The fifteen ideas the quiz is built on, how the questions work, and every term you need.
Take the practice test first. It will tell you which topics are weak. Then come here and read only those sections closely, rather than starting at the top and drifting.
This page is a summary, which means it is shorter than the unit. Use it to organize what you know and to find your gaps, then go back to your notes, the slide deck, and your model tracker for the detail. If you can read a section here and explain it out loud without looking, you know it. If you can only recognize it, you do not know it yet.
Everything on the quiz comes out of these. The line at the bottom of each one tells you how it tends to get asked.
ATP holds energy in the bond to its third phosphate. Break that bond off and you get ADP plus energy the cell can spend on moving, building, and pumping. Cellular respiration is the process that recharges ADP back into ATP.
On the quiz: You get asked to tell ATP apart from the electron carriers, and to say which has more energy, ATP or ADP.
They pick up high energy electrons during the earlier stages and carry them to the electron transport chain. They unload there, turn back into empty NAD+ and FAD+, and go back for more. They are not spent on cell work the way ATP is.
On the quiz: You get asked what they carry, where they pick it up, where they drop it off, and what happens when they cannot unload.
Glucose plus oxygen gives carbon dioxide plus water plus ATP. Every carbon in the glucose leaves as carbon dioxide, and the oxygen you breathe in ends up in the water. Photosynthesis runs the same equation backward.
On the quiz: You get asked to trace an atom. The carbon in your lunch leaves in your breath, and that is the path.
In the cytoplasm, a six carbon glucose is split into two three carbon pyruvate molecules. No mitochondrion required, which is why every living cell can do it.
On the quiz: You get asked for the location and whether a cell without mitochondria can still run it. It can.
Glycolysis spends 2 ATP to get started, then makes 4, so the net gain is 2. It also loads 2 NADH. The 2 ATP go to the cell's own work. Only the NADH continues to the chain.
On the quiz: You get asked for the net rather than the gross. Four is the number of ATP made, two is the number you keep.
This is the one stage that runs under every condition, in every cell, oxygen or not. It is the oldest part of the pathway and the reason anaerobic life was possible before the atmosphere had oxygen.
On the quiz: You get asked whether oxygen is required. It is not, and knowing that is how you answer half the fork questions.
After glycolysis the cell reaches a decision point. Oxygen present means pyruvate enters the mitochondrion for the Krebs cycle and the chain. Oxygen absent means fermentation in the cytoplasm.
On the quiz: You get asked to predict which path an organism takes given a scenario. Find the oxygen first.
Your muscle cells run lactic acid fermentation, which is the burn during a sprint. Yeast run alcoholic fermentation, producing ethanol and carbon dioxide, which is the gas that inflated the balloon and the bubbles in bread.
On the quiz: You get asked which organism makes which product. Humans do not make ethanol.
It makes only 2 ATP, so the ATP is not the point. The point is that dumping electrons onto pyruvate turns NADH back into empty NAD+, and without empty NAD+ glycolysis would stall within seconds.
On the quiz: This is the idea most students miss. Fermentation is a recycling step that keeps glycolysis alive.
Before the Krebs cycle can start, each three carbon pyruvate loses a carbon as carbon dioxide and becomes two carbon acetyl CoA. Two pyruvate means 2 CO2 released here.
On the quiz: You get asked where the missing carbon went. It left as carbon dioxide at this step.
It runs in the mitochondrial matrix and turns twice per glucose. Each turn releases 2 CO2, so 4 more carbon dioxide leave here. It produces 2 ATP for cell work and loads 6 NADH and 2 FADH2 for the chain.
On the quiz: You get asked for the location, the outputs, and where each output goes next.
The cycle never touches oxygen directly. It stops because the chain stops, so NADH and FADH2 stay loaded and no empty carriers come back. No empty carriers means the cycle has nothing to load.
On the quiz: This is the single most tested idea in the unit, and the wrong answer is always that oxygen is a reactant of the cycle.
On the inner mitochondrial membrane, electrons from NADH and FADH2 travel through a series of proteins. Their energy powers pumps that push H+ out, building a gradient. H+ then flows back through ATP synthase, and that flow is what makes the ATP. The carriers leave empty as NAD+ and FAD+, recycled and ready to go back, which is the same job fermentation does without oxygen.
On the quiz: You get asked what powers the pumps, which is the electrons, what the gradient is for, and where the empty carriers go next.
At the end of the chain, oxygen takes the spent electrons plus H+ and becomes water. Oxygen is consumed here and never released. Remove it and every step upstream backs up, which is why cyanide kills in minutes.
On the quiz: You get asked to predict what happens when the chain is blocked anywhere along it.
Aerobic respiration gets about 36 ATP per glucose. Fermentation gets 2. That roughly twenty fold advantage is why oxygen users took over the planet after the Great Oxidation Event, and why complex animal bodies became possible at all.
