GCSE Combined Science Notes

Cell Division

• Eukaryotic cells: Animal and plant cells with a nucleus and membrane-bound organelles.
• Prokaryotic cells: Bacteria with no nucleus, a single DNA loop, and plasmids.

Animal Cell Organelles

• Nucleus: Contains genetic material.
• Cytoplasm: Site of chemical reactions.
• Cell membrane: Controls entry/exit of substances.
• Mitochondria: Respiration.
• Ribosomes: Protein synthesis.

Plant Cell Organelles (extra)

• Cell wall (cellulose): Provides support and strength.
• Permanent vacuole: Stores cell sap.
• Chloroplasts: Photosynthesis (contains chlorophyll).

Cell Specialisation

• Cells differentiate to perform specific functions.
• Examples:
  • Sperm cell: Tail, mitochondria, acrosome (enzymes).
  • Nerve cell: Long, branched connections.
  • Muscle cell: Mitochondria, ability to contract.
  • Root hair cell: Large surface area.
  • Xylem/Phloem: Transport substances.

Microscopy

• Light microscope: Basic structures (living cells).
• Electron microscope: Higher resolution & magnification (smaller structures e.g., ribosomes).
• Magnification Formula:
  Magnification = \frac{Image \ size}{Real \ size}
• Use standard form and convert units (mm → µm = ×1000).

Cell Cycle

• Interphase: DNA replicates.
• Mitosis: Division for growth/repair.
  • Chromosomes line up → pulled apart → nuclei form → identical daughter cells

Stem Cells

• Embryonic: Can become any cell type.
• Adult stem cells: Limited (e.g., bone marrow).
• Uses: Treat diseases, replace damaged cells.
• Therapeutic cloning: Embryo with same DNA.
• Ethical concerns: Destruction of embryos, consent.
• Plant stem cells (meristems): Clone rare species, disease-resistant crops.

Transport in Cells

Diffusion

• Movement of particles from high → low concentration.
• Happens in gases/liquids (e.g., O2/CO2 in lungs).
• Factors affecting rate: Concentration gradient, temperature, surface area.

Osmosis

• Diffusion of water through a partially permeable membrane.
• High → low water concentration (dilute → concentrated).
• Turgid (swollen) / Flaccid (shrunken) cells.

Active Transport

• Movement against concentration gradient.
• Requires energy (from respiration).
• E.g., root hair cells absorbing minerals, gut absorbing glucose.

Organisation

Principles of Organisation

• Cell → Tissue → Organ → Organ system → Organism.
• Tissue = group of similar cells (e.g., muscular tissue).
• Organ = made of different tissues (e.g., stomach).

Enzymes

• Biological catalysts – speed up reactions.
• Made of proteins, specific shape (active site).
• Lock & key model: Enzyme binds to specific substrate.
• Affected by:
  • Temperature: Too high = denatured (changes shape).
  • pH: Extremes denature.
  • Substrate concentration: More = faster (until saturation).
• Required Practical: Effect of amylase on starch
  • Test with iodine (blue-black = starch present).
  • Measure time taken for starch to break down.

Digestive System

• Purpose: Break down large insoluble molecules into soluble ones.
• Main organs:
  • Mouth: Amylase in saliva.
  • Stomach: Pummels food, pepsin (protease), HCl (kills bacteria, pH 2).
  • Liver: Produces bile (neutralises acid, emulsifies fats).
  • Gall bladder: Stores bile.
  • Pancreas: Produces enzymes (amylase, protease, lipase).
  • Small intestine: Absorbs nutrients.
  • Large intestine: Absorbs water
• Enzymes in digestion:

Circulatory System

• Double circulatory system:
  • Right side → lungs → picks up O_2
  • Left side → body → delivers O_2

Heart

• Made of muscle tissue.
• Right = deoxygenated blood; Left = oxygenated blood.
• Valves prevent backflow.
• Coronary arteries supply heart muscle with blood.

Blood vessels

Blood components

• Red blood cells: Haemoglobin, no nucleus, biconcave (O_2 transport).
• White blood cells: Defend against pathogens.
• Platelets: Help clot blood.
• Plasma: Carries CO_2, urea, hormones, glucose.

