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2.1Topic 2 · Genes and Health

Gas Exchange, Cell Membranes and Transport — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

20 min read 31 parts Lesson 2 of 35 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Topic 2 · Genes and Health — 2.1 Gas Exchange, Cell Membranes and Transport — pages 1-7 — EDEXCEL A LEVEL BIOLOGY A (SNAB) 9BI0 — REVISION NOTES notes: Explain why large organisms need specialised exchange surfaces and use Fick's law to predict and calculate diffusion rates.All o…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • SA:V ratio
  • Diffusion distance

Section 1 of 31Why organisms exchange gases

All organisms exchange gases with their environment. Aerobic respiration requires oxygen and produces carbon dioxide as a waste product; photosynthesis requires carbon dioxide and produces oxygen as a waste product.

Section 2 of 31Surface area to volume ratio

Single-celled organisms have a high surface area to volume ratio (SA:V), which allows exchange by simple diffusion.

Surface area to volume ratio
The large surface area gives maximum absorption of nutrients and gases and removal of waste,
  • while the small volume means the diffusion distance to all organelles is short.
As organisms increase in size, SA:V decreases: there is less surface area for absorption and removal of waste in relation to the volume,
  • and therefore the requirements, of the organism.
Larger organisms therefore need specialised exchange surfaces and transport systems.

Be careful

  • the phrases surface area and surface area to volume ratio cannot be used interchangeably.
  • An elephant has a larger surface area than a bacterium, but its SA:V ratio is far smaller.

Section 3 of 31Maintaining the gradient

The alveoli have a good blood supply, which constantly removes oxygen from the capillary side of the exchange surface and brings carbon dioxide to it. The ventilation system in mammals ensures constant inhalation and exhalation, which supplies oxygen and removes carbon dioxide on the alveolar side. Together, these keep concentration gradients steep.

Section 4 of 31Fick's law of diffusion

Fick's law of diffusion
Fick's law relates the rate of diffusion to the concentration gradient, the diffusion distance and the surface area:
rate of diffusion is proportional to (surface area x concentration difference) / thickness of membrane
You will be given the equation in the exam, but you must understand how to interpret it.
Fick's law of diffusion
Worked example: rate = 0.012 x 3 x (1.05 x 10-16 / 1.5) = 0.012 x 3 x 7 x 10-17 = 2.52 x 10-18 molecules per square micrometre per second.
Doubling the surface area doubles the rate; doubling the membrane thickness halves it.

Section 5 of 31The mammalian lung

The lungs of air-breathing animals provide an ideal exchange surface for gases. They are located in the thorax, or chest cavity, protected by the rib cage.

Section 6 of 31Applying Fick's law to organisms

Fick's law explains why gas exchange organs are designed as they are: lungs have millions of alveoli for a large surface area; alveolar walls are one cell thick for a short diffusion distance; blood flow and breathing maintain the concentration gradient. Fish gills use countercurrent flow to maintain a gradient along the whole length of the lamellae.

Section 7 of 31Worked examples

  1. Explain why a bacterium does not need a ventilation system. [2]2 marks

  2. State Fick's law and what would increase diffusion rate. [3]3 marks

  3. How do blood flow and ventilation maintain concentration gradients at the alveoli? [3]3 marks

  4. A surface has area 4 units, concentration difference 5 and thickness 2. Calculate relative rate. [1]1 mark

Section 8 of 31Structure of the trachea and bronchi

The trachea is supported by C-shaped rings of cartilage.

1
The C-shape prevents friction from rubbing against the oesophagus, which lies close behind, and gives flexibility when food is swallowed.
2
A layer of mucus covering the lining traps dust and pathogens, preventing them from entering the lungs where they could cause infection.
3
Tiny hairs called cilia on the lining waft the mucus towards the top of the trachea, removing trapped particles and pathogens.
4
Bronchi (singular bronchus) have a similar structure to the trachea but thinner walls and a smaller diameter.

Section 9 of 31Alveoli

Human alveoli are adapted for efficient gas exchange: they have thin walls (one cell thick), a huge combined surface area, a rich capillary network and a moist lining. When describing alveoli, refer to the alveolar epithelium as the wall of the alveoli or alveolar wall, not as a cell wall; cell walls are found only in plants, fungi and bacteria.

Section 10 of 31Cell membranes

Membranes are vital structures found in all cells. The cell surface membrane separates the internal cell environment from the external environment, and intracellular membranes form compartments within the cell such as the nucleus, mitochondria and endoplasmic reticulum. Membranes also control the exchange of substances from one side to the other and act as an interface for communication.

