Key Concepts in Biology

What you'll learn

This topic forms the foundation of your Edexcel GCSE Biology course, covering fundamental concepts you'll apply throughout your studies. You'll explore cell structures, enzyme function, biological transport mechanisms, and practical microscopy skills. These principles underpin every other topic in the specification.

Key terms and definitions

Eukaryotic cell — a cell containing a nucleus and membrane-bound organelles; found in animals, plants, fungi and protists

Prokaryotic cell — a cell lacking a nucleus, with genetic material in a single loop of DNA and plasmids; bacteria are prokaryotes

Enzyme — a biological catalyst that speeds up the rate of chemical reactions without being used up

Active site — the specific region on an enzyme where the substrate binds; its shape is complementary to the substrate

Diffusion — the net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient

Osmosis — the net 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 against a concentration gradient using energy from respiration

Magnification — how many times larger an image appears compared to the actual size of the object

Core concepts

Cell structure and organisation

All living organisms are made of cells. There are two fundamental cell types that you must distinguish clearly.

Animal and plant cells (eukaryotic) share these structures:

• Nucleus — contains genetic material (DNA) arranged in chromosomes; controls cell activities
• Cytoplasm — gel-like substance where most chemical reactions occur
• Cell membrane — controls what enters and leaves the cell
• Mitochondria — site of aerobic respiration, releasing energy
• Ribosomes — site of protein synthesis

Plant cells additionally contain:

• Cell wall — made of cellulose, provides structural support
• Permanent vacuole — contains cell sap, helps maintain cell rigidity
• Chloroplasts — contain chlorophyll for photosynthesis (only in green parts)

Bacterial cells (prokaryotic) are much smaller and structurally simpler:

• No true nucleus — genetic material is a single DNA loop in the cytoplasm
• Plasmids — small rings of extra DNA
• Cell wall — different composition to plant cell walls
• May have a flagellum for movement
• No mitochondria or chloroplasts

The level of organisation in multicellular organisms follows this hierarchy: Cell → Tissue → Organ → Organ system → Organism

A tissue is a group of similar cells working together (e.g. muscle tissue). An organ is a collection of different tissues working together (e.g. the stomach). An organ system is a group of organs working together (e.g. the digestive system).

Microscopy and cell observation

Understanding how to use microscopes is essential both for practical work and exam questions.

Light microscopes:

• Use light and glass lenses to magnify specimens
• Maximum magnification approximately ×2000
• Can observe cells, nuclei and chloroplasts but not smaller organelles
• Relatively cheap and portable

Electron microscopes:

• Use electrons instead of light
• Much higher magnification (over ×500,000) and resolution
• Can observe mitochondria, ribosomes and plasmids
• Very expensive and require specialist facilities

Key calculations you must master:

Magnification = image size ÷ actual size

You can rearrange this triangle method:

• Image size = magnification × actual size
• Actual size = image size ÷ magnification

Unit conversions are critical:

• 1 millimetre (mm) = 1000 micrometres (μm)
• 1 micrometre (μm) = 1000 nanometres (nm)
• Therefore: 1 mm = 1,000,000 nm

When measuring cells under a microscope, ensure all measurements are in the same units before calculating.

Enzyme structure and function

Enzymes are biological catalysts made of protein. They have a specific three-dimensional shape determined by the sequence of amino acids.

The lock and key model explains enzyme specificity:

• The substrate fits into the active site like a key into a lock
• Only substrates with the correct complementary shape can bind
• An enzyme-substrate complex forms temporarily
• The reaction occurs and products are released
• The enzyme remains unchanged and can be reused

Factors affecting enzyme activity:

Temperature:

• At low temperatures, molecules move slowly with less kinetic energy
• Increasing temperature increases collision rate between enzymes and substrates
• Rate of reaction increases up to the optimum temperature
• Beyond the optimum, the enzyme denatures — the active site changes shape permanently
• The substrate can no longer bind, and the enzyme cannot function

pH:

• Each enzyme has an optimum pH
• Extremes of pH denature enzymes by altering the active site shape
• Pepsin (stomach) works best at pH 2; amylase (mouth) at pH 7

Substrate concentration:

• More substrate molecules increase collision frequency with enzymes
• Rate of reaction increases until all active sites are occupied
• Beyond this saturation point, rate plateaus

Diffusion and gas exchange

Diffusion is passive — it requires no energy from respiration. Particles move randomly, but net movement is down the concentration gradient until equilibrium is reached.

Factors affecting diffusion rate:

• Concentration gradient — steeper gradients increase rate
• Temperature — higher temperatures give particles more kinetic energy
• Surface area — larger areas allow more diffusion simultaneously
• Distance — shorter diffusion distances increase rate

Examples in living organisms:

• Oxygen diffuses from alveoli into blood capillaries
• Carbon dioxide diffuses from respiring cells into the blood
• Urea diffuses from liver cells into blood plasma
• Glucose and amino acids diffuse from the small intestine into blood

Surface area to volume ratio is crucial for understanding organism size limitations:

• Smaller organisms have a larger surface area compared to their volume
• Single-celled organisms can rely on diffusion alone for gas exchange
• Larger organisms need specialised exchange surfaces and transport systems
• Adaptations for efficient exchange include: large surface area, thin walls, good blood supply

Osmosis and water balance

Osmosis is a special case of diffusion involving only water molecules moving through a partially permeable membrane.

