Chapter 24: Metabolism – Study Notes

Chapter 24: Metabolism

Overview of Metabolism

Metabolism refers to the sum of all chemical reactions involved in maintaining the living state of the cells and the organism. It includes both anabolic (building up) and catabolic (breaking down) processes, primarily involving carbohydrates, lipids, and proteins.

• Carbohydrates: Main dietary sources include starch (grains, vegetables), sugars (fruits), and cellulose (fiber). Used primarily for energy production.

• Lipids: Dietary sources are triglycerides (animal and plant fats), cholesterol, and phospholipids. Used for energy storage, cell membrane structure, and hormone synthesis.

• Proteins: Dietary sources include animal products and legumes. Used for tissue building, enzymes, hormones, and energy (when necessary).

Main Nutrient Pathways

Cells utilize nutrients through three major pathways: glycolysis, Krebs cycle (citric acid cycle), and oxidative phosphorylation. These pathways are essential for ATP production.

• Glycolysis: Occurs in the cytosol; breaks down glucose into pyruvate, producing ATP and NADH.

• Krebs Cycle: Occurs in mitochondria; oxidizes acetyl CoA to CO2, generating NADH, FADH2, and ATP.

• Oxidative Phosphorylation: Occurs in mitochondria; uses NADH and FADH2 to produce ATP via the electron transport chain.

Carbohydrate Metabolism

Carbohydrate metabolism is the process by which glucose and other sugars are broken down to produce energy. The main steps include glycolysis, glycogenesis, glycogenolysis, and gluconeogenesis.

• Glycolysis: Converts glucose to pyruvate, yielding 2 ATP and 2 NADH per glucose molecule.

• Glycogenesis: Formation of glycogen from glucose for storage in liver and muscle.

• Glycogenolysis: Breakdown of glycogen to release glucose.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources (e.g., amino acids, glycerol).

Example: During fasting, the liver converts amino acids and glycerol into glucose via gluconeogenesis.

Lipid Metabolism

Lipid metabolism involves the breakdown and synthesis of fatty acids and triglycerides. It is crucial for energy storage and membrane structure.

• Beta-oxidation: Fatty acids are broken down in mitochondria to produce acetyl CoA, NADH, and FADH2.

• Lipogenesis: Synthesis of fatty acids from acetyl CoA, primarily in the liver and adipose tissue.

• Lipolysis: Breakdown of triglycerides into glycerol and fatty acids.

• Ketogenesis: Formation of ketone bodies from acetyl CoA during prolonged fasting or diabetes.

Example: In starvation, fatty acids are converted to ketone bodies for use by the brain and muscles.

Protein Metabolism

Protein metabolism includes the breakdown of proteins into amino acids and their subsequent use for energy, synthesis of new proteins, or conversion into glucose or fat.

• Deamination: Removal of the amino group from amino acids, producing ammonia and a keto acid.

• Transamination: Transfer of an amino group from one amino acid to a keto acid, forming a new amino acid.

• Urea Cycle: Ammonia produced from deamination is converted to urea in the liver for excretion.

• Protein Synthesis: Amino acids are used to build new proteins for growth and repair.

Example: Excess amino acids are deaminated and converted to glucose (gluconeogenesis) or fat (lipogenesis).

Energy Balance and Metabolic States

The body alternates between absorptive (fed) and postabsorptive (fasting) states to maintain energy balance.

• Absorptive State: Occurs after eating; nutrients are absorbed and stored. Insulin promotes glucose uptake and storage.

• Postabsorptive State: Occurs during fasting; stored nutrients are mobilized. Glucagon promotes glycogenolysis and gluconeogenesis.

Example: During exercise, muscle glycogen is broken down to provide glucose for ATP production.

Summary Table: Major Metabolic Pathways

Pathway	Main Substrate	Location	Major Products
Glycolysis	Glucose	Cytosol	Pyruvate, ATP, NADH
Krebs Cycle	Acetyl CoA	Mitochondria	CO2, NADH, FADH2, ATP
Oxidative Phosphorylation	NADH, FADH2	Mitochondria	ATP, H2O
Beta-oxidation	Fatty acids	Mitochondria	Acetyl CoA, NADH, FADH2
Gluconeogenesis	Amino acids, glycerol	Liver	Glucose

Key Equations

• ATP Yield from Glucose:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36-38\text{ATP}$

• Beta-Oxidation of Fatty Acids:

$\text{Fatty Acid} + \text{CoA} + \text{FAD} + \text{NAD}^+ \rightarrow \text{Acetyl CoA} + \text{FADH}_2 + \text{NADH}$

• Urea Formation:

$2\text{NH}_3 + \text{CO}_2 \rightarrow \text{urea} + \text{H}_2\text{O}$

Additional info:

• Metabolic pathways are tightly regulated by hormones such as insulin, glucagon, and epinephrine.

• Disorders of metabolism include diabetes mellitus, obesity, and inborn errors of metabolism.