Metabolic Processes and Nutrient Utilization

Carbohydrates

  • Uses:

    • Energy source

    • Parts of molecules

    • Receptors and marker molecules

Lipids

  • Uses:

    • Energy source

    • Cell components

    • Hormones

    • Bile salts

    • Chemical signals

Proteins

  • Uses:

    • Structural components

    • Metabolic catalysts

    • Transport molecules

    • Buffers

    • Blood clotting factors

    • Chemical signals

    • Energy source (not digested before absorbed)

Vitamins

  • Uses:

    • Coenzymes

    • Chemical signals

Minerals

  • Uses:

    • Influence membrane potential

    • Structural components

    • Coenzymes

    • Buffers

Water

  • Role in Metabolism:

    • Essential for metabolic processes

Metabolic Reactions

  • Definition:

    • Metabolism refers to all the chemical reactions in the body utilizing macromolecules such as proteins, fats, carbohydrates, and nucleic acids.

Types of Metabolic Reactions

  • Catabolism:

    • Includes all chemical reactions that break down complex organic molecules (e.g., carbohydrates, proteins, nucleic acids, lipids).

  • Anabolism:

    • Refers to chemical reactions that combine simple molecules to form complex molecules (e.g., amino acids, monosaccharides, fatty acids).

Mechanisms of ATP Generation

  • Phosphorylation:

    • The process of changing ADP into ATP which contains energy.

  • De-phosphorylation:

    • The process of changing ATP into ADP that has lost energy.

Carbohydrate Metabolism

  • During digestion, polysaccharides and disaccharides are converted to monosaccharides (primarily glucose).

  • Liver cells convert much of the remaining fructose and practically all of the galactose to glucose.

  • Carbohydrate metabolism is primarily concerned with glucose metabolism.

Carbohydrate Processing Overview

  • In the GI tract:

    • Polysaccharides are broken down into simple sugars.

    • Absorption of simple sugars (glucose, fructose, and galactose).

  • In the liver:

    • Fructose and galactose are transformed into glucose.

    • Storage of glycogen (also in muscle).

  • In body cells:

    • Functions of glucose:

    • Oxidized to produce energy.

Glycolysis Overview

  • Steps in Glycolysis:

    1. Glucose (6 carbon molecule) undergoes investment stage:

    • ATP is invested: ATP is hydrolyzed to ADP + inorganic phosphate (Pi).

    • The phosphate group attaches to glucose, forming glucose 6-phosphate.

    • Another ATP is invested: ATP is hydrolyzed to ADP + Pi.

    • This results in fructose 1,6-bisphosphate.

    1. Harvesting stage:

    • Fructose 1,6-bisphosphate is split into two 3-carbon molecules:

      • (A) Dihydroxyacetone, which will be converted into glyceraldehyde 3-phosphate (G3P).

      • (B) Glyceraldehyde 3-phosphate (G3P).

  • Each glyceraldehyde 3-phosphate will undergo further processing.

Glycolysis Continued

  • Glyceraldehyde 3-phosphate (G3P) continues harvesting:

    • NAD+ is reduced to NADH while adding a phosphate group to G3P.

    • Harvesting involves converting ADP + a phosphate taken from G3P to produce ATP.

    • The end product is 2 pyruvate (3-carbon molecule).

    • Overall glycolysis yields:

    • 2 NADH

    • 2 ATP

    • 2 pyruvate

  • Pyruvate will enter Krebs cycle; NADH will be utilized in the ETC stage.

Mitochondria and ATP Production

  • Energy Generation in Mitochondria:

    • The matrix within the inner mitochondrial membrane houses the Krebs cycle reactions.

  • Reactions of Krebs cycle and ETC require oxygen.

  • Aerobic respiration yields significantly more ATP than glycolysis.

Krebs Cycle Overview

  • Pyruvate oxidation:

    • Converts pyruvate (3-carbon molecule) to Acetyl-CoA (2-carbon molecule).

    • Reaction: Acetyl-CoA (2-carbon) + oxaloacetate (4-carbon) = citrate (6-carbon).

Steps in Krebs Cycle

  1. Pyruvate Oxidation:

    • Converts pyruvate to Acetyl-CoA.

    • Acetyl-CoA + Oxaloacetate = Citrate.

  2. From Citrate to Isocitrate:

    • Isocitrate is oxidized, releasing a carbon as CO2.

    • NAD+ is reduced to NADH, forming alpha-ketoglutarate (5-carbon molecule).

  3. From alpha-Ketoglutarate:

    • Oxidized, releasing CO2.

    • NAD+ is reduced to NADH, forming Succinyl-CoA (4-carbon).

  4. From Succinyl-CoA to Succinate:

    • Converted into succinate (4-carbon) while ADP is hydrolyzed to ATP.

  5. Succinate to Fumarate:

    • Succinate oxidized to fumarate (4-carbon), reducing FAD to FADH2.

  6. Fumarate to Malate:

    • Water adds to fumarate to produce malate (4-carbon).

