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This lesson provides a comprehensive review of all key concepts covered in this course as required by the Edexcel GCSE Combined Science specification (1SC0). Use this lesson to consolidate your understanding, check key definitions, review important equations and avoid common exam mistakes.
Make sure you can define each of the following terms precisely. In the exam, definitions are often worth 1–2 marks each.
| Term | Definition |
|---|---|
| Eukaryotic cell | A cell that has a true nucleus and membrane-bound organelles (e.g. animal, plant, fungal cells) |
| Prokaryotic cell | A cell that does not have a true nucleus; DNA is free in the cytoplasm (e.g. bacteria) |
| Organelle | A specialised structure within a cell that has a specific function |
| Enzyme | A biological catalyst (protein) that speeds up a chemical reaction without being used up |
| Active site | The region of an enzyme where the substrate binds |
| Denaturation | When an enzyme's active site changes shape (due to high temperature or extreme pH) so the substrate can no longer fit |
| Diffusion | The net movement of particles from an area of higher concentration to an area of lower concentration |
| Osmosis | The movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane |
| Active transport | The movement of substances from a region of lower concentration to a region of higher concentration (against the concentration gradient), requiring energy from respiration |
| Turgid | A plant cell that is firm and swollen because it has absorbed water by osmosis |
| Plasmolysis | When the cell membrane pulls away from the cell wall in a plant cell that has lost water by osmosis |
Exam Tip: Examiners mark definitions precisely. For osmosis, you must include all four elements: (1) movement of water, (2) from higher water potential to lower water potential, (3) through a partially permeable membrane, (4) it is passive. Missing any one element can lose you a mark.
You must be able to use and rearrange these equations:
Magnification=Actual sizeImage size
Rearrangements:
Percentage change=Initial massFinal mass−Initial mass×100
| Conversion | Multiply or Divide |
|---|---|
| mm → μm | × 1000 |
| μm → nm | × 1000 |
| nm → μm | ÷ 1000 |
| μm → mm | ÷ 1000 |
A cell has an actual diameter of 40 μm. Under a microscope, its image measures 16 mm. Calculate the magnification.
Step 1: Convert to the same units. 16 mm = 16,000 μm
Step 2: Magnification = 16,000 ÷ 40 = × 400
A potato cylinder has an initial mass of 3.20 g. After 24 hours in a sucrose solution, its mass is 2.88 g. Calculate the percentage change.
Percentage change = (2.88 − 3.20) ÷ 3.20 × 100 = −0.32 ÷ 3.20 × 100 = −10.0%
The negative value indicates the potato lost mass — water moved out of the cells by osmosis because the sucrose solution had a lower water potential.
Exam Tip: Always show your working in calculation questions — even if you get the final answer wrong, you can still pick up method marks. Include units in your final answer.
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Direction | High → Low concentration | High → Low water potential | Low → High concentration |
| Energy required | No | No | Yes (ATP from respiration) |
| Membrane required | Not always | Yes (partially permeable) | Yes (carrier proteins) |
| What moves | Any particle (gas or dissolved substance) | Water only | Specific substances |
| Examples | O₂ into blood, CO₂ out of cells | Water into root hair cells | Mineral ions into roots, glucose in gut |
Here are the most frequent errors students make in this topic. Learn them so you do not repeat them.
| Mistake | Correct Statement |
|---|---|
| "Mitochondria produce energy" | Mitochondria are the site of aerobic respiration where energy is transferred from glucose |
| "Prokaryotic cells have no DNA" | Prokaryotic cells have DNA — it is a single circular loop free in the cytoplasm (not in a nucleus) |
| "Enzymes are killed by heat" | Enzymes are denatured — their active site changes shape. Enzymes are not alive. |
| "Plant cells have a cell wall instead of a cell membrane" | Plant cells have both a cell wall (outer) and a cell membrane |
| "Osmosis is the movement of water from dilute to concentrated" | Osmosis is the movement of water from higher water potential to lower water potential through a partially permeable membrane |
| "Diffusion requires energy" | Diffusion is passive — it does not require energy from respiration |
| "Active transport goes from high to low concentration" | Active transport moves substances from low to high concentration (against the gradient) |
Exam Tip: Avoid vague language. Instead of "stuff moves", say exactly what substance moves, from where to where, and by which process. Precision wins marks.
Example: Define osmosis.
Model answer: Osmosis is the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Example: Describe how you would investigate the effect of sucrose concentration on potato mass.
Structure your answer: Aim → Method (step by step) → Variables → How to analyse results.
Example: Explain why a potato cylinder placed in pure water increased in mass.
Model answer: Pure water has a higher water potential than the cell sap inside the potato cells. Water moves into the potato cells by osmosis through the partially permeable cell membrane. The cells absorb water and swell, causing the mass to increase.
Always:
Exam Tip: For extended answer (6-mark) questions, plan your answer before writing. Include correct scientific terminology, a logical sequence, and a conclusion. Check spelling of key terms like "denaturation", "osmosis" and "partially permeable".
Use this checklist to confirm you understand every key concept:
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