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Biology · Cell Membranes

Flip through 20 full sample pages with substantial teaching notes, deep-dive explanations, worked practice, exam application and self-check questions.

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Overview

Cell membranes and transport

Cell membranes are central to cell survival because they regulate exchange with the environment. This sample develops the fluid-mosaic model, diffusion, osmosis, active transport and factors affecting movement, then connects them to practical and exam contexts.

  • Membrane structure determines selective permeability.
  • Passive transport does not require metabolic energy.
  • Active transport moves substances against a concentration gradient.
  • Surface area and diffusion distance strongly affect exchange rate.
Worked example / practice: Starter: explain why a membrane must be selectively permeable rather than freely permeable to every substance.

Deep-dive notes

The central idea on this page is cell membranes and transport. To use it confidently, connect the definition or rule above to the specific details listed here: Membrane structure determines selective permeability.; Passive transport does not require metabolic energy.; Active transport moves substances against a concentration gradient.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Cell membranes and transport” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain cell membranes and transport without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Biology explanations are strongest when they link structure → process → consequence.
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Structure

The phospholipid bilayer

Phospholipids have hydrophilic phosphate heads and hydrophobic fatty-acid tails. In water they arrange as a bilayer with heads facing aqueous environments and tails facing inward, creating a barrier to many charged or strongly polar substances.

  • Hydrophilic = attracted to water.
  • Hydrophobic = repelled by water.
  • The bilayer is flexible, not rigid.
  • Small non-polar molecules can diffuse through more easily than ions.
Worked example / practice: Reasoning: an ion cannot readily cross the hydrophobic interior, so membrane proteins are important for ion transport.

Deep-dive notes

The central idea on this page is the phospholipid bilayer. To use it confidently, connect the definition or rule above to the specific details listed here: Hydrophilic = attracted to water.; Hydrophobic = repelled by water.; The bilayer is flexible, not rigid.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “The phospholipid bilayer” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain the phospholipid bilayer without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Use precise terms: 'phospholipid bilayer' rather than simply 'fat layer'.
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Fluid mosaic

Proteins, cholesterol and membrane function

The fluid-mosaic model describes a dynamic phospholipid bilayer containing proteins and other molecules. Channel proteins, carrier proteins, receptors and enzymes give the membrane specialised functions.

  • Channel proteins provide hydrophilic pathways.
  • Carrier proteins change shape during transport.
  • Receptors bind specific signalling molecules.
  • Cholesterol influences membrane stability and fluidity in animal cells.
Worked example / practice: Example: glucose may enter cells through a specific carrier rather than passing freely through the phospholipid core.

Deep-dive notes

The central idea on this page is proteins, cholesterol and membrane function. To use it confidently, connect the definition or rule above to the specific details listed here: Channel proteins provide hydrophilic pathways.; Carrier proteins change shape during transport.; Receptors bind specific signalling molecules.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Proteins, cholesterol and membrane function” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain proteins, cholesterol and membrane function without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: When asked for membrane adaptations, name the component and its function.
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Diffusion

Net movement down a gradient

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration because of random particle motion. Individual particles move randomly in all directions, but the overall net movement follows the gradient.

  • Passive process: no direct ATP requirement.
  • Continues until dynamic equilibrium if conditions allow.
  • A steeper concentration gradient increases net diffusion rate.
  • Temperature affects particle kinetic energy.
Worked example / practice: Example: oxygen diffuses from alveolar air, where its concentration is higher, into blood where it is lower.

Deep-dive notes

The central idea on this page is net movement down a gradient. To use it confidently, connect the definition or rule above to the specific details listed here: Passive process: no direct ATP requirement.; Continues until dynamic equilibrium if conditions allow.; A steeper concentration gradient increases net diffusion rate.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Net movement down a gradient” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain net movement down a gradient without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Do not say particles 'want' to move. Describe random motion and net movement.
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Rate

Factors affecting diffusion rate

The rate of diffusion depends on the concentration gradient, surface area available for exchange, diffusion distance and temperature. In biological systems, maintaining a gradient can be as important as membrane properties.

