Life science: cells, DNA, energy and evolution
The big life-science ideas in plain words: what cells are made of, mitosis and meiosis, how DNA and mutations work, photosynthesis and respiration, the 10 percent energy rule in food chains, and how natural selection changes a population. Includes six questions with worked answers.
Why this matters on the test
About 40 percent of the GED® science test is life science. Most questions pair a life-science idea with a skill, such as reading a diagram, using numbers from a table, or judging a claim.
You rarely need long word lists. You do need the big ideas below, so you can tell what a diagram or a short report is about.
Cells are the building blocks
Cell theory has three parts. All living things are made of one or more cells. The cell is the smallest unit of life. New cells come only from cells that already exist.
Living things are organized in levels, from small to large: cell → tissue → organ → organ system → organism. For example: a heart muscle cell → heart muscle tissue → the heart → the circulatory system → you.
Key cell parts: the nucleus holds the DNA and directs the cell. The cell membrane controls what goes in and out. Mitochondria release energy from food. Ribosomes build proteins. Plant cells also have a cell wall for support and chloroplasts for photosynthesis. Bacteria are single cells with no nucleus.
Two ways cells divide
Mitosis: one cell divides once and makes two identical cells, each with the same number of chromosomes as the first cell. Human body cells have 46 chromosomes, so each new cell has 46. Mitosis is for growth and repair, like healing a cut.
Meiosis: a cell divides twice and makes four sex cells (eggs or sperm). Each one has half the chromosomes: 23 in humans. When an egg and a sperm join, the new cell has 23 + 23 = 46 again. Meiosis also mixes up genes, so brothers and sisters are not identical (unless they are identical twins).
DNA, genes and mutations
DNA is a long code. A gene is one section of DNA that holds the instructions for one protein. The cell copies the gene's message into RNA, then uses the RNA to build the protein. In short: DNA → RNA → protein. Proteins do most of the work that shows up as traits.
A mutation is a change in the DNA code. It can be harmful, helpful or have no effect. Only mutations in eggs or sperm can pass to children. A mutation in a skin cell stays with that person.
The environment can also change how a gene shows. For example, a Siamese cat's fur grows darker on its cooler body parts, like the ears and paws.
Energy in living things
Photosynthesis (in chloroplasts): carbon dioxide + water + light energy → sugar + oxygen. With chemical formulas: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Count the atoms: 6 carbon, 18 oxygen and 12 hydrogen on each side.
Cellular respiration (in mitochondria) runs the other way: sugar + oxygen → carbon dioxide + water + usable energy. Plants and animals both do this.
Fermentation releases a small amount of energy without oxygen. Yeast makes alcohol and carbon dioxide, and the gas makes bread rise. Hard-working muscles that run short of oxygen make lactic acid.
Energy in a food chain: the 10 percent rule
Energy starts with producers (plants), then moves to the animals that eat them, then to the animals that eat those animals. At each step, only about 10 percent of the energy passes up. The rest is used for living or lost as heat.
Worked numbers: grass holds 20,000 kilocalories. Rabbits get about 20,000 × 0.10 = 2,000. Foxes that eat the rabbits get about 2,000 × 0.10 = 200. This is why a food chain has fewer animals at the top.
Carrying capacity is the largest population an area can support for a long time. Food, water and space limit it.
Symbiosis means two species living closely together. In mutualism both gain, like bees and flowers. In commensalism one gains and the other is not affected, like barnacles riding on a whale. In parasitism one gains and the other is harmed, like a tick on a dog.
Evolution by natural selection
Natural selection needs four things. First, individuals differ in a trait. Second, the trait can be passed to offspring. Third, more young are born than can survive. Fourth, individuals with a helpful trait survive and have more young. Over many generations, the helpful trait becomes more common.
Example (made up for this lesson): on an island, a dry year leaves mostly hard seeds. Birds with thicker beaks crack them more easily, so more of them survive and have chicks. Years later, the average beak is thicker. No single bird changed its beak. The population changed.
Bacteria that resist an antibiotic work the same way. The drug does not create resistance. It kills the bacteria without it, so the resistant ones are left to multiply.
Evidence for common ancestry includes fossils, similar bone patterns (a human arm and a bat wing) and similar DNA. A cladogram is a branching diagram. Two groups that split from the same nearby branch point share a more recent common ancestor than groups that split off earlier.
Where this comes from
GED Testing Service's Assessment Guide for Educators: Science lists these life-science content topics: L.a (human body and health), L.b (energy for life functions), L.c (ecosystems), L.d (cells), L.e (heredity) and L.f (evolution). It says about 40 percent of the test is life science. We checked the guide and OpenStax Biology 2e on October 28, 2026.
The explanations, made-up examples and questions are original FinishHigh practice, not GED® test questions. GED® is a registered trademark of the American Council on Education. FinishHigh is independent and not affiliated with or endorsed by it. This lesson teaches a few skills. It is not a full course or an official readiness result.
Next, try the questions below. For Punnett squares, watch the FinishHigh Punnett square video in the video library.
Your turn.
Try each question before opening the explanation. These are FinishHigh practice examples, not GED® test questions.
Put these in order from smallest to largest: the heart, a heart muscle cell, the circulatory system, heart muscle tissue.
Show the answer and why
Heart muscle cell → heart muscle tissue → the heart → the circulatory system. Cells form tissue, tissue forms an organ, and organs work together in an organ system.
A human skin cell has 46 chromosomes. How many chromosomes are in each new cell after mitosis? How many are in each egg or sperm cell made by meiosis?
Show the answer and why
46 after mitosis, because mitosis makes identical copies. 23 after meiosis, because meiosis halves the number: 46 ÷ 2 = 23.
A water plant sits in a sealed jar in bright light for a day. Does the oxygen in the jar go up or down? Why?
Show the answer and why
Up. In bright light, the plant's photosynthesis releases more oxygen than its own respiration uses. Oxygen is a product of photosynthesis.
Grass in a field holds 50,000 kilocalories. Using the 10 percent rule, about how much energy reaches grasshoppers? About how much reaches birds that eat the grasshoppers?
Show the answer and why
Grasshoppers: 50,000 × 0.10 = 5,000 kilocalories. Birds: 5,000 × 0.10 = 500 kilocalories.
A farm sprays the same pesticide for ten years. At first most insects die. Now most insects survive it. Explain this with natural selection.
Show the answer and why
A few insects could already survive the pesticide because of a difference in their genes. They lived and had young, and the young got the same trait. Each year the pesticide removed more of the others. Over generations, resistant insects became most of the population. The pesticide did not create the trait. It selected for it.
In a cladogram, frogs branch off first, then lizards. Mice and humans split last, from the same fork. Which pair shares the most recent common ancestor?
Show the answer and why
Mice and humans. They split from the latest branch point, so their common ancestor lived more recently than the ancestor they share with lizards or frogs.
Trusted sources and further study
Sources guide the topics and factual information. The lesson wording, examples, and questions are original FinishHigh material.
- Science: GED Testing Service assessment targets for educators ↗GED Testing Service · checked 2026-10-20
- OpenStax Biology 2e ↗OpenStax, Rice University · checked 2026-10-28
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