Metabolism and Nutrition: Study Notes for Anatomy & Physiology

Metabolism and Nutrition

Overview of Metabolism and Nutrition

Metabolism encompasses all chemical reactions occurring in the body, enabling cells to extract energy from nutrients, build macromolecules, and maintain homeostasis. These reactions are organized into metabolic pathways, which are sequences of enzyme-catalyzed steps.

• Metabolism: The sum of all chemical reactions in the body.

• Four Basic Metabolic Processes:

  • Harnessing energy from nutrients to produce ATP.

  • Converting molecules for synthesis reactions.

  • Assembling macromolecules (proteins, polysaccharides, nucleic acids, lipids).

  • Breaking down macromolecules into monomers or smaller molecules.

• Metabolic Pathways: Series of enzyme-catalyzed reactions.

Phases of Metabolism: Catabolism and Anabolism

Metabolism is divided into two main phases: catabolism and anabolism. These phases are interconnected, with energy released from catabolic reactions fueling anabolic processes.

• Catabolism: Breakdown of molecules into smaller parts, releasing energy. Main nutrient monomers used:

  • Glucose: Preferred fuel for many cells, especially brain and liver.

  • Fatty acids: Derived from triglycerides, enter separate catabolic pathways.

  • Amino acids: Released from protein breakdown, enter catabolic pathways or used for other purposes.

• Anabolism: Synthesis of larger molecules from smaller ones, requiring energy. Used to build proteins, nucleic acids, lipids, and carbohydrates.

Energy Requirements of Metabolic Reactions

Metabolic reactions are paired to efficiently use energy. Exergonic reactions release energy, while endergonic reactions require energy input.

• Exergonic Reactions: Release energy; products have less energy than reactants. Most catabolic reactions are exergonic.

• Endergonic Reactions: Require energy input; products have more energy than reactants. Most anabolic reactions are endergonic.

• Relationship: Energy from exergonic catabolic reactions is used to fuel endergonic anabolic reactions, such as ATP synthesis.

Adenosine Triphosphate (ATP) and Phosphorylation

ATP is the cell's primary energy currency. Its hydrolysis releases energy for cellular work and phosphorylation reactions.

• ATP Structure: Contains three phosphate groups; bonds are unstable due to repulsion between negative charges.

• ATP Hydrolysis: Highly exergonic; only about 40% of energy is harnessed, rest lost as heat.

• Phosphorylation: ATP donates a phosphate group to a reactant, making it more reactive and favoring product formation.

Glucose Catabolism and ATP Synthesis

Introduction to Glucose Catabolism

Glucose catabolism involves breaking down glucose to release energy for ATP synthesis. It consists of glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Glycolysis: Occurs in cytosol; splits glucose into two pyruvate molecules.

• Citric Acid Cycle: Occurs in mitochondrial matrix; continues glucose breakdown.

• Oxidative Phosphorylation: Occurs in inner mitochondrial membrane; uses energy from electron transfer to synthesize ATP.

• Electron Transport Chain (ETC): Transfers electrons, leading to ATP synthesis.

Types of Phosphorylation

• Substrate-level Phosphorylation: Direct transfer of phosphate from substrate to ADP to form ATP.

• Oxidative Phosphorylation: Energy from electron flow is harnessed to generate ATP.

Classes of Glucose Catabolism

• Glycolytic (Anaerobic) Catabolism: Occurs without oxygen; involves glycolysis.

• Oxidative (Aerobic) Catabolism: Requires oxygen; includes ETC and citric acid cycle. Cellular respiration refers to oxygen consumption and production of CO2 and H2O.

Glycolysis

Glycolysis is a series of 10 enzyme-catalyzed reactions in the cytosol, splitting glucose into two pyruvate molecules.

• Energy Investment Phase: First 5 reactions; 2 ATP used; glucose phosphorylated and split into two 3-carbon compounds.

• Energy Payoff Phase: Last 5 reactions; phosphate groups transferred to ADP to yield ATP; compounds oxidized to produce NADH.

• Net Yield: 2 ATP spent, 4 ATP synthesized, 2 NADH produced, 2 pyruvate formed. Net gain: 2 ATP and 2 NADH per glucose.

Fate of Pyruvate

• Anaerobic Conditions: Pyruvate reduced to lactate.

• Aerobic Conditions: Pyruvate enters mitochondria, is oxidized, and enters citric acid cycle.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle consists of 8 reactions in the mitochondrial matrix, beginning oxidative glucose catabolism. Each glucose produces two acetyl-CoA, turning the cycle twice.

• Yield per Glucose: 10 NADH (2 glycolysis, 2 pyruvate oxidation, 6 citric acid cycle), 2 FADH2, 4 ATP (2 glycolysis, 2 citric acid cycle).

• Energy Conservation: Most energy is stored in NADH and FADH2 for use in ETC.

Electron Transport Chain and Oxidative Phosphorylation

The ETC is the final stage of glucose catabolism, where most ATP is produced. It involves electron transfer, proton pumping, and ATP synthesis via chemiosmosis.

• Electron Transfer: Electrons passed through 4 enzyme complexes (I-IV) in the inner mitochondrial membrane.

• Proton Pumping: H+ ions pumped into intermembrane space, creating an electrochemical gradient.

• ATP Synthesis: H+ flows back into matrix via ATP synthase, driving ATP production.

Fatty Acid and Amino Acid Catabolism

Fatty Acid Catabolism

Cells can break down fatty acids for energy, often yielding more ATP than glucose catabolism.

