Passive Transport: Diffusion and Osmosis Revision Notes | A Level AQA Biology

1. Why Membranes Are Not Just Walls

Picture a cell membrane not as a solid barrier, but as a dynamic, constantly shifting surface — more like a crowded dance floor than a brick wall. Molecules are crossing it every second, and the question is: how do they get through, and what decides which ones can?

Every cell membrane — whether it is the cell-surface membrane surrounding the whole cell or the membranes enclosing organelles such as mitochondria and the nucleus in eukaryotes — shares the same fundamental architecture. Understanding that architecture is the key to understanding every transport mechanism that follows.

2. The Fluid-Mosaic Model of Membrane Structure

The accepted model of membrane structure is called the fluid-mosaic model. The word 'fluid' tells you the membrane is not rigid — the phospholipid molecules can move laterally within each layer. The word 'mosaic' tells you the membrane is studded with a variety of different proteins, scattered like tiles in a mosaic pattern.

At the core of the membrane sits the phospholipid bilayer: two layers of phospholipid molecules arranged tail-to-tail. Each phospholipid has a hydrophilic (water-attracting) phosphate head that faces outward into the aqueous environment, and two hydrophobic (water-repelling) fatty acid tails that point inward, away from water. This arrangement is not random — it is thermodynamically stable, because the hydrophobic tails are shielded from water on both sides.

Embedded within and spanning this bilayer are proteins. Some span the entire membrane (intrinsic or integral proteins); others sit on the surface (extrinsic or peripheral proteins). These proteins carry out transport, act as receptors, and provide structural support. The specific proteins relevant to transport — channel proteins and carrier proteins — are covered in detail in the facilitated diffusion section.

On the outer surface of the cell-surface membrane, some proteins have short carbohydrate chains attached, forming glycoproteins. Similarly, some phospholipids have carbohydrate chains attached, forming glycolipids. Together, these carbohydrate-bearing molecules form the glycocalyx — a layer involved in cell signalling, cell recognition, and immune responses.

Cholesterol sits between phospholipid tails, acting as a kind of molecular spacer. At higher temperatures it prevents the membrane becoming too fluid; at lower temperatures it prevents it becoming too rigid. This temperature-buffering role is why cholesterol is described as a fluidity buffer. Critically for transport, cholesterol also reduces the permeability of the membrane to small polar molecules and ions by filling gaps between phospholipid tails.

With the membrane structure established, you can now see why different molecules cross membranes in different ways. The nature of the phospholipid bilayer — specifically its hydrophobic core — is the central constraint that shapes every transport mechanism.

3. Simple Diffusion: Crossing the Bilayer Directly

Some molecules can dissolve directly into the hydrophobic core of the bilayer and pass straight through — no proteins required. This is simple diffusion: net movement from a region of higher concentration to a region of lower concentration, down the concentration gradient, without any energy input from the cell.

The molecules that can do this share two key properties: they are small and they are non-polar (or lipid-soluble). Oxygen ($O_2$), carbon dioxide ($CO_2$), and lipid-soluble molecules such as steroid hormones are classic examples. Water is a partial exception — it is polar, but its molecules are small enough to slip through at a slow rate by simple diffusion (though most water movement occurs via osmosis through aquaporins, covered shortly).

Here is where the nature of the phospholipid bilayer imposes clear limitations. Large molecules cannot fit through the tightly packed tails. Charged ions (such as $Na^+$, $K^+$, $Cl^-$) and polar molecules (such as glucose and amino acids) are repelled by the hydrophobic interior — they cannot dissolve into it, so they cannot cross by simple diffusion. This is not a flaw in the membrane; it is precisely the selectivity that keeps the cell's internal environment controlled.

The rate of simple diffusion is governed by Fick's Law: rate is proportional to the surface area and the concentration gradient, and inversely proportional to the thickness of the membrane. A steeper gradient, a larger surface area, or a thinner membrane all increase the rate. This is why gas exchange surfaces such as alveoli are thin, moist, and highly folded.

In summary, simple diffusion is fast and energy-free for small, non-polar molecules, but the hydrophobic bilayer acts as an absolute barrier to ions and large polar molecules — a limitation that makes facilitated diffusion essential.

4. Facilitated Diffusion: Proteins Open the Gate

Because the hydrophobic core blocks ions and polar molecules, cells need an alternative route. That route is provided by specific membrane proteins — and the process is called facilitated diffusion. Like simple diffusion, it is passive: movement is still down the concentration gradient and requires no ATP. The difference is that molecules travel through proteins rather than through the lipid itself.

There are two types of transport protein involved, and AQA expects you to distinguish between them clearly.

