Quiz 12 — Cellular Respiration

The Guide

The same equation, read backward

Every living cell needs energy to do its work, and it gets that energy by taking sugar apart. That process is called cellular respiration. It goes on in your cells, in a tree, in a mushroom, and in yeast, all day and all night. Photosynthesis, the subject of Quiz 11, is how plants pack energy into sugar in the first place. Cellular respiration is how a living thing gets it back out and spends it. Both processes handle the same four substances. They just run in opposite directions.

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

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.

What the word "energy" is standing in for. A cell does not store loose energy. It loads it onto a small molecule called ATP, which travels to wherever the work is being done and is spent there — cash rather than a bank balance. Some of the energy escapes as heat instead, and does not get used at all. That heat is why you are warm to the touch, and why a room full of people needs less heating than an empty one.

Where it happens

In cells with a nucleus, most of the energy is released inside the mitochondria. A cell that works hard — a muscle cell, a heart cell — carries far more of them than a cell that does not.

Respiration is not breathing. The two words get used for each other and they are different things. Breathing moves air in and out of your lungs. Cellular respiration is chemistry happening inside every cell, all the time, and it would carry on for a while even if the breathing stopped. Breathing is the delivery service. Respiration is what the cell does with the delivery.

Plants respire too. This one costs points every year. Plants carry out photosynthesis when there is light, and they carry out respiration constantly, day and night, because their cells need energy like any others. A plant is not the opposite of an animal. It does both jobs; animals only do one.

When the oxygen runs short

Sometimes a cell needs energy faster than oxygen can reach it. It can then break glucose down without oxygen, which is called anaerobic respiration, or fermentation. It works, and it is quick, and it yields far less energy from the same sugar.

In a hard-working muscle the leftover is lactic acid, and that is the burn you feel in a sprint. In yeast the leftovers are ethanol and carbon dioxide, which is why bread rises, and why yeast is used to brew beer. Same shortage, two different waste products, depending on whose cells they are.

Worked Examples

Two questions, worked through

Example 1. You breathe out air containing more carbon dioxide than you breathed in. Where did the extra carbon dioxide come from?

Read the equation: glucose plus oxygen gives carbon dioxide plus water plus energy. The carbon dioxide is a product, made inside your cells as sugar is broken apart. The blood carries it to the lungs to be dumped.

The carbon dioxide you breathe out was, a few hours ago, part of the food you ate. The carbon in that gas was in food a few hours ago, and before that it was in a plant, and before that it was in the air.

Example 2. A sealed jar holds a green plant, water, and air, and sits on a sunny windowsill. Overnight, in the dark, the amount of oxygen in the jar falls. Why?

In the light the plant does both jobs: photosynthesis makes oxygen faster than respiration uses it, so oxygen builds up. In the dark photosynthesis stops, and respiration does not. The plant keeps using oxygen with nothing replacing it.

The plant did not switch from one process to the other. It was doing both all day; one of them stopped at nightfall and the other carried on.

The Quiz

Ten questions

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

Equation and diagram — questions 1 to 4

Use the two equations and the loop diagram from the Guide above.

Photosynthesis sits at the top, in the chloroplasts. Cellular respiration sits at the bottom, in the mitochondria. Glucose and oxygen pass down the right-hand side; carbon dioxide and water pass back up the left.

1.In the equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy, which substances are used up?

ACarbon dioxide and water
BGlucose and oxygen
CLight and water
DSugar and carbon dioxide

2.What is the relationship between the two processes in the diagram?

AEach one uses what the other one produces.
BThey are two names for the same process.
COne happens by day and the other by night.
DRespiration is limited to animal cells.

3.Where does a cell with a nucleus release most of its energy?

AChloroplast
BCytoplasm
CMitochondrion
DNucleus

4.A student says that plants do photosynthesis and animals do respiration. What is wrong with that?

AAnimals also carry out photosynthesis during the dark hours.
BPlants carry out respiration as well, day and night.
CNothing is wrong; the statement is correct.
DPlants do respiration but animals do not.

Table — questions 5 to 7

	With oxygen	Without oxygen
Fuel used	Glucose	Glucose
Where in the cell	Mitochondria	Cytoplasm
What is left over	Carbon dioxide and water	Lactic acid in muscle; ethanol and carbon dioxide in yeast
Energy from one glucose	A large amount	A small amount

5.According to the table, what do the two kinds of respiration have in common?

ABoth take place inside the mitochondria.
BBoth release the same amount of energy.
CBoth produce carbon dioxide as a product.
DBoth break down glucose as their fuel.

6.A runner sprints until her leg muscles burn. According to the table, what has built up?

ACarbon dioxide
BEthanol
CGlucose
DLactic acid

7.Why does a cell use the oxygen route whenever it can?

AIt produces far more energy from the same fuel.
BIt uses less glucose than the other kind does.
CIt works in the cytoplasm rather than elsewhere.
DIt produces lactic acid, which the body needs.

Passage — questions 8 to 10

A baker mixes flour, water, sugar, and yeast into dough and leaves the bowl in a warm corner of the kitchen. After an hour the dough has doubled in size, and it smells faintly of alcohol.

