Week 7 · Chapter

Chapter 7 — Photosynthesis

Introduction to Biology Free sample edition

The module's primary reading · ~35–45 minutes · Objective 4 (explain how cells obtain and transform energy — the laws of thermodynamics, ATP, and enzyme action with its temperature and pH limits — and trace cellular respiration and photosynthesis as ordered, located overviews, including fermentation and the relationship between the two processes), with Objective 3 (compare prokaryotic and eukaryotic cells, relate each eukaryotic organelle to its function, explain how substances cross the plasma membrane by passive and active transport (including osmosis and tonicity), and use the surface-area-to-volume ratio to explain why cells stay small) and 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) carried forward · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Every green leaf you walk past is building sugar out of air, water, and light, and this week you learn how. You will meet photosynthesis (the way a plant uses light energy to build sugar from carbon dioxide and water, giving off oxygen). You will see that it runs in two stages, in two places inside a small green structure in the leaf. You will also learn to track the atoms (follow one kind of atom from what goes in to what comes out), which settles two old puzzles: where the oxygen you breathe comes from, and where a tree gets its mass. Last week you followed sugar being taken apart; this week you watch it being built.

Why you already care about this

A redwood starts as a seed small enough to pinch between two fingers and becomes a towering tree. Where did all that mass come from? Ask a few friends and most will say the soil. It is a sensible guess. Roots go into the soil, the tree gets bigger, so the tree must be made of soil.

It is not. Most of the tree came out of the air. Every glucose a leaf builds pulls 6 carbon dioxide molecules out of the air around it. Inside the leaf, a cycle has to turn 3 times to send out one small sugar. And in this week's lab, a cup of 10 spinach disks rises to the surface, one disk at a time, lifted by oxygen the leaves make from water. Those three numbers are the whole week in miniature: carbon in from the air, a cycle that builds sugar, and oxygen out from water.

You meet this chemistry every day without naming it. The salad on your plate, the bread in your sandwich, and the oxygen in your next breath all trace back to a leaf in the light. This chapter gives that everyday fact its biology.

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

read the photosynthesis equation both ways, naming where each reactant comes from and what each product is;
sort living things into autotrophs and heterotrophs;
describe the chloroplast in words, part by part, and say what each part does;
name the two stages of photosynthesis in order, with the place each one runs;
explain why the Calvin cycle stops soon after the light goes out;
track the oxygen back to water and the carbon back to carbon dioxide, and use that to explain a tree's mass;
compare photosynthesis with cellular respiration and explain why a plant does both;
read the floating leaf disk experiment, compute its two averages, and name its variables and control.

Carry this line with you: the oxygen comes from water; the carbon comes from the air.

Where we are

Already covered: Week 1 taught the controlled experiment, and Weeks 2 and 3 covered water, bonds, and glucose. Week 4 named the chloroplast, Week 5 introduced ATP and enzymes, and Week 6 traced cellular respiration from glucose to ATP.
This week: photosynthesis, the equation, the chloroplast, its two stages in order with their locations, atom tracking, and how photosynthesis and respiration fit together.
How they connect: photosynthesis builds the glucose that Week 6 took apart, it runs on the ATP and enzymes of Week 5 inside the chloroplast of Week 4, and its lab uses the control group from Week 1.
Where we're going next: Week 8 reviews the first seven weeks for the midterm and adds one habit for any process: name each stage in order and the place it runs.

Week 4 gave you the chloroplast as one stop on the organelle tour, the place where a plant cell does photosynthesis, and it warned you that a plant cell has mitochondria too. Week 5 gave you ATP, the energy currency, and the idea that an enzyme speeds one job without being used up. Week 6 gave you an electron carrier, a molecule that picks up electrons in one step and drops them off in a later one. All three come back here. The difference is direction. Last week the cell spent sugar to charge ATP. This week the cell spends light to build sugar.

1. Read the equation: what goes in and what comes out

Picture a houseplant on a sunny kitchen windowsill. You water it every few days. It sits in the light. Week after week it puts out new leaves, and those leaves are made of something. The plant's recipe has only three ingredients, and you can name all of them by the end of this section.

