Chapter 7: How Cells Capture Light Energy via Photosynthesis

7.1 Overview of Photosynthesis

• Photosynthesis captures light energy to synthesize carbohydrates.

• CO_2 is reduced.

• H_2O is oxidized.

• General equation for photosynthesis:
  6CO2 + 12H2O + Light energy ——> C6H12O6 + 6O2 + 6H2O

• Light energy drives this endergonic reaction.

Photosynthesis Powers the Biosphere

• Biosphere: Regions on Earth's surface and atmosphere where life exists.

• Organisms: Autotrophs or heterotrophs.

• Autotrophs: Create organic molecules from inorganic sources; photoautotrophs use light.

• Heterotrophs: Consume food to acquire organic molecules.

• Photosynthesis by plants, algae, and cyanobacteria drives life.

Photosynthesis Occurs in the Chloroplasts of Plants and Algae

• Chloroplasts: Organelles for photosynthesis, containing chlorophyll.

• Photosynthesis occurs in leaves, specifically mesophyll cells.

• Stomata: Openings allowing CO2 and O2 passage.

• Chloroplast structures: Outer membrane, intermembrane space, inner membrane, stroma, thylakoid membranes (forming grana), thylakoid lumen.

Two Stages of Photosynthesis: Light Reactions and the Calvin Cycle

• Light reactions: Energy conversions from light to chemical energy in ATP and NADPH.

• ATP and NADPH: Energy for carbohydrate synthesis during the Calvin cycle.

7.2 Reactions That Harness Light Energy

Light is Electromagnetic Radiation

• Light: Electromagnetic radiation with electric and magnetic fields.

• Travels as waves and particles (photons).

• Visible light: Wavelengths detectable by human eyes.

• Shorter wavelengths: Higher energy; longer wavelengths: Lower energy.

Pigments Absorb Light Energy

• When light encounters a molecule:

  • It may pass through.

  • It may bounce off.

  • It may be absorbed.

• Pigments: Molecules that absorb light.

• Pigments absorb some light and reflect others; absorption depends on energy needed to boost an electron.

Pigments and Electron Excitation

• Electron excitation: Unstable state after energy absorption.

• Energy release:

  • As heat.

  • As light (fluorescence).

• Excited electrons can be transferred or captured by other molecules.

Photosynthetic Pigments in Plants

• Chlorophylls: chlorophyll a and chlorophyll b in green plants and algae.

• Porphyrin ring: Delocalized electron absorbs light.

• Hydrocarbon tail: Anchors pigment to thylakoid membrane proteins.

• Carotenoids: Another pigment type.

  • Abundant in fruits/flowers; produce yellow/orange/red.

  • Delocalized electrons present in colored regions.

Absorption and Action Spectra

• Different pigments allow light absorption at various wavelengths.

• Absorption spectrum: Graph of wavelengths absorbed by pigments.

• Chlorophylls: Strong absorption in blue-violet and red.

• Action spectrum: Rate of photosynthesis at specific wavelengths.

• High photosynthesis rates correlate with wavelengths strongly absorbed by chlorophylls and carotenoids.

Photosystems II and I and Linear Electron Flow

• Thylakoid membranes: Contain photosystem I (PS I) and photosystem II (PS II).

• PS I discovered first, but PS II initiates photosynthesis.

• Light excites pigment molecules in both.

Linear Electron Flow

• Combined action of PS II and PS I to produce O_2, ATP, and NADPH.

• Role of Photosystem II:

  • Initiates photosynthesis.

  • Excited electrons move from PS II to PS I.

  • Oxidizes water, producing O_2 and H^+.

  • Releases energized electrons to electron transport chain (ETC).

  • Energy builds H^+ electrochemical gradient.

• Role of Photosystem I:

  • Primary role: Make NADPH.

  • Addition of H^+ to NADP^+ enhances the H^+ gradient.

  • ATP production in the chloroplast: Photophosphorylation.

Cyclic Electron Flow

• Linear electron flow produces roughly equal amounts of ATP and NADPH; the Calvin cycle uses more ATP.

• Cyclic electron flow: An alternate pathway producing additional ATP.

• Electrons from PS I pass through ETC, contributing to the H^+ gradient, then return to PS I.

• Favored when NADP^+ levels are low, NADPH is high, and ATP levels are low.

7.3 Molecular Features of Photosystems

Photosystem II: Light Energy Capture and O_2 Production

• PS I and PS II have light-harvesting and reaction center complexes.

• Light-harvesting complex (antenna complex): Pigments anchored to transmembrane proteins.

  • Absorbs photons and transfers energy via resonance energy transfer.

• Reaction center: Redox reaction site containing P680.

  • P680 releases high-energy electron and is oxidized: P680^* \rightarrow P680^+ + e^-.

  • Water is oxidized to replace electrons on P680^+.

Photosystem II and Water Oxidation

• PS II: The only protein complex able to oxidize water, releasing O_2.

Electron Energy Levels and the Z Scheme

• Z scheme (1960): Photosynthesis involves two light absorption events; “Z” from zigzag energy curve.

• Consistent with linear flow: PS II to PS I to NADPH.

7.4 Synthesizing Carbohydrates via the Calvin Cycle

Calvin Cycle Overview

• ATP and NADPH from light reactions power the Calvin cycle to make carbohydrates.

• Calvin cycle: Incorporates atmospheric CO_2 into organic molecules.

• High energy input required.

  • 18 ATP and 12 NADPH used per 6 CO_2 incorporated.

Steps of the Calvin Cycle

• Product: glyceraldehyde-3-phosphate (G3P), a 3-carbon carbohydrate for glucose synthesis.

• Three phases:

  • Carbon fixation: CO_2 incorporated into RuBP (5-carbon sugar). Rubisco catalyzes, forming an unstable 6-carbon intermediate that splits into two 3-carbon molecules.

  • Reduction and carbohydrate production: ATP energy and NADPH electrons produce G3P.

  • Regeneration of RuBP: Most G3P regenerates RuBP to continue the cycle.

7.5 Variations in Photosynthesis

Environmental Factors

• Environmental conditions can alter the Calvin cycle.

  • Temperature.

  • Water availability.

  • Light intensity.

• C3 plants (~90%): First molecule with incorporated CO_2 (3-phosphoglycerate) is a 3-carbon molecule.

Photorespiration

• Rubisco has a higher affinity for CO2 but can add O2 to RuBP when CO2 is low and O2 is high.

• Photorespiration: Rubisco adds O2, releasing CO2.

• Wasteful because carbon loss limits plant growth.

• Hot/dry environments: 25-50% of photosynthetic work can be reversed by photorespiration in C3 plants.

C4 and CAM Plants

• C4 plants: Oxaloacetate (4-carbon molecule) is produced first during carbon fixation.

  • Two-cell layer organization: Mesophyll cells capture CO2 into oxaloacetate (using an enzyme specific to CO2) and transport carbon to bundle-sheath cells for the Calvin cycle.

• CAM plants: Separate processes temporally.

  • Stomata open at night to capture and store CO_2.

  • Stomata close during the day to conserve water; stored CO_2 is released for the Calvin cycle.

C3 vs. C4

• The best one depends on the environment.

• Cooler climates: C3 plants use less energy to fix CO_2.

• C4 and CAM plant adaptations exist to help plants in hot/dry environments conserve water and minimize photorespiration.