Human Physiology: Protein Synthesis and Cellular Metabolism Study Notes

Protein Synthesis

Overview of Protein Synthesis

Protein synthesis is the process by which cells generate new proteins, essential for cellular structure and function. This process involves two main stages: transcription and translation.

• Transcription: The genetic code in DNA is transcribed to form complementary messenger RNA (mRNA) in the nucleus.

• Translation: The mRNA moves from the nucleus to the cytoplasm, where ribosomes translate it to form the correct amino acid sequence of a protein.

• Example: Synthesis of hemoglobin in red blood cells.

Transcription: DNA to mRNA

Transcription is the first step in protein synthesis, where a segment of DNA is used as a template to produce mRNA.

• DNA Structure: DNA consists of two separate strands. One strand contains genes and serves as the template for mRNA synthesis.

• Promoter Sequence: A specific base sequence in DNA where RNA polymerase binds to initiate transcription.

• RNA Polymerase: The enzyme that binds to the promoter, separates DNA strands, and catalyzes the formation of mRNA.

• Post-Transcriptional Processing: Pre-mRNA undergoes modifications before becoming mature mRNA:

  • Introns: Non-coding regions removed from pre-mRNA.

  • Exons: Coding regions joined together to form mature mRNA.

• After Processing: Mature mRNA exits the nucleus and enters the cytoplasm.

• Example: Synthesis of insulin mRNA in pancreatic cells.

Translation: mRNA to Protein

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA.

• Ribosomes: Cellular structures composed of rRNA and proteins, responsible for protein synthesis.

• Initiation: Ribosomal subunits and initiation factors assemble at the start codon (AUG) on mRNA.

• tRNA: Transfer RNA molecules bring specific amino acids to the ribosome, matching mRNA codons via their anticodons.

• Peptidyl Transferase: Enzyme that catalyzes peptide bond formation between amino acids.

• Elongation: Ribosome moves along mRNA, adding amino acids to the growing polypeptide chain.

• Termination: Occurs when a stop codon is reached; the completed polypeptide is released.

• Example: Translation of mRNA coding for actin protein in muscle cells.

Summary Table: Key Steps in Protein Synthesis

Step	Location	Main Molecules Involved	Key Events
Transcription	Nucleus	DNA, RNA polymerase, pre-mRNA	DNA template used to synthesize pre-mRNA
Post-Transcriptional Processing	Nucleus	pre-mRNA, splicing enzymes	Introns removed, exons joined, mature mRNA formed
Translation	Cytoplasm	mRNA, ribosome, tRNA, amino acids	mRNA codons translated into polypeptide chain

Cellular Metabolism

Overview of Metabolism

Metabolism refers to the sum total of all chemical reactions that occur in cells, enabling growth, energy production, and maintenance of cellular functions.

• Anabolic Reactions: Synthesize larger molecules from smaller ones (e.g., proteins from amino acids).

• Catabolic Reactions: Break down larger molecules into smaller ones (e.g., glycogen to glucose).

• Hydrolysis: Water is used to break bonds in molecules (e.g., peptide bond breakdown).

• Condensation: Joining of smaller molecules to form larger ones, releasing water.

• Phosphorylation: Addition of a phosphate group (e.g., ATP synthesis).

• Dephosphorylation: Removal of a phosphate group (e.g., ATP hydrolysis).

• Oxidation-Reduction Reactions: Transfer of electrons between molecules; always coupled.

• Example: Glycolysis and Krebs cycle in cellular respiration.

Energy in Biological Systems

Cells require energy to perform work, which is provided by the conversion of chemical energy in nutrients to ATP.

• Kinetic Energy: Energy of motion (e.g., thermal, electrical).

• Potential Energy: Stored energy (e.g., chemical bonds in glucose).

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• Second Law of Thermodynamics: Natural processes increase the disorder (entropy) of a system.

• Example: Conversion of glucose to ATP during cellular respiration.

Enzymes and Reaction Rates

Enzymes are biological catalysts that increase the rate of chemical reactions by lowering activation energy.

• Enzyme-Substrate Complex: Enzyme binds to substrate to facilitate reaction.

• Specificity: Enzymes are specific to their substrates due to complementary shapes.

• Lock-and-Key Model: Substrate fits exactly into the enzyme's active site.

• Induced Fit Model: Enzyme changes shape to better fit the substrate upon binding.

• Cofactors: Non-protein components required for enzyme activity (e.g., iron, zinc).

• Coenzymes: Organic molecules that assist enzymes (e.g., NAD, FAD, CoA).

• Factors Affecting Enzyme Activity:

  • Enzyme catalytic rate

  • Substrate concentration

  • Enzyme concentration

  • Affinity of enzyme for substrate

• Example: Amylase catalyzing starch breakdown in saliva.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is the primary energy carrier in cells, synthesized from ADP and phosphate via condensation reactions.

• ATP Hydrolysis: Releases energy for cellular work.

• ATP Synthesis: Requires energy input, typically from nutrient breakdown.

• Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Example: Muscle contraction powered by ATP hydrolysis.

Summary Table: Metabolic Pathways

Pathway	Main Function	Location	Key Products
Glycolysis	Breakdown of glucose to pyruvate	Cytosol	2 ATP, 2 NADH, 2 pyruvate
Krebs Cycle	Oxidation of acetyl-CoA	Mitochondrial matrix	2 ATP, 6 NADH, 2 FADH2, 4 CO2
Oxidative Phosphorylation	ATP synthesis via electron transport chain	Inner mitochondrial membrane	~28 ATP, H2O

Additional info: These notes expand on the brief points in the original slides, providing definitions, examples, and context for key processes in protein synthesis and cellular metabolism relevant to General Biology students.