Health and Disease

• Health = physical and mental well-being.
• Disease types: Communicable (pathogens), non-communicable (e.g., cancer).
• Risk factors:
  • Lifestyle: Diet, exercise, smoking, alcohol.
  • Genetics: E.g., inherited disorders.
• Examples:
  • Smoking → lung disease, cancer, cardiovascular disease.
  • Obesity → Type 2 diabetes.
  • Alcohol → liver damage.
  • Carcinogens → cancer.

Plant Organisation

Plant tissues

• Epidermal: Covers plant.
• Palisade mesophyll: Photosynthesis.
• Spongy mesophyll: Gas exchange.
• Xylem: Transports water & minerals (transpiration stream).
• Phloem: Transports sugars (translocation).

Transpiration

• Water evaporates from leaves → pulls more up.
• Affected by: Light, temperature, wind, humidity.

Guard cells

• Control stomata (openings).
• Regulate gas exchange and water loss.

Atomic Structure & The Periodic Table

• Atom = smallest part of an element.
• Element = made of one type of atom.
• Compound = 2+ elements chemically bonded (e.g., H2O, CO2).
• Mixture = not chemically bonded (e.g., air).
• Formula examples:
  • Water = H_2O
  • Carbon dioxide = CO_2
  • Sodium chloride = NaCl

Separation Techniques

• Used to separate mixtures (not compounds).
• Filtration: Separates insoluble solids from liquids.
• Crystallisation: Separates soluble solid from solution.
• Distillation: Separates liquids by boiling point.
  • Simple: one liquid
  • Fractional: mixture (e.g. ethanol & water)
• Chromatography: Separates dyes in ink
  • R_f \ value = \frac{distance \ spot \ moved}{distance \ solvent \ moved}

Atomic Structure

Subatomic particles

• Atomic number = protons
• Mass number = protons + neutrons
• Number of electrons = same as protons in a neutral atom
• Isotopes – atoms of the same element with different numbers of neutrons

History of the Atom

• John Dalton: Solid spheres.
• JJ Thomson: Plum pudding (discovered electrons).
• Rutherford: Nuclear model (gold foil experiment, nucleus with positive charge).
• Bohr: Electrons in shells.
• Chadwick: Discovered neutrons.

Electron Shells

• Electron configuration (2, 8, 8…).
• Electrons fill lowest energy levels first.
• Determines chemical reactivity (valence electrons).

The Periodic Table

• Created by Mendeleev – left gaps, ordered by atomic mass but corrected anomalies.
• Modern table = ordered by atomic number.
• Groups (columns) = same number of outer electrons → similar properties.
• Periods (rows) = number of shells.

Group 1 – Alkali Metals

• Soft, low density
• 1 outer electron (very reactive)
• React with water → metal hydroxide + hydrogen gas
• Reactivity increases down the group
• Example:
  2Na + 2H2O \rightarrow 2NaOH + H2 \uparrow

Group 7 – Halogens

• Non-metals, 7 outer electrons
• Coloured vapours
• Form salts with metals (e.g., NaCl)
• Reactivity decreases down the group
• More reactive halogen displaces less reactive one from a compound
• E.g., Cl2 + KBr \rightarrow KCl + Br2

Group 0 – Noble Gases

• Full outer shell (8 electrons) → unreactive
• Boiling points increase down the group

The Particle Model of Matter

States of Matter

• Solids: Fixed shape, particles tightly packed, vibrate.
• Liquids: Fixed volume, particles can move/slide.
• Gases: No fixed shape/volume, particles move freely & quickly.

Changes of state

• Melting: solid → liquid
• Boiling: liquid → gas
• Condensing: gas → liquid
• Freezing: liquid → solid
• Sublimation: solid → gas
• ✅ Physical changes (not chemical): mass is conserved, particles rearranged but not changed
  Density = \frac{mass}{volume}
• Required Practical: Measuring density of irregular solids using:
  • Balance for mass
  • Water displacement (eureka can) for volume

Internal Energy

• Internal energy = total kinetic + potential energy of particles
• Heating increases internal energy:
  • Causes temperature rise (if changing kinetic energy)
  • OR change of state (if breaking bonds – potential energy)

Specific Heat Capacity (SHC)

• Energy to raise temperature of 1kg by 1°C
• Units: J/kg°C
• High SHC = takes more energy to heat up (e.g. water)
• Required Practical: Measuring SHC using a heater and thermometer

