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4. Cell Membranes and Transport

Syllabus
9700–2028–2029
Section
4
Level
AS

4.1 Fluid Mosaic Membranes

Syllabus
9700–2028–2029
Topic
4.1
Level
AS

The fluid mosaic model is a moving selective bilayer

The fluid mosaic model describes a cell membrane as a phospholipid bilayer containing different proteins. It is a flexible, moving interface rather than a static wall: the bilayer forms the basic boundary and the components together control exchange and communication.

  • Bilayer arrangement: Hydrophilic phosphate heads face the aqueous environments on either side, while hydrophobic fatty-acid tails face inward to form the membrane core.
  • Fluid: Phospholipids and many membrane proteins can move laterally within the membrane, although some proteins may be held in position.
  • Mosaic: Different proteins are distributed through the bilayer in a scattered pattern, giving a mosaic appearance; their hydrophilic and hydrophobic regions determine where they sit.
  • Selective barrier: The hydrophobic core makes passage difficult for many polar substances and ions, while membrane proteins can provide controlled routes for exchange or communication.
  • Compartmentalisation: Bilayers can enclose the cell or internal organelles, separating reaction environments without making the membrane immovable.

Amphipathic phospholipids self-organise into a bilayer, the hydrophobic core limits unassisted passage, and embedded or surface-associated proteins add selective functions. Because components can move and be arranged differently, the membrane can remain continuous while adapting its exchange and communication roles.

“Fluid” does not mean that the membrane dissolves or that every component moves freely in every direction. “Mosaic” does not mean that the arrangement is meaningless or wholly random. Selective permeability is produced by lipid chemistry plus the position and function of membrane proteins.

Cell-surface membrane components connect structure to function

A cell-surface membrane contains several components whose positions and chemical properties give the membrane its barrier, transport, stability and recognition functions. The phospholipid bilayer is the base; cholesterol, proteins and carbohydrate-containing molecules add specialised roles.

  • Phospholipids: Their hydrophilic heads face the aqueous surroundings and hydrophobic tails face inward, forming the bilayer and a barrier to many water-soluble substances. Individual molecules can move within their monolayer.
  • Cholesterol: It fits between phospholipids with its hydrophilic region near the surface and hydrophobic region among the tails. This helps regulate fluidity, limits excessive permeability and adds stability.
  • Membrane proteins: Intrinsic proteins are embedded in one or both monolayers; many span the membrane as transmembrane proteins. Extrinsic proteins associate with the inner or outer surface. Transport proteins provide selective routes for particular ions or polar molecules.
  • Glycolipids and glycoproteins: Carbohydrate chains project from the outer surface. They can act as receptors and cell markers, supporting specific binding and cell-to-cell recognition.

The hydrophobic phospholipid core blocks many polar substances, while specific transmembrane proteins provide controlled hydrophilic routes for substances that cannot cross the core unaided. Cholesterol tunes how tightly the lipid region behaves, and outward-facing carbohydrate chains expose molecular identities for recognition and receptor binding.

Not every membrane protein is a transport protein, and carbohydrate chains are not distributed equally on both sides: glycolipids and glycoproteins have their carbohydrate chains facing the extracellular side. This component card explains the parts and their jobs; the overall fluid-mosaic definition belongs to the neighbouring model card.

Each membrane component contributes a specific role

The cell-surface membrane combines phospholipids, cholesterol, glycolipids, proteins and glycoproteins. Their positions and chemistry create stability, controlled fluidity and permeability, selective transport, signalling and cell recognition.

Component Required roles linked to structure
phospholipids form the bilayer; hydrophobic core limits passage of ions and many polar molecules while allowing some lipid-soluble molecules through
cholesterol fits between phospholipid tails; stabilises the membrane, limits excessive permeability and buffers fluidity across temperature changes
channel proteins provide selective hydrophilic pores for particular ions or polar molecules
carrier proteins bind specific solutes and change shape to move them across the membrane
receptor proteins bind specific extracellular ligands and initiate cell signalling
glycolipids and glycoproteins expose carbohydrate chains as cell-surface antigens and recognition markers; some glycoproteins also act as receptors

The lipid core supplies the default permeability barrier. Channels provide continuous hydrophilic routes, while carriers alternate access after specific binding. These proteins therefore allow selective transport without making the bilayer freely permeable.

