Chapter 23: Photosynthesis: Light-dependent Reactions

Chapter 23: Photosynthesis: Light-dependent Reactions

Learning Objectives

  • Identify atoms that lose electrons (oxidation) and atoms that gain electrons (reduction) in photosynthesis.
  • Explain the impacts of oxidation and reduction on energy availability.
  • Model the transformation of light energy into chemical energy in C3 photosynthesis, including:
    • Chloroplast
    • Thylakoid
    • Stroma
    • Pigments
    • PSII
    • PSI
    • H2OH_2O
    • O2O_2
    • Electrons
    • Sunlight
    • H+H^+
    • ATP synthase
    • ADP/ATP
    • NADP+/NADPH
    • Reduction
    • Oxidation
  • Describe the main cellular structures and their functions required for C3 photosynthesis:
    • Chloroplast
    • Thylakoid
    • Granum
    • Stroma
    • Pigments

Introduction

  • Metabolic processes in organisms require energy.
  • This energy originates from photosynthesis.
  • Photosynthesis captures sunlight energy and converts it into chemical compounds (carbohydrates).
  • It also produces oxygen.
  • Light energy is converted into chemical energy during photosynthesis.

Energy in Living Systems

  • Energy production involves coordinated chemical pathways.
  • These pathways involve oxidation and reduction reactions (redox reactions).
  • Oxidation: Loss of electrons from an atom.
  • Reduction: Gain of electrons by an atom.
  • Oxidation and reduction occur simultaneously.
  • Figure 23.1: Stages of oxidation/reduction of a single carbon.
Electrons and Energy
  • Removing electrons (oxidation) decreases potential energy.
  • The electron is transferred to another compound, reducing it.
  • The oxidized compound loses potential energy, and the reduced compound gains potential energy.
  • High-energy electrons store energy used to fuel cell functions.
  • Transferring energy via high-energy electrons allows cells to use energy in small increments.
  • The module focuses on light-dependent reactions of photosynthesis.
Electron Carriers
  • Small compounds act as electron shuttles, carrying high-energy electrons.
  • Principal electron carriers are derived from B vitamins and nucleotides.
  • These compounds are easily reduced (accept electrons) or oxidized (lose electrons).
  • Nicotinamide adenine dinucleotide phosphate (NADP+) is derived from vitamin B3 (niacin).
    • NADP+ is the oxidized form.
    • NADPH is the reduced form (after accepting two electrons and a proton).
    • If a compound has an “H”, it is generally reduced.
    • Reduction: Addition of electrons.
    • Reducing agent: A compound that reduces another.
    • Oxidation: Removal of electrons.
    • Oxidizing agent: A compound that oxidizes another.
    • NADP+ plays a role in photosynthesis.
  • Flavin adenine dinucleotide (FAD) is derived from vitamin B2 (riboflavin).
    • The reduced form is FADH2.
  • NAD+ also acts as an oxidizing agent, forming NADH during carbohydrate catabolism in cellular respiration.
  • Figure 23.2: The oxidized form of the electron carrier (NAD+) and the reduced form (NADH).

Overview of Photosynthesis

  • Sunlight energy is captured to energize electrons, which is then stored in sugar molecules.
  • Energy stored by photosynthesis millions of years ago is extracted by burning coal and petroleum products today.
  • Plants, green algae, and cyanobacteria perform photosynthesis (Figure 23.3).
  • Photosynthesis stores energy from solar radiation into carbon-carbon bonds of carbohydrate molecules.
  • Carbohydrates power ATP synthesis via cellular respiration.
  • Photosynthesis powers 99% of Earth’s ecosystems.
  • The energy path goes from nuclear reactions on the sun to visible light to photosynthesis to vegetation to herbivores to carnivores.
  • Photosynthesis requires specific wavelengths of visible sunlight, carbon dioxide, and water (Figure 23.4).
  • It releases oxygen and produces glyceraldehyde-3-phosphate (G3P), which can be converted into glucose, sucrose, or other sugars.
  • Figure 23.5: The basic equation for photosynthesis.
  • Photosynthesis occurs in two stages: light-dependent and light-independent reactions (Figure 23.6).
  • Light-dependent reactions: Sunlight energy is absorbed by chlorophyll and converted into chemical energy.
  • Light-independent reactions: Chemical energy drives the assembly of sugar molecules from carbon dioxide.
Photosynthesis at the Grocery Store
  • Grocery store items link back to photosynthesis.
  • Meats and dairy: Animals eat plant-based foods.
  • Breads, cereals, and pastas: From starchy grains of photosynthesis-dependent plants.
  • Desserts and drinks: Contain sucrose, a plant product built from photosynthesis.
  • Paper goods and many plastics are derived from plants.
  • Spices and flavorings were produced by plants.
  • Photosynthesis connects to every meal.
Photosynthesis in Plants Takes Place in Chloroplasts
  • In plants, photosynthesis occurs in leaves, specifically in the mesophyll layer.
  • Gas exchange of carbon dioxide and oxygen occurs through stomata (Figure 23.7).
  • Stomata are typically on the underside of the leaf.
  • Guard cells regulate the opening and closing of stomata.
  • Photosynthesis takes place inside chloroplasts in plant cells (Figure 23.8).
  • Chloroplasts have a double membrane and are derived from cyanobacteria.
  • Thylakoids are stacked, disc-shaped structures within the chloroplast.
  • Chlorophyll, a pigment, is embedded in the thylakoid membrane.
  • The thylakoid membrane encloses the thylakoid lumen.
  • A stack of thylakoids is called a granum.
  • The liquid-filled space surrounding the granum is called stroma.
Research Connection: Marie Clark Taylor
  • Understanding light and plant development allows gardeners to select plants for conditions.
  • Dr. Marie Clark Taylor was the first woman to earn a scientific doctorate from Fordham University and the first African-American woman to earn a doctorate in botany (Figure 23.9).
  • Her dissertation examined the effect of light photoperiods on the development of cells that give rise to flowers.
  • Different plants (scarlet sage, cosmos, and orange cosmos) were exposed to different periods of daily light (6, 10, or 16 hours), and their seed and flower production was recorded.
  • Plants have adapted to thrive under particular conditions.
  • Plants in the shade have adapted to low levels of light by changing the relative concentrations of their chlorophyll pigments.
The light-dependent reactions of photosynthesis
  • The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP.
  • This chemical energy supports light-independent reactions and fuels the assembly of sugar molecules.
  • Protein complexes and pigment molecules work together to produce NADPH and ATP.
Absorption of Light Energy
  • Light energy of specific wavelengths is absorbed by pigments.
  • Chlorophylls and carotenoids are the two major classes of photosynthetic pigments.
  • Chlorophyll a and chlorophyll b are found in plants.
  • Carotenoids function as photosynthetic pigments that dispose of excess energy.
  • Carotenoids absorb excess energy and dissipate it as heat.
  • Many photosynthetic organisms have a mixture of pigments for a wider range of wavelengths.
  • When a photon of light hits a pigment molecule, it is absorbed.
  • The energy from the photon excites an electron, moving it to a higher energy orbital (Figure 23.10).