Section 11 of 31Phospholipids and membrane components

A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails, joined to glycerol by ester bonds. Spread over water, phospholipids form a single layer, a monolayer, with the heads in the water and the tails sticking up away from it. In membranes they form a bilayer with tails inside and heads facing the aqueous surroundings.

The bilayer contains non-lipid components.

Phospholipids and membrane components
Proteins are involved in transport and communication.
Glycolipids (lipids with carbohydrate attached) and glycoproteins (proteins with carbohydrate attached) are on the cell surface and aid cell-to-cell communication.
They bind substances such as hormones at the cell surface,
  • and some act as cell markers or antigens for recognition: for example, ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains.

Section 12 of 31Why membranes are selective

The hydrophobic core of the bilayer allows small non-polar molecules such as oxygen and carbon dioxide to diffuse straight through, but it blocks ions and large polar molecules, which need proteins. This makes the membrane partially permeable, so the cell can control what enters and leaves. Cholesterol in animal membranes sits between the phospholipids and regulates fluidity.

The lungs also include the diaphragm and intercostal muscles, which ventilate them, but the exam focus here is on the structure of the airway lining and the alveolar wall.

Section 13 of 31Worked examples

  1. Explain the function of cilia and mucus in the trachea. [3]3 marks

  2. Why are cartilage rings C-shaped? [2]2 marks

  3. Describe the arrangement of phospholipids in a bilayer. [2]2 marks

  4. Give a role for glycoproteins in membranes. [2]2 marks

Section 14 of 31The fluid mosaic model

The distribution of proteins within the membrane gives a mosaic appearance, and the structure of a protein determines its position. In exams, refer to the membrane surrounding the cell as the cell surface membrane or plasma membrane, to distinguish it from a cell's internal membranes.

Section 15 of 31Models of membrane structure

Scientists use models to represent real-world ideas, organisms, processes and systems that cannot be easily investigated.

1
The Gorter and Grendel model of the 1920s showed that phospholipids in the membrane are arranged in a bilayer: the number of phospholipids extracted from red blood cell membranes covered double the area of the plasma membrane.
2
Their model did not explain where proteins are located or how lipid-insoluble molecules enter and leave the cell.
3
Later models added proteins.
4
You do not need to recall names or dates,
  • but you should understand that models are interpretations of data that change when scientific advances bring new discoveries.

Section 16 of 31Practical: membrane permeability using beetroot

The permeability of cell membranes is affected by several factors, such as temperature and solvents.

1
Beetroot cells contain a red pigment, betalain, that leaks out if the membranes are damaged.
2
Method: use a cork borer and scalpel to cut five equal-sized sections of beetroot; equal dimensions ensure equal surface areas and volumes,
  • which would otherwise affect the rate of pigment leakage.
3
Rinse them, place each in a tube with the same volume of water, and place the tubes in water baths at different temperatures for the same time.
4
Remove the beetroot and use a colorimeter to measure the absorbance of the water, or compare colour.

Section 17 of 31Results and explanations

1High temperature
High temperature membrane proteins denature, increasing permeability.
  • Water inside the cells expands, pressing on the membrane and damaging components.
2Very low temperature (below 0 degrees C)
Very low temperature (below 0 degrees C) ice crystals can pierce the membrane, making it highly permeable once thawed.
  • Both high and very low temperatures increase permeability.
3Alcohol
Alcohol as concentration increases, permeability increases because alcohol dissolves lipids in the cell surface membrane,
  • so the membrane loses its structure and the pigment leaks out.
4Improving reliability
Improving reliability carry out several repeats, using different parts of the beetroot, and calculate a mean.

Section 18 of 31Analysing the beetroot results

Plot absorbance (or percentage transmission) against temperature.

Analysing the beetroot results
The graph usually shows little leakage at low temperatures, a sharp rise above about 45 degrees C as proteins denature and the bilayer is disrupted,
  • and a plateau when all pigment has leaked out.
Sources of error include differences in beetroot sections, temperature changes in the water baths and the difficulty of judging colour by eye,
  • which a colorimeter reduces.

