🔬 Year 9 Science

Cell Division & Inheritance

Mitosis

• Mitosis: cell division that produces two genetically identical daughter cells. Used for growth, repair, and asexual reproduction.
• Before mitosis, DNA is copied (replicated) so each daughter cell gets a full set of chromosomes
• Stages: Interphase (DNA replicated) → Prophase (chromosomes condense) → Metaphase (chromosomes align at equator) → Anaphase (chromatids pulled to opposite poles) → Telophase (nuclear envelopes reform) → Cytokinesis (cell splits)
• Human body cells have 46 chromosomes (23 pairs). After mitosis, each daughter cell still has 46.
• Cancer: uncontrolled mitosis caused by mutations. Tumours form when cells divide without the normal checks.

Meiosis

• Meiosis: cell division that produces four genetically different gametes (sex cells), each with half the normal number of chromosomes (23 in humans)
• Occurs in the testes (sperm) and ovaries (eggs)
• Two divisions: meiosis I separates homologous chromosome pairs; meiosis II separates chromatids
• Crossing over during meiosis I: chromatids exchange segments of DNA → new combinations of alleles → genetic variation

DNA and Inheritance

• DNA (deoxyribonucleic acid): the molecule that carries genetic information. A double helix made of nucleotides.
• Each nucleotide contains: a phosphate group, a deoxyribose sugar, and one of four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
• Base pairing rule: A always pairs with T; G always pairs with C
• A gene: a section of DNA that codes for a protein
• Alleles: different versions of a gene (e.g. brown eye and blue eye alleles)
• Dominant allele (capital letter): expressed when one or two copies are present
• Recessive allele (lower case letter): only expressed when two copies are present (homozygous recessive)
• Homozygous: both alleles the same (BB or bb); Heterozygous: alleles different (Bb)
• Genotype: the alleles an organism carries; Phenotype: the physical characteristic that results

Ecosystems & Interdependence

Ecosystem Terms

• Ecosystem: all the living organisms (biotic factors) and non-living factors (abiotic factors: temperature, light, water, pH, humidity) in an area, and their interactions
• Population: all individuals of one species in an area
• Community: all populations of different species in an area
• Habitat: the place where an organism lives
• Niche: the role an organism plays in its ecosystem (what it eats, what eats it, when it's active, etc.)

Competition and Interdependence

• Intraspecific competition: between members of the same species for the same resources
• Interspecific competition: between different species for similar resources
• Predator-prey relationships: classic oscillating cycles — as prey increases, predators increase; predators eat more prey, prey decreases, predators then decrease, prey recovers.
• Mutualism: both species benefit (e.g. clownfish and sea anemones, bees and flowers)
• Parasitism: one species benefits (parasite), one is harmed (host): tapeworms, fleas
• Commensalism: one benefits, one unaffected

Nutrient Cycling

• Carbon cycle: carbon moves between atmosphere (CO₂), plants (photosynthesis), animals (feeding), and decomposers (decay). Returned by respiration, combustion, and decay.
• Nitrogen cycle: nitrogen gas (N₂) in air → nitrogen-fixing bacteria in soil and root nodules convert it to ammonia → nitrifying bacteria convert ammonia to nitrates → absorbed by plant roots → eaten by animals → returned by decomposers and denitrifying bacteria
• Decomposers (bacteria and fungi): break down dead organic matter, releasing mineral ions back into the soil — essential for nutrient cycling

Chemical Bonding

Ionic Bonding

• Ionic bonding occurs between metals and non-metals. Electrons are transferred from the metal to the non-metal.
• Metal atoms lose electrons → form positive ions (cations). Non-metals gain electrons → form negative ions (anions).
• Ions arrange into a giant ionic lattice — a regular 3D structure held together by electrostatic forces of attraction between opposite ions
• Properties: high melting/boiling points (strong forces throughout lattice), conduct electricity when molten or dissolved (ions are free to move), soluble in water (many ionic compounds), brittle (layers of ions can slide and like charges repel)
• Example: sodium chloride (NaCl). Na loses 1 electron (Na⁺), Cl gains 1 electron (Cl⁻).

