Science: Science You Are Expected to Know

This is Unit Twenty-Two of ACT Preparation: science you are expected to know. Unit twenty-one judged explanations against evidence; this unit supplies the introductory facts that some science questions assume. One rule governs it: background knowledge supports the passage and never overrides it. By the end you will be able to explain the basics of life science, chemistry, physics, and earth and space science. And to combine that knowledge with a passage's data.

Eight entries on life science.

The cell.

ACT 2101 The Cell

The cell is the smallest unit of a living thing. All cells share a membrane, cytoplasm, DNA and ribosomes; eukaryotic cells also have a nucleus and organelles, such as mitochondria, which make ATP. Bacteria lack a nucleus.

Photosynthesis.

ACT 2102 Photosynthesis

Photosynthesis uses carbon dioxide, water and light to make glucose and oxygen, capturing the sun's energy in compounds nearly all life depends on. Carbon dioxide and water go in; glucose and oxygen come out.

Cellular respiration.

ACT 2103 Cellular Respiration

Cellular respiration makes ATP from the energy in glucose; in mitochondria it uses oxygen and releases carbon dioxide. It runs roughly opposite to photosynthesis, which is why energy-hungry muscle cells are packed with mitochondria.

DNA and genes.

ACT 2104 DNA and Genes

DNA is the hereditary material, organized into chromosomes. Genes are units of inheritance, and alleles are versions of a gene, such as a black or a red allele for beetle wing color. Each individual carries two alleles, one from each parent.

Dominant and recessive.

ACT 2105 Dominant and Recessive Alleles

With two different alleles, the dominant one is expressed and the recessive one hidden; a recessive trait shows only with two recessive alleles. Two identical alleles make an individual homozygous; two different, heterozygous.

A Punnett square, step by step.

ACT 2106 Predicting Offspring With a Punnett Square

Write each parent's alleles, capital for dominant, fill a two-by-two grid with one allele from each, and count: each box is one quarter. Two heterozygous beetles give three black to one red; crossing heterozygous with homozygous recessive gives half and half. The ratios are expected proportions, not guarantees, much as four coin flips can all land heads.

Natural selection.

ACT 2107 Natural Selection

Natural selection makes alleles that help survival or reproduction more common over time: darker beetles escaping birds on dark bark spread the dark allele. Selection acts only on heritable traits, not ones acquired during life.

Food webs.

ACT 2108 Food Webs and Energy Flow

A food chain passes energy from producers to consumers, and a food web joins many chains. Energy is lost as heat at each level, so grass, rabbit, fox each pass on less than they received.

Eight entries on chemistry.

Atoms and elements.

ACT 2109 Atoms, Elements and Compounds

An atom is the smallest unit of an element, with protons and neutrons in its nucleus and electrons around it; the number of protons, the atomic number, identifies the element. Eight protons means oxygen. A compound, such as water, can be broken into elements.

Reactions and mass.

ACT 2110 Chemical Reactions and Conservation of Mass

In a chemical reaction, reactants become products, and total mass stays the same. Ten grams of fuel and forty of oxygen yield fifty grams of products, counting the gases. Mass that seems to vanish in burning has escaped as gas.

Reaction rates.

ACT 2111 Reaction Rates

Reactions usually run faster at higher temperature, at higher concentration, with more exposed surface area, and with a catalyst: crushed antacid tablets fizz faster than whole ones. Rate is how fast products form, not how much.

States of matter.

ACT 2112 States of Matter and Phase Changes

A solid keeps its shape, a liquid takes its container's shape, and a gas fills its container. Melting and boiling are physical changes: a phase change alters the state, not the substance.

Density.

ACT 2113 Density

Density is mass divided by volume: a forty-gram block of fifty cubic centimeters has a density of 0.8 grams per cubic centimeter. Density does not depend on sample size; half the block has the same density.

Solutions.

ACT 2114 Solutions and Concentration

A solution is a uniform mixture of a solute in a solvent, and its concentration measures how much solute is in a given amount: twenty grams of salt in 100 milliliters is twice as concentrated as ten. Adding solvent dilutes it.

Density and concentration problems, step by step.

ACT 2115 Density and Concentration Problems

Write the defining ratio, substitute the known quantities with units, and solve, checking the units. A liquid of 1.2 grams per milliliter has a mass of 300 grams in 250 milliliters. Fifteen grams of sugar in 300 milliliters is 0.05 grams per milliliter, and diluted to 600, half that. Volume is mass divided by density, not times; the units catch the slip.

