🧬 A-Level Biology
Biology Complete Cheatsheet
Cells, genetics, body systems, ecology and evolution — complete biology revision.
📖 10 sections
⏱ 28 min read
✅ Quizzes included
🌙 Dark mode
01 Cell Structure ▼
Cell Types
Prokaryotic
No nucleus, no membrane-bound organelles. Bacteria.
Eukaryotic
Has nucleus and organelles. Plants, animals, fungi.
Animal cell
Nucleus, mitochondria, ribosomes, lysosomes, centrioles, cell membrane.
Plant cell
Animal cell features PLUS cell wall, chloroplasts, large vacuole.
Size
Prokaryotes: 1-10um. Eukaryotes: 10-100um.
Organelles
Nucleus
Contains DNA. Controls gene expression.
Mitochondria
Aerobic respiration. Produces ATP. 'Powerhouse of the cell'.
Chloroplast
Photosynthesis. Contains chlorophyll. Plants only.
Ribosome
Protein synthesis. On RER or free in cytoplasm.
Rough ER
Transports proteins. Studded with ribosomes.
Golgi
Packages and modifies proteins for secretion.
Vacuole
Plant: large, maintains turgor. Animal: small temporary.
02 Cell Division ▼
Mitosis result
2 genetically identical diploid cells. Growth and repair.
Meiosis result
4 genetically unique haploid cells (gametes).
PMAT stages
Prophase, Metaphase, Anaphase, Telophase (then Cytokinesis).
Crossing over
Prophase I: homologous chromosomes swap segments. Creates variation.
Cancer
Uncontrolled cell division due to mutations in tumour suppressor genes.
BIOLOGYCell cycle
Interphase (G1+S+G2): cell grows, DNA replicates
Mitosis (PMAT):        chromosomes separated
Cytokinesis:           cytoplasm divides into 2 cells
03 DNA & Genetics ▼
BIOLOGYCentral Dogma
DNA -- Transcription --> mRNA -- Translation --> Protein

DNA: Double helix. A-T (2 H-bonds), G-C (3 H-bonds).
Antiparallel: one strand 3'to5', other 5'to3'.

Transcription (nucleus):
RNA polymerase unwinds DNA, builds mRNA from template strand.
Codon = 3 bases on mRNA.

Translation (ribosome):
tRNA anticodon matches mRNA codon.
Each codon = 1 amino acid.
Start: AUG. Stop: UAA, UAG, UGA.
Genetics Terms
Gene
DNA sequence coding for a protein.
Allele
Alternative form of a gene (B or b).
Dominant
Expressed with just one copy (B).
Recessive
Only expressed when homozygous (bb).
Codominance
Both alleles expressed equally (AB blood group).
Sex-linked
On X chromosome. Males (XY) more likely to express recessive X-linked traits.
04 Human Body Systems ▼
Systems
Circulatory
Heart + blood vessels. Double circulation: pulmonary (lungs) + systemic (body).
Respiratory
Lungs + diaphragm. Gas exchange in alveoli: O2 in, CO2 out.
Digestive
Mouth to anus. Enzymes break down food. Villi absorb nutrients in small intestine.
Nervous
Brain + spinal cord + neurons. Fast electrical signals.
Endocrine
Glands + hormones. Slower chemical control. Insulin, adrenaline, oestrogen.
Immune
Phagocytes eat pathogens. Lymphocytes make antibodies. Memory cells = immunity.
Homeostasis
Thermoregulation
Vasodilation+sweating when hot. Vasoconstriction+shivering when cold.
Blood glucose
High: insulin released, glucose -> glycogen. Low: glucagon released, glycogen -> glucose.
ADH
High blood solute concentration -> more ADH -> less urine -> more water reabsorbed.
05 Enzymes & Biochemistry ▼
Active site
Specific shape. Only complementary substrate can bind. Lock and key model.
Optimum temperature
Increases rate up to optimum. Above: enzyme denatures (irreversible shape change).
Optimum pH
Deviation from optimum changes active site shape -> denatures.
Competitive inhibitor
Blocks active site. Add more substrate to overcome.
Non-competitive inhibitor
Binds elsewhere. Changes shape of active site. Cannot overcome by adding substrate.
Biological Molecules
Carbohydrates
Glucose monomer. Starch=plant store, Glycogen=animal store, Cellulose=plant wall.
Proteins
Amino acid monomers. Primary->secondary->tertiary->quaternary structure.
Lipids
Glycerol + 3 fatty acids (triglyceride). Phospholipids form cell membranes.
ATP
Adenosine triphosphate. Universal energy currency. Made in respiration.
06 Photosynthesis & Respiration ▼
BIOLOGYKey equations
PHOTOSYNTHESIS:
6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
Light reactions (thylakoids): water photolysis, ATP + NADPH made, O2 released
Calvin cycle (stroma): CO2 + ATP + NADPH -> glucose

