Science better

1. Biology

1.1 Biology Paper 1

1.1 Cell Biology
Eukaryotic cells (animal and plant cells) contain a nucleus; prokaryotic cells (bacterial cells) lack a nucleus and store DNA as single loops or plasmids.
Mitosis produces two identical diploid daughter cells for growth and repair.
Stem cells are undifferentiated cells found in human embryos and adult bone marrow.
Magnification formula:

$\text{Magnification} = \frac{\text{Image Size}}{\text{Real Size}}$

Diffusion moves molecules down a concentration gradient. Active transport requires energy to move substances against a concentration gradient.
1.2 Organisation
Enzymes are biological catalysts that lower activation energy via the lock and key model.
Heart structure: Four chambers (right atrium, right ventricle, left atrium, left ventricle).
Blood vessels: Arteries (thick elastic walls), veins (valves, thin walls), capillaries (one cell thick).
Plant transport: Xylem transports water and mineral ions; phloem transports dissolved sugars by translocation.
1.3 Infection and Response
Pathogens include viruses, bacteria, protists, and fungi.
Non-specific defences: Skin, stomach acid (

$HCl_{(aq)}$

), nasal hairs, and cilia.

White blood cells protect via phagocytosis, antibody production, and antitoxin production.
Vaccines contain inactive or dead pathogens to trigger antibody production and memory cell formation.
1.4 Bioenergetics
Photosynthesis is an endothermic reaction:

$6CO2 + 6H2O \rightarrow C6H{12}O6 + 6O2$

Aerobic respiration is an exothermic reaction:

$C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O$

Anaerobic respiration in muscles produces lactic acid; in yeast (fermentation), it produces ethanol and carbon dioxide.

1.2 Biology Paper 2

1.5 Homeostasis and Response
Homeostasis maintains optimal internal conditions for enzyme action and cell functions.
Reflex arc sequence: Receptor

$\rightarrow$

Sensory Neurone

$\rightarrow$

Relay Neurone

$\rightarrow$

Motor Neurone

$\rightarrow$

Effector.

Blood glucose control: Pancreas releases insulin to lower glucose levels, storing it as glycogen in the liver and muscles.
1.6 Inheritance, Variation, and Evolution
DNA is a double helix polymer storing genetic code.
Meiosis forms four genetically non-identical haploid gametes.
Alleles can be dominant or recessive; homozygous (identical alleles), heterozygous (different alleles).
Natural selection: Advantageous alleles produced by mutations are passed on to offspring.
1.7 Ecology
Ecosystems consist of biotic (living) and abiotic (non-living) factors.
Carbon cycle returns carbon to the atmosphere via respiration and combustion, and removes it via photosynthesis.

2. Chemistry

2.1 Chemistry Paper 1

2.1 Atomic Structure and the Periodic Table
Atoms consist of protons (

$+1$

charge), neutrons (

$0$

charge), and electrons (

$-1$

charge).

Group 1 (Alkali metals) reactivity increases down the group; Group 7 (Halogens) reactivity decreases down the group; Group 0 (Noble gases) are unreactive.
2.2 Bonding, Structure, and Properties of Matter
Ionic bonding: Transfer of electrons between metals and non-metals forming strong electrostatic attraction.
Covalent bonding: Sharing of pairs of electrons between non-metals.
Metallic bonding: Array of positive metal ions in a sea of delocalised electrons.
Giant covalent structures: Diamond (4 bonds per carbon), Graphite (3 bonds per carbon, delocalised electrons conduct electricity).
2.3 Quantitative Chemistry
Conservation of mass: Total mass of reactants equals total mass of products.
Relative formula mass (

$M_r$

) is the sum of relative atomic masses (

$A_r$

Mole equation:

$\text{Moles} = \frac{\text{Mass}}{M_r}$

2.4 Chemical Changes
Oxidation is loss of electrons; reduction is gain of electrons.
Reaction of acids with metals produces salt and hydrogen gas.
Neutralisation ionic equation:

$H^{+}{(aq)} + OH^{-}{(aq)} \rightarrow H2O{(l)}$

Electrolysis breaks down ionic compounds using electricity.
2.5 Energy Changes
Exothermic reactions release energy to surroundings; endothermic reactions absorb energy from surroundings.

2.2 Chemistry Paper 2

2.6 Rate and Extent of Chemical Change
Rate of reaction is affected by concentration, temperature, surface area, and catalysts.
Collision theory states particles must collide with energy greater than or equal to activation energy.
Reversible reactions reach dynamic equilibrium in a closed system:

$A + B \rightleftharpoons C + D$

2.7 Organic Chemistry
Crude oil is a finite mixture of hydrocarbons, mostly alkanes (

$Cn H{2n+2}$

Fractional distillation separates hydrocarbons by boiling point.
Cracking breaks down long-chain alkanes into shorter alkanes and alkenes.
2.8 Chemical Analysis
Retention factor:

$R_f = \frac{\text{Distance moved by spot}}{\text{Distance moved by solvent}}$

Gas tests: Oxygen (

$O_2$

, relights glowing splint), Hydrogen (

$H_2$

, squeaky pop), Carbon dioxide (

$CO_2$

, limewater turns cloudy), Chlorine (

$Cl_2$

, bleaches damp litmus paper).

2.9 Chemistry of the Atmosphere and Using Resources
Atmosphere composition: Approximately

$78\%$

nitrogen,

$21\%$

oxygen, and small amounts of carbon dioxide, water vapour, and noble gases.

