Movement across membranes occurs by: simple diffusion (limitations imposed by the phospholipid bilayer); facilitated diffusion (carrier proteins and channel proteins); osmosis (explained in terms of water potential); active transport (carrier proteins and ATP hydrolysis); co-transport (illustrated by absorption of sodium ions and glucose by cells lining the mammalian ileum)
Cells move substances across their membranes in five different ways. Each method suits different molecules and uses different proteins or energy sources.
Formula
Water potential (Ψ) = Solute potential (Ψs) + Pressure potential (Ψp)
Real World
Oral rehydration therapy (ORT) used in cholera treatment exploits sodium-glucose co-transport: adding glucose to a saline drink triggers co-transporters in the ileum to simultaneously absorb sodium ions and glucose, pulling water back into the blood.
Exam Focus
For co-transport questions, always state that sodium moves down its concentration gradient and glucose moves against its — and that the gradient is maintained by active transport of sodium out via the basolateral membrane.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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