Science 30 cheat sheet

Stimulus-response pathway: receptor detects a change, sensory neuron carries the impulse, CNS integrates it, motor neuron transmits it, effector (muscle or gland) responds.

Negative feedback restores a variable toward its set point (e.g., insulin lowers high blood glucose); positive feedback amplifies change away from the set point (e.g., oxytocin in labour).

Thermoregulation: vasodilation and sweating dissipate heat; vasoconstriction, shivering, and piloerection conserve and generate heat.

Osmoregulation: ADH increases water reabsorption in kidney tubules when blood solute concentration rises; aldosterone increases sodium reabsorption.

Blood glucose: insulin promotes cellular uptake and glycogen formation; glucagon promotes glycogen breakdown and raises blood glucose.

Neuron resting potential is maintained by the Na+/K+ pump (3 Na+ out, 2 K+ in); an action potential is depolarization past threshold followed by repolarization.

Synaptic transmission is chemical: neurotransmitter released from the presynaptic terminal crosses the cleft and binds receptors on the postsynaptic membrane; transmission is one-way.

Homeostasis depends on a sensor, a control centre, and an effector; failure of any component produces a homeostatic disorder.

Adaptation vs acclimatization: adaptation is a heritable population-level change over generations; acclimatization is a reversible physiological adjustment within one organism's lifetime.

pH = -log[H3O+]; each unit of pH represents a tenfold change in hydronium ion concentration. Neutral water at 25 °C has pH 7.

Acid deposition forms when SO2 and NOx react with water and oxygen to give H2SO4 and HNO3; wet deposition is rain, snow, or fog, dry deposition is gas or particle settling.

Liming neutralizes acidified water or soil: CaCO3 + 2H+ → Ca2+ + H2O + CO2.

Eutrophication sequence: nutrient enrichment (N, P) → algal bloom → decomposition consumes dissolved oxygen → hypoxia and fish kills.

BOD (biochemical oxygen demand) measures oxygen consumed by microbes decomposing organic matter; higher BOD indicates greater organic pollution.

Greenhouse mechanism: gases absorb outgoing infrared radiation and re-emit it, warming the lower atmosphere; key gases are CO2, CH4, N2O, and water vapour.

Photochemical smog requires sunlight, NOx, and volatile organic compounds; ground-level ozone is its principal harmful product.

Biomagnification: persistent, fat-soluble substances (e.g., PCBs, methylmercury) increase in concentration at each higher trophic level.

Catalytic converters convert CO, unburned hydrocarbons, and NOx into CO2, H2O, and N2; they do not remove CO2.

c = fλ, where c = 3.00 × 10^8 m/s; E = hf, where h = 6.63 × 10^-34 J·s. Higher frequency means higher photon energy and shorter wavelength.

Order of the spectrum by increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray.

Visible light spans about 400 nm (violet) to 700 nm (red); the order is red, orange, yellow, green, blue, indigo, violet by decreasing wavelength.

Refraction: light bends toward the normal entering a denser medium (n = c/v) and away from the normal exiting it; total internal reflection occurs above the critical angle.

Mirrors and lenses: real images form where rays actually converge and can be projected; virtual images form where rays only appear to diverge.

Ionizing radiation (UV, X-ray, gamma) carries enough photon energy to remove electrons from atoms; radio and microwave are non-ionizing.

Inverse-square law: intensity = P/(4πr^2), so doubling distance from a point source reduces intensity to one quarter.

Absorption and emission spectra are characteristic of each element; a line spectrum identifies composition of a distant source.

Ozone in the stratosphere absorbs most UV-B and UV-C; CFCs release chlorine atoms that catalytically destroy ozone.

First law: energy is conserved and only transformed; second law: each transformation increases total entropy, so no conversion is 100% efficient.

Efficiency = useful output energy / input energy × 100%; a typical coal plant is about 35–40% efficient, the remainder is waste heat.

Energy quality ranks electricity and chemical fuels above low-temperature heat; low-quality energy cannot be converted back to high quality without an external energy input.

Fossil fuel combustion: C + O2 → CO2 and CH4 + 2O2 → CO2 + 2H2O, both exothermic; CO2 released per unit energy is highest for coal and lowest for natural gas.

Renewable sources: solar photovoltaic converts light directly to electricity; wind and hydro drive turbines; geothermal and biomass use heat or combustion.

Energy return on investment (EROI) = energy delivered / energy invested; a source is only a net energy contributor when EROI > 1.

Reducing demand through insulation, efficient lighting (LEDs), and heat recovery lowers total primary energy use more cheaply than adding equivalent supply.

Nuclear fission splits heavy nuclei, releasing energy from mass defect (E = mc^2); waste remains hazardous for thousands of years.

Carbon footprint accounting converts all activity into CO2-equivalent using global warming potentials; methane's 100-year GWP is about 25–30 times CO2.