Photosynthesis

BCH 100 — Introductory Biochemistry · Dr. Radi

build Jul 17 · 14:27 · CC BY-NC-SA 4.0 · owned figures (RDKit / matplotlib / PyMOL)
Dr. Radi

By the end of this unit, you can…

  • Compare photosynthesis with cellular respiration, and locate the light reactions and Calvin cycle within the chloroplast
  • Explain how pigments harvest light and how Photosystems II and I drive the Z-scheme of electron flow
  • Describe photophosphorylation (cyclic vs non-cyclic) and the ATP + NADPH the light reactions produce
  • Walk the Calvin cycle — fixation, reduction, regeneration — and RuBisCO's role and limitation
  • Give the Calvin-cycle stoichiometry and the fate of the sugar (sucrose, starch, cellulose)
Dr. Radi

Today's route 🗺️

  1. Photosynthesis — The Light Reactions
  2. The Calvin Cycle
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1 · Photosynthesis — The Light Reactions

"Plants are better than us — they run respiration in reverse, powered by sunlight. Watch light split water and charge up ATP and NADPH."

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Plants are better than us

We burn glucose for energy; plants build it — running the same chemistry in reverse, powered by sunlight. Photosynthesis takes CO₂ + water + light and makes glucose + O₂; respiration takes glucose + O₂ and makes CO₂ + water + ATP. They're exact mirror images — and that O₂ you're breathing? A plant made it.

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Where it happens: the chloroplast

Photosynthesis runs in the chloroplast — think of it as a mitochondrion with an extra membrane. Stacked inside are the thylakoids (the green disks, piled into grana); their membrane holds the light-reaction machinery. Surrounding them is the stroma, where the Calvin cycle builds sugar. Thylakoid ≈ inner mitochondrial membrane.

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The real thing

Here's an actual chloroplast under the microscope — those green-and-gold stacks are grana, hundreds of thylakoid disks packed together to catch every photon. Between them stretch the stroma lamellae. This dense folding is why a single leaf cell can hold enough machinery to feed the whole plant.

Chloroplast STEM + reconstruction: Yuv345, CC BY-SA 4.0 (Wikimedia Commons)
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Catching the light

A single chlorophyll can't do much alone — so plants build a big antenna: hundreds of pigments (chlorophylls plus accessory carotenoids) that soak up photons and pass the energy inward, molecule to molecule, until it reaches one special reaction center. There, the energy finally ejects an electron — and the chemistry begins.

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Two photosystems in the membrane

The light reactions look a lot like the ETC — a chain of complexes in a membrane, pumping protons. Light hits Photosystem II, which splits water (that's where the O₂ comes from!) and sends electrons — ferried by plastoquinone and plastocyanin — through cytochrome b₆f to Photosystem I. A second hit of light at PSI boosts them again, all the way to NADPH. Protons pile up in the lumen and spin ATP synthase.

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Why it's called the "Z-scheme"

Plot the electrons' energy and you see why: they start low (in water), get boosted uphill by light at PSII, drift back down the chain, then get boosted again at PSI — up to NADPH. Two light-driven jumps, one downhill drift between them: it draws a "Z" on its side. Sunlight is doing the lifting.

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Two ways to make ATP with light

Making ATP with light energy is photophosphorylation, and there are two modes. Non-cyclic is the main line — PSII to PSI, splitting water and producing NADPH + ATP + O₂. Cyclic uses PSI alone, looping electrons back to make extra ATP only (no NADPH, no O₂) — a top-up for when the Calvin cycle needs more ATP than NADPH.

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2 · The Calvin Cycle

"Now spend that ATP and NADPH — turn thin air (CO₂) into sugar, one carbon at a time, with the planet's busiest enzyme."

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Building sugar from thin air

The light reactions charged up ATP and NADPH — now the Calvin cycle spends them to turn CO₂ into sugar. It runs in three phases: fixation (grab a CO₂), reduction (spend ATP + NADPH to make a sugar), and regeneration (rebuild the CO₂-catcher). And don't call these the "dark reactions" — they run in daylight and are completely dependent on the light reactions' ATP and NADPH!

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RuBisCO: the enzyme that feeds the planet

Fixation is the job of RuBisCO, which sticks a CO₂ onto the 5-carbon acceptor RuBP, splitting the product into two 3-carbon molecules. Nearly every carbon atom in your body passed through this enzyme. Its catch? It's slow, and it clumsily grabs O₂ instead of CO₂ (photorespiration) — so plants make tons of it, making it the most abundant protein on Earth.

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The price of a sugar

Carbon comes in one at a time, so it takes a lot of turns. Three turns fix three CO₂ and net one G3P — at a cost of 9 ATP + 6 NADPH. To build a full glucose (two G3P) takes six turns: 18 ATP + 12 NADPH. Every bit of that energy bill was paid by the light reactions.

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What the sugar becomes

That G3P is the plant's raw material for everything. Some the plant burns for its own energy (yes — plants run glycolysis too!). Some becomes sucrose, the sugar it ships around itself (and your maple syrup!). Some is stored as starch — the plant's glycogen. And a huge amount is spun into cellulose — the cell walls, wood, and cotton that are most of Earth's biomass.

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Why green potatoes are bad news

Leave a potato in the light and it turns green — it's making chlorophyll. Harmless on its own… but the plant makes a bitter toxin, solanine, right alongside it. So the green tinge is a warning label: where there's chlorophyll, there's solanine. Cut the green bits off — or, if it's very green, toss it.

Dr. Radi

Can you…?

  • ☐ compare photosynthesis with cellular respiration, and locate the light reactions and Calvin cycle within the chloroplast?
  • ☐ explain how pigments harvest light and how Photosystems II and I drive the Z-scheme of electron flow?
  • ☐ describe photophosphorylation (cyclic vs non-cyclic) and the ATP + NADPH the light reactions produce?
  • ☐ walk the Calvin cycle — fixation, reduction, regeneration — and RuBisCO's role and limitation?
  • ☐ give the Calvin-cycle stoichiometry and the fate of the sugar (sucrose, starch, cellulose)?

If any box stays empty, the practice site has a drill for it. 🧪

Dr. Radi