Metabolism and Cellular Respiration: Study Notes for ANP College Students

Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within the body, enabling energy extraction and utilization. These reactions are classified as either catabolic (breaking down molecules to release energy) or anabolic (building molecules, consuming energy).

• Catabolic reactions: Decomposition reactions; exergonic (release more energy than they consume).

• Anabolic reactions: Synthesis reactions; endergonic (consume more energy than they produce).

Metabolism occurs in three major stages:

1. Stage 1: Digestion and Absorption – Occurs in the GI tract; nutrients are transported via blood to tissue cells.

2. Stage 2: Catabolism/Anabolism of Nutrients – Occurs in the cytoplasm of tissue cells.

3. Stage 3: ATP Harvest – Primarily catabolic; occurs in mitochondria and requires oxygen.

Carbohydrate Metabolism

Key Processes in Carbohydrate Metabolism

Carbohydrates, especially glucose, are the preferred source for ATP production. Other sugars are converted to glucose for cellular use.

• Glycogenesis: Conversion of glucose to glycogen (anabolic); occurs in skeletal muscle and liver cells; stimulated by insulin from pancreatic islets.

• Glycogenolysis: Conversion of glycogen to glucose (catabolic); occurs in hepatocytes; stimulated by glucagon (pancreatic islets) and epinephrine (adrenal medulla).

• Gluconeogenesis: Formation of glucose from noncarbohydrate sources (anabolic); occurs in liver; stimulated by cortisol (adrenal cortex) and glucagon (pancreatic islets).

• Glycolysis: Breakdown of glucose into two pyruvic acid molecules (catabolic); used for ATP production.

Lipid Metabolism

Transport and Utilization of Lipids

Lipids are excellent for long-term energy storage. Due to their hydrophobic nature, they are transported in blood via lipoproteins.

• Low Density Lipoprotein (LDL): "Bad" cholesterol; 25% protein; carries cholesterol for membrane, steroid hormone, and bile production.

• High Density Lipoprotein (HDL): "Good" cholesterol; 40-45% protein; removes excess cholesterol from cells and transports it to the liver for elimination.

Lipid Metabolic Pathways

• Lipolysis: Breakdown of lipids (catabolic); products used for ATP; stimulated by cortisol, epinephrine/norepinephrine, and T3/T4.

• Lipogenesis: Formation of lipids from glucose and amino acids (anabolic); stimulated by insulin from the pancreas.

Protein Metabolism

Protein Catabolism and Anabolism

• Protein Catabolism: Breakdown of proteins into amino acids; amino acids can be converted to fats, ketone bodies, or glucose for ATP production.

• Protein Anabolism: Formation of peptide bonds between amino acids to produce new proteins.

Metabolic Rates

Measurement and Significance

Energy conversion in the body releases heat, often measured in kilocalories (kcal) per unit time.

• Basal Metabolic Rate (BMR): Measured when a person is rested and fasted; reflects energy needed for essential activities.

• Total Metabolic Rate (TMR): Includes energy for all activities; varies with age, gender, and activity level.

Average adult BMR: 1200-1800 kcal/day; TMR: 1600-3000 kcal/day.

Metabolic Pathways in Cellular Respiration

Redox Reactions

Cellular respiration relies on oxidation-reduction (redox) reactions, where electrons (energy) are transferred.

• Oxidation: Loss of electrons (gain oxygen or lose hydrogen).

• Reduction: Gain of electrons (lose oxygen or gain hydrogen).

• Tracking hydrogen atoms helps follow electron (energy) flow.

Coenzymes in Cellular Respiration

• Nicotinamide Adenine Dinucleotide (NAD+): Derived from niacin; reduced to NADH.

• Flavin Adenine Dinucleotide (FAD): Derived from riboflavin; reduced to FADH2.

• Coenzymes act as temporary electron/hydrogen acceptors; enter the electron transport chain (ETC).

• Each NADH yields 2.5 ATP; each FADH2 yields 1.5 ATP.

ATP Synthesis

• Substrate-Level Phosphorylation: Direct transfer of phosphate groups to ADP; does not require coenzymes; occurs in glycolysis (net 2 ATP) and Krebs cycle (2 ATP).

