GED Science Concept Review

Overview

This chapter covers how the human body works and stays healthy: body systems, homeostasis, nutrition, and disease. The GED Science test uses these ideas in its Human Health and Living Systems theme, often with a short passage, a food label, or a data table.

Body systems work together. Muscles pull on bones to move the body, and the nervous and endocrine systems send signals that control other organs. Homeostasis is the body's way of keeping inside conditions steady. Most of it works by negative feedback: the body senses a change and responds in the opposite direction, such as sweating when hot or releasing insulin when blood sugar rises.

Food gives the body energy and building materials. Carbohydrates are the main quick energy source, and fat stores the most energy per gram. Vitamins and minerals are needed in small amounts but contain no calories. Diseases are caused by pathogens (germs) that spread through the air, water, food, blood, or contact. They are prevented by methods such as vaccination, hand washing, sanitation, and protective equipment.

Overview

This chapter explains how living things get the energy they need. Plants capture energy from sunlight, and almost all living things release energy from food through cellular respiration.

In photosynthesis, plants and algae use light energy to turn carbon dioxide and water into glucose and oxygen. It takes place in chloroplasts, which contain the green pigment chlorophyll. In cellular respiration, cells use oxygen to break down glucose, releasing energy and giving off carbon dioxide and water. It takes place in the mitochondria.

The two processes are linked: each uses what the other produces. When oxygen is short, some cells, including human muscle cells during hard exercise, use fermentation, which releases a small amount of energy without oxygen and makes lactic acid in muscle. GED questions on this topic often describe a simple experiment, such as counting oxygen bubbles from a water plant under a lamp.

Overview

This chapter covers how energy and matter move through ecosystems and how populations affect one another. GED questions often show a food web or energy pyramid and ask what happens when one population changes.

Producers such as plants make their own food from sunlight. Consumers eat other organisms, and decomposers such as fungi and bacteria break down dead matter and return nutrients to the soil. In a food chain or food web, arrows point from the food to the organism that eats it, showing the direction energy flows. Only about 10% of the energy at one level passes to the next; the rest is used for life processes or lost as heat. That is why an energy pyramid gets narrower at the top.

Populations are limited by food, water, space, and predators. The largest population an area can support over time is its carrying capacity. Organisms also interact through symbiosis: in mutualism both benefit, in commensalism one benefits and the other is not affected, and in parasitism one benefits and the host is harmed. Invasive species, habitat destruction, and other disruptions can upset these relationships and cause local extinctions.

Overview

This chapter covers cells, the basic units of life, and how they are organized into larger structures. It also explains how cells divide.

The cell theory says that all living things are made of cells, the cell is the smallest unit of life, and new cells come only from existing cells. Inside a cell, the cell membrane controls what enters and leaves, the nucleus holds the DNA, ribosomes make proteins, and mitochondria release energy. Plant cells also have a cell wall and chloroplasts. Enzymes are proteins that speed up chemical reactions; they work best at a certain temperature, and too much heat changes their shape so they stop working well.

In many-celled organisms, similar cells form tissues, tissues form organs, and organs work together in organ systems. Cells divide by mitosis to grow and repair: one cell makes two identical cells. Meiosis makes sex cells (eggs and sperm) with half the usual number of chromosomes, 23 in humans instead of 46.

Overview

This chapter explains how traits are passed from parents to children. GED questions often ask you to complete a Punnett square, read a pedigree chart, or explain how genes and the environment work together.

DNA in chromosomes carries genes, the instructions for traits. Genetic information flows from DNA to RNA to protein, which is called the central dogma. Each person has two alleles (versions) of most genes, one from each parent. A dominant allele shows its trait even with one copy; a recessive trait shows only when both alleles are recessive. The genotype is the pair of alleles, such as Pp, and the phenotype is the trait you can see, such as purple flowers.

A Punnett square shows the possible offspring of two parents. A cross of two carriers (Pp × Pp) gives 1 PP : 2 Pp : 1 pp, so there is a 25% chance of the recessive trait. Mutations are changes in DNA that can create new alleles, and meiosis mixes alleles so that brothers and sisters differ. The environment can also affect how a gene is expressed; for example, the dark fur on a Siamese cat's cooler body parts depends on temperature.

Overview

This chapter covers how populations change over many generations and the evidence that living things share common ancestors. GED questions often describe a population over time or show a cladogram.

Natural selection needs inherited variation: individuals differ in traits that can be passed on. When some traits help individuals survive and reproduce in their environment, those traits become more common over generations. Examples include bacteria becoming resistant to antibiotics and dark peppered moths becoming more common on soot-darkened trees. When people choose which plants or animals breed, it is artificial selection.

Evidence of common ancestry includes similar bone structures in different species, such as the front limbs of humans, whales, and bats. A cladogram is a branching diagram of relationships; groups that share a more recent branch point are more closely related. When a population is split by a barrier, the groups can change until they can no longer interbreed, forming new species. This is speciation.

Overview

This chapter covers energy: its forms, how it changes from one form to another, how heat moves, and how waves carry energy. It is part of the Energy and Related Systems theme on the GED Science test.

Energy is not created or destroyed; it changes form. A raised object has gravitational potential energy, and a moving object has kinetic energy. A battery stores chemical energy that becomes electrical energy and then light and heat in a flashlight. Efficiency is the share of energy that becomes the useful form. Energy sources can be renewable, like sunlight and wind, or nonrenewable, like coal, oil, and natural gas.

