1.3Unit 1: Lifestyle, Transport, Genes and Health
SNAB Gas Exchange, Cell Membranes & Transport — IAL Edexcel Linear Revision Notes & Topical Questions
10 min read 11 parts Lesson 4 of 29 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
1.3 Gas Exchange, Cell Membranes and Transport — IAL EDEXCEL LINEAR notes: Explain how gas exchange surfaces, the lung and the cell membrane are adapted for exchange, and compare diffusion, facilitated diffusion, active transport and osmosis.Gas exchange — it occurs by diffusion …
- Surface area to volume ratio (SA:V)
- Adaptations to increase area
- Short diffusion pathway
- Steep concentration gradient
- Fick's law
- Doubling the rate
Part 1 of 9Gas exchange surfaces and surface area to volume ratio
1
Gas exchange
- Gas exchange it occurs by diffusion across an exchange surface, supplying oxygen for respiration and carbon dioxide for photosynthesis.
2
Surface area to volume ratio (SA:V)
Surface area to volume ratio (SA:V) surface area exposed to the environment compared with the internal volume.
- Volume increases faster than surface area as size increases, so the SA:V ratio decreases in larger organisms.
- Single-celled organisms have a high SA:V ratio and a short diffusion distance, so simple diffusion is enough.
- Large organisms have less surface per unit volume and longer diffusion distances to cells and tissues.
3
Adaptations to increase area
- Adaptations to increase area alveoli in lungs, lamellae in fish gills and spongy mesophyll in leaves.
- Internal diffusion distance is a separate problem solved by a mass transport system.
4
Short diffusion pathway
- Short diffusion pathway the exchange surface is often a single layer of flattened epithelial cells.
5
Steep concentration gradient
- Steep concentration gradient it is maintained by removing substances from one side and supplying them on the other.
- A good blood supply removes oxygen from the capillary side of the alveoli.
- Ventilation supplies oxygen and removes carbon dioxide on the alveolar side.
Part 2 of 9Fick's law of diffusion
Fick's law
- Fick's law rate of diffusion is proportional to ÷ thickness of the exchange surface.
- (surface area × concentration difference)
Equation
- Equation rate = P × A × ((C1 − C2) ÷ T).
- P is the permeability constant for that molecule and membrane.
- A is surface area, C1 − C2 is the concentration difference and T is the thickness of the exchange surface.
Doubling the rate
- Doubling the rate the rate doubles if surface area or concentration difference doubles, or if the diffusion pathway halves.
Part 3 of 9The mammalian lung
- Role
- lungs in the thorax maximise gas exchange while minimising water loss.
| Structure | Features and function |
|---|---|
| Trachea | C-shaped cartilage rings keep it open and allow flexibility when swallowing; mucus traps dust and pathogens and cilia waft it upwards |
| Bronchi | Similar to the trachea but thinner walls, smaller diameter and complete cartilage rings |
| Bronchioles | Narrow, thin-walled, self-supporting tubes; larger ones have smooth muscle and elastic fibres to adjust airflow, the smallest have elastic fibres only |
| Alveoli | Groups at the ends of bronchioles with a wall of a single layer of flattened squamous epithelium |
- Alveolar adaptations
- they combine a large surface area, thin permeable walls and a short diffusion distance.
- Elastic fibres around the alveoli let them stretch in inhalation and recoil.
- An extensive capillary network keeps a steep gradient as carbon dioxide enters and oxygen leaves the alveoli.
- A moist lining lets gases dissolve, so exchange occurs in solution.
Part 4 of 9Membrane structure
Membrane structure
Roles of membranes
- Roles of membranes they separate the cell from its surroundings, form compartments such as the nucleus and mitochondria, control exchange of substances and carry receptors and antigens.
Phospholipid
Phospholipid glycerol + phosphate head + two fatty acid tails.
- The phosphate head is polar and hydrophilic.
- The two tails are non-polar and hydrophobic.
- On water they form a monolayer, and in cells they form a bilayer with heads outwards and tails inwards.
Intrinsic (integral) proteins
- Intrinsic (integral) proteins they are embedded in the bilayer, arranged by their hydrophilic and hydrophobic regions.
Extrinsic (peripheral) proteins
- Extrinsic (peripheral) proteins they lie on the outer or inner surface of the membrane.
Cholesterol
- Cholesterol it sits between phospholipids and regulates fluidity.
- It stops tails packing too closely at low temperatures, so the membrane does not become rigid.
- It binds the tails at high temperatures, so the membrane is not too fluid.
- It increases mechanical strength and stability.
Glycoproteins and glycolipids
- Glycoproteins and glycolipids carbohydrate chains on the surface act in cell communication, binding hormones and acting as antigens such as the ABO blood group antigens.
Fluid mosaic
- Fluid mosaic phospholipids move sideways in their layer and proteins scattered through the bilayer give a mosaic pattern.
- Small non-polar molecules pass between the phospholipids.
- Large polar molecules and ions need channel or carrier proteins.