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. Here is the equation, read left to right with the arrow meaning "becomes":

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

Read the left side, the reactants. Carbon dioxide, CO₂, comes in from the air. It enters the leaf through stomata, tiny pores in the surface of a leaf through which carbon dioxide enters for photosynthesis (one pore is a stoma). Water, H₂O, comes up from the roots. Light is the energy that drives the whole process.

Now read the right side, the products. Glucose, C₆H₁₂O₆, is the sugar the plant builds; you met it in Week 3 as the monomer of starch and cellulose. Oxygen, O₂, leaves the plant and enters the air you breathe.

Living things split into two camps by how they get their food.

An autotroph is a living thing that makes its own food from simple inorganic materials, as plants and algae do by photosynthesis.
A heterotroph is a living thing that cannot make its own food and must eat food that other organisms made, as animals and fungi do.

You are a heterotroph. Your houseplant is an autotroph. Nearly every bite of food you eat traces back, step by step, to an autotroph that caught light.

Worked example — checking the equation's atoms. A balanced equation has the same number of each kind of atom on both sides. You check all three kinds in 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂.

Carbon. Each CO₂ has 1 carbon, so the left side has 6 × 1 = 6. Glucose on the right has 6. They match.
Hydrogen. Each H₂O has 2 hydrogens, so the left side has 6 × 2 = 12. Glucose on the right has 12. They match.
Oxygen. The left side has 6 CO₂ with 2 oxygens each plus 6 H₂O with 1 each: (6 × 2) + (6 × 1) = 18. The right side has 6 in the glucose plus 6 O₂ with 2 each: 6 + (6 × 2) = 18. They match.

In words: 6 carbons, 12 hydrogens, and 18 oxygens on each side, so no atom is created or destroyed. Week 5's first law said the same about energy; atoms obey the same bookkeeping.

Misconception check. Students often list glucose and oxygen as the reactants of photosynthesis. The slip is tempting because those are the reactants of cellular respiration, which you learned last week, and the two equations look alike. The cure is to ask what the plant takes in. A plant takes in carbon dioxide, water, and light. Glucose and O₂ are what it makes, so they sit on the right side, never the left.

Memory hook: air, water, and light go in; sugar and oxygen come out.

2. Picture the chloroplast: membranes, stacks, and fluid

Think of a sealed plastic bag full of water, with several short stacks of coins floating inside it. Now shrink that bag until it fits inside one leaf cell, with room for many more beside it. That is the shape of a chloroplast, the organelle Week 4 named as the place where photosynthesis happens.

Here is the chloroplast described as a labeled figure, from the outside in.

Two membranes. An outer membrane and an inner membrane wrap the whole organelle.
The fluid inside. The stroma is the fluid that fills a chloroplast around the thylakoids, where the Calvin cycle builds sugar.
The discs. A thylakoid is a flat, disc-shaped membrane sac inside a chloroplast; its membrane holds chlorophyll and runs the light-dependent reactions.
The stacks. A granum is a stack of thylakoids inside a chloroplast, piled like coins; the plural is grana.
The pigment. Chlorophyll is the green pigment in the thylakoid membrane of a chloroplast; it absorbs red and blue light and reflects green, which is why leaves look green.

So the coins in the bag are the thylakoids, each stack of coins is a granum, the water around them is the stroma, and the bag itself is the double membrane.

Why do leaves look green? Light from the sun carries many colors. Chlorophyll absorbs the red and the blue and puts that energy to work. It reflects the green, and the reflected green is what reaches your eyes. The color you see is the color the leaf is not using.

Structure determines function here as everywhere. Chlorophyll sits in the thylakoid membrane, so the thylakoid membrane is where light is caught. The stroma holds RuBisCO, the enzyme that starts the Calvin cycle, so the stroma is where sugar is assembled. Two places, two jobs.

Worked example — which part of the chloroplast? You name the part from a description.

A flat sac whose membrane holds chlorophyll. A thylakoid.
A pile of those sacs, stacked like coins. A granum.
The fluid around the stacks, where sugar is built. The stroma.
The pigment that reflects green. Chlorophyll.

In words: discs, stacks, fluid, pigment. The discs and their pigment catch light; the fluid builds sugar.

