Key Concepts in Biology — Edexcel GCSE Biology Revision Notes

What you'll learn

This topic forms the foundation of Edexcel GCSE Biology, introducing the fundamental principles that underpin all living organisms. You'll study cell structures, how substances move across membranes, enzyme function, and the use of microscopes to observe biological specimens. These concepts appear throughout Paper 1 and Paper 2, often combined with other topics in multi-step questions worth 4-6 marks.

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 free in the cytoplasm, characteristic of bacteria.

Diffusion — the net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient.

Osmosis — the diffusion of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

Active transport — the movement of substances against a concentration gradient, from lower to higher concentration, requiring energy from respiration.

Enzyme — a biological catalyst that speeds up the rate of chemical reactions without being used up, made of protein with a specific active site.

Substrate — the molecule that binds to an enzyme's active site and is converted into products.

Resolution — the ability to distinguish between two separate points, determining the level of detail visible through a microscope.

Core concepts

Animal and plant cell structures

All eukaryotic cells share common structures that you must be able to identify and describe:

Structures in both animal and plant cells:

Nucleus — contains genetic material (DNA) arranged in chromosomes, controls cellular activities
Cytoplasm — gel-like substance where most chemical reactions occur, contains enzymes
Cell membrane — controls what enters and exits the cell, partially permeable
Mitochondria — site of aerobic respiration, releasing energy from glucose
Ribosomes — site of protein synthesis, where amino acids are assembled

Additional structures in plant cells only:

Cell wall — made of cellulose, provides structural support and prevents the cell from bursting
Permanent vacuole — contains cell sap (water with dissolved sugars and salts), maintains internal pressure
Chloroplasts — contain chlorophyll for photosynthesis, absorb light energy to make glucose

Exam questions frequently ask you to identify these structures from diagrams or explain their functions. A 4-mark question might require you to describe differences between animal and plant cells, expecting you to name structures and explain their functions.

Bacterial cells and specialised cells

Prokaryotic cells (bacteria) differ significantly from eukaryotic cells:

Much smaller (typically 1-10 μm compared to 10-100 μm for eukaryotic cells)
Genetic material consists of a single loop of DNA plus small rings called plasmids
No nucleus — DNA floats free in the cytoplasm
Cell wall made of peptidoglycan (not cellulose)
Some bacteria have a flagellum for movement

Specialised cells are adapted for specific functions:

Sperm cell — streamlined head, long tail for swimming, many mitochondria for energy, acrosome with digestive enzymes to penetrate egg
Egg cell — large food reserves in cytoplasm, cell membrane changes after fertilisation to prevent entry of more sperm
Ciliated epithelial cell — tiny hair-like cilia beat to move mucus and trapped particles up the throat
Root hair cell — large surface area for water absorption, many mitochondria for active transport of mineral ions
Xylem cell — dead cells forming continuous tubes, lignin strengthening in cell walls for water transport
Phloem cell — sieve plates between cells allow dissolved sugars to move, companion cells provide energy

Using microscopes

Microscopy skills are tested through both practical work and written examination questions.

Light microscope use:

Place slide on stage and select lowest power objective lens
Use coarse focus to move stage close to lens
Look through eyepiece and adjust coarse focus to move stage away until roughly focused
Adjust fine focus to sharpen image
To see greater detail, switch to higher power objective lens and refocus

Magnification calculations appear regularly in exams:

Magnification = Image size ÷ Actual size

You must be able to rearrange this equation and convert between units (millimetres to micrometres: ×1000).

Electron microscopes have much higher magnification and resolution than light microscopes:

Light microscopes: maximum magnification ×2000, resolution ~200 nm
Electron microscopes: magnification up to ×2,000,000, resolution ~0.2 nm
Electron microscopes allow scientists to see internal structures of mitochondria and chloroplasts, ribosomes, and plasmids in bacteria

Enzymes and the lock and key model

Enzymes are crucial for controlling metabolism. Each enzyme has a specific active site with a complementary shape to its substrate — this is the lock and key model.

How enzymes work:

Substrate molecule has a shape complementary to the enzyme's active site
Substrate binds to active site forming an enzyme-substrate complex
Enzyme catalyses the reaction, converting substrate to products
Products are released and enzyme is unchanged, ready to bind another substrate molecule

Factors affecting enzyme activity:

Temperature:

Increasing temperature increases enzyme activity initially
Higher temperature means more kinetic energy, more frequent successful collisions between enzyme and substrate
Above the optimum temperature (often around 37°C for human enzymes), enzyme activity decreases rapidly
High temperatures break bonds holding the enzyme's shape, changing the active site shape
The enzyme is denatured — this is permanent and irreversible

pH:

Each enzyme has an optimum pH where activity is highest
pH values above or below the optimum decrease enzyme activity
Extreme pH values denature the enzyme by breaking bonds in the protein structure
Different enzymes have different optimum pH values (pepsin in stomach: pH 2; amylase in mouth: pH 7)

Edexcel exams commonly present enzyme data as graphs, requiring you to identify optimum conditions, describe trends, and explain changes in activity using the denaturation concept.

