Metabolism, Nutrition, and Energetics: Study Notes for Anatomy & Physiology II

Metabolism and Nutrient Processing

Introduction to Metabolism

Metabolism encompasses all chemical reactions occurring within an organism or cell, essential for maintaining life. These reactions are divided into two main categories: catabolism (breakdown of organic substances) and anabolism (synthesis of new organic molecules). The continuous replacement of organic molecules is termed metabolic turnover.

• Catabolism: Decomposition of complex molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy input.

• Organic molecules: Compounds containing carbon, such as carbohydrates, lipids, and proteins.

Nutrient Pool and Distribution

The nutrient pool refers to the accessible reserve of organic substrates used for metabolic turnover or energy production. Nutrients are absorbed by the gastrointestinal (GI) tract and distributed to various tissues. When dietary intake is insufficient, the body mobilizes energy reserves from the liver (triglycerides and glycogen), adipose tissue (triglycerides), and skeletal muscle (glycogen and proteins).

Cellular Metabolism and Energy Production

Metabolic Pathways

Cellular metabolism involves a series of metabolic pathways that convert nutrients into energy. These pathways are classified as anaerobic (not requiring oxygen) or aerobic (requiring oxygen).

• Anaerobic metabolism: Occurs in the cytoplasm, does not require oxygen (e.g., glycolysis).

• Aerobic metabolism: Occurs in mitochondria, requires oxygen (e.g., citric acid cycle, electron transport chain).

Glycolysis

Glycolysis is the anaerobic breakdown of one glucose molecule in the cytoplasm, producing a net gain of 2 ATP, 2 NADH (electron carriers), and 2 pyruvate molecules.

• Location: Cytoplasm

• Oxygen requirement: None

• Products: 2 ATP, 2 NADH, 2 pyruvate

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle (Krebs cycle) is an aerobic process that occurs in the mitochondria. Pyruvate from glycolysis is converted to acetyl-CoA, which enters the cycle. Each turn of the cycle produces 1 ATP, 3 NADH, and 1 FADH2 per pyruvate (2 ATP per glucose).

• Location: Mitochondrial matrix

• Oxygen requirement: Yes

• Products (per glucose): 2 ATP, 6 NADH, 2 FADH2

Oxidative Phosphorylation and Electron Transport Chain (ETC)

The electron transport chain (ETC) is the final stage of aerobic respiration, occurring in the inner mitochondrial membrane. It uses electrons from NADH and FADH2 to generate a proton gradient, driving ATP synthesis. Oxygen is the final electron acceptor, forming water. The ETC produces about 32 ATP per glucose molecule, accounting for over 90% of cellular ATP production.

• Location: Inner mitochondrial membrane

• Oxygen requirement: Yes

• ATP yield: ~32 ATP per glucose

Overall ATP Yield from Glucose

One molecule of glucose generates a total of 30–32 ATP through glycolysis, the citric acid cycle, and oxidative phosphorylation.

Nutrient Metabolism

Carbohydrate Metabolism

Carbohydrates are the preferred substrate for ATP production during rest. The body stores carbohydrates as glycogen and can mobilize or synthesize glucose as needed.

• Glycogenesis: Formation of glycogen from glucose.

• Glycogenolysis: Breakdown of glycogen into glucose.

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

Forms of Carbohydrates

• Monosaccharides: Glucose, fructose, galactose

• Disaccharides: Maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose)

• Polysaccharides: Glycogen, starch, fiber

Lactose Intolerance

Lactose intolerance results from insufficient production of lactase, the enzyme required to digest lactose. Symptoms include bloating, gas, cramps, and diarrhea after consuming lactose-containing foods. Management includes avoiding lactose, using enzyme supplements, or consuming lactose-free products.

Carbohydrate Absorption and Transport

Carbohydrates are absorbed as monosaccharides in the small intestine, transported into epithelial cells, then into villi capillaries, and finally to the liver via the hepatic portal vein. The liver releases glucose into the bloodstream or stores it as glycogen.

Lipid Metabolism

Lipid Digestion

Lipid digestion begins in the mouth with lingual lipase and continues in the stomach. In the duodenum, bile salts emulsify fats, and pancreatic lipase digests triglycerides into fatty acids and monoglycerides, which form micelles for absorption.

Lipid Absorption and Transport

Lipids from micelles diffuse into small intestine epithelial cells, where they are reassembled into triglycerides and packaged into chylomicrons. Chylomicrons enter lymphatic vessels and are transported to the bloodstream, where lipoprotein lipase releases fatty acids into tissues.

Cholesterol and Lipoproteins

Cholesterol is a sterol lipid transported in the blood by lipoproteins. Low-density lipoproteins (LDL) deliver cholesterol to tissues and are considered "bad" cholesterol, while high-density lipoproteins (HDL) remove cholesterol from tissues and are considered "good" cholesterol.

Protein Metabolism

Protein Digestion

Protein digestion begins with mechanical processing in the mouth, followed by denaturation and enzymatic breakdown in the stomach (pepsin). In the duodenum, pancreatic enzymes further digest proteins into amino acids and small peptides for absorption.

