Metabolism and Thermodynamics in Biology

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within an organism, enabling the transformation of matter and energy. It is an emergent property of life, arising from the orderly interactions between molecules.

• Metabolic Pathways: A series of chemical reactions where a specific molecule is altered stepwise to produce a product. Each step is catalyzed by a specific enzyme.

• Catabolic Pathways: Release energy by breaking down complex molecules into simpler compounds (e.g., cellular respiration).

• Anabolic Pathways: Consume energy to build complex molecules from simpler ones (e.g., synthesis of proteins from amino acids).

Metabolic Pathways in Detail

Metabolic pathways are interconnected, forming a complex network that regulates cellular function and energy flow.

• Enzymes: Biological catalysts that speed up reactions without being consumed.

• Example: The breakdown of glucose in cellular respiration is a catabolic pathway; the synthesis of glycogen from glucose is an anabolic pathway.

Forms of Energy in Biological Systems

Types of Energy

Energy is the capacity to cause change and exists in various forms relevant to biological processes.

• Kinetic Energy: Energy associated with motion (e.g., water turning turbines).

• Thermal Energy: Kinetic energy from random movement of atoms/molecules; transfer is called heat.

• Light Energy: Used in photosynthesis.

• Potential Energy: Energy due to location or structure (e.g., water behind a dam, arrangement of electrons in bonds).

• Chemical Energy: Potential energy available for release in a chemical reaction (e.g., glucose breakdown).

The Laws of Thermodynamics

First Law of Thermodynamics

The energy of the universe is constant; energy can be transferred and transformed, but not created or destroyed. This is also known as the principle of conservation of energy.

Second Law of Thermodynamics

Every energy transfer increases the entropy (disorder) of the universe. Some energy is lost as heat and becomes unavailable to do work. Living organisms increase the disorder of their surroundings through metabolism.

• Spontaneous Processes: Increase entropy and occur without energy input.

• Nonspontaneous Processes: Decrease entropy and require energy input.

Biological Order and Disorder

Cells create complex structures from simpler materials, demonstrating local order while increasing overall entropy.

Free Energy and Spontaneity

Free Energy Change (ΔG)

Free energy (G) is the portion of a system’s energy that can do work. The change in free energy during a reaction determines whether it is spontaneous.

• Equation: Where: = change in free energy = change in enthalpy (total energy) = change in entropy = temperature in Kelvin

• Spontaneous Reactions: is negative.

• Nonspontaneous Reactions: is zero or positive.

Exergonic and Endergonic Reactions

Chemical reactions are classified by their free-energy changes:

• Exergonic Reaction: Proceeds with a net release of free energy (), spontaneous.

• Endergonic Reaction: Absorbs free energy (), nonspontaneous.

ATP and Energy Coupling

ATP Structure and Function

ATP (adenosine triphosphate) is the cell’s energy currency, composed of ribose, adenine, and three phosphate groups. It couples exergonic and endergonic reactions, powering cellular work.

ATP Hydrolysis and Cellular Work

Energy is released when ATP’s terminal phosphate bond is broken by hydrolysis. This energy is used to drive endergonic reactions via phosphorylation, making recipient molecules more reactive.

The ATP Cycle

ATP is regenerated by phosphorylation of ADP, using energy from catabolic (exergonic) reactions. The ATP cycle couples energy-yielding and energy-consuming processes.

Enzymes and Activation Energy

Activation Energy Barrier

Every chemical reaction requires an initial input of energy to break bonds, known as activation energy (EA).

Enzymes as Biological Catalysts

Enzymes are proteins that lower the activation energy barrier, speeding up reactions without being consumed. They do not alter but make reactions occur faster.

Substrate Specificity and Catalytic Cycle

Enzymes bind to specific substrates at their active site, forming an enzyme-substrate complex. The induced fit model describes how enzymes change shape to enhance catalysis.

• Active Site: Region on the enzyme where substrate binds.

• Induced Fit: Enzyme changes shape to fit substrate, enhancing reaction.

Factors Affecting Enzyme Activity

Environmental Effects

Enzyme activity is influenced by temperature, pH, and specific chemicals (cofactors and inhibitors).

• Optimal Temperature: Each enzyme has a temperature at which it works best; beyond this, it denatures.

• Optimal pH: Varies by enzyme and environment (e.g., pepsin in stomach, trypsin in intestine).

• Cofactors: Nonprotein helpers; inorganic (metal ions) or organic (coenzymes, often vitamins).

Enzyme Inhibition

Enzyme inhibitors reduce enzyme activity:

• Competitive Inhibitors: Resemble substrate and bind to active site, blocking substrate.

• Noncompetitive Inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

*Additional info: The notes have been expanded with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.*