πŸ”¬ Year 8 Science

Deeper biology, chemistry, and physics β€” building from Year 7 foundations towards GCSE.

Biology – Digestion & Nutrition

The Digestive System

  • Digestion is the process of breaking down large, insoluble food molecules into small, soluble ones that can be absorbed into the blood
  • Mechanical digestion: physical breaking down (teeth, stomach churning)
  • Chemical digestion: enzymes break chemical bonds in food molecules

The Journey of Food

  • Mouth: teeth mechanically break food; salivary amylase begins starch digestion; tongue mixes food into a bolus
  • Oesophagus: carries food to stomach via peristalsis (wave-like muscle contractions)
  • Stomach: muscular walls churn food; gastric juice (contains hydrochloric acid and protease) breaks down proteins; acid kills bacteria
  • Small intestine: most digestion and all absorption occurs here. Bile from the liver emulsifies fats (breaks into droplets). Pancreatic enzymes (amylase, protease, lipase) complete digestion. Villi and microvilli increase surface area for absorption into the blood
  • Large intestine: reabsorbs water from undigested food; remaining waste becomes faeces
  • Rectum and anus: faeces stored then expelled

Enzymes and Digestion

  • Amylase: breaks down starch into sugars (maltose). Produced in salivary glands and pancreas.
  • Protease: breaks down proteins into amino acids. Found in stomach and pancreas.
  • Lipase: breaks down lipids (fats) into fatty acids and glycerol. Found in pancreas and small intestine.
  • Enzymes work best at their optimum temperature (body temperature β‰ˆ 37Β°C) and pH. Too hot or too acidic/alkaline = enzyme denatures (loses its shape and stops working).

Villi β€” Adaptations for Absorption

  • Villi are small finger-like projections lining the small intestine
  • Adaptations: large surface area, thin walls (one cell thick), rich blood supply (capillaries and lacteal), microvilli (brush border) for even more surface area
  • Glucose and amino acids absorbed into blood capillaries β†’ transported to liver via hepatic portal vein
  • Fatty acids and glycerol absorbed into lacteals (lymph vessels)

Biology – Respiration & Breathing

Aerobic Respiration

Word Equation Glucose + Oxygen β†’ Carbon dioxide + Water (+ energy) Glucose: C₆H₁₂O₆ | Oβ‚‚ β†’ COβ‚‚ + Hβ‚‚O Occurs in the mitochondria
  • Releases energy for: movement, warmth, chemical reactions, active transport, growth
  • Continuous process in every living cell

Anaerobic Respiration

  • Occurs when oxygen is not available (e.g. during intense exercise)
  • In animals and humans: Glucose β†’ Lactic acid (+ small amount of energy)
  • Lactic acid causes muscle fatigue and the burning sensation during intense exercise
  • An oxygen debt builds up β€” must be repaid after exercise (heavy breathing continues to oxidise lactic acid)
  • In yeast: Glucose β†’ Ethanol + Carbon dioxide (fermentation β€” used in bread-making and brewing)

The Respiratory System

  • Nose/mouth β†’ trachea β†’ bronchi β†’ bronchioles β†’ alveoli
  • Alveoli adaptations: thin walls (one cell thick), moist surface, large number (β‰ˆ300 million) for huge surface area, rich network of capillaries
  • Gas exchange at alveoli: oxygen diffuses from alveolar air into blood; COβ‚‚ diffuses from blood into alveolar air
  • Breathing mechanism: diaphragm and intercostal muscles control the volume of the chest cavity. Inhalation: diaphragm contracts/flattens; ribcage rises; pressure drops; air rushes in. Exhalation: diaphragm relaxes; ribcage falls; pressure rises; air pushed out.
  • The trachea, bronchi, and bronchioles are lined with cilia and mucus to trap and remove particles and bacteria (mucociliary escalator)

