Life Science: Start Here — The People’s Share

Before you begin

What this page is for

Life science is the study of living things: what they are made of, how they get and use energy, how they grow and make more of themselves, how they pass their traits to their young, how they change over long stretches of time, and how they depend on one another. Another name for life science is biology.

On the GED science test, about 40 questions out of every 100 are about life science. You do not need to be a biologist to answer them. You need to know a small number of big ideas well, and to be able to read a short passage, a table, or a diagram about living things and say what it shows.

This page walks through those big ideas in eleven parts, in the same order as the Map of the Terrain and the Look Again quizzes. Each part has a short reading, a key idea to remember, a longer reading you can open if you want more, a few videos, three practice questions, and a list of the pages on this site that go deeper.

You do not have to do everything at once. Read one part, watch a video if it helps, try the practice, and stop. If a part makes sense right away, skip its videos and move on. If it feels confusing, a video often makes it clear. Two or three parts is a good sitting, so the whole page is four or five sittings. Keep paper and a pencil beside you.

The page assumes nothing except that you can read carefully. Every science word is explained the first time it appears, and all of them are collected in Words to know near the end. At the very end, Check yourself has fifteen questions on the whole page.

Before the reading: two films

If you have ten minutes, start with something to look at. See a Salamander Grow From a Single Cell (National Geographic) shows one cell becoming a whole animal, sped up. Room one, Alive or Not?, opens with The Inner Life of the Cell, an animation made for Harvard University of what goes on inside one of your own cells. Both are a reminder that everything on this page is happening in you right now.

The videos

If you are short on time: fifteen videos

Each part below lists its own videos. If you want one list to work through, these fifteen cover all of GED life science in order, in about three hours of watching. Watch one or two at a time, not all at once.

The videos are on YouTube and open in a new tab, so this page stays where you left it. Most are made by teachers for high school biology classes. They move quickly; pausing and rewinding is normal and expected.

Scientific Method (updated) Beverly Biology
Characteristics of Life Amoeba Sisters
Biological Levels in Biology: The World Tour Amoeba Sisters
Cell History (updated) Beverly Biology
Organelles of the Cell (updated) Beverly Biology
Photosynthesis (Overview), updated Beverly Biology
Cellular Respiration (Overview), updated Beverly Biology
Fermentation Overview (updated) Beverly Biology
Mitosis & the Cell Cycle (updated) Beverly Biology
Meiosis and Crossing Over (updated) Beverly Biology
11 Organ Systems of the Human Body (Made Easy!) Siebert Science
Immune System (updated) Beverly Biology
Punnett Squares: Basic Introduction The Organic Chemistry Tutor
Natural Selection Bozeman Science
Energy Flow in Ecosystems Bozeman Science

Part 1

How scientists find out

Science is a way of finding out what is true by testing ideas against what actually happens. It starts with a question, such as does this fertilizer make tomato plants grow taller? A scientist then makes a prediction, called a hypothesis. A hypothesis is usually written as an “if … then” sentence: If I give tomato plants fertilizer, then they will grow taller than plants without it.

To test the hypothesis, the scientist runs an experiment. The most important rule of an experiment is to change only one thing at a time. The one thing the scientist changes on purpose is called the independent variable. In the tomato experiment, that is the fertilizer. The thing the scientist measures to see what happened is called the dependent variable, because it depends on the change. Here, that is the height of the plants.

Everything else must be kept the same for every plant: the same kind of tomato, the same pots, the same soil, the same water, the same sunlight. These are the controlled variables. Often there is also a control group: plants that get no fertilizer at all, so there is something to compare against.

Why so strict? Suppose the fertilized plants sat in a sunny window in a Bronx apartment and the other plants sat in a dark hallway. If the fertilized plants grew taller, you could not tell whether the fertilizer did it or the sunlight did. When two things change at once, you cannot know which one caused the result. The GED test often describes an experiment like this and asks you to find the mistake. Usually the mistake is that more than one thing was changed.

Finally, scientists report what they did and what they found, so that other people can repeat the experiment. A result that others can repeat is one people can trust.

Key idea

A fair experiment changes only one thing, measures the result, and keeps everything else the same.

Read more: graphs, and the two variables

When the results of an experiment are drawn on a line graph, there is a standard way to place the two variables. The independent variable, the one the scientist changed, goes along the bottom of the graph, on the x-axis. The dependent variable, the one that was measured, goes up the side, on the y-axis.

Some teachers use the memory aid DRY MIX for this: Dependent, Responding, Y-axis; Manipulated (changed), Independent, X-axis. The aid only helps if you already know which variable is which, so always ask first: which one did the scientist change, and which one did the scientist measure?

A graph of plant height against weeks of growth, for example, puts the weeks along the bottom and the height up the side, because the height depends on how much time has passed, and not the other way around.

