Chapter 4.1-4.4

Chapter 4: Cellular Metabolism

4.1 Metabolic Reactions

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

  • Types of metabolism:

  1. Anabolism: Synthesis of larger molecules from smaller ones, requiring energy (ATP).

  2. Catabolism: Breakdown of larger molecules into smaller ones, producing energy (ATP).

Anabolism

• Definition: The metabolic pathway that constructs molecules from smaller units. It requires energy in the form of ATP.

• Functions:

  • Provides materials necessary for cell maintenance, growth, and repair.

• Dehydration Synthesis:

  • Process in which water (H₂O) is produced during the formation of larger molecules.

  • Used to produce:

  • Polysaccharides

  • Proteins

  • Triglycerides

Anabolism: Triglycerides

• Chemical Structure:

  • Depicted as:

  • Glycerol combined with 3 fatty acid molecules to form a triglyceride (fat) molecule.

  • Example Reaction:

  • ext{Glycerol} + 3 ext{ fatty acids}
    ightarrow ext{Triglyceride} + 3 ext{H}_2 ext{O}

• Visual Representation:

  • Structure includes a glycerol backbone and three long-chain fatty acids attached.

Anabolism: Proteins

• Peptide Bond Formation:

  • Amino acids combine to form dipeptides through dehydration synthesis.

  • Water (H₂O) is released during the formation of peptide bonds.

• Example Reaction:

  • ext{Amino acid} + ext{Amino acid}
    ightarrow ext{Dipeptide} + ext{H}_2 ext{O}

Catabolism

• Definition: The metabolic pathway that breaks down larger molecules into smaller ones, releasing energy (ATP).

• Hydrolysis:

  • Uses water (H₂O) to split larger substances into smaller units.

  • This is the reverse of dehydration synthesis.

• Processes:

  • Breakdown of carbohydrates into monosaccharides.

  • Breakdown of triglycerides into glycerol and fatty acids.

  • Breakdown of proteins into amino acids.

4.2 Control of Metabolic Reactions

• Importance: The rates of catabolic and anabolic reactions must be carefully controlled to avoid cellular damage or death.

• Role of Enzymes:

  • Enzymes are globular proteins that catalyze specific reactions and increase the rate of chemical reactions.

  • Activation Energy: Enzymes lower the activation energy necessary to initiate reactions, allowing them to occur more easily.

  • Reusability: Enzymes are not consumed in reactions and can be reused multiple times.

• Substrate Specificity:

  • Each enzyme is specific to a particular substrate based on the shape of its active site.

  • Many enzymes are named after their substrates with the suffix “-ase”; for example:

  • Lipase: Breaks down lipids.

  • Lactase: Breaks down lactose.

Enzyme-Driven Metabolic Pathways

• Sequence: A series of enzyme-controlled reactions leads to the formation of a product.

  • Each substrate is the product of the previous reaction.

  • Each step in a pathway is catalyzed by different enzymes.

• Rate-Limiting Enzyme: The slowest step in a pathway that sets the rate for the entire sequence and controls the flow of molecules. Often the first enzyme in the reaction sequence, its activity can be inhibited by the product in a negative feedback loop.

4.3 Energy for Metabolic Reactions

• Energy Source: Metabolic reactions utilize chemical energy stored in ATP.

• Oxidation of Glucose:

  • Involves the loss of hydrogen atoms (along with electrons).

  • Bonds break, releasing energy that is utilized for anabolic reactions.

  • Enzymes help reduce the activation energy required for oxidation during cellular respiration.

Release of Chemical Energy

• Cellular Respiration:

  • Energy released from the breaking of glucose bonds is transferred to other molecules.

  • Cells capture approximately 40% of the released energy to synthesize ATP; the remaining 60% is lost as heat.

ATP Molecules

• Definition: The primary energy-carrying molecule in the cell.

• Structure:

  • Consists of three components:

  1. Adenine (a nitrogenous base)

  2. Ribose (a five-carbon sugar)

  3. Three phosphate groups in a chain, where the second and third phosphates are bonded by high-energy bonds.

• Energy Release:

  • Energy is needed to break the terminal phosphate bond, releasing chemical energy for cellular processes.

