Metabolism and Nutrition: An Overview for Anatomy & Physiology Students

Metabolism and Nutrition

Overview of Metabolism and Nutrition

Metabolism encompasses all chemical reactions that occur within the body to maintain life. These reactions are essential for energy production, synthesis of molecules, and cellular maintenance. Nutrition provides the raw materials and energy required for these metabolic processes.

Learning Outcomes

• Define the terms metabolism, catabolism, and anabolism, and identify the nutrients the body is able to use for fuel.

• Compare and contrast endergonic and exergonic reactions.

• Describe the process of phosphorylation.

Key Concepts in Metabolism

Definitions and Types of Metabolic Reactions

Metabolism is divided into two main categories: catabolism and anabolism.

• Catabolic reactions: Break down complex molecules into simpler ones, releasing energy. Example: Breakdown of glucose during cellular respiration.

• Anabolic reactions: Build complex molecules from simpler ones, requiring energy input. Example: Synthesis of proteins from amino acids.

These reactions are often coupled, with energy released from catabolic (exergonic) reactions fueling anabolic (endergonic) reactions.

Energy Transfer: Exergonic vs. Endergonic Reactions

Metabolic reactions involve the transfer of energy:

• Exergonic reactions: Release energy (e.g., catabolic processes).

• Endergonic reactions: Require energy input (e.g., anabolic processes).

ATP (adenosine triphosphate) acts as the main energy currency, linking these reactions.

Oxidation-Reduction (Redox) Reactions

Many metabolic reactions involve the transfer of electrons:

• Oxidation: Loss of electrons, usually associated with energy release.

• Reduction: Gain of electrons, usually associated with energy gain.

Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

ATP: The Energy Currency of the Cell

ATP stores energy in its high-energy phosphate bonds. When ATP is hydrolyzed to ADP and inorganic phosphate, energy is released for cellular work.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP (e.g., glycolysis, citric acid cycle).

• Oxidative phosphorylation: Indirect formation of ATP via the electron transport chain and chemiosmosis in mitochondria.

Digestion and Absorption of Nutrients

Chemical Digestion and Absorption

Macronutrients are broken down into absorbable units:

• Carbohydrates: Digested into monosaccharides (e.g., glucose, galactose).

• Proteins: Digested into amino acids.

• Lipids: Digested into fatty acids and monoglycerides.

Glucose Catabolism (Cellular Respiration)

Stages of Glucose Catabolism

Glucose catabolism is the primary pathway for ATP production and occurs in four main stages:

1. Glycolysis: Splits glucose into two molecules of pyruvic acid in the cytosol. Net gain: 2 ATP.

2. Formation of Acetyl Coenzyme A: Pyruvic acid is converted to acetyl CoA, producing CO2 and NADH.

3. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; acetyl CoA is oxidized, producing ATP, NADH, FADH2, and CO2.

4. Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through a series of carriers, driving ATP synthesis and forming water.

Summary Equation for Cellular Respiration

The overall reaction for aerobic cellular respiration is:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 36\ (or\ 38)\ ATP}$

ATP yield: 2 from glycolysis, 2 from citric acid cycle, and the remainder from the electron transport chain.

Catabolism of Other Nutrients

Amino Acid Catabolism

Amino acids can be used for energy by deamination and entry into the citric acid cycle, or converted into glucose, fatty acids, or ketone bodies.

Fatty Acid Catabolism (Lipolysis)

Fatty acids are broken down via beta-oxidation to acetyl CoA, which enters the citric acid cycle. Glycerol can be converted to glucose.

Anabolic Pathways

Functions of Anabolism

Anabolic pathways are essential for:

• Nutrient storage (e.g., glycogen, fat)

• Synthesis of structural and functional molecules (e.g., proteins, nucleic acids)

• Synthesis of special molecules (e.g., FAD, NAD+)

Glucose Anabolism

• Glycogenesis: Formation of glycogen from glucose, stimulated by insulin.

• Gluconeogenesis: Formation of new glucose from non-carbohydrate sources, stimulated by glucagon and cortisol.

Lipid and Amino Acid Anabolism

• Lipogenesis: Synthesis of lipids for storage, stimulated by insulin.

• Amino acids are used for protein synthesis; excess can be converted to other storage molecules.

Metabolic States and Regulation

Absorptive State

During the absorptive state (about 12 hours per day), ingested nutrients are absorbed and used for ATP production or stored as glycogen or fat. Glucose is the primary energy source.

Post-Absorptive State

During the post-absorptive state, stored nutrients are mobilized to maintain blood glucose levels (80–100 mg/dL). Glucagon stimulates glycogenolysis and gluconeogenesis.

Metabolic Rate and Thermoregulation

Heat and Energy Balance

Metabolic rate is the overall rate of heat production, measured in kilocalories (Calories). Basal metabolic rate (BMR) is the energy expended at rest. Body temperature is regulated by negative feedback mechanisms to maintain homeostasis.

Nutrition

Macronutrients

Macronutrients provide energy and structural molecules:

• Proteins

• Lipids

• Carbohydrates

Different types of macronutrients can influence metabolic function.

Micronutrients

Micronutrients do not provide energy but are essential as enzyme cofactors and catalysts:

• Vitamins: Organic compounds; fat-soluble (A, D, E, K) are stored, water-soluble (B group, C) are excreted in urine when in excess.

• Minerals: Inorganic elements needed in small amounts (e.g., Ca, Mg, K, Na, iodine, iron).

Water is required for metabolic reactions, and fiber is necessary for GI tract motility (insoluble vs. soluble fiber).

Lipoproteins and Cholesterol

Lipids are transported in the blood as lipoproteins:

• VLDLs (Very Low-Density Lipoproteins)

• LDLs (Low-Density Lipoproteins): Should be under 130 mg/dL

• HDLs (High-Density Lipoproteins): Should be above 40 mg/dL

Cholesterol is obtained from the diet (15%) and synthesized by the liver. Normal total cholesterol levels should be below 200 mg/dL.

Macronutrient	Digestive End Product	Main Function
Carbohydrates	Monosaccharides (e.g., glucose)	Energy source
Proteins	Amino acids	Structural, functional molecules
Lipids	Fatty acids, monoglycerides	Energy storage, cell membranes

Additional info: This summary integrates and expands upon the provided slides and images, ensuring a comprehensive, exam-ready overview of metabolism and nutrition for Anatomy & Physiology students.