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Enzymes
Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up. Each enzyme has an active site with a shape complementary to its substrate (lock and key), forming an enzyme–substrate complex. Activity rises with temperature up to an optimum (about 37 °C for human enzymes) as molecules collide more often; above it the enzyme denatures: the active site changes shape and the substrate no longer fits. Each enzyme also has an optimum pH.
Enzyme Action and Kinetics
Enzymes are globular proteins that lower activation energy by forming enzyme–substrate complexes at a specific active site. Induced fit: the active site changes shape slightly as the substrate binds. Rate rises with temperature (more collisions) up to an optimum, then falls as bonds break and the enzyme denatures. Away from the optimum pH, ionic and hydrogen bonds change, altering the active site. Rate increases with substrate concentration until all active sites are occupied (Vmax). Km is the substrate concentration at ½Vmax: a low Km means high affinity. Competitive inhibitors bind the active site (overcome by more substrate; Vmax unchanged, Km increases). Non-competitive inhibitors bind elsewhere, changing the active site (Vmax decreases). Immobilised enzymes can be reused and give product free of enzyme.
Microbial Growth and Forensics
Microorganisms are cultured on agar or in broth using aseptic technique. A growth curve has a lag phase (enzymes being made), log (exponential) phase (doubling each generation), stationary phase (births = deaths; nutrients limited, waste builds up) and death phase. . Counting: serial dilution then plate count (colonies × dilution factor ÷ volume plated); haemocytometer; turbidity (colorimeter). Forensics: time of death is estimated from body temperature (falls ~1.5 °C per hour to surroundings), rigor mortis (starts 2–6 h, lasts ~36 h, as ATP runs out), decomposition by bacteria and enzymes, and forensic entomology (blowfly larvae stages), plus a predictable succession of insects on the body. DNA profiling identifies victims and suspects.
Genetic Modification
Genetic modification changes an organism's DNA by inserting a gene from another organism. To make human insulin: the gene is cut out with restriction enzymes, leaving sticky ends; a bacterial plasmid is cut with the same enzyme; DNA ligase joins the gene into the plasmid (recombinant plasmid); the plasmid is put into bacteria, which multiply in fermenters and make insulin. GM crops can be pest-resistant, herbicide-resistant or have added nutrients (Golden Rice, vitamin A). Concerns include effects on wild populations and food safety.
Recombinant DNA and PCR
To make recombinant DNA: obtain the gene by restriction enzymes (cut at specific palindromic sites, leaving sticky ends), by reverse transcriptase (mRNA → cDNA, without introns) or by synthesis. Cut a plasmid vector with the same restriction enzyme, so sticky ends are complementary; DNA ligase joins them. Add a promoter so the gene is transcribed. Transform host cells (heat shock with Ca²⁺, electroporation). Identify transformed cells with marker genes (antibiotic resistance, GFP fluorescence). PCR copies DNA: denature at 95 °C, anneal primers at ~55 °C, extend at 72 °C with Taq polymerase; each cycle doubles the DNA (). Gel electrophoresis separates DNA fragments by size: DNA (negative phosphates) moves to the anode; smaller fragments travel further.
Practical Skills in Biology
A good investigation changes one independent variable, measures the dependent variable, and keeps other variables controlled (e.g. temperature with a water bath). A control experiment (e.g. boiled enzyme) shows the effect is caused by the factor tested. Repeats improve reliability. The CORMS plan: Change, Organisms (same species, age, size), Repeat, Measure (what and how), Same (control variables). Biological drawings use clear single lines, no shading, labelled with ruled lines. Evaluate by identifying anomalies and suggesting specific improvements.
Eukaryotic and Prokaryotic Cell Structure
Eukaryotic cells have a nucleus (double envelope with pores, chromatin, nucleolus that makes ribosomes) and membrane-bound organelles: rough ER (ribosomes; makes and transports proteins), smooth ER (lipids, steroids), Golgi body (modifies, packages proteins into vesicles), mitochondria (aerobic respiration, ATP; folded cristae), lysosomes (digestive enzymes), 80S ribosomes, centrioles and microtubules. Plant cells also have a cellulose cell wall, chloroplasts and a large vacuole. Prokaryotic cells are smaller, with no nucleus (circular DNA, plasmids), 70S ribosomes, a peptidoglycan (murein) wall and sometimes a capsule and flagellum. Viruses are non-living: nucleic acid in a protein capsid, sometimes an envelope.
Digestion and Absorption
Digestion breaks large insoluble molecules into small soluble ones that can be absorbed. Physical digestion (teeth chewing, stomach churning) increases surface area. Chemical digestion uses enzymes: amylase (mouth, pancreas) turns starch into maltose; protease (pepsin in stomach, trypsin from pancreas) turns proteins into amino acids; lipase (pancreas) turns fats into fatty acids and glycerol. Bile from the liver neutralises stomach acid and emulsifies fats. Products are absorbed in the small intestine, whose villi give a huge surface area; water is absorbed mainly in the small intestine and colon.
Infectious Diseases and Antibiotics
Cholera (bacterium Vibrio cholerae): contaminated water; toxin causes severe diarrhoea; controlled by clean water, sanitation, oral rehydration. Malaria (Plasmodium, a protoctist): spread by female Anopheles mosquitoes; controlled by nets, insecticides, drugs, draining water. Tuberculosis (Mycobacterium tuberculosis): airborne droplets; BCG vaccine, long antibiotic courses, contact tracing. Measles (virus): droplets; MMR vaccine. HIV/AIDS (retrovirus): body fluids; destroys T-helper cells; antiretroviral drugs, condoms, screening. Antibiotics target features of bacteria absent in human cells; penicillin inhibits enzymes building peptidoglycan cross-links, so cells burst by osmosis. Antibiotics don't affect viruses. Resistance arises by random mutation and spreads by natural selection and plasmid transfer, worsened by overuse and unfinished courses.
