Every living thing is made of cells. There are two main types: prokaryotic (no nucleus, like bacteria) and eukaryotic (has a nucleus, like plant and animal cells).

Feature	Plant Cell	Animal Cell
Cell Wall	Yes	No
Chloroplasts	Yes	No
Large Vacuole	Yes	No (small)
Nucleus	Yes	Yes
Mitochondria	Yes	Yes

Photosynthesis happens in chloroplasts (plants, sunlight in). Cellular respiration happens in mitochondria (all cells, energy out). They are essentially reverse reactions.

DNA (deoxyribonucleic acid) is a double helix molecule found in the nucleus. It carries the instructions for building and running every living organism. A gene is a specific segment of DNA that codes for a trait. Humans have about 20,000 genes on 23 pairs of chromosomes.

A Punnett square predicts the probability of offspring traits. Cross a carrier (Bb) with another carrier (Bb):

Humans have 46 chromosomes (23 pairs). The 23rd pair determines sex: XX = female, XY = male. Sex-linked traits are carried on the X chromosome. Because males only have one X, recessive traits on it (like color blindness) show up more often in males.

Mutations are changes in DNA sequence. They can be caused by radiation, chemicals, or copying errors. Most are harmless; some affect traits; a few can cause disease.

Only about 10% of energy passes from one trophic level to the next. The other 90% is lost as heat. This is why large predators are rare - there is not enough energy at the top to support many of them.

Major biomes include: tropical rainforest (hot, wet, most biodiverse), temperate forest, grassland/savanna, desert (low precipitation), tundra (cold, permafrost), and taiga (coniferous forest).

Symbiosis = two species living in close relationship. Types: mutualism (both benefit), commensalism (one benefits, other unaffected), parasitism (one benefits, other harmed).

Carrying capacity = the maximum population an environment can support given available resources. Biodiversity = variety of species in an ecosystem. More biodiversity = more stable ecosystem.

The adult human skeleton has 206 bones. Functions: structure, protection (skull protects brain), movement, blood cell production (in bone marrow), mineral storage.

Voluntary muscles are under conscious control (moving your arm). Involuntary muscles work automatically (heart, digestive muscles). Tendons connect muscle to bone. Ligaments connect bone to bone.

The heart has 4 chambers: right atrium, right ventricle, left atrium, left ventricle. The left side pumps oxygenated blood to the body; the right side pumps deoxygenated blood to the lungs.

Arteries carry blood away from the heart (usually oxygenated). Veins carry blood back to the heart (usually deoxygenated). Capillaries are tiny vessels where exchange of oxygen and nutrients happens with tissues.

Air enters through the nose/mouth, travels down the trachea, into the bronchi, and reaches the alveoli (tiny air sacs in the lungs). Gas exchange happens here: O₂ enters the blood, CO₂ leaves the blood and is exhaled.

The diaphragm is the dome-shaped muscle below the lungs. When it contracts, it flattens and increases lung volume, drawing air in.

Path of food: Mouth (chewing + saliva begins digestion) → Esophagus (muscular tube) → Stomach (acid + enzymes break food down) → Small Intestine (most nutrient absorption) → Large Intestine (water absorption, compacting waste) → elimination.

Central Nervous System (CNS): brain + spinal cord. Peripheral Nervous System (PNS): all nerves outside CNS. Neurons are nerve cells that transmit signals electrically and chemically.

The endocrine system uses hormones (chemical messengers) released by glands into the bloodstream. Key glands: pituitary (master gland, controls others), thyroid (metabolism), adrenal (stress response, adrenaline).

The immune system defends against pathogens (bacteria, viruses, fungi). White blood cells are the main defenders. Antibodies are proteins that recognize and neutralize specific pathogens. Vaccines work by introducing a harmless version of a pathogen so the immune system learns to recognize it before a real infection.

Charles Darwin's theory of natural selection works through four key steps:

An adaptation is any trait that helps an organism survive and reproduce in its environment (camouflage, sharp claws, drought resistance). "Survival of the fittest" does not mean strongest - it means best adapted to the current environment.

Speciation occurs when populations of one species become so different (often due to geographic isolation) that they can no longer interbreed. Darwin observed this with finches in the Galapagos Islands - different beak shapes evolved to match different food sources.

Mass is the amount of matter in an object (kg), same everywhere. Weight is the gravitational force on an object (N), changes with gravity. You weigh less on the Moon, but your mass is the same.

Friction is a force that opposes motion between surfaces in contact. Static friction (holds things still) is greater than kinetic friction (opposes sliding motion).

Gravity is a force of attraction between objects with mass. It depends on the masses of both objects and the distance between them. All objects fall at the same rate in a vacuum regardless of mass (Galileo's discovery).

Kinetic energy is energy of motion. Potential energy is stored energy due to position or condition. A roller coaster at the top of a hill has maximum PE and minimum KE. At the bottom, minimum PE and maximum KE.