On the quiz: You get asked to explain why the gap is so large, and the answer is that oxygen lets the cell finish extracting energy that was in the glucose all along.
This fills in as you mark the ideas above. Work from the top down. Nothing here is saved, so print it or write it down before you close the tab.
Nothing marked yet. Go back up and rate each of the fifteen ideas honestly. Guessing high here only costs you later.
Expect questions that ask you to connect what you watched happen to the chemistry behind it.
Yeast, sugar, and warm water in a sealed bottle with a balloon on top. The balloon inflates with carbon dioxide from alcoholic fermentation. The bottle with no sugar is the control, and it tells you sugar is the fuel driving the reaction. A warmer bottle inflates faster because enzymes speed up as temperature rises toward their optimum.
Pulse before and after activity. Heart rate and breathing climb because working muscles need more oxygen delivered to the chain and have more carbon dioxide to clear. Push hard enough and oxygen delivery falls behind demand, the muscles add lactic acid fermentation, and you feel the burn.
The content is half the battle. These are the habits that pick up the other half.
Most scenario questions turn on one fact: is oxygen available to this cell right now? Answer that before you read the choices and half the options rule themselves out.
Wrong choices on this quiz are usually correct for most of a sentence and then wrong in two or three words. Use the red pen on the practice test to cross out exactly which words break a choice.
When two choices are the same sentence with two terms traded, one of them is the answer and the other is the trap. Slow down and check which direction the mechanism actually runs.
On a data question, find the control row, compare the numbers, and do any subtraction the data invite. Decide what you think happened, then go find the choice that says it.
The stages are chained. If a question blocks something anywhere, trace backward: what cannot unload, what piles up, what runs out. Most of the harder questions are the same idea in different clothes.
A wrong choice will often take a real pattern in the data and reverse which thing caused which. If an answer says A caused B, check that B did not come first.
You should be able to give a one sentence definition of each of these without hesitating.
| Aerobic | Any process that uses oxygen. |
| Anaerobic | Any process that does not use oxygen. |
| ATP | Adenosine triphosphate. The cell's usable energy currency, with three phosphate groups. |
| ADP | Adenosine diphosphate. ATP after the third phosphate has been removed and the energy spent. |
| NAD+ and NADH | An electron carrier. NAD+ is empty, NADH is loaded with electrons and a hydrogen. |
| FAD+ and FADH2 | A second electron carrier that works the same way as NAD+ and NADH. |
| Glucose | The six carbon sugar that cellular respiration breaks down for energy. |
| Glycolysis | The first stage. Splits glucose into two pyruvate in the cytoplasm. Needs no oxygen. |
| Pyruvate | The three carbon molecule glycolysis produces. Two come from each glucose. |
| Cytoplasm | The fluid filling the cell, outside the organelles. Where glycolysis and fermentation happen. |
| Mitochondrion | The organelle where aerobic respiration happens. Plural is mitochondria. |
| Matrix | The fluid inside the inner membrane of a mitochondrion. Where the Krebs cycle runs. |
| Inner membrane | The folded membrane inside the mitochondrion that holds the electron transport chain. |
| Cristae | The folds of the inner membrane. More folds means more surface area for the chain. |
| Acetyl CoA | The two carbon molecule pyruvate becomes before entering the Krebs cycle. |
| Krebs cycle | The second stage, in the matrix. Also called the citric acid cycle or the TCA cycle. |
| Electron transport chain | The final stage. Proteins in the inner membrane that pass electrons along and pump H+. |
| Oxidative phosphorylation | Another name for the electron transport chain stage. |
| ATP synthase | The protein that H+ flows back through, which attaches a phosphate to ADP to make ATP. |
| Concentration gradient | A difference in concentration across a membrane. Here, more H+ outside than inside. |
| Active transport | Moving particles from low concentration to high. Requires energy. The H+ pumps do this. |
| Passive transport | Moving particles from high concentration to low. Requires no energy. H+ returning does this. |
| Final electron acceptor | Oxygen. It takes the spent electrons at the end of the chain and becomes water. |
| Fermentation | The anaerobic pathway after glycolysis. Makes little ATP but recycles NAD+. |
| Lactic acid fermentation | The version in your muscle cells. Produces lactic acid, which is the burn. |
| Alcoholic fermentation | The version in yeast. Produces ethanol and carbon dioxide. |
| Ethanol | The alcohol yeast produce during fermentation. |
| Metabolic activity | How hard a cell or organism is working. Higher activity means a higher respiration rate. |
| Enzyme | A protein that speeds up a reaction. Enzymes run faster as temperature rises toward their optimum. |
| Great Oxidation Event | The rise of atmospheric oxygen about 2.4 billion years ago that made aerobic life possible. |
You do not have to memorize these. You need to read one and say where a molecule came from, where it goes next, and what would back up if a step stopped.
If you can answer all of these out loud, you are ready.