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Week 8 · Midterm study guide

Midterm Exam — Study Guide

Introduction to Biology Free sample edition

Coverage — what the midterm covers

The midterm is cumulative over Weeks 1–7. It opens in the Week 8 module and closes at the due time your instructor sets. There is no time limit, and you get one attempt. The questions appear one at a time, and you cannot go back once you move on.

Format: 25 questions, and every question counts the same. Eighteen ask you to pick one option, four ask you to judge a statement as true or false, and three ask you to pick every option that applies. Any data table or labeled figure a question needs is described in words in the question, and the arithmetic is short enough for scratch paper.

What is in scope, with the number of midterm questions on each course objective:

Midterm coverage by course objective: the weeks the drawn questions come from, the main skills, and the number of questions out of 25
Course objective Weeks the questions come from Main skills Questions
1. The practice of science 1 and 7 The characteristics of life as a set; the parts of a controlled experiment; averaging trials; hypothesis and theory; evolution as the framework 4
2. The chemistry of life and the macromolecules 2, 3, and 5 The three bonds; water's polarity and its properties; the pH scale and buffers; the four classes; enzymes as proteins whose shape is their job 7
3. The cell 4 and 6 Prokaryote and eukaryote; organelles and their jobs; passive and active transport; tonicity; the falling surface-area-to-volume ratio; the mitochondrion's parts 5
4. Energy, enzymes, respiration, and photosynthesis 5, 6, and 7 Energy and its two laws; ATP; enzymes; the four stages of respiration in order with their places; fermentation; the two stages of photosynthesis; atom tracking 9

By week, the questions come out as 3 from Week 1, 4 from Week 2, 1 from Week 3, 4 from Week 4, 5 from Week 5, 4 from Week 6, and 4 from Week 7. Energy carries the most questions, 9 of the 25, so put the most review time on Weeks 5 to 7. Chemistry and the macromolecules come next with 7. Week 3 has only one question of its own, but its proteins come back inside the Week 5 enzyme questions. Week 8's order-and-location habit is not scored on its own, and practicing it is still good review, because it runs on the Week 4, 6, and 7 processes the midterm does check.

Topic-by-topic review

Each topic gives the key idea in plain words, the course's own definitions, a course example, and the mistakes students make most often with the cure for each. The deeper review for every topic is that week's chapter, lecture page, slide deck, and readings.

Week 1 — The science of biology

  • Key idea: life is a whole checklist, not one box, and science tests an idea against a control before it trusts it.
  • The characteristics of life: the seven properties biologists use together to recognize a living thing are order, energy processing, growth and development, reproduction, response to the environment, regulation and homeostasis, and evolutionary adaptation. Something counts as alive only when it shows the whole set. Fire grows and uses energy, and a crystal is ordered, but neither is alive. The cell is the smallest unit that is unambiguously alive; a single cell can be a complete organism on its own.
  • Levels of organization, in order: atom → molecule → organelle → cell → tissue → organ → organ system → organism → population → community → ecosystem → biosphere. An emergent property is an ability that appears at one level but not in its separate parts: a heart cell twitches, but only the whole heart pumps.
  • The scientific method: the loop of observation → question → hypothesis → prediction → experiment → analysis → conclusion, with each conclusion raising a new question. A hypothesis is a proposed explanation that can be tested and must be falsifiable, able to be shown wrong. Evidence supports or fails to support a hypothesis; it never proves it.
  • A controlled experiment: the independent variable is the factor the experimenter deliberately changes; the dependent variable is the result the experimenter measures. The controlled variables are kept the same for every group. The control group is the no-treatment baseline. A confound is a second, unplanned difference between the groups that could also explain the result.
  • Hypothesis and theory: a theory is a broad explanation backed by a large body of repeated tests and observations, such as the cell theory, the germ theory, and evolution. It is the opposite of a hunch.
  • Evolution: change in the heritable traits of a population over generations, the unifying framework of biology. Natural selection is the process in which individuals with traits better suited to their environment survive and reproduce more. Life's unity is the evidence: a near-universal genetic code, the same building-block molecules, and the same cell machinery.
  • Course example (illustrative, not real data): in the drops on a penny example, plain water held 28 + 30 + 32 = 90 drops in three trials, and 90 ÷ 3 = 30. Soapy water held 12 + 14 + 16 = 42, and 42 ÷ 3 = 14. The difference is 30 − 14 = 16 drops, and the evidence supports the soap hypothesis without proving it.
  • Predictable mistakes → cures: (1) "if it moves and grows, it is alive." Cure: run the whole checklist of seven. (2) a theory called "a guess." Cure: a theory is the most supported kind of idea in science. (3) "one experiment proves it." Cure: say supports or fails to support. (4) the control group read as the group that gets the treatment. Cure: it is the no-treatment baseline. (5) the independent and dependent variables swapped. Cure: you change the independent one; the result depends on it.