• Recycling of ATP:

  • ATP can lose its terminal phosphate to form Adenosine Diphosphate (ADP), and the cycle continues through phosphorylation generated during cellular respiration where the phosphate group can be added back.

4.4 Cellular Respiration

• Overview: Contains distinct but interconnected catabolic reaction sequences:

  1. Glycolysis: A stepwise reaction occurring in the cytosol.

  2. Citric Acid Cycle: A metabolic cycle.

  3. Electron Transport Chain: A stepwise reaction.

• Requirements: Cellular respiration of glucose requires a supply of glucose and oxygen.

• Final Products of Cellular Respiration

  • Carbon dioxide (CO₂)

  • Water (H₂O)

  • ATP (40% chemical energy)

  • Heat (60%)

• Types of Reactions:

  • Anaerobic Reactions: Do not require O₂ and produce minimal ATP.

  • Aerobic Reactions: Require O₂ and produce the majority of ATP.

Glycolysis

• Definition: The process of breaking down glucose.

• Location: Occurs in the cytosol of the cell.

• Process: Includes 10 enzyme-catalyzed reactions resulting in:

  • Breakdown of 6-carbon glucose into two 3-carbon pyruvic acid molecules.

  • Does not require oxygen (anaerobic phase).

  • Yield of 2 ATP molecules per glucose molecule.

Phases of Glycolysis

1. Phosphorylation: Conversion of glucose into its phosphorylated form.

2. Splitting/Cleavage: Division of glucose into two 3-carbon molecules.

3. Production Phase: Generation of NADH, 4 ATP, and 2 molecules of pyruvic acid.

Electron/Hydrogen Carriers

• Function: During reactions, hydrogen is released (which contains energy).

  • Hydrogen atoms are transferred as pairs (2 protons and 2 electrons) to hydrogen carrier molecules NAD+ or FAD+ to form NADH and FADH2.

  • NADH/FADH2 carry these atoms with high energy electrons to the electron transport chain where oxygen acts as the final electron acceptor.

Anaerobic Reactions

• Conditions: Occur when no oxygen is present.

  • Cannot proceed to the electron transport chain as there’s no place to unload hydrogen atoms.

  • During this process, NADH and H+ return electrons and protons back to pyruvic acid to form lactic acid.

• Yield: There is a net gain of 2 ATP per glucose molecule (2 ATP are needed to start and 4 ATP are generated). Although inefficient, this process is still beneficial.

Aerobic Reactions

• Conditions: Occur in the presence of oxygen, allowing pyruvic acid to proceed through aerobic pathways.

• Efficiency: More energy-efficient with three main events:

  1. Formation of Acetyl CoA

  2. Citric Acid Cycle

  3. Electron Transport Chain (Oxidative Phosphorylation)

Citric Acid Cycle

• Initiation: Begins when acetyl CoA combines with oxaloacetic acid to produce citric acid.

• Cycle Dynamics: Citric acid is transformed back into oxaloacetic acid through a series of enzymatic reactions, allowing the cycle to repeat, provided pyruvic acid and O2 are available.

• Yield per Citric Acid Molecule (2 produced per glucose):

  • 1 ATP, 8 hydrogen atoms transferred to NAD+ and FAD, and 2 CO₂ released which enter the bloodstream for exhalation.

Electron Transport Chain (ETC)

• Role: NADH and FADH2 deliver hydrogen and high-energy electrons to the electron transport chain, which consists of a series of enzyme complexes located in the inner membrane of mitochondria.

• Energy Transfer: Energy from electrons is used to power ATP synthase, which phosphorylates ADP to form ATP. Water (H₂O) is generated as a byproduct (with oxygen as the final electron acceptor).

Summary of ATP Production in Complete Oxidation of Glucose

• Total Yield:

  • 2 ATP produced in glycolysis.

  • 2 ATP produced in citric acid cycle.

  • 28 ATP produced in the electron transport chain.

• Overall Total: 32 ATP produced from the complete oxidation of one glucose molecule.