Quiz 11 — Photosynthesis

The Guide

Building food out of air and light

Animals eat. Plants do not. A plant builds its own food out of three things it collects: a gas from the air, water from the ground, and light. That process is photosynthesis, and almost every calorie eaten by anything on Earth started there.

What goes in, what comes out

In words: carbon dioxide + water + light energy → glucose + oxygen.

Chemists write it with symbols rather than words, and it is worth being able to read the symbols, because the test uses them and because the two equations only reveal themselves as mirrors when they are lined up side by side.

Two kinds of number, and they mean different things. A number in front of a formula counts whole molecules: 6CO₂ is six of them. A small number tucked below counts atoms inside one molecule: CO₂ holds two oxygen atoms. So 6CO₂ means six molecules, each carrying one carbon and two oxygens. Read the front number first, then look inside.

Where the mirror stops being true. The two summary equations match exactly, and that is genuinely useful. The chemistry underneath does not. Photosynthesis and respiration run through different steps, in different parts of the cell, using different tools; neither is the other one played backward. What mirrors is the bookkeeping — what goes in and what comes out — and that is enough for this test and for most purposes. Keep the mirror; do not push it.

Glucose is a sugar, and sugar is stored energy. So the real work of photosynthesis is a conversion: light energy arrives, and chemical energy leaves, locked into a molecule the plant can keep, spend later, or turn into wood.

Where each piece comes from

The carbon dioxide comes from the air, through small openings on the underside of each leaf. The water comes up from the roots. The light is captured by chlorophyll, the green substance packed inside the chloroplasts you met in Quiz 9. The oxygen is a leftover — the plant has no use for it and releases it, and animals, people included, need that oxygen to live.

Why leaves are green. Chlorophyll absorbs red light and blue light and uses their energy. Green light it mostly reflects, which is to say it throws the green away. The color of a leaf is the one part of sunlight the plant could not use.

The mistake nearly everyone makes. Plants do not eat soil. Soil supplies water and a small quantity of minerals, and that is all. The bulk of a tree — the trunk, the branches, the tons of wood — is built mostly from carbon that arrived as an invisible gas. A log burning in a fireplace is releasing air that a tree took out of the sky. The passage in this quiz is about the man who first tested this and got it half right.

Worked Examples

Two questions, worked through

Example 1. A potted plant is kept in a dark cupboard with plenty of water for two weeks. It turns pale and limp. Why?

Check the three inputs one at a time. Water: supplied. Carbon dioxide: the cupboard has air, so supplied. Light: missing. With one of the three gone the process stops, and the plant lives on stored sugar until that runs out.

When a process needs several things at once, a failure means finding which one is absent. Going down the list beats guessing at the whole.

Example 2. A student says photosynthesis "makes energy." What is wrong with that?

Energy is not made. It arrives as light, and photosynthesis changes its form — light energy becomes chemical energy held inside a sugar molecule. Nothing new was created; something was converted and stored.

The right verb is captures or stores, not makes. This matters more than it sounds, and Quiz 42 comes back to it: energy changes form and moves around, and it is never created.

The Quiz

Ten questions

Answer all ten, then press the button at the bottom. Nothing is timed.

Diagram and equation — questions 1 to 4

1.In the equation for photosynthesis, what does the small 2 in CO₂ tell you?

AEach molecule contains two oxygen atoms.
BTwo molecules of the gas are needed.
CThe gas is used twice in the process.
DTwo separate atoms of carbon sit in each one.

2.Where does the water used by the leaf come from?

AFrom the air
BFrom the roots
CFrom the sugar
DFrom sunlight

3.What becomes of the sunlight that the leaf takes in?

AIt is turned into water inside the leaf.
BIt escapes again as green light and heat.
CIt is used to pull water up from the roots.
DIt is stored as chemical energy in sugar.

4.Which statement describes what leaves the plant during photosynthesis?

ACarbon dioxide leaves and oxygen enters.
BWater and sugar both leave the plant.
COxygen leaves and sugar stays behind.
DNothing leaves; the plant keeps it all.

Graph — questions 5 to 7

A water plant in a beaker. The oxygen it gives off is counted as bubbles.

5.At 40 units of light, about how many bubbles per minute does the plant give off?

6.What is the overall shape of the line?

AThe rate keeps rising at the same speed throughout.
BThe rate rises quickly at first, then levels off.
CThe rate falls once the light passes 30 units.
DThe rate stays the same no matter how much light.

7.Beyond 30 units, extra light adds almost nothing. What does that suggest?

AThe plant has stopped needing light altogether.
BThe graph was measured wrongly beyond 30 units.
CMore light would damage the leaf, so it slows down.
DSomething other than light is now limiting the rate.

Passage — questions 8 to 10

In the 1640s Jan Baptist van Helmont planted a young willow weighing 5 pounds in a pot holding 200 pounds of dried soil. He covered the pot so that nothing could blow in, and for five years he gave the tree nothing but water.

At the end of five years he weighed the willow: 169 pounds. He dried the soil and weighed that too: 199 pounds and 14 ounces.

Van Helmont concluded that the 164 pounds the tree had gained must have come from the water, since nothing else had been added.

8.What did van Helmont's second weighing show about the soil?

