Cell Biology Study Guide: Metabolism, ATP, and Enzyme Kinetics

Energy and Thermodynamics in Cells

Thermodynamics

Thermodynamics is the study of energy and its transformations in biological systems. It is fundamental to understanding how cells obtain, use, and transfer energy.

• Energy: The capacity to do work or cause change.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• Second Law of Thermodynamics: Entropy (disorder) increases in spontaneous processes.

• Kinetic Energy: Energy of motion (e.g., movement of molecules).

• Potential Energy: Stored energy (e.g., chemical bonds).

Example: ATP hydrolysis releases energy for cellular work.

Free Energy and Spontaneity

Free energy (Gibbs free energy) determines whether a reaction is spontaneous. The change in free energy ($\Delta G$) predicts the direction of chemical reactions.

• Exergonic Reaction: Releases energy; $\Delta G < 0$; spontaneous.

• Endergonic Reaction: Requires energy input; $\Delta G > 0$; non-spontaneous.

• Formula: $\Delta G = \Delta H - T\Delta S$

Example: Cellular respiration is exergonic; photosynthesis is endergonic.

ATP: Structure and Function

Adenosine Triphosphate (ATP)

ATP is the primary energy carrier in cells. It consists of adenine, ribose, and three phosphate groups.

• Structure: Adenine (nitrogenous base), ribose (sugar), three phosphates.

• Function: Stores and transfers energy for cellular processes.

• ATP Hydrolysis: ATP + H2O → ADP + Pi + energy

Example: Muscle contraction, active transport, biosynthesis.

Metabolic Pathways and Enzyme Function

Types of Metabolic Pathways

Metabolism includes all chemical reactions in cells, divided into catabolic and anabolic pathways.

• Catabolic Pathways: Breakdown molecules, release energy (e.g., glycolysis).

• Anabolic Pathways: Build molecules, require energy (e.g., protein synthesis).

Example: Glycolysis (catabolic), DNA synthesis (anabolic).

Enzymes: Biological Catalysts

Enzymes are proteins that speed up biochemical reactions by lowering activation energy.

• Active Site: Region where substrate binds and reaction occurs.

• Specificity: Enzymes are specific to substrates.

• Induced Fit: Enzyme changes shape to fit substrate.

Example: Amylase catalyzes starch breakdown.

Enzyme Regulation and Kinetics

Enzyme Regulation

Enzyme activity is regulated by various mechanisms to control metabolic pathways.

• Allosteric Regulation: Binding of molecules at sites other than the active site alters enzyme activity.

• Feedback Inhibition: End product inhibits an earlier step in the pathway.

• Positive Regulation: Activators increase enzyme activity.

Example: ATP inhibits phosphofructokinase in glycolysis.

Enzyme Kinetics

Enzyme kinetics studies the rates of enzyme-catalyzed reactions. The Michaelis-Menten equation describes the relationship between substrate concentration and reaction rate.

• Michaelis-Menten Equation: $v = \frac{V_{max}[S]}{K_m + [S]}$

• Vmax: Maximum reaction rate.

• Km: Substrate concentration at half Vmax; indicates enzyme affinity.

Example: Enzyme kinetics experiments measure Vmax and Km.

Enzyme Inhibition

Enzyme inhibitors decrease enzyme activity. There are two main types: competitive and non-competitive.

• Competitive Inhibition: Inhibitor binds to active site, blocking substrate.

• Non-Competitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape.

Example: Drugs often act as enzyme inhibitors.

Cooperativity and Temperature Effects

Cooperativity

Some enzymes show cooperativity, where binding of one substrate affects binding of others.

• Positive Cooperativity: Binding increases affinity for additional substrates.

• Negative Cooperativity: Binding decreases affinity.

Example: Hemoglobin shows positive cooperativity in oxygen binding.

Temperature Coefficient (Q10)

Q10 measures the rate change of a reaction with a 10°C temperature increase.

• Formula: $Q_{10} = \frac{R_2}{R_1}$ where R2 and R1 are rates at different temperatures.

• Application: Used to compare metabolic rates in organisms.

Example: Fish metabolism increases with temperature.

Enzyme Nomenclature

Classification and Types

Enzymes are classified by the reactions they catalyze. Common types include:

• Hydrolases: Catalyze hydrolysis reactions (e.g., proteases).

• Kinases: Transfer phosphate groups (e.g., ATP kinases).

• Isomerases: Catalyze isomerization (e.g., glucose to fructose).

Example: ATP synthase synthesizes ATP from ADP and Pi.

Summary Table: Enzyme Types and Functions

Enzyme Type	Function	Example
Hydrolase	Hydrolysis of bonds	Protease
Kinase	Phosphate transfer	Hexokinase
Isomerase	Isomerization	Phosphoglucoisomerase
Ligase	Bond formation	DNA ligase
Oxidoreductase	Redox reactions	Lactate dehydrogenase

Additional info: Enzyme nomenclature is based on substrate and reaction type.