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Higher Level Revision Notes
Leaving Cert Higher Level Biology
Chapter 5: Cell Diversity & Membrane Transport
Cell diversity + membrane transport
Subtopics Covered
- Learning Outcomes
- Big Picture: Why Cell Diversity and Transport Matter
- Cell Diversity and Specialisation
- Stem Cells
- Prokaryotic vs Eukaryotic Cells
- Plasma Membrane Structure
What This Pack Includes
- Structured chapter notes formatted for ExamsLogic website reading
- Exam-focused diagrams, definitions, and worked examples
- Interactive practice sections carried over from the source notes
- Independent study guidance based on the official curriculum
- Print-friendly layout for future PDF export when needed
This publication is an independent educational resource developed by ExamsLogic and compiled by experienced educators. It is based on publicly available official curricula, including Cambridge, Pearson Edexcel, IB, and the Irish Leaving Certificate. This product is not endorsed by, affiliated with, or sponsored by any examination board or governing authority. All registered trademarks remain the property of their respective owners.
1. Learning Outcomes
- Describe cell diversity and explain why cells become specialised.
- Compare prokaryotic and eukaryotic cells.
- Relate specialised cell structures to their functions.
- Describe the plasma membrane as a selectively permeable boundary.
- Explain diffusion, osmosis and active transport.
- Compare the effects of hypotonic, hypertonic and isotonic solutions on plant and animal cells.
- Apply transport concepts to unfamiliar biological examples.
HL focus: Always link structure to function, and always include concentration gradient / energy / selectively permeable membrane when explaining transport.
2. Big Picture: Why Cell Diversity and Transport Matter
Division of labour
Multicellular organisms contain many specialised cells, each adapted to perform a particular job efficiently.
Exchange
Cells must take in useful substances and remove wastes through the plasma membrane.
Homeostasis
Membrane transport helps maintain suitable internal conditions for enzyme activity and survival.
Biology link: Transport across membranes explains many later topics, including absorption in the small intestine, gas exchange in lungs, kidney function, plant water movement and nerve impulses.
3. Cell Diversity and Specialisation
Cell diversity means that cells in a multicellular organism are not all the same. They differ in shape, size, organelles and function.
Cell specialisation means a cell has structural features that allow it to perform a specific function efficiently.
| Specialised cell | Main function | Key adaptation | Structure-function link |
|---|---|---|---|
| Red blood cell | Oxygen transport | Biconcave shape, no nucleus, haemoglobin | More surface area and more space for haemoglobin. |
| Nerve cell | Transmit impulses | Long axon and branched endings | Allows rapid communication over long distances. |
| Root hair cell | Absorb water and minerals | Long hair-like extension | Large surface area for absorption. |
| Sperm cell | Fertilisation | Flagellum, many mitochondria, acrosome | Movement, energy supply and enzyme release. |
| Palisade mesophyll cell | Photosynthesis | Many chloroplasts | Maximises light absorption for photosynthesis. |
Exam Tip: For specialisation questions, do not only name the adaptation. Explain how the adaptation helps the cell perform its function.
4. Stem Cells
Stem cells are unspecialised cells that can divide and develop into specialised cell types.
| Term | Meaning | Exam detail |
|---|---|---|
| Unspecialised | Not yet adapted for one specific function | They can become different types of cells. |
| Differentiation | Process where a cell becomes specialised | Genes are switched on/off to produce a particular structure and function. |
| Potential medical use | Repair damaged tissues | Examples include blood disorders, spinal injury research and replacing damaged cells. |
Examiner trap: A specialised cell usually cannot become any cell type. Stem cells are important because they retain the ability to differentiate.
5. Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No true nucleus | True nucleus present |
| DNA | Circular DNA, free in cytoplasm | Linear chromosomes inside nucleus |
| Membrane-bound organelles | Absent | Present |
| Size | Usually smaller | Usually larger |
| Examples | Bacteria | Animal, plant and fungal cells |
Common mistake: Do not say prokaryotes have no DNA. They do have DNA, but it is not enclosed in a nucleus.
