Light - Dependent

Light-Dependent Reactions

Two Types of Energy Reactions

Endergonic Reactions

• Definition: Chemical reactions that require a net input of energy.

• Characteristics:

  • Absorb free energy and store it.

  • Example: Photosynthesis represented by the equation: 6CO2 + 6H2O
    to C6H{12}O6 + 6O2

  • Sunlight provides photons (light energy) to facilitate this process.

Exergonic Reactions

• Definition: Chemical reactions that release energy.

• Characteristics:

  • Example: Cellular respiration represented by the equation: C6H{12}O6 + 6O2
    ightarrow 6CO2 + 6H2O + ATP

  • The process involves energy released from glucose.

Metabolic Reactions of Cells

What is Metabolism?

• Definition: The sum total of the chemical activities of all cells.

• Functions:

  • Managing the material and energy resources of the cell.

Two Types of Metabolism

• Catabolic Pathways:

  • Involve breaking down molecules to release energy (exergonic).

  • Example: Cellular respiration as noted above.

  • Hydrolysis is the process where a water molecule is added to break chemical bonds, resulting in simpler compounds.

• Anabolic Pathways:

  • Involve building up complex molecules from simpler ones, typically consuming energy (endergonic).

Adenosine Triphosphate (ATP)

• Structure: Consists of three phosphate groups.

• Function: A key molecule that stores and provides energy for cellular processes.

Objectives of Light-Dependent Reactions

• Discuss the importance of chlorophyll and other pigments.

• Discuss the pathway of electrons in light-dependent reactions.

• Recognize the value of photosynthesis in the perpetuation of life.

Key Terms and Acronyms

• ACRONB IODIXDE: An abbreviation related to carbon dioxide.

• SPLITTING OF WATER: A crucial step in photosynthesis.

• ANDEOISNE PSOHPAHTIRT: Misspelled reference to Adenosine phosphate.

Structure of the Chloroplast

• Outer Membrane: Encases the chloroplast.

• Inner Membrane: Contains thylakoids and stroma.

• Granum: A stack of thylakoids, site for light reactions.

• Thylakoid: Membranous structures where light-dependent reactions occur.

• Stroma: Aqueous space surrounding thylakoids, where the Calvin cycle occurs.

Light Reactions: In the Thylakoids

• Photosystems:

  • Light harvesting complexes embedded in thylakoid membrane.

  • Composed of clusters of photopigments including chlorophyll a, chlorophyll b, and carotenoids.

• Photosystem I (P700): One type of photosystem.

• Photosystem II (P680): Another type of photosystem.

Electron Transport Chain (ETC)

• Components: Includes various molecules and enzymes such as:

  • Ferredoxin-NADP reductase: Enzyme involved in transferring electrons.

  • cytochrome: A class of enzymes important in the electron transport chain.

  • ATP synthase: Enzyme that synthesizes ATP from ADP and inorganic phosphate.

  • Plastoquinone: A molecule involved in transferring electrons.

  • Plastocyanin: A copper-containing protein involved in electron transport.

• How It Works:

  • Electrons are passed along from one molecule to the next.

  • Energy released during these transfers contributes to ATP formation.

  • Light energy is essential for replenishing electrons by splitting water through the oxygen-evolving complex.

Overview of Light Reactions

• Inputs:

  • Light energy

  • Water ($H_2O$)

• Outputs:

  • ATP

  • NADPH

  • Oxygen ($O_2$)

• Process:

  • Traps energy from sunlight to form ATP and high-energy electrons.

  • Splitting of water replenishes electrons and H+ ions.

  • Release of oxygen as a byproduct.

Cyclic and Non-Cyclic Pathways

• Cyclic Pathway: Utilizes photosystem I (P700) to produce ATP without generating NADPH.

• Non-Cyclic Pathway: Involves both photosystem II (P680) and photosystem I (P700) resulting in ATP and NADPH production.

  • This pathway also includes the splitting of water and the release of oxygen.