Week 1 · Chapter

Chapter 1 — The Science of Biology

Introduction to Biology Free sample edition

The module's primary reading · ~35–45 minutes · Objective 1 (describe the practice of science in biology — the characteristics of life as a set, the levels of biological organization, the scientific method and a controlled experiment's variables and control, and the difference between a hypothesis and a theory — and explain evolution by natural selection as the unifying framework of biology) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Biology is the science of living things, and this week you learn how biologists decide what counts as alive. You will meet the characteristics of life (the seven properties that, taken together, mark something as living) and the ladder of sizes biologists use to place any living structure. You will learn the scientific method (the loop of observing, asking, predicting, testing, and concluding that turns a hunch into evidence) and how to read a simple experiment from start to finish. You will also see why a theory (a broad explanation backed by a large body of repeated tests) is the strongest idea in science, not a guess. Finally, you will meet evolution (change in the inherited traits of a population over generations), the idea that ties all of biology together.

Why you already care about this

Picture a penny on a kitchen counter and a dropper of water. Drop by drop, the water piles into a shiny dome on the coin. In one illustrative run made for this course, a penny held an average of 30 drops of plain water before the dome spilled. With a little dish soap stirred into the water, it held an average of only 14 drops. That is 16 fewer drops, from one small change.

Now ask the questions a biologist asks. Did the soap cause the difference, or something else? How sure can you be after three tries? Would you bet that the result would happen again tomorrow? Those questions are the heart of this week. Every claim you will meet in this course was earned this way. Someone changed one thing, held everything else steady, measured carefully, and let the evidence decide.

You already make claims like this in daily life. A new running shoe "makes you faster." A houseplant "likes" the east window. A cold medicine "works." This chapter gives you the tools to test claims like those instead of trusting them.

By the end of this chapter you will be able to:

sort a thing as alive or not alive by checking all seven characteristics of life;
put the levels of organization in order from atom to biosphere and name an emergent property;
trace the steps of the scientific method and write a falsifiable hypothesis;
name the independent variable, dependent variable, controlled variables, control group, and any confound in an experiment;
compute the average and range of three trials, with every step shown, and say what the numbers mean;
tell a hypothesis from a theory, and explain why evolution by natural selection is the unifying framework of biology.

Carry this line with you: life is the whole checklist, not one box.

Where we are

This week: what makes something alive, how life is organized, how controlled experiments test ideas, hypothesis versus theory, and evolution.
How they connect: this chapter is the foundation; every later week uses the habits you build here.
Where we're going next: Week 2, The Chemistry of Life, zooms in to the bottom rungs of the ladder, atoms and molecules, and explains the penny result through water's chemistry.

This is the first week of the course, so nothing earlier is assumed. You need no background beyond high-school science and the arithmetic of adding and dividing. If a word is new, this chapter defines it the first time it appears. Next week you will meet the water molecule up close, so the penny experiment in Section 5 sets that up.

1. Recognize life: check the whole list

Think about a candle flame. It grows when you feed it wax. It uses energy and gives off heat. It even seems to respond to a breeze by bending away. Yet nobody calls a candle flame alive. Now think about a salt crystal growing in a jar of salty water as the water dries. It has a neat, repeating order, and it gets bigger. It is not alive either.

So what does a living thing have that a flame and a crystal lack? Biologists answer with a list, and the key rule is that a living thing shows every item on it.

The characteristics of life are the seven properties biologists use together to recognize a living thing:

Order. A living thing is built from highly organized parts.
Energy processing. It takes in energy and uses it to do work.
Growth and development. It grows and changes in a pattern set by its inherited instructions.
Reproduction. It makes more of its own kind.
Response to the environment. It reacts to changes around it, such as light, touch, or temperature.
Regulation and homeostasis. It holds its inside conditions steady. Homeostasis is that steady-state regulation, such as your body holding its temperature on a cold day.
Evolutionary adaptation. Over many generations, its kind changes in ways that suit its environment.

Something counts as alive only when it shows the whole set. Lists in other sources group these properties a little differently; this course teaches these seven.

The smallest thing that shows all seven beyond doubt is a cell, the smallest unit that is unambiguously alive. A single cell can be a complete organism on its own. A single bacterium, for example, takes in energy, grows, responds, and divides to make more bacteria, all as one cell. Your body is built from a huge number of cells, but each of them is alive in its own right.