Biology – Genetics & Variation

Key Terms

  • DNA (deoxyribonucleic acid): the molecule that contains genetic information; found in the nucleus
  • Gene: a section of DNA that codes for a specific protein (and therefore a characteristic)
  • Chromosome: a long strand of coiled DNA. Humans have 46 chromosomes in 23 pairs; sex cells (sperm and egg) have 23
  • Allele: different versions of the same gene (e.g. an allele for brown eyes and an allele for blue eyes)
  • Genotype: the genetic makeup (which alleles you have)
  • Phenotype: the physical characteristic you show
  • Dominant allele: expressed even if only one copy is present. Represented by a capital letter (e.g. B)
  • Recessive allele: only expressed when two copies are present. Represented by a lowercase letter (e.g. b)
  • Homozygous: both alleles are the same (BB or bb)
  • Heterozygous: two different alleles (Bb) β€” also called a carrier if recessive is hidden

Punnett Squares

  • A grid used to predict the probability of offspring inheriting each genotype
  • e.g. Two heterozygous parents (Bb Γ— Bb): offspring ratios = 1 BB : 2 Bb : 1 bb = 3 dominant phenotype : 1 recessive phenotype
  • A recessive genetic condition (e.g. cystic fibrosis): child must inherit two recessive alleles (cc) to be affected

Variation

  • Continuous variation: a range of values with no distinct categories; controlled by many genes and the environment. e.g. height, weight, intelligence
  • Discontinuous variation: distinct categories; usually controlled by one or few genes. e.g. blood group (A, B, AB, O), tongue rolling, attached/free earlobes
  • Variation is caused by: genetic differences (mutations, sexual reproduction mixing alleles) and environmental factors (diet, exercise, disease)

Biology – Evolution & Classification

Darwin's Theory of Evolution by Natural Selection

  • Variation: individuals within a species show variation in their characteristics
  • Struggle for survival: organisms produce more offspring than can survive; competition for resources (food, mates, space)
  • Survival of the fittest: individuals with characteristics better suited to the environment are more likely to survive and reproduce
  • Inheritance: favourable characteristics are passed to offspring; over many generations they become more common
  • Over millions of years, species change (evolve) and new species arise
  • Evidence: fossil record, comparative anatomy, DNA comparisons, observed evolution in bacteria (antibiotic resistance)

Classification

  • Linnaean classification: Kingdom β†’ Phylum β†’ Class β†’ Order β†’ Family β†’ Genus β†’ Species
  • Mnemonic: "King Philip Came Over For Good Soup"
  • The five kingdoms: Animalia, Plantae, Fungi, Protoctista, Prokaryota (bacteria)
  • Modern classification is based on evolutionary relationships (phylogenetics) and DNA analysis
  • Species definition: organisms that can interbreed to produce fertile offspring

Antibiotic Resistance β€” Evolution in Action

  • Random mutation in bacteria may give resistance to an antibiotic
  • When antibiotics are used, non-resistant bacteria die; resistant ones survive and reproduce
  • Over time, the population is dominated by resistant bacteria (e.g. MRSA β€” methicillin-resistant Staphylococcus aureus)
  • Prevention: only use antibiotics when necessary; complete the full course; never share antibiotics

Chemistry – Metals & the Reactivity Series

The Reactivity Series

  • Metals arranged in order of decreasing reactivity: K, Na, Li, Ca, Mg, Al, Zn, Fe, Ni, Sn, Pb, Cu, Ag, Au, Pt
  • Mnemonic: "Potassium Sodium Lithium Calcium Magnesium Aluminium Zinc Iron Nickel Tin Lead Copper Silver Gold Platinum" β€” Please Stop Letting Children Make A Zebra Into Nickers, Sir George's Puppy"
  • More reactive metals: react vigorously with water and dilute acids, displace less reactive metals from compounds

Reactions of Metals

  • Metal + water β†’ metal hydroxide + hydrogen. e.g. 2Na + 2Hβ‚‚O β†’ 2NaOH + Hβ‚‚
  • Metal + dilute acid β†’ salt + hydrogen. e.g. Mg + Hβ‚‚SOβ‚„ β†’ MgSOβ‚„ + Hβ‚‚
  • Metal + oxygen β†’ metal oxide. e.g. 2Mg + Oβ‚‚ β†’ 2MgO (bright white flame)
  • Test for hydrogen: lit splint β€” hydrogen burns with a squeaky pop