Part 5 of 9Evolution of membrane models
| Model | Proposal | Evidence | Problem |
|---|---|---|---|
| Gorter and Grendel (1920s) | Phospholipids form a bilayer | Extracted phospholipid area from red cells was double the membrane area | Did not explain proteins or movement of lipid-insoluble molecules |
| Davson and Danielli (1930s) | Protein layers above and below the bilayer | Membranes control transport; electron micrographs showed two dark lines with a lighter band | Freeze-etching showed globular structures in the centre; proteins were globular, varied in size and partly hydrophobic |
| Singer and Nicolson (1972) | Fluid mosaic with peripheral and integral globular proteins | Proteins extend into the membrane centre and are free to move | Currently best explains structure and function |
- Models
- they are interpretations of data that change when new discoveries do not fit.
Part 6 of 9Practical: membrane permeability with beetroot
- Principle
- beetroot cells contain the red pigment betalain, and more leaks out when membrane permeability is higher.
- A colorimeter measures absorbance of the coloured liquid, and a higher absorbance means more pigment.
- The colorimeter is zeroed with distilled water in a cuvette and a colour filter selects the correct wavelength.
1
Cut five equal-sized beetroot pieces with a cork borer and scalpel so surface area and volume are equal.
2
Rinse the pieces to remove pigment released by cutting.
3
Place each piece in a test tube with the same volume of water (for example 5 cm³).
4
Put the tubes in water baths at different temperatures (for example 10, 20, 30, 40 and 50 °C) for the same time, about 30 minutes.
5
Remove the beetroot and use a fresh pipette to transfer each liquid sample to a cuvette.
6
Measure absorbance of each sample with the colorimeter.
Result
- Result permeability increases with temperature.
- Phospholipids gain kinetic energy and pack less tightly.
- Membrane proteins denature at high temperatures.
- Expanding water damages the membrane, and ice crystals below 0 °C pierce it.
Limitations
- Limitations cuvettes of different thickness, pieces of different size and uneven pigment distribution.
- Use the same cuvette, cut pieces accurately, and repeat with different parts of the beetroot to find a mean.
Variation
- Variation increasing alcohol concentration increases permeability because alcohol dissolves membrane lipids.
Part 7 of 9Diffusion and facilitated diffusion
Diffusion
Diffusion net movement of a substance from a region of higher concentration to lower concentration down a concentration gradient.
- It is passive and uses the kinetic energy of molecules, continuing until equilibrium.
- Simple diffusion across the bilayer suits small, non-polar molecules.
Factors affecting rate
- Factors affecting rate concentration gradient, temperature, surface area and the properties of the substance.
Facilitated diffusion
Facilitated diffusion diffusion of large polar molecules (glucose, amino acids) and ions through transport proteins.
- Channel proteins are pores through the membrane, most of which are gated and open or close to control ion movement.
- Carrier proteins switch between two shapes, so the binding site opens first to one side and then the other.
- Both types are specific, and net movement is down the concentration gradient.
Part 8 of 9Active transport and bulk transport
1Active transport
Active transport movement of molecules or ions across a membrane from low to high concentration using ATP.
- It needs specific carrier proteins, and ATP hydrolysis provides energy for the shape change.
- Examples are kidney tubule reabsorption, absorption of digestion products, phloem loading and uptake of ions by root hair cells.
2Endocytosis
- Endocytosis the membrane engulfs large molecules and pinches off a vacuole.
- (proteins, lipids, some carbohydrates)
- Phagocytosis is endocytosis of a pathogen or particle.
3Exocytosis
- Exocytosis vesicles from the Golgi fuse with the cell surface membrane to secrete hormones, enzymes and lipids in bulk.
4Both bulk processes
- Both bulk processes they are active and need energy.
Part 9 of 9Osmosis
Osmosis
Osmosis net movement of water molecules from a region of higher water potential (dilute solution) to lower water potential (concentrated solution) through a partially permeable membrane.
- It is a specialised diffusion of water down its concentration gradient.
- Dissolved solutes bind water molecules, so fewer free water molecules are available.
- Water passes through the bilayer because its molecules are small, even though they are polar.
In pure water
- In pure water water enters the cell, which swells.
- An animal cell can burst, but a plant cell wall prevents bursting.
In concentrated solution
- In concentrated solution water leaves the cell, which shrinks.
- An animal cell shrivels, whereas in a plant cell the vacuole and cytoplasm shrink while the wall keeps the shape.
Key terms
| Term | Meaning |
|---|---|
| Fick's law | Rate of diffusion is proportional to surface area × concentration difference ÷ thickness. |
| Phospholipid | Lipid with a hydrophilic phosphate head and two hydrophobic fatty acid tails. |
| Fluid mosaic model | Membrane model of a mobile phospholipid bilayer with scattered proteins. |
| Facilitated diffusion | Diffusion across a membrane through channel or carrier proteins. |
| Active transport | Movement against a concentration gradient using carrier proteins and ATP. |
| Osmosis | Net movement of water across a partially permeable membrane from dilute to concentrated solution. |
| Endocytosis | Bulk uptake of material by engulfing it in the cell surface membrane. |
| Exocytosis | Bulk secretion of material by vesicles fusing with the cell surface membrane. |
Avoid these mistakes
1
Using surface area and SA:V ratio interchangeably — larger organisms have a larger surface area but a smaller ratio.
2
Calling the alveolar wall a cell wall — cell walls occur only in plants, so say alveolar wall or epithelium.
3
Confusing channel and carrier proteins in active transport — active transport needs carrier proteins that change shape using ATP.
4
Describing osmosis as movement of solute — it is movement of water.
Marked practiceQuizzes for this lesson
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.