Misconception check — two swaps. Students often swap the thylakoid and the stroma. The words are new and sound alike. The cure is to tie each word to a picture: the thylakoid is a disc (a solid shape, a membrane), and the stroma is the fluid around it. Students also say chlorophyll absorbs green, because leaves are green. It is the reverse. Chlorophyll reflects green, which is why green reaches your eye, and it absorbs red and blue.

Memory hook: thylakoid is the disc; stroma is the sea around it.

3. Run stage one: the light-dependent reactions

Think of a solar panel on a garage roof charging a battery during the day. The panel does not build anything. It catches light and stores the energy so something else can do the work later. Stage one of photosynthesis does the same job.

The light-dependent reactions are the first stage of photosynthesis, in the thylakoid membrane, where chlorophyll absorbs light, water is split to release oxygen, and ATP and NADPH are made.

Here is what happens, in order.

Chlorophyll absorbs light. The red and blue light caught in the thylakoid membrane supplies the energy.
Water is split. The plant breaks water molecules apart. Each water gives up electrons, hydrogen ions (H⁺), and oxygen.
Oxygen leaves. The oxygen from the split water is released as O₂. This is the oxygen that leaves the plant and enters the air.
The energy is stored. The captured light energy is used to make two loaded molecules: ATP, the energy currency you met in Week 5, and NADPH.

NADPH is a loaded electron carrier that the light-dependent reactions make and the Calvin cycle spends, together with ATP, to build sugar. It does the same kind of job as last week's NADH: a carrier that picks up electrons in one step and delivers them to a later one.

Notice what stage one does not make. It makes no sugar at all. Its outputs are oxygen, which leaves, and ATP and NADPH, which are the charged batteries the next stage will spend.

Worked example — the inputs and outputs of stage one. You fill in a two-column card for the light-dependent reactions.

Where. The thylakoid membrane.
In. Light and water.
Out and gone. O₂, released from the leaf.
Out and passed on. ATP and NADPH, handed to stage two.

In words: light and water go in; oxygen leaves; ATP and NADPH carry the energy forward. No glucose yet.

Misconception check. Students often think the oxygen a plant gives off comes from the carbon dioxide it takes in. The idea is tempting because CO₂ has two oxygens in its name, and O₂ looks like the CO₂ with the carbon pulled off. But the O₂ comes from splitting water in stage one. Section 5 shows how scientists traced it there.

Memory hook: stage one charges the batteries and lets the oxygen go.

4. Run stage two: the Calvin cycle builds sugar

Picture a bakery that runs on the electricity a rooftop solar panel stored during the day. Flour comes in by the sack, the ovens draw on the charged batteries, and loaves go out the door. If the batteries run flat, the ovens stop, even though the bakery itself never looks at the sun. Stage two of photosynthesis is that bakery.

The Calvin cycle is the second stage of photosynthesis, in the stroma, where carbon dioxide is fixed and ATP and NADPH from the light-dependent reactions are spent to build the three-carbon sugar G3P. It runs as a loop, so it can keep turning as long as it is supplied.

Three moves happen in the loop.

Carbon fixation is the step that attaches carbon from carbon dioxide in the air to a molecule already in the Calvin cycle, turning a gas into part of a sugar.
RuBisCO is the enzyme in the stroma that fixes carbon dioxide at the start of the Calvin cycle. You need its name and its job; you do not need its chemistry.
G3P is the three-carbon sugar the Calvin cycle builds; three turns of the cycle export one G3P, and glucose and other sugars are assembled from it.

So the Calvin cycle takes in CO₂, fixes it with RuBisCO, spends ATP and NADPH from stage one, and sends out G3P. The order of the two stages is a fact you can be graded on:

light-dependent reactions (thylakoid membrane) → Calvin cycle (stroma)

Worked example — counting turns of the cycle. Three turns of the Calvin cycle fix 3 CO₂ and export one G3P with 3 carbons. How many turns does it take to build one glucose?

