energy, metabolism and enzymes

Metabolism refers to the sum of all chemical reactions occurring within a cell.
It encompasses both the breakdown of food (digestion) and the utilization of that food to synthesize new cellular substances.
It serves as a transformation of matter and energy in accordance with the laws of thermodynamics.

A metabolic pathway is a series of biochemical steps where the product of one reaction becomes the substrate for the next reaction. Each step is mediated by a specific enzyme.
- Diagram illustrating a metabolic pathway:
- Enzyme 1 → Enzyme 2 → Enzyme 3 → Enzyme 4
- Initial substrate → Intermediate substrate A → Intermediate substrate B → Intermediate substrate C → End product.

Catabolic Pathways: These pathways break down large molecules into smaller ones, liberating energy in the process.
Anabolic Reactions: These pathways consume energy to assemble complex molecules from simpler ones.
Free Energy (G) Changes:
- Catabolic Reaction: Energy is released; products have less free energy than reactants.
- Anabolic Reaction: Energy is consumed; products have more free energy than reactants.

Energy is defined as the capacity to induce change; all chemical reactions in biological systems involve energy transformations.

Two main states:
1. Kinetic Energy: The energy of motion.
2. Potential Energy: The stored energy in an object.

Energy flows into biological systems primarily from the sun:
- Photosynthesis process:
- Sunlight + 6CO₂ (Carbon Dioxide) + 6H₂O (Water) → C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen).
Photosynthetic organisms convert sunlight into stored chemical energy, which is stored in chemical bonds of glucose.

Chemical energy is the energy stored in the chemical bonds of substances. It is released during chemical reactions, often leading to the emission of heat.
Example: Food, as a form of chemical energy, illustrates this concept with glucose (C₆H₁₂O₆) being a specific instance.

Thermodynamics is the study of energy transfer in natural systems. It looks at how energy is transformed from one form to another across different systems.

States that energy cannot be created or destroyed but can only change forms. This principle is known as the conservation of energy.
- Total energy in the universe remains constant.
- During energy transformations, some energy is dissipated as heat.

States that every energy transfer increases the entropy (or disorder) of the universe.
- Energy transformations tend to proceed in a manner that increases the overall disorder. Some energy is typically rendered unusable in each transformation, often lost as heat.

Entropy (S): A measure of disorder within a system; as time progresses, the entropy of a closed system increases unless external work is done to restore order.
Enthalpy (H): Represents the total heat content of a system; it reflects the total energy available in a system. The relationship of changes in enthalpy and entropy determines the spontaneity of processes.

Gibbs free energy (G) is a thermodynamic quantity that represents the maximum reversible work obtainable from a system at constant temperature and pressure.
It is used in predicting the spontaneity of chemical reactions:
- Formula: G = H - TS
- Where:
  - H = Enthalpy
  - T = Absolute Temperature (Kelvin)
  - S = Entropy

Exergonic Reactions: Reactions that release energy; they have a negative Gibbs free energy change (ΔG < 0) and proceed spontaneously.
- Example: Cellular respiration, where glucose is broken down into CO₂ and H₂O releasing energy.
Endergonic Reactions: Reactions that require energy; they have a positive Gibbs free energy change (ΔG > 0) and are not spontaneous without an external energy source.
- Example: The synthesis of glucose from CO₂ and H₂O, which absorbs energy.

Enzymes are macromolecules, primarily proteins (some are RNA), that act as biological catalysts to facilitate chemical reactions by lowering the activation energy required.

A substrate (e.g., sucrose) binds to the active site of an enzyme (e.g., sucrase) forming an enzyme-substrate complex.
This binding induces changes that facilitate the breaking of bonds (e.g., glucose-fructose bond), leading to the formation of products.
The enzyme is then free to interact with new substrate molecules.

Enzymes are influenced by cellular conditions such as temperature, pH, and salinity.
Enzyme reactions are affected by:
- Concentration of substrate and enzyme.
- Specificity for their substrates.

Competitive Inhibition: An inhibitor competes with the substrate for binding to the active site of the enzyme.
Noncompetitive Inhibition: An inhibitor binds to an enzyme at a location other than the active site, causing changes that hinder substrate binding.
Feedback Inhibition: The accumulation of the end product of a metabolic pathway inhibits an earlier step in that pathway to prevent waste of resources.

These complexes allow enzymes to work efficiently by channeling the product to the next enzyme while minimizing unwanted side reactions, enabling efficient control of biochemical pathways.