In animal cells:

• If placed in pure water (higher water potential), water enters by osmosis
• The cell swells and may burst (lysis)
• In concentrated salt solution (lower water potential), water leaves
• The cell shrinks and becomes crenated

In plant cells:

• In pure water, water enters the cell by osmosis
• The vacuole swells, pushing cytoplasm against the cell wall
• The cell becomes turgid (firm) — the cell wall prevents bursting
• In concentrated solution, water leaves by osmosis
• The cytoplasm pulls away from the cell wall — the cell becomes plasmolysed

Turgidity is essential for plant support in non-woody tissue. This is why plants wilt when short of water.

Active transport

Unlike diffusion and osmosis, active transport moves substances against their concentration gradient — from low to high concentration.

Key features:

• Requires energy from respiration (ATP)
• Uses carrier proteins in cell membranes
• Can achieve very high concentration differences

Examples:

• Mineral ions absorbed from dilute solutions in soil into concentrated root hair cells
• Glucose reabsorbed from kidney tubules into blood
• Movement of glucose from intestine into blood when concentrations are equal

Active transport is essential when organisms need to accumulate substances in higher concentrations than their surroundings.

Cell differentiation and specialisation

Differentiation is the process by which cells become specialised for specific functions. In animals, most cells differentiate early in development then lose this ability. Plant cells can differentiate throughout life.

Examples of specialised cells:

Sperm cells:

• Long tail for swimming to egg
• Mitochondria provide energy for movement
• Acrosome contains enzymes to digest egg membrane
• Haploid nucleus contains genetic information

Egg cells:

• Large cytoplasm contains nutrients for developing embryo
• Haploid nucleus
• Cell membrane changes after fertilisation to prevent more sperm entering

Ciliated epithelial cells:

• Line airways
• Cilia beat to move mucus and trapped particles out of lungs

Root hair cells:

• Long extension increases surface area for water and mineral absorption
• Thin wall for short diffusion distance
• Many mitochondria for active transport of minerals

Worked examples

Example 1: Magnification calculation

Question: A student observes a plant cell under a microscope. The image of the cell measures 24 mm. The actual length of the cell is 60 μm. Calculate the magnification. [3 marks]

Solution: Step 1: Convert units to the same scale 24 mm = 24,000 μm [1 mark]

Step 2: Use the magnification formula Magnification = image size ÷ actual size [1 mark]

Step 3: Calculate Magnification = 24,000 ÷ 60 = ×400 [1 mark]

Mark scheme notes: You must show unit conversion, correct formula and final answer. Always include the × symbol with magnification.

Example 2: Explaining enzyme denaturation

Question: Explain why an enzyme stops working when heated above 50°C. [3 marks]

Model answer:

• High temperature causes the enzyme to denature [1 mark]
• The active site changes shape [1 mark]
• The substrate can no longer fit into the active site / enzyme-substrate complex cannot form [1 mark]

Mark scheme notes: Simply stating "the enzyme denatures" earns only 1 mark. You must explain the consequence for the active site and substrate binding.

Example 3: Comparing diffusion and active transport

Question: Root hair cells absorb mineral ions from the soil. The concentration of mineral ions is higher inside the root hair cell than in the soil.

(a) Name the process by which mineral ions are absorbed. [1 mark] (b) Explain why this process requires mitochondria. [2 marks]

Model answer: (a) Active transport [1 mark]

(b) Active transport requires energy / ATP [1 mark], which is released by respiration in mitochondria [1 mark]

Common mistakes and how to avoid them

• Confusing diffusion, osmosis and active transport — Remember: diffusion involves any particles down a gradient; osmosis involves only water through a membrane; active transport goes against the gradient and needs energy

• Saying enzymes "die" when denatured — Enzymes are not alive. Use the term "denature" and explain the change to the active site shape

• Forgetting to convert units in magnification calculations — Always convert to the same unit (usually micrometres) before dividing or multiplying

• Thinking plant cells burst in pure water — The cell wall prevents this. Animal cells can burst, but plant cells become turgid

• Confusing "concentration of solution" with "concentration of water" — A concentrated sugar solution has low water potential (less water molecules). Students often get this backwards

• Not using precise terminology for cell structures — "The green bits" won't earn marks; use "chloroplasts". "The jelly" should be "cytoplasm"

Exam technique for "Key Concepts in Biology"

• Command words matter: "State" needs a simple fact; "Explain" requires reasoning with because/therefore links; "Calculate" needs working shown

• Magnification questions: Always show: unit conversion, formula, calculation. Even if your final answer is wrong, you can earn method marks (usually 2 out of 3)

• Describing enzyme action: Include these key terms for full marks: active site, complementary, enzyme-substrate complex, products. Draw on the lock and key model

• Comparing cells: Make clear comparisons — "Animal cells do not have chloroplasts whereas plant cells do" scores better than listing features separately

Quick revision summary

Cells are the basic units of life, categorised as prokaryotic (bacteria) or eukaryotic (animal, plant). Enzymes are protein catalysts with specific active sites that denature at extreme temperatures or pH. Diffusion moves particles down concentration gradients passively; osmosis is water movement through membranes; active transport requires energy to move substances against gradients. Use magnification = image size ÷ actual size with correct unit conversions. Specialised cells have adaptations for specific functions through differentiation.