  • Larger surface area → faster total exchange.
  • Shorter diffusion path → faster exchange.
  • Steeper gradient → faster net diffusion.
  • Higher temperature → faster particle motion.
Worked example / practice: Application: alveoli have a large surface area and thin walls, while ventilation and blood flow help maintain concentration gradients.

Deep-dive notes

The central idea on this page is factors affecting diffusion rate. To use it confidently, connect the definition or rule above to the specific details listed here: Larger surface area → faster total exchange.; Shorter diffusion path → faster exchange.; Steeper gradient → faster net diffusion.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Factors affecting diffusion rate” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain factors affecting diffusion rate without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Explain each feature separately instead of listing adaptations without linking them to rate.
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Osmosis

Water movement through membranes

Osmosis is the net movement of water molecules through a partially permeable membrane from a region of higher water potential to lower water potential. At school level it is often described in terms of dilute and concentrated solutions, but water potential is more precise.

  • Only water movement is described by osmosis.
  • A partially permeable membrane is required.
  • Solute concentration changes water potential.
  • Net movement stops at equilibrium, though molecules continue moving.
Worked example / practice: Example: water enters a plant cell in a dilute external solution because external water potential is higher.

Deep-dive notes

The central idea on this page is water movement through membranes. To use it confidently, connect the definition or rule above to the specific details listed here: Only water movement is described by osmosis.; A partially permeable membrane is required.; Solute concentration changes water potential.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Water movement through membranes” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain water movement through membranes without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Do not define osmosis as 'movement from high concentration to low concentration' without specifying water.
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Plant cells

Osmosis in plant cells

Plant cells respond to water movement differently from animal cells because the cellulose cell wall resists excessive expansion. Water entering the vacuole increases turgor pressure and can make the cell turgid.

  • Turgid cells support non-woody plant tissues.
  • Loss of water makes cells flaccid.
  • Severe water loss can cause plasmolysis.
  • The cell wall prevents bursting under normal osmotic conditions.
Worked example / practice: Application: a wilted plant has reduced turgor because cells have lost water.

Deep-dive notes

The central idea on this page is osmosis in plant cells. To use it confidently, connect the definition or rule above to the specific details listed here: Turgid cells support non-woody plant tissues.; Loss of water makes cells flaccid.; Severe water loss can cause plasmolysis.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Osmosis in plant cells” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain osmosis in plant cells without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Distinguish the cell membrane from the cell wall in diagrams and explanations.
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Animal cells

Osmosis in animal cells

Animal cells lack a rigid cell wall, so large osmotic water movements can alter cell volume substantially. In a strongly hypotonic environment an animal cell may swell and lyse; in a hypertonic environment it loses water and shrinks.

  • Isotonic: no net change in cell volume.
  • Hypotonic external solution: net water entry.
  • Hypertonic external solution: net water loss.
  • Medical solutions must be chosen to avoid damaging cells.
Worked example / practice: Example: red blood cells placed in pure water take in water and may burst.

Deep-dive notes

The central idea on this page is osmosis in animal cells. To use it confidently, connect the definition or rule above to the specific details listed here: Isotonic: no net change in cell volume.; Hypotonic external solution: net water entry.; Hypertonic external solution: net water loss.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Osmosis in animal cells” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain osmosis in animal cells without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Use 'net movement of water' rather than implying all water molecules move one way.
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Active transport

Moving against a gradient

Active transport moves substances across membranes against their concentration gradient using energy from cellular respiration and specific carrier proteins or pumps.

  • Requires metabolic energy, commonly ATP.
  • Can move from low concentration to high concentration.
  • Uses membrane proteins.
  • Rate can depend on respiration and availability of transport proteins.
Worked example / practice: Example: mineral ions may be absorbed by root hair cells even when their concentration is lower in the soil than inside the cell.