  7. Malate to Oxaloacetate:

    • Malate oxidized to oxaloacetate, reducing NAD+ to NADH.

Krebs Cycle Output

  • The Krebs cycle occurs twice per glucose molecule:

    • 1st Round: 3 NADH, 1 ATP, 1 FADH2.

    • 2nd Round: 3 NADH, 1 ATP, 1 FADH2.

    • Total after two rounds: 6 NADH, 2 ATP, 2 FADH2.

Electron Transport Chain (ETC)

  • Definition:

    • Oxidative phosphorylation involves the transfer of electrons from electron acceptors (NADH and FADH2).

  • Electrons travel from a higher to lower energy state, driving protons across the membrane, creating a proton gradient.

Complexes of the ETC

  • Overview of Complexes I-IV:

    • Complex I: Involves the oxidation of NADH, pumping H+ from the matrix into the intermembrane space.

    • Complex II: Involves the oxidation of FADH2; both contribute electrons to coenzyme Q (CoQ).

  • Electrons pass through CoQ to Complex III.

    • Complex III pumps protons into the intermembrane space, creating a positive charge.

  • Electrons from Complex III transfer to Cytochrome C, then to Complex IV, where further protons are pumped into the intermembrane space.

  • Oxygen as final electron acceptor creates two water molecules.

Chemiosmosis

  • ATP Synthase Function:

    • Uses protons from the intermembrane space to convert ADP into ATP.

Lipid Metabolism

  • Definition:

    • Lipids include fats such as triglycerides (both unsaturated and saturated).

  • Breakdown Process:

    • Lipids undergo beta-oxidation to be synthesized/oxidized.

  • Triglycerides convert into glycerol and free fatty acids that enter mitochondria as Acetyl-CoA.

Steps of Beta-Oxidation

  1. Transport:

    • Fatty acids transported from adipose tissue to target cells.

  2. Entry into Cells:

    • Fatty acids converted into Acetyl-CoA in the cytoplasm and enter mitochondria.

  3. Oxidative Catabolism:

    • Acetyl-CoA enters Krebs cycle in the mitochondrial matrix generating NADH and FADH2 for the ETC.

Protein Metabolism

  • Definition:

    • Proteins consist of amino acids essential for bodily functions (contractile proteins, antibodies, hormones, enzymes).

  • Polypeptides:

    • A long chain of amino acids (polypeptide chain) is formed during protein synthesis.

  • Metabolism of Excess Amino Acids:

    • Excess amino acids are not stored but are converted to glucose or fatty acids for energy.

Amino Acids

  • Essential Amino Acids:

    • Of the 20 amino acids, 10 are essential (cannot be synthesized by the body and must be included in the diet).

  • Protein Metabolism Processes:

    • Transamination: Transferring of the amine group from one amino acid to another.

    • Deamination: Conversion of glutamate to alpha-ketoglutarate, releasing toxic ammonia.

    • Urea Cycle: Conversion of ammonia to urea for excretion.

Lipogenesis, Glycogenesis, and Glycogenolysis

  • Glycogenesis: Storage of glucose as glycogen.

  • Glycogenolysis: Release of glucose by converting glycogen back into glucose.

  • Lipogenesis: Synthesis of fatty acids from excess carbohydrates consumed.

Gluconeogenesis

  • Definition:

    • Conversion of proteins or fats into glucose when it is needed as energy by nervous tissue and red blood cells.

  • Pathways in Gluconeogenesis:

    • Glycolysis and Krebs cycle intermediary reactions lead to glucose production.

Metabolic States

  • Absorptive State:

    • Occurs immediately after eating; nutrients are absorbed into the bloodstream.

    • Liver processes nutrients into energy-storage molecules like glycogen and triglycerides.

  • Postabsorptive State:

    • Occurs when nutrients are not being absorbed, maintaining blood glucose levels; fatty acids and lactic acid are utilized for energy.

Body Temperature Homeostasis

  • Definition of Core Temperature:

    • The body’s temperature that should be maintained for proper function (around 37°C / 98.6°F).

  • Shell Temperature:

    • The temperature at the skin surface tends to be lower.

  • Implications of Temperature Changes:

    • High temperatures can denature proteins; low temperatures can cause cardiac arrhythmias.

Energy Homeostasis and Regulation of Food Intake

  • Hypothalamus Function:

    • Manages food intake through feeding and satiety centers; leptin, the satiety hormone, regulates energy expenditure and hunger response.

Mineral Functions

  • Key Minerals:

    • Calcium and phosphorus form bone matrix and are involved in enzymatic reactions.

    • Magnesium serves as a catalyst in ATP production and regulates water osmosis.

Vitamin Absorption and Function

  • Fat-soluble Vitamins:

    • Absorbed with dietary fats and stored in liver (A, D, E, K).

  • Water-soluble Vitamins:

    • Absorbed with water and are not well-stored in the body (B vitamins, C).

    • Excess is typically excreted in urine.