Transport across Cell Membranes
Explain diffusion, osmosis and active transport through a selectively permeable phospholipid-and-protein membrane.
How to study National 5 Biology
Move between biological structures, processes, experimental evidence, data and justified explanations while using current official materials for exact paper and assignment conditions.
Core concepts
Concept 1
The cell membrane contains phospholipids and proteins and is selectively permeable.
Exam cue: Name the transported substance and state the direction of its concentration gradient.
Concept 2
Diffusion moves molecules down a concentration gradient without an energy requirement.
Exam cue: For osmosis, compare water concentrations and include the selectively permeable membrane.
Concept 3
Osmosis moves water from higher to lower water concentration through a selectively permeable membrane.
Exam cue: Predict whether animal cells burst or shrink and whether plant cells become turgid or plasmolysed.
Concept 4
Active transport uses energy and membrane proteins to move substances against a concentration gradient.
Risk pitfalls and guardrails
Defining osmosis as movement of any solute.
Guardrail: Name the transported substance, direction of the gradient and whether energy or a selectively permeable membrane is required.
Saying passive transport requires ATP.
Guardrail: Check the biological level, direction, location, units and whether the evidence supports the strength of the claim.
Using turgid and plasmolysed for animal cells.
Guardrail: Check the biological level, direction, location, units and whether the evidence supports the strength of the claim.
Memory anchors
Selective permeability
Some substances cross the membrane more readily than others.
Diffusion
Net movement of molecules from higher to lower concentration.
Osmosis
Movement of water from higher to lower water concentration through a selectively permeable membrane.
Active transport
Energy-dependent movement against a concentration gradient using membrane proteins.
Plant cell in dilute solution
Water enters and the cell becomes turgid.
Plant cell in concentrated solution
Water leaves and the cell may become plasmolysed.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
Which two components make up most of a cell membrane?
What does selectively permeable mean?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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