Protein Absorption

Amino acids are absorbed into small intestine epithelial cells and transported to capillaries, similar to carbohydrate absorption.

States of Nutrient Absorption

Absorptive and Postabsorptive States

The absorptive state occurs after a meal when nutrients are being absorbed and stored, primarily regulated by insulin. The postabsorptive state occurs when nutrient absorption is not occurring, and the body relies on stored energy reserves, regulated by hormones such as glucagon.

Vitamins and Minerals

Vitamins

Vitamins are organic compounds required in small amounts for essential metabolic activities. They are classified as fat-soluble (A, D, E, K) or water-soluble (B vitamins and C).

Vitamin	Significance	Sources	Recommended Daily Allowance (mg)	Effects of Deficiency	Effects of Excess
A	Required for synthesis of visual pigments, epithelial maintenance	Leafy greens, yellow vegetables	0.7–0.9	Night blindness, epithelial deterioration	Headache, nausea, liver damage
D	Normal bone growth, calcium absorption	Sunlight, fortified dairy, egg yolks	0.005–0.015*	Rickets, skeletal deterioration	Calcium deposits in soft tissues
E	Prevents breakdown of vitamin A and fatty acids	Vegetables, seeds	10–15	Anemia, other problems	Nausea, stomach cramps, fatigue
K	Essential for liver synthesis of prothrombin and other clotting factors	Vegetables, intestinal bacteria	0.03–0.12	Bleeding disorders	Liver dysfunction

Vitamin	Component or Precursor of	Sources	Recommended Daily Allowance (mg)	Effects of Deficiency	Effects of Excess
B1 (Thiamine)	Coenzyme in many pathways	Milk, meat, bread	1.1–1.2	Muscle weakness, CNS and cardiovascular problems	Hypotension
B2 (Riboflavin)	Part of FAD, coenzyme in multiple pathways	Milk, meat, eggs	1.3–1.7	Epithelial and mucosal deterioration	Itching, tingling
B3 (Niacin)	Part of NAD, coenzyme in multiple pathways	Meat, bread, potatoes	13–17	CNS, GI, epithelial, and mucosal deterioration	Vasodilation, itching, burning
B6 (Pyridoxine)	Coenzyme in amino acid and lipid metabolism	Meat, whole grains, vegetables	1.8–2.2	Retarded growth, CNS problems	Fatigue, numbness, loss of coordination
B12 (Cobalamin)	Coenzyme in nucleic acid metabolism	Milk, meat	0.0024	Pernicious anemia	Polycythemia
C (Ascorbic acid)	Coenzyme, antioxidant	Citrus fruits	75–90	Scurvy	Kidney stones

Electrolytes

Electrolytes are minerals that conduct electricity in the body, regulate fluid balance, transport nutrients, support muscle and nerve function, and help convert calories into energy.

Nutrition and Energetics

Balanced Diet

A balanced diet contains all ingredients required for homeostasis, including substrates for ATP production, essential amino acids, fatty acids, vitamins, electrolytes, and water. Dietary guidelines recommend appropriate proportions of food groups for optimal health.

Caloric Value of Nutrients

• Proteins: 4 cal/g

• Carbohydrates: 4 cal/g

• Lipids: 9 cal/g

A calorie is the amount of energy required to raise the temperature of 1 gram of water by 1°C.

Metabolic and Eating Disorders

Eating Disorders

• Anorexia nervosa: Self-induced starvation or loss of appetite, resulting in significant weight loss.

• Bulimia: Binge eating followed by purging (vomiting, laxatives, diuretics); more common than anorexia.

Metabolic Disorders

• Obesity: Body weight >20% above ideal; increases risk for diabetes and hypertension.

• High cholesterol: Associated with low HDL and high LDL; recommended intake is under 300 mg/day.

• Protein deficiency disease (e.g., Kwashiorkor): Results in edema due to low blood albumin; common in malnutrition.

• Malnutrition: State of inadequate or excessive nutrient absorption.

Energetics and Appetite Regulation

Basal Metabolic Rate (BMR)

BMR is the minimum resting energy expenditure of an awake, alert person. The average is about 70 cal/hr or 1680 cal/day, but it varies with weight, size, and activity level.

Control of Appetite

Appetite is regulated by the hypothalamus, which contains feeding and satiety centers. The hormone leptin, secreted by adipose tissue, signals satiety and helps regulate energy balance. Neural signals, hormones, body temperature, and psychological factors also influence feeding behavior.

Summary Table: Five Basic Food Groups and Their Effects on Health

Nutrient Group	Provides	Health Effects
Grains	Carbohydrates, vitamins, minerals, fiber	Rapid rise in blood glucose, reduced risk of cardiovascular disease
Vegetables	Carbohydrates, vitamins, minerals, fiber	Reduced risk of cardiovascular disease, cancer, and obesity
Fruits	Carbohydrates, vitamins, minerals, fiber	Reduced risk of cardiovascular disease, cancer, and obesity
Dairy	Calcium, protein, vitamins	Bone health, reduced risk of osteoporosis
Meat, Fish, Eggs, Legumes	Protein, vitamins, minerals	Muscle maintenance, immune function