Biochemical Energy Production and Digestion
At a Glance
STAGE 1 - Digestion transforms ingested food into smaller, simpler molecules:
Fats are broken down into fatty acids and glycerol.
This process, known as lipolysis, is crucial for energy storage mobilization.
Lipolysis is stimulated by hormones like epinephrine and glucagon during periods of energy demand.
Fatty acids are transported in the blood bound to albumin for delivery to tissues.
Glycerol can be used for gluconeogenesis in the liver.
Carbohydrates are broken down into glucose and other sugars.
Glycolysis is the primary pathway for glucose metabolism.
Glycolysis occurs in the cytoplasm and involves a series of enzymatic reactions.
Glucose is converted into pyruvate, generating ATP and NADH.
Pyruvate can be further oxidized in the mitochondria or converted to lactate under anaerobic conditions.
Proteins are broken down into amino acids.
Proteolysis is essential for protein turnover and amino acid supply.
Protein turnover is a dynamic process involving continuous protein synthesis and degradation.
Amino acids are used for protein synthesis, energy production, or conversion to other metabolites.
Excess amino acids are deaminated, and the resulting carbon skeletons can enter metabolic pathways.
STAGE 2 - Small molecules from digestion are further degraded into smaller units, primarily acetyl groups, which become part of acetyl CoA.
Process: Acetyl Group Formation (acetyl CoA)
Acetyl CoA serves as a central metabolite linking glycolysis, fatty acid oxidation, and amino acid catabolism.
Pyruvate dehydrogenase complex (PDC) converts pyruvate to acetyl CoA in the mitochondria.
Fatty acids are broken down by beta-oxidation to produce acetyl CoA.
Amino acid catabolism also generates acetyl CoA or other intermediates that can enter the citric acid cycle.
STAGE 3 - Acetyl CoA is oxidized to produce and reduced coenzymes (NADH, FADH₂) in the citric acid cycle.
The citric acid cycle (Krebs cycle) is the final common pathway for the oxidation of fuel molecules.
The citric acid cycle occurs in the mitochondrial matrix.
Acetyl CoA combines with oxaloacetate to form citrate, which undergoes a series of reactions to regenerate oxaloacetate.
Each cycle generates 2 molecules of , 3 molecules of NADH, 1 molecule of FADH₂, and 1 molecule of GTP.
STAGE 4 - NADH and FADH₂ facilitate ATP production through the electron transport chain and oxidative phosphorylation.
The electron transport chain harnesses the energy from NADH and FADH₂ to generate a proton gradient, which drives ATP synthesis.
The electron transport chain is located in the inner mitochondrial membrane.
Electrons from NADH and FADH₂ are transferred through a series of protein complexes, releasing energy to pump protons from the mitochondrial matrix to the intermembrane space.
The resulting proton gradient drives ATP synthesis by ATP synthase.
Oxidative phosphorylation is highly efficient, generating approximately 32 ATP molecules per glucose molecule.
Protein Digestion
Mouth: Saliva has an effect on digestion.
Salivary enzymes initiate the breakdown of certain proteins.
Saliva contains enzymes like amylase, but its effect on protein digestion is minimal.
Mechanical chewing in the mouth helps to break down food into smaller particles.
Stomach:
HCl denatures proteins.
Disrupts the proteins' secondary and tertiary structures, making them more accessible to enzymatic hydrolysis.
Parietal cells in the stomach secrete HCl, which lowers the pH of the stomach to around 2.
The acidic environment denatures proteins, unfolding their polypeptide chains.
Pepsin hydrolyzes peptide bonds.
Pepsin is an aspartic protease that cleaves peptide bonds at specific amino acid residues.
Chief cells in the stomach secrete pepsinogen, the inactive precursor of pepsin.
Pepsinogen is activated by HCl or by pepsin itself through autocatalysis.
Pepsin cleaves peptide bonds preferentially at aromatic amino acid residues.
Small Intestine:
Trypsin, chymotrypsin, carboxypeptidase, and aminopeptidase hydrolyze peptide bonds.
These pancreatic proteases work synergistically to break down proteins into free amino acids and small peptides.
Pancreatic proteases are secreted as inactive zymogens and activated in the small intestine.
Enterokinase, an enzyme produced by the intestinal cells, activates trypsinogen to trypsin.
Trypsin then activates other zymogens, including chymotrypsinogen, procarboxypeptidase, and proelastase.
Active transport occurs in the intestinal lining.
Amino acid transporters mediate the uptake of amino acids from the intestinal lumen into the enterocytes.
Various amino acid transporters with different specificities are located in the apical membrane of enterocytes.
These transporters utilize the sodium gradient to transport amino acids into the cells.
Amino acids are absorbed into the bloodstream.
Amino acids are transported from the enterocytes into the bloodstream for distribution to tissues.
Amino acids exit the enterocytes via facilitated diffusion or active transport mechanisms in the basolateral membrane.
From the bloodstream, amino acids are taken up by various tissues for protein synthesis and other metabolic processes.
Triacylglycerols (TAGs) Digestion
Mouth: Saliva has no effect on digestion.
Salivary amylase primarily acts on carbohydrates, not lipids.
Saliva contains lingual lipase, but its activity is minimal in adults.
Mechanical chewing in the mouth helps to break down food into smaller particles.
Stomach:
Churning action produces small fat droplets (chyme).
Mechanical digestion in the stomach helps to emulsify fats, increasing their surface area for enzymatic action.
The churning action of the stomach muscles mixes the food with gastric secretions, forming chyme.
The emulsification of fats increases their accessibility to lipases.
Gastric lipases hydrolyze some (approximately 10%) of TAGs.
Gastric lip