Energy cannot be created or destroyed - it can only change form. Total energy in a closed system remains constant. The universe's total energy is fixed.

Common energy transformations: chemical → thermal (burning wood), electrical → light (lightbulb), gravitational PE → KE (falling object).

No machine is 100% efficient because some energy is always lost as heat. A car engine is typically 20-40% efficient.

Transverse waves: particles move perpendicular to wave direction. Light waves, water waves. Can travel through a vacuum.

Longitudinal waves: particles move parallel to wave direction. Sound waves. Require a medium (cannot travel in space).

Speed of light = 3 x 10⁸ m/s (300,000 km/s). The fastest speed in the universe. Sound travels about 343 m/s in air. This is why you see lightning before you hear thunder.

From lowest frequency to highest (lowest energy to highest energy): Visible light is a tiny portion of the spectrum. Gamma rays are the most energetic and dangerous. Radio waves have the longest wavelength.

Reflection: wave bounces off a surface (mirrors, echoes). The angle of incidence equals the angle of reflection.
Refraction: wave bends when it moves from one medium to another (why a straw looks bent in water, why lenses work).
Diffraction: wave bends around obstacles or through openings. Explains why you can hear sound around corners.

Atoms have protons (+), neutrons (neutral), and electrons (-). When electrons move, we get electric current. Opposite charges attract; like charges repel.

Conductors allow electrons to flow easily (copper, gold, most metals). Insulators resist electron flow (rubber, glass, wood, plastic).

Voltage (V) - electrical pressure/potential difference, unit: Volts. Current (I) - flow of charge, unit: Amps (A). Resistance (R) - opposition to current flow, unit: Ohms (Ω).

Series circuit: components connected in one loop. Current is the same throughout. If one bulb burns out, all go out. Total resistance = sum of all resistances.

Parallel circuit: components connected in separate branches. Voltage is the same across each branch. If one branch fails, others continue working. Used in home wiring.

Every magnet has a north and south pole. Opposite poles attract; like poles repel. A magnetic field is the region around a magnet where magnetic forces act. Electric current creates a magnetic field (electromagnetism). Generators convert mechanical energy to electrical energy using magnetic fields.

Most earthquakes and volcanoes occur at plate boundaries. The Ring of Fire is a zone of intense seismic and volcanic activity around the Pacific Ocean. Earthquakes are measured on the Richter scale (magnitude) and produce seismic waves (P-waves and S-waves). The point of origin underground is the focus; the point on the surface directly above is the epicenter.

From lowest to highest: Troposphere (weather happens here, 0-12 km), Stratosphere (ozone layer here, 12-50 km), Mesosphere (meteors burn up here, 50-80 km), Thermosphere (auroras, space shuttles orbit here, 80-700 km), Exosphere (outer limit, merges with space).

The ozone layer in the stratosphere absorbs harmful UV radiation from the Sun.

Inner rocky planets (Mercury, Venus, Earth, Mars): small, dense, solid surfaces. Outer gas giants (Jupiter, Saturn): mostly hydrogen and helium, no solid surface. Ice giants (Uranus, Neptune): composed of water, methane, and ammonia ices. Pluto is now classified as a dwarf planet.

Seasons are caused by Earth's 23.5-degree axial tilt, not distance from the Sun. When the Northern Hemisphere tilts toward the Sun, it is summer there.

Moon phases occur because we see different portions of the sunlit side as the Moon orbits Earth (~29.5 days).

Solar eclipse: Moon passes between Earth and Sun, blocking sunlight. Lunar eclipse: Earth passes between Sun and Moon, casting a shadow on the Moon.

Asteroids: rocky bodies mostly in the asteroid belt between Mars and Jupiter. Comets: icy bodies with elliptical orbits; develop a tail as they near the Sun. Meteoroids become meteors when they enter Earth's atmosphere; those that reach the ground are meteorites.

Weather = short-term atmospheric conditions (today's temperature, tomorrow's rain). Climate = the long-term average weather patterns of a region over decades.

Factors affecting climate: latitude (closer to equator = warmer), altitude (higher elevation = cooler), ocean currents (warm or cold currents moderate nearby climates), geography (mountains create rain shadows).

The greenhouse effect: Earth's atmosphere traps some of the Sun's heat (like a greenhouse). Greenhouse gases (CO₂, methane, water vapor) absorb and re-emit infrared radiation. This is natural and necessary; human-added greenhouse gases are enhancing it, driving climate change.

Tropical zone (near equator): hot and wet. Temperate zone (mid-latitudes): four seasons, moderate temperatures. Polar zones (high latitudes): cold year-round.

Cold front: cold air mass moves in and pushes under warm air. Brings sudden storms, then cooler temperatures. Shown on maps as a blue line with triangles.

Warm front: warm air mass moves in over cold air. Brings gradual precipitation. Shown as a line with semicircles.

Humidity = amount of water vapor in air. High air pressure = fair weather; low air pressure = stormy weather. Severe weather types: tornadoes, hurricanes, blizzards, thunderstorms.