Week 2 — The chemistry of life

  • Key idea: a bond's verb (share or transfer) names the bond, and water's lopsided charge explains almost everything water does.
  • Atoms and molecules: an atom is the smallest unit of an element that still has that element's properties, built from protons, neutrons, and electrons. A molecule is two or more atoms joined by chemical bonds: H is an atom, and H₂O is a molecule. Oxygen, carbon, hydrogen, and nitrogen make up about 96% of the body by mass (oxygen 65%, carbon 18%, hydrogen 10%, nitrogen 3%).
  • The three bonds: a covalent bond is a strong bond in which two atoms share a pair of electrons. An ionic bond forms when one atom transfers an electron to another and the two oppositely charged ions attract, as in table salt. A hydrogen bond is a weak attraction between a slightly positive hydrogen on one molecule and a slightly negative atom on another; one is weak, but many together are powerful.
  • Polarity: in water, oxygen pulls the shared electrons toward itself, so the oxygen is δ− and each hydrogen is δ+. That lopsided charge lets water molecules hydrogen-bond to each other.
  • Water's properties: cohesion is the attraction of water molecules to each other, which produces surface tension. Adhesion is the attraction of water to a different surface. Capillary action is water climbing a narrow tube as adhesion pulls and cohesion drags more water behind. Water's high specific heat lets oceans and bodies change temperature slowly. Evaporative cooling is why sweat cools you. Ice floats because hydrogen bonds lock frozen molecules farther apart. Water is the solvent of every cell.
  • pH: a measure of hydrogen-ion concentration on an inverse logarithmic scale from 0 to 14, where 7 is neutral, a lower number means more hydrogen ions, and each whole unit is a tenfold change. Count the steps first, then raise ten to that power. Low number, high acid. Pure water at pH 7 has a hydrogen-ion concentration of 1 × 10⁻⁷ moles per liter.
  • Buffers: a buffer resists a change in pH; it slows a change but never prevents one, and it can be overwhelmed. The carbonic acid and bicarbonate buffer holds blood near pH 7.4.
  • Course example: pH 4 against pH 7 is three steps (4 to 5, 5 to 6, 6 to 7), so 10³ = 1,000 times more hydrogen ions at pH 4.
  • Predictable mistakes → cures: (1) "ionic bonds share electrons." Cure: covalent shares; ionic transfers. (2) cohesion and adhesion swapped. Cure: co is water with water; ad is attached to another surface. (3) "a higher pH is more acidic." Cure: low number, high acid. (4) the pH scale read as a ruler. Cure: pH 4 is 1,000 times more acidic than pH 7, not a bit more. (5) "a buffer prevents any change." Cure: it resists, and it can be overwhelmed.

