Diffusion

Diffusion and concentration gradients

A concentration tells you how much of a substance is present in a particular region. When one region contains more particles of a substance than a neighbouring region, there is a concentration gradient between them, and that gradient is the whole reason diffusion happens at all.

The exact definition is worth learning precisely, because Cambridge marks against it closely:

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.

Notice what that definition does notsay. It does not say particles “travel towards” the region of lower concentration, because they do not — particles move randomly, in every direction, with no sense of where they are or where fewer of them happen to be. The reason there is still an overall movement is purely a matter of numbers: because more particles start out in the high-concentration region, more particles move out of that region than move back into it, purely by chance. Add up every individual, random movement and the result is a net movementfrom high concentration to low concentration. That word “net” is doing real work in the definition, and dropping it is one of the most common ways students lose the mark for stating diffusion accurately.

As diffusion continues, the difference in concentration between the two regions gets smaller, so fewer particles are “left over” to produce an imbalance. Eventually the particles may become evenly distributed. At that point the particles have not stopped moving — they are still moving just as randomly as before — but because roughly equal numbers now move in each direction, there is no longer any netmovement. An answer that says diffusion “stops” once concentrations are equal is describing the wrong thing: the random movement never stops, only the net movement does.

Where the energy for diffusion comes from

Particles such as molecules and ions are always moving, and that movement means they possess kinetic energy — energy associated with movement. This is the entire energy source behind diffusion: the random motion the particles already have, simply by existing above absolute zero.

The point Cambridge specifically wants you able to state is that a cell does not need to supply energy from respiration for diffusion to happen. That single distinction is what separates diffusion from active transport, which does use energy released by respiration to move particles the opposite way, against their concentration gradient. If a question asks you to compare the two processes, the energy source is very often the exact line an examiner is checking for — get it backwards and you can describe every other part of diffusion correctly and still lose the mark.

Diffusion across the cell membrane

The cell membrane forms the boundary between the contents of a cell and its surroundings. Some substances can cross that boundary by diffusion, but only when a suitable concentration gradient exists across it.

Suppose the concentration of a substance is higher outside a cell than inside it, and that substance is able to cross the membrane. There will be a net movement into the cell, by diffusion. If instead its concentration is higher inside the cell, the net movement can just as easily be out ofthe cell. The direction is never fixed to “in” or “out” as a rule you memorise — it always depends on which side has the higher concentration:

higher concentration → lower concentration

Get into the habit of identifying the two concentrations first and letting the direction follow from them, rather than guessing which way a substance “should” move based on what it is.

Why diffusion matters in living organisms

Cells are constantly using up some substances and producing others, and that activity is what keeps concentration gradients in place so diffusion can keep happening. A few examples cover most of what Cambridge expects you to be able to apply this to:

• Oxygen can diffuse from a region where its concentration is higher towards cells where its concentration is lower — those cells are using oxygen in respiration, which keeps the gradient going.
• Carbon dioxide produced by cells can diffuse away from them, moving from where its concentration is higher, in or around the cells producing it, to where it is lower.
• Dissolved substances — solutes, meaning substances dissolved in a solvent — can diffuse across membranes wherever a suitable concentration gradient exists.

This is why diffusion turns up again and again in later chapters, applied to specific exchange surfaces and transport systems rather than restated from scratch: gas exchange in the lungs, absorption after digestion, excretion of waste. The biological setting changes each time, but the underlying mechanism never does — it is still a concentration gradient and random particle movement doing all the work, with no energy spent by the cell.

How quickly any of this happens depends on a separate set of four factors — surface area, temperature, concentration gradient and distance — covered in full, with the practical investigations Cambridge expects, on factors affecting diffusion.