Movement of substances into and out of cells
BiologyΒ· 2.15β2.17 (Issue 3 spec)Β· 25 min read
1. Core Cell Transport Processes (2.15)β β ββββ± 8 min
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Diffusion
Net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient), no energy required (passive process)
Osmosis
Net movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) across a partially permeable membrane, passive process
Active Transport
Movement of substances against a concentration gradient (from lower to higher concentration), requires energy released from respiration, uses carrier proteins in the cell membrane
A student defines osmosis as 'the diffusion of water from a dilute solution to a concentrated solution'. State one missing element of this definition required for full marks.
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Step 1: Recall Edexcel's mandatory requirements for osmosis definitions.
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Step 2: Identify that the definition mentions water movement and concentration direction, but omits the critical partially permeable membrane requirement.
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Answer: The definition does not state that osmosis occurs across a partially permeable membrane.
2. Factors Affecting Transport Rate (2.16)β β β βββ± 7 min
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Four key factors affect how quickly substances move into or out of cells, all assessed regularly in exam questions:
Concentration gradient steepness: The steeper the difference in concentration between two regions, the faster the rate of diffusion and osmosis.
Temperature: Higher temperatures give particles more kinetic energy, so they move faster, increasing transport rate.
Diffusion pathway distance: The shorter the distance particles have to travel, the faster the rate of transport.
Surface area to volume ratio (SA:V): Smaller cells/organisms have a larger surface area relative to their volume, so exchange of substances happens faster. Larger organisms require specialized exchange surfaces to compensate for their small SA:V.
Explain why a mouse has a faster rate of gas exchange per gram of body mass than an elephant.
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Step 1: Link organism size to surface area to volume ratio.
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Step 2: A mouse is much smaller than an elephant, so it has a far higher surface area to volume ratio.
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Step 3: A higher SA:V means more surface area is available for gas exchange per gram of body mass, leading to faster exchange rates.
3. Required Practical: Diffusion and Osmosis (2.17)β β β βββ± 7 min
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You will be assessed on your understanding of both living (e.g. potato tissue) and non-living (e.g. agar, Visking tubing) practical investigations of diffusion and osmosis.
For the common potato osmosis practical: potato cores are cut to equal size, blotted dry, weighed, then placed in solutions of varying sugar/salt concentration for a fixed time, then re-blotting and re-weighing to calculate percentage change in mass.
Mass/length increase: Net movement of water into potato cells, so the external solution is more dilute than potato cell sap. Cells become turgid.
Mass/length decrease: Net movement of water out of potato cells, so the external solution is more concentrated than potato cell sap. Cells become flaccid or plasmolysed.
No change in mass/length: The external solution has the same concentration as the potato cell sap, so there is no net movement of water.
A potato core placed in a 0.5M sucrose solution loses 8% of its mass. Explain this result.
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Step 1: Link mass loss to water movement via osmosis.
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Step 2: The 0.5M sucrose solution is more concentrated than the potato cell sap, so water moves out of the potato cells across their partially permeable cell membranes.
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Step 3: The net loss of water from the potato cells causes the core to decrease in mass.
4. Comparing Passive and Active Transportβ β ββββ± 3 min
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Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
Energy required? | No (passive) | No (passive) | Yes (from respiration) |
Gradient direction | Down (highβlow) | Down (highβlow water) | Against (lowβhigh) |
Partially permeable membrane required? | No | Yes | No (uses carrier proteins) |
Substances transported | Small soluble particles (Oβ, glucose) | Water only | Ions, large molecules (minerals) |
Which process requires energy from respiration to move substances against a concentration gradient?
Diffusion
Osmosis
Active transport
Reveal answer
2 βActive transport is the only active process listed, using energy from respiration to move substances up a concentration gradient.
5. Common Pitfalls
Wrong move:
Defining osmosis as 'diffusion of water' without mentioning a partially permeable membrane
Why:
Edexcel marking schemes explicitly require reference to the membrane for full marks, this is a common discriminator question
Correct move:
Always include 'across a partially permeable membrane' in any osmosis definition
Wrong move:
Stating active transport uses energy from the concentration gradient
Why:
Active transport moves against the gradient, so it cannot use gradient energy
Correct move:
Explicitly state active transport uses energy released from aerobic respiration
Wrong move:
Mixing up water movement direction in osmosis practicals
Why:
Many students mix up dilute/concentrated solutions and mass change, losing marks in practical interpretation questions
Correct move:
Remember water moves to the more concentrated solution: if external solution is more concentrated than cells, water leaves and mass decreases
Wrong move:
Stating larger organisms have a larger surface area to volume ratio
Why:
SA:V decreases as organism size increases, a common multiple choice misconception
Correct move:
Smaller cells/organisms have higher SA:V, larger ones have lower SA:V
Wrong move:
Using absolute mass change instead of percentage change for practical comparisons
Why:
Starting masses of potato cores are never identical, so absolute change does not allow fair comparison
Correct move:
Calculate percentage change in mass using % change = ((final mass - initial mass)/initial mass) Γ 100
6. Quick Reference Cheatsheet
Term | Exam-Ready Key Definition |
|---|---|
Diffusion | Net movement of particles down concentration gradient, passive, no energy |
Osmosis | Net movement of water down water concentration gradient, across partially permeable membrane, passive |
Active Transport | Movement against concentration gradient, uses energy from respiration |
Rate Factors | SA:V, temperature, concentration gradient, diffusion distance |
Practical Rule | Mass β = water in (external dilute), mass β = water out (external concentrated) |
7. Frequently Asked
Can I call osmosis 'diffusion of water' in exams?
No. Edexcel marking schemes require you to explicitly mention that osmosis occurs across a partially permeable membrane alongside the net movement of water down a water concentration gradient to get full marks.
What energy source does active transport use?
Active transport uses energy released from aerobic respiration in cells, not energy from the concentration gradient like passive transport processes.
Going deeper
- syllabusEdexcel IGCSE Biology 4BI1 SpecificationRefer to section 2.15β2.17 for official guidance
- practical guideEdexcel IGCSE Biology Practical HandbookContains full method for the osmosis potato core practical
What's Next
Now that you have mastered core cell transport processes, you can apply this foundational knowledge to specialized exchange surfaces and transport systems across living organisms, a frequent topic in Edexcel IGCSE Biology exam papers. You will encounter diffusion in gas exchange systems in animals (alveoli) and plants (leaf stomata), osmosis in root hair cell water uptake, and active transport in mineral ion absorption in plant roots and nutrient uptake in the small intestine villi. Be sure to practice past paper questions on this topic to familiarize yourself with strict marking scheme requirements, especially for definition and practical interpretation questions, which are common high-mark questions across both Paper 1 and Paper 2.