Week 3 — Biological macromolecules

  • Key idea: big molecules are built from small units by removing water and broken apart by adding it back, and a molecule's shape decides its job.
  • Building and breaking: a monomer is one small building-block unit; a polymer is a long chain of monomers. Dehydration synthesis builds a polymer by joining two monomers and removing one water molecule for each new bond. Hydrolysis breaks a polymer by adding a water molecule back across a bond; digestion is hydrolysis.
  • The four classes and their building blocks: carbohydrates are built from monosaccharides such as glucose; proteins from amino acids (20 common kinds) joined by peptide bonds; nucleic acids from nucleotides. Lipids are the one class that is not a polymer: a triglyceride is one glycerol and three fatty acids, an assembly rather than a chain.
  • Carbohydrates: monosaccharide → disaccharide → polysaccharide. Starch is stored fuel and cellulose is structural fiber, both built from glucose linked differently.
  • Lipids: a saturated fatty acid has a straight tail that packs tightly, so its fat stays solid; an unsaturated one has a kinked tail, so its fat stays liquid. A phospholipid has a water-loving head and two water-fearing tails. A steroid is built on four fused rings.
  • Proteins: the four levels in order are primary structure (the exact order of amino acids) → secondary structure (helices and pleated sheets) → tertiary structure (the three-dimensional fold of one chain) → quaternary structure (two or more chains assembled). Denaturation is the loss of a protein's shape, and with it its function; the amino-acid sequence stays intact.
  • Nucleic acids: DNA has the sugar deoxyribose, uses thymine, and forms a double helix. RNA has the sugar ribose, uses uracil where DNA uses thymine, and is usually a single strand.
  • Course example: in sickle cell disease, valine takes the place of glutamic acid at position 6 of hemoglobin's beta chain, one change out of roughly 600 amino acids. Structure determines function. In the iodine starch test (illustrative, qualitative), bread and potato turn blue-black, while sugar water and cooking oil stay amber, and plain water is the negative control.
  • Predictable mistakes → cures: (1) "lipids are polymers like the other three." Cure: a triglyceride is an assembly. (2) dehydration synthesis and hydrolysis swapped. Cure: build by removing water; break by adding water. (3) "all carbs are sugar." Cure: cellulose is a carbohydrate and is the dietary fiber. (4) "RNA contains thymine." Cure: RNA uses uracil. (5) denaturation read as breaking the chain. Cure: the fold is lost, and the sequence stays.

Week 4 — Cell structure and function

  • Key idea: every cell shares a few basic parts, each organelle does one job, and the membrane is a gate that a growing cell outgrows.
  • Two kinds of cell: a prokaryotic cell has no nucleus and no membrane-bound organelles; its DNA sits in the nucleoid, and it is typically about 0.1 to 5.0 μm across. A eukaryotic cell has a true nucleus and membrane-bound organelles and is typically about 10 to 100 μm across. Every cell shares a plasma membrane, cytoplasm, DNA, and ribosomes.
  • Organelles and their jobs: the nucleus holds the DNA and directs the making of proteins. A ribosome builds proteins. The rough endoplasmic reticulum makes and folds proteins for export; the smooth endoplasmic reticulum makes lipids and helps detoxify. The Golgi apparatus modifies, sorts, and ships. The mitochondrion makes ATP, and its cristae add surface for the job. The chloroplast carries out photosynthesis. A lysosome breaks down waste. A plant's central vacuole stores water and keeps the cell firm. A cell wall supports plants, fungi, and bacteria; animal cells have none. A plant cell has chloroplasts and mitochondria, both.
  • The membrane: a phospholipid bilayer with proteins set into it, described by the fluid mosaic model (Singer and Nicolson, 1972), a classroom model. It is selectively permeable: a gate, not a wall.
  • Passive transport (no energy spent, down the gradient): diffusion, from more concentrated to less; facilitated diffusion, still downhill but through a protein channel or carrier; and osmosis, the diffusion of water toward the side with more dissolved solute. Osmosis moves water, not the solute.
  • Tonicity, always read from the cell's side: hypotonic describes a solution with less solute than the inside of the cell, so water moves in; an animal cell can swell and burst. Hypertonic describes a solution with more solute, so water moves out and the cell shrivels. Isotonic means no net change.
  • Active transport: movement against the gradient, paid for with ATP by a pump. The sodium-potassium pump uses one ATP in each cycle to move three Na⁺ out and two K⁺ in. Passive is downhill and free; active is uphill and paid.
  • Why cells stay small: the surface-area-to-volume ratio falls as a cell grows, because surface area grows as the square and volume as the cube. For a cube of side s, surface area = 6s², volume = s³, and the ratio simplifies to 6 ÷ s.
  • Course example (illustrative): the cube cell of side 3 has surface area 6 × 3 × 3 = 54 and volume 3 × 3 × 3 = 27, so the ratio is 54 ÷ 27 = 2, written 2:1. The four cubes give 6:1, 3:1, 2:1, and 1.5:1, falling as the side grows.
  • Predictable mistakes → cures: (1) "plant cells have chloroplasts instead of mitochondria." Cure: they have both. (2) "animal cells have a cell wall." Cure: plants, fungi, and bacteria do. (3) "osmosis moves the salt." Cure: osmosis moves water. (4) hypotonic and hypertonic flipped. Cure: the reference is the inside of the cell. (5) "a bigger cell has more surface, so it works better." Cure: volume outgrows surface, so the ratio drops. (6) a ratio left as 54:27 or divided backwards. Cure: surface first, volume second, divide, simplify.

