7.6 Chapter 7 Summary

Energy Forms: Potential, Kinetic, and Chemical

• Energy is defined as the ability to do work.
• Kinetic Energy is the energy associated with motion (e.g., moving water).
• Potential Energy is stored energy due to an object’s position or structure (e.g., water behind a dam, or energy in chemical bonds).
• Chemical Energy is the potential energy stored within the chemical bonds of molecules. This energy is released when the bonds are broken, a process vital for fueling cellular work.
• Adenosine Triphosphate (ATP) is the cell’s primary energy currency. Hydrolyzing ATP releases a quick burst of free energy which can be harnessed to power cellular processes, an event known as energy coupling.
• Redox Reactions (Oxidation-Reduction) are chemical reactions essential for energy transfer; Oxidation Is Loss of electrons, and Reduction Is Gain of electrons (OIL RIG).

Photosynthesis

Photosynthesis is the process by which plants convert light energy (solar radiation) into chemical energy stored in carbohydrate molecules. This process supplies energy to nearly all of Earth’s ecosystems.

• Overall Chemical Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆

• There are two stages of photosynthesis:
  1. Light-Dependent Reactions: Convert light energy into the temporary chemical energy carriers ATP and NADPH, which are both produced in the mitochondrial matrix. The enzymes that drive these reactions are located in the thylakoid membranes inside the chloroplast.
  2. Light-Independent Reactions (Calvin Cycle): Use the energy from ATP and NADPH to assemble G3P from carbon dioxide. The enzymes that drive the Calvin Cycle are located in the chloroplast stroma.

Light-Dependent Reactions

The light-dependent reactions convert solar energy into chemical energy (ATP and NADPH) and produce oxygen gas.

• Process:
  1. Light Absorption: Pigments like chlorophyll absorb specific wavelengths of visible light, exciting electrons in a reaction center within Photosystem II (PSII) and Photosystem I (PSI) in the thylakoid membrane.
  2. Water Splitting and Release: PSII’s lost electron is replaced by splitting a water molecule (H₂O), which yields electrons, H⁺ ions (protons), and oxygen gas (O₂).
  3. ATP Synthesis: Electrons move through an electron transport chain, and their energy is used to pump ions into the thylakoid lumen, creating a high concentration gradient. The flow of these protons back out through ATP synthase drives the formation of ATP from ADP and inorganic phosphate (P_i).
  4. NADPH Production: Electrons are re-energized by PSI and passed to NADP reductase, which reduces NADP to form the high-energy electron carrier NADPH.
• Products: ATP, NADPH, and Oxygen (O₂).

Photosynthesis: Calvin Cycle

The Calvin cycle (light-independent reactions) uses the chemical energy from the light reactions to fix atmospheric carbon dioxide into a stable organic form. This cycle occurs in the stroma.

• Stage 1: Fixation: The enzyme RuBisCO catalyzes the reaction of a CO₂ molecule with a five-carbon molecule, RuBP, resulting in two molecules of the three-carbon compound 3-PGA. This is called carbon fixation.
• Stage 2: Reduction: ATP and NADPH supply the energy and electrons needed to convert 3-PGA into Glyceraldehyde 3-phosphate (G3P). The “empty” carriers ( ADP and NADP) return to the light-dependent reactions.
• Stage 3: Regeneration: One G3P molecule exits the cycle to be used by the plant. The remaining G3P molecules are rearranged, consuming more ATP, to regenerate the starting molecule, RuBP, allowing the cycle to continue.

Products of Photosynthesis

The immediate, net output of the Calvin cycle is the three-carbon sugar Glyceraldehyde-3-phosphate (G3P).

• G3P Fate: The plant uses G3P as a building block to synthesize larger, more stable carbohydrate molecules for energy storage and structural needs.
• Final Products: Two molecules of G3P are combined to form a six-carbon sugar like glucose (C₆H₁₂O₆), which can then be converted into:
  • Sucrose for transport throughout the plant.
  • Starch for long-term storage.
  • Other complex organic molecules.