Active transport

Writing "down" or "along" a concentration gradient describes diffusion and scores nothing, even if the answer also mentions energy. Active transport moves particles against the gradient, from lower to higher concentration. Candidates who have just revised diffusion reverse the direction under pressure — "against" is the single most important word in this definition.

The mark scheme wants the source named. "Uses energy" alone, or "energy from food", falls short: glucose from food is only the substrate, and the usable energy is released by respiration inside the cell. Write "using energy from respiration" every time, and add mitochondria (or ATP, at Extended) if the question asks how that energy is supplied.

Saying no energy is needed flatly contradicts the definition, and usually means the process has been confused with osmosis or diffusion, which are passive. Active transport is the only one of the three that requires an energy input — that is what "active" means. The many mitochondria in transporting cells are the structural evidence for that requirement.

Root hair cells take up water by osmosis and mineral ions by active transport, so both processes appear in the same question. Osmosis moves water molecules only, through a partially permeable membrane, down a water potential gradient, and is passive — it can never move ions against their gradient. "Ions are absorbed by osmosis" scores zero.

The carrier proteins sit in the cell membrane. A plant cell wall is fully permeable and contains no pumps, so it cannot control what enters the cell. Examiners have flagged the same slip for osmosis, where candidates named the cell wall as the partially permeable membrane. In both processes the membrane is the selective barrier.

Two facts carry the Extended marks: the carriers are proteins (not lipids or carbohydrates), and each is specific — complementary in shape to one type of particle, like an enzyme and its substrate. A pump for nitrate ions will not carry glucose or water. Vague answers about "holes" or "gaps in the membrane" gain no credit.

A common wrong answer is that ions enter simply by diffusion because the soil contains them. Diffusion slows and stops once concentrations equalise, so it can never raise the internal concentration above the external one. Whenever data show a cell holding more of a substance than its surroundings, active transport must be involved.