Week 5 — Energy, enzymes, and metabolism

  • Key idea: a cell never makes energy; it captures and transfers it, and enzymes let it do so fast enough to stay alive.
  • Energy: energy is the capacity to do work. Kinetic energy is the energy of motion, including heat; potential energy is stored energy, as in a stretched bowstring or the chemical bonds of food.
  • The two laws: the first law of thermodynamics says energy is transferred and transformed but never created or destroyed. The second law of thermodynamics says every transfer loses some energy as heat that can no longer do work, which raises entropy.
  • Metabolism: catabolic reactions break molecules down and release energy, so they are exergonic. Anabolic reactions build molecules and require energy, so they are endergonic.
  • ATP: adenosine carrying a chain of three phosphates, the cell's energy currency. ATP → ADP + phosphate + energy, and food recharges ADP back to ATP. ADP carries two phosphates and is the spent form. ATP is the energy currency and DNA is the instruction set.
  • Enzymes: an enzyme is a protein that speeds up a reaction by lowering its activation energy, without changing where the reaction starts or ends. It grips its substrate at the active site, does one job, and comes out unchanged to be reused. The lock-and-key model and the induced fit model are both classroom models.
  • What changes the rate: temperature rises the rate to an optimum near 37 °C for human enzymes, then heat past the optimum denatures the enzyme and the rate crashes to zero. Cold only slows it. Most human enzymes work best near neutral pH, but pepsin works in the acidic stomach, roughly pH 1.5 to 3.5. More substrate speeds the reaction until saturation, when every active site is busy. Feedback inhibition lets a pathway's end product slow an earlier enzyme.
  • Course example (illustrative, not real data): the catalase and temperature averages are 2 cm cold, 5 cm at room temperature, 10 cm at body temperature, and 0 cm boiled. The shape is a rise to a peak and then a crash, and 10 ÷ 2 = 5, so the body-temperature foam is 5 times the cold foam.
  • Predictable mistakes → cures: (1) "a cell makes energy." Cure: the first law forbids it. (2) "enzymes get used up." Cure: an enzyme is reused. (3) "an enzyme adds energy." Cure: it only lowers the hill. (4) "hotter is always faster." Cure: only up to the optimum. (5) denatured read as slowed. Cure: a cooked egg does not un-cook; cold slowing reverses. (6) ATP and ADP swapped. Cure: three phosphates is charged; two is spent.

