Topic 7: Energy and Enzymes

Thermodynamics in Biology

Thermodynamics is the study of energy transformations in matter. In biological systems, it helps explain how cells obtain, use, and transfer energy to sustain life.

First Law of Thermodynamics

• Definition: The total amount of energy in the Universe is constant.

• Corollary: Energy can be transferred or transformed, but never created or destroyed.

• Every living cell must find a source of energy to sustain its operation.

Second Law of Thermodynamics

• Definition: The entropy of the Universe as a whole tends to increase with time.

• Corollary: Cells keep their entropy low, but energy transformations increase the entropy of their surroundings.

• Entropy is often analogized as "disorder" in a system.

Free Energy and Chemical Reactions

Biological reactions are driven by changes in free energy, specifically Gibbs free energy (G).

• Reactants and products differ in their free energy content.

• Molecules only react if the change in free energy is favorable.

Exergonic vs. Endergonic Reactions

• Exergonic reactions: Release energy; products have less free energy than reactants.

• Endergonic reactions: Require energy input; products have more free energy than reactants.

Example: Sucrose Hydrolysis

Sucrose, a disaccharide made by plants, is broken down into glucose and fructose in the body.

• Reaction: Sucrose + H2O → Glucose + Fructose

• This reaction is exergonic ( kJ), but does not occur spontaneously in water.

• Enzyme required: Sucrase catalyzes the reaction in the digestive tract.

Role of Enzymes in Biological Reactions

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy (Ea).

• Most enzymes are proteins; some are RNA or combinations.

• Enzymes do not change the overall free energy change () of a reaction.

• Enzymes are named for their substrate + "-ase" (e.g., sucrase).

• Enzymes can be sensitive to environmental conditions, which may cause denaturation (loss of shape and function).

Activation Energy and Reaction Rates

• Even thermodynamically favorable reactions may not occur spontaneously if the activation energy is too high.

• Enzymes lower the activation energy, allowing reactions to proceed at biologically relevant rates.

Protein Denaturation

Proteins, including enzymes, can lose their functional shape due to heat or chemical changes.

• Denaturation: The process by which proteins lose their structure and function.

• Example: Cooking egg whites (albumen) changes their structure irreversibly.

Enzyme Mechanism and Specificity

• Enzymes bind specific substrates and catalyze specific reactions.

• Enzyme activity can be affected by inhibitors (competitive and non-competitive).

Endergonic Reactions and ATP

Cells use ATP (adenosine triphosphate) to drive endergonic reactions that require energy input.

• ATP consists of adenine (base), ribose (sugar), and three phosphate groups.

• Hydrolysis of ATP releases energy:

• ATP couples with endergonic reactions to make them energetically favorable.

Cellular Work Powered by ATP

ATP is used for various types of cellular work:

• Transport work: Moving solutes across membranes via transport proteins.

• Mechanical work: Moving vesicles along cytoskeletal tracks.

• ATP hydrolysis provides the energy for these processes.

Summary Table: Exergonic vs. Endergonic Reactions

Key Terms

• Thermodynamics: Study of energy transformations.

• Entropy: Measure of disorder in a system.

• Gibbs Free Energy (): Energy available to do work.

• Activation Energy (): Energy required to start a reaction.

• Enzyme: Biological catalyst that speeds up reactions.

• Denaturation: Loss of protein structure and function.

• ATP: Molecule that stores and transfers energy in cells.

Example Application

• When you eat sucrose, sucrase in your intestine breaks it down into glucose and fructose, which are then absorbed by cells using transport proteins powered by ATP.