Facilitated Diffusion: Study Guide (Biology 2.6)

Structure decides the route

Whether a substance can cross a membrane on its own depends on its structure: its size, its polarity and whether it carries a charge. The centre of the phospholipid bilayer is made of nonpolar hydrocarbon tails. Anything that is charged or strongly polar is attracted to water and repelled by that hydrophobic layer.

Type of substance	Examples	Crosses the bilayer on its own?	Route
Small, nonpolar	O₂, CO₂, N₂	yes, freely	simple diffusion
Small, polar, uncharged	H₂O, NH₃	slowly, in small amounts	simple diffusion; water mostly through aquaporins
Ions, any size	Na⁺, K⁺, Cl⁻, Ca²⁺	no	channel proteins
Large, polar	glucose, amino acids	no	carrier proteins

An ion such as Na⁺ is smaller than a water molecule, yet it cannot slip through the bilayer. Its charge attracts a shell of water molecules (a hydration shell, Topic 1.1), and pushing a charged particle into the nonpolar core would take far too much energy. Size alone does not decide; charge and polarity matter more.

Facilitated diffusion is the answer for these substances. It is the passive movement of a substance across a membrane through a transport protein, from high to low concentration, with no energy input from the cell. The protein provides a hydrophilic route; the concentration gradient provides the drive.

Channel proteins and carrier proteins

There are two kinds of protein that carry out facilitated diffusion.

Channel proteins form a pore lined with hydrophilic (polar or charged) amino acids that runs right through the membrane. Ions or water molecules flow through it, usually in single file. Channels are very fast. They are also specific: the width of the pore and the charges lining it decide what can pass. A potassium channel, for example, lets K⁺ through but very few Na⁺ ions. Many channels are gated: they open or close in response to a signal, such as a change in voltage or the binding of a molecule. This lets a cell switch ion movement on and off.

Carrier proteins bind a specific molecule on one side of the membrane, change shape, and release it on the other side. They are slower than channels, because each carrier must change shape for every molecule (or small group of molecules) it moves. Glucose transporters are carrier proteins that move glucose into many cells, such as red blood cells, by facilitated diffusion.

Ions, channels and polarized membranes

Ions such as sodium (Na⁺) and potassium (K⁺) can only cross a membrane through channel proteins (or through pumps, Topic 2.8). This has an important side effect. Every ion carries a charge, so when ions move through channels, charge moves too.

Take a cell with K⁺ more concentrated inside than outside. If its membrane has open K⁺ channels, K⁺ diffuses out down its gradient. The negative ions it was balancing, mostly large negatively charged molecules such as proteins, cannot follow. So the inside of the cell becomes negative compared with the outside. The membrane is now polarized: there is a difference in charge, a voltage, across it. In a typical nerve cell at rest, the inside is about 70 millivolts more negative than the outside.

Once a membrane is polarized, an ion’s movement depends on two things: its concentration gradient and the charge across the membrane. As the inside becomes more negative, it starts to attract positive K⁺ back. Net movement of K⁺ slows and stops before the concentrations become equal. Together, these two influences make up the electrochemical gradient (developed in Topic 2.8).

If Na⁺ channels then open, Na⁺ rushes in, down both its concentration gradient and its charge gradient, and the inside becomes less negative. Opening and closing gated Na⁺ and K⁺ channels in this way is the basis of nerve impulses, which you meet later in the course.

Large polar molecules: carriers and saturation

Glucose is polar and too large to cross the bilayer at a useful rate, so it moves through carrier proteins. The process is still passive: glucose only moves from where it is more concentrated to where it is less concentrated. Many cells keep their internal glucose low by converting it into other molecules as soon as it arrives. This keeps the gradient, so glucose keeps flowing in without any energy being spent on transport itself.

Because each carrier can only bind and move molecules at a certain speed, facilitated diffusion has a maximum rate. At low concentrations, adding more solute gives a big rise in rate. At high concentrations, almost every carrier is busy all the time, and adding more solute makes little difference. The rate levels off (the carriers are saturated). Simple diffusion through the bilayer does not level off in the same way: its rate keeps rising in proportion to the gradient.

Aquaporins: fast lanes for water

Water is small and uncharged, so a little of it crosses the bilayer directly (Topic 2.4). But many cells need to move large quantities of water quickly. They use aquaporins: channel proteins that let water molecules pass in single file, while keeping out ions.

• In the kidney, cells lining the collecting ducts can add aquaporins to their surface membrane when the body needs to save water. The hormone ADH (antidiuretic hormone) triggers this: vesicles carrying aquaporins fuse with the membrane by exocytosis (Topic 2.5). More water is then reabsorbed into the blood, and the urine becomes more concentrated.
• In plants, root cells use aquaporins to take in water from the soil quickly.

Water movement through aquaporins is still passive. It is osmosis through a protein, and its direction depends on the solute concentrations on each side (Topic 2.7). An aquaporin speeds water up; it never decides the direction.