Covalent Bonding

• Covalent bonds form between non-metals. Atoms share pairs of electrons.
• Each bond is a shared pair of electrons. A double bond is two shared pairs.
• Simple molecular structures (e.g. H₂O, CO₂, CH₄): low melting/boiling points (weak intermolecular forces between molecules), do not conduct electricity (no free ions or electrons)
• Giant covalent structures (e.g. diamond, graphite, silicon dioxide): extremely high melting points (many strong covalent bonds throughout the structure)
• Diamond: each carbon bonded to 4 others in a tetrahedral arrangement; very hard, does not conduct electricity
• Graphite: each carbon bonded to 3 others in layers; one delocalised electron per carbon atom → conducts electricity; layers can slide → soft, lubricant

Metallic Bonding

• In metals, atoms release their outer electrons into a "sea of delocalised electrons"
• The positive metal ions are held together by their attraction to the delocalised electrons
• Properties: good conductors of electricity and heat (delocalised electrons carry charge/energy), malleable and ductile (layers of ions can slide without disrupting the bonding), high melting points (strong forces)

Introduction to Organic Chemistry

What Is Organic Chemistry?

• Organic chemistry is the study of compounds containing carbon (with a few exceptions like CO₂)
• Carbon can form 4 covalent bonds and can bond to other carbon atoms — forming chains, branches, and rings. This gives an enormous variety of compounds.
• Crude oil is a mixture of hydrocarbons (compounds containing only carbon and hydrogen)

Alkanes

• Alkanes: saturated hydrocarbons (single C–C bonds only). General formula: CₙH₂ₙ₊₂
• Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀)
• Saturated: no double bonds — full complement of hydrogen atoms
• Combustion: alkanes are fuels. Complete combustion: hydrocarbon + oxygen → carbon dioxide + water. Incomplete combustion (limited oxygen): produces carbon monoxide (toxic) and/or carbon (soot)

Alkenes

• Alkenes: unsaturated hydrocarbons (contain a C=C double bond). General formula: CₙH₂ₙ
• Ethene (C₂H₄), Propene (C₃H₆), Butene (C₄H₈)
• The double bond makes alkenes more reactive than alkanes
• Test for alkenes: decolourise bromine water (orange/brown → colourless). Alkanes do not decolourise bromine water.
• Addition reactions: a molecule adds across the double bond. Ethene + Br₂ → dibromoethane

Fractional Distillation of Crude Oil

• Crude oil is separated into fractions based on their different boiling points in a fractionating column
• Short carbon chains: lower boiling point, more flammable, less viscous, more in demand (petrol)
• Long carbon chains: higher boiling point, less flammable, more viscous (bitumen for roads)
• Fractions (from top of column, smallest to largest): petroleum gas, petrol, naphtha, kerosene (jet fuel), diesel, fuel oil, bitumen
• Cracking: breaking long-chain alkanes into shorter alkanes and alkenes. Uses high temperature and a catalyst. Increases the supply of useful fractions.

Waves & The Electromagnetic Spectrum

Wave Properties

• Amplitude: the maximum displacement from the equilibrium position (related to energy and loudness/brightness)
• Wavelength (λ): the distance between two successive identical points (e.g. crest to crest) in metres
• Frequency (f): the number of complete waves per second, measured in Hertz (Hz)
• Period (T): time for one complete wave. T = 1/f
• Wave speed equation: v = fλ (wave speed = frequency × wavelength)
• Transverse waves: oscillation perpendicular to direction of travel (light, water waves, EM waves)
• Longitudinal waves: oscillation parallel to direction of travel (sound waves). Compressions and rarefactions.

