8.3 Using Light Energy to Make Organic Molecules

Photosynthesis and Energy Flow

Photosynthesis converts sunlight into chemical energy, transforming inorganic carbon dioxide into organic molecules like glucose. This process is the foundation of energy flow in nearly every ecosystem on Earth. Primary producers capture light energy, which then moves through food webs to support diverse ecological communities.

This section covers the Calvin cycle (where carbon fixation actually happens), how light-dependent reactions power that process, and how photosynthetic energy flows through ecosystems.

Steps of the Calvin Cycle

The Calvin cycle takes place in the stroma of the chloroplast and uses the ATP and NADPH produced by the light-dependent reactions. It runs in three stages:

1. Carbon fixation

   • The enzyme RuBisCO catalyzes the addition of $CO_2$ to ribulose bisphosphate ($RuBP$), a 5-carbon sugar
   • This produces two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound
   • This is the actual moment inorganic carbon becomes part of an organic molecule

2. Reduction

   • ATP and NADPH from the light-dependent reactions convert 3-PGA into glyceraldehyde 3-phosphate (G3P)
   • G3P is a simple 3-carbon sugar that serves as the building block for glucose and other organic compounds (starch, cellulose, amino acids, fatty acids)

3. Regeneration of RuBP

   • Most of the G3P molecules are recycled to regenerate RuBP, which keeps the cycle running
   • For every 3 turns of the cycle (3 $CO_2$ molecules fixed), only 1 net G3P exits the cycle
   • That net G3P is used to build glucose and other organic molecules

Carbon Fixation in Energy Conversion

Carbon fixation is the step where inorganic $CO_2$ gets incorporated into an organic molecule. It's the bridge between the atmosphere's carbon and the carbon in living things.

Here's how it connects to energy conversion:

• During the light-dependent reactions, light energy is captured and stored temporarily in ATP and NADPH
• In the Calvin cycle, that ATP and NADPH power the reduction of 3-PGA to G3P
• G3P is then used to build glucose and other organic compounds that store energy in their chemical bonds
• That stored energy is later released through cellular respiration to power cellular processes like growth and reproduction

So carbon fixation is really the step where light energy gets converted into stable chemical energy in organic molecules.

Photorespiration: RuBisCO sometimes binds $O_2$ instead of $CO_2$, especially when oxygen concentrations are high relative to carbon dioxide. This is called photorespiration, and it reduces photosynthetic efficiency because no carbon is fixed and energy is wasted. Some plants (C4 and CAM plants) have evolved mechanisms to minimize this problem.

Light-Dependent Reactions and Energy Capture

The light-dependent reactions happen in the thylakoid membranes of the chloroplast. Their job is to capture light energy and convert it into the chemical energy carriers (ATP and NADPH) that the Calvin cycle needs.

The process works in a chain of events:

1. Light absorption: Light-harvesting complexes (antenna pigments) in the thylakoid membrane capture photons and funnel that energy to the reaction center of Photosystem II (PSII)

2. Water splitting: PSII uses the light energy to split water molecules ($2H_2O \rightarrow 4H^+ + 4e^- + O_2$), releasing oxygen as a byproduct

3. Electron transport chain: Excited electrons pass from PSII through an electron transport chain to Photosystem I (PSI), releasing energy along the way that pumps $H^+$ ions into the thylakoid lumen

4. ATP synthesis (photophosphorylation): The $H^+$ gradient drives ATP synthase to produce ATP through chemiosmosis

5. NADPH production: PSI re-energizes electrons using light, and those electrons are transferred to $NADP^+$, reducing it to NADPH

The ATP and NADPH then move into the stroma to fuel the Calvin cycle.

Photosynthesis in Ecosystem Energy Flow

Photosynthetic organisms are the primary producers of most ecosystems. They capture light energy and lock it into organic compounds, making that energy available to everything else in the food web.

• Herbivores (primary consumers) obtain energy by eating primary producers like plants and algae
• Carnivores (secondary and tertiary consumers) obtain energy by eating herbivores or other carnivores
• Decomposers (bacteria, fungi) break down dead organisms, releasing energy as heat and recycling nutrients back into the soil and water for primary producers to use again

Energy transfer between trophic levels is inefficient. Only about 10% of the energy at one trophic level gets passed to the next. The rest is lost as heat or used for the organism's own metabolic processes (respiration, movement, excretion).