Watch

Scientific Method (updated) Beverly Biology

Practice: Part 1

Choose an answer, then press Check. The explanation opens either way.

A student tests whether salt water or fresh water makes seeds sprout faster. She waters one tray of seeds with salt water and one with fresh water. What is the independent variable?
Two groups of volunteers try a new sleep routine. Group A sleeps with the new routine in a quiet room. Group B sleeps without it, next to a noisy street in Queens. Group A sleeps better. What is wrong with this experiment?
Why do scientists report their methods and results so that others can repeat the experiment?

On this site

Change One Thing the app: experiments to take apart, and mistakes to find
Look Again, Quiz 2: The Scientific Method test questions on this part
The Method Wheel a handout to print and keep
Look Again, Quizzes 1 to 7 reading passages, tables, graphs and diagrams, the skills under every science question

Part 2

What makes something alive

Anyone can tell a dog from a rock. But some cases are harder: a seed in a packet, a candle flame, a virus. So biologists use a list of signs. Something is alive if it shows all of them. The GED test expects you to know the list.

1. It is made of cells. A cell is the smallest unit that is alive on its own. Some living things are a single cell, like a bacterium. Your body is about thirty trillion cells.

2. It uses energy. Living things take in energy, from food or from sunlight, and use it to keep themselves running. All the chemical work a living thing does to use energy is called its metabolism.

3. It keeps its inside steady. On a cold January morning waiting for the bus and on a hot August afternoon, your body holds its temperature close to 98.6 degrees. Keeping the inside steady while the outside changes is called homeostasis. Part 8 is about it.

4. It responds to its surroundings. A plant on a windowsill bends toward the light. You pull your hand back from a hot pan.

5. It grows and develops. Living things grow, and they also change form in a set order: a seed becomes a tree, a child becomes an adult.

6. It reproduces. Living things make more of their own kind and pass on their instructions, their DNA, to the young. DNA is the set of instructions every living thing carries in its cells; Part 9 is about it.

7. Its kind changes over many generations. Over long stretches of time, kinds of living things change to suit their surroundings. This is called evolutionary adaptation, and Part 10 is about it.

The list works as a whole. A candle flame uses energy and spreads, and a salt crystal grows, but neither is made of cells, so neither is alive. The signs also describe a kind of living thing, not one individual: a person who never has children is still alive, because people as a kind reproduce.

Key idea

Living things are made of cells, use energy, keep their inside steady, respond, grow, reproduce, and change over generations. Something must show all the signs to count as alive.

How life is organized

Living things are built in levels, from the smallest pieces to the whole animal or plant. Each level is made of pieces from the level below it.

At the bottom are atoms, the tiny particles that all matter is made of. Atoms join to make molecules, such as water or DNA. Molecules build cells. Here is the most important point on this ladder: atoms and molecules are not alive, and the cell is the first level that is alive.

In a body made of many cells, cells do not work alone. A group of similar cells doing the same job is a tissue, such as muscle tissue. Different tissues working together make an organ, such as the heart or the stomach. Organs working together make an organ system, such as the digestive system, which takes food apart. All the organ systems together make an organism: one complete living thing, such as you.

The levels of life: the largest at the top, the smallest at the bottom. The dashed line marks where life begins, at the cell.

Key idea

Atoms → molecules → cells → tissues → organs → organ systems → organism. The cell is the first level that is alive.

Cells: the building blocks of life

Every living thing is made of cells, and every cell comes from another cell. Scientists call these two facts, with one more, cell theory: all living things are made of cells; the cell is the basic unit of life; and all cells come from cells that already existed.

Two kinds of cell. Cells come in two basic kinds, and the difference is where they keep their DNA. A prokaryotic cell (say pro-KAIR-ee-OT-ik) has no nucleus: its DNA floats inside the cell. Bacteria are prokaryotes. They are small, simple, and always a single cell. A eukaryotic cell (say you-KAIR-ee-OT-ik) keeps its DNA inside a nucleus, a compartment with its own wall. Eukaryotic cells are larger and have many other small parts with walls of their own. Animals, plants, fungi such as mushrooms, and many single-celled living things are eukaryotes. The names help: pro means before and eu means true, so a prokaryote is from before the nucleus, and a eukaryote has a true one.

The parts of a cell. The small parts inside a cell are called organelles, which means little organs. The GED test asks about a few of them:

The cell membrane is the thin outer skin of every cell. It controls what goes in and what comes out.
The cytoplasm is the jelly that fills the cell, where the organelles sit.
The nucleus holds the cell’s DNA, its instructions.
Ribosomes are tiny machines that build proteins, following those instructions. Every cell has them.
The mitochondria (one is a mitochondrion) release energy from food for the cell to use. Part 5 explains how.
Plant cells also have a stiff cell wall outside the membrane, which holds the plant up, and chloroplasts, which capture sunlight to make food. Animal cells have neither.

