Human Physiology - Energy Production and Cell Function

Energy Currency of Cells

• ATP (Adenosine Triphosphate)

  • The primary energy currency of the cell.
  • When ATP is split, it forms:
  • ADP (Adenosine Diphosphate)
  • Pi (Inorganic Phosphate)
  • Energy (usable for various cellular activities such as muscle contraction, active transport, etc.)

• Equation:
  ATP
  ightarrow ADP + Pi + ext{Energy}

Pathways of ATP Production

1. Substrate-Level Phosphorylation

• Direct transfer of a phosphate group to ADP to form ATP.
• Occurs in:
  • Glycolysis
  • TCA cycle (Tricarboxylic Acid Cycle)
• Characteristics:
  • Does not require oxygen (anaerobic process).

2. Anaerobic Glycolysis

• Breakdown of glucose into pyruvate without oxygen.
• Produces a small amount of ATP via substrate-level phosphorylation.
• If oxygen is present, pyruvate can enter the mitochondria for further processing.

3. Pyruvate Decarboxylation

• Process:
  • Pyruvate is converted into Acetyl-CoA before entering the TCA cycle.
• Products:
  • Produces NADH (energy carrier) but does not produce ATP directly.

Cell Structure and Human Physiology

Overview

• Larger species have more cells, but individual cell size remains consistent across species.
• Approximately 200 cell types exist in the human body.
• Despite diversity, many cells share common features.

Major Subdivisions of a Cell

1. Plasma Membrane
   • The outer layer controlling substance entry and exit.
2. Cytoplasm
   • Jelly-like fluid where organelles reside.
3. Nucleus
   • Control center housing the cell's DNA.

Intermediary Metabolism

• Refers to the collection of all chemical reactions within the cell.
• Definition:
  • The set of biochemical reactions that convert nutrients into energy and building blocks for growth, repair, and normal function.

Two Main Components

1. Catabolism
   • Breakdown of molecules (e.g., sugars, fats) to release energy.
2. Anabolism
   • Energy-using processes that build up complex molecules (e.g., proteins, DNA).

Key Concepts

• Anabolic Processes: Favor the construction of molecules necessary for the development of organs and tissues.
• Catabolic Processes: Favor the reduction of complex molecules into simpler units.

Energy Production Processes

4. Tricarboxylic Acid Cycle (TCA Cycle or Krebs Cycle)

• Acetyl-CoA undergoes further breakdown.
• Produces ATP primarily through substrate-level phosphorylation.
• Generates high-energy electrons via NADH and FADH2 for the next stage (oxidative phosphorylation).

Additional Processes

1. Creatine Phosphate
   • First energy reserve utilized at the initiation of muscle contraction.
2. Glycolysis
   • An anaerobic process occurring in the cytosol. Glucose is converted into two pyruvate molecules across 10 reactions, resulting in:
     • A net yield of 2 ATP per glucose, indicating lower energy extraction efficiency.
     • Conditions: Dysfunctional glycolysis may occur due to diseases (e.g., McArdle disease, caused by enzyme deficiencies affecting energy production in muscles).
3. Pyruvate Decarboxylation
   • Occurs in the mitochondrial matrix: Pyruvate is processed by the pyruvate dehydrogenase complex, removing a carbon atom (decarboxylation).
   • Produces
     • Carbon dioxide (CO2), which exits the body via respiration.
     • Hydrogen transferred to NAD+ forms NADH as an energy carrier for later respiration stages.
4. Tricarboxylic Acid Cycle
   • Occurs post-conversion of pyruvate to Acetyl-CoA in the mitochondrial matrix, using aerobic metabolism to generate ATP.
   • Produces significant energy carriers: NADH and FADH2 for further ATP production.

Electron Transport Chain

• Process: Electrons from NADH and FADH2 are transferred through proteins in the inner mitochondrial membrane.
• As electrons progress, energy is utilized to:
  • Pump hydrogen ions (H+) into the intermembrane space, establishing a concentration gradient.
  • This gradient drives ATP synthase, converting ADP and Pi into ATP.
• This reaction, termed oxidative phosphorylation, is fundamentally responsible for producing most cellular ATP.
• Final Products:
  • Electrons combine with oxygen (O2) and hydrogen ions, generating water as a byproduct.

ATP Yield Per Glucose

• The maximum potential ATP yield from one glucose molecule is 38 ATP.
• Actual yield may vary due to different metabolic conditions.

Aerobic vs Anaerobic Conditions

• Anaerobic: No oxygen available.
  • Glycolysis is the terminal pathway; pyruvate is converted into lactic acid.
• Aerobic: Utilizes oxygen, with significantly higher energy yield compared to anaerobic pathways.

Plasma Membrane

• Also referred to as the cell membrane, encompasses each cell and regulates internal and external exchanges, marking the boundary between cell contents and environment.

Types of Equilibrium Across Plasma Membrane

1. Osmotic Equilibrium

   • Concerned with water movement across a semi-permeable membrane.
   • Occurs when water and solute concentrations equalize, leading to no net movement.
   • Example: Red blood cell in an isotonic solution maintains size due to equal water movement.
   • Key Concept: Osmotic equilibrium is fundamentally about water balance.

2. Chemical Equilibrium

   • Pertains to solutes (not water).
   • Achieved when substance concentrations are equal on both membrane sides with continuous molecular movement but no net concentration change.
   • Example: Oxygen diffuses into cells until internal concentration equals external.
   • Key Concept: Chemical equilibrium relates to solute balance.

3. Electrical Equilibrium

   • Relates to the distribution of electrical charges (ions) across a membrane.
   • Achieved when positive and negative ion concentrations are balanced, resulting in no net ionic movement.
   • Example: Equal positive and negative charges on both sides result in balanced electrical potential.
   • Key Concept: Electrical equilibrium concerns ion charge balance.