Chapter 24: Metabolism

• Overview of metabolic processes including the citric acid cycle and electron transport system.
• Discussion of the nutrient pool and its role in energy production.
• Summarization of carbohydrate metabolism.
• Examination of lipid transport and distribution.
• Overview of protein metabolism and the utilization of proteins as an energy source.

Cellular Respiration

• Definition: The process of utilizing nutrients to produce ATP (energy) while releasing waste products.

Key Components:

• Starting Point: Chemical reactions in the cytoplasm transform organic nutrients into fragments that are capable of being processed by mitochondria.
• Oxidative Phosphorylation:
  • Coenzymes deliver hydrogen atoms containing high-energy electrons from the citric acid cycle to the electron transport system (ETS).
  • The ETS is responsible for generating ATP, consuming oxygen, and producing water.

Mitochondrial Function:

• The primary substrate for mitochondria is a 2-carbon molecule known as acetate, which attaches to coenzyme A to form acetyl-CoA.
  • Once formed, Coenzyme A is released through a reaction leading to the production of citric acid, signifying the commencement of the citric acid cycle (also known as Krebs cycle).

Citric Acid Cycle:

• The cycle begins with the formation of citric acid (a 6-carbon molecule) from acetyl-CoA.
• Function:
  • The citric acid cycle extracts hydrogen atoms from organic molecules and transfers them to coenzymes, facilitating the uptake and processing of energy.
  • Energy Output:
  • The cycle produces one ATP molecule for each cycle completion.

Electron Transport System (ETS):

• Composed of 4-carbon molecules which facilitate further ATP production:
  • Inputs include ADP and inorganic phosphate (P) along with oxygen (O₂).
  • Outputs include ATP, carbon dioxide (CO₂), and water (H₂O).

Nutrient Pool

• Comprised of nutrients that provide materials for:
  • Anabolism: Building new structures.
  • Catabolism: Breaking down existing structures for energy or recycling purposes.

Carbohydrate Metabolism

• Carbohydrates serve as the preferred source for ATP production.
  • They can be converted into various other structural components.
  • Major storage form includes glycogen, predominantly stored in the liver and muscles.

Lipid Metabolism

Lipids (Fats):

• Function: Concentrated energy source; contains twice the energy compared to carbohydrates and proteins.
• Utilization: Fats can either be used to produce ATP or stored by fat cells for future energy needs.
  • Cholesterol derived from chylomicrons is processed by the liver.

Lipid & Cholesterol Pathway:

• Chylomicron Absorption:

  • Chylomicrons enter the bloodstream via the left subclavian vein, circulating through the pulmonary circuit before entering the systemic circulation.
  • Capillaries contain lipases that break down chylomicrons, releasing fatty acids and monoglycerides, which diffuse into the interstitial fluid.

• Fat Utilization in Skeletal Muscles:

  • Resting skeletal muscles absorb fatty acids for decomposition, using the derived ATP for muscle contractions and converting glucose to glycogen.

Fat Cell Function:

• Fat cells absorb monoglycerides and fatty acids to synthesize triglycerides, storing them for future energy use.

Lipoproteins and Lipid Distribution:

• The liver absorbs chylomicrons to remove triglycerides, combines cholesterol from chylomicrons with synthesized or recycled cholesterol, and alters surface proteins.
• The liver releases low-density lipoproteins (LDLs) into circulation for transporting cholesterol to peripheral tissues, where excess cholesterol can be excreted in bile.
• HDL (High-Density Lipoproteins) function includes returning cholesterol to the liver for packaging into new LDLs or excretion with bile salts.
  • Excess cholesterol diffuses out from cells and into the bloodstream, where it is collected by HDLs.
  • Once in peripheral tissues, LDLs are absorbed, and cholesterol is extracted for membrane synthesis, hormone production, and more.

Protein Metabolism

• Proteins are broken down into amino acids for absorption.
  • The majority of amino acids go toward protein synthesis.
  • Excess protein can be converted into energy or stored as fat for future use.