Carbohydrate Metabolism: Bioenergetics and Pathways

Introduction to Carbohydrate Metabolism

• The course will cover bioenergetics and carbohydrate metabolism.
• Specific metabolic pathways to be discussed include glycolysis, fructose and galactose metabolism, gluconeogenesis, and the hexose monophosphate shunt (HMP).
• The tricarboxylic acid cycle (TCA) will also be covered.
• Glycogen metabolism is a key topic, including glycogenesis and glycogenolysis.
• Specialized pathways like the glyoxylate, phosphoketolase, and Rappoport-Luebering pathways.
• Oxidative phosphorylation and the respiratory chain.
• Inhibition of oxidative phosphorylation and the respiratory chain.

Bioenergetics

• Energetics involves studying energy requirements within systems.
• Bioenergetics is the balance between energy intake and utilization for life-sustaining processes.
• Energy is required for tissue synthesis, osmoregulation, digestion, respiration, reproduction, and locomotion.
• Animals must obtain energy from the chemical bonds of complex molecules through oxidation with oxygen.

Energy Sources

• The original energy source for food is the sun.
• Plants convert solar energy into glucose via photosynthesis.
• Glucose acts as a hydrocarbon source for synthesizing other organic compounds in plants.

Metabolism and Metabolic Pathways

• Metabolism is the totality of an organism's chemical reactions resulting from molecular interactions.
• Metabolism follows the laws of thermodynamics by transforming matter and energy.
• A metabolic pathway involves multiple steps, starting with a specific molecule and ending with a product, each catalyzed by an enzyme.
• Anabolic reactions join small molecules into larger ones, while catabolic reactions break down large molecules into smaller subunits.

Bioenergetics and Thermodynamics

• Bioenergetics explains how organisms handle energy via metabolic pathways.
• Biological energy transformations adhere to the laws of thermodynamics.
• Catabolic pathways release energy by breaking down complex molecules while anabolic pathways consume energy to build complex ones.

Energy: Forms and Transformation

• The capacity to do work or cause change is defined as energy.
• Energy manifests in two forms: potential (stored) and kinetic (causing change through motion).
• Potential energy includes chemical energy stored in molecular structure.
• Energy can be converted from one form to another.

Laws of Thermodynamics

• Thermodynamics studies energy changes, governed by two fundamental laws.
• The first law states that energy cannot be created or destroyed, only transferred or transformed.
• A cheetah converts the chemical energy of food into kinetic energy for movement.
• The second law explains that the universe's disorder (entropy) is continuously increasing.
• Energy transformations spontaneously convert matter from ordered to disordered forms.
• Entropy or disorder increases with spontaneous changes requiring no external energy.

Second Law of Thermodynamics and Energy Transfer

• Some energy dissipates as heat during each conversion.
• Energy is often lost as unusable heat during energy transfer or transformation.
• Cellular respiration follows the second law of thermodynamics, increasing the universe's entropy with every energy transfer or transformation.

Thermodynamic Quantities

• Gibbs free energy (G) measures the energy available to do work at constant temperature and pressure.
• A release of free energy (ΔG is negative) indicates an exergonic reaction.
• Requiring free energy (ΔG is positive) means the reaction is endergonic.
• Enthalpy (H) is the heat content of a system.
• An exothermic reaction releases heat; products have less heat content than reactants (ΔH is negative).
• An endothermic reaction absorbs heat from surroundings (ΔH is positive).
• Entropy (S) is a measure of randomness or disorder.
• Reactions that produce simpler, more disordered products have a gain in entropy (ΔS > 0).

Biological Order and Disorder

• Cells create order from less ordered materials.
• Organisms use energy to replace matter and energy with less ordered forms.
• The evolution of complex organisms does not violate the second law of thermodynamics, and although entropy may decrease in an organism, total entropy in universe increases.

Free Energy in Biological Systems

• Living systems increase the entropy of the universe and use energy to maintain order.
• Gibbs free energy (G) – in a cell, is the amount of energy contained in a molecule’s chemical bonds (T&P constant).
• The change in free energy (∆G) can be negative (exergonic) or positive (endergonic).
• Free energy that can do work under cellular conditions is a living system's free energy.

Gibbs Free Energy

• Gibbs' free energy is the portion of a system's energy that can perform work under uniform temperature and pressure.
• Free energy is needed to break and form chemical bonds.
• Change in free energy - ΔG can be:
• Endergonic - This is when any reaction requires an energy input
• Exergonic - This is when any reaction releases free energy

Exergonic and Endergonic Reactions

• Exergonic reactions have reactants with more free energy than the products, which cause a net release of energy and/or an increase in entropy.
• Exergonic reactions also occur spontaneously with no net energy input.
• Endergonic reactions have reactants with less free energy than the products, which require a net input of energy and/or a decrease in entropy.
• Endergonic reactions do not occur spontaneously.

Equilibrium in Metabolism

• Reactions in a closed system will always reach equilibrium and do no work.
• Cells are open systems with a constant flow of materials, not at equilibrium.
• Hydroelectric systems are used as an analogy to describe open and closed systems.
• A catabolic pathway in a cell releases free energy in a series of reactions.
• A multistep open hydroelectric system provides an analogy for cellular respiration where no reaction reaches equilibrium.

Energy Coupling

• Living organisms combine exergonic with endergonic reactions.
• Energy released by exergonic reactions drives ATP production from ADP and Pi.
• ATP can be broken down to ADP and Pi, releasing power for the cell's endergonic reactions.
• ATP hydrolysis to ADP + Pi yields energy.
• ATP synthesis from ADP + Pi requires energy.

ATP: Structure and Function

• ATP (adenosine triphosphate) is the cell's energy shuttle and provides energy for cellular functions.
• The terminal phosphate bond in ATP breaks to release energy.

How ATP Performs Work

• Cells perform mechanical, transport, and chemical work.
• Energy coupling is key to the way cells manage energy resources.
• ATP powers cellular work by coupling exergonic reactions to endergonic reactions.
• ATP drives endergonic reactions via phosphorylation, like the transfer of a phosphate group to a reactant.
• The recipient molecule is phosphorylated.
• The three types of cellular work are powered by ATP hydrolysis.

ATP/ADP Cycle

• Releasing the third phosphate from ATP to make ADP generates energy.
• Linking the phosphates together requires energy, so making ATP from ADP requires energy in turn.
• Catabolic pathways regenerate ATP from ADP and phosphate.

Activation Energy

• Both endergonic and exergonic reactions need activation energy to start, and molecules usually lack enough kinetic energy to reach the transition state when they collide.
• High activation energy means most collisions will be non-productive and the reaction will go slowly if at all.
• The activation energy is the initial amount of energy is often supplied in the form of heat which allows the reactants to come together.

Reaction Rates

• Energy (heat) can increase reaction rates as molecules move faster and collide more frequently and with greater force.
• A catalyst can also increase reaction rates as it reduces the energy is needed to reach the activation state, without being changed itself; proteins that function as catalysts are called enzymes.

Enzymes and Activation Energy

• Enzymes lower activation energy creating a lower route for the reaction.