Carbons in glucose. Glucose, C₆H₁₂O₆, has 6 carbons.
G3P per glucose. Each G3P carries 3 carbons, so 6 ÷ 3 = 2 G3P make one glucose.
Turns per glucose. Each G3P takes 3 turns, so 2 × 3 = 6 turns.
Check against the equation. Each turn fixes one CO₂, so 6 turns fix 6 CO₂, exactly the 6 CO₂ on the left of the equation.

In words: one glucose needs 6 turns of the cycle versus 3 turns for one G3P, twice as many, and those 6 turns pull in the 6 carbon dioxide molecules the equation promised.

Why the Calvin cycle still needs light

The Calvin cycle is sometimes called the "light-independent" stage, because no step in it absorbs light. That name causes trouble. The cycle runs on ATP and NADPH, and those come only from the light-dependent reactions. Turn off the light and stage one stops making them. The supply runs down, and the Calvin cycle stops soon after, like the bakery ovens when the batteries go flat.

Misconception check. Students often say the Calvin cycle does not need light. The idea is tempting because no step of the cycle absorbs light, and the name "light-independent" seems to say so. The cure is to follow the supply. The cycle spends ATP and NADPH, and only the light-dependent reactions make them, so in the dark the cycle stops soon after the light does. Students also put the stages out of order. Light comes first, because the second stage spends what the first stage makes.

Memory hook: light first, then sugar; the stroma spends what the thylakoid makes.

5. Track the atoms: where the oxygen and the carbon come from

Back to the redwood. If the tree is not made of soil, what is it made of? You can answer that the way a detective follows a suspect: pick one kind of atom and follow it from where it starts to where it ends up.

Atom tracking is following one kind of atom from a reactant into a product to learn where it goes, as in showing that the oxygen plants release comes from water and the carbon in sugar comes from carbon dioxide.

The oxygen comes from water

Look at the equation again. Oxygen atoms appear in both reactants, CO₂ and H₂O. So which one supplies the O₂ the plant gives off? You cannot tell by reading the equation. You have to tag one source and follow it.

A tracer is a detectable form of an atom, such as a heavy form of oxygen, that scientists follow through a process to see where that atom ends up. Ruben, Randall, Kamen, and Hyde (1941) used a heavy form of oxygen as a tracer. Following that tagged oxygen showed that the O₂ plants release comes from water. The oxygen comes from splitting water in the light-dependent reactions, exactly as Section 3 described.

The carbon comes from the air

Now follow the carbon. The only carbon-carrying reactant in the equation is CO₂. The Calvin cycle fixes that carbon into G3P, and G3P is assembled into glucose. So every carbon atom in the sugar came out of the air.

Glucose is the starting material a plant builds its body from. Week 3 showed that cellulose, the structural fiber of plants, and starch, the plant's stored fuel, are both built from glucose. So the sugar the leaves make becomes the plant itself. That answers the puzzle.

Worked example — the redwood puzzle, atom by atom. You explain where a tree's mass comes from by following each kind of atom.

Carbon. It comes from CO₂ in the air, entering through the stomata and fixed in the Calvin cycle.
Hydrogen. The only hydrogen-carrying reactant is water, drawn up by the roots and split in the light-dependent reactions, so the hydrogen comes from water.
Where the bulk comes from. Carbon pulled from the air, plus water, makes up most of the tree's mass.
What the soil adds. The soil supplies minerals, not bulk.

In words: a tree's mass is mostly carbon pulled from the air, plus water, and the soil supplies minerals, not the bulk of the wood.

Honesty note: this explanation is a plain-language model of a long chain of chemistry. It is the right answer for this course, and the details you would need to tell every atom's full story belong to a later biochemistry course.

Misconception check. Students often think a plant's mass comes from the soil. It is the most natural guess there is, because roots go into the soil and plants need soil to grow well. But soil supplies minerals, not bulk. The bulk is mostly carbon from CO₂ in the air, plus water. If you ever doubt it, ask where the carbon in glucose could come from: CO₂ is the only reactant that carries any.

Memory hook: trees are built from thin air, plus water.

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

(a) Before this week, where would you have said a tree's mass comes from? What made that answer feel right, and what would you now tell someone who gave it?
(b) Think of a food you ate today. Trace one carbon atom in it back as far as you can, step by step, until you reach the air.