Worked example — alive or not? Run the full checklist on three things you could find around a home.

A houseplant. It is ordered (leaves, stem, roots), processes energy from light, grows and develops new leaves, can reproduce, bends toward a window (response), regulates water loss (homeostasis), and belongs to a kind that has adapted over generations. Every box is checked, so it is alive.
A candle flame. It processes energy and grows, but it does not reproduce its own kind from inherited instructions, does not regulate its inside conditions, and does not adapt over generations. Several boxes are empty, so it is not alive.
A salt crystal. It is ordered and it grows, but it does not process energy to do work, respond, regulate itself, reproduce from instructions, or adapt. It is not alive.

In words: the houseplant passes because it checks every box; the flame and the crystal each check one or two and fail the rest.

Misconception check. Students often think, "If it moves and grows, it is alive." The idea is tempting because the living things we see every day, pets, people, plants, move and grow. But fire grows and uses energy, and a crystal is ordered and grows, and neither is alive. The cure is to require the whole set of seven, every time. One checked box proves nothing.

Memory hook: life is the whole checklist, not one box.

2. Climb the levels of organization

Think about a soccer team. A player can run and kick. But only the whole team can run a passing play down the field. The team can do something no single player can do on their own.

Living things are organized the same way. Biologists place every structure on one ladder of sizes, from the smallest piece to the whole living planet. The levels of organization, from smallest to largest, are:

atom → molecule → organelle → cell → tissue → organ → organ system → organism → population → community → ecosystem → biosphere

Read the arrow as "builds up to." Atoms join into molecules. Molecules build an organelle, a small working part inside a cell. Organelles work together inside a cell. Similar cells working together form a tissue, and tissues form an organ such as the heart. Organs that share a job form an organ system, and organ systems make up an organism, one whole living thing. Organisms of the same kind living in one area form a population. The populations of different kinds sharing that area form a community. A community together with its nonliving surroundings, such as water, soil, and air, forms an ecosystem. All of Earth's ecosystems together form the biosphere.

At each level, something new appears. An emergent property is an ability that appears at one level of organization but not in its separate parts. A single heart muscle cell can twitch. Only the whole organ, the heart, can pump blood. The pumping is an emergent property of the organ.

Worked example — place it on the ladder. Put these five items in order from smallest to largest: a heart, a heart muscle cell, the people living in one town, a water molecule, and one person.

Find the smallest: the water molecule (molecule level).
Next: the heart muscle cell (cell level).
Next: the heart (organ level).
Next: one person (organism level).
Largest: the people living in one town (population level).

In words: molecule → cell → organ → organism → population, the same order as the ladder, with some rungs skipped.

Misconception check. Students often think a bigger level is "more alive" than a smaller one, or that the ladder is a list of separate things. Both ideas miss the point. The cell is already fully alive; the higher levels are cells organized into larger working groups. What changes as you climb is not "how alive" something is but what it can do. Each new level adds emergent properties its parts lack.

Memory hook: a heart cell twitches; only the heart pumps.

3. Test ideas with the scientific method

Imagine a friend who grows basil. One pot sits on a sunny windowsill and another on a shaded bookshelf, and the windowsill pot seems taller. Your friend says, "Basil grows taller in more light." Maybe. Or maybe the windowsill pot got more water, or better soil, or a stronger seedling. How would you find out?

You would use the scientific method, the loop of observation, question, hypothesis, prediction, experiment, analysis, and conclusion that scientists use to test ideas against evidence. It is a loop, not a straight line, because every conclusion raises a new question.

Here are the steps in order:

Observation. You notice something: the windowsill basil looks taller.
Question. You ask about it: does light change how tall basil grows?
Hypothesis. You propose a testable explanation. A hypothesis is a proposed explanation that can be tested and must be falsifiable, often written as "if … then … because …".
Prediction. You state what you expect to see. A prediction is the specific result you expect if the hypothesis is right.
Experiment. You run a fair test.
Analysis. You organize and summarize the results.
Conclusion. You decide whether the evidence supports the hypothesis, and you ask what to test next.

