Movement into and out of cells — common mistakes

Be precise with terminology! Use 'partially permeable' for cell membranes, not 'semi-permeable'. Always refer to water moving down a 'water potential gradient', not a 'water concentration gradient'. For active transport, explicitly state that it moves substances 'against a concentration gradient' and 'requires ATP'.

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!

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.

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.