The membrane and what crosses it. Small particles spread out on their own from where there are many of them to where there are few, the way the smell of coffee spreads through an apartment. This is called diffusion, and it costs the cell no energy. When water does this across a membrane, it has its own name, osmosis. Sometimes a cell needs to pull something in that it already has plenty of, which means moving it from where there is little to where there is a lot. That is uphill, and it costs energy. It is called active transport.

Key idea

All living things are made of cells. Prokaryotic cells, like bacteria, have no nucleus; eukaryotic cells, like yours, keep their DNA in a nucleus and have many organelles.

Read more: the cell as a city, and the three domains

Teachers often compare a cell to a city. The membrane is the city’s border, with checkpoints deciding who comes in and who goes out. The nucleus is city hall, where the plans are kept. The ribosomes are factories, building what the city needs. The mitochondria are power plants. The comparison is only a help for remembering; a cell has no one in charge, and every part works by chemistry.

Scientists sort all living things into three large groups called domains. Bacteria are single-celled prokaryotes. Archaea (say ar-KEE-uh) are also single-celled prokaryotes and look like bacteria, but their chemistry and genes are different; many live in places almost nothing else can, such as boiling springs and very salty lakes. Eukarya are all the living things made of eukaryotic cells: animals, plants, fungi, and others. Older books sort living things into five or six kingdoms instead; the domains are the larger, newer grouping, and the kingdoms fit inside them.

Watch

Organelles of the Cell (updated) Beverly Biology
Prokaryotic vs. Eukaryotic Cells (Updated) Amoeba Sisters

Also good, if you want another explanation

Introduction to Cells: The Grand Cell Tour Amoeba Sisters
Cell History (updated) Beverly Biology

Practice: Part 4

Choose an answer, then press Check. The explanation opens either way.

A scientist looks at a cell under a microscope. Its DNA floats loose inside it, and there is no nucleus. What kind of cell is it most likely to be?
Which two parts would you find in a leaf cell but not in a cell from your arm?
A cell pulls in more of a substance that it already has much more of than its surroundings. What is this, and what does it cost?

On this site

Room two: Two Kinds of Cells a bacterium beside an animal cell, the three domains, a plant cell beside an animal cell
Room three: Made to Order inside one cell: every part by name, and a protein built and shipped out
Look Again, Quiz 9: The Cell test questions on cells
Look Again, Quiz 10: The Membrane diffusion, osmosis, and active transport

Part 5

Energy: photosynthesis and cellular respiration

Every living thing needs energy, and almost all of it starts with the sun. This part is about the two processes that move that energy through life. The GED test asks about them often, usually together.

Photosynthesis. A plant does not eat. It makes its own food, a sugar called glucose, out of three things: carbon dioxide, a gas it takes in from the air through tiny openings in its leaves; water, which comes up from its roots; and light, which it captures with a green substance called chlorophyll. This happens inside the chloroplasts. The plant also gives off oxygen, which it does not need. This whole process is called photosynthesis: photo means light, and synthesis means putting together.

Cellular respiration. Now the living thing has sugar with energy stored in it. To use that energy, its cells take the sugar apart, using oxygen. This happens inside the mitochondria, and it gives off carbon dioxide and water, along with energy the cell can spend. This process is called cellular respiration. It happens in your cells, in a dog’s cells, in a mushroom, and in plants too, all day and all night.

Look at what goes in and what comes out of each. Photosynthesis takes in carbon dioxide and water and gives off sugar and oxygen. Cellular respiration takes in sugar and oxygen and gives off carbon dioxide and water. The same four substances appear in both; only the direction is reversed. That is why they are often called mirror images.

The same four substances, in opposite directions. Photosynthesis stores the sun’s energy in sugar; cellular respiration releases it.

carbon dioxide + water + light → sugar + oxygenPhotosynthesis. In plants and algae it happens in the chloroplasts. Some bacteria can do it too.

sugar + oxygen → carbon dioxide + water + energyCellular respiration, in mitochondria. Nearly every living thing does it, plants included.

Two mistakes to watch for. First, plants do both. They photosynthesize when there is light, and their cells carry out cellular respiration all the time, just as yours do. Second, cellular respiration is not the same as breathing. Breathing brings oxygen into your lungs; cellular respiration is what your cells do with that oxygen.

Key idea

Plants use sunlight to make sugar and give off oxygen (photosynthesis). Nearly all living things, plants included, break sugar down with oxygen to get energy (cellular respiration). The two are mirror images.

Read more: makers and takers, ATP, and working without oxygen

Living things that make their own food, as plants do, are called autotrophs (self-feeders), or producers. Living things that must eat other living things to get their sugar, as animals and fungi do, are called heterotrophs (other-feeders), or consumers. Every sandwich you eat, even the meat in it, began as sugar made by a plant.