Do not reduce every membrane protein to transport: receptors signal, and glycoproteins can support recognition. Cholesterol does not simply make the membrane rigid; it stabilises and buffers fluidity while also reducing permeability.

Cell signalling runs from ligand secretion to a specific response

Cell signalling links a source cell to a target cell: a specific chemical ligand is secreted, transported to cells, binds only to a complementary receptor on a target cell and triggers a specific response.

  1. A signalling cell secretes a specific chemical ligand.
  2. The ligand is transported away from the source cell through an extracellular fluid or other transport medium.
  3. Only target cells with a complementary cell-surface receptor bind that ligand.
  4. Binding changes the receptor and initiates an intracellular signalling change; the ligand need not cross the bilayer.
  5. The target cell produces a specific response, such as changing enzyme activity, membrane transport or gene expression.

Secretion makes the signal available, transport brings it to potential targets, and receptor complementarity selects which cells respond. The receptor then transduces extracellular binding into an internal change, explaining why a neighbouring cell without the receptor does not respond.

A receptor is not automatically a channel or carrier, and the ligand does not need to enter the cell in this surface-receptor pathway. Do not omit secretion or transport: receptor binding is the third major stage, not the beginning of the full signalling sequence.

4.2 Movement Into and Out of Cells

Syllabus
9700–2028–2029
Topic
4.2
Level
AS

Classify membrane transport by gradient, route and energy

Membrane transport is classified by what moves, its direction relative to a gradient, the route across the membrane and whether cellular energy is required. Not every process moves from high concentration to low concentration, and not every process uses a membrane protein.

  • Simple diffusion: Small non-polar molecules move directly through the phospholipid bilayer from higher to lower concentration; no transport protein or cellular energy is required.
  • Facilitated diffusion: A specific ion or polar molecule moves down its concentration gradient through a channel or carrier; a membrane protein is required, but cellular energy is not.
  • Osmosis: Water moves across a partially permeable membrane from higher water potential to lower water potential; the driving force is the water-potential gradient, not ATP.
  • Active transport: A specific substance is moved against its concentration gradient through a carrier/pump using energy supplied by the cell, commonly from ATP hydrolysis.
  • Endocytosis: The membrane encloses material and pinches off a vesicle to bring bulk material into the cell; this is vesicular transport and requires energy.
  • Exocytosis: A vesicle fuses with the cell-surface membrane and releases its contents outside; this is vesicular transport and requires energy.

A gradient can drive net diffusion when the membrane route is available. A hydrophobic bilayer may block a polar solute, so facilitated diffusion supplies a protein route without reversing the gradient. Active transport uses energy to build or maintain a gradient, while endocytosis and exocytosis move material too large or numerous for a channel or carrier by changing membrane shape.

Protein involvement alone does not prove active transport: facilitated diffusion also uses proteins but follows a gradient and does not require cellular energy. Osmosis is water movement, not solute movement, and endocytosis/exocytosis are bulk vesicle processes rather than high-to-low diffusion.

Design a fair diffusion or osmosis investigation

A transport investigation must connect a measurable change to movement across a boundary. Use matched samples, a controlled difference between the two sides, a fixed exposure and repeated measurements so that the effect of the transport variable can be separated from handling or environmental variation.