6. Compare photosynthesis with respiration

Hold last week's equation and this week's side by side, and they look like reflections in a mirror. What one takes in, the other gives off. That likeness is real, and it is also the source of the most common mix-up of the unit.

Photosynthesis and cellular respiration compared by inputs, outputs, energy, location, and which organisms run each
Feature	Photosynthesis	Cellular respiration
Takes in	carbon dioxide, water, light energy	glucose, oxygen
Gives off	glucose, oxygen	carbon dioxide, water, ATP
Energy story	stores light energy in sugar	releases the sugar's energy to recharge ATP
Where in a plant cell	chloroplast	cytoplasm, then mitochondrion
Who runs it	plants, algae, and other autotrophs	nearly every living thing, plants included

Photosynthesis and cellular respiration are roughly mirror processes. Photosynthesis stores light energy in sugar and releases O₂. Respiration releases the sugar's energy as ATP and uses O₂. Each one's products are close to the other one's reactants. "Roughly" matters: they run in different organelles, with different enzymes and carriers, so they are partners and not one process run backwards.

A plant does both. Its chloroplasts make sugar in the light. Its mitochondria burn sugar day and night, because the plant needs ATP every minute, not only when the sun is up. A potted fern in a dark hallway at midnight is still respiring.

Worked example — what is the plant doing? You decide which process or processes run in a leaf cell in each setting.

A sunny afternoon. Both. The chloroplasts photosynthesize, and the mitochondria respire.
Midnight, no light. Respiration only. Photosynthesis stops when the ATP and NADPH from the light run out; respiration keeps going.
A heterotroph, such as you, at any hour. Respiration only. You have no chloroplasts.

In words: respiration never stops in a living plant; photosynthesis runs only while there is light.

The evolutionary lens: Earth's oxygen

Here is the long view. The oxygen in Earth's air was not always there. Early oxygen-producing photosynthesis is how Earth gained the oxygen in its air. The aerobic respiration you learned last week, with oxygen as the final electron acceptor, depends on that oxygen. So the two processes are linked not only inside one leaf but across the history of life: photosynthesis filled the air with oxygen, and respiration came to depend on it. Evolution, the unifying framework from Week 1, is the story that connects them.

Misconception check. Students often say plants photosynthesize instead of respiring, as if a plant had a sugar factory but no furnace. The idea is tempting because photosynthesis is the thing plants are famous for. The cure is Week 4's rule: a plant cell has chloroplasts and mitochondria. Chloroplasts make sugar; mitochondria burn it, day and night. Plants do both.

Memory hook: the chloroplast builds the sugar; the mitochondrion spends it.

7. Read the floating leaf disk test

Set two clear cups on a kitchen counter. Fill both with the same baking-soda solution. Into each, drop 10 small disks punched from a spinach leaf, with the air drawn out of them so they sink. Put one cup under a lamp. Put the other in a dark cupboard. Then watch.

The baking soda supplies dissolved CO₂, the carbon source for the Calvin cycle. Under the lamp, the light-dependent reactions split water and release oxygen. The oxygen collects inside the disk's tissue, and when enough has gathered, the disk floats. In the dark cup there is little light to drive stage one, so little oxygen forms. The dark cup is the control group: the no-light baseline that shows what happens without the factor being tested.

The readings below are an illustrative dataset constructed for this course, not real data. Your own cups at home will give their own numbers.

Leaf disks floating out of 10 at five times, in the light and in the dark (illustrative, constructed for this course, not real data)
Minute	Light	Dark
0	0	0
3	2	0
6	5	0
9	8	1
12	10	1

Worked example — the floating leaf disk experiment. You read the experiment from the question to the conclusion.