The energy a cell gets from cellular respiration is packed into small molecules called ATP. You can think of the sugar as money in a savings account and ATP as cash in your pocket: the cell cannot spend the sugar directly, so it turns it into ATP, which it can spend on moving, building, and everything else it does.

When there is not enough oxygen, some cells can still get a small amount of energy from sugar without it. This is called fermentation. During a hard sprint, your muscle cells do this and make a substance called lactic acid, which is part of the burning feeling in tired muscles. Yeast does it too, and gives off alcohol and carbon dioxide gas; the gas is what makes bread dough rise. Fermentation releases much less energy than cellular respiration with oxygen.

The two processes also connect you to plants through the air. The oxygen you breathe in was given off by plants and algae, and the carbon dioxide you breathe out can be taken in by a tree on your block and built into wood. This loop is called the carbon-oxygen cycle.

Watch

Photosynthesis (Overview), updated Beverly Biology
Cellular Respiration (Overview), updated Beverly Biology

Also good, if you want another explanation

Fermentation Overview (updated) Beverly Biology
Photosynthesis and Cellular Respiration (playlist) Amoeba Sisters
Cellular Respiration Bozeman Science

Practice: Part 5

Choose an answer, then press Check. The explanation opens either way.

Which process takes in carbon dioxide and water and gives off sugar and oxygen?
A plant is kept in a dark closet for two days. What happens in its cells?
Where in the cell does cellular respiration take place?

On this site

Look Again, Quiz 11: Photosynthesis test questions, and where the weight of a tree comes from
Look Again, Quiz 12: Cellular Respiration getting the energy back out, with oxygen and without

Part 6

Cell division: mitosis and meiosis

Your body makes new cells every day. When you scrape your knee, new skin cells fill in the wound. When a child grows taller, new bone and muscle cells are being made. New cells are made by an older cell dividing in two, and there are two different ways a cell can divide.

Before either one, the cell copies its DNA. The DNA is packed into long threads called chromosomes. A human body cell has 46 chromosomes, in 23 pairs: one of each pair came from the person’s mother and one from the father.

Mitosis (say my-TOE-sis) is division for growth and repair. One cell divides once and makes two cells, each with a full set of 46 chromosomes. The two new cells are identical to the first. It works like a copy machine.

Meiosis (say my-OH-sis) is division for making sex cells: sperm in men and eggs in women. One cell divides twice and makes four cells, each with only 23 chromosomes, half the usual number. The four cells are all different from one another.

Why half? When a sperm and an egg join, their chromosomes add together: 23 from the father and 23 from the mother make 46, the right number for a new person. If each sex cell carried 46, the child would start with 92, and the number would keep doubling with every generation. And why different? During meiosis, the pairs of chromosomes swap pieces and are shuffled into the new cells at random. That is why brothers and sisters with the same parents do not look exactly alike.

Mitosis makes two cells that match the first. Meiosis makes four cells with half the chromosomes, all different.

	Mitosis	Meiosis
What it is for	Growth and repair	Making sex cells (sperm and eggs)
How many new cells	2	4
Chromosomes in each, in humans	46, a full set	23, half
Alike or different	Identical to the first cell	All different

Key idea

Mitosis: growth and repair, two identical cells, a full set of chromosomes. Meiosis: sex cells, four different cells, half the chromosomes.

Read more: bacteria, and the words diploid and haploid

Bacteria have no nucleus, so they divide in a simpler way. The bacterium copies its one loop of DNA, grows longer, and pinches in two, making two identical bacteria. This is called binary fission. If a GED question describes bacteria dividing, the answer is binary fission, not mitosis.

Biologists have names for the two chromosome counts. A cell with the full set, two of each chromosome, is diploid; in humans that is 46. A cell with one of each, half the full set, is haploid; in humans that is 23. Body cells are diploid; sex cells are haploid. The two words look alike, so it helps to say them aloud with their numbers: diploid, 46; haploid, 23.

Watch

Mitosis & the Cell Cycle (updated) Beverly Biology
Meiosis and Crossing Over (updated) Beverly Biology

Also good, if you want another explanation

Mitosis and Meiosis (playlist) Amoeba Sisters
Meiosis Bozeman Science

Practice: Part 6

Choose an answer, then press Check. The explanation opens either way.

A cut on your hand heals over two weeks. Which kind of cell division made the new skin cells?
A human egg cell has how many chromosomes?
Why do two sisters with the same parents look different?

On this site

Look Again, Quiz 13: Cell Division test questions on mitosis and meiosis
Room four: Growing It Back how some animals regrow a lost leg, and why people mostly cannot

Part 7

The human body

Your body is made of eleven organ systems. Each one is a group of organs doing one large job. You do not need to memorize every organ in every system. You do need to know what each system does, and how the systems depend on one another.

