2.2Cells
Cell Membranes & Transport — AP BIOLOGY Revision Notes & Topical Questions
5 min read 12 parts Lesson 4 of 12 1 quiz
Lesson summary
2.2 Cell Membranes & Transport — AP BIOLOGY notes: Describe membrane structure, selective permeability, passive and active transport, osmosis with water potential calculations, and bulk transport.Phospholipids — a polar hydrophilic phosphate head and two nonpolar hydrophobic fatt…
- Glycoproteins and glycolipids
- Cholesterol
- Fatty acids
- Fluid mosaic model
- Transport proteins
- Composition
Section 1 of 12Membrane components and the fluid mosaic model
Membrane components and the fluid mosaic model
Phospholipids
- Phospholipids a polar hydrophilic phosphate head and two nonpolar hydrophobic fatty acid tails,
- forming a bilayer with heads facing water and tails inwards.
Proteins
- Proteins hydrophilic regions face the aqueous surroundings and hydrophobic regions face the interior;
- roles include transport, enzymes, recognition and signalling.
- Integral proteins are embedded in the bilayer (transmembrane if they span it);
- peripheral proteins are loosely bound to the surface.
- Membrane proteins include channels, carriers, pumps, receptors, enzymes and cell-adhesion and recognition glycoproteins.
Glycoproteins and glycolipids
- Glycoproteins and glycolipids carbohydrate chains on the outer surface used in cell recognition and signalling.
Cholesterol
- Cholesterol a steroid that regulates fluidity, reducing it at high temperature and preventing tight packing at low temperature.
Fatty acids
- Fatty acids unsaturated tails increase fluidity.
Fluid mosaic model
Fluid mosaic model a phospholipid framework containing embedded components that can flow around the surface within their layer.
Section 2 of 12Selective permeability
Selective permeability
Selective permeability a membrane lets some substances cross and blocks others, controlling exchange and allowing concentration gradients to form.
Cause
- Cause nonpolar tails block charged and polar substances, and tightly packed phospholipids block large molecules.
Transport proteins
- Transport proteins carriers and channels let large or polar substances and ions cross, so the number and type of proteins decide what crosses.
| Substance | How it crosses | Examples |
|---|---|---|
| Small, nonpolar | Passes freely between phospholipids | N2, O2, CO2 |
| Small, polar | Passes in small volumes; water mainly uses aquaporins | H2O |
| Large, polar or ions | Only via transport proteins | Glucose, sucrose, Na+, Ca2+ |
Section 3 of 12The cell wall as a barrier
1Composition
- Composition complex carbohydrates lying outside the plasma membrane.
- (cellulose in plants, chitin in fungi, peptidoglycan in bacteria)
2Permeability
- Permeability freely permeable to small molecules but can hold back large molecules.
3Rigidity
- Rigidity limits the volume of water a cell can take in, so the cell does not burst.
4Water still enters by osmosis
- Water still enters by osmosis the wall only supplies the rigidity that stops the cell breaking.
Section 4 of 12Passive transport
Passive transport
Passive transport net movement from high to low concentration without direct metabolic energy.
Simple diffusion
- Simple diffusion direct movement across the bilayer down a gradient.
Facilitated diffusion
- Facilitated diffusion movement down a gradient through specific channel or carrier proteins.
- Channel proteins are pores that may be gated (open or closed by voltage or a bound ligand).
- Carrier proteins have a binding site on one side and change shape to release the substance on the other.
- Examples are aquaporins and Na+ and K+ ions.
Osmosis
- Osmosis movement of water down a water potential gradient, between phospholipids or via aquaporins.
Rate of diffusion
- Rate of diffusion increases with a steeper gradient, higher temperature, larger surface area and shorter distance.
Importance
- Importance passive transport imports useful materials and exports waste, e.g. oxygen and CO2 exchange in the lungs.
Section 5 of 12Active transport
1Active transport
Active transport movement against a concentration gradient (low to high) using energy from ATP and carrier proteins; only carriers, not channels, can do it.
2Uses
- Uses builds and maintains gradients, e.g. proton pumps create H+ gradients used by ATP synthase.
3Cotransport (secondary active transport)
- Cotransport (secondary active transport) one substance moves against its gradient coupled to another moving down a gradient set up by an ATP pump, e.g. Na+–glucose symport.
4Bulk transport
- Bulk transport exocytosis and endocytosis move large molecules in vesicles and need energy.
- Exocytosisa vesicle fuses with the membrane and releases neurotransmitters, insulin or digestive enzymes.