Question. Does light change how fast spinach leaf disks make oxygen?
Hypothesis. If leaf disks sit under a lamp, then they will float sooner than disks kept in the dark, because light drives the light-dependent reactions that split water and release oxygen.
Light average. 0 + 2 + 5 + 8 + 10 = 25, and 25 ÷ 5 = 5.0 disks floating across the five readings.
Dark average. 0 + 0 + 0 + 1 + 1 = 2, and 2 ÷ 5 = 0.4 disks floating across the five readings.
Endpoint. At 12 minutes, 10 − 1 = 9 more disks float in the light than in the dark.
Halfway. Half of the 10 disks is 10 ÷ 2 = 5, and the light cup reaches 5 floating disks at about 6 minutes.
Increments in the light. 2 − 0 = 2, 5 − 2 = 3, 8 − 5 = 3, and 10 − 8 = 2 disks per 3-minute interval; 2 + 3 + 3 + 2 = 10, every disk in the cup.
Source of the lift. The oxygen that floats the disks comes from the light-dependent reactions splitting water, not from the CO₂.
Variables. The independent variable is light versus dark. The dependent variable is the number of disks floating. The controlled variables are the number of disks, the leaf, the baking-soda solution, the cups, and the timing. The dark cup is the control group.
Conclusion. The evidence supports the hypothesis. It does not prove it.

In words: under the lamp the disks rise steadily until all 10 float, while the dark cup stays nearly flat. The gradable facts are the rising trend in the light, the flat dark control, and the source of the oxygen, never any one disk count. If your class repeats the test, the spread of the class data will show how much one cup can vary.

Caveat: a single pair of cups is a small test. One run supports or fails to support a claim and invites a repeat; it never settles it.

Misconception check. Students often say the bubbles that lift the disks are carbon dioxide, because the cup is full of baking soda and baking soda is the CO₂ source. The idea is tempting because CO₂ is the gas you put in. But the CO₂ is used up in the Calvin cycle. The gas that collects and lifts the disks is oxygen, released when stage one splits water. A second slip is to call the dark cup a failed experiment. It is the control, and a flat control is exactly what makes the light result mean something.

Memory hook: light in, oxygen out, disks up.

Key terms

Photosynthesis — The process in which a plant, alga, or other autotroph uses light energy to build sugar from carbon dioxide and water, releasing oxygen. (first used: 1. Read the equation: what goes in and what comes out)
Stomata — Tiny pores in the surface of a leaf through which carbon dioxide enters for photosynthesis; one pore is a stoma. (first used: 1. Read the equation: what goes in and what comes out)
Autotroph — A living thing that makes its own food from simple inorganic materials, as plants and algae do by photosynthesis. (first used: 1. Read the equation: what goes in and what comes out)
Heterotroph — A living thing that cannot make its own food and must eat food that other organisms made, as animals and fungi do. (first used: 1. Read the equation: what goes in and what comes out)
Stroma — The fluid that fills a chloroplast around the thylakoids, where the Calvin cycle builds sugar. (first used: 2. Picture the chloroplast: membranes, stacks, and fluid)
Thylakoid — A flat, disc-shaped membrane sac inside a chloroplast; its membrane holds chlorophyll and runs the light-dependent reactions. (first used: 2. Picture the chloroplast: membranes, stacks, and fluid)
Granum — A stack of thylakoids inside a chloroplast, piled like coins; the plural is grana. (first used: 2. Picture the chloroplast: membranes, stacks, and fluid)
Chlorophyll — The green pigment in the thylakoid membrane of a chloroplast; it absorbs red and blue light and reflects green, which is why leaves look green. (first used: 2. Picture the chloroplast: membranes, stacks, and fluid)
Light-dependent reactions — The first stage of photosynthesis, in the thylakoid membrane, where chlorophyll absorbs light, water is split to release oxygen, and ATP and NADPH are made. (first used: 3. Run stage one: the light-dependent reactions)
NADPH — A loaded electron carrier that the light-dependent reactions make and the Calvin cycle spends, together with ATP, to build sugar. (first used: 3. Run stage one: the light-dependent reactions)
Calvin cycle — The second stage of photosynthesis, in the stroma, where carbon dioxide is fixed and ATP and NADPH from the light-dependent reactions are spent to build the three-carbon sugar G3P. (first used: 4. Run stage two: the Calvin cycle builds sugar)
Carbon fixation — The step that attaches carbon from carbon dioxide in the air to a molecule already in the Calvin cycle, turning a gas into part of a sugar. (first used: 4. Run stage two: the Calvin cycle builds sugar)
RuBisCO — The enzyme in the stroma that fixes carbon dioxide at the start of the Calvin cycle. (first used: 4. Run stage two: the Calvin cycle builds sugar)
G3P — The three-carbon sugar the Calvin cycle builds; three turns of the cycle export one G3P, and glucose and other sugars are assembled from it. (first used: 4. Run stage two: the Calvin cycle builds sugar)
Atom tracking — Following one kind of atom from a reactant into a product to learn where it goes, as in showing that the oxygen plants release comes from water and the carbon in sugar comes from carbon dioxide. (first used: 5. Track the atoms: where the oxygen and the carbon come from)
Tracer — A detectable form of an atom, such as a heavy form of oxygen, that scientists follow through a process to see where that atom ends up. (first used: 5. Track the atoms: where the oxygen and the carbon come from)