Deep-dive notes

The central idea on this page is moving against a gradient. To use it confidently, connect the definition or rule above to the specific details listed here: Requires metabolic energy, commonly ATP.; Can move from low concentration to high concentration.; Uses membrane proteins.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Moving against a gradient” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain moving against a gradient without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: State why diffusion cannot achieve the same movement: the movement is against the concentration gradient.
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Facilitated diffusion

Protein-assisted passive transport

Some molecules move down their concentration gradients but cannot cross the lipid bilayer easily. Channel or carrier proteins allow facilitated diffusion without direct energy expenditure.

  • Still passive.
  • Moves down an electrochemical or concentration gradient.
  • Specific proteins increase selectivity.
  • Channels and carriers operate differently.
Worked example / practice: Comparison: active transport and facilitated diffusion both use proteins, but only active transport can drive net movement against the gradient using energy.

Deep-dive notes

The central idea on this page is protein-assisted passive transport. To use it confidently, connect the definition or rule above to the specific details listed here: Still passive.; Moves down an electrochemical or concentration gradient.; Specific proteins increase selectivity.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Protein-assisted passive transport” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain protein-assisted passive transport without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Comparison questions need both similarities and differences.
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Exchange surfaces

Why specialised exchange surfaces work

Large organisms require specialised exchange surfaces because their surface-area-to-volume ratio is lower and diffusion distances are greater. Efficient surfaces share recurring structural features.

  • Large surface area.
  • Thin barrier.
  • Good blood supply or transport system.
  • Ventilation or another mechanism to maintain gradients.
Worked example / practice: Application: villi increase intestinal surface area; capillaries remove absorbed molecules and help maintain gradients.

Deep-dive notes

The central idea on this page is why specialised exchange surfaces work. To use it confidently, connect the definition or rule above to the specific details listed here: Large surface area.; Thin barrier.; Good blood supply or transport system.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Why specialised exchange surfaces work” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain why specialised exchange surfaces work without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Relate each adaptation to the exact process being accelerated.
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Practical

Investigating osmosis with plant tissue

A common investigation places equal-sized plant tissue samples in solutions of different concentration, then measures mass or length change. Control starting size, solution volume, time and temperature.

  • Blot tissue consistently before weighing.
  • Calculate percentage mass change, not just raw change, when starting masses differ.
  • Repeat and calculate a mean.
  • Plot percentage change against solution concentration.
Worked example / practice: Percentage change = (final mass − initial mass)/initial mass × 100%. The concentration giving approximately 0% change estimates the tissue's isotonic point.

Deep-dive notes

The central idea on this page is investigating osmosis with plant tissue. To use it confidently, connect the definition or rule above to the specific details listed here: Blot tissue consistently before weighing.; Calculate percentage mass change, not just raw change, when starting masses differ.; Repeat and calculate a mean.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Investigating osmosis with plant tissue” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain investigating osmosis with plant tissue without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Never dry the tissue aggressively; blotting should remove surface liquid without drawing water from cells.
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Enzymes and membranes

Membrane proteins as biological machinery

Some membrane proteins act as enzymes or form complexes that catalyse reactions. Others participate in cell recognition or signalling. The membrane is therefore an active interface, not simply a barrier.

  • Receptor shape is complementary to signalling molecules.
  • Protein conformation matters.
  • Membrane proteins can cluster into functional complexes.
  • Changes in membrane protein structure can alter transport or signalling.
Worked example / practice: Reasoning: a mutation changing the shape of a channel protein could reduce transport even if the concentration gradient is unchanged.

Deep-dive notes

The central idea on this page is membrane proteins as biological machinery. To use it confidently, connect the definition or rule above to the specific details listed here: Receptor shape is complementary to signalling molecules.; Protein conformation matters.; Membrane proteins can cluster into functional complexes.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Membrane proteins as biological machinery” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain membrane proteins as biological machinery without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Use molecular structure to explain functional consequences.
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Surface area

Surface-area-to-volume ratio

As an organism or cell becomes larger, volume increases faster than surface area. A lower surface-area-to-volume ratio makes exchange across the outer surface less efficient relative to metabolic demand.