AEROBIC RESPIRATION:
C6H12O6 + 6O2 -> 6CO2 + 6H2O + 36-38 ATP
Glycolysis -> Krebs cycle -> Electron transport chain

ANAEROBIC (animals): glucose -> lactic acid + 2 ATP
ANAEROBIC (yeast): glucose -> ethanol + CO2 + 2 ATP
💡
Plants do BOTH photosynthesis and respiration. Net gas exchange depends on light intensity.
07 Ecology ▼
Food chain
Producer -> Primary -> Secondary -> Tertiary consumer
Energy transfer
Only ~10% of energy passes to next trophic level. ~90% lost as heat.
Carbon cycle
Photosynthesis removes CO2. Respiration, decomposition, combustion release CO2.
Nitrogen cycle
N2 -> NH3 (fixation) -> NO3- (nitrification) -> plant protein (assimilation) -> N2 (denitrification).
Succession
Pioneer species -> intermediate -> climax community over time.
Biodiversity
Variety of species in ecosystem. Measured by Simpson's Diversity Index.
08 Evolution ▼
BIOLOGYNatural selection mechanism
1. VARIATION within population (mutations, meiosis, sexual reproduction)
2. COMPETITION for limited resources (food, mates, space)
3. SURVIVAL of better-adapted individuals
4. REPRODUCTION: survivors pass on advantageous alleles
5. Over generations: change in allele frequencies = EVOLUTION

Speciation: geographic isolation -> different selection -> reproductive isolation -> new species

Evidence: fossil record, comparative anatomy, DNA comparison, antibiotic resistance
Hardy-Weinberg
p^2 + 2pq + q^2 = 1. Allele frequencies stable in large random-mating population.
Genetic drift
Random allele frequency change. More significant in small populations.
Bottleneck
Population crash -> reduced genetic diversity -> founder effect.
09 Classification ▼
Domain
Broadest: Archaea, Bacteria, Eukarya
Kingdom
Animalia, Plantae, Fungi, Protista
Phylum
Chordata, Arthropoda, Mollusca
Class
Mammalia, Reptilia, Aves, Amphibia
Order
Primates, Carnivora
Family
Hominidae, Felidae
Genus
Homo, Panthera
Species
sapiens, leo, tigris
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Memory: 'Dear King Philip Came Over For Good Soup' = Domain Kingdom Phylum Class Order Family Genus Species
10 Mini Quizzes ▼
❓ Quiz 1
What is the difference between mitosis and meiosis?
Mitosis: 2 identical diploid cells for growth/repair. Meiosis: 4 genetically unique haploid gametes for reproduction.
❓ Quiz 2
What organelle is the 'powerhouse of the cell'?
Mitochondria perform aerobic respiration (Krebs cycle + electron transport chain) producing 36-38 ATP per glucose molecule.
❓ Quiz 3
C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP represents:
Aerobic respiration breaks down glucose with oxygen to release energy. Photosynthesis is the reverse equation.