Study Notes on Energy Carriers

Focus: Understanding how energy is harvested and stored in carrier molecules in cells.
Goals:
- Describe energy harvesting in cells.
- Recognize and describe four common energy carrier molecules in living organisms.

Energy Conservation Principle: Energy cannot be created or destroyed but can be transformed from one form to another.
- Example: In plants, energy from sunlight is converted into chemical energy to produce energy carriers.

Plants harness energy from sunlight through photosynthesis.
- Light Harvesting Proteins: Capture sunlight, facilitating conversion into carrier molecules, specifically ATP and NADPH.
- Outcome of Photosynthesis: Energy stored is used to manufacture sugars.
In all organisms, food is an essential energy source.
- Catabolism: Breakdown of food through redox reactions generates energy carriers.
- These carriers transport energy to sites in the cell where it can be utilized for various metabolic processes.

Energy conversion occurs in small, manageable steps to facilitate chemical bond formation and energy recovery.
- Analogy: Burning sugar (marshmallow) in a campfire versus cellular respiration.
- Direct burning releases energy as heat, carbon dioxide, and water without storage.
- Cellular respiration follows a stepwise process avoiding total energy loss, instead capturing energy in carrier molecules.
Activation Energy: Overcome by enzymes functioning at body temperature within the cellular machinery to harvest energy efficiently through these small steps.

Structure of ATP:
- Composed of adenosine (adenine + ribose sugar) and three phosphate groups.
Mechanism of Energy Storage and Release:
- Cleavage of one phosphate group results in ADP (Adenosine Diphosphate) and inorganic phosphate, releasing energy ( $ext{ΔG} < 0$ ).
- Reaction is spontaneous and drives unfavorable processes.
- Energy in ATP is held in phosphoanhydride bonds.
- Repulsion from negative charges of neighboring phosphates increases entropy and stability upon breakdown:
- From ATP to ADP + P, entropy increases from one molecule to two.
- ADP + P are more stable than ATP, allowing energy storage and transfer during cellular processes.
ATP Hydrolysis Examples:
- Used in condensation reactions to activate monomers (e.g., A phosphorylated by ATP).
- Results in the formation of macromolecules by increasing the activation energy, allowing for favorability of reactions.

Function: Carry energy through redox reactions.
- Concept: LEO says GER (Loss of Electrons is Oxidation, Gain of Electrons is Reduction).
- Redox reactions involve oxidation (exergonic, energy released) and reduction (endergonic, energy absorbed) processes.
Reduction and Oxidation:
- Example: Molecule A donates an electron to NADP, forming NADPH.
- Oxidation of A releases energy; subsequent reduction of NADP requires energy but captures it, keeping energy within the cell.
Electron Transfer:
- Movement of electrons occurs as hydrogens in biological systems, which is essential to track energy transfer.
Molecular Structure:
- NAD consists of nicotinamide (hydrogens transferred) and adenine components, existing as a dinucleotide.
- NADPH contains an additional phosphate group.
- Conversions between oxidized (3 double bonds) and reduced forms (2 double bonds) carrying energy during redox processes.

Role: An energy intermediate in metabolic pathways.
Structure: Contains an adenine nucleotide (ADP + extra phosphate) and a long coenzyme section.
Energy Transfer:
- Energy resides predominantly in the acetyl group.
- Participates in metabolic processes, such as lengthening fatty acid hydrocarbon chains or facilitating conversions in the Krebs cycle by adding carbons to oxaloacetate.

Represents primary energy carrier between plants and animals.
Process in Plants:
- Generated through photosynthesis via the reduction of carbon dioxide.
Process in Animals:
- Ingestion: Glucose oxidation provides energy via redox reactions transferring energy to NADH.
Connection to Metabolism:
- Glucose serves as the starting substrate for NADH generation, which is crucial for ATP production in the electron transport chain.

The metabolic sequence transforms energy from sugar to NADH and acetyl-CoA, and ultimately to ATP.
Transport Mechanism:
- Energy carriers (like NADH) move energy from the cytoplasm to mitochondria, where ATP production occurs.
- ATP disperses throughout the cell and organism for various energy needs.

Future exploration will focus on the origins of these energy carriers and their specific mechanisms within cellular energy transactions.