Unit 11: Photosynthesis
How a chloroplast captures light, turns it into a proton gradient, and spends that gradient's ATP and NADPH pulling carbon out of the air.
6 topics · 22 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.
Chloroplast Structure and the Thylakoid Membrane
- How it worksHow does the thylakoid membrane's stacked disks pack enormous surface area for capturing light into a small space?
- Common mistakeWhy isn't the stroma the same compartment as the thylakoid lumen, even though both sit inside the chloroplast?
- Going furtherWhat happens to a chloroplast's light-capturing capacity when its thylakoid stacks fail to form properly?
Your turn1 of 2
Both the stroma and the thylakoid lumen are fluid-filled spaces found inside the chloroplast. A student assumes they must be the same compartment, just referred to by two different names. Why isn't the stroma the same compartment as the thylakoid lumen?Light Absorption and the Light Reactions
- How it worksHow does chlorophyll absorb a specific wavelength of light and boost an electron to a higher energy level?
- How it worksWalk me through how photosystem II passes an excited electron down a chain to photosystem I.
- Common mistakeWhy isn't the oxygen released during photosynthesis coming from the carbon dioxide being fixed?
- Worked exampleFind how many photons are needed to fix 5 molecules of carbon dioxide, given a commonly cited estimate of about 8 photons required per CO2 fixed.
Your turn1 of 2
Find how many photons are needed to fix 15 molecules of carbon dioxide, given a commonly cited estimate of about 9 photons required per CO2 fixed.Chemiosmosis in the Chloroplast
- How it worksHow does splitting water and passing electrons down the thylakoid chain pump protons into the thylakoid lumen?
- How it worksHow does that proton gradient drive ATP synthase to make ATP on the stromal side of the membrane?
- Common mistakeWhy isn't the proton gradient in a chloroplast built in the same direction as the one in a mitochondrion?
- Ties togetherHow does chemiosmosis in the chloroplast mirror the same logic that powers oxidative phosphorylation in the mitochondrion?
Your turn1 of 2
Both mitochondria and chloroplasts use chemiosmosis, pumping protons across a membrane to drive ATP synthase. A student assumes the proton gradient in a chloroplast must be built in exactly the same direction as in a mitochondrion, since both use the same basic mechanism. Why is this wrong?The Calvin Cycle and Carbon Fixation
- How it worksWalk me through how the Calvin cycle fixes carbon dioxide onto a five-carbon sugar and rebuilds that sugar to keep the cycle going.
- Worked exampleFind how many turns of the Calvin cycle are needed to net one molecule of glucose, given that each turn fixes one carbon dioxide.
- Common mistakeWhy isn't the Calvin cycle actually restricted to happening only in the dark?
- Going furtherWhat happens to Calvin cycle output when the ATP or NADPH supply from the light reactions is cut off?
Your turn1 of 2
Find how many turns of the Calvin cycle are needed to net 11 molecules of glucose, given that each turn fixes one carbon dioxide and glucose has six carbons.Rubisco and Photorespiration
- How it worksHow does rubisco accidentally grab oxygen instead of carbon dioxide and start photorespiration?
- How it worksWalk me through how photorespiration burns ATP and releases carbon dioxide instead of fixing it.
- Common mistakeWhy isn't photorespiration simply the plant version of cellular respiration happening for the same reason?
Your turn1 of 2
On a hot, dry day, a plant's stomata partly close to conserve water, which lowers the carbon dioxide available inside its leaves while oxygen remains abundant. Under these conditions, how does rubisco end up starting photorespiration instead of normal carbon fixation?C4 and CAM Adaptations to Photorespiration
- How it worksHow does a C4 plant separate carbon fixation from the Calvin cycle into two different cell types to concentrate carbon dioxide around rubisco?
- How it worksHow does a CAM plant separate those same two steps in time instead of space by fixing carbon only at night?
- Common mistakeWhy doesn't a C4 or CAM plant run a completely different Calvin cycle than an ordinary C3 plant?
- Ties togetherHow do C4 and CAM plants both solve the exact same photorespiration problem using opposite strategies, space versus time?
Your turn1 of 2
C4 and CAM plants both use extra biochemical steps to concentrate carbon dioxide around rubisco before the Calvin cycle runs. A student assumes this must mean C4 and CAM plants run an entirely different version of the Calvin cycle from an ordinary C3 plant. Why is this wrong?