  1. Define the question and variables: Choose the independent variable, such as solution concentration or concentration gradient, and the dependent readout, such as mass/length change, colour or solute concentration. Keep the transport process being tested explicit.
  2. Prepare matched samples: Use plant-tissue pieces of comparable starting size and surface area, or a bounded model such as dialysis tubing or agar. Record the starting mass, length or concentration consistently.
  3. Apply the treatment fairly: Place samples in the planned range of solutions or gradients for the same time and under the same relevant temperature, volume and handling conditions. Use a suitable control or comparison condition.
  4. Measure the change: Remove and handle every sample in the same way, then record final mass or length, colour/penetration or concentration as appropriate. Calculate a change or percentage change when that is the chosen readout.
  5. Repeat and compare: Use repeats at each condition, summarise the results consistently and show variation before inferring a trend. Compare like with like rather than selecting a convenient result.
  6. Interpret the process: Diffusion is net movement of particles down a concentration gradient; osmosis is net movement of water across a partially permeable boundary down a water-potential gradient. In a plant-tissue mass experiment, mass change is an operational proxy for net water movement, not direct observation of individual molecules.

A steeper controlled gradient can change the net movement, but the conclusion is only defensible when sample geometry, time, temperature, solution volume and measurement handling are matched. Plant tissue can show osmosis through mass or length change; dialysis tubing or agar can model diffusion across a boundary, but the model does not automatically reproduce every property of a living cell.

Mass, length or colour change alone does not identify the transport process. State what crossed the boundary, whether the boundary was partially permeable, and which controls exclude evaporation, leakage, damaged tissue, unequal blotting or inconsistent handling. No net change means equal opposing net flows, not that molecules stopped moving.

Calculate surface-area-to-volume ratio and predict exchange limits

Surface-area-to-volume ratio (SA:V) is the surface area available for exchange divided by the volume that must be supplied. A higher ratio gives more surface per unit volume, but exchange also depends on diffusion distance, gradients and membrane properties.

  1. Find the surface area: Calculate the total area of every exposed face of the geometric solid, using consistent length units.
  2. Find the volume: Calculate the volume of the same solid in the same length units.
  3. Form the ratio: Write SA:V as surface area : volume, then divide both terms by any common factor to simplify. The ratio carries inverse-length units before a purely numerical ratio is reported.
  4. Symbolic worked example — cube: For a cube with side length a, SA = 6a² and V = a³. Therefore SA:V = 6a²:a³ = 6:a.
  5. Scale check: If the same cube’s side becomes 2a, SA:V = 24a²:8a³ = 3:a, half the original ratio. Increasing linear size increases total surface area but increases volume faster, so there is less surface per unit volume for exchange.

Materials cross an exchange surface, while the surface must supply the volume behind it. As SA:V falls, a smaller proportion of the volume lies close to the surface and diffusion alone becomes less effective unless cells reduce distance, increase exchange surface or use additional transport mechanisms.

Do not compare absolute surface area alone: a larger object can have more total surface but a lower SA:V. SA:V is not a complete rate law; state the geometry and keep units consistent before connecting the ratio to biological exchange.

Use agar blocks to test surface-area-to-volume effects on diffusion

An agar-block investigation models how surface-area-to-volume ratio and diffusion distance affect exchange. Blocks with different dimensions are exposed to the same coloured or indicator solution, so the fraction reached or the penetration distance can be compared after a controlled exposure.

  1. Prepare matched blocks: Make agar blocks of different dimensions from the same agar mixture and indicator/colour condition. Keep the block shape comparable and calculate each block’s surface area, volume and SA:V.
  2. Set the fair comparison: Use the same external solution, temperature, volume and exposure time for every block. Handle and remove blocks consistently so the only planned difference is block size/SA:V.
  3. Expose the blocks: Place the blocks in the solution so that the solution can reach all intended exposed surfaces. Use a control or reference condition where appropriate to identify the starting colour and any background change.
  4. Measure the diffusion front: After exposure, section blocks consistently and measure penetration depth or the coloured fraction using the same rule for every block. Record the result rather than relying on a visual impression.
  5. Repeat and compare: Repeat each block size, summarise variation and compare penetration or coloured fraction with calculated SA:V. A higher SA:V should expose a larger fraction of the block and shorten the average diffusion path in the fixed exposure.
  6. State the model boundary: Agar shows a geometric diffusion pattern; it does not directly test a living cell’s membrane selectivity, active transport, metabolism or cytoplasm.

