🔬 GCSE Combined Science

Biology – Cells & Organisation

Eukaryotic vs Prokaryotic Cells

• Eukaryotic: plant and animal cells — have a nucleus and membrane-bound organelles
• Prokaryotic: bacteria — no nucleus, circular DNA, smaller (1–10 µm)

Animal Cell Organelles

• Nucleus: contains DNA; controls cell activity
• Mitochondria: site of aerobic respiration (ATP production)
• Ribosomes: protein synthesis
• Cell membrane: controls what enters and leaves

Additional Plant Cell Structures

• Cell wall: cellulose; provides support
• Chloroplasts: contain chlorophyll; site of photosynthesis
• Permanent vacuole: contains cell sap; maintains turgor

Diffusion, Osmosis & Active Transport

• Diffusion: movement of particles from high → low concentration (passive)
• Osmosis: movement of water through a partially permeable membrane from dilute → concentrated solution
• Active transport: movement against concentration gradient — requires energy (ATP)

Biology – Infection & Response

Pathogens

• Bacteria: reproduce rapidly; produce toxins. Treated with antibiotics.
• Viruses: invade cells and replicate inside. No cure — only vaccination prevents.
• Fungi: e.g. athlete's foot. Treated with antifungals.
• Protists: e.g. malaria (spread by mosquito vectors)

The Immune System

• White blood cells: phagocytes (engulf pathogens) and lymphocytes (produce antibodies)
• Antibodies are specific to one antigen (lock-and-key model)
• Memory cells remain after infection → faster response on re-infection (immunity)
• Vaccination: introduces dead/weakened pathogen to stimulate antibody production without illness

Biology – Bioenergetics

• Limiting factors: light intensity, CO₂ concentration, temperature
• Light-dependent stage: in thylakoid membrane; splits water, produces ATP and NADPH
• Light-independent (Calvin cycle): in stroma; uses CO₂ to produce glucose

Biology – Homeostasis

Homeostasis is the maintenance of a stable internal environment. All control systems use negative feedback.

• Blood glucose: insulin (lowers) and glucagon (raises), secreted by the pancreas
• Body temperature: hypothalamus detects change → sweating/vasodilation (hot) or shivering/vasoconstriction (cold)
• Kidneys: filter blood; control water and ion balance; ADH controls water reabsorption

Biology – Inheritance, Variation & Evolution

• DNA is a double helix made of nucleotides (base, sugar, phosphate)
• Base pairs: A–T, C–G
• Genes: sections of DNA that code for proteins
• Alleles: different versions of a gene
• Dominant allele expressed if one or two copies present (capital letter)
• Recessive allele only expressed if two copies present (lowercase)
• Punnet squares show the probability of offspring genotypes
• Sex determination: XX = female, XY = male

Biology – Ecology

• Food chain: producer → primary consumer → secondary consumer → tertiary consumer
• Only ~10% of energy is passed between trophic levels
• Carbon cycle: photosynthesis (absorbed), respiration/combustion (released), decomposition
• Nitrogen cycle: nitrogen-fixing bacteria → nitrates → taken up by plants → decomposers return to soil
• Biodiversity is maintained by reducing deforestation, land use, and pollution

Chemistry – Atomic Structure & the Periodic Table

• Atom: protons + neutrons (nucleus) + electrons (shells)
• Atomic number = number of protons; Mass number = protons + neutrons
• Isotopes: same element, different number of neutrons
• Group number = number of outer electrons; Period = number of electron shells
• Group 1 (alkali metals): very reactive, 1 outer electron, react with water to give H₂ and metal hydroxide
• Group 7 (halogens): 7 outer electrons; more reactive further up the group
• Group 0 (noble gases): full outer shell, very unreactive