Acids and bases.

ACT 2116 Acids, Bases and pH

The pH scale measures how acidic or basic a solution is: at twenty-five degrees Celsius, below seven is acidic, seven neutral, above seven basic. The neutral point shifts at other temperatures.

Eight entries on physics.

Motion.

ACT 2117 Speed, Velocity and Acceleration

Speed is how fast; velocity adds direction; acceleration is the rate velocity changes. A car going from zero to twenty meters per second in five seconds accelerates at four meters per second each second. Constant speed around a curve is still acceleration.

Newton's laws.

ACT 2118 Newton's Laws of Motion

An object stays at rest or in steady motion unless a net force acts; acceleration grows with force and shrinks with mass; and forces come in equal, opposite pairs. The same push accelerates an empty cart more than a loaded one. The pairs act on different objects, so they do not cancel.

Energy.

ACT 2119 Kinetic and Potential Energy

Kinetic energy, one half m v squared, grows with the square of speed, so doubling speed quadruples it. Gravitational potential energy depends on height. Total energy is constant; it only changes form.

Following energy, step by step.

ACT 2120 Following Energy Through a System

List the forms of energy at the start and the end, and account for the change, since the total is constant. A ball with ten joules of potential energy at the top of a ramp and nine of kinetic energy at the bottom turned one joule into heat through friction. Energy that seems lost has changed form.

Waves.

ACT 2121 Waves

A wave's speed equals its wavelength times its frequency: two meters at three cycles per second travels six meters per second. For a fixed speed, higher frequency means shorter wavelength.

Circuits.

ACT 2122 Electric Circuits

Current is the flow of charge, driven by voltage, and by Ohm's law current equals voltage divided by resistance: twelve volts across four ohms drives three amperes. More resistance means less current.

Heat and temperature.

ACT 2123 Heat and Temperature

Temperature is related to the average kinetic energy of a substance's particles; heat is energy moving because of a temperature difference, from hotter to colder, as from a hot spoon into cool water.

Gravity.

ACT 2124 Gravity and Orbits

Two masses attract with a force that grows with their masses and weakens with the square of their distance, so doubling the distance cuts the pull to a quarter. Gravity is mutual, and it holds planets in elliptical orbits.

Six entries on earth and space, and on using background knowledge in a passage.

The seasons.

ACT 2125 The Seasons

Seasons come from the tilt of Earth's axis, about 23.5 degrees: in June the Northern Hemisphere leans toward the sun and has summer, while the south has winter. Seasons do not come from Earth's distance from the sun.

The solar system.

ACT 2126 The Solar System

The four inner planets, Mercury, Venus, Earth and Mars, are small and rocky; the four outer, Jupiter, Saturn, Uranus and Neptune, are giants of light ices, liquids and gases.

The water cycle.

ACT 2127 The Water Cycle

Driven by the sun, water evaporates, condenses into clouds, and falls as rain or snow. Ice can also pass straight into vapor, by sublimation.

The rock cycle.

ACT 2128 The Rock Cycle

Igneous rock forms as magma cools, sedimentary rock as buried sediment is cemented, and metamorphic rock under deep heat and pressure; each can become the others. Slow cooling underground makes intrusive rock; fast cooling at the surface, volcanic.

Plate tectonics.

ACT 2129 Plate Tectonics

Earth's outer layer is broken into moving plates, with boundaries that move apart, divergent; together, convergent; or side by side, transform. Name a boundary by the plates' relative motion.

Background knowledge in a passage, step by step.

ACT 2130 Using Background Knowledge in a Passage

Identify the concept the question touches, recall the core fact, and combine it with the passage's data. An invented reaction that finished in eighty seconds at twenty degrees should be faster at forty, so a choice of 120 seconds contradicts background knowledge. Background knowledge supports the passage; it never overrides it, and if the data show a different pattern, follow the data.

Look at what you can now do. You can explain how cells capture and use energy, how traits are inherited, and how selection and food webs work. You can explain atoms, reactions, density, concentration and pH, checking units as you go. You can explain motion, force, energy, waves, circuits, heat and gravity. You can explain the seasons, the planets, the water and rock cycles, and the plates. And you can put that knowledge to work beside a passage's data, letting the data lead. Unit twenty-three brings the course to test day: pacing, guessing and the full method.