What you need to know
- 2.1–2.5Exchange surfaces, membranes, permeability, osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane. and transport.
By the end, you should be able to…
- Retrieve key mechanisms before checking the answers.
- Apply ideas to unfamiliar numerical and experimental examples.
- Identify and correct missing steps in an explanation.
Build your understanding
Name the transported substance
osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane. refers to water, not to pigment or dissolved soluteA substance that is dissolved in a solvent.. State the water-potential gradient and partial permeability when explaining net movement.
Separate mechanism from evidence
A permeability measurement can support a membrane explanation, but it is affected by tissue preparation, exposure time and the way pigment is measured. Use controls before attributing all differences to treatment.
Make your reasoning visible
Answer the command word
Describe the pattern, explain the mechanism or evaluate the evidence as requested. A correct fact may still miss the question.
Show workings and units
Write a calculation before its result, and distinguish fractions, percentages and percentage-point changes.
Use answers diagnostically
Attempt the question first, then identify the missing step rather than only checking whether your final number matches.
Avoid this misconception
The claim: A longer answer automatically earns more credit.
The correction: Relevance, accuracy and the requested reasoning matter.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Explain one reason small non-polar molecules cross a phospholipid bilayer more readily than ions.
Show suggested answer
The bilayer has a hydrophobic interior. Non-polar molecules can dissolve in it, whereas charged ions usually require membrane proteins.
Practice question
Retrieval and explanation · suggested answer
A tissue cell has water potential −500 kPa and its surrounding solution −200 kPa. Predict initial net water movement across its partially permeable membrane.
Show suggested answer
Water moves into the cell, from the higher (less negative) potential −200 kPa to the lower potential −500 kPa.
Practice question
Retrieval and explanation · suggested answer
Compare facilitated diffusion with active transport.
Show suggested answer
Both can involve specific membrane proteins. Facilitated diffusion moves down a concentration or electrochemical gradient without direct ATP use; active transport can move against a gradient and requires energy, here supplied by ATP.
Practice question
Retrieval and explanation · suggested answer
Using rate proportional to area × concentration difference / thickness, predict the relative rate if area halves, thickness doubles and the concentration difference stays constant.
Show suggested answer
Relative rate = 0.5/2 = 0.25: one quarter of the original rate, with other factors held constant.
Practice question
Retrieval and explanation · suggested answer
Heated beetroot samples release more pigment than a control. Explain the membrane interpretation and one alternative experimental explanation to control.
Show suggested answer
Heat can disrupt membrane lipids and proteins, increasing pigment leakage through the tonoplast and cell-surface membrane. Unequal tissue size or cutting damage could also increase pigment release, so standardise pieces and rinse them first.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A gas-exchange surface doubles in area, but its diffusion distance triples. With the same concentration difference, does diffusion rate necessarily improve?
Show a hint
Calculate the proportional change before making a claim.
Explore the reasoning
Predicted rate is 2/3 of the original, so it decreases under these assumptions. More area alone does not guarantee improvement if another factor changes.
Recall this lesson
Lesson recall questions
Question 1 of 5: State Fick’s law of diffusion in words.
Gas exchange surfaces and Fick’s law
Practice question
Retrieval and explanation · suggested answer
State Fick’s law of diffusion in words.
Lesson: 2A 5: The Need for Gas Exchange Surfaces
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Joining exposure data to biology
2A · Thinking Bigger: Asthma and Air PollutionNHLBI identifies air pollution among possible asthma triggers. Environmental health research links measurements of exposure with biological outcomes. Suppose a fictional survey finds more reported wheezing near a…Specification 2.1(iii) · 1.15 · 1.16Read & use the scienceClose article
Why this matters
NHLBI identifies air pollution among possible asthma triggers. Environmental health research links measurements of exposure with biological outcomes. Suppose a fictional survey finds more reported wheezing near a busy road. The association suggests a question; it does not establish that traffic pollution caused every symptom. Researchers would need comparable exposure measurements, consistent symptom definitions and information about alternative explanations such as smoking exposure. Narrowed airways can reduce ventilation, connecting the population pattern to a plausible biological mechanism. Keeping the mechanism separate from the strength of the study is essential when interpreting environmental health headlines.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.1(iii) understand how the structure of the mammalian lung is adapted for rapid gaseous exchange
- 1.15 be able to analyse and interpret quantitative data on illness and mortality rates to determine health risks, including distinguishing between correlation and causation and recognising conflicting evidence
- 1.16 be able to evaluate the design of studies used to determine health risk factors, including sample selection and sample size used to collect data that is both valid and reliable
Use the science
A fictional survey finds more self-reported wheezing near a busy road than farther away. What measurements and comparisons would help test a pollution explanation, and why could narrowed airways affect gas exchange?
Explore a possible response
Measure pollution over comparable periods and record symptoms consistently in comparable groups. Consider smoking exposure, age and other confounders. Narrowed airways can reduce ventilation and maintenance of the oxygen gradient at exchange surfaces. A plausible mechanism supports investigation but does not remove confounding or prove causation in this survey.
Explore the source
A lung interface on a microchip
2A · The Need for Gas Exchange SurfacesResearchers built a small device with human lung and blood-vessel cells on opposite sides of a flexible membrane. They could expose one side to air and the other…Specification 2.1(i) · 2.1(ii)Read & use the scienceClose article
Why this matters
Researchers built a small device with human lung and blood-vessel cells on opposite sides of a flexible membrane. They could expose one side to air and the other to flowing liquid, and apply stretching movements. The aim was to model selected properties of the lung interface for experiments, not to make an entire replacement lung. The design makes familiar variables experimental choices: barrier thickness, movement and exposure can be controlled and measured. A bioengineer asks which features must be reproduced for the scientific question being investigated, and which missing features limit what the model can tell us.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.1(i) know the properties of gas exchange surfaces in living organisms (large surface area to volume ratio, thickness of surface and difference in concentration)
- 2.1(ii) understand how the rate of diffusion is dependent on these properties and can be calculated using Fick’s Law of Diffusion
Use the science
If a model’s exchange barrier were much thicker than the real lung barrier, what effect would you predict on diffusion, assuming other factors were held constant?
Explore a possible response
A longer diffusion distance should reduce transfer rate under Fick’s law. A model may still answer some questions, but absolute rates need careful interpretation.
Explore the source
- Reconstituting Organ-Level Lung Functions on a ChipHuh et al., Science (2010)