Photosynthesis Notes

Life on Earth is Solar Powered

• Photosynthesis converts light energy into chemical energy.

• Chloroplasts in plants and photosynthetic organisms facilitate photosynthesis.

• Chloroplasts capture light and convert it into chemical energy stored in sugars.

Nutrition Modes

• Organisms gain organic compounds through:

  • Autotrophic nutrition: self-feeders.

  • Heterotrophic nutrition: consuming others.

Autotrophs

• Autotrophs produce organic molecules from CO2 and inorganic materials.

• They're the biosphere's producers, the source of organic compounds.

• Plants are photoautotrophs, using light to synthesize organic substances.

• Photosynthesis occurs in algae, unicellular eukaryotes, and some prokaryotes.

Heterotrophs

• Heterotrophs rely on other organisms for food.

• They are the biosphere’s consumers; some are decomposers.

• Most fungi and many prokaryotes decompose.

• Almost all heterotrophs depend on photoautotrophs for food and oxygen.

Fossil Fuels

• Fossil fuels store solar energy from the past.

• Researchers explore photosynthesis for alternative fuels.

Photosynthesis Overview

• Two stages:

  • Light reactions: convert solar energy to chemical energy.

  • Calvin cycle: uses chemical energy to make food molecules.

Structural Organization in Photosynthesis

• Photosynthetic enzymes are in biological membranes.

• Process likely originated in bacteria; endosymbiont theory.

Chloroplasts

• Chloroplasts are in photosynthetic organisms, mainly plants.

• Leaves are major photosynthesis sites with mesophyll cells containing chloroplasts.

• CO2 enters, and O2 exits via stomata; water and sugar transported through veins.

• A typical mesophyll cell contains 30–40 chloroplasts.

Chloroplast Structure

• Chloroplasts have two membranes and stroma.

• Thylakoids form a third membrane system, stacked as grana.

• Chlorophyll in thylakoid membranes drives organic molecule synthesis.

Tracking Atoms Through Photosynthesis

• Photosynthetic equation: 6 CO2 + 12 H2O + Light energy \rightarrow C6H{12}O6 + 6 O2 + 6 H_2O

• Net consumption equation: 6 CO2 + 6 H2O + Light energy \rightarrow C6H{12}O6 + 6 O2

• Simplified equation: CO2 + H2O \rightarrow [CH_2O] + O*2

  • Where [CH_2O] is a carbohydrate.

The Splitting of Water

• O2 released by plants comes from H2O, not CO2.

• Chloroplasts split water into hydrogen and oxygen.

• Van Niel's proposal:

  • Sulfur bacteria: CO2 + 2 H2S \rightarrow [CH_2O] + H*2O + 2 S

  • Plants: CO2 + 2 H2O \rightarrow [CH2O] + H*2O + O2

  • General: CO2 + 2 H2X \rightarrow [CH_2O] + H*2O + 2 X

• Scientists used oxygen-18 (^{18}O) as a tracer.

Photosynthesis as a Redox Process

• Photosynthesis reverses electron flow of cellular respiration.

• Water is split, transferring electrons to carbon dioxide, reducing it to sugar.

  • 6 CO2 \rightarrow C6H{12}O6 (reduced)

  • 6 H2O \rightarrow 6 O2 (oxidized)

• Requires energy from light.

Two Stages of Photosynthesis

• Light reactions:

  • Split water, release O2.

  • Use light to transfer electrons and H+ from water to NADP+, reducing it to NADPH.

  • Generate ATP via chemiosmosis (photophosphorylation).

• Calvin cycle:

  • Incorporates CO2 into organic molecules (carbon fixation).

  • Reduces fixed carbon to carbohydrate using NADPH and ATP.

• Light reactions occur in thylakoids; Calvin cycle in the stroma.

• NADP+ and ADP pick up electrons and phosphate outside thylakoids.

The Nature of Sunlight

• Light is electromagnetic energy in rhythmic waves.

• Wavelength ranges from nanometers to kilometers.

• Visible light spans 380 nm to 750 nm.

• Light acts as photons with fixed energy.

• Photon energy is inversely related to wavelength.

• The atmosphere allows visible light through.

Photosynthetic Pigments

• Pigments absorb visible light differently.

• The color seen is reflected or transmitted.

• Chlorophyll absorbs violet-blue and red light, reflecting green.

• Spectrophotometers measure light absorption versus wavelength.

Types of Pigments:

• Chlorophyll a: key light-capturing pigment.

• Chlorophyll b: accessory pigment.

• Carotenoids: accessory pigments for photoprotection.

