Question 1

What is the difference between diffusion and osmosis in Cambridge Biology 9700?

Accepted Answer

Diffusion is the general net movement of *any* particles from a region of higher concentration to lower concentration. Osmosis is a *special type of diffusion* specifically for **water molecules** across a **partially permeable membrane** from a region of higher water potential to lower water potential. Master these concepts with our free revision lessons and practice with past papers to excel in your 9700 exams!

Question 2

How does active transport differ from facilitated diffusion?

Accepted Answer

Both active transport and facilitated diffusion involve membrane proteins. However, facilitated diffusion is a passive process that occurs *down* a concentration gradient and doesn't require ATP. Active transport, conversely, moves substances *against* a concentration gradient and *requires metabolic energy (ATP)* to power the carrier proteins. Understanding this distinction is crucial for Paper 1 MCQs and Paper 2 structured questions.

Question 3

Why is understanding water potential essential for osmosis questions?

Accepted Answer

Water potential (Ψ) provides a quantitative way to predict the direction of water movement. Instead of simply saying 'high water concentration', you must use 'higher water potential' (less negative) and 'lower water potential' (more negative). Cambridge examiners expect you to use this term precisely, especially when explaining changes in plant and animal cell states. Consistently applying water potential concepts, including Ψ = Ψs + Ψp, will earn you full marks in detailed explanations.

Movement into and out of cells

What you need to know

Movement into and out of cells