Biology lessons
A level & IB Biology: cells, biological molecules, exchange, genetics, energy, and exam skills.
Key topics
- Biological molecules
- Cells & microscopy
- Exchange & transport
- Genetics & variation
- Energy: photosynthesis & respiration
- Practical & exam skills
Lesson course
Work through each lesson, run the interactive practice, and tick it off as you go.
1. Biological molecules
Core
Carbohydrates & lipids
Describe monomers, polymers, and bonding in carbs and lipids.
Monosaccharides (e.g. glucose) join by condensation forming glycosidic bonds and water. Lipids are non-polymers: triglycerides = glycerol + 3 fatty acids via ester bonds.
Draw and label one condensation reaction forming a glycosidic bond.
Proteins & enzymes
Relate protein structure to enzyme function.
Proteins fold from amino acids (peptide bonds) into specific 3D shapes. Enzymes lower activation energy; the active site is complementary to the substrate (induced fit).
Explain one factor affecting enzyme rate using collision theory.
Water & inorganic ions
Explain water's properties and roles of key ions.
Water is polar, a good solvent, has high specific heat, and cohesion (for transport). Ions like Fe²⁺ (haemoglobin) and PO₄³⁻ (DNA/ATP) have specific roles.
List four properties of water and link each to a biological role.
2. Cells
Core
Cell structure & microscopy
Compare eukaryotic/prokaryotic cells and calculate magnification.
Eukaryotes have a nucleus and membrane-bound organelles; prokaryotes don't. Magnification = image size ÷ actual size; watch units (1 mm = 1000 µm).
Use magnification = image/actual on two worked examples.
Cell membranes & transport
Explain diffusion, osmosis, and active transport.
The membrane is a fluid mosaic. Diffusion and osmosis are passive (down a gradient); active transport uses ATP and carrier proteins to move against the gradient.
Design one osmosis experiment with a clear variable.
Cell division
Describe mitosis and its role in growth and repair.
Mitosis produces two genetically identical diploid cells for growth/repair: prophase, metaphase, anaphase, telophase, then cytokinesis.
Order the stages of mitosis and state what happens in each.
3. Exchange & transport
Core
Surface area & gas exchange
Link SA:V ratio to exchange surfaces.
As size increases, surface area to volume ratio falls, so diffusion alone is too slow — hence lungs, gills, and circulatory systems with large, thin, moist surfaces.
Explain why large organisms need specialised exchange surfaces.
The heart & circulation
Describe the cardiac cycle and double circulation.
Mammals have a double circulation: pulmonary (heart–lungs) and systemic (heart–body). The cardiac cycle is diastole (fill) then systole (contract).
Label a heart diagram and trace blood through both circuits.
Transport in plants
Explain transpiration and translocation.
Water moves up the xylem by transpiration pull (cohesion-tension). Sugars move in phloem by translocation (source to sink).
Explain how four factors change transpiration rate.
4. Genetics & variation
Core
DNA, RNA & protein synthesis
Describe transcription and translation.
DNA is transcribed to mRNA (transcription); ribosomes read codons to build a polypeptide (translation). Each codon = 3 bases coding one amino acid.
Transcribe and translate one short DNA sequence.
Inheritance
Use genetic diagrams for monohybrid crosses.
Alleles are versions of a gene; dominant alleles mask recessive. A monohybrid cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotype ratio.
Complete two Punnett squares and state phenotype ratios.
Variation & evolution
Explain natural selection and types of variation.
Variation arises from mutation and meiosis. Natural selection: individuals best suited to the environment survive and reproduce, passing on advantageous alleles.
Explain one example of natural selection step by step.
5. Energy
Core
Photosynthesis
Describe the light-dependent and light-independent reactions.
Light-dependent reactions (thylakoids) make ATP and reduced NADP using light. The Calvin cycle (stroma) fixes CO₂ into glucose using those products.
Summarise where each stage occurs and its products.
Respiration
Outline glycolysis, the Krebs cycle, and oxidative phosphorylation.
Glycolysis (cytoplasm) → link & Krebs (mitochondrial matrix) → oxidative phosphorylation (inner membrane). Aerobic respiration yields far more ATP than anaerobic.
State the net ATP yield and where each stage occurs.
Energy & ecosystems
Explain energy transfer and ecological efficiency.
Energy enters via producers and is lost (respiration, heat, waste) at each trophic level — typically only ~10% passes on, limiting food-chain length.
Calculate efficiency between two trophic levels.
6. Practical & exam skills
Skills
Experimental design
Identify variables and improve validity and reliability.
A valid experiment changes one independent variable, measures the dependent, and controls the rest. Repeats improve reliability; large samples reduce anomalies.
Critique one method for control variables and repeats.
Data & statistics
Process data and choose a statistical test.
Pick the test by data type: correlation (Spearman), difference (t-test), association (chi-squared). Always state the conclusion against the hypothesis.
Plot data correctly and state which test fits one scenario.
Extended answers
Structure 6-mark questions with linked points.
Read the command word (describe/explain/evaluate), make distinct linked points, use correct terminology, and answer exactly what's asked.
Plan and write one 6-mark answer using the command word.
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