Potable water is safe to drink; produced by filtration and sterilisation.

3. Physics

3.1 Physics Paper 1

3.1 Energy
Kinetic energy:

$E_k = \frac{1}{2} m v^2$

Gravitational potential energy:

$E_p = m g h$

Elastic potential energy:

$E_e = \frac{1}{2} k e^2$

Power:

$P = \frac{E}{t} = \frac{W}{t}$

Efficiency:

$\text{Efficiency} = \frac{\text{Useful Output Energy}}{\text{Total Input Energy}}$

Law of Conservation of Energy: Energy can be transferred usefully, stored, or dissipated, but cannot be created or destroyed.
Thermal conductivity measures how rapidly a material conducts energy; higher conductivity increases the rate of thermal transfer.
3.2 Electricity
Charge flow:

$Q = I t$

Potential difference:

$V = I R$

Ohmic conductors have constant resistance at constant temperature; filament lamps increase resistance as temperature rises; diodes allow current in one direction only.
Series circuits: Current is constant throughout; potential difference is shared; total resistance is the sum of component resistances (

$R{\text{total}} = R1 + R_2$

Parallel circuits: Potential difference is equal across branches; total current is the sum of branch currents; adding parallel resistors reduces overall resistance.
Electrical power:

$P = I V = I^2 R$

Energy transferred:

$E = P t = Q V$

Mains electricity in the UK: Alternating current (AC) at

$230\,V$

and

$50\,Hz$

. Wiring: Live wire (brown,

$230\,V$

), Neutral wire (blue,

$0\,V$

), Earth wire (green/yellow stripe, safety feature at

$0\,V$

National Grid: Step-up transformers increase potential difference for transmission to reduce energy loss as heat; step-down transformers lower voltage to

$230\,V$

for domestic safety.

3.3 Particle Model of Matter
Density:

$\rho = \frac{m}{V}$

Specific heat capacity: Energy required to raise temperature of

$1\,kg$

of a substance by

$1\,^\circ\text{C}$

$\Delta E = m c \Delta \theta$

Specific latent heat: Energy required to change the state of

$1\,kg$

of a substance without a change in temperature:

$E = m L$

Internal energy is the total kinetic and potential energy stored by particles in a system.
Gas pressure results from collisions of gas particles with container walls; increasing temperature increases kinetic energy and pressure at constant volume.
Boyle's Law for fixed mass of gas at constant temperature:

$p V = \text{constant}$

3.4 Atomic Structure
Rutherford's alpha-scattering experiment showed that atoms have a small, dense, positively charged nucleus surrounded by electrons.
Alpha particles (

$\alpha$

, helium nuclei, highly ionising, stopped by paper), beta particles (

$\beta$

, fast-moving electrons, moderately ionising, stopped by aluminium), gamma rays (

$\gamma$

, electromagnetic waves, weakly ionising, absorbed by thick lead).

Half-life is the time taken for half the radioactive nuclei in a sample to decay or for the activity to halve.
Contamination is the unwanted presence of radioactive atoms on materials; irradiation is exposure to radiation without making the object radioactive.

3.2 Physics Paper 2

3.5 Forces
Scalar quantities have magnitude only; vector quantities have magnitude and direction.
Weight:

$W = m g$

; Work done:

$W = F s$

; Hooke's Law:

$F = k e$

Distance is scalar; displacement is vector. Speed is scalar; velocity is vector.
Acceleration:

$a = \frac{\Delta v}{t}$

; Uniform acceleration equation:

$v^2 - u^2 = 2 a s$

Newton's First Law: An object remains at rest or at constant velocity unless acted upon by a resultant force.
Newton's Second Law:

$F = m a$

Newton's Third Law: When two objects interact, they exert equal and opposite forces on each other.
Stopping distance = Thinking distance + Braking distance. Thinking distance is affected by speed and reaction time (fatigue, drugs, alcohol). Braking distance is affected by speed, road conditions, and vehicle condition.
Momentum:

$p = m v$

. In a closed system, total momentum before an event equals total momentum after (Conservation of Momentum).

3.6 Waves
Transverse waves have oscillations perpendicular to energy transfer (e.g., light, water waves); Longitudinal waves have oscillations parallel to energy transfer (e.g., sound waves).
Wave speed equation:

$v = f \lambda$

Period of a wave:

$T = \frac{1}{f}$

Electromagnetic spectrum (increasing frequency, decreasing wavelength): Radio, Microwave, Infrared, Visible light, Ultraviolet, X-rays, Gamma rays. All EM waves travel at

$3 \times 10^8\,m/s$

in a vacuum.

Refraction occurs when waves change speed and direction as they cross a boundary between media of different densities.
3.7 Magnetism and Electromagnetism
Permanent magnets produce their own magnetic field; induced magnets become magnetic when placed in a field. Field lines point from North to South.
Current through a wire creates a magnetic field. Solenoids enhance field strength; adding an iron core forms an electromagnet.
Motor Effect: A current-carrying conductor in a magnetic field experiences a force:

$F = B I L$

. Fleming's Left-Hand Rule determines force direction (Thumb = Motion/Force, First Finger = Magnetic Field, Second Finger = Current).

Generator Effect: Inducing a potential difference in a conductor by moving it through a magnetic field or changing the field around it.
Transformers use AC in the primary coil to produce a changing magnetic field in an iron core, inducing an AC potential difference in the secondary coil:

$\frac{Vp}{Vs} = \frac{Np}{Ns}$