Displacement Reactions

  • A more reactive metal displaces a less reactive metal from its salt solution
  • e.g. Zn + CuSOβ‚„ β†’ ZnSOβ‚„ + Cu (zinc displaces copper because Zn is more reactive)
  • Cu + ZnSOβ‚„: no reaction (Cu is less reactive than Zn)
  • Uses: thermite reaction (Al + Feβ‚‚O₃ β†’ Alβ‚‚O₃ + Fe) used for welding railway tracks

Extracting Metals from Ores

  • Very reactive metals (K, Na, Al): extracted by electrolysis (expensive, high energy)
  • Moderately reactive metals (Zn, Fe): extracted by reduction with carbon (coke) in a blast furnace
  • Iron ore (haematite, Feβ‚‚O₃) β†’ blast furnace β†’ pig iron β†’ steel (iron with small amounts of carbon)
  • Less reactive metals (Cu, Ag, Au, Pt): can be found native (unreacted) or easily reduced

Corrosion & Rusting

  • Rusting: iron reacts with oxygen and water to form iron(III) oxide (Feβ‚‚O₃·xHβ‚‚O)
  • Both oxygen AND water are needed for rusting
  • Prevention: painting, oiling, galvanising (coating with zinc β€” zinc sacrificially corrodes instead), electroplating, alloying (stainless steel)

Chemistry – Rates of Reaction

What Affects the Rate?

  • Rate of reaction = how fast reactants are converted to products
  • Measured by: how quickly reactants are used up, or how quickly products are formed
  • Rate = amount of product formed (or reactant used) Γ· time

Factors that Increase the Rate

  • Temperature: higher temperature β†’ particles have more kinetic energy β†’ move faster β†’ collide more frequently and with more energy β†’ more successful collisions β†’ faster rate
  • Concentration (of solution) / pressure (of gas): more particles in the same space β†’ more frequent collisions β†’ faster rate
  • Surface area: smaller pieces = larger surface area β†’ more collisions at the surface β†’ faster rate. e.g. powdered marble reacts faster than lumps with HCl
  • Catalyst: a substance that increases the rate without being used up. It provides an alternative reaction pathway with lower activation energy. e.g. manganese dioxide catalyses decomposition of hydrogen peroxide

Collision Theory

  • For a reaction to occur, particles must collide with sufficient energy (at least the activation energy) and correct orientation
  • Activation energy: the minimum energy needed to break bonds and start a reaction
  • A catalyst lowers the activation energy, so more collisions are successful

Physics – Electricity & Circuits

Current, Voltage, and Resistance

  • Current (I): the flow of charge (electrons) around a circuit. Measured in Amperes (A) using an ammeter in series.
  • Voltage / Potential difference (V): the energy given to each unit of charge by the battery; the "push" that drives current. Measured in Volts (V) using a voltmeter in parallel.
  • Resistance (R): opposition to current flow. Measured in Ohms (Ξ©).
Ohm's Law V = I Γ— R (Voltage = Current Γ— Resistance) I = V / R | R = V / I

Series and Parallel Circuits

  • Series: components in one loop. Same current throughout. Voltage shared. If one component breaks, all stop. Total resistance = R₁ + Rβ‚‚ + R₃
  • Parallel: components in separate branches. Voltage the same across each branch. Current shared (splits). If one branch breaks, others continue. 1/R_total = 1/R₁ + 1/Rβ‚‚
  • Household circuits are wired in parallel so each appliance can be switched on/off independently at the same voltage

Electrical Power and Energy

Power and Energy Power (W) = Voltage (V) Γ— Current (A) [P = VI] Energy transferred (J) = Power (W) Γ— Time (s) [E = Pt] Also: P = IΒ²R | P = VΒ²/R
  • The kilowatt-hour (kWh) is used by energy companies: 1 kWh = using 1kW for 1 hour
  • Cost of electricity = power (kW) Γ— time (h) Γ— cost per kWh

Static Electricity

  • Caused by the transfer of electrons between materials by friction
  • Like charges repel; unlike charges attract
  • Uses: photocopiers, inkjet printers, electrostatic paint spraying, lightning conductors
  • Dangers: sparks near flammable materials (e.g. fuelling aircraft); electric shocks

