Bioenergetics: Energy Flow in Living Systems

• Bioenergetics is the study of energy flow through living systems
• It involves energy production, storage, and utilization within cells and organisms
• It is central to understanding metabolism, the sum of all chemical reactions in a cell or organism

Energy and Thermodynamics

• Energy is the capacity to do work
• Thermodynamics governs energy transformations in biological systems
• The first law of thermodynamics states energy cannot be created or destroyed, only converted
• The second law of thermodynamics states that every energy transfer increases entropy (disorder)
• Living organisms must increase the entropy of their surroundings to maintain their organization
• Gibbs free energy (G) is the energy available to do work at constant temperature and pressure
• Change in Gibbs free energy (ΔG) determines the spontaneity of a reaction:
  • A negative ΔG indicates a spontaneous (exergonic) reaction, releasing energy
  • A positive ΔG indicates a non-spontaneous (endergonic) reaction, requiring energy input
  • A ΔG of zero indicates a reaction at equilibrium

ATP: The Energy Currency of the Cell

• Adenosine triphosphate (ATP) is the primary energy currency of the cell
• ATP consists of adenine, ribose, and three phosphate groups
• The bonds between phosphate groups are high-energy bonds
• Hydrolysis of ATP to ADP (adenosine diphosphate) and inorganic phosphate (Pi) releases energy
• This energy drives endergonic reactions in the cell
• ATP can also be hydrolyzed to AMP (adenosine monophosphate) and pyrophosphate (PPi); PPi is further hydrolyzed to 2 Pi
• ATP is regenerated from ADP and Pi through cellular respiration and photosynthesis
• The ATP cycle involves continuous ATP hydrolysis and regeneration to power cellular activities

Metabolic Pathways

• Metabolic pathways are series of interconnected chemical reactions
• Each reaction is catalyzed by a specific enzyme
• Pathways can be linear, branched, or cyclic
• Catabolic pathways break down complex molecules into simpler ones, releasing energy (exergonic)
• Anabolic pathways build complex molecules from simpler ones, requiring energy (endergonic)
• Catabolism provides the energy and building blocks for anabolism

Enzymes and Catalysis

• Enzymes are biological catalysts that speed up chemical reactions
• Most enzymes are proteins
• Enzymes lower the activation energy of a reaction, making it more likely to occur
• Enzymes are highly specific for their substrates
• The active site is the region of the enzyme where the substrate binds and catalysis occurs
• Enzyme activity can be regulated by various factors:
  • Temperature: Enzymes have an optimal temperature range
  • pH: Enzymes have an optimal pH range
  • Substrate concentration: Increasing substrate concentration increases reaction rate up to a saturation point
  • Inhibitors: Inhibitors can decrease enzyme activity (competitive or noncompetitive)
  • Activators: Activators can increase enzyme activity

Oxidation-Reduction Reactions (Redox Reactions)

• Redox reactions involve the transfer of electrons from one molecule to another
• Oxidation is the loss of electrons
• Reduction is the gain of electrons
• Redox reactions always occur together: one substance is oxidized while another is reduced
• Electron carriers, such as NAD+ and FAD, play a crucial role in redox reactions
• NAD+ (nicotinamide adenine dinucleotide) accepts electrons and becomes NADH
• FAD (flavin adenine dinucleotide) accepts electrons and becomes FADH2
• NADH and FADH2 carry electrons to the electron transport chain for ATP production

Cellular Respiration

• Cellular respiration is the process by which cells break down organic molecules to produce ATP
• It includes glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation
• Glycolysis occurs in the cytoplasm and converts glucose into pyruvate, producing a small amount of ATP and NADH
• The citric acid cycle occurs in the mitochondrial matrix and oxidizes pyruvate to CO2, producing ATP, NADH, and FADH2
• Oxidative phosphorylation occurs in the inner mitochondrial membrane and uses the electron transport chain and chemiosmosis to produce a large amount of ATP

Electron Transport Chain and Chemiosmosis

• The electron transport chain (ETC) is a series of protein complexes that transfer electrons from NADH and FADH2 to oxygen
• Electrons are passed from one complex to another, releasing energy
• This energy is used to pump protons (H+) from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient
• Chemiosmosis is the movement of H+ ions down their electrochemical gradient through ATP synthase
• ATP synthase uses the energy from H+ flow to phosphorylate ADP to ATP
• Oxidative phosphorylation produces the majority of ATP in cellular respiration

Photosynthesis

• Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose
• It occurs in chloroplasts
• Photosynthesis includes the light-dependent reactions and the Calvin cycle (light-independent reactions)
• The light-dependent reactions occur in the thylakoid membranes and use light energy to split water, producing ATP, NADPH, and oxygen
• The Calvin cycle occurs in the stroma and uses ATP and NADPH to fix carbon dioxide into glucose

Anabolic Pathways Examples

• Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors
• Protein synthesis involves building proteins from amino acids
• Nucleic acid synthesis involves building DNA and RNA from nucleotides

Regulation of Metabolic Pathways

• Metabolic pathways are tightly regulated to maintain cellular homeostasis
• Regulation can occur at several levels:
  • Enzyme activity: Allosteric regulation, feedback inhibition
  • Enzyme synthesis: Gene expression control
  • Compartmentalization: Separating pathways in different cellular compartments
  • Hormonal control: Insulin and glucagon regulate glucose metabolism
• Feedback inhibition is a common regulatory mechanism where the end product of a pathway inhibits an earlier step in the pathway