Blood Glucose Control
Blood glucose is kept near 5 mmol dm⁻³ by the islets of Langerhans in the pancreas. High glucose: β cells release insulin, which binds receptors on liver, muscle and fat cells; more GLUT4 transporters move to membranes; glucose is converted to glycogen (glycogenesis) and fat, and respiration increases. Low glucose: α cells release glucagon, which binds liver receptors; via the second messenger cyclic AMP and an enzyme cascade (protein kinase), glycogen is broken down (glycogenolysis) and glucose is made from amino acids and glycerol (gluconeogenesis). Adrenaline also raises glucose via cAMP. Type 1 diabetes: no insulin (autoimmune), treated with insulin injections. Type 2: cells less responsive to insulin, linked to obesity; treated with diet, exercise and drugs. Glucose is measured with test strips/biosensors using glucose oxidase.
Syllabus topics
| Topic | Board | Paper | Lesson |
|---|---|---|---|
| Enzyme action | Cambridge 5090 | Multiple Choice | Available |
| Enzyme action | Cambridge 5090 | Theory | Available |
| Enzyme action | Cambridge 0610 | Extended | Available |
| Enzyme action | Cambridge 0610 | Core | Available |
| Enzyme action | Edexcel 4BI1 | Paper 1 (core content) | Available |
| Enzyme action | Edexcel 4BI1 | Paper 2 (all content) | Available |
| Enzyme action and kinetics | Cambridge 9700 | AS Level Structured Questions | Available |
| Enzyme action and kinetics | Cambridge 9700 | Multiple Choice | Available |
| Enzyme action and kinetics | Edexcel YBI11 | Unit 1: Molecules, Diet, Transport and Health | Available |
| Enzyme action and kinetics | AQA 7402 | Paper 1: Topics 1–4 | Available |
| Enzymes | Cambridge 5090 | Multiple Choice | Planned |
| Enzymes | Cambridge 5090 | Theory | Planned |
| Enzymes | Cambridge 0610 | Extended | Planned |
| Enzymes | Cambridge 0610 | Core | Planned |
| Enzymes | Cambridge 9700 | AS Level Structured Questions | Planned |
| Enzymes | Cambridge 9700 | Multiple Choice | Planned |
Questions
| ID | Question | Topic | Marks | Difficulty |
|---|---|---|---|---|
| SA-OB-BT-012 | Biofuel ethanol is made by… | Biology: microorganisms in biotechnology Uses of yeast and bacteria; enzymes in industry | 1 | L1 Foundation |
| SA-AB-EK-005 | A reaction reaches its end point in 40 s. Calculate the rate (s⁻¹).… | A Level Biology: enzymes Effects of temperature, pH, enzyme and substrate concentration | 1 | L1 Foundation |
| SA-OB-GM-002 | The enzyme that joins DNA fragments is… | Biology: genetic modification Steps in genetically modifying bacteria | 1 | L1 Foundation |
| SA-OB-BT-002 | Yoghurt is made using… | Biology: microorganisms in biotechnology Uses of yeast and bacteria; enzymes in industry | 1 | L1 Foundation |
| SA-AB-GT-001 | Restriction enzymes… | A Level Biology: recombinant DNA, PCR and electrophoresis Restriction enzymes, ligase, vectors, reverse transcriptase | 1 | L1 Foundation |
| SA-AB-EK-001 | Enzymes speed up reactions by… | A Level Biology: enzymes Lock-and-key and induced fit; activation energy | 1 | L1 Foundation |
| SA-OB-DG-001 | Amylase breaks down… | Biology: digestion and absorption Digestive enzymes, bile and their sites | 1 | L1 Foundation |
| SA-OB-DG-002 | The products of fat digestion are… | Biology: digestion and absorption Digestive enzymes, bile and their sites | 1 | L1 Foundation |
| SA-OB-EN-001 | Enzymes are made of… | Biology: enzymes Lock and key model and specificity | 1 | L1 Foundation |
| SA-OB-BT-001 | Bread rises because yeast produces… | Biology: microorganisms in biotechnology Uses of yeast and bacteria; enzymes in industry | 1 | L1 Foundation |
| SA-OB-EN-002 | The part of the enzyme where the substrate binds is the… | Biology: enzymes Lock and key model and specificity | 1 | L1 Foundation |
| SA-AB-DR-012 | Semi-conservative replication means each new DNA molecule… | A Level Biology: DNA structure and replication Semi-conservative replication and its enzymes | 1 | L1 Foundation |
| SA-AB-CA-001 | The enzyme that fixes CO₂ in the Calvin cycle is… | A Level Biology: Calvin cycle and limiting factors Carbon fixation, reduction and regeneration of RuBP | 1 | L1 Foundation |
| SA-OB-EN-012 | A catalyst… | Biology: enzymes Lock and key model and specificity | 1 | L1 Foundation |
| SA-OB-EN-008 | An enzyme produces 30\ cm^3 of oxygen in 2 minutes. Calculate the mean rate in cm^3 per minute.… | Biology: enzymes Calculating rates from enzyme experiments | 2 | L2 Standard |