- Endocytosisthe membrane engulfs material by phagocytosis, pinocytosis or receptor-mediated uptake such as cholesterol-LDL.
Section 6 of 12The sodium-potassium pump
- Result
- moves 3 Na+ out and 2 K+ in per ATP, setting up the membrane potential and gradients for cotransport.
1
Three Na+ from inside the axon bind to the pump.
2
ATP transfers a phosphate to the protein, which changes shape and opens to the outside.
3
The three Na+ are released outside.
4
Two K+ from outside bind.
5
The phosphate is released and the pump returns to its original shape.
6
The two K+ are released inside.
Section 7 of 12Tonicity and osmosis
Tonicity
- Tonicity always described relative to the cell: hypotonic has lower solute concentration than the cell, hypertonic higher and isotonic equal.
Osmolarity
Osmolarity total concentration of solute particles per litre; water moves from lower to higher osmolarity (higher to lower water potential).
| Solution | Animal cell | Plant cell |
|---|---|---|
| Hypotonic | Swells, may lyse | Turgid (normal) |
| Isotonic | Normal | Flaccid |
| Hypertonic | Shrivels (crenates) | Plasmolysed |
Osmoregulation
- Osmoregulation controls internal water balance; failure bursts red blood cells and wilts plants.
Paramecium
- Paramecium freshwater is hypotonic to it, so a contractile vacuole pumps out the water that keeps entering.
Central vacuole
- Central vacuole in plants it supports the cell and regulates ion concentration in the cytoplasm.
Section 8 of 12Water potential
Water potential
- Ψ = ΨP + ΨS, measured in MPa or bar (1 MPa = 10 bar), and water moves from higher (less negative) to lower (more negative) Ψ.
Solute potential
- ΨS = −iCRT, where i is the ionisation constant, C the molar concentration, R = 0.0831 L bar mol−1 K−1 and T the temperature in K (°C + 273).
- (1 for sucrose or glucose, 2 for NaCl)
Effect of solute
- Effect of solute pure water has ΨS = 0 and adding solute makes ΨS more negative.
Pressure potential
- ΨP is 0 in an open beaker, positive in turgid plant cells and negative in xylem vessels where water is pulled under tension.
Percentage change
- ÷ initial × 100%.
- (final − initial)
Section 9 of 12Potato core method for tissue water potential
1
Cut equal-size potato cores, blot dry and record initial masses.
2
Place cores in a range of sucrose solutions (e.g. 0.0–1.0 M) for a fixed time at constant temperature.
3
Remove, blot dry in the same way and record final masses.
4
Calculate percentage change in mass.
5
Plot percentage change against sucrose molarity and draw a line of best fit.
6
Read the x-intercept (0% change), where the solution is isotonic with the tissue, and calculate the tissue ΨS with −iCRT.
Section 10 of 12Choosing the transport route
| Mechanism | Gradient | Protein | ATP | Example |
|---|---|---|---|---|
| Simple diffusion | Down | No | No | O2, CO2 across the bilayer |
| Facilitated diffusion | Down | Channel or carrier | No | Aquaporins, K+ channels |
| Osmosis | Down Ψ | Aquaporins or bilayer | No | Water into root hair cells |
| Primary active transport | Against | Pump | Yes, direct | Na+/K+ ATPase |
| Secondary active transport | Against for one solute | Cotransporter | Indirect | Na+–glucose symport |
| Endo/exocytosis | Bulk | Vesicle formation | Yes | Phagocytosis, insulin secretion |
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Fluid mosaic model | membrane of a fluid phospholipid bilayer with moving embedded proteins |
| Selective permeability | some substances cross more easily than others |
| Facilitated diffusion | passive transport through channel or carrier proteins |
| Active transport | movement against the gradient using ATP and carriers |
| Osmosis | net movement of water from higher to lower water potential |
| Water potential | potential energy of water per unit volume, Ψ |
| Endocytosis | bulk uptake by a membrane vesicle |
| Plasmolysis | shrinking of the protoplast from the wall in hypertonic solution |
| Osmoregulation | control of internal water and solute levels |
Section 12 of 12Exam technique
1
Confusing facilitated diffusion with active transport — both can use carriers, but only active transport goes against the gradient using ATP.
2
Describing tonicity without saying relative to what — always compare the solution with the cell contents.
3
Saying water enters by active transport — osmosis is passive.
4
Saying the cell wall stops water entering — it only prevents bursting.
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