The Electromagnetic Spectrum

• All electromagnetic (EM) waves: travel at the speed of light (3 × 10⁸ m/s) in a vacuum, are transverse, can travel through a vacuum, transfer energy
• The spectrum (lowest frequency/longest wavelength → highest frequency/shortest wavelength):
• Radio waves: TV/radio broadcasting, communication
• Microwaves: satellite communication, cooking (microwave ovens)
• Infrared (IR): thermal imaging, TV remotes, optical fibres, heating
• Visible light: the only part the human eye detects; ROYGBIV (red to violet)
• Ultraviolet (UV): causes sunburn and skin cancer; used in UV-curing, fluorescent lighting, tanning
• X-rays: medical imaging (bone fractures, dental), airport security
• Gamma rays: emitted by radioactive nuclei; sterilising medical equipment, cancer radiotherapy, killing bacteria in food

Reflection, Refraction, and Absorption

• Reflection: angle of incidence = angle of reflection (both measured from the normal)
• Refraction: waves change speed when entering a different medium → change direction (unless hitting the boundary perpendicularly). Light slows in denser media (glass, water) and bends towards the normal.
• Total internal reflection: at or beyond the critical angle, light is entirely reflected inside the denser medium. Basis of optical fibres (fibre-optic cables for internet and medical endoscopes).

Motion & Forces

Describing Motion

• Distance: total length of path travelled (scalar — no direction)
• Displacement: straight-line distance from start to finish in a given direction (vector)
• Speed: distance ÷ time (scalar)
• Velocity: displacement ÷ time (vector — has direction)
• Acceleration: change in velocity ÷ time. a = (v−u)/t. Vector quantity. Deceleration = negative acceleration.

Distance–Time and Velocity–Time Graphs

• Distance-time graph: gradient = speed. Horizontal line = stationary. Curve = changing speed (acceleration).
• Velocity-time graph: gradient = acceleration. Horizontal line = constant velocity. Area under the graph = displacement.

Newton's Laws

• Newton's 1st Law: an object remains at rest or in uniform motion unless acted on by a resultant force. (Inertia)
• Newton's 2nd Law: F = ma. Resultant force (N) = mass (kg) × acceleration (m/s²). A greater force produces a greater acceleration; a greater mass reduces acceleration for the same force.
• Newton's 3rd Law: for every action there is an equal and opposite reaction. The two forces act on different objects (e.g. the foot pushes the ground backward; the ground pushes the foot forward).

Weight, Gravity, and Terminal Velocity

• Weight (W) = mass (m) × gravitational field strength (g). W = mg. On Earth, g ≈ 9.8 N/kg.
• Objects fall freely under gravity accelerating at g, but air resistance increases with speed
• Terminal velocity: when drag (air resistance) = weight, resultant force = 0 → no further acceleration → constant speed (terminal velocity). Parachutes increase drag to lower terminal velocity.

Atomic Structure & Radiation

Structure of the Atom

• Atom: nucleus (protons + neutrons) surrounded by electrons in shells/energy levels
• Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass ~0 (negligible)
• Atomic number (proton number, Z): number of protons. Defines the element.
• Mass number (A): total number of protons + neutrons
• Isotopes: atoms of the same element with the same number of protons but different numbers of neutrons (different mass numbers). Same chemical properties (same electron arrangement), different physical properties.
• Ions: atoms that have gained or lost electrons, giving them a charge

Radioactive Decay

• Radioactivity: unstable nuclei emit radiation to become more stable. Spontaneous and random — cannot be predicted for an individual nucleus.
• Alpha (α) radiation: a helium nucleus (2 protons, 2 neutrons). Mass 4, charge +2. Stopped by paper or a few cm of air. Highly ionising. Dangerous if ingested.
• Beta (β) radiation: a fast electron emitted when a neutron changes to a proton. Mass ~0, charge −1. Stopped by a few mm of aluminium. Moderately ionising.
• Gamma (γ) radiation: a high-energy electromagnetic wave (no particle). No mass, no charge. Stopped by many cm of lead or several metres of concrete. Least ionising per unit distance.

Half-Life

• Half-life: the time taken for half the radioactive nuclei in a sample to decay
• After 1 half-life: 50% remains. After 2 half-lives: 25%. After 3: 12.5%. After n half-lives: (½)ⁿ remains.
• Carbon dating: uses carbon-14 (half-life ~5,730 years) to date organic materials up to ~50,000 years old
• Medical uses: short half-life isotopes used as tracers (e.g. technetium-99m, half-life 6 hours) — enough time for imaging, but decays quickly to minimise patient exposure
• Nuclear power: uranium-235 undergoes fission (splitting) releasing enormous energy to generate electricity