The word falsifiable means able to be shown wrong: some possible result of a test would count against the idea. "Basil grows taller in more light" is falsifiable, because a test could show no difference in height. "Basil grows best when it feels happy" is not falsifiable, because no measurement could ever show it wrong.

Worked example — the basil loop. Walk the basil question through all seven steps.

Observation: the windowsill pot looks taller than the bookshelf pot.
Question: does light change how tall basil grows?
Hypothesis: if basil plants get more light, then they will grow taller, because plants use light energy to build new tissue.
Prediction: plants on a bright windowsill will be taller after a set number of weeks than identical plants kept in shade.
Experiment: set up several matching pots in each place, with the same soil, seedlings, and watering.
Analysis: measure every plant's height on the same day and compare the two groups.
Conclusion: say whether the results support or fail to support the hypothesis, and ask a new question, such as how much light is enough.

In words: the loop turned a casual "it looks taller" into a test anyone could repeat.

Notice the verbs in step 7. Evidence supports or fails to support a hypothesis. It never proves it. A later test, a bigger sample, or a better method could always add something new. Honesty note: this is not a weakness of science. It is what keeps science open to correction.

Misconception check. Students often think, "One experiment proves it." The idea is tempting because a clean result feels final. But one experiment can be thrown off by chance, a mistake, or a factor nobody noticed. Scientists repeat tests and say the evidence supports or fails to support a hypothesis. Save "proves" for mathematics.

Memory hook: evidence supports; it never proves.

4. Build a controlled experiment

Suppose a gym-goer wonders whether a cup of coffee raises heart rate during a workout. The fair way to find out is to compare people who have the coffee with people who do not, while keeping everything else the same. That is the core of a controlled experiment: a test that compares groups that differ in only one deliberately changed factor, so any difference in the result can be traced to that factor.

Every controlled experiment has the same five parts:

The independent variable is the factor the experimenter deliberately changes between groups. You change it.
The dependent variable is the result the experimenter measures, which may change in response to the independent variable. The result depends on it.
The controlled variables are the conditions kept the same for every group, so they cannot explain a difference in the result.
The control group is the no-treatment baseline group, which shows what happens without the change being tested.
A confound is a second, unplanned difference between the groups that could also explain the result, so the experiment cannot tell which difference mattered.

The one-sentence test for the first two: I change the independent variable; the result depends on it. When you define a variable, define it so it can be measured: not "fitness" but "heart rate in beats per minute after ten minutes of cycling."

Worked example — naming the parts. Label each part of the coffee test.

Independent variable: whether a person drinks a cup of coffee with caffeine or a cup without caffeine before the workout.
Dependent variable: heart rate, in beats per minute, after ten minutes on the same exercise bike.
Controlled variables: the same bike, the same resistance setting, the same time of day, and the same size of cup.
Control group: the people who drink the cup without caffeine. They are the baseline.
A possible confound: suppose the caffeine group exercised in a hot room and the control group in a cool one. Room temperature would then be a second difference, and you could not tell whether caffeine or heat raised heart rate.

In words: one change (caffeine), one measurement (heart rate), everything else held steady, and a baseline group to compare against.

Misconception check. Two slips show up again and again. First, students often call the group that gets the treatment the "control group." It is the reverse: the control group is the no-treatment baseline. Second, students swap the independent and dependent variables. Use the test sentence: I change the independent variable; the result depends on it. Coffee is changed, so coffee is independent; heart rate is measured, so heart rate is dependent.

Memory hook: change one thing; measure one thing; hold the rest steady.

5. Read the drops on a penny experiment from start to finish

Now you can read a full experiment, from question to conclusion. This is the course's first lab and the example you will see in the lecture, the slides, the tutorial, the quiz, and the Biology Lab & Inquiry this week. The numbers below are an illustrative dataset constructed for this course, not real data; your own trials at home will land somewhere nearby.

The question: does soap change how many drops of water a penny holds before the water spills over the edge?

The hypothesis: if soap is added to the water, then the penny will hold fewer drops, because soap weakens the pull of water molecules on each other. That pull is called cohesion, and you will learn exactly how it works in Week 2. For now it is enough to know that it holds the dome of water together.

The prediction: the soapy-water trials will average fewer drops than the plain-water trials.