  • For similar shapes, surface area scales with length².
  • Volume scales with length³.
  • Small cells have relatively more membrane area per unit volume.
  • Large organisms need transport systems and exchange organs.
Worked example / practice: Cube example: doubling side length multiplies surface area by 4 but volume by 8, so SA:V falls.

Deep-dive notes

The central idea on this page is surface-area-to-volume ratio. To use it confidently, connect the definition or rule above to the specific details listed here: For similar shapes, surface area scales with length².; Volume scales with length³.; Small cells have relatively more membrane area per unit volume.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Surface-area-to-volume ratio” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain surface-area-to-volume ratio without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Show the relationship with numbers if a question asks why size matters.
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Data skills

Interpreting transport graphs

Graphs may show rate against concentration, temperature or time. Passive simple diffusion can increase roughly with gradient, while carrier-mediated transport may plateau when proteins become saturated.

  • Read axes and units before interpreting.
  • Describe overall trend before explaining it.
  • Plateaus may indicate a limiting factor.
  • Distinguish correlation from mechanism.
Worked example / practice: Example: if uptake rate reaches a maximum despite rising external concentration, carrier availability may be limiting.

Deep-dive notes

The central idea on this page is interpreting transport graphs. To use it confidently, connect the definition or rule above to the specific details listed here: Read axes and units before interpreting.; Describe overall trend before explaining it.; Plateaus may indicate a limiting factor.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Interpreting transport graphs” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain interpreting transport graphs without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Use graph evidence in your wording: 'between X and Y, rate increases...'
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Comparison

Diffusion, osmosis and active transport

These processes are easy to confuse. Organise them by what moves, direction relative to a gradient, membrane requirement and energy requirement.

  • Diffusion: particles down a concentration gradient.
  • Osmosis: water through a partially permeable membrane down a water-potential gradient.
  • Active transport: specific substances against a gradient using energy.
  • Facilitated diffusion: down a gradient through proteins.
Worked example / practice: Self-test: glucose moving down its gradient through a carrier is facilitated diffusion, not active transport.

Deep-dive notes

The central idea on this page is diffusion, osmosis and active transport. To use it confidently, connect the definition or rule above to the specific details listed here: Diffusion: particles down a concentration gradient.; Osmosis: water through a partially permeable membrane down a water-potential gradient.; Active transport: specific substances against a gradient using energy.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Diffusion, osmosis and active transport” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain diffusion, osmosis and active transport without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: When naming a process, justify it with at least two defining features.
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Exam language

Writing biological explanations

Biology marking schemes often reward linked statements. A good explanation names the structure, describes the process and states the consequence for the cell or organism.

  • Avoid vague words like 'stuff' or 'things'.
  • Use 'increases the rate of diffusion' rather than 'helps diffusion'.
  • Name the gradient.
  • Use 'net movement' where appropriate.
Worked example / practice: Model: 'The alveolar wall is one cell thick, reducing diffusion distance, so oxygen diffuses into the blood more rapidly.'

Deep-dive notes

The central idea on this page is writing biological explanations. To use it confidently, connect the definition or rule above to the specific details listed here: Avoid vague words like 'stuff' or 'things'.; Use 'increases the rate of diffusion' rather than 'helps diffusion'.; Name the gradient.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Writing biological explanations” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain writing biological explanations without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Every adaptation should be connected to a measurable advantage.
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Practice

Transport practice questions

Try: 1) Define diffusion. 2) Explain why root hair cells contain many mitochondria. 3) Predict what happens to a plant cell in concentrated salt solution. 4) State two adaptations of an efficient gas-exchange surface.

