Metabolism, Nutrition, and Energetics: Study Notes for Anatomy & Physiology (BIO 141)

Metabolism, Nutrition, and Energetics

Introduction to Metabolism

Metabolism refers to all chemical and physical changes that occur within living cells and tissues. It encompasses the processes by which the body acquires, stores, and utilizes energy and nutrients.

• Essential elements and nutrients are required for cellular function and energy production.

• Abundant nutrients are used to build energy reserves.

• Energy reserves are mobilized when needed for cellular activities.

Types of Metabolic Processes

• Anabolism: The synthesis of large molecules from smaller ones; requires energy. Example: building proteins from amino acids.

• Catabolism: The breakdown of large molecules into smaller ones; releases energy. Example: breaking down glucose during cellular respiration.

Metabolic reactions occur in specific cellular locations:

• Initial steps often take place in the cytosol.

• Further breakdown and energy production occur in the mitochondria.

Major Pathways of Catabolism

• Lipolysis: Breakdown of fats.

• Proteolysis: Breakdown of proteins.

• Glycolysis: Breakdown of glucose.

Cellular Respiration and Energy Production

Overview of Glucose Catabolism

The complete oxidation of glucose to carbon dioxide and water is the primary source of ATP in most cells. This process involves several stages:

• Glycolysis (Anaerobic): Occurs in the cytosol; does not require oxygen.

• Krebs Cycle (Citric Acid Cycle) (Aerobic): Occurs in the mitochondria; requires oxygen.

• Electron Transport Chain (ETC) (Aerobic): Occurs in the inner mitochondrial membrane; requires oxygen.

Overall reaction:

$ext{C}_6 ext{H}_{12} ext{O}_6 + 6 ext{O}_2 ightarrow 6 ext{CO}_2 + 6 ext{H}_2 ext{O}$

One molecule of glucose yields approximately 30–32 molecules of ATP.

Glycolysis

Glycolysis is the process of breaking down one glucose molecule (C6H12O6) into two molecules of pyruvate (each with three carbons). This process occurs in the cytosol and does not require oxygen.

• Involves a series of enzymatic steps.

• Requires the input of two ATP molecules.

• Produces four ATP molecules (net gain: two ATP).

• Generates two NADH molecules.

Key reaction:

$ext{Glucose} + 2 ext{NAD}^+ + 2 ext{ADP} + 2 ext{P}_i ightarrow 2 ext{Pyruvate} + 2 ext{NADH} + 2 ext{ATP}$

Phosphorylation Steps in Glycolysis

• A phosphate group is attached to the sixth carbon of glucose, forming glucose-6-phosphate.

• Another ATP is used to add a second phosphate group, forming fructose-1,6-bisphosphate.

• Subsequent steps lead to the formation of pyruvate and the production of ATP and NADH.

Fate of Pyruvate

• If oxygen is low: Pyruvate is converted to lactate (lactic acid) in the cytosol; NADH is oxidized to NAD+ (anaerobic conditions).

• If oxygen is adequate: Pyruvate enters the mitochondria for further oxidation in the Krebs cycle (aerobic conditions).

Oxidation and Reduction Reactions

Definitions and Importance

Oxidation-reduction (redox) reactions are essential for energy transfer in cells. These reactions involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons, gain of oxygen, or loss of hydrogen; decrease in potential energy.

• Reduction: Gain of electrons, loss of oxygen, or gain of hydrogen; increase in potential energy.

• Oxidation and reduction are always paired (one molecule is oxidized, another is reduced).

Key coenzymes:

• NAD (Nicotinamide adenine dinucleotide): Accepts electrons and hydrogen atoms.

• FAD (Flavin adenine dinucleotide): Also accepts electrons and hydrogen atoms.

Krebs Cycle (Citric Acid Cycle)

Overview

The Krebs cycle is a series of reactions in the mitochondria that oxidize pyruvate, producing NADH, FADH2, CO2, and a small amount of ATP (as GTP).

• Each cycle yields:

  • 3 NADH

  • 1 FADH2

  • 2 CO2

  • 1 GTP (converted to ATP)

• Low direct ATP yield; most energy captured in NADH and FADH2.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

Mechanism

The ETC is located in the inner mitochondrial membrane and consists of a series of protein complexes and cytochromes that transfer electrons from NADH and FADH2 to oxygen.

• Electrons are passed through complexes I-IV, ultimately reducing oxygen to water.

• Proton gradient is created across the inner membrane.

• ATP synthase uses this gradient to produce ATP from ADP and inorganic phosphate.

Key reaction:

$ext{ADP} + ext{P}_i ightarrow ext{ATP}$

Summary Table: Major Steps and Products of Cellular Respiration

Stage	Location	Oxygen Required?	Main Products
Glycolysis	Cytosol	No	2 ATP, 2 NADH, 2 Pyruvate
Krebs Cycle	Mitochondrial Matrix	Yes	2 ATP (as GTP), 6 NADH, 2 FADH2, 4 CO2
Electron Transport Chain	Inner Mitochondrial Membrane	Yes	~26-28 ATP, 6 H2O

Anabolism: Gluconeogenesis and Glycogen Metabolism

Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors, such as amino acids, lactate, and glycerol. This process is essential when glucose levels are low and is irreversible.

• Occurs mainly in the liver.

• Uses substrates like pyruvate, lactate, glycerol, and certain amino acids.

• Cannot use fatty acids, leucine, or lysine for glucose synthesis.

Glycogenesis and Glycogenolysis

• Glycogenesis: Formation of glycogen from glucose when glucose levels are high; occurs in liver and skeletal muscle.

• Glycogenolysis: Breakdown of glycogen to release glucose when energy is needed.

Glycogenesis uses uridine triphosphate (UTP) to activate glucose for polymerization into glycogen. Glycogenolysis releases glucose for metabolic needs.

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The primary energy currency of the cell.

• NADH and FADH2: Electron carriers that store energy for the ETC.

• Pyruvate: End product of glycolysis; can be further metabolized aerobically or anaerobically.

• Coenzymes: Organic molecules that assist enzymes in catalyzing reactions (e.g., NAD, FAD).

Example Application: During intense exercise, muscle cells rely on anaerobic glycolysis for rapid ATP production, leading to lactate accumulation. In contrast, during rest or moderate activity, aerobic pathways predominate, maximizing ATP yield from glucose.

Additional info: Some details, such as the specific steps of glycolysis and the full mechanism of the ETC, were expanded for academic completeness.