Week 6 — Cellular respiration

  • Key idea: respiration releases food energy to recharge ATP in four ordered stages, each in its own place.
  • The definition: cellular respiration is the chemistry inside cells that breaks down glucose with oxygen, releasing carbon dioxide and water and using the released energy to recharge ATP. It is not breathing, and nearly every living thing, plants included, does it day and night. The equation: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP.
  • The four stages, in order, with their places: glycolysis (cytoplasm) → pyruvate oxidation (mitochondrion) → citric acid cycle (mitochondrial matrix) → electron transport chain (inner mitochondrial membrane). Glycolysis splits one glucose into two pyruvate and nets 2 ATP and 2 NADH without oxygen. The citric acid cycle releases CO₂ and loads NADH and FADH₂. The electron transport chain makes the most ATP, and oxygen is the final electron acceptor, joining with hydrogen to form water.
  • The total: about 30 to 32 ATP per glucose in current estimates (36 to 38 in older ones), an estimate to learn in words, not a number to memorize.
  • Fermentation: a low-energy backup for when oxygen runs out, glycolysis plus a side path that turns NADH back into NAD⁺. Lactic acid fermentation runs in hard-working muscle and in red blood cells; alcoholic fermentation runs in yeast, pyruvate → ethanol + CO₂. Each yields only glycolysis's 2 ATP.
  • Course example (illustrative, not real data): in the yeast balloon example, 0, 1, 2, and 4 teaspoons of sugar gave 0, 8, 14, and 18 cm. The increments are 8 − 0 = 8, 14 − 8 = 6, and 18 − 14 = 4 cm, a pattern of diminishing returns, and the 0-teaspoon control reads 0.
  • Predictable mistakes → cures: (1) "cellular respiration is breathing." Cure: breathing moves the gases; respiration uses them inside cells. (2) "plants do not respire." Cure: they respire around the clock. (3) the citric acid cycle placed in the cytoplasm. Cure: it runs in the matrix. (4) "most of the ATP comes from glycolysis." Cure: the chain makes the most. (5) "oxygen is used in glycolysis." Cure: oxygen acts at the very end. (6) fermentation read as a high-ATP alternative. Cure: it is a 2-ATP backup.

Week 7 — Photosynthesis

  • Key idea: a plant builds sugar from carbon pulled out of the air, in two ordered stages, and releases oxygen that came from water.
  • The definition: photosynthesis is the process in which a plant, alga, or other autotroph uses light energy to build sugar from carbon dioxide and water, releasing oxygen. The equation: 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂. CO₂ enters through the stomata, and water comes up from the roots. An autotroph makes its own food; a heterotroph eats food something else made.
  • The chloroplast: two membranes wrap the fluid stroma; disc-shaped thylakoids are stacked into grana; chlorophyll in the thylakoid membrane absorbs red and blue light and reflects green.
  • The two stages, in order, with their places: the light-dependent reactions (thylakoid membrane) → the Calvin cycle (stroma). The light-dependent reactions split water, release O₂, and make ATP and NADPH. The Calvin cycle fixes CO₂ with the enzyme RuBisCO and spends that ATP and NADPH to build the three-carbon sugar G3P; three turns export one G3P. The Calvin cycle does not use light directly, but it stops soon in the dark, because its ATP and NADPH run out.
  • Atom tracking: the O₂ a plant releases comes from water, not from CO₂, and the carbon in sugar comes from CO₂. A tree's mass is mostly carbon from the air, plus water; soil supplies minerals, not bulk.
  • Two processes side by side: photosynthesis and cellular respiration are roughly mirror images, and a plant does both. Early oxygen-producing photosynthesis is how Earth gained the oxygen that aerobic respiration depends on.
  • Course example (illustrative, not real data): in the floating leaf disk example, the light cup read 0, 2, 5, 8, and 10 disks out of 10 at 0, 3, 6, 9, and 12 minutes. The dark cup read 0, 0, 0, 1, and 1. The light average is 25 ÷ 5 = 5.0, the dark average is 2 ÷ 5 = 0.4, and at 12 minutes 10 − 1 = 9 more disks float in the light.
  • Predictable mistakes → cures: (1) "a plant's mass comes from the soil." Cure: mostly carbon from CO₂, plus water. (2) "the oxygen comes from the carbon dioxide." Cure: it comes from splitting water. (3) "the Calvin cycle does not need light." Cure: it needs the light reactions' ATP and NADPH. (4) thylakoid and stroma swapped. Cure: light is caught on the thylakoid membrane; sugar is built in the stroma. (5) "plants photosynthesize instead of respiring." Cure: they do both. (6) glucose and O₂ listed as reactants. Cure: they are products.

Week 8's new habit (not scored on its own)

The order-and-location habit is a two-part check run on any biological process: name its stages in the order they happen, and name the compartment of the cell where each stage runs, before adding any other detail. A compartment is a space or membrane inside a cell where one stage runs, such as the cytoplasm, the matrix, the inner mitochondrial membrane, the thylakoid membrane, or the stroma. A pathway is a chain of steps a cell runs in a fixed order, written as an arrow chain with each stage's compartment. The course runs the habit on respiration (four stages, four places), photosynthesis (two stages, two places), and an exported protein (nucleus → a ribosome on the rough ER → the rough ER → the Golgi apparatus → the plasma membrane).