Heat flows from hot to cold by conduction (direct contact), convection (moving liquid or gas), and radiation (waves that can cross empty space). Temperature is not the same as heat: more matter needs more heat for the same temperature change. Exothermic reactions release heat, and endothermic reactions absorb it.

Waves are described by frequency and wavelength, and wave speed = frequency × wavelength. The electromagnetic spectrum runs from radio waves (lowest frequency) through visible light to X-rays and gamma rays (highest). Ultraviolet light causes sunburn. Sound is a mechanical wave that needs matter to travel.

Overview

This chapter covers how things move and the forces that change their motion. On the GED test, a question gives you any formula it needs, so practice using a formula and reading motion from a table or graph.

Speed is distance divided by time, and acceleration is the change in speed divided by time. Momentum is mass times velocity, so a light object moving fast can have more momentum than a heavy object moving slowly. Newton's first law says objects keep their motion unless a force acts (inertia). The second law says force = mass × acceleration. The third law says every action has an equal and opposite reaction.

Mass is the amount of matter and stays the same everywhere; weight is the pull of gravity on that mass (weight = mass × g, with g about 9.8 m/s² on Earth). Without air resistance, all objects fall at the same rate. Friction acts against motion.

Work is force times distance, and power is work divided by time. Simple machines, such as levers, pulleys, ramps (inclined planes), wedges, screws, and wheels and axles, make work easier by trading force for distance. Mechanical advantage = output force ÷ input force.

Overview

This chapter covers what matter is made of, how it changes, and how chemical reactions are written. You do not need to memorize the periodic table, but you should recognize common element names and symbols.

Atoms contain protons (positive) and neutrons (no charge) in the nucleus, with electrons (negative) around it. A physical change, such as melting, boiling, or cutting, changes form but not the substance. A chemical change makes a new substance; signs include gas bubbles, a color change, heat or light, or a new solid.

Density is mass divided by volume. An object floats in a liquid if it is less dense than the liquid, and liquids that do not mix stack with the least dense on top. The volume of an odd-shaped solid can be found by how much it raises the water level.

A balanced chemical equation has the same number of each kind of atom on both sides, because mass is conserved: atoms are rearranged, not created or destroyed. The limiting reactant is the one that runs out first and sets how much product can form. In a solution, the solvent dissolves the solute. Most solids dissolve better in hotter solvents, and a saturated solution holds all the solute it can at that temperature.

Overview

This chapter connects Earth's systems with living things: how matter cycles, where our energy and resources come from, and how people prepare for natural hazards. Questions often use a data table or a short description of a town's plan.

Matter moves in cycles. In the water cycle, water evaporates, condenses into clouds, falls as precipitation, and flows back to the ocean. In the carbon cycle, photosynthesis removes carbon dioxide from the air, while respiration, burning, and volcanoes add it. In the nitrogen cycle, soil bacteria change nitrogen gas into forms plants can use. Fossil fuels formed over millions of years from buried ancient plants and sea organisms, so they are nonrenewable.

Renewable resources such as sunlight, wind, and trees are replaced naturally in a short time. Using a resource sustainably means not using it faster than it is replaced, such as pumping no more groundwater than rain refills. Natural hazards include earthquakes, hurricanes, floods, and volcanic eruptions. Mitigation reduces their harm, for example with stronger building codes, raised homes, levees, dunes, and wetlands that absorb storm water. Strong earthquakes are much rarer than weak ones.

Overview

This chapter covers Earth's major parts, the atmosphere, the oceans, and the solid Earth, and how they interact. Many questions ask you to read a table of measurements or explain a cause and effect.

The atmosphere is about 78% nitrogen and 21% oxygen. From the ground up, its layers are the troposphere (where weather happens), the stratosphere (which contains the ozone layer that absorbs much of the Sun's ultraviolet radiation), the mesosphere, and the thermosphere. Greenhouse gases such as carbon dioxide and water vapor trap heat near the surface; adding more of them warms the climate. Wind blows from high air pressure toward low air pressure.

The oceans hold salt water and move heat around the planet. Warm currents such as the Gulf Stream make nearby coasts milder. When water stays too warm, corals can bleach.

Inside Earth are the crust, mantle, outer core, and inner core. The crust is broken into tectonic plates, and most earthquakes and volcanoes happen near plate boundaries. Where plates move apart at mid-ocean ridges, new ocean crust forms. Weathering by wind, water, and ice breaks rock down over time.

Overview

This chapter covers Earth's place in space and how scientists know Earth's age. Questions often give a short table of planet data or describe rock layers.

The Sun is a star, one of billions in the Milky Way galaxy. A star like the Sun becomes a red giant near the end of its life and then a white dwarf; only much more massive stars can become neutron stars or black holes. Earth spins on its axis about once every 24 hours, which causes day and night, and orbits the Sun once a year. The tilt of Earth's axis causes the seasons.

The Moon's gravity is the main cause of tides. In a solar eclipse the Moon is between the Sun and Earth; in a lunar eclipse Earth is in the middle.

Planets farther from the Sun take longer to orbit it. Earth's history is read from rocks. In undisturbed layers of sedimentary rock, the bottom layer is oldest, and fossils show what lived at different times. Radiometric dating uses half-life: each half-life, half of a radioactive element's atoms change, so counting how much is left tells the age of a rock.