Specific Latent Heat

• Energy to change state of 1kg without temperature change
• Fusion = melting/freezing
• Vaporisation = boiling/condensing
• Units: J/kg

Particle Motion in Gases

• Gases have random motion & collisions
• Increasing temperature → particles move faster = higher pressure
• Pressure increases with temperature (if volume constant)
  Pressure ∝ temperature (in Kelvin)
• Reducing volume → increases pressure (particles hit walls more often)
  Inversely proportional (when temp is constant)
• Units:
  • Pressure: Pa (pascals)
  • Volume: m³

Atomic Structure

• Atoms: tiny particles, radius ≈ 1 × 10^{-10} m
• Made of:
  • Nucleus: protons (+1), neutrons (0) → tiny but most mass
  • Electrons: (-1), orbit in shells, negligible mass

• Atomic number = protons
• Mass number = protons + neutrons
• Isotopes = same protons, different neutrons

The History of the Atom

• Dalton: solid spheres
• JJ Thomson: plum pudding model (discovered electron)
• Rutherford: gold foil experiment → nuclear model
• Bohr: electrons in energy levels/shells
• Chadwick: discovered neutron

Radioactive Decay

• Unstable isotopes decay randomly to become more stable
• Emit ionising radiation

Types of radiation

• Alpha: 2 protons, 2 neutrons (big and heavy)
• Beta: a neutron becomes a proton + electron
• Gamma: electromagnetic wave, no mass or charge

Nuclear Equations

• Show radioactive decay:

Alpha decay example

Beta decay example

• ✅ Total mass & atomic number must balance.

Half-Life

• Time for half the radioactive nuclei to decay
  Also: time for count rate to fall by half
• Used for:
  • Dating objects
  • Diagnosing/monitoring patients (medical tracers)
• Measured in Becquerels (Bq) = decays per second

Uses & Risks of Radiation

Uses

• Smoke detectors: alpha radiation
• Medical tracers: gamma (passes out body easily)
• Radiotherapy: high doses to kill cancer cells
• Sterilising equipment: gamma

Risks

• Ionising radiation can:
  • Damage cells/DNA
  • Cause mutations → cancer
  • Kill cells
• Contamination = radioactive particles on/in you
• Irradiation = exposed to radiation, but not radioactive yourself
• ✅ Precautions: shielding (lead), protective suits, tongs

Bonding, Structure & the Properties of Matter

Types of Bonding

• Ionic Bonding:
  • Between metals and non-metals
  • Electrons are transferred
    • Metal loses electrons → becomes positive ion (cation)
    • Non-metal gains electrons → becomes negative ion (anion)
  • Strong electrostatic forces between oppositely charged ions
• Covalent Bonding:
  • Between non-metals
  • Electrons are shared between atoms
  • Strong bonds within molecules
  • Weak forces between molecules
• Metallic Bonding:
  • Between metal atoms
  • Positive metal ions in a sea of delocalised electrons
  • Strong electrostatic attraction between ions and electrons

Ionic Compounds

• Form a giant ionic lattice
• High melting & boiling points
• Conduct electricity when molten or in solution
• Don’t conduct when solid

Simple Molecular Substances

• Small molecules (e.g. H2O, CO2, CH_4)
• Low melting/boiling points
• Don’t conduct electricity

Giant Covalent Structures

• E.g. Diamond, Graphite, Silicon Dioxide (SiO_2)
• Very high melting points
• Diamond: 4 bonds per carbon → very hard, no free electrons
• Graphite: 3 bonds per carbon → layers, conducts electricity
• Graphene: single layer of graphite

Metallic Structures

• Layers of positive metal ions with delocalised electrons
• High melting & boiling points
• Conductors of heat & electricity
• Malleable (layers slide)

Polymers & Fullerenes

• Polymers = long chains of covalently bonded molecules (plastics)
  Strong intermolecular forces = solid at room temp
• Fullerenes = carbon molecules in hollow shapes (e.g. C_{60})
  • Used in nanotubes, drug delivery, lubricants

States of Matter

• Solid: particles fixed in regular pattern
• Liquid: particles close, can move past each other
• Gas: particles far apart, move randomly

Properties Summary Table