many channels are gated: they open and close in response to a voltage, a signal molecule binding, or a stretch of the membrane. Aquaporins are water channels.
A carrier (also called a transporter) binds the substance on one side of the membrane, changes shape, and releases it on the other side, rather as an enzyme binds its substrate. Like enzymes, carriers are specific, and like enzymes, they can be saturated: once every carrier is busy, raising the concentration further cannot speed things up. The glucose carrier of red blood cells, GLUT1, is a carrier of this kind, and it carries glucose in either direction, always down the gradient.
The carrier’s curve is the same shape as the curve of an enzyme’s rate against substrate concentration, described by Leonor Michaelis and Maud Menten in 1913. At a concentration equal to Km, the carrier runs at half its maximum rate.
Body cells take up glucose from the blood by facilitated diffusion, down its gradient. But a carrier that works in both directions should stop moving glucose in, on net, once the glucose inside matches the glucose outside. Cells keep taking it up anyway. The trick is that glucose barely stays glucose once inside: the enzyme hexokinase attaches a phosphate group to it, making glucose 6-phosphate. The glucose carrier does not carry glucose 6-phosphate, and with its negatively charged phosphate it cannot cross the bilayer either. Free glucose inside the cell is kept low, the gradient never runs down, and glucose keeps flowing in.
Bacteria have their own version. Many take up sugars through the phosphotransferase system, which attaches the phosphate as the sugar crosses the membrane. The sugar arrives already trapped, a mode of transport called group translocation.
The sodium–potassium pump of animal cells is the best-known pump. In each cycle it moves three sodium ions out of the cell and two potassium ions in, both against their gradients, paid for by one ATP. The result is a cell with much less sodium and much more potassium inside than out. Animal cells spend a sizable share of their ATP running these pumps. The pump was discovered by Jens Christian Skou, who shared the 1997 Nobel Prize in Chemistry for it.
The pump works by changing shape. Phosphorylation by ATP shifts it from a form that opens toward the inside of the cell and binds sodium tightly, to a form that opens toward the outside and binds potassium instead. Put the steps of one cycle in order, starting with the pump open toward the inside.
Plants and fungi run their plasma membranes on a different pump, one that uses ATP to push protons (H+) out of the cell. Animal cells have proton pumps too, for example in the membranes of lysosomes, which keep the inside of a lysosome acidic.
A gradient built by a pump is a store of energy, and a cell can spend it by letting the pumped ion flow back down its gradient through a carrier that brings something else along. The second substance can be moved up its own gradient that way, with no ATP used at that step. When both move in the same direction, the carrier is a symporter; when they move in opposite directions, it is an antiporter. (A carrier that moves only one substance, such as GLUT1, is a uniporter.)
For each crossing, choose how it happens.
This tab adds the arithmetic: equilibrium potentials, the resting potential, and proton motive force. Each calculator comes with presets, so the numbers can be explored without being typed in.
Pumps do more than build concentration gradients. Because the ions they move carry charges, they also separate charge across the membrane. The sodium–potassium pump moves three positive charges out for every two it brings in, and the inside of an animal cell ends up negative relative to the outside. A difference in charge across a membrane is a voltage, the membrane potential, and the membrane, with charges held apart on its two faces, is much like a battery.
An ion that can cross the membrane then feels two pushes at once: its concentration gradient, and the pull of the voltage on its charge. Together they make up its electrochemical gradient. For sodium, both pushes point the same way. There is far more sodium outside than in, and the inside is negative, so sodium rushes into a cell whenever a sodium channel opens. That rush is the start of a nerve impulse.
Suppose a membrane lets only potassium through. Potassium, much more concentrated inside, leaks out down its gradient. Each potassium ion that leaves takes a positive charge with it and leaves the inside a little more negative, and that growing negative charge pulls back on the remaining potassium. Very soon, after only a tiny fraction of the potassium has left, the electrical pull inward exactly balances the concentration push outward. The voltage at which that happens is potassium’s equilibrium potential, given by the equation of Walther Nernst:
where z is the ion’s charge (+1 for K+, −1 for Cl−, +2 for Ca2+), F is the Faraday constant, and E is the voltage of the inside relative to the outside. At body temperature, RT/F is about 26.7 mV, which works out to roughly 61.5 mV for each tenfold concentration difference of a singly charged ion.
Presets are typical, approximate values for a mammalian nerve cell; real cells vary.
