Photosynthesis Steps

Why This Matters

Photosynthesis is the foundation of nearly all life on Earth—it's the process that converts light energy into the chemical energy stored in glucose, and it's responsible for the oxygen you're breathing right now. On the AP Biology exam, you're being tested on your understanding of energy transformations, chemiosmosis, enzyme function, and the relationship between structure and function in chloroplasts. These concepts connect directly to cellular respiration (they're essentially mirror processes) and to broader themes about how organisms capture and use energy.

Don't just memorize that "light hits chlorophyll and glucose comes out." You need to understand why electrons get excited, how a proton gradient drives ATP synthesis, and what each molecule's role is in the overall process. The exam loves to ask you to compare the light-dependent and light-independent reactions, trace the flow of electrons and protons, and explain what would happen if a specific step were blocked. Know the mechanisms, not just the vocabulary.

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Capturing Light Energy

The first challenge of photosynthesis is harvesting light energy and converting it into a form cells can use. This happens in the thylakoid membranes, where pigments are precisely arranged to maximize energy capture. The key principle here is that specific wavelengths of light excite electrons, and that energy must be transferred efficiently before it dissipates as heat.

Light Absorption by Pigments

• Chlorophyll a and b absorb blue and red wavelengths—they reflect green light, which is why plants appear green to us
• Accessory pigments like carotenoids expand the range of usable light and protect chlorophyll from photo-oxidative damage
• Pigments are organized into photosystems within thylakoid membranes, with antenna complexes funneling energy toward reaction centers

Electron Excitation in Photosystems

• Light energy excites electrons to higher energy states—this is the critical first step in converting light to chemical energy
• Two photosystems work in series: Photosystem II (PSII) absorbs light at 680 nm, while Photosystem I (PSI) absorbs at 700 nm
• Excited electrons leave the reaction center and enter the electron transport chain, initiating the light-dependent reactions

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Building the Proton Gradient

Once electrons are excited, they pass through an electron transport chain (ETC) embedded in the thylakoid membrane. The energy released as electrons move "downhill" is used to pump protons, creating the electrochemical gradient that powers ATP synthesis—this is chemiosmosis, and it's the same principle used in mitochondria.

Electron Transport Chain

• Electrons move through membrane-bound protein complexes—including plastoquinone, cytochrome b6f complex, and plastocyanin
• Energy release pumps $H^+$ into the thylakoid lumen—creating a high concentration of protons inside the thylakoid space
• $NADP^+$ is the final electron acceptor, reduced to NADPH by the enzyme NADP+ reductase at the end of PSI

Photolysis (Water Splitting)

• Water molecules are split at PSII to replace electrons lost from chlorophyll—this is called photolysis
• Oxygen is released as a byproduct—the $O_2$ we breathe comes from water, not $CO_2$
• Protons from water contribute to the gradient—they're released into the thylakoid lumen, intensifying the electrochemical gradient

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ATP Synthesis via Chemiosmosis

The proton gradient is potential energy waiting to be harvested. ATP synthase is the molecular turbine that converts this gradient into the universal energy currency of cells. This process—called photophosphorylation in chloroplasts—demonstrates how cells couple exergonic processes (proton flow) to endergonic ones (ATP synthesis).

ATP Synthase and Photophosphorylation

• Protons flow down their gradient through ATP synthase—from the thylakoid lumen back into the stroma
• The flow of $H^+$ causes conformational changes in ATP synthase that catalyze $ADP + P_i \rightarrow ATP$
• This is non-cyclic photophosphorylation—it produces both ATP and NADPH, which are essential for the Calvin cycle

Light-Dependent Reactions Summary

• Location: thylakoid membranes—structure is critical; the enclosed space allows gradient formation
• Inputs: light, water, $ADP$, $P_i$, $NADP^+$—outputs are ATP, NADPH, and $O_2$
• Purpose: convert light energy to chemical energy stored in ATP and NADPH, which power carbon fixation

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Carbon Fixation and the Calvin Cycle

The Calvin cycle takes the ATP and NADPH produced by light-dependent reactions and uses them to build organic molecules from $CO_2$. This occurs in the stroma and is sometimes called the "light-independent reactions"—but don't be fooled; it depends entirely on products from the light-dependent reactions and stops without them.

Carbon Fixation by RuBisCO

• $CO_2$ is "fixed" to RuBP by the enzyme RuBisCO—this is the most abundant enzyme on Earth and the entry point for carbon into the biosphere
• The product is an unstable 6-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA)
• This step doesn't require ATP or NADPH directly—it's purely the attachment of inorganic carbon to an organic molecule

Reduction Phase: Building G3P

• ATP and NADPH reduce 3-PGA to glyceraldehyde-3-phosphate (G3P)—this is where light-reaction products are consumed
• G3P is a 3-carbon sugar that serves as the building block for glucose, starch, cellulose, and other organic molecules
• For every 3 $CO_2$ fixed, 6 G3P are produced—but only 1 G3P exits the cycle; the rest regenerate RuBP

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Regeneration and Cycle Continuation

The Calvin cycle must regenerate its starting molecule, RuBP, to keep fixing carbon. This regeneration phase uses ATP and involves a complex series of molecular rearrangements—it's why the cycle is a cycle and not a linear pathway.

RuBP Regeneration

• 5 of every 6 G3P molecules are used to regenerate RuBP—only 1 G3P represents net carbon gain
• ATP is required for regeneration—this is the second use of ATP in the Calvin cycle
• The cycle must turn 3 times to produce one G3P that can exit and contribute to glucose synthesis

Calvin Cycle Summary

• Location: stroma of chloroplasts—the aqueous environment where enzymes like RuBisCO operate
• Three phases: fixation, reduction, regeneration—each with distinct inputs and outputs
• Net equation: 3 $CO_2$ + 9 ATP + 6 NADPH → 1 G3P—this represents the energy investment needed to build organic molecules

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Factors Affecting Photosynthesis Rate

Understanding what limits photosynthesis helps you predict how plants respond to environmental changes. Each factor affects specific steps in the process—the exam often asks you to explain WHY a factor matters, not just that it does.

Environmental Limiting Factors

• Light intensity increases photosynthesis rate until saturation—at high light, other factors (like $CO_2$ or enzyme capacity) become limiting
• $CO_2$ concentration directly affects carbon fixation—more $CO_2$ means more substrate for RuBisCO, up to a point
• Temperature affects enzyme function—RuBisCO and other enzymes have optimal ranges; extreme heat denatures proteins, extreme cold slows reactions

Water and Stomatal Regulation

• Water stress causes stomata to close—this conserves water but limits $CO_2$ entry
• Closed stomata reduce carbon fixation rates—even if light and temperature are optimal
• This trade-off explains why drought reduces plant growth—it's a direct link between water availability and photosynthetic output

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Quick Reference Table

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Self-Check Questions

1. Trace the path of electrons from water to NADPH. Which photosystem comes first in the sequence, and why is water split at that location?

2. Compare the roles of ATP and NADPH in the Calvin cycle. In which specific phases is each molecule consumed?

3. If a plant is exposed to high light intensity but very low $CO_2$ concentration, which reactions would be most affected—light-dependent or light-independent? Explain your reasoning.

4. How is chemiosmosis in chloroplasts similar to and different from chemiosmosis in mitochondria? Consider the direction of proton pumping and the location of ATP synthase.

5. A researcher uses a chemical that blocks the cytochrome b6f complex. Predict the effects on (a) the proton gradient, (b) ATP production, (c) NADPH production, and (d) oxygen release. Which would be affected first?