Representative practice

These are fresh variants, not the midterm's questions. Work each worked example yourself first, then compare your steps. Every number below comes from the course's own pinned values or from arithmetic on them, computed and checked in advance, and every model dataset is labelled illustrative.

Worked example — alive or not, and the parts of an experiment (Week 1)

Alive or not? A robot vacuum hums across a kitchen floor. It runs on stored battery energy, it senses a wall, and it turns away.

  • It seems to show two boxes: energy processing and a response to the environment.
  • It fails the rest of the set: it does not grow and develop, reproduce, hold its insides steady by regulation, or adapt over generations, and it is not built of cells.
  • Verdict: not alive. The rule is the whole checklist of seven, never one or two boxes.

Name the parts. A gardener asks whether light changes how tall basil grows. Six identical pots of basil get the same soil, the same water, and the same pot size. Three sit under a grow lamp, and three sit in a shaded corner.

  • Independent variable: the light (lamp versus shade), the factor deliberately changed.
  • Dependent variable: the height of the basil, the result measured.
  • Controlled variables: the soil, the water, and the pot size.
  • Control group: the shaded pots, the baseline without the extra light.
  • A confound to avoid: if the lamp pots also got extra fertilizer, a second difference would hide which change mattered.

Recompute a course figure (illustrative): the penny example's two ranges are 32 − 28 = 4 drops for plain water and 16 − 12 = 4 drops for soapy water. The gap between the averages is 30 − 14 = 16 drops, and 16 ÷ 4 = 4, so the gap is four times as wide as either condition's spread.

Why it matters: a gap that dwarfs the spread is what lets the evidence support the hypothesis, and the honest verb is still supports, never proves.

Worked example — steps on the pH ladder and water at work (Week 2)

Count the steps, then raise ten:

  • Lemon juice at about pH 2 against a liquid at pH 4: count the steps from 2 up to 4 (2 to 3, 3 to 4), which is two steps. So 10² = 100 times more hydrogen ions sit in the lemon juice.
  • Tap water at about pH 7 against baking-soda solution at about pH 9: two steps again, so 10² = 100 times more hydrogen ions in the tap water. The lower number holds more hydrogen ions.
  • Concentration: [H⁺] = 10 raised to the power of minus the pH, so pH 5 means 1 × 10⁻⁵ moles per liter.

Match the property to the scene:

  • Sugar stirred into iced tea disappears: water is the solvent.
  • On a long hike, sweat on your skin cools you as it dries: evaporative cooling.
  • A water strider stands on a pond without sinking: cohesion, which makes surface tension.

Why it matters: the pH scale counts tenfold steps, not the even spacing of a ruler, and each property of water traces back to hydrogen bonds between polar molecules.

Worked example — build it, break it, sort it (Week 3)

Count the water. Picture a short chain built from one of each of the 20 common amino acids, joined in a row.

  • Joining 20 amino acids in a row makes 20 − 1 = 19 peptide bonds.
  • One water molecule leaves at every new bond, so 19 water molecules are removed.
  • Breaking the whole chain by hydrolysis adds the same 19 water molecules back.

Sort four molecules into their classes:

  • Glycogen: a carbohydrate, a polysaccharide built from glucose.
  • Olive oil: a lipid, an assembly of glycerol and fatty acids, not a polymer. It stays liquid, the sign of kinked, unsaturated tails.
  • A strand of DNA: a nucleic acid, built from nucleotides.
  • An enzyme: a protein, built from amino acids.

Why it matters: "bonds = units − 1" keeps the water count honest, and lipids are the one class that is not a chain of one repeating monomer.

Worked example — osmosis, the pump, and the cube (Week 4)

Read tonicity from the cell's side:

  • A raisin dropped into a glass of water plumps up. The water is hypotonic to the raisin's cells, so water moves in by osmosis.
  • A freshwater aquarium fish placed in seawater loses water from its cells. Seawater is hypertonic to those cells, so water moves out. In both cases the water moves, not the salt.