System	What it does
Skin (integumentary)	The outer covering. Keeps germs and harmful chemicals out, keeps water in, and helps control temperature with sweat.
Skeletal	About 206 bones. Hold the body up, protect organs such as the brain and heart, and make blood cells inside the bone marrow.
Muscular	Moves the body by pulling on bones. Also pumps the heart and pushes food through the gut.
Nervous	The brain, the spinal cord, and the nerves. Fast electrical signals, such as pulling your hand from a hot stove.
Endocrine	Glands that release hormones, chemical messages carried in the blood. Slower, longer-lasting control of growth, energy use, and blood sugar.
Circulatory (cardiovascular)	The heart, blood vessels, and blood. Carries oxygen and food to every cell and carries waste away.
Respiratory	The nose, windpipe, and lungs. Brings oxygen into the blood and lets carbon dioxide out.
Digestive	The mouth, stomach, and intestines. Takes food apart into pieces small enough for the blood to carry.
Excretory (urinary)	The kidneys and bladder. Filter waste out of the blood and remove it as urine.
Immune and lymphatic	White blood cells and the vessels that carry them. Find and destroy germs, and remember them for next time.
Reproductive	Makes sex cells, sperm or eggs, so that new people can be born.

How they work together. Take one ordinary moment: you run up the subway stairs to catch a train. Your muscular system moves your legs, pulling on your skeletal system. The muscles need more energy, so your respiratory system breathes faster to bring in more oxygen, and your circulatory system pumps harder to carry that oxygen, and the sugar your digestive system took from your breakfast, to the muscle cells. Your nervous system runs all of this, and your skin sweats to get rid of the extra heat. In one climb up the stairs, at least seven systems are working together.

Staying healthy. The GED test also asks about disease. An infectious disease is caused by a germ, such as a bacterium or a virus, and can spread from person to person: the flu, COVID-19, tuberculosis. A noninfectious disease does not spread from person to person; it comes from a person’s genes, habits, or surroundings: heart disease, most cancers, type 2 diabetes. A vaccine trains the immune system to recognize a germ before you ever meet it, so your body is ready. Antibiotics kill bacteria but do nothing against viruses.

Key idea

Each organ system does one large job, and the systems depend on one another. Know what each one does, not every detail.

Homeostasis: staying in balance

Your body is always adjusting to keep its inside steady. Too hot? You sweat. Too cold? You shiver. Blood sugar too high after a meal? An organ called the pancreas releases a hormone called insulin, which helps the cells take sugar out of the blood. Blood sugar too low? The pancreas releases a different hormone, glucagon, which tells the liver to put stored sugar back into the blood. Keeping the inside steady like this is homeostasis (say HO-mee-oh-STAY-sis).

Most of this works by negative feedback. Here “negative” does not mean bad. It means opposite: the body pushes back against a change, to bring things back to normal. The thermostat in an apartment works the same way. When the room gets colder than the setting, the heat comes on; when the room warms back up to the setting, the heat shuts off. The change itself switches off the response.

Once in a while the body uses positive feedback, which does the reverse: instead of pushing back against a change, it makes the change bigger and bigger until a job is finished. The standard example is childbirth. Contractions push the baby down; that pressure causes the body to make more of a hormone that makes the contractions stronger; stronger contractions push harder, and so on, until the baby is born and the loop stops. Blood clotting is another example.

Negative feedback: a change is sensed, the body responds, and the response undoes the change.

Key idea

Homeostasis is the body keeping its inside steady. Negative feedback pushes back against a change; positive feedback makes a change bigger until a job is done.

DNA and heredity

Heredity is the passing of traits from parents to their children: why you might have your mother’s nose or your father’s eyes. The instructions for those traits are carried in DNA.

DNA. A DNA molecule is shaped like a twisted ladder, a shape called a double helix. The rungs of the ladder are made of four chemicals, called bases, written with the letters A, T, C, and G. They always pair the same way: A with T, and C with G. The order of the letters along the ladder is the code that spells out the instructions, the way the order of letters in a word spells its meaning.

Genes and alleles. A gene is a section of DNA that holds the instructions for one trait, such as blood type. You have two copies of every gene, one from each parent. The two copies are not always the same; the different versions of a gene are called alleles (say uh-LEELZ).

Dominant and recessive. Some alleles are dominant: if you have even one copy, its trait shows. It is written with a capital letter, such as P. Others are recessive: the trait shows only if you have two copies. It is written with a small letter, such as p. A person with one of each, Pp, shows the dominant trait but carries the recessive allele and can pass it on. Such a person is called a carrier.

Genotype and phenotype. The two letters a person has, such as Pp, are the genotype. The trait you can actually see, such as purple flowers, is the phenotype.

Punnett squares. A Punnett square is a small grid for predicting what the offspring of two parents might inherit. One parent’s two alleles go across the top, the other parent’s two go down the side, and each box gets one letter from its column and one from its row. The four boxes show the four equally likely combinations. The GED test gives Punnett squares often and asks you to read them.

