Metabolism Overview

Define metabolism, anabolism, catabolism, and how these processes relate to one another.
Describe the catabolic pathways that utilize nutrient molecules as fuel sources.
Describe the anabolic pathways through which nutrients are synthesized or stored.
Explain the hormonal control of absorptive and post-absorptive states and how nutrient levels are regulated during each state, including the roles of insulin and glucagon.

Metabolism: The entire set of cellular reactions that involves energy transformations, categorized into two main types:
- Anabolism: The process of building complex molecules from simpler ones (synthesis).
- Catabolism: The breakdown of complex molecules into simpler ones (degradation).

Anabolism and catabolism are interconnected processes which together make up metabolism.
Anabolic reactions require energy (endergonic) provided by ATP, while catabolic reactions release energy (exergonic) that can be captured in ATP molecules.

Nutrient macromolecules are broken down into monomers through the digestive system:
- Monomers produced:
  - Glucose
  - Fatty acids
  - Amino acids
These monomers serve as fuel to generate ATP, which powers various cellular processes.
Cellular respiration: A significant catabolic pathway.
1. Glycolysis: Glucose ($C6H{12}O_6$) is broken down into pyruvate producing NADH and a net gain of 2 ATP.
2. Citric Acid Cycle (Krebs Cycle): Acetyl-CoA is further oxidized yielding NADH, FADH2, and ATP.
3. Electron Transport Chain (ETC): NADH and FADH2 electrons are transferred along a chain, leading to ATP formation through oxidative phosphorylation.
Overall Reaction for Cellular Respiration:
C6H{12}O6 + 6O2
ightarrow 6H2O + 6CO2 + 38 ATP + ext{heat}

Anabolism: Utilizes energy stored in ATP to synthesize larger, more complex molecules. Examples include:
- Synthesis of proteins from amino acids.
- Lipogenesis: The conversion of excess glucose and fatty acids into triglycerides for storage.
- Glycogenesis: The conversion of glucose into glycogen.
Key anabolic reactions:
- Glycogenesis (making glycogen)
- Lipogenesis (making lipids)
- Protein synthesis (making proteins)

Energie is stored principally as:
- Glycogen in liver and muscle.
- Triglycerides in adipose tissue.

Absorptive State: Anabolism dominates following food ingestion.
- Nutrients absorbed lead to increased insulin secretion, promoting glucose uptake, glycogenesis, lipogenesis, and protein synthesis.
- Key Hormone: Insulin controls glucose uptake and storage in muscle and adipose tissues.
Post-Absorptive State: Catabolism dominates during fasting or between meals.
- Hormones like glucagon promote glycogenolysis (breakdown of glycogen) and gluconeogenesis (synthesis of glucose from non-carbohydrate sources).
- Key Hormone: Glucagon raises blood glucose levels by stimulating liver to release glucose.

Absorptive State (Immediately post-meal):
- Glucose: Main energy source for cells.
- Amino acids are directed towards protein synthesis.
- Excess glucose is stored as glycogen, and additional energy is stored as triglycerides.
Post-Absorptive State (4 hours post-meal):
- Maintains blood glucose levels through:
  - Glycogenolysis: Breakdown of glycogen into glucose.
  - Gluconeogenesis: Formation of glucose from non-carbohydrates like amino acids and glycerol.
  - Lipolysis: Breakdown of triglycerides into fatty acids for energy.

Pathways	Anabolic Reaction	Catabolic Reaction
Glycogen	Glycogenesis (glucose → glycogen)	Glycogenolysis (glycogen → glucose)
Triglycerides	Lipogenesis (glucose → triglycerides)	Lipolysis (triglycerides → fatty acids)
Protein	Protein Synthesis (amino acids → proteins)	Protein Hydrolysis (proteins → amino acids)
Glucose	Gluconeogenesis (non-glucose → glucose)	Glycolysis (glucose → pyruvate)

Added sugars (e.g., high-fructose corn syrup) contributes to increased lipogenesis and may lead to metabolic syndrome conditions such as:
- Abdominal obesity
- Hypertension (HTN)
- Hyperglycemia
- Insulin resistance
- Hyperlipidemia (high cholesterol)
Increased intake of fructose can elevate the risk of fatty liver disease and type II diabetes.

Figure 23.1: Overview of metabolic processes
Figures 23.4 - 23.10: Related to glycolysis, citric acid cycle, and oxidative phosphorylation.
Figures 23.12 - 23.14: Anabolic pathways, such as glycogenesis and lipogenesis.

Understanding metabolism is crucial in comprehending how energy is produced, stored, and utilized in the body, and the role of hormones in regulating these processes. This foundational knowledge underlies many health and nutrition discussions today.