The pump, per ATP: each cycle spends 1 ATP to move 3 Na⁺ out and 2 K⁺ in. That is 3 + 2 = 5 ions moved for each ATP, and 3 − 2 = 1 more positive ion leaves than enters on each cycle. It moves them against their gradients, so it is active transport.

The cube, worked (illustrative):

  • Side 4: surface area 6 × 4 × 4 = 96 square units; volume 4 × 4 × 4 = 64 cubic units; ratio 96 ÷ 64 = 1.5, written 1.5:1.
  • Side 1 against side 4: 6 ÷ 1.5 = 4, so the smallest cube has four times as much surface for each unit of volume as the largest.

Why it matters: a big cell has too little membrane for the volume it must feed, which is why cells stay small or divide, flatten, or fold.

Worked example — energy sorts and the enzyme curve (Week 5)

Sort the energy:

  • A stretched bowstring at an archery range: potential energy, stored.
  • The arrow in flight: kinetic energy, motion.
  • Digesting a sandwich at lunch: catabolic and exergonic, breaking down and releasing energy.
  • A cell building a protein: anabolic and endergonic, building up and requiring energy.

Recompute the catalase peak (illustrative): the body-temperature trials were 9, 10, and 11 cm. 9 + 10 + 11 = 30, and 30 ÷ 3 = 10 cm. Against the room average, 10 ÷ 5 = 2, so the body-temperature foam is twice the room foam.

A fever, read through the curve: a thermometer at a campus health center shows a fever a few degrees above the body's set point of about 37 °C. Enzymes tuned to about 37 °C are pushed off their optimum, which helps explain feeling weak and foggy. Cold would only slow them; enough heat past the optimum denatures them.

Why it matters: the gradable facts are the shape (a rise to a peak, then a crash) and that boiling denatures for good while cold only slows.

Worked example — bread, balloons, and a lost pound (Week 6)

At a bakery: dough rises because yeast run alcoholic fermentation, pyruvate → ethanol + CO₂, and the CO₂ gas lifts the bread. The yeast make only glycolysis's 2 ATP per glucose this way.

Check the yeast balloon totals (illustrative): the increments add to 8 + 6 + 4 = 18 cm, which matches 18 − 0 = 18 cm, the change from the control to the fullest bottle. Doubling the sugar from 2 to 4 teaspoons added 18 − 14 = 4 cm, and 8 ÷ 2 = 4, so the last doubling added half of what the first teaspoon did.

Where does a lost pound of fat go? Mostly out as the CO₂ you breathe out. Respiration breaks fuel down to carbon dioxide and water.

Order and place: glycolysis runs in the cytoplasm, pyruvate oxidation in the mitochondrion, the citric acid cycle in the matrix, and the electron transport chain on the inner membrane.

Why it matters: fermentation keeps glycolysis running with very little ATP, and the four stages always run in the same order in the same places.

Worked example — the two-box card and the leaf disks (Week 7)

A houseplant on a sunny windowsill, as a two-box card:

  • Box 1, light-dependent reactions (thylakoid membrane): in go light and water; out come O₂, ATP, and NADPH.
  • Box 2, Calvin cycle (stroma): in go CO₂, ATP, and NADPH; out comes the three-carbon sugar G3P.

Count the Calvin cycle turns: three turns export one G3P with 3 carbons. Glucose has 6 carbons, so 6 ÷ 3 = 2 G3P make one glucose, and 2 × 3 = 6 turns fix the 6 CO₂ in the equation.

Recompute the leaf disks (illustrative): the light cup's increments are 2 − 0 = 2, 5 − 2 = 3, 8 − 5 = 3, and 10 − 8 = 2. Together 2 + 3 + 3 + 2 = 10, every disk in the cup. At 12 minutes, 10 ÷ 1 = 10, so ten times as many disks float in the light as in the dark.

Why it matters: the oxygen that lifts each disk came from split water, and the carbon in the sugar came from CO₂.

Worked example — the order-and-location habit (Week 8)

Run the habit on the windowsill houseplant at night, in one of its root cells. The lamp is off, but the cell still respires.