Diffusion proceeds inward from each exposed surface. A smaller block has more surface relative to its volume and a shorter distance to its centre, so the same exposure can reach a greater fraction of it than a larger block. The conclusion is about geometry and diffusion under matched conditions, not about a precise biological rate constant.

Do not compare blocks after different exposure times or with different agar/solution conditions. Colour reaching the centre shows indicator diffusion in agar; it is not evidence that agar has a cell membrane or that active transport occurred.

Estimate plant-tissue water potential from zero mass change

Plant-tissue water potential can be estimated by finding the external solution condition that produces no net change in tissue mass. At that point, water enters and leaves the tissue at equal net rates, so the tissue and external solution have approximately matching water potentials.

  1. Prepare the series: Make a range of solutions with known concentrations. Cut tissue pieces from the same plant material with comparable dimensions and starting condition.
  2. Record starting values: Blot or handle every piece consistently, then record each piece’s initial mass (or length) before immersion.
  3. Expose fairly: Place matched pieces in separate solutions for the same exposure period, keeping temperature, solution volume, tissue dimensions, handling and other relevant conditions constant.
  4. Measure the change: Remove and handle pieces consistently, record final mass or length, and calculate percentage change when mass is used: percentage change = ((final value − initial value) ÷ initial value) × 100.
  5. Repeat and plot: Use repeats at each concentration, show variation and plot percentage mass change against solution concentration. Use the trend or justified best-fit line to locate the concentration where change is zero.
  6. Infer cautiously: The zero-change intercept estimates the external condition with no net water movement and therefore an approximately matching water potential. It is an estimate, not necessarily one of the measured concentrations.

Positive mass change indicates net water entry; negative mass change indicates net water loss. The intercept is informative because it marks equal opposing net water flows across the tissue’s partially permeable boundaries, not because water molecules stop moving there. If the solution’s water potential is known or can be related to its concentration within the course method, it provides an estimate for the tissue condition.

Do not choose the nearest tube without using the trend, and do not treat a single mass reading as tissue water potential. Damaged tissue, solute leakage, evaporation, inconsistent blotting, unequal sample size or non-linear data can shift the estimate; report the control and limitation rather than inventing precision.

Water potential predicts different plant and animal cell outcomes

Water moves by osmosis across a partially permeable membrane from higher water potential to lower water potential. The direction of water movement comes first; the cell outcome then depends on whether the cell has a supporting cell wall.

  • External solution has higher water potential than the cell: Water enters the cell. A plant cell becomes turgid as the wall resists further expansion; an animal cell swells and may burst because it has no cell wall.
  • External solution has approximately equal water potential to the cell: There is no net water movement. A plant cell is flaccid rather than strongly supported by turgor; an animal cell remains near its normal volume. Individual water molecules still cross in both directions.
  • External solution has lower water potential than the cell: Water leaves the cell. A plant cell loses turgor and may plasmolyse as the cell surface membrane pulls away from the wall; an animal cell loses volume and may crenate.
  • Why the outcomes differ: The cellulose cell wall resists expansion and helps convert water entry into turgor pressure, but it does not stop water loss or prevent plasmolysis. Animal cells have no wall, so swelling and shrinking change the whole-cell shape more directly.

Compare the two water potentials to predict the direction of water movement, then apply the structural boundary. More dilute external conditions usually have higher water potential and drive entry; more concentrated conditions usually have lower water potential and drive exit. These concentration descriptions are relative conditions, not fixed numerical thresholds.

Do not say that water always moves into dilute cells or out of concentrated cells without comparing water potentials on both sides. No net change means equal opposing water flows, not no movement, and plant-cell walls prevent bursting but do not make plant cells immune to water loss.