6. Plasma Membrane Structure
The plasma membrane controls movement of substances into and out of the cell. It is selectively permeable, meaning it allows some substances through more easily than others.
| Component | Role | HL wording |
|---|---|---|
| Phospholipid bilayer | Main membrane structure | Hydrophilic heads face water; hydrophobic tails point inward. |
| Proteins | Transport, receptors, enzymes | Channel and carrier proteins help specific substances cross. |
| Cholesterol | Stability | Helps maintain membrane fluidity. |
| Carbohydrate chains | Recognition | Involved in cell communication and identification. |
Exam Tip: In membrane answers, use the phrase selectively permeable, not simply "thin wall" or "lets things pass".
7. Diffusion
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient.
High concentration→Down gradient→Low concentration→No energy needed
| Example | Substance diffusing | Direction |
|---|---|---|
| Gas exchange in lungs | Oxygen | Alveoli to blood |
| Gas exchange in tissues | Carbon dioxide | Cells to blood |
| Leaf gas exchange | Carbon dioxide | Air spaces to photosynthesising cells |
Examiner trap: Diffusion does not require energy from respiration. If energy is needed, the process is likely active transport.
8. Osmosis
Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a selectively permeable membrane.
| Solution type | Meaning | Animal cell effect | Plant cell effect |
|---|---|---|---|
| Hypotonic | More dilute outside the cell | Water enters; cell may burst | Water enters; cell becomes turgid |
| Hypertonic | More concentrated outside the cell | Water leaves; cell shrinks | Water leaves; cell becomes plasmolysed |
| Isotonic | Equal water concentration | No net movement; normal | No net movement; flaccid compared with turgid |
Common mistake: Do not write "salt moves by osmosis." Osmosis is movement of water only.
9. Active Transport
Active transport is the movement of substances from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy from respiration.
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| Substance moved | Molecules / ions | Water only | Molecules / ions |
| Gradient | High to low | High water concentration to low water concentration | Low to high |
| Energy needed? | No | No | Yes |
| Membrane needed? | Not always | Yes, selectively permeable | Yes, with carrier proteins |
HL detail: Active transport is essential when cells need to accumulate minerals, glucose or ions even when the concentration inside the cell is already high.
Example: Root hair cells use active transport to absorb mineral ions from soil when the concentration of ions is lower in the soil than inside the root cell.
10. Factors Affecting Rate of Transport
| Factor | Effect | Reason |
|---|---|---|
| Concentration gradient | Steeper gradient increases diffusion/osmosis rate | Greater difference between two regions. |
| Temperature | Higher temperature usually increases diffusion rate | Particles have more kinetic energy. |
| Surface area | Larger surface area increases exchange | More space for particles to cross. |
| Distance / thickness | Shorter distance increases rate | Particles travel a shorter path. |
| Number of transport proteins | More proteins can increase facilitated diffusion / active transport | More pathways through the membrane. |
| Respiration rate | Affects active transport | More ATP is available. |
11. Interactive Simulator: Membrane Transport Detective
Choose the Transport Type
Clue: Oxygen moves from alveoli into the blood down a concentration gradient.
Feedback
Ready
Decide whether the clue involves water, energy, or movement down a gradient.
12. Interactive Simulator: Osmosis Outcome
Predict the Cell Response
Scenario: Animal cell placed in a hypotonic solution.
Feedback
Ready
Remember: water moves from dilute solution to concentrated solution through a selectively permeable membrane.
13. High-Value Exam Guidance
Exam Tip 1: In diffusion answers, include higher to lower concentration and down a concentration gradient.
Exam Tip 2: In osmosis answers, include water molecules and selectively permeable membrane.
Exam Tip 3: In active transport answers, include against the concentration gradient and energy from respiration.
Examiner Trap 1: Osmosis is not the movement of solute. It is the movement of water.
Examiner Trap 2: Arguing that all movement through membranes needs energy is wrong. Diffusion and osmosis are passive.