• Oxidative Phosphorylation: Produces most ATP; energy from oxidation of food fuels ATP synthase in ETC.

Cellular Respiration

Summary Equation

Cellular respiration is a series of catabolic redox reactions producing ATP.

Overall equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP + heat)}$

Phases: Glycolysis, Krebs (Citric Acid) Cycle, Oxidative Phosphorylation (ETC).

Glycolysis

• Occurs in cytoplasm.

• Glucose is phosphorylated to glucose-6-phosphate (traps glucose in cell).

• Final products: 2 pyruvic acid, 2 NADH, 2 net ATP (4 total, 2 invested).

• Fate of pyruvic acid depends on oxygen:

  • Aerobic: Enters Krebs cycle and ETC; 2 NADH go to ETC.

  • Anaerobic: Converted to lactic acid; ATP yield remains at 2; mitochondrial processes do not function.

• Aerobic conditions yield more ATP.

Citric Acid/Krebs Cycle

• Transition: 2 pyruvic acid (3C) converted to 2 Acetyl CoA (2C); 2 CO2 released; 2 NADH produced.

• Acetyl CoA combines with oxaloacetic acid to form citric acid.

• End of cycle: citric acid converted back to oxaloacetic acid; extra carbons released as CO2.

• Yield: 2 ATP, 6 NADH, 2 FADH2, 4 CO2 (per glucose).

• Other nutrients (fats, proteins) can feed into the Krebs cycle.

Electron Transport Chain (ETC)

• Final catabolic reactions; coenzymes (NADH, FADH2) act as electron carriers.

• Electrons passed to membrane-bound proteins in the inner mitochondrial membrane.

• Oxidative phosphorylation: electrons fuel proton pumps, creating H+ gradient.

• H+ ions flow back through ATP synthase, generating ATP.

• Oxygen is the final electron acceptor, forming water.

• Yield: 28 ATP (per glucose); 2 NADH from glycolysis = 5 ATP; 8 NADH from transition/Krebs = 20 ATP; 2 FADH2 from Krebs = 3 ATP.

Cellular Aerobic Respiration: Big Picture

Summary of ATP Yield and Reactant/Product Flow

• 1 glucose molecule yields net 32 ATP through glycolysis, Krebs cycle, and ETC.

• Reactants:

  • Glucose: from digested food.

  • Oxygen: inhaled by lungs.

• Products:

  • CO2: exhaled by lungs.

  • H2O: used or excreted.

  • ATP: used for cellular processes (e.g., muscle contraction, sodium-potassium pump).

  • Heat: released to environment, helps regulate body temperature.

Table: Summary of Metabolic Pathways

Pathway	Type	Location	Stimulated By	Main Products
Glycogenesis	Anabolic	Skeletal muscle, liver	Insulin	Glycogen
Glycogenolysis	Catabolic	Liver (hepatocytes)	Glucagon, Epinephrine	Glucose
Gluconeogenesis	Anabolic	Liver	Cortisol, Glucagon	Glucose
Lipolysis	Catabolic	Various tissues	Cortisol, Epinephrine, T3/T4	Fatty acids, glycerol
Lipogenesis	Anabolic	Various tissues	Insulin	Lipids
Protein Catabolism	Catabolic	Various tissues	Various hormones	Amino acids
Protein Anabolism	Anabolic	Various tissues	Various hormones	Proteins

Diagram Guidance

Cellular Respiration Steps and Locations

• Glycolysis: Occurs in cytoplasm; produces 2 ATP, 2 NADH, 2 pyruvic acid.

• Krebs Cycle: Occurs in mitochondrial matrix; produces 2 ATP, 6 NADH, 2 FADH2, 4 CO2.

• Electron Transport Chain: Occurs in inner mitochondrial membrane; produces 28 ATP, water.

• Fates of Pyruvic Acid: With oxygen (aerobic), enters mitochondria for Krebs cycle; without oxygen (anaerobic), converted to lactic acid in cytoplasm.

Additional info: For exam preparation, students should be able to label these steps on a cell diagram, indicating glycolysis in the cytoplasm, Krebs cycle in the mitochondrial matrix, and ETC in the inner mitochondrial membrane, as well as the two fates of pyruvic acid.