2.1.5Unit 2: Cells, Development, Biodiversity and Conservation
Diffusion, Facilitated Diffusion and Active Transport — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 12 of 26 Unit 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.
Part 1 of 5Simple and Facilitated Diffusion
Passive processes move substances down a gradient without the cell spending ATP.
Simple diffusion
- Simple diffusion Net movement of molecules down a concentration gradient directly through the phospholipid bilayer, needing no energy from the cell.
Small non-polar molecules
- Small non-polar molecules Oxygen, carbon dioxide and lipid-soluble substances dissolve in the hydrophobic core of the bilayer and diffuse across easily.
Facilitated diffusion
- Facilitated diffusion Passive movement of polar molecules and ions down their gradient through specific channel or carrier proteins in the membrane.
Channel proteins
- Channel proteins These form water-filled pores that allow particular ions to pass, and some gates open only in response to a signal.
Carrier proteins
- Carrier proteins These bind a specific molecule, change shape, and release it on the other side, so a limited number can become saturated.
No ATP needed
- No ATP needed Both forms are passive because the energy comes from the kinetic energy of the particles moving down the gradient.
Part 2 of 5Active Transport
Active Transport
Against the gradient
- Against the gradient Active transport moves molecules or ions from a region of lower concentration to a region of higher concentration.
Energy from ATP
- Energy from ATP The carrier protein is driven by ATP hydrolysis, so cells with high transport activity contain many mitochondria.
Specific carriers
- Specific carriers Each carrier protein binds only its own substance, and the rate levels off once all carriers are working at once.
Sensitive to respiration
- Sensitive to respiration Respiratory inhibitors such as cyanide stop ATP production, and active transport stops while diffusion carries on.
| Feature | Simple diffusion | Facilitated diffusion | Active transport |
|---|---|---|---|
| Direction | Down gradient | Down gradient | Against gradient |
| Energy (ATP) | No | No | Yes |
| Protein needed | No | Yes, channel or carrier | Yes, carrier |
| Rate can saturate | No | Yes | Yes |
Part 3 of 5Factors Affecting Diffusion Rate
1
A steeper concentration gradient increases the rate, because more particles move from the high side than return from the low side.
2
A larger surface area increases the rate, which is why exchange surfaces such as villi and alveoli are folded or highly divided.
3
A thinner exchange surface shortens the diffusion distance and so increases the rate of diffusion.
4
Higher temperature increases particle kinetic energy, and diffusion becomes faster although cell proteins may be damaged if it gets too hot.
5
These factors combine in Fick's law: rate of diffusion is proportional to surface area × concentration difference ÷ diffusion distance.
Part 4 of 5Worked Example
Question
- An exchange surface has its surface area doubled and its thickness halved, with the concentration difference unchanged.
- By what factor does the rate of diffusion change according to Fick's law?
- Rate ∝ surface area × concentration difference ÷ thickness.
- Doubling surface area multiplies the rate by 2.
- Halving the thickness multiplies the rate by 1 ÷ 0.5 = 2.
- Combined effect = 2 × 2 = 4.
Answer The rate of diffusion increases by a factor of 4.
Part 5 of 5Exam Checklist & Tips
2.1.5 – Key Points for the Exam
- Diffusion Net movement of particles from higher concentration to lower concentration, due to their random motion.
- Facilitated diffusion Passive transport of ions or polar molecules through channel or carrier proteins.
- Active transport Movement of substances against a concentration gradient using ATP and carrier proteins.
- Concentration gradient The difference in concentration between two regions.
- Fick's law Rate of diffusion is proportional to surface area × concentration difference ÷ thickness.
Common Mistakes
- Stating that diffusion stops at equilibrium, when particles actually keep moving but there is no net movement.
- Saying facilitated diffusion uses energy; it is passive because it only goes down the gradient.
- Forgetting to say that active transport needs ATP from respiration and carrier proteins.
- Describing carriers as pores; channels are pores and carriers change shape.
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