Physics – Magnetism & Electromagnetism

Permanent Magnets

  • Magnetic materials: iron, steel, nickel, cobalt (and alloys). Aluminium, copper, wood, plastic are non-magnetic.
  • Every magnet has a north pole and a south pole. Like poles repel; unlike poles attract.
  • Magnetic field: the region around a magnet where magnetic force acts. Shown by field lines from N to S. Closer lines = stronger field.
  • The Earth has a magnetic field β€” its geographic north pole is a magnetic south pole (which is why compass needles point north)

Electromagnets

  • A current-carrying wire creates a magnetic field around it (right-hand rule: wrap right hand around wire with thumb pointing in current direction β€” fingers show field direction)
  • Electromagnet: a coil of wire (solenoid) around an iron core; becomes magnetic when current flows; can be switched on/off
  • Increase magnetic strength: more turns of wire, larger current, soft iron core
  • Uses: electric bells, scrapyard cranes, MRI scanners, loudspeakers, electric motors

The Motor Effect & Electric Motors

  • A current-carrying conductor in a magnetic field experiences a force (the motor effect)
  • Direction of force: using Fleming's Left-Hand Rule β€” thumb (force/motion), index finger (field Nβ†’S), middle finger (conventional current)
  • Electric motor: uses the motor effect to convert electrical energy β†’ kinetic energy; coil rotates in a magnetic field; commutator reverses current direction to maintain continuous rotation

Electromagnetic Induction

  • Moving a conductor through a magnetic field induces a voltage (EMF) β€” this is the reverse of the motor effect
  • Generator: mechanical energy β†’ electrical energy (opposite of motor)
  • Transformer: changes the voltage of an AC supply; uses electromagnetic induction between two coils

Physics – Space & the Universe

The Solar System

  • Our Sun is a medium-sized star; the Solar System contains 8 planets orbiting it
  • Planets in order: Mercury, Venus, Earth, Mars (rocky/terrestrial), Jupiter, Saturn, Uranus, Neptune (gas giants)
  • Mnemonic: "My Very Educated Mother Just Served Us Nachos"
  • A planet orbits a star; a moon orbits a planet; a satellite can be natural (moon) or artificial
  • The asteroid belt lies between Mars and Jupiter: rocky debris left over from the formation of the Solar System
  • Comets: icy bodies with highly elliptical orbits; develop a tail as they approach the Sun
  • Dwarf planets: Pluto, Eris, Ceres β€” too small and unable to clear their orbital neighbourhood

Gravity and Orbits

  • Gravity is a universal attractive force between all masses; it holds the Solar System together
  • Planets orbit the Sun because gravity provides the centripetal force for circular motion
  • Orbital speed: closer planets have shorter orbital periods (Mercury = 88 days; Neptune = 165 years)
  • Artificial satellites: orbit Earth in circular paths; lower orbits β†’ faster orbital speed and shorter period
  • Geostationary orbit (β‰ˆ35,786 km): satellite orbits at the same rate as Earth rotates β€” appears stationary. Used for TV satellites and communications.

The Life Cycle of Stars

  • Nebula β†’ (gravity pulls gas together) β†’ protostar β†’ main sequence star (long stable period; nuclear fusion converts H to He)
  • For a star like our Sun: main sequence β†’ red giant β†’ planetary nebula β†’ white dwarf
  • For a massive star: main sequence β†’ red supergiant β†’ supernova β†’ neutron star or black hole
  • Nuclear fusion in stars creates all elements heavier than helium β€” we are literally made of stardust

The Universe

  • The universe is approximately 13.8 billion years old; began with the Big Bang
  • The universe is still expanding β€” galaxies are moving away from each other
  • Evidence for the Big Bang: the universe is expanding (discovered by Edwin Hubble), cosmic microwave background radiation (faint "afterglow" of the Big Bang)
  • The Milky Way galaxy contains approximately 200–400 billion stars; the observable universe contains approximately 2 trillion galaxies
  • Light year: the distance light travels in one year (approximately 9.46 Γ— 10¹⁡ metres); used to measure astronomical distances