Each condition was run three times. A trial is one complete run of the experiment under one condition.

Drops of water held by a penny before spilling, three trials per condition (illustrative, constructed for this course, not real data)
Condition	Trial 1	Trial 2	Trial 3
Plain water	28	30	32
Soapy water	12	14	16

Three numbers per row are hard to compare at a glance, so you summarize each row two ways. The average is the sum of a set of trials divided by the number of trials. The range is the largest trial minus the smallest trial, which shows how far a set of repeated measurements spreads.

Worked example — the penny data. Work through the steps in order.

Plain-water average. Add the three trials: 28 + 30 + 32 = 90. Divide by the number of trials: 90 ÷ 3 = 30. The plain-water average is 30 drops.
Soapy-water average. Add: 12 + 14 + 16 = 42. Divide: 42 ÷ 3 = 14. The soapy-water average is 14 drops.
Difference. Subtract the soapy average from the plain average: 30 − 14 = 16. The penny held 16 fewer drops on average with soap.
Reduction. Divide the difference by the plain-water baseline: 16 ÷ 30 ≈ 0.53. The soapy water held about 53% fewer drops.
Range. Plain water: 32 − 28 = 4 drops. Soapy water: 16 − 12 = 4 drops. Each condition spreads over only 4 drops, while the gap between the averages is 16 drops.

In words: plain water averaged 30 drops versus 14 for soapy water, more than twice as many, and the trials within each condition sat close together, so the difference is much bigger than the trial-to-trial wobble.

Now name the parts, exactly as Section 4 taught you:

Independent variable: plain versus soapy water.
Dependent variable: the number of drops before the spill.
Controlled variables: the same penny, the same dropper, and the same height for every drop.
Control group: plain water.
A confound to avoid: if the soapy trials had used a different dropper with larger drops, the dropper would be a second difference between the groups, and you could not tell which difference caused the result.

The conclusion: the evidence supports the hypothesis. It does not prove it. One experiment with three trials per condition supports or fails to support a claim and invites a repeat. Honesty note: the penny result also depends on water's chemistry, which you have not met yet, so this week you can say what happened with confidence and only preview why.

In practice (your tools). In a free spreadsheet such as Google Sheets, type the three plain-water trials into cells B2, C2, and D2. In cell E2, type =AVERAGE(B2:D2) to get the average, and in cell F2, type =MAX(B2:D2)-MIN(B2:D2) to get the range. Repeat in row 3 for the soapy trials. Always check the spreadsheet's answer against your own hand calculation; a typo in one cell changes everything after it.

Misconception check. Students often compute the reduction as 16 ÷ 14 instead of 16 ÷ 30. The slip is tempting because 14 is the newer, more interesting number. But a reduction is measured against the baseline, and the baseline is the control group, plain water. Dividing by the control's average, 30, gives about 53%; dividing by 14 answers a different question.

Memory hook: add the trials and divide by the count; then compare to the control.

6. Tell a hypothesis from a theory

In everyday talk, people say "I have a theory" when they mean a hunch: "My theory is that the bus is late because of the game." In science, the word means almost the opposite.

A theory, in science, is a broad explanation backed by a large body of repeated tests and observations; it is the opposite of a hunch. A hypothesis is a single testable idea. A theory ties together many hypotheses that have survived test after test, often for generations, and it explains a wide range of observations at once.

Biology has several theories you will lean on all term:

the cell theory, the explanation that living things are built from cells and that the cell is the basic unit of life, which you met in Section 1;
the germ theory of disease; and
evolution, the unifying framework of all of biology, which is the subject of the next section.

Worked example — sort the statement. Decide whether each statement is a hypothesis or a theory.

"If tomato seedlings get a liquid plant food, then they will grow taller in four weeks than seedlings without it, because the food supplies material for growth." This is a hypothesis: one specific, testable, falsifiable idea about one situation.
"Living things are built from cells, and the cell is the basic unit of life." This is a theory: a broad explanation backed by an enormous body of observations across every kind of living thing.

In words: the first statement could be tested in a garden in a month; the second sums up a huge body of tested evidence.

A theory is still open to revision if strong new evidence appears. Caveat: that openness does not make a theory weak. A theory is the most supported kind of idea science has.

