BMS 123 · Common Core (Tronc commun)Section 1.2 · Pharmacokinetics and drug absorption
Section 2 of 27
Crossing biological membranes
Describe the structure of a biological membrane and compare filtration, simple diffusion, facilitated diffusion, active transport and endocytosis as ways for a drug to cross it.
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Why this matters
Every pharmacokinetic process begins with a membrane. A drug that cannot cross the gut wall is never absorbed, and a drug that cannot cross a cell membrane never reaches its target inside the cell. Knowing which of the five mechanisms is at work tells you whether absorption will be fast or slow, whether it can be saturated by a large dose, and whether another drug or a food can compete with it. This section supplies the vocabulary used by every later unit on distribution, metabolism and excretion.
What you need to know first
You need only the everyday ideas of oil and water not mixing, and of a substance moving from a place where it is concentrated to a place where it is dilute, like a smell spreading across a room.
Words to know (5)
Biological membrane
A thin sheet of cells and lipids that separates two compartments of the body and controls what passes between them. The wall of the small intestine is a biological membrane that a swallowed drug must cross.
Phospholipid bilayer
Two layers of fat-like molecules whose water-loving heads face outward and whose fat-like tails form the inner core. The bilayer explains why fat-soluble drugs cross easily and water-soluble charged drugs do not.
Concentration gradient
The difference in concentration between two sides of a membrane, which pushes a substance from the higher side to the lower. A high drug concentration in the gut drives simple diffusion into the blood.
Passive transport
Movement across a membrane that uses only the concentration gradient and requires no energy from the cell. Filtration and simple diffusion are both passive, so they stop when the gradient disappears.
Carrier protein
A protein embedded in the membrane that binds a substance and moves it across; it is selective and can become saturated. A carrier for a sugar will not carry an unrelated drug, and it stops working when fully occupied.
One idea at a timeIdea 1 / 6
In one sentence To enter the body a drug must pass the epithelial membranes that line the organs and body cavities, and those membranes are built of cells joined by water-filled junctions.
Epithelial membranes are cells joined by water-filled junctions
The body has several biological membranes: the walls of the gastrointestinal tract (stomach, intestine, colon, anus), blood capillaries, the skin, the alveoli of the lungs and the mucous membranes. They differ in composition and thickness, and therefore in their absorption properties. To get into the body, drugs have to penetrate the epithelial membranes that line the major organs and body cavities. Epithelial membranes consist of a series of cells joined by water-filled junctions, so there are two possible routes across them: through the cells or between them.
For example
A small water-soluble molecule may slip between two intestinal cells, while a fat-soluble molecule dissolves through the cells themselves.
Pause and check yourself
What are the two possible routes across an epithelial membrane?
Show the answer
Through the cells themselves (transcellular) or between the cells, through the water-filled junctions (paracellular).
Read detailed explanations
Why carriers saturate, and why that creates competition
A carrier is a protein with a limited number of binding places, so the rate of carrier-mediated transport is not a straight line against concentration. At low concentration many places are free and transport rises steeply; as the concentration climbs, more carriers are occupied and the rate flattens; once every carrier is working, the rate is maximal and adding more drug changes nothing. This saturation has two practical consequences. First, a large dose can overwhelm a carrier route, so absorption or secretion stops increasing in proportion to the dose. Second, two substances that use the same carrier compete for it: the one with the higher affinity or the higher concentration occupies more places and slows the transport of the other. Saturation and competition are therefore not curiosities; they are the reason carrier-mediated steps behave differently from simple diffusion, which never saturates because it has no carrier to fill.
Efflux transporters: the membrane as a gatekeeper
Not every transporter carries a drug inward. Efflux transporters sit in the membrane and use energy to push molecules back out of the cell. They are a defence system: the cell recognises a foreign molecule, often a lipophilic one that diffused in easily, and expels it before it can act. Because a lipophilic drug crosses the bilayer quickly in both directions, an efflux pump can produce a steady outward flow that keeps the intracellular concentration low even when the blood concentration is high. The best known example, P-glycoprotein, is abundant in the cells that line the intestine, in the capillaries of the brain and in the kidney and liver. Its presence explains several otherwise puzzling observations: an orally taken drug may be absorbed through the gut cell and then pumped straight back into the gut lumen, reducing bioavailability; a drug may distribute to almost every tissue yet hardly enter the brain; and a second drug that occupies the pump can raise the level of the first by blocking its expulsion. Efflux is thus a third variable, alongside diffusion and uptake, that decides how much drug actually stays where it is needed.
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The phospholipid bilayer
A cross-section of a cell membrane: water outside, a double layer of phospholipids with a fatty core, embedded proteins and a water-filled channel.
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01 / 5Polar phosphate head
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Polar phosphate head
The water-loving end of each phospholipid. The heads of the two layers face the aqueous media on each side, which keeps the membrane stable and explains why water itself crosses easily.
Schematic cross-section, not to scale. Real membranes vary in thickness, protein content and the number of junctions, and cell membranes are shown here as a single flat sheet.
Key points to remember
✓Epithelial membranes are cells joined by water-filled junctions, so drugs may pass through the cells or between them.
✓The phospholipid bilayer has polar heads outward and a lipid core, and it is semi-permeable.
✓Filtration uses aqueous pores for small water-soluble, ionised or polar substances.
✓Simple diffusion is the commonest mechanism: down the gradient, no carrier, no energy, for lipophilic unionised drugs.
✓Facilitated diffusion is carrier-mediated, selective and saturable but energy-free; active transport adds energy and can go against the gradient.
✓Endocytosis engulfs large molecules that cannot use the other routes.
Go deeper
Worked example
Choosing the likely transport mechanism. Four substances reach the same membrane: (A) a small ionised salt, (B) a small uncharged fat-soluble drug, (C) a nutrient that needs a carrier but moves down its gradient, and (D) a large protein molecule. You are asked which mechanism is most likely for each.
Substance A is small, water-soluble and charged, so filtration through the water-filled junctions between cells is the likely route.
Substance B is small, uncharged and lipophilic, so simple diffusion straight through the lipid core is the likely route — and it needs no carrier and no energy.
Substance C uses a carrier but follows its concentration gradient, so the mechanism is facilitated diffusion: selective and saturable, but still energy-free.
Substance D is too large for pores and cannot dissolve in the lipid core, so it is taken in by endocytosis, in which the membrane engulfs the molecule.
The same membrane uses different mechanisms for different substances. The deciding features are size, water or fat solubility, charge, and whether a carrier and energy are available.
Common misconception
A very common beginner model is that a membrane is a simple sieve through which anything small enough can pass. Size helps, but the lipid core blocks small charged molecules and lets larger fat-soluble ones through, and carriers add selectivity and a saturation limit.
Check your reasoning
A drug is small but completely ionised at intestinal pH. Explain why it is poorly absorbed by simple diffusion, and name one other mechanism that might still carry it across the membrane.
Reveal an answer
Simple diffusion requires a lipophilic, non-ionised molecule because the drug must dissolve through the lipid core of the bilayer; an ionised molecule is water-soluble and is held back. It may still cross by filtration through the water-filled junctions between cells if it is small enough, or by a carrier-mediated process such as facilitated diffusion or active transport if a selective transporter for it exists.
Where this comes from
BMS 123 General Pharmacology II lecture notes — Part 1, Principles of Pharmacokinetics · Drug absorption: structure of the biological membrane; passage of drugs through membranes; Table 1
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About this course and its sources
Bilingual lessons organised from the supplied BMS 123 General Pharmacology II lecture notes (Principles of Pharmacokinetics and Principles of Pharmacodynamics). Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.