A second batch, mixed the same way, is left in the refrigerator. After the same hour it has barely risen.

The baker also stirs some yeast into two jars of warm water. One jar has sugar added; the other has none. After the same hour, the sugar jar is covered in foam, and the plain jar has none.

8.What made the first batch of dough rise?

AAlcohol produced by the yeast expanding.
BWater turning to steam in the warm air.
CCarbon dioxide given off by the yeast.
DAir whipped into the dough by mixing.

9.Why did the plain jar make no foam?

ACold slowed the yeast down too much to work.
BThe yeast had no sugar to break down for energy.
CPlain water cannot hold gas bubbles.
DThe yeast used up its oxygen before it started.

10.The dough smells faintly of alcohol. What does that tell you about the yeast?

AThe yeast was working with plenty of oxygen.
BThe yeast has died and is breaking down.
CThe yeast was working without much oxygen.
DThe alcohol came from the flour, not the yeast.

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. B — glucose and oxygen. Follow the right-hand arrow. It runs downward, from photosynthesis into respiration, carrying those two. A is what respiration gives off, traveling up the other side. The four substances are the same in both directions, so the only thing that tells you which is which is the direction of the arrow.

2. A — each uses what the other produces. The outputs of one are the inputs of the other, which is why the diagram closes into a loop rather than running off the page. C is the belief that plants photosynthesize by day and respire by night, as though they took turns. They do not. Respiration runs continuously in a plant; only photosynthesis stops when the light does.

3. C — the mitochondrion. Named on the lower box of the diagram, and met in Quiz 9 as the part that releases energy from sugar. A is the chloroplast, which does the opposite job in the opposite box. Both words are long and start with C, and under time pressure that is enough.

4. B — plants respire as well. Plant cells need energy exactly as animal cells do, and they get it the same way, all day and all night. What sets plants apart is that they also make the sugar in the first place. C is the answer most people give before they meet this, and it is worth knowing why it feels right: plants and animals look like opposites, so the processes look like they should divide up neatly between them. They do not. Plants do both jobs.

5. D — both break down glucose. Read across the top row: glucose on both sides. That is the one row where the two columns match. C is true of one column and half of the other — yeast does give off carbon dioxide without oxygen, but muscle gives off lactic acid. A row that matches partly is not a row that matches.

6. D — lactic acid. A sprinting muscle cannot get oxygen fast enough, so it uses the route that does not need any, and lactic acid is what that route leaves behind in muscle. B is the yeast leftover from the same row. The table gives two answers in one box, and which one applies depends on whether the cell belongs to a runner or to a loaf of bread.

7. A — far more energy from the same fuel. The last row of the table says it directly: a large amount against a small amount, from one glucose either way. The oxygen route is not faster, and it is not simpler. It is more thorough. B reverses it. The route without oxygen wastes glucose rather than saving it, because it leaves most of the energy still locked inside the lactic acid or the ethanol it throws away.

8. C — carbon dioxide from the yeast. Yeast working without much oxygen gives off carbon dioxide and ethanol. The gas is trapped in the stretchy dough, and thousands of small bubbles push it up. A picks the other product of the same reaction. The ethanol is there — you can smell it — but it stays liquid and does no lifting. Only the gas lifts.

9. B — no sugar to break down. Three batches, and only one thing was changed in each. This batch was missing the fuel, so nothing was broken down and no gas was produced. A explains the second batch, not the third. The baker ran a controlled comparison without calling it one: same recipe, one change at a time, three results. Match each result to the one thing that was changed for it.

10. C — working without much oxygen. Ethanol appears only on the no-oxygen side of the table. Buried in a bowl of dough, the yeast has very little air, so that is the route it takes. A reverses it. With plenty of oxygen the yeast would produce carbon dioxide and water and no alcohol at all — and the bread would still rise, since carbon dioxide comes out either way. The smell of alcohol is what tells you which route was used.

Your Score

What the number means

8 to 10	Solid. Check topic 12 on your map and go on to Quiz 13, on cell division.
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. Draw the loop from memory with both arrows labeled — that one drawing carries Quizzes 11 and 12 together.

Misses on 1 or 3 mean the two halves of the loop are still swapping places, and drawing it by hand fixes that faster than reading it again. A miss on 2 or 4 is the one that matters most: both are the belief that plants and animals divide the work between them, and the test asks about it often. Misses on 6, 8, or 10 mean the two leftovers are still blurred — lactic acid in muscle; ethanol and carbon dioxide in yeast.

The plate for this quiz

The gallery image is an electron micrograph of a single mitochondrion, its inner membrane folded back and forth into deep pleats. The folding is the point: it packs an enormous working surface into a very small space, and that surface is where the energy is released.

There is something else worth telling students about this picture. A mitochondrion carries its own small loop of DNA, separate from the cell's, and it divides on its own schedule. The leading explanation is that it began as a free-living bacterium that was taken in by a larger cell and never left. Every breath you take feeds a passenger. Micrographs of this kind are held in public collections including the National Institutes of Health image library.