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.
6 CO2 + 6 H2O + light energy gives C6H12O6 + 6 O2. Carbon dioxide and water go in, light drives it, and sugar and oxygen come out. Cellular respiration is the same equation read backward, which is the fastest way to remember either one.
On the quiz: You get asked to pick the correct equation out of four that look alike. Three of them are the real one with two things swapped, so read the arrow first and then check which side the oxygen is on.
Sunlight carries energy into the chloroplast and no atoms at all. Every atom in a plant came from carbon dioxide, water, or the minerals in soil. The carbon in a tree trunk came out of the air.
On the quiz: This is the Van Helmont question. A plant gains mass, the soil barely changes, and students reach for water or sunlight. The answer is carbon dioxide.
Thylakoids are the flattened discs, stacked into grana, and their membranes hold the pigments and the light reaction machinery. The stroma is the fluid around them, where the Calvin cycle runs.
On the quiz: You get asked which stage happens where. Membrane work is light reactions, fluid work is the Calvin cycle.
Chlorophyll absorbs blue and red strongly and green hardly at all. Green light bounces off the leaf and into your eye, which is why leaves look green. Leaves also carry carotene and xanthophyll, which absorb in ranges chlorophyll misses.
On the quiz: You get asked why green light drives almost no photosynthesis. The color you see is the color the leaf is throwing away.
Light and water go in. ATP and NADPH come out, along with the oxygen released as waste. These two products exist for one purpose, which is to be spent by the Calvin cycle.
On the quiz: You get asked what the light reactions hand over. It is never glucose. Glucose is built later, in the other stage.
Photosystem II pulls water apart to get electrons. The leftover oxygen atoms pair up and leave as O2. Every oxygen molecule in the air you are breathing came out of a water molecule inside a chloroplast.
On the quiz: The classic wrong answer is carbon dioxide. Its oxygen ends up in sugar and water, never in the gas released.
Energy from the electrons powers pumps that push H+ into the thylakoid. The ions then flow back out through ATP synthase, and that flow is what makes the ATP. Same machinery as the mitochondrion, pumping the opposite way.
On the quiz: You get asked what powers the pumps, which is the electrons, and which way H+ moves. In the chloroplast it goes in. In the mitochondrion it goes out.
It picks up electrons at the end of the light reactions, carries them to the Calvin cycle, unloads, and goes back as empty NADP+. It is the photosynthesis version of NADH, and like NADH it is not spent on work the way ATP is.
On the quiz: You get asked what it carries, where it loads, where it unloads, and what happens when it cannot unload.
It runs in the stroma and needs three things delivered: carbon dioxide for the carbon, and ATP and NADPH for the energy and the electrons. It turns six times to build one glucose.
On the quiz: You get asked what has to arrive before it can run. Miss any one of the three and the cycle stops.
RuBisCO is the enzyme that grabs carbon dioxide gas out of the air and locks it onto a molecule already in the cycle. That single step is where a gas becomes part of a solid plant, and it is why RuBisCO is the most abundant protein on Earth.
On the quiz: You get asked which enzyme does what. RuBisCO attaches carbon. ATP synthase makes ATP. Keep those two apart.
It uses no light directly, which is why the old name dark reactions was misleading. It stops after sunset anyway, because the ATP and NADPH stop arriving. The dependency is on the carriers, not on the light itself.
On the quiz: This is the most tested idea in the photosynthesis half of the unit, and it is the same shape as the Krebs cycle question from the respiration half. Trace what runs out, not what is absent.
Block the light reactions and the Calvin cycle starves for carriers. Block the Calvin cycle and the carriers stay loaded, the light reactions have nowhere to unload, and oxygen output stalls. Each stage depends on getting its empties back.
On the quiz: You get a drug or a herbicide blocking one stage and have to predict what happens to the other. Work out what cannot unload, what piles up, and what runs out.
Light, carbon dioxide, and temperature can each be the bottleneck. Whichever is scarcest sets the rate, and adding more of anything else changes almost nothing. When a graph flattens, something other than the variable on the x axis has taken over.
On the quiz: You get a curve that rises and then levels off, and have to name what became limiting after the flattening point. Adding more of something already plentiful is the trap answer.
Plants have chloroplasts and mitochondria both, and they respire day and night. Glucose is stored energy rather than usable energy, so a plant still has to break it down for ATP. In daylight photosynthesis simply outpaces respiration.
On the quiz: The wrong answer is always that the two processes take turns, photosynthesis by day and respiration by night. They overlap constantly.
Photosynthesis is the only process that pulls carbon dioxide out of the air at scale, which makes forests, crops, and ocean algae genuine carbon sinks. Carbon stored in a plant is not stored permanently, since respiration, decay, and fire all return it.