Chemistry – Bonding & Structure

• Ionic bonding: transfer of electrons between metal and non-metal; forms lattice — high melting point, conducts when dissolved/melted
• Covalent bonding: sharing of electrons between non-metals; can be simple molecular (low mp) or giant covalent (diamond — very high mp)
• Metallic bonding: delocalised electrons in a sea; conducts electricity; malleable
• Nanoparticles (1–100 nm): very high surface area to volume ratio; used in medicine, sun cream, electronics

Chemistry – Quantitative Chemistry

• Atom economy = (mass of desired product ÷ total mass of products) × 100
• % yield = (actual yield ÷ theoretical yield) × 100
• Avogadro's number: 6.02 × 10²³ particles per mole

Chemistry – Rates of Reaction & Equilibrium

• Rate increases with: higher temperature, higher concentration, larger surface area, catalyst
• Collision theory: more frequent/energetic collisions = faster rate
• Activation energy: minimum energy needed for a reaction to occur
• Le Chatelier's Principle: if conditions change, equilibrium shifts to oppose that change
• Increasing temperature → shifts towards endothermic direction
• Increasing pressure → shifts towards fewer moles of gas

Chemistry – Organic Chemistry

• Alkanes (CₙH₂ₙ₊₂): saturated; single bonds; e.g. methane, ethane
• Alkenes (CₙH₂ₙ): unsaturated; C=C double bond; e.g. ethene
• Test for alkenes: bromine water turns from orange to colourless
• Crude oil: fractional distillation separates into fractions by boiling point
• Cracking: breaks long-chain hydrocarbons into shorter, more useful ones (+ alkenes)
• Polymers made by addition polymerisation of alkene monomers

Physics – Forces & Motion

• Resultant force: vector sum of all forces on an object
• Newton's 3rd law: every action has an equal and opposite reaction
• Terminal velocity: when drag = weight; acceleration = 0
• Stopping distance = thinking distance + braking distance

Physics – Energy

• Energy is conserved (cannot be created or destroyed)
• Specific heat capacity: E = mcΔT (energy to raise temperature)
• Specific latent heat: E = mL (energy for change of state, no temperature change)
• Renewable: solar, wind, tidal, hydroelectric — low carbon but variable output
• Non-renewable: fossil fuels, nuclear — reliable but finite/polluting

Physics – Waves

• Transverse waves: oscillation perpendicular to direction of travel (light, water)
• Longitudinal waves: oscillation parallel to direction of travel (sound)
• EM spectrum (low → high frequency): radio, microwave, infrared, visible, UV, X-ray, gamma
• Refraction: change of speed at boundary causes bending of waves
• Total internal reflection: occurs when angle > critical angle inside a denser medium (optical fibres)

Physics – Electricity

• Charge Q = It (charge = current × time)
• Resistors in parallel have lower total resistance than any individual resistor
• Diode: only allows current in one direction; resistance is very high in reverse
• LDR: resistance decreases as light increases
• Thermistor: resistance decreases as temperature increases
• Mains electricity: 230V AC at 50Hz in the UK

Physics – Magnetism & Electromagnetism

• Magnetic field lines go from North to South pole (outside the magnet)
• Fleming's Left-Hand Rule: thumb = force (motion), index = field, middle = current
• Motor effect: current-carrying wire in magnetic field experiences a force
• Electromagnetic induction: moving a conductor in a field induces an EMF (generator)
• Transformers: step voltage up/down using coil ratio: V₁/V₂ = N₁/N₂

Physics – Atomic & Nuclear Physics

• Rutherford's gold foil experiment showed the atom is mostly empty with a dense positive nucleus
• Alpha (α): 2 protons + 2 neutrons; stopped by paper; highly ionising
• Beta (β): fast electron; stopped by thin aluminium; moderately ionising
• Gamma (γ): EM wave; stopped by thick lead; weakly ionising
• Half-life: time for activity/mass to halve
• Nuclear fission: large nucleus splits (e.g. U-235); releases energy in nuclear reactors
• Nuclear fusion: two small nuclei combine (e.g. in the Sun); releases more energy than fission