  • Stages in order: glycolysis → pyruvate oxidation → citric acid cycle → electron transport chain.
  • Places: cytoplasm → mitochondrion → matrix → inner membrane.
  • The check: four stages, four places, said aloud before any detail. A citric acid cycle placed in the cytoplasm fails the habit at once.

Why it matters: the habit catches the two classic slips, a stage out of order and a stage in the wrong place.

Self-check questions

Try each one, then read the check and the one-line why.

  1. Which level of organization comes right after tissue? Check: organ. Why: tissues build organs, and organs build organ systems.
  2. A claim says no possible result could ever count against it. Can it serve as a hypothesis? Check: no. Why: a hypothesis must be falsifiable.
  3. During digestion, a starch chain is broken into glucose. Dehydration synthesis or hydrolysis? Check: hydrolysis. Why: breaking a polymer adds water back across each bond.
  4. Does a cell from a dog's skin have a cell wall? Check: no. Why: animal cells have none; plants, fungi, and bacteria do.
  5. Which organelle makes lipids: the rough or the smooth endoplasmic reticulum? Check: the smooth ER. Why: the rough ER, studded with ribosomes, makes and folds proteins for export.
  6. Which molecule is the energy currency, and which is the instruction set? Check: ATP is the currency; DNA is the instructions. Why: the course never lets the two blur.
  7. Which two stages of respiration release CO₂, and how many CO₂ in all per glucose? Check: pyruvate oxidation and the citric acid cycle; 2 + 4 = 6 CO₂. Why: two leave in pyruvate oxidation and four in the cycle.
  8. Which stage of photosynthesis makes NADPH, and which spends it? Check: the light-dependent reactions make it; the Calvin cycle spends it. Why: stage one loads the carrier that stage two uses to build sugar.

Check yourself — choose one. Write two or three sentences to a friend.

  • (a) Explain why the boiled potato read 0 cm of foam while the cold potato still read 2 cm.
  • (b) Explain why the cube of side 4 has less surface for each unit of volume than the cube of side 1.

Either one rehearses an explain-why skill the midterm checks.

Watch before the exam

These eight videos come from the course's Readings & Resources pages for Weeks 1 to 8, and every one has captions. Use them to review and to connect weeks, not to learn anything new.

Study plan

A short countdown. Count back from the day you plan to sit the midterm.

  • Two weeks out: as you finish Week 6, start a one-page list of the course's swapped pairs: independent and dependent variable, covalent and ionic bond, cohesion and adhesion, dehydration synthesis and hydrolysis, hypotonic and hypertonic, ATP and ADP. Add thylakoid and stroma when Week 7 arrives.
  • One week out: as you finish Week 7, reread the topic reviews above for Weeks 1 to 3, the weeks furthest back.
  • At the start of Week 8: take the practice exam once, cold and closed-note. Note which objectives cost you the most.
  • Three or four days before you sit it: reread the topic reviews for your two weakest objectives, and redo the matching worked examples from scratch without looking.
  • Two days before: run the paired exam-prep tutorial (the copy-and-paste chatbot tutor, or its no-AI route) for active recall. Tell it which areas the practice exam exposed.
  • The day before: retake the practice exam, explain every miss out loud, and work the self-check questions above.
  • Right before: say the two pathways aloud in order with their places, skim the "predictable mistakes → cures" lines for all seven weeks, and then stop studying.

This guide is for review. The exam-prep tutorial and the practice exam are where you rehearse. Pulling ideas back out of memory, not rereading, is what makes them stick.

Exam format

Every one of the 25 questions counts the same, and the midterm checks your own understanding without AI. The questions come one at a time, and there is no going back, so settle each one before you move on.

Test-taking strategies for this material:

  • Name the pair before you choose. Most questions turn on two neighboring ideas, such as covalent and ionic, hypotonic and hypertonic, or thylakoid and stroma. Say which pair is in play first.
  • Say the order and the place. For any process, name the stages in order and where each one runs before you read the options.
  • Read tonicity from the cell's side. Ask which way the water moves, not the salt.
  • Count pH steps, then raise ten. Never subtract and stop there.
  • Average with every step on scratch paper. Add the trials, then divide by the number of trials.
  • For a "pick every option that applies" question, judge each statement on its own, as if it were a true-or-false question.

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