Summary

Photosynthesis: carbon dioxide, water, and light go in; glucose and oxygen come out.
Autotrophs make their own food; heterotrophs, like you, eat food others made.
In the chloroplast, thylakoids are the discs, grana are the stacks, and the stroma is the fluid.
Chlorophyll absorbs red and blue light and reflects green.
Stage one, the light-dependent reactions, runs in the thylakoid membrane: water is split, oxygen leaves, ATP and NADPH are made.
Stage two, the Calvin cycle, runs in the stroma: RuBisCO fixes CO₂, ATP and NADPH are spent, G3P is built.
Three turns of the Calvin cycle export one G3P; the cycle stops soon in the dark.
The oxygen comes from water; the carbon in sugar comes from carbon dioxide.
A tree's mass is mostly carbon from the air, plus water; soil supplies minerals.
Plants both photosynthesize and respire; early photosynthesis gave Earth its oxygen.

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 tomato plant on a balcony exports 4 G3P from its Calvin cycle (an illustrative number). How many turns of the cycle did that take, how many CO₂ did it fix, and how many glucose molecules could be assembled from those 4 G3P?
A pond plant sits in a jar of water under a desk lamp, and a second jar is wrapped in foil. In an illustrative run, a student counts the bubbles rising from each plant in four one-minute readings: lamp 6, 8, 7, 7; foil 1, 0, 1, 0. Find each average, name the gas in the bubbles and where its atoms came from, and name the control.
Sort each event into stage one (the light-dependent reactions) or stage two (the Calvin cycle), and give its location: water is split; CO₂ is fixed by RuBisCO; NADPH is made; G3P is built; ATP is spent to build sugar.
A friend at a pumpkin-growing contest says, "A giant pumpkin is mostly soil that the roots pulled up." Explain, using atom tracking, what is wrong with that claim and what the pumpkin's mass is mostly made of.

Answers (worked reasoning — read after attempting)

Each G3P takes 3 turns, so 4 G3P take 4 × 3 = 12 turns. Each turn fixes one CO₂, so the plant fixed 12 CO₂. Glucose has 6 carbons and each G3P has 3, so it takes 2 G3P per glucose: 4 ÷ 2 = 2 glucose molecules. In words, 12 carbon dioxide molecules from the air became enough sugar for 2 glucose.
Lamp: 6 + 8 + 7 + 7 = 28, and 28 ÷ 4 = 7 bubbles per minute on average. Foil: 1 + 0 + 1 + 0 = 2, and 2 ÷ 4 = 0.5 bubbles per minute on average. The gas is oxygen, released by the light-dependent reactions, and its atoms came from splitting water, not from carbon dioxide. The foil-wrapped jar is the control, the no-light baseline. The evidence supports the idea that light drives oxygen production; it does not prove it.
Water is split: stage one, in the thylakoid membrane. CO₂ is fixed by RuBisCO: stage two, in the stroma. NADPH is made: stage one, in the thylakoid membrane. G3P is built: stage two, in the stroma. ATP is spent to build sugar: stage two, in the stroma. Stage one makes the ATP and NADPH; stage two spends them.
Follow the carbon. The only carbon-carrying reactant in photosynthesis is CO₂ from the air, which the Calvin cycle fixes into sugar, and the pumpkin builds its flesh largely from that sugar. Its hydrogen comes from water. So the pumpkin's mass is mostly carbon from the air, plus water. The soil supplies minerals, not the bulk.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz; next week, the order-and-location method ties every process together.

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