Biology Lecture 3/4

Introduction to Metabolism

• Metabolism is complex, involving various pathways.

• Focused on ATP production from glucose but recognizes other nutrient sources.

  • Citric Acid Cycle: Central pathway in metabolism.

Energy Production from Nutrients

• Glucose as primary energy source; however, fats (fatty acids) and proteins (amino acids) also contribute.

• Acetyl CoA and Pyruvate can be derived from:

  • Certain amino acids depending on their structure.

  • Direct conversion of some amino acids into Acetyl CoA.

  • Conversion of fatty acids into Acetyl CoA by breaking them down two carbons at a time.

Conversion and Processing of Nutrients

• Different pathways exist for producing Acetyl CoA from various macromolecules (carbs, fats, proteins).

• The citric acid cycle allows for the production of other molecules, not just ATP:

  • Intermediates like alpha-ketoglutarate can be used to synthesize amino acids when energy and nutrients are sufficient.

Aerobic vs. Anaerobic Respiration

• The majority of ATP is produced through aerobic respiration.

• During intense activity (e.g., sprinting), anaerobic processes can temporarily supplement ATP production when oxygen is scarce.

  • Glycolysis: Initial ATP production occurs here, primarily relying on NAD+ for continuation.

Role of NAD+ in Glycolysis

• Essential for glycolysis as it accepts electrons to become NADH.

• High energy demand (e.g., sprinting) can deplete NAD+ reserves, jeopardizing ATP production.

• Conversion of Pyruvate:

  • In the absence of oxygen, pyruvate acts as an electron acceptor, converting NADH back to NAD+, allowing glycolysis to continue.

  • This process produces lactic acid.

Fermentation Types

• Lactic Acid Fermentation: In animals, leads to muscle fatigue due to lactic acid accumulation during high-intensity efforts.

• Ethanol Fermentation: Used by yeast and some bacteria; converts pyruvate to ethanol, producing NAD+ for glycolysis.

  • Interestingly performed by some fish like goldfish under low oxygen conditions.

Energy Transfer Reactions

• Free Energy Diagrams:

  • Useful for visualizing the energy changes during reactions such as:

    • Formation of dipeptides from amino acids (requires energy input).

    • Breakdown of dipeptides to amino acids (often spontaneous).

• Enzymes: Needed to lower activation energy, allowing reactions to proceed more efficiently

  • Non-spontaneous reactions require both enzymes and an energy input (e.g., ATP).

G-Protein Coupled Receptors (GPCRs)

• Complex signaling pathways occur via GPCRs, involving multiple steps and components:

  • Ligand binds to the receptor, activating the G protein (converts GDP to GTP).

  • The active G protein interacts with an early effector (e.g., adenylate cyclase) to produce cyclic AMP (cAMP).

  • cAMP serves as a second messenger, amplifying the signal by activating downstream effectors (e.g., protein kinase A).

• Importance of signal amplification in physiological responses to low concentrations of hormones in the bloodstream.

Summary of Cellular Signaling and ATP Production

• Integral to understanding how energy is harvested, utilized, and how cellular communication occurs in biological systems.

• Key focus is on metabolism, energy transformation, and the role of enzymes and receptors in biological processes.

• Be familiar with concepts of fermentation, energy diagrams, and GPCR signaling for exam prep.