Transport Across the Cell Membrane

Lessons

Senior 5 · Biology

Transport Across the Cell Membrane

5 lessons

Lesson 1 of 5

The fluid mosaic membrane, diffusion and facilitated diffusion

25 min

A membrane made only of fat

Biochemists can make an artificial membrane from phospholipids alone, with no proteins in it, and test what crosses. Oxygen and carbon dioxide pass through almost at once. Water crosses slowly. Glucose barely crosses at all, and ions such as Na⁺ and K⁺ are stopped almost completely. Yet a red blood cell takes up glucose in seconds and a nerve cell lets Na⁺ flood in during every impulse. Something in a real membrane must let these substances through. In Senior 2 you named the parts of the membrane; this lesson explains how its structure decides what crosses it, and how fast.

Substance	Type	Crosses a pure phospholipid bilayer?
O₂, CO₂, N₂	Small, non-polar gases	Rapidly
Steroid hormones, ethanol	Lipid-soluble	Readily
H₂O, urea	Small, polar, uncharged	Slowly
Glucose, amino acids	Larger, polar	Hardly at all
Na⁺, K⁺, Cl⁻, H⁺	Ions (charged)	Practically not at all

The rule: the hydrophobic core of the bilayer lets through small and lipid-soluble molecules, and blocks charged and large polar ones.

Definition

Fluid mosaic model: the model of membrane structure proposed by S. J. Singer and G. L. Nicolson in 1972. The membrane is a fluid bilayer of phospholipids in which individual molecules move sideways, with proteins scattered through it like the pieces of a mosaic.

Intrinsic protein: a protein that spans the bilayer (channels, carriers). Extrinsic protein: a protein attached to one surface only.

Glycoprotein and glycolipid: a protein or a lipid with a carbohydrate chain attached, on the outer surface; they act as receptors and recognition sites.

The parts of the membrane and their jobs

Phospholipids form the bilayer. Their hydrophilic phosphate heads face the water on both sides; their hydrophobic fatty-acid tails face each other inside. The core is a barrier to ions and polar molecules.
Cholesterol sits between the tails. It reduces fluidity at high temperature and stops the tails packing solid at low temperature, keeping the membrane stable.
Channel proteins form water-filled pores for particular ions or for water.
Carrier proteins bind particular molecules and change shape to move them across.
Glycoproteins and glycolipids form the glycocalyx: receptors for hormones such as insulin, and markers that let cells recognise each other, as the immune system does.

Definition

Diffusion: the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion. It is passive: it uses no energy from ATP.

Facilitated diffusion: diffusion of ions or polar molecules across a membrane through specific channel or carrier proteins. It is also passive and goes only down the gradient.

How fast? Fick's law

The rate of diffusion across a surface such as the wall of an alveolus follows Fick's law:

\text{rate of diffusion} \propto \frac{\text{surface area} \times \text{difference in concentration}}{\text{thickness of the surface}}

So diffusion is faster across a larger area, down a steeper gradient, and across a thinner barrier. Other factors also matter:

Temperature: warmer molecules have more kinetic energy and move faster.
Size of the molecule: small molecules diffuse faster than large ones.
Lipid solubility: lipid-soluble and non-polar molecules cross the bilayer more easily.

Every exchange surface in biology is built to obey Fick's law: the alveoli of the lungs, the villi of the small intestine, the gills of a tilapia and the spongy mesophyll of a leaf all have a large area, a thin barrier and a gradient kept steep by ventilation or blood flow.

Worked example 1: Fick's law and lung disease

In a lung damaged by emphysema, the total alveolar surface area falls to 60 % of normal, and scarring thickens the barrier between air and blood from 0.5 µm to 0.8 µm. Assuming the concentration gradient is unchanged, calculate the new rate of oxygen diffusion as a percentage of normal.

Rate is proportional to area and inversely proportional to thickness, so the new rate relative to normal is $\frac{0.60 \times 1}{0.8 \div 0.5}$ .
Thickness factor: $\frac{0.8}{0.5} = 1.6$ .
New rate $= \frac{0.60}{1.6} = 0.375$ , so about 38 % of normal.

Interpretation: two changes that each look moderate combine to cut oxygen uptake by almost two-thirds, which is why such patients are short of breath even at rest.

Facilitated diffusion

Ions and polar molecules cross through proteins:

Channel proteins are pores lined with polar groups. Each lets through only certain ions, and many are gated: they open only in response to a voltage change (the Na⁺ channels of neurones) or the binding of a chemical. Aquaporins are channels for water; there are many in the collecting ducts of the kidney.
Carrier proteins bind a specific molecule, change shape and release it on the other side. The glucose carrier GLUT1 lets red blood cells take up glucose from plasma.

