Unit 8: Photosynthesis and Metabolic Integration
The mirror image of respiration, and how a cell decides whether to store fuel or burn it.
6 topics · 20 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Capturing Light: Chlorophyll and the Light-Dependent Reactions
- How it worksHow does a photon striking chlorophyll knock one of its electrons up to a higher energy level?
- Going furtherWhat happens to a plant's light capture when it's given only wavelengths of light that chlorophyll doesn't absorb well?
- Common mistakeWhy isn't a plant's green color the wavelength of light it's actually using for photosynthesis?
Your turn1 of 2
A photon of light strikes a chlorophyll molecule inside a leaf, and moments later that energy is on its way toward building sugar. A student pictures the photon as somehow physically pushing the whole molecule, the way wind pushes a leaf. What actually happens when the photon is absorbed?Splitting Water and Building a Proton Gradient in the Chloroplast
- How it worksHow does photosystem II rip electrons off of water to replace the ones it just lost to light?
- How it worksWalk me through how an electron transport chain in the thylakoid membrane pumps protons the same way a mitochondrion does.
- Common mistakeWhy isn't the oxygen a plant releases coming from the carbon dioxide it takes in?
- Ties togetherHow does a chloroplast's proton gradient turning ATP synthase reuse the exact same chemiosmotic mechanism as a mitochondrion?
Your turn1 of 2
Photosystem II loses electrons to light and needs to replace them to keep functioning. A student assumes the plant must pull replacement electrons from the carbon dioxide it is about to fix into sugar. Where does photosystem II actually get its replacement electrons?The Calvin Cycle and Carbon Fixation
- How it worksHow does the enzyme rubisco attach a molecule of carbon dioxide onto an existing five-carbon sugar?
- How it worksWalk me through how ATP and NADPH from the light reactions power the Calvin cycle's sugar-building steps.
- Worked exampleFind how many turns of the Calvin cycle, each fixing one carbon dioxide, are needed to build one six-carbon glucose molecule.
- Going furtherWhat happens to the Calvin cycle's rate once a plant's ATP and NADPH supply from the light reactions runs low?
Your turn1 of 2
Each turn of the Calvin cycle fixes one carbon dioxide molecule. Find how many turns are needed to build 11 six-carbon glucose molecules.Why Photosynthesis and Respiration Are Chemical Mirror Images
- How it worksHow does photosynthesis store energy by reducing carbon dioxide while respiration releases energy by oxidizing sugar?
- Common mistakeWhy isn't a plant cell one that only photosynthesizes and never respires?
- Ties togetherHow does the electron transport chain shared by chloroplasts and mitochondria connect these two pathways through one common mechanism?
Your turn1 of 2
Photosynthesis's overall equation and cellular respiration's overall equation use the exact same molecules, just with reactants and products swapped. A student assumes this must mean the two pathways run through the identical set of enzymatic steps, just performed in reverse order. Why is this assumption wrong?Metabolic Integration: Fed and Fasted States
- How it worksHow do insulin and glucagon flip a cell's metabolic pathways between storing fuel and releasing it?
- Going furtherWhat happens to a cell's fuel choice hours into a fast once its glycogen stores run out?
- Common mistakeWhy isn't burning fat and burning carbohydrate running through two completely unconnected sets of chemistry?
Your turn1 of 2
Right after a meal, a cell's metabolism shifts toward storing incoming fuel, while hours into a fast it shifts toward releasing stored fuel instead. A student wonders how a cell knows which mode to be in at any given moment. What actually controls this switch?Gluconeogenesis and Glycogen Storage
- How it worksHow does gluconeogenesis reroute around glycolysis's irreversible steps to effectively run the pathway backward?
- Going furtherWhat happens to a cell's stored glycogen supply during the first hours of fasting, before gluconeogenesis ramps up?
- Worked exampleFind how many glucose units a liver could release from a glycogen chain of 3000 units if 10 percent were used in the first hour.
Your turn1 of 2
Find how many glucose units a liver could release from a glycogen chain of 3937 units if 18 percent were used in the first hour.