Metabolism, Nutrition, and Energetics: Study Guide for Anatomy & Physiology

Metabolism, Nutrition, and Energetics

Metabolism: Core Concepts

Metabolism encompasses all chemical reactions occurring within the cells of the body, supporting life and maintaining homeostasis. It is divided into two main processes:

• Catabolism: The breakdown of organic molecules, releasing energy used to synthesize ATP. This is an exergonic process.

• Anabolism: The synthesis of new organic molecules, requiring energy. This endergonic process is essential for maintenance, growth, secretion, and building nutrient reserves.

Cellular metabolism refers to the chemical reactions that maintain cellular functions, involving metabolic turnover—the continuous breakdown and replacement of cellular components. The nutrient pool is the primary source of organic substrates for these processes.

The Nutrient Pool

The nutrient pool acts as a metabolic reserve, utilized according to the body's nutritional needs:

• Insufficient Digestive Absorption: Body mobilizes reserves from the nutrient pool.

• Excessive Digestive Absorption: Surplus nutrients are stored in body reserves.

• Tissue-Specific Dynamics:

  • Liver: Converts and stores nutrients (e.g., glycogen), maintains blood glucose levels.

  • Adipocytes: Store excess lipids as triglycerides.

  • Skeletal Muscle Cells: Maintain glycogen reserves for energy.

  • Neural Tissue: Requires a continuous supply of glucose.

Aerobic Metabolism: Fundamental Pathways

Aerobic metabolism involves catabolic reactions requiring molecular oxygen ($O_2$) to break down organic substrates, producing ATP. The process consists of three main phases:

1. Glycolysis: Occurs in the cytosol; breaks down glucose (6C) into two pyruvate (3C) molecules. Anaerobic.

2. Citric Acid Cycle: Occurs in the mitochondrial matrix; generates reduced coenzymes (NADH, $FADH_2$) for energy transfer.

3. Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane; uses energy from reduced coenzymes to produce most cellular ATP via the electron transport chain and chemiosmosis.

Digestion Overview

Digestion is the mechanical and chemical breakdown of large organic molecules into smaller substrates for replenishing the nutrient pool. The body digests three primary macronutrients:

• Carbohydrates

• Lipids

• Proteins

Carbohydrate Digestion and Absorption

Carbohydrate digestion involves enzymatic breakdown at various locations:

Location	Specific Enzymes	Biochemical Process
Mouth	Salivary amylase	Complex Carbs → Disaccharides/Trisaccharides
Duodenum	Pancreatic alpha-amylase	Complex Carbs → Disaccharides/Trisaccharides
Jejunum	Brush border enzymes (Maltase, Sucrase, Lactase)	Disaccharides/Trisaccharides → Monosaccharides

Monosaccharides are absorbed in the jejunum and transported to the liver via the hepatic portal vein. The liver converts these sugars to glucose, which is either released to maintain blood glucose (~90 mg/dL) or stored as glycogen.

Lipid Digestion and Transport

Lipid digestion begins in the oral cavity and stomach (lingual lipase), but most occurs in the duodenum (pancreatic lipase). Lipid absorption and transport involve:

1. Bile Salts: Emulsify large lipid droplets in the duodenum.

2. Micelles: Complexes of bile salts and lipids that diffuse across intestinal epithelium.

3. Chylomicrons: Lipids reassembled and coated with proteins in intestinal cells.

4. Exocytosis: Chylomicrons secreted into interstitial fluid.

5. Lacteals: Chylomicrons enter lymphatic vessels (lacteals) due to their size, eventually reaching venous circulation.

Lipoprotein	Main Function
LDL (Low-Density Lipoprotein)	Delivers cholesterol to peripheral tissues
HDL (High-Density Lipoprotein)	Transports excess cholesterol back to liver

Clinical Indicators: Total cholesterol ($>200$ mg/dL) and LDL:HDL ratio are key markers for cardiovascular risk.

Lipid Metabolism

• Lipolysis: Catabolic breakdown of lipids for energy; yields more energy than glucose catabolism.

• Lipogenesis: Anabolic synthesis of lipids from sugars or amino acids.

• Essential Fatty Acids: Omega-3 and Omega-6 must be obtained from the diet.

Protein Digestion and Amino Acid Metabolism

Protein digestion involves several enzymes:

• Pepsin: Stomach enzyme.

• Trypsin: Pancreatic enzyme active in the duodenum.

• Peptidases: Brush border enzymes in the jejunum.

Amino acid synthesis occurs via amination and transamination. Catabolism (deamination) releases toxic ammonium ions, which are processed by the urea cycle in the liver and excreted as urea by the kidneys.

Metabolic States

• Absorptive State: Lasts ~4 hours after eating; active nutrient absorption and storage. Regulated by Insulin, Growth Hormone, and sex hormones.

• Postabsorptive State: Occurs when absorption is absent; mobilization of energy reserves. Regulated by Glucagon, Epinephrine, glucocorticoids, and Growth Hormone.

Vitamins and Minerals

• Vitamins: Organic compounds required in trace amounts for metabolism.

• Minerals: Inorganic substances functioning as electrolytes.

Type	Examples	Properties
Fat-Soluble	A, $D_3$, E, K	Absorbed/stored with fats; risk of toxic accumulation
Water-Soluble	B series, C	Distributed in fluids; excess excreted in urine

Diet and Nutrition

• Balanced Diet: Provides all essential nutrients and water in proper proportions.

• Malnutrition: Dysfunction from inadequate or excessive nutrient absorption.

• Calorie (kilocalorie): Unit of food energy and metabolic expenditure.

Nutrient	Energy Content (Cal/g)
Carbohydrates	4.18
Proteins	4.32
Lipids	9.46

Energetics and Basal Metabolic Rate (BMR)

Energetics is the study of energy flow and transformation in the body. BMR is the minimum resting energy expenditure of an awake, alert individual, influenced by:

• Body size and weight

• Physical activity and mental state

• Metabolic state (absorptive vs. postabsorptive)

• Hormonal concentrations (e.g., thyroid hormone, epinephrine)

Appetite Control

The hypothalamus regulates appetite via two centers:

• Feeding Center: Controls hunger sensation.

• Satiety Center: Controls fullness sensation.

Appetite regulation mechanisms:

• Short-Term Control:

  • Satiety Factors: Elevated blood glucose, hormone CCK, stretch receptors in digestive tract.

  • Hunger Factors: Neuropeptide Y (NPY), hormone Ghrelin (secreted when stomach is empty).

• Long-Term Control:

  • Leptin: Peptide hormone from adipose tissue; stimulates satiety center, suppresses appetite.