Comparing the routes

Feature	Simple diffusion	Facilitated diffusion	Active transport (Topic 2.8)
Protein needed	no	yes: channel or carrier	yes: pump
Direction	down the gradient	down the gradient	can be against the gradient
Energy from the cell	none	none	direct input (ATP)
Levels off at high concentration?	no	yes (saturation)	yes
Typical substances	O₂, CO₂	ions, glucose, water (aquaporins)	ions, some nutrients

Worked example 1: reading a saturation curve

Question. Fictional cells take up glucose only through carrier proteins. The table gives the rate of uptake at different outside glucose concentrations (arbitrary units; the same values appear as circles in Figure 2).

Outside glucose / mmol L⁻¹	1	2	4	8	16	32
Rate of uptake	2.4	4.0	6.0	8.0	9.6	10.7

(a) Calculate the percentage increase in rate when the concentration doubles from 1 to 2 mmol L⁻¹, and from 16 to 32 mmol L⁻¹. (b) Explain the difference. (c) A mutation halves the number of carriers in the membrane. Predict the rate at 4 and at 32 mmol L⁻¹, and justify.

(a) Calculations.

1. From 1 to 2: (4.0 − 2.4) ÷ 2.4 × 100 = 1.6 ÷ 2.4 × 100 = 67%.
2. From 16 to 32: (10.7 − 9.6) ÷ 9.6 × 100 = 1.1 ÷ 9.6 × 100 = 11% (11.5% before rounding).

(b) Explanation. Both steps double the concentration, but the second gives a much smaller rise. At 1–2 mmol L⁻¹, most carriers are free at any moment, so more glucose means more carriers in use and a much faster rate. At 16–32 mmol L⁻¹, nearly all carriers are already busy. The rate is limited by the number of carriers and how fast each works, not by the glucose supply. The carriers are close to saturation: the rate at 32 mmol L⁻¹ is already 89% of the maximum of about 12.

(c) Prediction. Each carrier still works in the same way, so with half as many carriers the rate at every concentration should halve:

• at 4 mmol L⁻¹: 6.0 ÷ 2 = 3.0
• at 32 mmol L⁻¹: 10.7 ÷ 2 = 5.35 (about 5.4), with the maximum falling from about 12 to about 6.

Check. The answer separates the two limits. At low concentration, the rate rises steeply as glucose is added. At high concentration, the number of carriers sets the ceiling. Halving the carriers halves the rate at both, because at any one concentration each carrier is busy for the same share of the time. The prediction also keeps the process passive: fewer carriers slow uptake, but glucose still only moves down its gradient.

Worked example 2: predicting the effect of blocking aquaporins

Question. A layer of fictional kidney collecting-duct cells is grown on a filter. Water flow across the layer is measured (µL cm⁻² h⁻¹) under a fixed difference in solute concentration.

Condition	Water flow
No hormone	2.0
With ADH	14.0
With ADH and Compound B (blocks aquaporins)	2.6

(a) By what factor does ADH increase water flow? (b) Calculate the percentage decrease in flow when Compound B is added with ADH. (c) Explain the results, and explain why flow does not fall to zero. (d) Predict the effect on a person whose collecting-duct aquaporins do not work.

(a) 14.0 ÷ 2.0 = 7.0 times.

(b) (14.0 − 2.6) ÷ 14.0 × 100 = 11.4 ÷ 14.0 × 100 = 81% (81.4% before rounding).

(c) ADH causes vesicles containing aquaporins to fuse with the membrane, so the cells have more water channels. Water still moves only down its gradient, but much faster, because many more molecules can pass each second. Compound B blocks the aquaporins, so flow drops back close to the no-hormone value. It does not fall to zero because some water still crosses the phospholipid bilayer directly, and the cells may keep a few unblocked channels.

(d) Less water would be reabsorbed from the urine back into the blood. The person would produce large volumes of dilute urine, lose water quickly and feel very thirsty, with a risk of dehydration. Giving more ADH would not help, because the problem is the channel, not the signal. A rare inherited condition, nephrogenic diabetes insipidus, can arise in this way.

This is the reasoning pattern for “predict the effect of a disruption”: name the component that changed, say what it normally does, follow the effect to the cell, then to the organism.

Common misconceptions

• “Facilitated diffusion uses energy because it uses a protein.” The protein gives a route, not a push. No ATP is used.
• “Channel proteins pump ions.” Channels only let ions flow down their gradient. Pumps (Topic 2.8) use energy.
• “Facilitated diffusion can move substances against their gradient if there are enough carriers.” More carriers only make movement down the gradient faster.
• “Na⁺ is tiny, so it can pass between the lipids.” Its charge and hydration shell keep it out of the hydrophobic core.
• “Water can only cross a membrane through aquaporins.” A small amount crosses the bilayer directly; aquaporins add a fast route.
• “The rate of facilitated diffusion keeps rising as concentration rises.” It levels off when the proteins are saturated.
• “Any channel lets any ion through.” Channels and carriers are specific to particular ions or molecules.
• “Ion movement only changes concentrations.” Ions carry charge, so their movement can polarize the membrane.

Where this leads

Aquaporins lead straight into Topic 2.7, Tonicity and Osmoregulation, where you work out which way water moves. Topic 2.8 explains the pumps that use ATP to build the ion gradients that channels let ions run down. If the difference between passive and active transport is not secure, re-read the Topic 2.5 study guide. Then try the practice questions, and consolidate with the revision notes and the checklist.