10.2 Metabolism

Metabolism

• Refers to all chemical reactions in an organism
  • Anabolic is building up
  • Catabolic is breaking down
  • Both processes happen in cells
• Cells must spend ATP to:
  • perform routine maintenance
  • remove and replace structures and components
• Cells spend additional energy for vital functions:
  • growth
  • secretion
  • contraction

Energy

• Cells break down molecules to obtain energy:
  • used to generate ATP
• Most ATP production takes place in the mitochondria

Materials Transport

• The cardiovascular system carries materials through the body
• Materials diffuse from the bloodstream into cells

Organic Compounds

• Glycogen: the most abundant storage carbohydrate in the body; a branched chain of glucose molecules
• Triglycerides: the most abundant storage lipids; primarily composed of fatty acids
• Proteins: the most abundant organic components in the body; perform many vital cellular functions

Nutrient Usage

• Carbohydrate Metabolism: Generates ATP and other high-energy compounds by breaking down carbohydrates:
  • Glucose + Oxygen → Carbon Dioxide + Water
  • C6H12O6 + 6O2 → 6CO2 + 6H2O
• The process happens in several parts:
  • Glycolysis
  • Citric acid cycle
  • Oxidative phosphorylation

Glucose Breakdown

• Occurs in small steps which release energy to convert ADP to ATP
• Breaking down 1 molecule of glucose nets 36 molecules of ATP
• For 1 glucose molecule processed, the cell gains 36 molecules of ATP:
  • 2 from glycolysis
  • 4 from NADH generated in glycolysis
  • 2 from the Citric acid cycle (through GTP)
  • 28 from Oxidative Phosphorylation

Glycolysis

• The first stage of carbohydrate metabolism, where glucose is broken down into pyruvate
• Occurs in the cytosol
• Does not require oxygen (anaerobic)

Citric Acid Cycle (Krebs Cycle)

• The second stage of carbohydrate metabolism, where pyruvate is broken down into carbon dioxide, water, and ATP
• Occurs in the mitochondria
• Requires oxygen (aerobic)

Oxidative Phosphorylation

• The final stage of carbohydrate metabolism
• Takes place in the mitochondria
• Involves the electron transport chain and chemiosmosis
• Produces the majority of ATP from glucose breakdown

Anabolism

• Reversing the direction of catabolism is not directly possible due to enzymatic processes
• Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precursors, such as:
  • Lactic acid
  • Glycerol
  • Amino acids
• Glucose is stored as glycogen in the liver and skeletal muscle

Glycogenesis

• The second stage of replenishing glycogen supplies
• The formation of glycogen from glucose
• Occurs slowly

Glycogenolysis

• The breakdown of glycogen
• Occurs quickly
• Involves a single enzymatic step
• Allows access to energy by muscle when needed

Lipid Metabolism

• Lipid molecules contain carbon, hydrogen, and oxygen in different proportions than carbohydrates
• Triglycerides are the most abundant lipid in the body

Lipid Catabolism

• Also called lipolysis, breaks lipids down into smaller pieces
• Hydrolysis splits a triglyceride into:
  • 1 molecule of glycerol
  • 3 fatty acid molecules
• Important as energy reserves
• Can provide large amounts of ATP, but slowly
• Saves space, but hard for water-soluble enzymes to reach

Beta-Oxidation

• Fatty acid breakdown
• The cell can gain 144 ATP molecules from one 18-carbon fatty acid molecule
• Breakdown yields about 1.5 times the energy of glucose breakdown

Lipid Usage

• Lipid Synthesis (Lipogenesis): can use almost any organic substrate
• Lipid Storage: Important energy reserves, providing large amounts of ATP, but slowly
  • Saves space, but hard for water-soluble enzymes to reach

Lipid Transport

• Cells require lipids to maintain membranes
• Steroid hormones must reach target cells in many different tissues
• Most lipids are not soluble in water and need special transport mechanisms
• Most lipids circulate through the bloodstream as lipoproteins
• Free fatty acids make up a fraction of the total circulating lipids

Lipoproteins

• Are lipid-protein complexes
• Contain large insoluble glycerides and cholesterol
• 5 classes:
  • Chylomicrons
  • Very low-density lipoproteins (VLDLs)
  • Intermediate-density lipoproteins (IDLs)
  • Low-density lipoproteins (LDLs)
  • High-density lipoproteins (HDLs)

Role of LDL and HDL

• LDL (Low-density lipoprotein) delivers cholesterol to cells; sometimes called “bad” lipids
• HDL (High-density lipoprotein) removes excess cholesterol and returns it to the liver; sometimes called “good” lipids

Proteins

• The body synthesizes 100,000 to 140,000 proteins, each with different form, function, and structure
• All proteins are built from the 20 amino acids
• 10 essential amino acids:
  • 8 that are not synthesized by the body: isoleucine, leucine, lysine, threonine, tryptophan, phenylalanine, valine, and methionine
  • 2 that are insufficiently synthesized: arginine and histidine

Protein Synthesis

• The body synthesizes half of the amino acids needed to build proteins
• Nonessential amino acids can be made by the body on demand

Protein Catabolism

• Cellular proteins are recycled in the cytosol: peptide bonds are broken, and free amino acids are used in new proteins
• If other energy sources are inadequate, the mitochondria generate ATP by breaking down amino acids in the TCA cycle

Summary

• Metabolism is the sum of all chemical reactions in an organism.
• Energy is obtained by breaking down molecules, primarily through respiration.
• Carbohydrate Metabolism is broken down into three stages: glycolysis, citric acid cycle, and oxidative phosphorylation.
• Lipid Metabolism involves the storage, breakdown (lipolysis), and transport (lipoproteins) of lipids.
• Protein Metabolism involves synthesis, degradation (catabolism), and recycling of proteins.