Cellular Energy and Enzyme Function: Study Notes

Cellular Energy and Enzyme Function

Thermodynamics in Biology

Thermodynamics is the study of energy transformations in biological systems. The laws of thermodynamics govern how energy is transferred and transformed in living organisms.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

• Free Energy Change (ΔG): The change in free energy of a reaction determines whether it can occur spontaneously. If ΔG is negative, the reaction is spontaneous; if positive, it is non-spontaneous.

• Exergonic vs. Endergonic Reactions: Exergonic reactions release energy (ΔG < 0), while endergonic reactions require energy input (ΔG > 0).

Example: Cellular respiration is an exergonic process, while photosynthesis is endergonic.

ATP and Cellular Work

Adenosine triphosphate (ATP) is the primary energy currency of the cell. It powers cellular work by coupling exergonic reactions (which release energy) to endergonic reactions (which require energy).

• ATP Structure: ATP consists of adenine, ribose, and three phosphate groups.

• ATP Hydrolysis: The breakdown of ATP to ADP and inorganic phosphate releases energy that can be used for cellular work.

• Coupling Reactions: Cells use the energy released from ATP hydrolysis to drive endergonic processes such as active transport and biosynthesis.

Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Enzymes and Biological Catalysis

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy barrier.

• Enzyme Structure: Enzymes are proteins with specific three-dimensional shapes that determine their function.

• Active Site: The region of the enzyme where substrate molecules bind and undergo a chemical reaction.

• Enzyme Specificity: Each enzyme catalyzes a specific reaction due to the unique shape of its active site.

• Induced Fit Model: The enzyme changes shape slightly to fit the substrate more closely upon binding.

• Factors Affecting Enzyme Activity: Temperature, pH, cofactors, and inhibitors can influence enzyme function.

Example: The enzyme sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Regulation of Enzyme Activity and Metabolism

Cells regulate enzyme activity to control metabolic pathways and maintain homeostasis.

• Allosteric Regulation: Enzyme activity is regulated by molecules that bind to sites other than the active site, causing conformational changes.

• Feedback Inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction.

• Example (Feedback Inhibition): In isoleucine biosynthesis, isoleucine acts as an allosteric inhibitor of the first enzyme in its pathway, thus regulating its own production.

Table: Comparison of Exergonic and Endergonic Reactions

Property	Exergonic Reaction	Endergonic Reaction
Energy Change (ΔG)	Negative (ΔG < 0)	Positive (ΔG > 0)
Spontaneity	Spontaneous	Non-spontaneous
Energy Flow	Releases energy	Requires energy input
Example	Cellular respiration	Photosynthesis

Additional info: The isoleucine feedback inhibition example is a classic case of metabolic regulation, ensuring resources are not wasted in overproduction of amino acids.