Movement across membranes

Diffusion is a passive process requiring no energy input. The rate of diffusion is affected by:

Concentration gradient — larger difference increases rate
Temperature — higher temperature increases rate
Surface area — larger area increases rate

Examples of diffusion in living organisms:

Oxygen diffusing from alveoli into blood capillaries
Carbon dioxide diffusing from respiring cells into blood
Urea diffusing from liver cells into blood for excretion

Osmosis is a special case of diffusion involving water movement:

Water moves from dilute solution (high water concentration) to concentrated solution (low water concentration)
Movement occurs across a partially permeable membrane that allows water but not large solute molecules to pass
In animal cells, too much water entering causes cell to burst; too much water leaving causes cell to shrink
Plant cells have a cell wall preventing bursting — cells become turgid when full of water, plasmolysed when they lose water

Active transport moves substances against the concentration gradient:

Requires energy from respiration (supplied by mitochondria)
Uses carrier proteins in the cell membrane
Examples include root hair cells absorbing mineral ions from dilute soil solution, and glucose absorption from the small intestine when gut concentration is lower than blood concentration

Exam questions often ask you to distinguish between these three processes, particularly comparing diffusion and active transport or osmosis and diffusion.

Practical investigations

Edexcel GCSE Biology assesses practical skills worth 15% of your qualification. Key concepts investigations include:

Investigating osmosis using potato cylinders:

Cut identical potato cylinders and measure initial mass
Place in different concentrations of sugar/salt solution
After 30 minutes, remove, blot dry and measure final mass
Calculate percentage change in mass
Higher concentration solution causes water loss (negative % change); lower concentration causes water gain (positive % change)

Investigating enzyme activity:

Amylase breakdown of starch tested using iodine solution (blue-black colour disappears when starch is fully digested)
Can investigate effect of temperature or pH on reaction rate
Control variables: enzyme concentration, substrate concentration, volume

Worked examples

Example 1: A student observes a cell using a light microscope. The actual length of the cell is 0.05 mm. The length of the image is 25 mm. Calculate the magnification. (2 marks)

Answer: Magnification = Image size ÷ Actual size Magnification = 25 ÷ 0.05 (1 mark) Magnification = ×500 (1 mark)

Example 2: Explain why increasing temperature from 20°C to 40°C initially increases enzyme activity but further increases to 60°C cause enzyme activity to decrease. (4 marks)

Answer: At higher temperatures, molecules have more kinetic energy (1 mark). This leads to more frequent collisions between enzyme and substrate molecules (1 mark). Above the optimum temperature, bonds holding the enzyme structure together break (1 mark). This changes the shape of the active site so the substrate no longer fits / the enzyme is denatured (1 mark).

Example 3: A student investigated osmosis by placing potato cylinders in different concentrations of salt solution. Explain why the potato cylinder in pure water increased in mass. (3 marks)

Answer: Water moved into the potato cells by osmosis (1 mark). Water moved from a region of higher water concentration (pure water) to a region of lower water concentration (inside potato cells) (1 mark). Water moved across a partially permeable membrane / cell membrane (1 mark).

Common mistakes and how to avoid them

Mistake: Saying prokaryotic cells have no DNA or genetic material. Correction: Prokaryotic cells do contain DNA, but it is not enclosed in a nucleus — instead it forms a single loop in the cytoplasm, plus additional DNA in plasmids.
Mistake: Writing that enzymes are "killed" or "die" at high temperatures. Correction: Enzymes are not alive. Use the term "denatured" to describe the permanent change to the active site shape caused by broken bonds in the protein structure.
Mistake: Confusing diffusion and osmosis definitions, or saying osmosis requires energy. Correction: Osmosis is specifically the diffusion of water molecules across a partially permeable membrane. Both diffusion and osmosis are passive processes requiring no energy input from respiration.
Mistake: Forgetting to convert units in magnification calculations. Correction: Ensure both image size and actual size use the same units before calculating. Remember: 1 mm = 1000 μm. Check the question carefully for required units in your answer.
Mistake: Stating that active transport moves substances "with" or "up" the concentration gradient. Correction: Active transport moves substances against the concentration gradient, from a region of lower concentration to higher concentration.
Mistake: Describing mitochondria as "making" or "producing" energy. Correction: Energy cannot be created or destroyed. Mitochondria are the site of aerobic respiration, which releases energy from glucose through chemical reactions.

Exam technique for Key Concepts in Biology

"Explain" questions require you to give reasons or mechanisms. For enzyme questions, link structure (active site shape) to function. For membrane transport, always state the direction of movement and reference concentration. Expect 2-4 marks with one mark per valid point.
Calculation questions on magnification always show your working. Even if your final answer is incorrect, you can gain marks for correct method. Include units in your final answer. Typical marks: 1 for rearrangement/substitution, 1 for correct answer with unit.
Practical questions may ask you to identify variables (independent, dependent, control), describe methods, or evaluate procedures. Use precise language: "use a water bath to maintain temperature at 30°C" not "keep it warm". Know the difference between accuracy, precision and repeatability.
Command words matter: "State" needs a brief answer with no explanation. "Describe" requires you to write about what happens. "Explain" needs reasons with scientific detail, often using "because", "therefore" or "this causes". Higher mark questions (4-6 marks) usually require extended explanations linking multiple concepts.

Quick revision summary

Eukaryotic cells contain a nucleus and organelles including mitochondria and ribosomes; plant cells additionally have chloroplasts, cell wall and permanent vacuole. Prokaryotic bacterial cells lack a nucleus and are much smaller. Diffusion moves particles down a concentration gradient; osmosis is water diffusion across a membrane; active transport moves substances against the gradient using energy. Enzymes are biological catalysts with specific active sites complementary to their substrate. Temperature and pH affect enzyme activity; extreme conditions cause denaturation. Light microscopes magnify up to ×2000; electron microscopes achieve much higher magnification and resolution.