On the quiz: You get asked to evaluate a climate claim. The honest answer is usually that plants slow the problem rather than solving it, and the reason is that the carbon comes back.
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.
Straw punched spinach disks, a syringe vacuum to pull the air out of them, and bicarbonate solution as a dissolved carbon dioxide supply. A fresh disk floats on its trapped air. Pull that air out and the disk sinks. Put it under light and photosynthesis fills the air spaces with oxygen until it rises again, so time to float is a stand in for photosynthetic rate. The foil wrapped cup is the control for light, and a plain water cup is the control for the carbon source. Disks cut through a vein infiltrate badly, which is the usual reason one group lags behind another.
Plants or disks under red, blue, green, and white light. Blue and red perform well and green performs barely better than the dark control, because chlorophyll reflects green rather than absorbing it. Light that bounces off the leaf cannot do any work, and that is the whole point of the lab.
The content is half the battle. These are the habits that pick up the other half.
Almost every question is really asking about one stage. Decide whether it is light reactions or Calvin cycle first, and half the options rule themselves out.
Wrong choices here are usually correct for most of a sentence and then wrong in two or three words. Use the red pen 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. Check which direction the process actually runs.
On an experiment question, the covered leaf, the foil wrapped tube, or the tube with no plant is there to eliminate one explanation. Name which one and the conclusion follows.
A curve that rises and then levels off is a limiting factor handing off. The question is almost always which factor became the bottleneck after that point.
If a factor is already in surplus, adding more changes almost nothing. Find what is actually scarce before you predict an effect, and be suspicious of any answer that adds to a surplus.
You should be able to give a one sentence definition of each of these without hesitating.
| Photosynthesis | Building sugar from carbon dioxide and water using light energy. |
| Autotroph | An organism that makes its own food. Plants, algae, and some bacteria. |
| Heterotroph | An organism that must eat other organisms for food. Animals and fungi. |
| Chloroplast | The organelle where photosynthesis happens. Found in plant and algal cells. |
| Thylakoid | A flattened disc inside the chloroplast. Its membrane holds the light reaction machinery. |
| Granum | A stack of thylakoids. Plural is grana. |
| Stroma | The fluid around the thylakoids, inside the chloroplast. Where the Calvin cycle runs. |
| Lumen | The space inside a thylakoid, where H+ builds up during the light reactions. |
| Pigment | A molecule that absorbs some wavelengths of light and reflects the rest. |
| Chlorophyll a | The main pigment of photosynthesis. Absorbs blue and red, reflects green. |
| Chlorophyll b | A second chlorophyll that absorbs slightly different wavelengths and passes energy to chlorophyll a. |
| Carotene and xanthophyll | Accessory pigments, orange and yellow, that catch wavelengths chlorophyll misses. |
| Wavelength | What makes one color of light different from another. Measured in nanometers. |
| Absorption spectrum | A graph of how much light a pigment absorbs at each wavelength. |
| Light reactions | The first stage, in the thylakoid membrane. Makes ATP and NADPH and releases oxygen. |
| Photosystem II | The protein complex that splits water and starts the electron flow. |
| Photosystem I | The second complex in the chain. Re energizes the electrons so NADPH can be made. |
| Electron transport chain | Proteins in the thylakoid membrane that pass electrons along and pump H+. |
| ATP synthase | The protein H+ flows back through, which attaches a phosphate to ADP to make ATP. |
| NADP+ and NADPH | An electron carrier. NADP+ is empty, NADPH is loaded. The photosynthesis version of NAD+ and NADH. |
| ATP | Adenosine triphosphate. The cell's usable energy currency. |
| Calvin cycle | The second stage, in the stroma. Builds sugar from carbon dioxide. Also called the light independent reactions. |
| Carbon fixation | Attaching carbon dioxide gas onto a molecule, turning a gas into part of a solid. |
| RuBisCO | The enzyme that fixes carbon dioxide. The most abundant protein on Earth. |
| RuBP | The five carbon molecule RuBisCO attaches carbon dioxide to. |
| G3P | The three carbon sugar the Calvin cycle produces, which the plant builds glucose and everything else from. |
| Glucose | The six carbon sugar photosynthesis ultimately produces. C6H12O6. |
| Cellulose | The tough molecule plants build from glucose to make cell walls. Most of a tree's mass. |
| Starch | How a plant stores glucose. Turns blue black with iodine, which is how you test a leaf for it. |
| Stomata | Pores on the underside of a leaf that let carbon dioxide in and water vapor out. |
| Bicarbonate | A dissolved source of carbon dioxide. What makes the leaf disk lab work. |
| Limiting factor | Whichever requirement is scarcest, and therefore the one setting the rate. |
| Carbon sink | Anything that takes more carbon out of the air than it puts back. Forests and ocean algae are the big ones. |
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.