A real membrane at rest lets several ions through, but not equally. Most resting cells have many open potassium channels and only a few routes for sodium. The resting potential therefore lands near potassium’s equilibrium potential, pulled a little toward sodium’s. The equation that weighs each ion by how freely it crosses was derived by David Goldman and applied to nerve by Alan Hodgkin and Bernard Katz:
Each P is that ion’s permeability. Only the ratios matter, so potassium’s is set to 1 here.
Notice what the pump does and does not do here. Its direct contribution to the voltage, from moving three charges out for every two in, is in most cells only a few millivolts. Its main job is to build and maintain the concentration gradients. The voltage itself comes mostly from potassium leaking out down the gradient the pump built. Likewise, a single nerve impulse moves so few ions that it barely dents the gradients, and the membrane is brought back to rest after each impulse by potassium channels opening, not by the pump. The pump keeps the battery charged over the longer run, across many impulses.
Bacteria, mitochondria, and chloroplasts run on a gradient of protons. Electron transport chains, covered on the Doing Biochemistry page, use energy taken from electrons to pump protons across a membrane: out of a bacterium, out of the inner compartment (the matrix) of a mitochondrion, or into the flattened sacs (thylakoids) of a chloroplast. Plant and fungal plasma membranes build a proton gradient with ATP-driven proton pumps instead. Either way, the protons are then more concentrated on one side, and usually the other side is also negative. The combined push on protons to flow back is the proton motive force.
Like any electrochemical gradient, it has two parts, a voltage (Δψ) and a difference in pH (ΔpH), and adding them up gives the proton motive force, Δp, in millivolts. Here “inside” is the side the protons flow back into: a bacterium’s cytoplasm, a mitochondrion’s matrix, a chloroplast’s stroma.
with Δψ = ψin − ψout and ΔpH = pHin − pHout. A negative Δp means protons are pushed inward. At 37 °C, 2.303 RT/F is about 61.5 mV, so each unit of pH is worth about as much as 61.5 mV of voltage.
Presets are illustrative; measured values vary with the organism and its conditions.
The proton motive force is spent in several ways. In bacteria it drives cotransport, such as lactose uptake by E. coli, and turns the motors of bacterial flagella. (Some bacteria, such as Vibrio species, run their flagella on a sodium gradient instead.) Above all, it makes ATP. The enzyme ATP synthase is, in effect, a proton pump running in reverse. Protons flowing back through it, down their electrochemical gradient, turn part of it like a turbine, and the turning drives the making of ATP from ADP and phosphate. Run the other way, by splitting ATP, the same enzyme pumps protons.
The idea that ATP is made this way, by way of a gradient across a membrane rather than by direct chemistry, was proposed in 1961 by Peter Mitchell, who called it chemiosmosis. It was met with skepticism for years and won him the 1978 Nobel Prize in Chemistry. How ATP synthase does its part was worked out by Paul Boyer and John Walker, who shared the 1997 prize with Jens Christian Skou, discoverer of the sodium–potassium pump.
Some things are far too big for any channel or carrier: a bacterium, a droplet of fluid, a particle of cholesterol-carrying lipoprotein, a load of hormone or digestive enzyme. Cells move these in and out in bulk, wrapped in membrane. Taking things in this way is endocytosis: the plasma membrane folds inward around the material and pinches off as a bag, a vesicle, inside the cell. Sending things out is exocytosis: a vesicle inside the cell fuses with the plasma membrane and opens to the outside.
Neither moves anything through a lipid bilayer. A bacterium engulfed by an amoeba is inside the amoeba but not in its cytoplasm. It sits in a membrane bag whose inside is, in effect, a pocket of the outside world, and the amoeba has to digest it there and move the products across the bag’s membrane before they are truly in.
The cell reaches out around a particle, often another cell, and engulfs it in a vesicle called a food vacuole or phagosome. Enzymes are then delivered into the vacuole, usually by fusion with a lysosome, and the particle is digested inside it. Amoebae and many other protists feed this way. Digesting food in a small, closed space has an advantage for a cell living in water: enzymes released into the surrounding water would simply diffuse away, while enzymes delivered into a food vacuole stay concentrated around the food. Among animals, sponges rely on digestion inside their cells entirely, having no gut. Cnidarians and flatworms digest food partly in a gut cavity and partly inside the cells lining it, while most other animals, ourselves included, digest food mainly in a gut, outside their cells.