Channel proteins work like a tunnel: the aqueous pore shields the ion or polar molecule from the hydrophobic tails as it passes through. The channel is selective because its diameter and the charge distribution of its lining determine which particles fit and which are excluded. Ion channels may be gated — they open or close in response to a stimulus such as a change in voltage or the binding of a signalling molecule.

Think of a carrier protein as a revolving door. The molecule binds to a specific site on the protein, triggering a shape change that moves the molecule to the other side and releases it. Because the protein must change shape for each molecule transported, carrier proteins are slower than channel proteins. Crucially, both types only facilitate movement down the concentration gradient — no energy is consumed.

A useful comparison: if you increase the concentration gradient, the rate of simple diffusion keeps rising proportionally. With facilitated diffusion, the rate also rises as the gradient steepens — but only up to a point. Once all the available channel or carrier proteins are occupied (saturated), the rate plateaus. This saturation effect is a key piece of evidence that proteins are involved, and it is a favourite examiner distinction between simple and facilitated diffusion.

The plateau in facilitated diffusion occurs because the number of transport proteins in the membrane is finite. This is a key distinction from simple diffusion, where no such saturation occurs because the entire lipid bilayer surface is available.

5. Osmosis and Water Potential

Water is the most abundant molecule in any cell, and its movement across membranes is governed by a specific form of diffusion called osmosis. To explain osmosis precisely at A Level, you need the concept of water potential.

Water potential (symbol $\Psi$, pronounced 'psi') is a measure of the tendency of water molecules to move from one region to another. It is measured in kilopascals (kPa). Pure water at atmospheric pressure has a water potential of zero. Adding any solute lowers the water potential, making it a negative value. The more solute dissolved, the more negative the water potential.

Osmosis is then defined as the net movement of water molecules from a region of higher water potential (less negative, or zero) to a region of lower water potential (more negative), across a partially permeable membrane, without the use of metabolic energy. Water always moves down its water potential gradient.

Worked example: Predicting the direction of osmosis between two solutions

A cell has an internal water potential of -800 kPa. It is placed in a solution with a water potential of -300 kPa. Predict the direction of net water movement and explain your reasoning.

1. Cell interior: $\Psi = -800$ kPa External solution: $\Psi = -300$ kPa — Write both values clearly before comparing. The units are kPa.
2. $-300 > -800$ on the number line — -300 is less negative than -800, so the external solution has the higher water potential. Think of the number line: -300 is to the right of -800, therefore higher.
3. Water moves from higher $\Psi$ to lower $\Psi$ Therefore: external solution $\rightarrow$ cell interior — Net water movement is always from the region of higher water potential to the region of lower water potential across the partially permeable membrane.
4. Water enters the cell by osmosis. — The cell gains water and may swell. In a plant cell this creates turgor pressure; in an animal cell, excess water entry can cause lysis (bursting).

Answer: Net water movement is from the external solution (higher water potential, -300 kPa) into the cell (lower water potential, -800 kPa) by osmosis. The cell gains water.

A common source of confusion is the sign of water potential. Remember: pure water = 0 kPa. Any solution has a negative water potential. A value of -100 kPa is higher than -500 kPa — water molecules have a greater tendency to move from the -100 kPa region to the -500 kPa region (down the water potential gradient). Always compare values on a number line rather than by their absolute magnitude.

Water crosses the membrane primarily through aquaporins — channel proteins that form water-specific pores. Although aquaporins are channel proteins, AQA treats osmosis as a distinct process from facilitated diffusion in exam answers. The movement of water across the membrane in response to a water potential gradient is always called osmosis, not facilitated diffusion, even when aquaporins are involved. The partially permeable membrane allows water through but restricts the passage of solute molecules, which is what creates the net directional movement.

Osmosis underpins critical biological processes: the uptake of water by plant roots from soil, the reabsorption of water in the kidney tubules, and the maintenance of cell volume in all living cells. Understanding water potential is therefore not an abstract exercise — it explains how organisms regulate their water balance.

6. Comparing the Three Passive Transport Mechanisms

Now that all three mechanisms are established, it is worth drawing them together. All three are passive — no ATP is consumed. The differences lie in what is transported, how it crosses the membrane, and what drives the movement.

7. Exam Technique and Common Mistakes

AQA questions on this topic test both recall and application. The most reliable way to lose marks is to use imprecise language — so the following guidance focuses on the exact wording that earns marks and the traps that cost them.

For extended-response questions asking you to compare transport mechanisms, structure your answer around: the type of molecule transported, the route taken, the driving force, whether proteins are required, and whether ATP is consumed. Covering all five dimensions in a logical sequence will address the full range of marking points.

Passive transport — simple diffusion, facilitated diffusion, and osmosis — is the foundation on which all membrane transport is built. Every mechanism that follows in the specification, including active transport, builds on the principles established here: the nature of the bilayer, the role of proteins, and the direction of movement down gradients.