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9700 · 7.2

Transport mechanisms — practice questions

Practice and worked examples for 9700 Transport mechanisms. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

The diagram shows a cell surface membrane. Explain how the movement of glucose into a cell differs when transported via facilitated diffusion compared to active transport.

Show solution outline
  1. Fac facilitated Diffusion: Glucose moves down its concentration gradient (from higher to lower concentration). It uses a specific carrier protein but does not require metabolic energy (ATP). The carrier protein binds to glucose, changes shape, and releases it inside the cell.
  2. Active Transport: Glucose moves against its concentration gradient (from lower to higher concentration). It also uses a specific carrier protein but requires metabolic energy (ATP). ATP is hydrolysed to provide the energy for the carrier protein to change shape and 'pump' the glucose into the cell.

Worked example 2

A plant cell has Ψs=−800 kPa\Psi_s = -800\,\text{kPa} and Ψp=+300 kPa\Psi_p = +300\,\text{kPa}. Calculate the water potential Ψ\Psi. [2 marks]

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Ψ=Ψs+Ψp=−800+300=−500 kPa\Psi = \Psi_s + \Psi_p = -800 + 300 = -500\,\text{kPa}.

Water moves from regions of higher (less negative) Ψ\Psi to lower Ψ\Psi across a partially permeable membrane.

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