In our own bodies, phagocytosis is a defense. White blood cells such as macrophages and neutrophils engulf and destroy bacteria and debris. The process was discovered in 1882 by Élie Metchnikoff, who pushed rose thorns into transparent starfish larvae and watched mobile cells gather around them. He shared the 1908 Nobel Prize in Physiology or Medicine.
The cell takes in small droplets of the surrounding fluid, with whatever happens to be dissolved in it, in small vesicles. Pinocytosis is not selective: what comes in is simply a sample of the outside.
Here the cell takes in particular substances. They first bind to receptor proteins on the cell surface, the receptors gather into pits coated on the inside with the protein clathrin, and the pits pinch off as vesicles carrying the bound cargo, concentrated many times over. Cells take up cholesterol this way, in the form of the particles called low-density lipoprotein (LDL), the “bad cholesterol” of blood tests. Michael Brown and Joseph Goldstein worked out the LDL receptor, sharing the 1985 Nobel Prize in Physiology or Medicine. People with familial hypercholesterolemia have defective LDL receptors, so LDL builds up in their blood. Developing egg cells, which are huge and must take in great quantities of material, gather their yolk protein, vitellogenin, in the same way. Many viruses enter cells by getting themselves taken in through endocytosis, too.
Exocytosis is endocytosis in reverse. A vesicle inside the cell moves to the plasma membrane and fuses with it, emptying its contents to the outside, while its membrane becomes part of the plasma membrane. Some cells do this continuously, as when a growing plant cell sends materials for its cell wall to its surface; that is constitutive exocytosis. Others store their products in vesicles and release them only on a signal, usually a rise in calcium inside the cell; that is regulated exocytosis. Nerve cells release neurotransmitters at synapses this way when a nerve impulse arrives, and the beta cells of the pancreas release insulin when blood glucose rises.
In eukaryotic cells, the vesicles of exocytosis come from the endomembrane system: the endoplasmic reticulum (ER), the Golgi apparatus, the vesicles that shuttle between them and to the plasma membrane, and the lysosomes and vacuoles. Proteins bound for the plasma membrane, for secretion, or for lysosomes are made by ribosomes on the rough ER and threaded into or across the ER membrane as they are made. From there they travel by vesicle to the Golgi, which modifies and sorts them, and on to their destinations. Proteins that work in the cytoplasm itself are made on free ribosomes and never enter the endomembrane system. Mitochondria and chloroplasts are not part of it either: they import most of their proteins directly from the cytoplasm.
Follow the membranes and something surprising appears. The inside (the lumen) of the ER, of the Golgi, and of each vesicle is equivalent to the outside of the cell, since when a vesicle fuses with the plasma membrane, its lumen becomes part of the outside. And because membrane proteins never flip, a protein’s end facing the lumen of the ER will face the outside of the cell once it reaches the plasma membrane. Sugars are added to proteins in the lumen of the ER and Golgi, which is why the sugars of membrane proteins end up on the outer face of the plasma membrane and never on the inner one.
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Many terms link to a Wikipedia article (↗) for more.
“Water-fearing” and “water-loving.” Nonpolar substances, such as oils, are hydrophobic; polar and charged substances, such as sugars and ions, are hydrophilic.
Having a hydrophilic part and a hydrophobic part in the same molecule. Phospholipids, soaps, and detergents are amphipathic (also called amphiphilic).
A lipid with two fatty acid tails and a charged, phosphate-containing head. The main building block of membranes.
A small sphere of amphipathic molecules in water, tails inward and heads outward. Formed mainly by molecules with a single tail, such as soaps and detergents.
A sheet of lipid two molecules thick, tails facing each other in the middle and heads facing water on both sides. The basic structure of biological membranes.
A hollow sphere of lipid bilayer, enclosing water, made in the laboratory. Used to study membranes and to deliver drugs.
The model of membrane structure proposed by Singer and Nicolson in 1972: a fluid lipid bilayer in which proteins and other molecules are embedded and can move about, like the pieces of a mosaic.
How freely a membrane’s molecules move within it. Raised by unsaturated (kinked) and shorter fatty acid tails and by warmth; buffered by cholesterol in animal cells.
A fatty acid with one or more carbon–carbon double bonds. In natural fats these are usually cis double bonds, which put kinks in the tail.
The sterol of animal cell membranes. It makes membranes less fluid at moderate and high temperatures and keeps them from setting at low ones.