Misconception check. Students often think a hypothesis and a theory are both "only guesses." The idea is tempting because everyday speech uses "theory" for a hunch. In science, a theory is the most heavily tested and supported kind of explanation there is. If someone says evolution is "only a theory," they are using the everyday meaning of the word, not the scientific one.

Memory hook: a hypothesis is a test; a theory is what survived the tests.

7. Evolution: the framework that ties biology together

Look at the living things around you: a cat on a windowsill, mold on old bread, an oak tree, a single bacterium, you. They look nothing alike. Yet inside, they run on remarkably similar parts. Evolution is the idea that explains both facts at once: why life is so varied and why it is so alike underneath.

Evolution is change in the heritable traits of a population over generations; it is the unifying framework of biology. A heritable trait is one that can pass from parents to offspring. The main process that shapes that change is natural selection.

Natural selection is the process in which individuals with traits better suited to their environment survive and reproduce more, so those heritable traits become more common in the population over generations. Variation in the population is supplied by mutation, a random change in the DNA, and natural selection sorts that variation. Over long spans of time, the result is a population that is better suited, or adapted, to its environment. That is the seventh characteristic of life from Section 1.

Worked example — natural selection in a model population (illustrative). Picture a made-up population of beetles living on dark soil, constructed for this course to show the logic, not a real study.

Variation: some beetles inherit a dark shell and some a light shell.
Pressure: birds hunting on the dark soil spot light beetles more easily.
Survival and reproduction: dark beetles, on average, live longer and leave more offspring.
Inheritance: the offspring inherit their parents' shell color.
Change over generations: generation after generation, a larger share of the population has dark shells.

In words: no single beetle changed its color; the population changed, because dark-shelled beetles left more offspring.

Evolution explains life's diversity: populations in different environments are shaped in different directions over many generations, producing the variety of living things. It also explains life's unity, and that unity is the evidence. Nearly all living things share the same genetic code, build themselves from the same kinds of building-block molecules, and run the same basic cell machinery. The simplest explanation for so much shared equipment is that all living things descend from common ancestors.

How the traits are passed down, the genetics behind the beetles, waits until Weeks 10 through 12. For now, the framework is the point.

Misconception check. Students often think evolution means an individual organism changing during its lifetime, as if a beetle could darken its shell to hide. The idea is tempting because we see individuals adjust all the time. But evolution, as biologists define it, is change in a population's heritable traits across generations. Individuals survive or do not; populations evolve.

Memory hook: individuals are selected; populations evolve.

Pause and reflect. Choose one of these two prompts and think it through, or jot a few sentences in your notes.

(a) Think of a claim you have heard about health, food, or fitness. How would you turn it into a falsifiable hypothesis and test it with a control group?
(b) Think of a living thing you see often. Which of the seven characteristics of life is easiest to spot in it, and which is hardest to see?

