Energy and Metabolism in Living Systems

The Energy of Life

Cells are dynamic chemical factories where thousands of reactions occur, extracting energy from nutrients and converting it to forms usable for cellular work. Metabolism encompasses all chemical reactions within an organism, enabling life processes.

• Cellular respiration extracts energy stored in sugars and other fuels.

• Cells use this energy to perform work, such as movement and synthesis.

• Some organisms can convert energy to light (e.g., bioluminescence).

Organization of Metabolic Pathways

Metabolic pathways are sequences of chemical reactions, each catalyzed by a specific enzyme, that transform molecules within cells.

• Anabolic pathways: Build complex molecules from simpler ones (e.g., protein synthesis).

• Catabolic pathways: Break down complex molecules into simpler compounds (e.g., cellular respiration).

• Each step is catalyzed by a specific enzyme.

Forms and Laws of Energy

Forms of Energy

Energy is the capacity to cause change and exists in various forms:

• Kinetic energy: Energy of motion (e.g., heat is kinetic energy from molecular movement).

• Potential energy: Stored energy due to position or structure.

• Chemical energy: Potential energy available for release in chemical reactions.

The Laws of Thermodynamics

Thermodynamics governs energy transformations in biological systems.

• First law (Conservation of energy): Energy cannot be created or destroyed, only transformed from one form to another.

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

• Living cells convert organized forms of energy to heat, a more disordered form.

• Organisms are open systems, exchanging energy and matter with their surroundings.

Biological Order and Disorder

Organisms create ordered structures from less organized forms of energy and matter, but the overall entropy of the universe increases.

• Complex molecules are built from simpler ones, but energy release increases disorder.

• The evolution of complexity does not violate the second law, as total entropy increases.

Free Energy and Spontaneity

Free-Energy Change ()

Free energy () measures a system's capacity to do work. The change in free energy () during a process determines whether it occurs spontaneously.

• = change in enthalpy (total energy), = absolute temperature, = change in entropy.

• Spontaneous processes have and can perform work.

Free Energy, Stability, and Equilibrium

Systems move toward maximum stability (equilibrium). Free energy decreases as stability increases.

• At equilibrium, a system is at maximum stability and cannot perform work.

• Spontaneous processes move systems toward equilibrium.

Exergonic and Endergonic Reactions

Metabolic reactions are classified by their energy changes:

• Exergonic reaction: Releases free energy; spontaneous ().

• Endergonic reaction: Absorbs free energy; nonspontaneous ().

Equilibrium and Metabolism

Cells are open systems, maintaining a constant flow of materials and never reaching equilibrium, allowing continuous work.

• Metabolism is never at equilibrium; a cell at equilibrium is dead.

ATP: The Energy Currency of the Cell

Structure and Hydrolysis of ATP

ATP (adenosine triphosphate) is the main energy carrier in cells.

• Composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups.

• Hydrolysis of ATP releases energy by breaking the terminal phosphate bond.

• The energy released is used to drive cellular work.

How ATP Performs Work

ATP hydrolysis powers cellular work by transferring a phosphate group to other molecules (phosphorylation).

• Coupled reactions use ATP hydrolysis to drive endergonic processes.

• The recipient molecule becomes phosphorylated, increasing its reactivity.

The Regeneration of ATP

ATP is regenerated from ADP and inorganic phosphate via energy from catabolic reactions.

• The ATP cycle transfers energy from catabolic to anabolic pathways.

Enzymes: Biological Catalysts

Enzyme Function and Activation Energy

Enzymes are proteins that speed up chemical reactions by lowering the activation energy barrier ().

• Catalyst: A chemical agent that speeds up a reaction without being consumed.

• Enzymes do not affect ; they only hasten reactions that would occur eventually.

• Activation energy (): The initial energy needed to start a reaction.

How Enzymes Speed Up Reactions

Enzymes lower by binding substrates in their active site, orienting them correctly, and providing a favorable microenvironment.

• The active site is the region where the substrate binds.

• Enzyme-substrate complex forms, leading to product formation.

• Enzyme activity is highly specific.

Effects of Local Conditions on Enzyme Activity

Enzyme activity is influenced by environmental factors:

• Each enzyme has an optimal temperature and pH for function.

• Extreme conditions can denature enzymes, reducing activity.

Cofactors and Coenzymes

Cofactors are nonprotein helpers required for enzyme function.

• Inorganic cofactors: Metal ions (e.g., Mg2+).

• Organic cofactors: Coenzymes (e.g., vitamins).

Enzyme Inhibition

Enzyme activity can be regulated by inhibitors:

• Competitive inhibitors: Bind to the active site, competing with the substrate.

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

• Some inhibitors are toxins, poisons, pesticides, or antibiotics.

Allosteric Regulation of Enzymes

Allosteric regulation involves molecules binding to sites other than the active site, affecting enzyme activity.

• Can inhibit or stimulate enzyme activity.

• Most allosterically regulated enzymes are made from multiple subunits.

• Cooperativity: Substrate binding to one active site increases activity at other sites.

Feedback Inhibition

Feedback inhibition prevents a cell from wasting resources by shutting down metabolic pathways when enough product is made.

• The end product of a pathway inhibits an earlier step.

Key Terms and Definitions

• Anabolic pathway: Consumes energy to build complex molecules from simpler ones.

• Catabolic pathway: Releases energy by breaking down complex molecules.

• Exergonic reaction: Releases energy; spontaneous.

• Endergonic reaction: Absorbs energy; nonspontaneous.

• Substrate: The reactant an enzyme acts on.

• Active site: Region on enzyme where substrate binds.

• Cofactor: Nonprotein molecule required for enzyme function.

• Coenzyme: Organic cofactor (often vitamins).

• Competitive inhibitor: Substance that reduces enzyme activity by competing for the active site.

• Noncompetitive inhibitor: Substance that reduces enzyme activity by binding elsewhere and altering enzyme shape.

• Feedback inhibition: End product inhibits an earlier step in a pathway.

Comparison Table: Exergonic vs. Endergonic Reactions

Example: ATP Hydrolysis

• ATP hydrolysis:

• Energy released is used for cellular work, such as muscle contraction and active transport.

Additional info:

• Enzyme specificity is due to the unique shape of the active site, which fits only specific substrates.

• Environmental factors such as temperature and pH can denature enzymes, affecting their function.

• Allosteric regulation is a key mechanism for controlling metabolic pathways in response to cellular needs.