Movement of a lipid from one layer of the bilayer to the other. Rare on its own; carried out deliberately by enzymes called flippases.
A protein that enters the lipid bilayer itself. Most span it (transmembrane proteins); a few enter only one layer.
An integral protein that spans the bilayer from one face to the other, once (single-pass) or several times (multi-pass).
A protein held to a membrane by a lipid attached to it, the lipid buried in the bilayer and the protein itself outside it. Some textbooks count these as integral proteins.
A protein attached to a membrane’s surface, usually to an integral protein, without entering the bilayer.
Letting some substances cross more readily than others.
Net movement of a substance from where it is more concentrated to where it is less concentrated, down its concentration gradient, by the random motion of its molecules.
A state in which movement continues in both directions at equal rates, so that nothing changes overall.
Net movement of water across a selectively permeable membrane, from where water is more concentrated (less solute) to where it is less concentrated (more solute).
The pressure that would have to be applied to a solution to stop the net flow of water into it from pure water across a membrane that holds back the solute.
The concentration of dissolved particles in a solution, counting each ion of a salt separately. Body fluids are about 290 milliosmoles per liter.
A comparison of the solution outside a cell with the inside, counting only solutes that cannot cross the membrane. A hypotonic solution has less such solute (water moves in), an isotonic one the same (no net movement), and a hypertonic one more (water moves out).
The bursting of a cell. Hemolysis is the bursting of red blood cells, as in a hypotonic solution.
The shrinking and puckering of a cell without a wall, such as a red blood cell, in a hypertonic solution. Essentially the opposite of lysis.
The state of a walled cell whose membrane is pressed against its wall by water taken in from a hypotonic solution. The pressure is turgor pressure.
Limp: the state of a walled cell in an isotonic solution, as in a wilting plant.
The shrinking of a walled cell’s cytoplasm away from its wall in a hypertonic solution. Essentially the opposite of turgidity.
A water channel: a membrane protein that lets water cross far faster than through the bilayer alone.
A measure of how much water in a food is free for microbes to use, from 1 (pure water) down. Salting, sugaring, and drying all lower it.
Movement down a gradient, needing no energy: simple diffusion and facilitated diffusion.
Diffusion straight through the lipid bilayer, without the help of a protein.
Diffusion down a gradient made easier (facilitated) by a protein: through a channel or by a carrier. The protein speeds the crossing but does not change its direction, and no energy is spent.
A membrane protein with a pore through which particular ions or water flow down their gradients. Many open and close (are gated).
A membrane protein that binds a substance on one side, changes shape, and releases it on the other. Carriers can be saturated.
Carriers that move one substance, two in the same direction, and two in opposite directions.
Movement up a gradient, paid for with energy: directly from ATP (primary active transport) or by another substance moving down its own gradient (secondary active transport, or cotransport).
The pump of animal cells that moves three Na+ out and two K+ in for each ATP it uses (Na+/K+-ATPase).
A pump that moves protons (H+) across a membrane, powered by ATP, by light, or by electron transport.
Bacterial transport in which a sugar is chemically changed, by having a phosphate attached, as it crosses the membrane.
The voltage across a membrane, usually given as the inside relative to the outside.
The combined push on an ion from its concentration gradient and from the membrane potential.
The membrane potential at which an ion’s electrical and concentration pushes balance, so that it has no net movement. Given by the Nernst equation.
The membrane potential of an unstimulated cell, typically about −60 to −80 mV in nerve cells, set mainly by potassium leaking out.
The electrochemical gradient of protons across a membrane, combining a voltage and a pH difference.
The enzyme that makes ATP using protons flowing down their gradient through it; in effect, a proton pump running in reverse.
Taking material into a cell by folding the plasma membrane around it and pinching it off as a vesicle.
“Cell eating”: engulfing a large particle, such as another cell, in a food vacuole (phagosome).
“Cell drinking”: taking in droplets of surrounding fluid, with whatever is dissolved in them, nonselectively.
Taking in particular substances that first bind to receptors on the cell surface, as with LDL.
Release of a vesicle’s contents outside the cell when the vesicle fuses with the plasma membrane; constitutive when continuous, regulated when triggered by a signal.
The connected membranes of a eukaryotic cell, linked directly or by vesicles: the nuclear envelope, endoplasmic reticulum, Golgi apparatus, vesicles, lysosomes, vacuoles, and plasma membrane.