Because there is a limited number of carrier proteins, facilitated diffusion has a maximum rate. When every carrier is busy, raising the concentration further does not speed it up: the carriers are saturated (Figure 2). Simple diffusion through the bilayer shows no such limit.

Worked example 2: is a carrier involved?

Red blood cells were placed in glucose solutions and the rate of glucose uptake was measured (illustrative data, arbitrary units).

Glucose outside / mmol dm⁻³	0	2	4	8	16	32
Rate of uptake	0	40	64	84	96	100

(a) Calculate the percentage increase in rate when the concentration doubles from 2 to 4, and from 16 to 32 mmol dm⁻³. (b) What do the results show?

(a) 2 to 4: $\frac{64 - 40}{40} \times 100 = 60\,\%$ . 16 to 32: $\frac{100 - 96}{96} \times 100 = 4.2\,\%$ .
(b) If glucose crossed by simple diffusion, doubling the concentration would double the rate (100 % increase) at every concentration. Instead the increase shrinks and the rate levels off near 100: the glucose carriers are close to saturation. The uptake is facilitated diffusion by carrier proteins.

Warning

Common mistakes

"Facilitated diffusion uses ATP because it uses proteins." It does not. The energy comes from the random motion of the molecules; the protein only provides a route. Movement is always down the gradient.
"Diffusion stops at equilibrium." Molecules keep moving both ways; only the net movement stops.
Writing that a larger molecule "cannot diffuse". It diffuses, but more slowly, and may not fit through the bilayer.

Cognito: "A-Level Biology - Fluid Mosaic Model". Watch for the role of each membrane component, especially cholesterol and the glycoproteins, and why the model is called "fluid" and "mosaic".

Tip

Fick's law questions: write the proportionality first, then change one factor at a time. "Area × 2 and thickness × 2" leaves the rate unchanged; "area ÷ 2 and gradient ÷ 2" quarters it.

✦

Practice: the membrane and diffusion

Explain why oxygen crosses a phospholipid bilayer readily but Na⁺ ions do not.
Give the functions of (a) cholesterol, (b) glycoproteins in a cell membrane.
Why is the fluid mosaic model described as "fluid" and as a "mosaic"?
State Fick's law in words, and name two features of the alveoli that follow from it.
The surface area of an exchange surface doubles and its thickness also doubles. What happens to the rate of diffusion?
Distinguish between channel proteins and carrier proteins.
A gill's surface area is halved by a parasite while the concentration gradient doubles because of faster ventilation. Calculate the new rate as a percentage of the original.
Sketch and explain the shape of a graph of rate of facilitated diffusion against concentration difference.

Show answersHide answers

O₂ is small and non-polar, so it dissolves in and passes through the hydrophobic core. Na⁺ is charged and surrounded by water molecules, so it is repelled by the hydrophobic tails; it can cross only through a channel or carrier.
(a) Cholesterol regulates fluidity and stability: it restrains movement of the tails at high temperature and stops them packing solid at low temperature. (b) Glycoproteins act as receptors (e.g. for insulin) and as recognition markers between cells.
Fluid: the phospholipids and many proteins move sideways within the layer. Mosaic: proteins are scattered through the bilayer like tiles in a mosaic pattern.
Rate is proportional to surface area × concentration difference ÷ thickness. Alveoli have a huge total area and very thin walls (one flattened cell each, next to a one-cell capillary wall); blood flow and ventilation keep the gradient steep.
It is unchanged: $\frac{2}{2} = 1$ .
Channels are pores that let specific ions or water through without binding them and are often gated; carriers bind a specific molecule and change shape to move it across, so they can become saturated.
$\frac{1}{2} \times 2 = 1$ , so 100 %: the rate is unchanged.
The curve rises steeply and roughly in proportion at low concentration difference, then levels off: the number of carrier proteins is limited and at high concentrations all are in use (saturated).

Key points

Fluid mosaic membrane: a phospholipid bilayer about 7 nm thick, with cholesterol, intrinsic and extrinsic proteins, glycoproteins and glycolipids.
Small, non-polar or lipid-soluble molecules diffuse through the bilayer; ions and polar molecules need channel or carrier proteins.
Fick's law: rate ∝ area × concentration difference ÷ thickness; temperature, molecular size and lipid solubility also matter.
Facilitated diffusion is passive and specific, and saturates when all carriers are busy.

Sources and credits

Written by Ganzaa. The model described is from S. J. Singer and G. L. Nicolson, "The fluid mosaic model of the structure of cell membranes", Science 175 (1972). No text was copied.
Video: A-Level Biology - Fluid Mosaic Model, Cognito, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples and the practice questions are written by Ganzaa. The data in Worked example 2 are illustrative.

1A membrane made only of fat

2The parts of the membrane and their jobs

3How fast? Fick's law

4Facilitated diffusion

A membrane made only of fat

The parts of the membrane and their jobs

How fast? Fick's law

Facilitated diffusion