Excitation of Chlorophyll by Light

• Absorbing light boosts electrons to higher energy levels (excited state).

• Electrons drop back, releasing energy as heat or fluorescence.

Photosystems

• Chlorophyll and molecules organize into photosystems in the thylakoid membrane.

• Photosystems have a reaction-center complex and light-harvesting complexes.

• Light-harvesting complexes gather light; reaction-center complex transfers electrons.

• Photosystem II (PS II) and photosystem I (PS I) cooperate.

• PS II's chlorophyll a is P680, PS I's is P700.

Linear Electron Flow

• Light drives ATP and NADPH synthesis via two photosystems.

1. Light excites PS II pigment molecules, relaying energy to P680.

2. Electron transferred from P680 to primary electron acceptor, forming P680+.

3. Enzyme splits water into electrons, H+, and oxygen; electrons replace those lost by P680+; H+ released into thylakoid space; oxygen forms O2.

4. Electrons pass from PS II to PS I via electron transport chain, pumping H+ into thylakoid space.

5. Proton gradient drives ATP production via chemiosmosis.

6. Light excites PS I pigment molecules, exciting P700.

7. Photoexcited electrons pass to PS I's primary electron acceptor, creating P700+.

8. Electrons passed from PS I to ferredoxin (Fd), then to NADP+, forming NADPH.

• Light reactions generate ATP and NADPH for the Calvin cycle.

Cyclic Electron Flow

• Electrons cycle from Fd to cytochrome complex, back to P700 in PS I.

• Generates ATP, no NADPH, no oxygen.

• Seen in photosynthetic bacteria; may be photoprotective.

Chemiosmosis in Chloroplasts and Mitochondria

• Both generate ATP by chemiosmosis.

• Electron transport chains pump H+ across membranes, creating a proton-motive force.

• ATP synthase couples H+ diffusion to ATP phosphorylation.

• In chloroplasts, electrons from water; in mitochondria, from organic molecules.

• Mitochondria transfer chemical energy to ATP; chloroplasts transform light energy to ATP.

• H+ reservoir: intermembrane space in mitochondria, thylakoid space in chloroplasts.

• ATP forms in the stroma.

Calvin Cycle

• Anabolic, building carbohydrates from smaller molecules using energy.

• CO2 enters, sugar exits; ATP and NADPH reduce CO2 to sugar (G3P).

• Three repeats fix one G3P molecule.

Calvin Cycle Steps:

• Phase 1: Carbon Fixation:

  • CO2 attached to RuBP by rubisco, forming 3-phosphoglycerate.

• Phase 2: Reduction:

  • 3-phosphoglycerate becomes 1,3-bisphosphoglycerate using ATP, then G3P using NADPH.

  • One G3P is net gain; others regenerate RuBP.

• Phase 3: Regeneration of RuBP:

  • G3P skeletons rearrange into RuBP using ATP.

• Net synthesis of one G3P requires nine ATP and six NADPH.

• G3P becomes starting material for organic compounds like glucose and sucrose.

• Photosynthesis integrates light reactions and the Calvin cycle.

Alternative Mechanisms of Carbon Fixation

• Plants balance photosynthesis with water loss.

• Stomata regulate CO2, O2, and transpiration; closing them favors photorespiration.

Photorespiration

• C3 plants fix carbon via rubisco, forming 3-phosphoglycerate.

• Closing stomata reduces CO2, increases O2, leading rubisco to add O2 instead of CO2.

• This releases CO2, consumes ATP, and produces no sugar, decreasing photosynthetic output.

• May be an evolutionary relic.

C4 Plants

• C4 plants use alternate carbon fixation forming a four-carbon compound.

• Have bundle-sheath and mesophyll cells.

• CO2 is fixed in mesophyll cells, then released in bundle-sheath cells for the Calvin cycle.

(1) PEP carboxylase adds CO2 to PEP, forming oxaloacetate.

• PEP carboxylase has a higher affinity for CO2 than rubisco and no affinity for O2.

(2) Four-carbon compounds are exported to bundle-sheath cells.

(3) Four-carbon compounds release CO2 for the Calvin cycle.

CAM Plants

• Open stomata at night, close during the day.

• Take up CO2 at night, storing organic acids in vacuoles.

• During the day, CO2 is released for the Calvin cycle.

• C4 plants separate steps structurally; CAM plants separate them temporally.

• All use the Calvin cycle to make sugar.

Importance of Photosynthesis

• Light reactions make ATP and NADPH; Calvin cycle uses them to produce sugar.

• Sugar provides plants with energy and carbon skeletons.

• About 50% of organic material fuels cellular respiration.

• Carbohydrate is