• Triglycerides: Main storage form; broken down by lipolysis into fatty acids and glycerol.

• β-oxidation: Fatty acids bound to coenzyme A, enter mitochondrial matrix, and are broken down into acetyl-CoA.

• Ketogenesis: In liver cells, acetyl-CoA is converted to ketone bodies during starvation or carbohydrate restriction. Excess ketone bodies cause ketosis and can lead to ketoacidosis (dangerous drop in blood pH).

Amino Acid Catabolism

Proteins are broken down into amino acids, which are catabolized for energy or converted to other compounds.

• Transamination: Amino group transferred to α-ketoglutarate, forming a carbon skeleton and glutamate.

• Oxidative Deamination: Glutamate is deaminated in mitochondria; ammonia is either used for new amino acids or removed via the urea cycle.

• Urea Cycle: Ammonia combined with CO2 to form urea, excreted by kidneys.

Anabolic Pathways

Introduction to Anabolic Pathways

Anabolism is essential for nutrient storage, synthesis, and structural maintenance. Excess nutrients are stored as glycogen or adipose tissue.

• Glycogen: Storage form of glucose.

• Adipose: Storage form of triglycerides.

Glucose Anabolism

• Glycogenesis: Synthesis of glycogen from glucose, mainly in liver and skeletal muscle.

• Glycogenolysis: Breakdown of glycogen to release glucose.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources (glycerol, pyruvate, lactate, citric acid cycle intermediates, glucogenic amino acids). Fatty acids cannot be converted to glucose.

Fatty Anabolism

• Lipogenesis: Synthesis of fatty acids in cytosol, catalyzed by fatty acid synthase.

• Sources: Glycerol and fatty acids can be derived from amino acids and glucose.

• Excess Amino Acids: Converted to triglycerides for storage.

Amino Acid Anabolism

• Essential Amino Acids: 9 must be obtained from diet; 11 can be synthesized by the body.

• Synthesis: Addition of amino group to carbon skeletons (α-ketoglutarate, pyruvate, oxaloacetate).

• Protein Synthesis: Limited; excess amino acids converted to other molecules.

Metabolic States and Regulation of Feeding

Metabolic States

The body alternates between absorptive and postabsorptive states to ensure continuous energy supply.

• Absorptive State: Occurs after feeding (up to 4 hours); nutrients absorbed and used for oxidation, glycogenesis, lipogenesis, and protein synthesis.

• Postabsorptive State: Begins after absorption is complete; anabolic processes slow, catabolic processes (ketogenesis, gluconeogenesis, glycogenolysis, lipolysis, glucose sparing) dominate.

Macronutrients and Micronutrients

Proteins and Amino Acids

• Proteins: Serve as molecular fuels, structural molecules, and enzymes.

• Recommended Intake: 10–35% of daily calories from protein-rich foods.

• Non-essential Amino Acids: 11 can be synthesized from carbon skeletons.

• Essential Amino Acids: 9 must be obtained from diet.

Micronutrients: Vitamins and Minerals

• Vitamins: 13 organic compounds required for body functions.

  • Fat-soluble: A, D, E, K

  • Water-soluble: C, B vitamins

• Minerals: Elements (other than C, H, O, N) required for physiological processes.

Structural Lipid: Cholesterol

Cholesterol Processing

• Sources: Found in animal-based foods; liver synthesizes 85% of required cholesterol, diet provides 15%.

• Lipoproteins: Carrier proteins for cholesterol in blood.

  • VLDLs: Very low-density lipoproteins

  • LDLs: Low-density lipoproteins

  • HDLs: High-density lipoproteins

• Hypercholesterolemia: Elevated blood cholesterol, often due to excessive dietary intake.

Diet and Body Mass

Body Mass and BMI

• Body Mass: Amount of matter in the body.

• Body Weight: Force exerted on body mass by gravity.

• Body Mass Index (BMI): Equation accounting for height to assess relative body mass.

• Energy Balance: Difference between energy intake and expenditure determines weight gain or loss.

• Healthy Diet: Meets needs for micronutrients, macronutrients, essential amino acids, and cholesterol. MyPlate (USDA, 2011) illustrates proportions for a nutritious meal.

• Obesity: BMI > 30.0; increases risk for type 2 diabetes, coronary artery disease, cancer, hypertension, osteoarthritis.

Key Equations

• ATP Hydrolysis: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

• Body Mass Index (BMI): $\mathrm{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}$

• Urea Formation: $2\mathrm{NH}_3 + \mathrm{CO}_2 \rightarrow \mathrm{urea} + \mathrm{H}_2\mathrm{O}$

Table: Essential vs. Non-Essential Amino Acids

Type	Number	Source
Essential	9	Dietary
Non-Essential	11	Synthesized by body

Table: Fat-Soluble vs. Water-Soluble Vitamins

Type	Examples
Fat-Soluble	A, D, E, K
Water-Soluble	C, B vitamins

Table: Types of Lipoproteins

Lipoprotein	Main Function
VLDL	Transport triglycerides from liver to tissues
LDL	Deliver cholesterol to cells
HDL	Remove excess cholesterol from tissues

Example: During fasting, the liver increases ketogenesis, producing ketone bodies for energy, but excessive production can lead to ketoacidosis.

Example: Glycogenolysis is activated during the postabsorptive state to maintain blood glucose levels.

Example: A healthy diet includes a balance of macronutrients and micronutrients, as illustrated by the USDA's MyPlate.