What is moving, and why rate matters
Cells constantly swap substances with their surroundings — speed is the focus here.
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:
- Surface area to volume ratio
- Distance the substance travels
- Temperature
- 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 cross membranes by diffusion, osmosis and active transport.
- Spec 2.16 is about the RATE (speed) of movement, not the process itself.
- Four factors: SA:V ratio, distance, temperature, concentration gradient.