Energy and Cellular Metabolism

Core Concepts of Biological Energy

• Energy Requirements: Living organisms require energy for work, reproduction, and maintaining order against entropy.
• Work Categorization:
  • Chemical work: Making and breaking chemical bonds.
  • Transport work: Moving particles and creating concentration gradients.
  • Mechanical work: Movement of organelles, cilia, flagella, and muscle contraction.
• Forms of Energy:
  • Kinetic energy: Energy of motion.
  • Potential energy: Stored energy found in chemical bonds and concentration gradients.
• Thermodynamics:
  • First Law: Total energy in the universe is constant (conservation of energy).
  • Second Law: Natural processes move toward randomness or disorder ($entropy$).

Chemical Reactions and Enzymes

• Reaction Energetics:
  • Activation Energy: The initial energy required to start a reaction.
  • Exergonic Reactions: Energy-producing reactions where products have lower free energy than reactants.
  • Endergonic Reactions: Energy-utilizing reactions requiring an external energy input.
• Enzymes: Specialized proteins (or RNA) that act as catalysts by lowering activation energy without being consumed.
  • Isozymes: Enzymes that catalyze the same reaction but function under different conditions or in different tissues.
  • Modulators: Activity is influenced by temperature, $\text{pH}$ (optimal human $\text{pH} \approx 7.4$), coenzymes, and feedback inhibition.
• Reaction Types:
  • Oxidation-reduction: Gains ($reduced$) or loses ($oxidized$) electrons.
  • Hydrolysis-dehydration: Addition or removal of water molecules.
  • Addition-subtraction-exchange: Addition, removal, or transfer of functional groups (e.g., $kinases$ add phosphate groups).
  • Ligation: Joining two molecules using energy from $\text{ATP}$.

Metabolism and ATP Production

• Metabolic Pathways:
  • Catabolism: Energy-releasing breakdown of biomolecules.
  • Anabolism: Energy-utilizing synthesis of biomolecules.
• ATP Yield:
  • Aerobic metabolism: Of one glucose molecule yields between $30-32 \text{ ATP}$.
  • Anaerobic metabolism: Yields only $2 \text{ ATP}$ per glucose.
• Key Catabolic Stages:
  • Glycolysis: Converts one $6$-carbon glucose into two $3$-carbon pyruvate molecules; occurs in the cytosol.
  • Citric Acid Cycle: Takes place in the mitochondrial matrix; produces $\text{ATP}$, $\text{CO}_2$, and high-energy electrons stored in $\text{NADH}$ and $\text{FADH}_2$.
  • Electron Transport System (ETS): Uses high-energy electrons and $\text{O}_2$ as the final acceptor to produce the majority of $\text{ATP}$ and $\text{H}_2\text{O}$.

Protein Synthesis

• Transcription: The process of converting $DNA$ into $mRNA$, $tRNA$, or $rRNA$ using $RNA \text{ polymerase}$.
• mRNA Processing: Includes alternative splicing where non-coding $introns$ are removed and coding $exons$ are joined.
• Translation: matches $mRNA$ codons with $tRNA$ anticodons to assemble amino acids into a protein chain at the ribosome.
• Post-translational Modification: Final steps including protein folding, cleavage, addition of side groups (lipids, sugars, phosphates), or assembly into polymeric proteins.

Questions & Discussion

• Q: What is another symptom of Tay-Sachs disease besides loss of muscle control and brain function?
  • A: Damage to light-sensitive cells of the eye due to accumulation of gangliosides can cause vision problems and blindness.
• Q: How could you test whether Sarah and David are carriers of the Tay-Sachs gene?
  • A: By measuring blood levels of $hexosaminidase A$ or conducting genetic screening for specific mutations.
• Q: Why might the genetic test for mutations be more accurate than the enzyme levels test?
  • A: The genetic test is a direct measure of the carrier state, whereas enzyme levels can be influenced by other physiological factors like protein breakdown.
• Q: In what situation might the enzyme test be more accurate than the genetic test?
  • A: If a person has a rare mutation not covered by the specific markers in the standard genetic screening.
• Q: What is the chance of a child being a carrier if one parent is a carrier ($Tt$) and one is not ($TT$)?
  • A: Based on a Punnett square ($Tt \times TT$), there is a $50\%$ chance the child will be a carrier ($Tt$).
• Q: What are the chances for children of two carriers ($Tt \times Tt$)?
  • A: There is a $25\%$ chance the child will have the disease ($tt$) and a $50\%$ chance the child will be a carrier ($Tt$).