Cell membranes and transport
IB Biology SLΒ· 6 min read
1. 1. The Fluid Mosaic Model of Membrane Structureβ β ββββ± 15 min
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Fluid Mosaic Model
Describes the cell membrane as a dynamic phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol. Most components can move laterally within the bilayer, giving the membrane fluidity.
Example:
Amphipathic phospholipids arrange with hydrophilic phosphate heads facing aqueous cytoplasm/extracellular fluid, and hydrophobic fatty acid tails facing inward.
Cholesterol modulates fluidity: restricts movement at high temperatures, prevents packing at low temperatures
Integral proteins span the full bilayer; peripheral proteins bind to the membrane surface
Glycoproteins/glycolipids function in cell recognition and signaling
Predict how a lack of cholesterol would affect an animal cell membrane at high temperatures.
- 1
Recall the core function of cholesterol in animal cell membranes
- 2
At high temperatures, cholesterol normally restricts movement of fatty acid tails in the bilayer
- 3
If cholesterol is absent, there is no restriction on molecular movement within the bilayer
- 4
Conclusion: The membrane becomes overly fluid, loses structural integrity, and has uncontrolled permeability
2. 2. Passive Transport: Simple Diffusion and Osmosisβ β ββββ± 20 min
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Passive Transport
Net movement of substances across a membrane down their concentration or water potential gradient, that does not require ATP energy input from the cell.
Simple diffusion occurs for small, non-polar molecules that can pass directly through the hydrophobic phospholipid bilayer. Common examples include oxygen, carbon dioxide, and steroid hormones.
Osmosis
Net passive movement of free water molecules across a semipermeable membrane, from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).
A human red blood cell with 0.9% solute concentration is placed into a 10% saline solution. Predict and explain the outcome.
- 1
Classify the tonicity: the extracellular solution is hypertonic to the red blood cell cytoplasm
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Water potential is higher inside the cell than outside the cell
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By osmosis, free water moves out of the cell down the water potential gradient
- 4
Outcome: The red blood cell loses water and undergoes crenation (shriveling)
Which substance crosses cell membranes by simple diffusion?
A) Glucose
B) Oxygen
C) Sodium ions
D) Insulin
Reveal answer
B βGlucose is too large/polar, sodium ions are charged, and insulin is a large protein: all cannot cross via simple diffusion. Only small non-polar oxygen diffuses directly through the bilayer.
3. 3. Facilitated Diffusionβ β β βββ± 15 min
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Facilitated Diffusion
Passive movement of large, polar, or charged substances down their concentration gradient, via specific channel or carrier proteins embedded in the membrane.
The hydrophobic core of the phospholipid bilayer repels polar and charged molecules, so these substances cannot cross via simple diffusion, even when moving down their gradient. Channel proteins form hydrophilic pores for specific ions, while carrier proteins change shape to transport larger molecules like glucose.
Explain why glucose cannot cross cell membranes via simple diffusion.
- 1
Glucose is a large polar molecule
- 2
The hydrophobic core of the phospholipid bilayer repels all polar molecules
- 3
Glucose cannot pass directly through the hydrophobic core
- 4
It must therefore use a specific carrier protein for facilitated diffusion, even when moving down its concentration gradient
4. 4. Active Transport and Bulk Transportβ β β βββ± 20 min
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Active Transport
Movement of substances across a membrane against their concentration gradient, which requires ATP energy and specific transmembrane protein pumps.
The most well-studied example is the sodium-potassium pump in animal cells, which moves 3 sodium ions out of the cell and 2 potassium ions into the cell against their gradients, using one ATP per cycle. Bulk transport (endocytosis and exocytosis) is also an active process that moves large volumes of material via membrane vesicles.
Iodine concentration in thyroid gland cells is 100x higher than in blood plasma. What transport mechanism moves iodine into thyroid cells, and why is this required?
- 1
Iodine is moving against its concentration gradient (higher inside the cell than outside)
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Movement against a concentration gradient requires energy input in the form of ATP
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This process is carried out by a specific membrane protein pump
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Conclusion: The mechanism is active transport, required to accumulate iodine against its gradient
Endocytosis brings material into the cell (phagocytosis for large particles, pinocytosis for fluid), while exocytosis releases material from the cell, for example insulin secretion from pancreatic beta cells.
5. Common Pitfalls
Wrong move:
Stating osmosis is the movement of solute down a concentration gradient.
Why:
Osmosis specifically describes the movement of free water molecules, not solute.
Correct move:
Osmosis is the net movement of free water across a semipermeable membrane from higher to lower water potential.
Wrong move:
Claiming facilitated diffusion requires ATP because it uses proteins.
Why:
All passive transport, including facilitated diffusion, moves down a gradient and does not need ATP.
Correct move:
Facilitated diffusion is passive, only requiring specific proteins to enable movement down the concentration gradient.
Wrong move:
Predicting a plant cell placed in pure water will burst.
Why:
Plant cells have a rigid cell wall that resists osmotic pressure and swelling.
Correct move:
A plant cell placed in pure water will become turgid (firm), the cell wall prevents bursting.
Wrong move:
Claiming all membrane proteins are free to move in the bilayer.
Why:
Some proteins are anchored to the cytoskeleton and are fixed in place.
Correct move:
Most membrane components move laterally, but not all are free to move in the fluid mosaic model.
6. Quick Reference Cheatsheet
Process | ATP Required? | Direction | Needs Protein? | Example |
|---|---|---|---|---|
Simple diffusion | No | Down gradient | No | Oβ, COβ crossing membrane |
Osmosis | No | Down water potential | No (aquaporins optional) | Water movement into plant roots |
Facilitated diffusion | No | Down gradient | Yes | Glucose entering body cells |
Active transport | Yes | Against gradient | Yes (pump) | NaβΊ/KβΊ pump in nerve cells |
Endo/Exocytosis | Yes | Any direction | No (uses vesicles) | Insulin secretion from pancreas |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· 1
Membrane structure multiple choice
- 2024 Β· 2
Osmosis practical data analysis
- 2023 Β· 1
Transport mechanism comparison
Going deeper
- practical guideIB Biology SL Osmosis Practical GuideRequired core practical for this topic
What's Next
Understanding cell membranes and transport is foundational for almost all other topics in IB Biology, from cell signaling and division to physiological processes like gas exchange and nerve conduction. The principles of osmosis are key for understanding water movement in plants, while active transport underpins processes like glucose reabsorption in the kidney and electrical action potentials in neurons. This topic also includes a required core practical on osmosis that is frequently assessed in experimental questions on paper 2 and 3. Build on this knowledge with the related topics below.