Worked example

In pea plants, purple flowers (P) are dominant over white flowers (p). Two plants, each Pp, are crossed. What fraction of the offspring are likely to have white flowers?

Put one parent’s alleles, P and p, across the top, and the other parent’s, P and p, down the side. Fill each box with one letter from its column and one from its row: PP, Pp, Pp, pp.

Only pp shows the recessive trait, white. That is 1 box out of 4, so 1 out of 4, or 25 percent, of the offspring are likely to have white flowers. The other 3 out of 4 have purple flowers.

Sense check: two purple parents can have a white-flowered offspring, because each parent carries a hidden p.

Key idea

DNA is the set of instructions for life. You get one allele of each gene from each parent. A dominant allele shows with one copy; a recessive allele needs two. Punnett squares predict the chances.

Read more: from DNA to a trait, and genetics in the news

A gene works by telling the cell how to build one protein. Proteins do most of the work in a cell and give a body many of its traits. The cell first copies the gene’s message out of the nucleus, and then the ribosomes read the message and build the protein. Room three on this site walks through a protein being built and shipped out of a cell.

A mutation is a change in the DNA code. Most mutations make no difference; some cause disease; a few turn out to help. A pedigree is a family tree that shows which members of a family had a trait, and GED questions sometimes ask you to read one.

Genetics is often in the news. DNA fingerprinting compares DNA from different people and is used in paternity tests and court cases, and it has freed people who were wrongly convicted. Genetic engineering moves genes from one living thing into another; insulin for people with diabetes has been made this way, by bacteria, since the 1980s. A newer tool called CRISPR can edit a single gene. These raise real questions that a society has to decide together: who may see your genetic information, who can afford gene treatments, and whether we should ever change the genes a person passes on to their children.

Watch

DNA, Chromosomes, Genes, and Traits: An Intro to Heredity Amoeba Sisters
Punnett Squares: Basic Introduction The Organic Chemistry Tutor

Also good, if you want another explanation

DNA Khan Academy
DNA Replication (Updated) Amoeba Sisters

Practice: Part 9

Choose an answer, then press Check. The explanation opens either way.

In DNA, which base always pairs with A?
A plant has the genotype pp for flower color, where P (purple) is dominant. What color are its flowers?
A parent who is PP is crossed with a parent who is pp. What fraction of the offspring will show the dominant trait?

On this site

Room three: Made to Order how a cell builds a protein from its instructions
Flash cards: Cells and Genetics forty cards to check yourself
The Map of the Terrain, topics 21 to 24 the Look Again quizzes on heredity are coming next

Part 10

Evolution by natural selection

Kinds of living things change over time. Not within one animal’s life, but over many generations. This change is called evolution, and the main cause of it is natural selection. Charles Darwin described it in 1859.

Natural selection needs three things:

1. Variation. The members of any group of living things are not all the same. Some rabbits are faster, some mice are darker, some finches have thicker beaks.

2. A struggle to survive. Life is hard: there are predators, there is not enough food for everyone, the weather is harsh. Not every animal survives long enough to have young.

3. Inheritance. Parents pass their traits to their young.

Put the three together. The individuals whose traits happen to help them in their surroundings are more likely to survive and have young, and they pass those helpful traits on. Over many generations, the helpful trait becomes common in the group, and the unhelpful one becomes rare.

Darwin’s phrase “survival of the fittest” is often misunderstood. Fittest does not mean strongest. It means best suited to the surroundings. A white rabbit survives better in snow than a brown one, because a hawk cannot see it as easily. In a brown forest, the brown rabbit is the fittest.

One more point the test likes: animals do not change on purpose, and one animal does not change during its life to suit its surroundings. A giraffe did not get a long neck by stretching. Giraffes that happened to be born with longer necks reached more leaves, survived better, and had more young.

Key idea

Variation + a struggle to survive + inheritance = natural selection. Over many generations, a population changes to suit its surroundings.

Read more: the peppered moth, Darwin’s finches, and the evidence

In England before the 1800s, most peppered moths were pale with dark specks, which hid them against pale tree bark. Then coal smoke from factories blackened the trees. The pale moths now stood out, birds ate them, and within a few decades almost all the moths near the factory cities were dark. When clean-air laws cleared the smoke in the 1950s and 60s, pale moths became common again. The moths did not choose their color; the birds did the selecting.

On the Galápagos Islands, Darwin found finches that were almost alike except for their beaks. On islands with hard seeds, the finches had thick, strong beaks for cracking them; where insects hid in bark, the beaks were long and thin. All had come from one kind of finch that reached the islands long ago. Scientists have since watched beak sizes change in just a few years during droughts.

