Introduction to Metabolism

Introduction to Metabolism

• Metabolism is the totality of an organism's chemical reactions
• Metabolism transforms matter and energy, subject to the laws of thermodynamics
• A metabolic pathway starts with a specific molecule and ends with a specific product, each step catalysed by a specific enzyme.

Metabolic Pathways

• Catabolic pathways release energy by breaking down complex molecules into simpler compounds
• Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of catabolism
• Anabolic pathways consume energy to build complex molecules from simpler ones
• The synthesis of protein from amino acids is an example of anabolism
• Bioenergetics is the study of how organisms manage their energy resources

Forms of Energy

• Energy is the capacity to cause change
• Energy exists in various forms, some of which perform work
• Kinetic energy is energy associated with motion
• Thermal energy (heat) is kinetic energy associated with random movement of molecules or atoms
• Potential energy is energy that matter possesses due to its location or structure
• Chemical energy is potential energy available for release in a chemical reaction
• Energy can be converted from one form to another

What Can Cells Do With Energy?

• Cells use energy for:
  • Chemical work (forming or breaking chemical bonds)
  • Mechanical work (moving things within the cell via cellular components)
  • Transport work (moving substances across a cell membrane)

The Laws of Energy Transformation

• Thermodynamics is the study of energy transformations
• A closed system is isolated from its surroundings
• In an open system, energy and matter can be transferred between the system and its surroundings
• Organisms are open systems

The First Law of Thermodynamics

• The first law of thermodynamics states that the energy of the universe is constant
• Energy can be transferred and transformed, but it cannot be created or destroyed

The Second Law of Thermodynamics

• The second law of thermodynamics states that every energy transfer or transformation increases the entropy (disorder) of the universe
• During every energy transfer or transformation, some energy is unusable and is often lost as heat

Exergonic and Endergonic Reactions in Metabolism

• Exergonic reactions proceed with a net release of free energy and are spontaneous
• Endergonic reactions absorb free energy from their surroundings and are nonspontaneous

Another Way to View the Reactions

• Exergonic reactions release energy as they proceed (ΔG < 0)
• Endergonic reactions require energy input to proceed (ΔG > 0)

The Importance of ATP in Metabolism

• ATP (Adenosine triphosphate) is composed of ribose sugar, adenine, and three phosphate groups
• Cells use energy coupling, which is mediated by ATP to do work

Coupled Reactions

• ATP powers cellular work by transferring a phosphate group from ATP to another molecule
• This exergonic reaction is coupled with an endergonic reaction
• The overall effect is that the coupled reactions are exergonic (release energy).

ATP

• ATP drives endergonic reactions by transferring a phosphate group
• This process forms a phosphorylated intermediate, which is a molecule storing energy from ATP for later use.

The Regeneration of ATP

• ATP is reformed by the addition of a phosphate group to ADP
• The energy for this reaction comes from catabolic reactions in the cell

REDOX Reactions

• Oxidation is the process of losing electrons, while reduction is gaining electrons
• Many important reactions within cells involve the transfer of electrons through REDOX reactions
• Enzymes involved in redox reactions include dehydrogenases and oxidases.

Electron Transport Chains

• A chain of redox reactions can occur, where the product of one redox reaction is the reactant for the next
• These transfer electrons and release energy

ATP Production

• ATP is produced during electron transport along a chain