3.1

Metabolism & Energy

Metabolism: The sum of all chemical reactions that occur in the cell, encompassing both the processes that break down molecules to produce energy and those that construct cellular components from simpler precursors.
Metabolic Pathway: A sequential series of chemical reactions within a living organism, where each step is catalyzed by specific enzymes that lower the activation energy required for reactions to proceed.
- Metabolism can be divided into two categories:
  - Catabolism: The breakdown of complex molecules into smaller units, which releases energy. This process typically occurs during cellular respiration, where glucose and other organic substrates are oxidized to generate ATP.
  - Anabolism: The use of energy to synthesize larger molecules from smaller ones. Anabolic pathways, such as photosynthesis and protein synthesis, are essential for growth and development.

Energy

Definition: The capacity to perform work, which is crucial for maintaining life processes and cellular functions.
Types of Energy:
- Kinetic Energy: The energy of motion, which is involved in processes like muscle contraction and the movement of molecules.
- Potential Energy: Energy that is stored in a system or within chemical bonds, which can be transformed into kinetic energy during metabolic reactions.
- Work: Refers to the transfer of energy to move an object over a distance or to rearrange matter, occurring mainly through the transformation of potential energy into kinetic energy in living cells.

Laws of Thermodynamics

Applicability: Both laws apply to a defined system (object of study) and its surroundings (everything outside the system including the universe itself).

1st Law of Thermodynamics

Statement: Energy cannot be created or destroyed; it can only change forms or transfer between objects without a net loss. This principle of energy conservation is foundational in understanding energy transfer in biological systems.
- For instance, in cellular respiration, the energy released during the breakdown of glucose is transformed into ATP energy; any thermal energy released must be balanced by energy entering the surrounding environment.

Energy and Chemical Bonds

Chemical Energy: The potential energy stored in the arrangement of atoms within a compound, which can be released during chemical reactions.
Bond Energy: The amount of energy that is required to break a specific type of bond within a molecule, typically measured in kilojoules per mole (kJ/mol). Unbonded atoms possess more chemical energy than when they are part of a compound due to the stability gained through bonding.

2nd Law of Thermodynamics

Statement: The entropy of the universe always increases in any energy transfer or transformation process; energy transfers are never 100% efficient. This law implies that natural processes tend to progress towards a state of greater disorder and randomness.
- Useful energy tends to dissipate into unusable heat or random particle motion, leading to an overall increase in entropy within a system.

Entropy (S)

Definition: A quantitative measure of disorder or randomness within a system; every energy transformation that occurs also leads to an increase in overall entropy in the universe.
Systems naturally progress toward states that have higher entropy, demonstrating a tendency towards disorder and a more relaxed state.

Free Energy

Definition: The amount of energy available to perform useful work within a system after accounting for energy lost due to entropy generation.
Gibbs Free Energy (G): A key thermodynamic quantity, calculated as G = H - TS (where H is enthalpy, T is temperature in Kelvin, and S is entropy). Changes in Gibbs free energy help predict whether a reaction will occur spontaneously.

Endergonic & Exergonic Reactions

Exergonic Reactions:
- Characterized by the release of energy, making these reactions spontaneous under standard conditions.
- These reactions have a negative change in free energy (ΔG < 0), indicating that the products contain less free energy than the reactants.
Endergonic Reactions:
- Require an input of energy, making them non-spontaneous without a continual source of energy.
- These reactions have a positive change in free energy (ΔG > 0), indicating that the products have more free energy than the reactants.

Thermodynamics & Metabolism

A spontaneous reaction does not imply it occurs immediately; the rate might be slow if the activation energy is high.
Biological catalysts, known as enzymes, play a vital role in metabolic pathways by lowering the activation energy needed for reactions, thereby increasing the reaction rates and efficiency of metabolism.

Adenosine Triphosphate (ATP)

Structure: A nucleotide composed of adenine, a ribose sugar, and three phosphate groups linked by high-energy bonds.
Function: ATP acts as the primary energy currency of cells, providing the energy needed for various cellular reactions and processes, including muscle contraction, ion transport, and biosynthesis.
ATP Hydrolysis: The reaction catalyzed by the enzyme ATPase that converts ATP into ADP (adenosine diphosphate) and inorganic phosphate, releasing approximately -30.6 kJ/mol of energy that can then be utilized in biological work.

Redox Reactions

Definition: Chemical reactions involving the transfer of electrons between substances, fundamental for cellular energy processes; redox reactions release energy stored in organic molecules used for the synthesis of ATP.
Oxidation: Defined as the loss of electrons; conversely, Reduction signifies the gain of electrons.
Mnemonic Devices: OIL RIG (Oxidation Is Loss, Reduction Is Gain) and LEO (Lose Electrons = Oxidation; Gain Electrons = Reduction) serve as helpful tools for remembering these definitions.

Electron Transfer

Each redox reaction consists of an electron donor (reducing agent) and an electron acceptor (oxidizing agent).
Energy transfer during oxidation-reduction reactions involves energy inputs to detach electrons from atoms, often requiring more energy for more electronegative atoms.
The release of energy in redox reactions is crucial for driving ATP synthesis during cellular respiration.

Redox & Cellular Respiration

During the oxidation of glucose, electrons are transferred to oxygen, facilitating the release of energy that is harnessed for producing ATP.
Organic molecules that are rich in hydrogen serve as excellent fuels, providing significant energy during their oxidation processes.
The transfer of electrons along an energy gradient to oxygen ultimately releases energy used for the synthesis of ATP, the energy currency of cells.

Electron Carriers

NAD+: A key electron acceptor in cellular respiration, which functions by carrying high-energy electrons generated during the breakdown of glucose and other substrates. Each NADH molecule formed can ultimately contribute to ATP production during oxidative phosphorylation, playing a pivotal role in energy metabolism.

Metabolism involves all chemical reactions in cells, divided into:

Catabolism: Energy-releasing breakdown of molecules.
Anabolism: Energy-consuming synthesis of larger molecules.

Energy Types:

Kinetic: Energy of motion.
Potential: Stored energy.

Thermodynamics:

1st Law: Energy cannot be created or destroyed, only transformed.
2nd Law: Entropy increases in energy transfers.

Free Energy: Drives cellular work, calculated by Gibbs free energy (G = H - TS).

Exergonic Reactions: Release energy (ΔG < 0).
Endergonic Reactions: Consume energy (ΔG > 0).

Enzymes: Catalyze reactions, enhancing metabolic pathways.

ATP: Primary energy currency of cells.

Redox Reactions: Involve electron transfers, crucial for energy release in cellular respiration, supported by carriers like NAD+.