The evidence for evolution comes from several directions at once: fossils, which show how living things looked long ago; the similar bone structure in a human arm, a bat’s wing, and a whale’s flipper; the way early embryos of very different animals look alike; and DNA, which shows which living things are most closely related. Bacteria that survive antibiotics and pass that ability on are natural selection happening today, in days rather than centuries.

Watch

Natural Selection Bozeman Science

Also good, if you want another explanation

Natural Selection Amoeba Sisters
Darwin’s Theory of Evolution: A Really Simple and Brief Explanation Science ABC

Practice: Part 10

Choose an answer, then press Check. The explanation opens either way.

Foxes in the desert have much larger ears than foxes in the Arctic. Large ears give off body heat. Which explanation is correct?
In natural selection, what does “fittest” mean?
Doctors worry that using antibiotics too often makes them stop working. Why?

On this site

Flash cards: Evolution forty cards to check yourself
The Map of the Terrain, topics 25 and 26 the Look Again quizzes on evolution are coming

Part 11

Ecosystems: how living things depend on one another

No living thing lives alone. An ecosystem is all the living things in one place, together with the nonliving things around them: the soil, the water, the air, the sunlight. A pond is an ecosystem; so is Jamaica Bay, and so is a vacant lot in Brooklyn.

Energy flows one way. The energy in an ecosystem starts with the sun. Producers, such as grass, trees, and algae, capture it by photosynthesis. Primary consumers eat the producers: rabbits eat grass. Secondary consumers eat the primary consumers: foxes eat rabbits. Decomposers, such as bacteria, fungi, and worms, break down dead things and waste. A food chain shows one line of who eats whom; a food web shows all the lines together, since most animals eat more than one thing.

The 10 percent rule. At each step up a food chain, most of the energy is used up. A rabbit spends most of the energy from the grass it eats just staying alive: moving, breathing, keeping warm. That energy leaves as heat. Only about 10 percent of the energy at one level is passed to the level above it. This is why food chains are short, and why there are always far fewer foxes than rabbits.

An energy pyramid, from the bottom up: producers (grass), primary consumers (grasshoppers), secondary consumers (snakes), and tertiary consumers (hawks). Each level gets only about one tenth of the energy of the level below it.

Matter goes around. Energy flows through an ecosystem and is lost as heat, but matter, the atoms that living things are made of, is used over and over. In the carbon cycle, plants take carbon dioxide from the air, animals eat the plants, and animals, plants, and decomposers all return carbon dioxide to the air. The atoms in your body have been part of other living things many times before.

Living together. Some pairs of living things have a close, long-lasting relationship. In mutualism, both benefit: bees get food from flowers, and the flowers get pollinated. In parasitism, one benefits and the other is harmed: a tick feeding on a dog. In commensalism, one benefits and the other is neither helped nor harmed: a bird nesting in a tree.

Limits. An environment can support only so many of one kind of living thing, because food, water, and space run out. That largest number is its carrying capacity. When a population grows past it, many die, and the population falls back.

Key idea

Energy flows one way, from the sun to producers to consumers, and about 90 percent is lost at each step. Matter is recycled. Living things depend on one another.

Read more: reading food webs, populations, and people

In a food web diagram, the arrows point the way the energy goes: from the thing being eaten to the thing that eats it. The arrow from grass to rabbit means the rabbit eats the grass. If a question asks what happens when one kind of animal disappears, follow the arrows. What ate it will have less food and decline; what it ate will be eaten less and increase.

Predators and their prey rise and fall together. In Canada, the numbers of snowshoe hares and of the lynx that hunt them rise and fall about every ten years, with the lynx a little behind the hares: more hares mean more food for lynx, then more lynx mean fewer hares, then fewer hares mean fewer lynx.

People change ecosystems more than any other living thing: by clearing land, polluting air and water, bringing in invasive species from other places, overfishing, and burning fossil fuels, which adds carbon dioxide to the air faster than plants can take it back. A varied ecosystem, with many kinds of living things, called high biodiversity, recovers from damage better than a simple one. GED questions on this topic usually ask you to name a cause, predict an effect, or judge a proposed solution.

Watch

Energy Flow in Ecosystems Bozeman Science
Community Interactions (Competition, Predation, Symbiosis) Beverly Biology

Also good, if you want another explanation

Energy Pyramid Christopher Rafalik
Biogeochemical Cycles (updated) Beverly Biology
Ecology Series (playlist) Amoeba Sisters
Human Impacts on Earth Systems Bozeman Science
World Biomes Explained MooMooMath and Science

Practice: Part 11

Choose an answer, then press Check. The explanation opens either way.

Grass in a field holds 20,000 units of energy. About how much of that reaches the rabbits that eat the grass?
Clownfish live among the stinging arms of sea anemones, which protect them from predators, and the clownfish chase away fish that eat the anemones. What kind of relationship is this?
Deer are brought to an island with plenty of food and no predators. Their numbers grow quickly for years, then suddenly fall. What most likely happened?