AThe soil lost about 164 pounds over five years.
BThe soil gained weight as the tree grew larger.
CThe soil lost about two ounces over five years.
DThe soil was left unweighed at the end of the trial.

9.Van Helmont credited the gain to water. What had he left out of account?

AThe minerals the roots drew from the soil.
BThe carbon dioxide the leaves took from the air.
CThe oxygen the tree released into the air.
DThe sunlight the leaves absorbed on each of those days.

10.What does this experiment show about measurement?

AA careful measurement can still support a wrong conclusion.
BMeasurements taken over five years are too old to be trusted.
CThe experiment should have been repeated by another researcher.
DWeighing the soil was the wrong thing to have measured.

Send this line to your teacher

The line records which questions you missed and which answer you chose. That is more useful to your teacher than the score, because it shows where a question went wrong. If a question felt unclear even though you got it right, add its number with a question mark — for example 5? — before you send it.

Score ______ / 10 Missed — write the question number and the letter you chose:
______________________________________________________________

The Key

Answers, and the trap in each one

1. A — each molecule contains two oxygen atoms. A small number sitting below the line counts atoms inside one molecule. CO₂ is one carbon joined to two oxygens, which is where the name carbon dioxide comes from. B is what the number would mean if it were in front instead of below. That is the whole distinction, and the test relies on it: 6CO₂ is six molecules; the 2 is inside each one. D moves the small number to the wrong element — it is attached to the O, not the C.

2. B — from the roots. The blue arrow runs up the stem from the roots into the leaf. A is where the carbon dioxide comes from, not the water. Both arrive at the same leaf and people mix up which came by which route.

3. D — stored as chemical energy in sugar. This is the whole point of the process. Light energy arrives, and it leaves as energy locked inside a sugar molecule where the plant can keep it. B is half true and misses the point: some light does bounce off, which is why leaves look green. But the light the leaf absorbs is the light it uses, and the question asked about that.

4. C — oxygen leaves and sugar stays. Oxygen is the leftover and goes out. The sugar is the product worth keeping and stays inside. A has both gases running backward. That reversal describes respiration, which is Quiz 12, and getting the two backward is the single most common error in this part of the course.

5. D — about 36 bubbles per minute. Find 40 along the bottom, go up to the line, then across to the numbers. C is the reading at 30 units, one point to the left. The line is nearly flat there, so the two points sit close together and it is easy to land on the wrong one. Trace with a finger.

6. B — rises quickly, then levels off. Steep from 0 to 20, gentler from 20 to 30, flat after that. A describes a straight line. This one is not straight, and the difference between a straight rise and a rise that flattens is the entire subject of question 7.

7. D — something other than light is limiting it. Photosynthesis needs three things. Once light is plentiful, the shortage is somewhere else — carbon dioxide, or warmth, or the amount of chlorophyll the plant has. Adding more of what is already sufficient changes nothing. A misreads a flat line as "no longer needed." The plant still needs light and is still using it; it simply cannot use any more. This idea shows up again in Part VI, where a population stops growing for exactly the same kind of reason.

8. C — about two ounces. 200 pounds at the start, 199 pounds 14 ounces at the end. The soil barely changed. A is the tree's gain, not the soil's loss. The two numbers sit in neighboring sentences, and the whole force of the experiment is that they do not match: the tree gained 164 pounds and the soil lost two ounces.

9. B — carbon dioxide from the air. About half the weight of a living tree is water, so van Helmont was half right. Most of the rest was built from carbon dioxide, an invisible gas that came in through the leaves. He could not have known this: no one yet knew that plants take in a gas from the air. A is a real thing that happens and cannot account for the weight, since the soil lost only two ounces. D is also real and also not the answer: light delivers energy, not material. Energy has no weight to contribute.

10. A — a careful measurement can still support a wrong conclusion. Van Helmont did everything right. He controlled the pot, ruled out additions, weighed both the tree and the soil before and after, and ran it for five years. His numbers were sound and his conclusion was wrong, because the thing that mattered was invisible and he had no way to weigh it. C sounds like the Quiz 2 answer and is a good instinct in general. It is not the lesson here: another researcher repeating this experiment would have gotten the same numbers and drawn the same wrong conclusion. Repetition catches accidents. It does not catch a factor that nobody has thought of yet.

Your Score

What the number means

8 to 10	Solid. Check topic 11 on your map and go on to Quiz 12, which runs this same equation backward.
6 or 7	Close. Read the whole key, then take this again in a few days before you check the box.
5 or fewer	Worth another pass. Write the word equation out by hand, then say aloud where each item comes from and where it goes.

Misses on 1, 2, or 5 mean an arrow or a graph line was read too fast. A miss on 4 is worth attention before you start Quiz 12, because that item is exactly where photosynthesis and respiration get reversed. Misses on 7 or 10 are the reasoning items: one is about why more of a good thing stops helping, the other about why a sound experiment can still point the wrong way.

The plate for this quiz

The gallery image is a leaf skeleton — a leaf left in water until the soft tissue rots away and only the vein network remains, branching and rebranching down to threads. Nineteenth-century botanists prepared them for teaching, and the plates survive in several collections.

It belongs here because it shows the plumbing. Every one of those veins was carrying water up from the roots to cells that were splitting it apart with sunlight. What is left is the delivery system with the factory removed.