A concentration gradient is the difference in concentration between two regions. Particles move down the gradient by diffusion, and the steeper it is, the faster the net movement.
This lesson is part of movement across membranes. It builds on predicting what happens to cells in different solutions.
What does a full explanation include?
A complete answer has three parts: the direction (from high to low concentration), the reason (random movement gives a net movement down the gradient), and the limit (movement slows as the gradient falls).
For active transport, the direction reverses: substances move from low to high concentration, using energy and carrier proteins.
Worked example: which gradient is steeper?
An original comparison. Cell A has oxygen at 2 units inside and 8 units outside. Cell B has oxygen at 3 units inside and 5 units outside.
The gradient for A is 8 − 2 = 6 units. For B it is 5 − 3 = 2 units. A has the steeper gradient, so oxygen diffuses into A faster.
In words: there are more oxygen particles outside A than inside, so more particles cross inward than outward each second. The difference is larger for A, so the net rate is higher.
Diffusion against active transport
Root hair cells can take in mineral ions even when the soil has fewer ions than the cell. Suppose the soil has 1 unit and the cell has 4 units.
Diffusion would move ions out, from 4 to 1. So the uptake must be active transport: carrier proteins use energy from respiration to move ions against the gradient.
The mistake that loses the second mark
A student writes: “Particles move from high to low concentration because they want to balance out.” “Want” is not a mechanism, and “balance out” ignores that particles keep moving.
A better answer: “Particles move randomly. More particles are on the high-concentration side, so more cross toward the low side, giving a net movement down the gradient.”
How do I check my answer against the marks?
Count your ideas. A two-mark “explain” needs two separate ideas. For three marks, add the limit or the factor that changes the rate.
A self-check question
Explain why a cell in cold water takes in oxygen more slowly than in warm water, assuming the same gradient.
Answer
At a lower temperature, particles have less kinetic energy and move more slowly. Fewer particles cross the membrane each second, so the rate of diffusion is lower. The gradient is the same, so the difference comes from the speed of movement, not the steepness.
Try interpreting membrane transport experiment data to use these ideas on numbers. The biological data interpretation trainer has more practice.
If your explanations stop one sentence short, a teacher in SPM Biology one-to-one tuition can check each answer against the marks. You can start with the one-hour trial class (from RM50).