On this site

Flash cards: Ecosystems forty cards to check yourself
The Map of the Terrain, topics 27 to 30 the Look Again quizzes on ecosystems are coming

Words to know

All the terms on this page, in one place

Each word is explained where it first appears. Here they are together, in alphabetical order.

Active transport moving something across a cell membrane from where there is little of it to where there is more. It costs energy.

Allele one version of a gene. You have two alleles of each gene, one from each parent.

Carrying capacity the largest number of one kind of living thing an environment can support.

Cell the smallest unit that is alive on its own. All living things are made of cells.

Cell membrane the thin outer skin of a cell, which controls what goes in and out.

Cellular respiration taking sugar apart with oxygen to release energy. Happens in the mitochondria of nearly every living thing.

Chloroplast the part of a plant cell where photosynthesis happens.

Chromosome a long thread of DNA. Human body cells have 46; sex cells have 23.

Consumer a living thing that gets its energy by eating other living things.

Control group the part of an experiment that is left alone, so there is something to compare against.

Decomposer a living thing, such as a fungus or bacterium, that breaks down dead things and waste.

Dependent variable what a scientist measures in an experiment.

Diffusion particles spreading from where there are many to where there are few. It costs no energy.

DNA the molecule that carries the instructions for building and running a living thing.

Dominant an allele whose trait shows even when there is only one copy. Written with a capital letter.

Ecosystem all the living things in a place, together with the nonliving things around them.

Eukaryotic cell a cell that keeps its DNA in a nucleus. Animals, plants, and fungi are made of them.

Evolution the change in a kind of living thing over many generations.

Gene a section of DNA with the instructions for one trait.

Genotype the two alleles a living thing has for a trait, such as Pp.

Homeostasis keeping the inside of the body steady while the outside changes.

Hypothesis a prediction that can be tested, often written as an “if … then” sentence.

Independent variable the one thing a scientist changes on purpose in an experiment.

Meiosis cell division that makes sex cells: four different cells with half the chromosomes.

Mitochondria the parts of a cell where cellular respiration releases energy from food.

Mitosis cell division for growth and repair: two cells identical to the first.

Natural selection the process by which helpful inherited traits become common over generations, because the living things that have them survive and have more young.

Negative feedback a response that pushes back against a change, bringing things back to normal.

Nucleus the part of a eukaryotic cell that holds its DNA.

Organ a body part made of different tissues working together, such as the heart.

Osmosis diffusion of water across a membrane.

Phenotype the trait you can actually see, such as white flowers.

Photosynthesis using light to make sugar from carbon dioxide and water, giving off oxygen. Happens in chloroplasts.

Positive feedback a response that makes a change bigger until a job is finished, as in childbirth.

Producer a living thing that makes its own food, usually by photosynthesis.

Prokaryotic cell a cell with no nucleus, such as a bacterium.

Punnett square a grid for predicting what offspring may inherit from two parents.

Recessive an allele whose trait shows only when there are two copies. Written with a small letter.

Ribosome a tiny machine in every cell that builds proteins.

Tissue a group of similar cells doing the same job.

Check yourself

Fifteen questions on the whole page

One or two questions from each part, in order. Each explanation names the part to go back to if you missed it.

1.
An experiment tests whether music helps plants grow. What should be kept the same for every plant?
2.
Which sign of life does a salt crystal NOT show?
3.
Which is an organ system?
4.
A cell has a nucleus, mitochondria, a cell wall, and chloroplasts. What is it?
5.
Water moves into a cell across its membrane, with no energy spent. What is this called?
6.
What gas do plants give off during photosynthesis?
7.
Which living things carry out cellular respiration?
8.
Which process makes cells with half the usual number of chromosomes?
9.
Which system filters waste out of the blood and removes it as urine?
10.
You shiver on a freezing day and your body warms up. This is an example of
11.
Two parents are both Pp for flower color. What is the chance that an offspring is pp?
12.
What is a gene?
13.
Dark moths became common near English factory cities when soot darkened the trees. Why?
14.
Why are there fewer hawks than mice in a field?
15.
In a food web, which way do the arrows point?

Where to go next

After this page

Take these in the order you need them, not all at once.

The four rooms Alive or Not?, Two Kinds of Cells, Made to Order, and Growing It Back: apps you work with your hands, in order.
The Look Again quizzes Quizzes 8 to 20 cover Parts 2 to 8 of this page in the test’s own kind of question. Quizzes on heredity, evolution, and ecosystems are coming.
The Map of the Terrain all fifty-five science topics, to mark off as you go.
The Life Science flash cards two hundred cards in five sets, for checking yourself once a part is read.
The study guides on the Life Science shelf longer readings on photosynthesis and respiration, cell division, genetics, the human body, and evolution and ecosystems.