  • 1) Include net movement and concentration gradient.
  • 2) Link respiration/ATP to active transport.
  • 3) Water leaves; cell becomes flaccid and may plasmolyse.
  • 4) Large surface area and short diffusion distance are acceptable examples.
Worked example / practice: Extension: explain why a carrier-mediated uptake graph may level off at high substrate concentration.

Deep-dive notes

The central idea on this page is transport practice questions. To use it confidently, connect the definition or rule above to the specific details listed here: 1) Include net movement and concentration gradient.; 2) Link respiration/ATP to active transport.; 3) Water leaves; cell becomes flaccid and may plasmolyse.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Transport practice questions” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain transport practice questions without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Mark your answer against the command word: a 'state' question needs less detail than an 'explain' question.
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Summary

Membrane transport checklist

You should now be able to describe membrane structure, distinguish transport mechanisms, explain factors affecting exchange and analyse common osmosis experiments.

  • Structure: phospholipids + proteins + other components.
  • Passive: diffusion, osmosis, facilitated diffusion.
  • Active: energy-dependent movement against a gradient.
  • Practical: control variables, percentage change and graph interpretation.
Worked example / practice: Final challenge: compare oxygen uptake by diffusion with mineral-ion uptake by a root hair cell in four precise sentences.

Deep-dive notes

The central idea on this page is membrane transport checklist. To use it confidently, connect the definition or rule above to the specific details listed here: Structure: phospholipids + proteins + other components.; Passive: diffusion, osmosis, facilitated diffusion.; Active: energy-dependent movement against a gradient.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Membrane transport checklist” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain membrane transport checklist without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Use the full notes to extend these ideas into cell signalling, respiration, homeostasis and organ-level exchange.
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Extension

Applying membrane transport to unfamiliar contexts

Higher-level biology questions often place familiar transport ideas into an unfamiliar organism, tissue or experimental setup. The key is to identify the gradient, membrane properties, energy requirement and exchange-surface features from the information given rather than relying on memorised examples.

  • Identify what substance is moving and in which direction.
  • Decide whether movement is passive or energy-dependent.
  • Use evidence from the question to infer membrane or protein involvement.
  • Link any structural adaptation to its effect on rate or control of transport.
Worked example / practice: If uptake continues when external concentration is lower than internal concentration and falls sharply when respiration is inhibited, active transport is strongly indicated because movement is against the gradient and depends on metabolic energy.

Deep-dive notes

The central idea on this page is applying membrane transport to unfamiliar contexts. To use it confidently, connect the definition or rule above to the specific details listed here: Identify what substance is moving and in which direction.; Decide whether movement is passive or energy-dependent.; Use evidence from the question to infer membrane or protein involvement.. These are not separate facts to memorise in isolation; they form the reasoning chain you should be able to reconstruct without looking.

A strong revision method is to close the notes after reading this section and reproduce the key idea from memory. Then compare your version with the page, identify any missing terminology, and correct the explanation before moving to practice. This converts passive reading into active retrieval and makes the page useful for both first learning and later revision.

Exam-style application

Possible question: Explain, apply or use the idea of “Applying membrane transport to unfamiliar contexts” in a new situation. Start by stating the relevant rule or definition precisely, then use the information in the question, show the intermediate reasoning, and finish with a conclusion that answers the command word.

  1. Identify exactly what the question is asking and underline the command word.
  2. Write the relevant definition, relationship, rule or principle before substituting or applying it.
  3. Use the information given rather than relying on vague general statements.
  4. Show the reasoning in a logical sequence so method marks remain visible.
  5. Check the final answer for units, terminology, plausibility and relevance to the question.

Before you turn the page

  • Can you define or explain applying membrane transport to unfamiliar contexts without looking?
  • Can you give one correct example and one common mistake?
  • Can you recognise when this idea should be used in a question?
  • Can you explain your method clearly enough for someone else to follow?
Exam / study tip: Use the evidence supplied in the unfamiliar context. Do not name a process without justifying it from direction, gradient and energy dependence.
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