Section 19 of 31Worked examples

  1. Explain why the fluid mosaic model is called a model. [2]2 marks

  2. Why must beetroot pieces be the same size? [2]2 marks

  3. Explain the effect of high temperature on membrane permeability. [3]3 marks

  4. How would the results change with increasing ethanol concentration? [2]2 marks

Section 20 of 31Diffusion

Diffusion is the net movement of a substance from a region of its higher concentration to a region of its lower concentration.

Diffusion
It can occur across a cell membrane, but also within a cellular compartment, so a membrane is not essential.
In the example of glucose, a steep concentration gradient at the start gives way, over time, to equilibrium.
The rate depends on several factors,
  • including concentration gradient, surface area, thickness of the exchange surface, temperature and the nature of the molecule.

Section 21 of 31Facilitated diffusion

Some substances cannot diffuse directly through the phospholipid bilayer, for example large polar molecules such as glucose and amino acids, and ions such as sodium (Na+) and chloride (Cl-).

1
They cross by facilitated diffusion through protein channels or carriers.
2
Carrier proteins are highly specific,
  • allowing only one type of molecule or ion through, and change shape to move the molecule from one side of the membrane to the other.
3
Channel proteins are pores that extend through the membrane and allow charged substances such as ions to diffuse.
4
Remember: movement from high to low concentration is diffusion; if it requires a protein because the molecule is charged or polar, it is facilitated diffusion.

Section 22 of 31Active transport

Active transport is the movement of molecules and ions through a membrane from a region of lower concentration to a region of higher concentration.

Active transport
It requires energy in the form of ATP from respiration, because substances move against their concentration gradient, and needs carrier proteins.
Do not confuse carrier proteins with channel proteins: active transport requires carrier proteins (transmembrane proteins that undergo a conformational change), not channel proteins.

Section 23 of 31Bulk transport: endocytosis and exocytosis

Some molecules are too large to cross via membrane proteins.

1
In endocytosis the cell surrounds the substance with a section of cell surface membrane,
  • which engulfs it and pinches off inside the cell to form a temporary vacuole containing the substance.
2
Phagocytosis is an example of endocytosis.
3
Exocytosis is the reverse: vesicles fuse with the cell surface membrane and release their contents, allowing bulk secretion of substances from cells.
4
Both are active processes and require energy.
ProcessDirectionEnergy neededProteins
DiffusionHigh to lowNoNone
Facilitated diffusionHigh to lowNoChannel or carrier
Active transportLow to highYes (ATP)Carrier
Endocytosis / exocytosisBulk, into / out of cellYesNone (vesicles)

Section 24 of 31Worked examples

  1. Explain why glucose cannot diffuse directly through the bilayer. [2]2 marks

  2. State two differences between facilitated diffusion and active transport. [2]2 marks

  3. What is the role of ATP in active transport? [1]1 mark

  4. Describe phagocytosis in terms of the membrane. [2]2 marks

Section 25 of 31What osmosis is

Every cell is surrounded by a partially permeable cell surface membrane.

Osmosis
Water crosses this membrane by osmosis: the net movement of water molecules from a region of higher water potential (a more dilute solution, with a lower solute concentration) to a region of lower water potential (a more concentrated solution, with a higher solute concentration) through a partially permeable membrane.
In other words, water moves down its water potential gradient.

Water potential describes the tendency of water molecules to move, i.e. how freely available the water is.

What osmosis is
Pure water has the highest water potential (0 kPa); adding solute lowers it, so solutions have negative values.
Osmosis matters because it acts constantly on the cells of living organisms: cell cytoplasm is water with dissolved substances,
  • so any difference between the cytoplasm and the surrounding fluid drives water in or out.

Section 26 of 31Animal cells in different solutions

An animal cell has no cell wall, so it cannot resist large volume changes.

Animal cells in different solutions
Solution with higher water potential than the cell (dilute)
Solution with higher water potential than the cell (dilute) water enters, the cell swells and may burst (lysis).
Solution with the same water potential
Solution with the same water potential no net movement; the cell keeps its shape.
Solution with lower water potential (concentrated)
Solution with lower water potential (concentrated) water leaves, the cell shrinks and the membrane appears crinkled (crenation).

Section 27 of 31Plant cells in different solutions

A plant cell has a cellulose cell wall outside the membrane.

