Movement of Substances

Every living cell must take in the substances it needs (oxygen, glucose, water, mineral ions) and get rid of waste (carbon dioxide, urea). These substances cross the cell membrane by three processes you have already met:

• Diffusion — net movement of particles from a high to a low concentration (down a gradient).
• Osmosis — diffusion of water across a partially permeable membrane.
• Active transport — movement against a gradient, using energy from respiration.

This subtopic (spec 2.16) is not about how substances move — it is about how FAST they move. A bigger organism, or a more active one, needs substances delivered quickly. Four factors control the rate:

1. Surface area to volume ratio
2. Distance the substance travels
3. Temperature
4. Concentration gradient

Exam tip. When a question says "explain the rate", you must talk about these factors — not just name the process. Every mark here is about faster or slower.

Substances enter and leave through the surface (the membrane), but they are used up by, and must reach, the whole volume inside. So what matters is the surface area to volume ratio (SA:V).

• A large SA:V ratio → faster movement: there is lots of surface "doorway" for each unit of volume that needs supplying.
• A small SA:V ratio → slower movement: the inside is too far from the surface and not enough membrane to supply it.

Key idea: as objects get bigger, SA:V gets smaller. Volume grows faster than surface area. Work it out for cubes:

Cube side	Surface area	Volume	SA : V ratio
1 cm	6 cm²	1 cm³	6 : 1 (large)
2 cm	24 cm²	8 cm³	3 : 1
3 cm	54 cm²	27 cm³	2 : 1 (small)

The small 1 cm cube has the largest SA:V, so substances reach its centre fastest. This is why single cells are tiny, and why large organisms need special exchange surfaces (see the last section).

Exam tip. Always write the ratio the right way round — surface area : volume. "It has a large surface area" alone is not enough; a large object has a large surface area too, but a small ratio.

Distance — how far the substance has to travel through the barrier (and across to where it is used).

• Shorter distance → faster movement.
• That is why exchange surfaces are thin (often just one cell thick). The membrane of an alveolus and the wall of the capillary next to it are each a single, flat cell — oxygen has almost no distance to cross.
• A thick barrier slows movement down, because particles must diffuse through more material.

Temperature — how much energy the particles have.

• Higher temperature → faster movement.
• The particles (and water molecules) gain more kinetic energy, so they move faster and collide / spread more often. They cross the membrane more quickly.
• (For active transport, warmth also speeds up respiration, which supplies the energy — though if it gets too hot, enzymes denature and the cell is damaged.)

Exam tip. For temperature, the marking point is usually "particles have more kinetic energy so move faster" — say why, not just "it's hotter".

The concentration gradient is the difference in concentration between two regions (e.g. inside and outside the cell).

• A steep (large) gradient → faster movement.
• A shallow (small) gradient → slower movement; if concentrations become equal, net diffusion stops (particles still move, but equally both ways).

Living organisms keep the gradient steep so movement stays fast. For example:

• Blood flow carries absorbed oxygen and food away as soon as they cross a surface, keeping their concentration low on the inside — so the gradient (high outside → low inside) stays steep.
• Breathing brings fresh air into the alveoli, keeping oxygen high outside the blood.

Exam tip. "Maintains a concentration gradient" is a high-value phrase. If asked why blood keeps flowing past an exchange surface, the answer is to keep the gradient steep so diffusion stays fast.

Large organisms cannot rely on their outer surface — their SA:V ratio is too small. Instead they have specialised exchange surfaces that combine the rate factors to make movement as fast as possible. The same three design rules appear everywhere:

1. Large surface area (so a large SA:V for exchange).
2. Thin barrier (short distance to cross).
3. Good supply (blood flow, breathing or water uptake) to keep the concentration gradient steep.

Exchange surface	What it absorbs	Large surface area	Thin / short distance	Keeps gradient steep
Villi (small intestine)	digested food	finger-like villi (and microvilli)	wall one cell thick	rich blood supply carries food away
Alveoli (lungs)	oxygen in, CO₂ out	millions of tiny air sacs	wall one cell thick	blood flow + breathing renew the air
Root hair cells (roots)	water + mineral ions	long, thin hair extension	thin cell wall/membrane	water moves on into the plant

How to use this in the exam. If asked "how is the [villus / alveolus / root hair cell] adapted for fast [absorption / gas exchange / uptake]", structure your answer around the three rules and link each one back to a rate factor from 2.16:

• Large surface area → larger SA:V ratio → faster exchange.
• Thin → short diffusion distance → faster.
• Good supply → maintains a steep concentration gradient → faster.

Watch out. A root hair cell is long and thin to give a large surface area for absorbing water and ions — not the same as villi/alveoli, but the principle (large SA, short distance, steep gradient) is identical.