Key terms

Characteristics of life — The seven properties biologists use together to recognize a living thing: order, energy processing, growth and development, reproduction, response to the environment, regulation and homeostasis, and evolutionary adaptation. (first used: 1. Recognize life: check the whole list)
Homeostasis — The regulation that holds a living thing's internal conditions steady, such as body temperature, even when the surroundings change. (first used: 1. Recognize life: check the whole list)
Cell — The smallest unit that is unambiguously alive; a single cell can be a complete organism on its own. (first used: 1. Recognize life: check the whole list)
Levels of organization — The ordered ladder biologists use to place any living structure, from atom to molecule, organelle, cell, tissue, organ, organ system, organism, population, community, ecosystem, and biosphere. (first used: 2. Climb the levels of organization)
Emergent property — An ability that appears at one level of organization but not in its separate parts, such as the pumping of a whole heart. (first used: 2. Climb the levels of organization)
Scientific method — The loop of observation, question, hypothesis, prediction, experiment, analysis, and conclusion that scientists use to test ideas against evidence. (first used: 3. Test ideas with the scientific method)
Hypothesis — A proposed explanation that can be tested and must be falsifiable, often written in an if-then-because form. (first used: 3. Test ideas with the scientific method)
Prediction — The specific result you expect to see in a test if the hypothesis is right. (first used: 3. Test ideas with the scientific method)
Falsifiable — Able to be shown wrong, because some possible result of a test would count against the idea. (first used: 3. Test ideas with the scientific method)
Controlled experiment — A test that compares groups that differ in only one deliberately changed factor, so any difference in the result can be traced to that factor. (first used: 4. Build a controlled experiment)
Independent variable — The factor the experimenter deliberately changes between groups. (first used: 4. Build a controlled experiment)
Dependent variable — The result the experimenter measures, which may change in response to the independent variable. (first used: 4. Build a controlled experiment)
Controlled variables — The conditions kept the same for every group, so they cannot explain a difference in the result. (first used: 4. Build a controlled experiment)
Control group — The no-treatment baseline group, which shows what happens without the change being tested. (first used: 4. Build a controlled experiment)
Confound — A second, unplanned difference between the groups that could also explain the result. (first used: 4. Build a controlled experiment)
Average — The sum of a set of trials divided by the number of trials. (first used: 5. Read the drops on a penny experiment from start to finish)
Range — The largest trial minus the smallest trial, which shows how far a set of repeated measurements spreads. (first used: 5. Read the drops on a penny experiment from start to finish)
Theory — In science, a broad explanation backed by a large body of repeated tests and observations; the opposite of a hunch. (first used: 6. Tell a hypothesis from a theory)
Evolution — Change in the heritable traits of a population over generations; the unifying framework of biology. (first used: 7. Evolution: the framework that ties biology together)
Natural selection — The process in which individuals with traits better suited to their environment survive and reproduce more, so those traits become more common over generations. (first used: 7. Evolution: the framework that ties biology together)

Summary

Something is alive only if it shows all seven characteristics of life.
The cell is the smallest unit that is unambiguously alive.
The levels run atom to biosphere, and each level adds emergent properties.
The scientific method is a loop; a hypothesis must be falsifiable.
Evidence supports or fails to support a hypothesis; it never proves it.
A controlled experiment changes one independent variable and measures one dependent variable against a control group.
A confound is a second difference between groups; design it out.
An average is the sum of the trials divided by their count; a range is the largest minus the smallest.
A theory is a broad, heavily tested explanation, not a guess.
Evolution by natural selection explains both life's diversity and its unity.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

A robot vacuum moves around a room, uses electricity, and turns when it bumps into a chair. Is it alive? Use the checklist to explain.
Put these in order from smallest to largest: a forest's oak trees, a leaf cell, a whole oak tree, a leaf, all the living things and the soil, water, and air of the forest.
A gardener tests a liquid plant food on tomato seedlings (illustrative numbers). Three fed seedlings reach 21, 24, and 27 cm after four weeks; three unfed seedlings reach 15, 18, and 21 cm. Find each group's average and the difference. Then name the independent variable, the dependent variable, and the control group.
A student tests whether chewing gum improves memory. The gum group takes the memory test at 9 a.m.; the no-gum group takes it at 9 p.m. The gum group scores higher. What is the problem with this design, and how would you fix it?

Answers (worked reasoning — read after attempting)

No. The robot vacuum checks a few boxes: it has order, it processes energy (electricity), and it responds to the chair. But it does not grow and develop, does not reproduce its own kind, does not regulate its inside conditions the way a living thing does, and does not adapt over generations. Life is the whole checklist, so a few checked boxes are not enough.
Smallest to largest: a leaf cell (cell), a leaf (organ), a whole oak tree (organism), the forest's oak trees (population), and all the living things plus the soil, water, and air (ecosystem). The order matches the ladder: cell → organ → organism → population → ecosystem.
Fed average: 21 + 24 + 27 = 72, and 72 ÷ 3 = 24 cm. Unfed average: 15 + 18 + 21 = 54, and 54 ÷ 3 = 18 cm. Difference: 24 − 18 = 6 cm, so the fed seedlings averaged 6 cm taller. The independent variable is whether a seedling gets plant food; the dependent variable is seedling height after four weeks; the control group is the unfed seedlings. The evidence supports the idea that the plant food helps; it does not prove it.
Time of day is a confound: the groups differ in gum and in testing time, so you cannot tell which one made the difference. Fix it by testing both groups at the same time of day, so gum is the only difference between them.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz — and next week the hydrogen-bond mechanism behind water's cohesion explains the penny result.

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