Examiner Trap 3: Plant cells become turgid in hypotonic solution because the cell wall prevents bursting.
Common Mistake 1: Saying "particles want to move". Use scientific wording: particles move randomly from high to low concentration.
Common Mistake 2: Forgetting to mention the membrane in osmosis definitions.
Examiner Secret 1: Many marks come from correct comparison language: diffusion = down gradient; active transport = against gradient.
Examiner Secret 2: Osmosis questions often test plant versus animal cells, not only the definition.
14. MCQs with Instant Answers
1. Which process requires energy from respiration?
Answer: C. Active transport moves substances against a concentration gradient using energy.
2. Osmosis is the movement of:
Answer: B. Osmosis is water movement through a selectively permeable membrane.
3. A plant cell in a hypotonic solution becomes:
Answer: B. Water enters and the plant cell becomes turgid.
4. Prokaryotic cells lack:
Answer: C. Prokaryotic cells have DNA but not enclosed in a true nucleus.
5. Which adaptation helps a root hair cell absorb water and minerals?
Answer: B. The root hair extension increases surface area for absorption.
15. Structured Questions
- Define diffusion and give one biological example. [4]
- Define osmosis and explain what happens to an animal cell in a hypotonic solution. [5]
- Compare diffusion, osmosis and active transport. [9]
- Explain how a root hair cell is adapted for absorption. [4]
- Compare prokaryotic and eukaryotic cells. [6]
- Explain why the plasma membrane is described as selectively permeable. [3]
16. Mark Scheme
Q1. Movement of particles [1] from high concentration to low concentration [1] down a concentration gradient [1]. Valid example such as oxygen from alveoli to blood [1].
Q2. Movement of water molecules [1] from high water concentration to low water concentration [1] through a selectively permeable membrane [1]. In hypotonic solution water enters animal cell [1]; cell swells and may burst/lyse [1].
Q3. Diffusion: particles high to low, no energy [3]. Osmosis: water only, through selectively permeable membrane, high water to low water concentration [3]. Active transport: substances low to high, against gradient, energy from respiration [3].
Q4. Long extension/large surface area [1] increases absorption [1]; thin cell surface/short diffusion pathway [1]; many mitochondria provide energy for active transport of mineral ions [1].
Q5. Prokaryotes have no true nucleus [1], circular DNA free in cytoplasm [1], no membrane-bound organelles [1]. Eukaryotes have true nucleus [1], linear chromosomes [1], membrane-bound organelles [1].
Q6. Membrane allows some substances through [1] but restricts others [1], depending on size/charge/lipid solubility or presence of transport proteins [1].
Q2. Movement of water molecules [1] from high water concentration to low water concentration [1] through a selectively permeable membrane [1]. In hypotonic solution water enters animal cell [1]; cell swells and may burst/lyse [1].
Q3. Diffusion: particles high to low, no energy [3]. Osmosis: water only, through selectively permeable membrane, high water to low water concentration [3]. Active transport: substances low to high, against gradient, energy from respiration [3].
Q4. Long extension/large surface area [1] increases absorption [1]; thin cell surface/short diffusion pathway [1]; many mitochondria provide energy for active transport of mineral ions [1].
Q5. Prokaryotes have no true nucleus [1], circular DNA free in cytoplasm [1], no membrane-bound organelles [1]. Eukaryotes have true nucleus [1], linear chromosomes [1], membrane-bound organelles [1].
Q6. Membrane allows some substances through [1] but restricts others [1], depending on size/charge/lipid solubility or presence of transport proteins [1].
17. Mastery Checklist
- I can explain cell diversity and specialisation.
- I can link specialised cell structures to functions.
- I can compare prokaryotic and eukaryotic cells.
- I can describe the plasma membrane using the fluid mosaic model.
- I can define diffusion accurately.
- I can define osmosis accurately.
- I can explain active transport using energy and concentration gradient language.
- I can predict plant and animal cell responses in different solutions.
- I can answer comparison questions using clear biological terminology.