Example 1
A tissue sample changes from 5.0g to 5.5g. Calculate percentage change.
55.5−5.0.0×100=+10%. Net water uptake is consistent with the gain.
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Year 11 · SACE Stage 1
Membranes, diffusion and osmosis
Predict movement from concentration information and calculate percentage mass change.
Method and worked examples
A plasma membrane contains a phospholipid bilayer and proteins. Selective permeability helps maintain internal conditions. Diffusion is net movement down a concentration gradient through random motion. Facilitated diffusion uses proteins without a direct energy input. For ions, electrical potential also matters: passive movement follows the electrochemical gradient. Active transport can move a substance against its relevant gradient using cellular energy.
Osmosis is net movement of water across a selectively permeable membrane towards lower water potential. In simple solutions, water moves towards the higher concentration of solutes that cannot cross. Animal cells may swell or shrink; plant walls resist expansion and allow turgor. Percentage mass change equals final minus initial mass, divided by initial mass, multiplied by 100.
Example 1
Example 2
A cell imports an ion against its electrochemical gradient. Classify transport.
Net movement against the electrochemical gradient requires energy-coupled active transport.
Watch out: Channels and carriers can enable passive facilitated diffusion down a relevant gradient; transport direction and energy use distinguish the processes.