Turgid
In a dilute solution water enters the vacuole and cytoplasm and pushes the membrane against the wall; the wall resists,
  • so the cell becomes turgid (firm), and it does not burst.
Plasmolysed
In a concentrated solution water leaves, the vacuole and cytoplasm shrink and the membrane pulls away from the wall: the cell is plasmolysed,
  • although the cell wall maintains the overall shape of the cell.
When the cell is just at the point of starting to plasmolyse it is described as incipient plasmolysis,
  • and the water potential of the cell then equals that of the surrounding solution.
Surrounding solutionAnimal cellPlant cell
Dilute (higher water potential)Swells, may lyseTurgid
EqualNo changeFlaccid, incipient plasmolysis
Concentrated (lower water potential)Shrinks, crenatesPlasmolysed

Section 28 of 31Worked reasoning and applications

To predict the direction of osmosis, compare the water potential on each side of the membrane.

Worked reasoning and applications
Water always moves towards the more negative value.
Examples: a plant root hair cell has a lower water potential than the soil water,
  • so water enters by osmosis; a lettuce leaf placed in water becomes crisp as its cells become turgid,
  • while salt sprinkled on a slug draws water out of its cells; turgor from osmosis supports non-woody plant stems and keeps leaves flat,
    • and if the cells lose water the plant wilts.

In an investigation, plant tissue (e.g. potato cylinders) is placed in a range of sucrose concentrations.

Worked reasoning and applications
The percentage change in mass is plotted against concentration; the point where the line crosses zero shows the concentration with the same water potential as the tissue.
Cylinders must be the same size, dried in the same way, and left for the same time and at the same temperature.
Using percentage change rather than raw change in mass allows fair comparison between cylinders of different starting mass.

Section 29 of 31Worked examples

  1. Explain why a red blood cell placed in pure water bursts but a plant cell does not. [4]4 marks

  2. A potato cylinder loses mass in a concentrated sucrose solution. Explain this. [3]3 marks

Section 30 of 31Key terms

TermMeaning
SA:V ratioSurface area divided by volume
Fick's lawRate of diffusion depends on surface area, concentration gradient and distance
Diffusion distanceDistance a substance diffuses across
Concentration gradientDifference in concentration between two regions
VentilationMovement of air into and out of the lungs
ThoraxChest cavity
TermMeaning
TracheaWindpipe supported by cartilage
CiliaHair-like projections that move mucus
AlveolusGas exchange air sac
PhospholipidLipid with a hydrophilic head and hydrophobic tails
GlycoproteinProtein with carbohydrate chains
AntigenMolecule that stimulates an immune response
TermMeaning
Fluid mosaic modelModel of membrane with phospholipid bilayer and scattered proteins
PermeabilityEase with which substances pass through a membrane
DenatureChange shape irreversibly
Cell surface membraneMembrane surrounding the cell
ColorimeterInstrument measuring absorbance of a solution
ModelRepresentation of a real system
TermMeaning
DiffusionNet movement down a concentration gradient
Facilitated diffusionDiffusion through channel or carrier proteins
Active transportMovement against a gradient using ATP
Carrier proteinProtein that changes shape to move a specific substance
EndocytosisBulk uptake into a cell by membrane engulfing
ExocytosisBulk secretion out of a cell by vesicles
TermMeaning
OsmosisNet movement of water across a partially permeable membrane from higher to lower water potential.
Water potentialA measure of the tendency of water molecules to move; highest for pure water (0 kPa).
TurgidFirm plant cell full of water, with the membrane pressing on the cell wall.
PlasmolysisMembrane and cytoplasm of a plant cell pulling away from the wall after water loss.
Partially permeable membraneMembrane that lets some substances through but not others.

Section 31 of 31Exam technique

1
Treating surface area and SA:V as the same.
2
Saying diffusion needs energy.
3
Saying thick membranes speed up diffusion.
4
Forgetting that ventilation and blood supply maintain the gradient.
1
Calling the alveolar wall a cell wall.
2
Saying the phosphate heads are hydrophobic.
3
Saying the cartilage is a complete ring.
4
Saying bronchi have thick walls.
1
Saying that the model is fixed and final.
2
Saying that low temperatures always preserve membranes.
3
Calling the membrane a barrier that nothing crosses.
4
Forgetting to use repeats and a mean.
1
Saying channel proteins need ATP.
2
Saying facilitated diffusion is active.
3
Saying active transport goes down a gradient.
4
Saying diffusion needs a membrane.
Avoid these mistakes
Saying water moves from high concentration to low concentration — water moves from a region of higher water potential (lower solute concentration) to lower water potential.
Saying osmosis moves solute — only water moves; the membrane is partially permeable.
Saying plant cells burst in pure water — the cell wall prevents this and the cell becomes turgid.

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