🔬Biophysics Unit 8 – Energy Transformations in Living Systems

Key Concepts and Definitions

• Bioenergetics studies energy flow through living systems, from molecular interactions to ecosystem dynamics
• Thermodynamics describes energy changes in systems, including laws of energy conservation and entropy
• Free energy represents the usable energy available for work in a system, often quantified as Gibbs free energy (G)
• Metabolism encompasses all chemical reactions involved in maintaining living cells and organisms
  • Catabolism breaks down complex molecules to release energy
  • Anabolism uses energy to construct components of cells (proteins, nucleic acids, carbohydrates, lipids)
• Enzymes are biological catalysts that lower activation energy barriers and speed up metabolic reactions
• Coupled reactions link energetically unfavorable processes to favorable ones, driving vital cellular functions
• Redox reactions involve the transfer of electrons between molecules, playing a central role in energy conversion pathways

Thermodynamics in Biological Systems

• The first law of thermodynamics states that energy cannot be created or destroyed, only converted between forms
• The second law of thermodynamics asserts that entropy (disorder) tends to increase in closed systems over time
• Living organisms are open systems that exchange matter and energy with their surroundings to maintain order
• Gibbs free energy change (ΔG) predicts the spontaneity of reactions in biological systems
  • Negative ΔG indicates a spontaneous reaction that can perform work
  • Positive ΔG signifies a non-spontaneous reaction requiring an input of energy
• Enzymes and coupled reactions allow cells to carry out thermodynamically unfavorable processes essential for life
• Biological systems utilize energy-rich molecules (ATP, NADH) to drive anabolic reactions and maintain homeostasis
• Heat dissipation from metabolic processes contributes to thermal regulation in endothermic organisms (mammals, birds)

Energy Sources and Storage in Organisms

• Autotrophs (plants, algae, some bacteria) capture light or chemical energy to synthesize organic compounds
• Heterotrophs (animals, fungi, most microorganisms) obtain energy by consuming organic molecules produced by other organisms
• Carbohydrates, primarily glucose, serve as a major energy source for cells through glycolysis and cellular respiration
• Lipids, especially triglycerides, store large amounts of energy in their chemical bonds for long-term use
• Proteins can be catabolized for energy in times of starvation or during intense exercise
• ATP (adenosine triphosphate) is the primary energy currency in cells, used to power numerous cellular processes
  • Hydrolysis of ATP to ADP (adenosine diphosphate) and inorganic phosphate (Pi) releases energy for work
  • ATP is continuously regenerated through substrate-level phosphorylation and oxidative phosphorylation
• Reduced coenzymes (NADH, FADH2) store energy from catabolic reactions and donate electrons for ATP production

Metabolic Pathways and Energy Conversion

• Glycolysis is a 10-step pathway that breaks down glucose into two pyruvate molecules, generating ATP and NADH
• The citric acid cycle (Krebs cycle) oxidizes acetyl-CoA derived from pyruvate, fatty acids, or amino acids
  • Produces CO2, reduced coenzymes (NADH, FADH2), and a small amount of ATP
  • Regenerates oxaloacetate to continue the cyclic pathway
• Oxidative phosphorylation is the primary source of ATP in aerobic organisms
  • Electron transport chain transfers electrons from NADH and FADH2 to oxygen, establishing a proton gradient
  • ATP synthase uses the proton gradient to drive ATP synthesis through chemiosmosis
• Beta-oxidation catabolizes fatty acids to produce acetyl-CoA, which enters the citric acid cycle
• Amino acid catabolism removes nitrogen groups (deamination) and funnels carbon skeletons into glycolysis or the citric acid cycle
• Anabolic pathways (gluconeogenesis, lipogenesis, protein synthesis) consume energy to build complex molecules from simpler precursors

Cellular Respiration and ATP Production

• Cellular respiration is the process of breaking down organic molecules to generate ATP in the presence of oxygen
• Glycolysis occurs in the cytosol and does not require oxygen (anaerobic)
  • Net yield of 2 ATP and 2 NADH per glucose molecule
  • Pyruvate can be converted to lactate (lactic acid fermentation) or ethanol (alcoholic fermentation) in the absence of oxygen
• Pyruvate dehydrogenase complex links glycolysis to the citric acid cycle by converting pyruvate to acetyl-CoA
• The citric acid cycle and oxidative phosphorylation occur in the mitochondrial matrix and inner membrane, respectively
• Oxidative phosphorylation yields the majority of ATP in aerobic respiration (up to 34 ATP per glucose)
  • Chemiosmosis couples the proton gradient to ATP synthesis via ATP synthase
  • Proton leak across the inner mitochondrial membrane generates heat instead of ATP, contributing to thermogenesis
• Substrate-level phosphorylation directly transfers a phosphate group to ADP from high-energy intermediates (e.g., phosphoenolpyruvate in glycolysis)

Photosynthesis and Light Energy Capture

• Photosynthesis is the process by which autotrophs convert light energy into chemical energy stored in organic compounds
• Light-dependent reactions occur in the thylakoid membranes of chloroplasts
  • Photosystems I and II absorb light energy to excite electrons, which are then transferred through an electron transport chain
  • Electron flow generates a proton gradient used for ATP synthesis (photophosphorylation) and reduces NADP+ to NADPH
  • Oxygen is released as a byproduct of splitting water molecules to replace electrons lost from photosystem II
• Light-independent reactions (Calvin cycle) take place in the stroma of chloroplasts
  • CO2 is fixed by the enzyme RuBisCO to produce 3-phosphoglycerate, which is reduced to form simple sugars using ATP and NADPH from the light reactions
  • Regeneration of RuBP (ribulose bisphosphate) allows the cycle to continue
• C3, C4, and CAM photosynthesis are adaptations to different environmental conditions (temperature, water availability, CO2 concentration)
  • C4 plants (maize, sugarcane) concentrate CO2 in bundle sheath cells to minimize photorespiration
  • CAM plants (cacti, succulents) fix CO2 at night and store it as malic acid to conserve water

Membrane Potentials and Ion Gradients

• Cell membranes maintain concentration gradients of ions (Na+, K+, Ca2+, Cl-) through selective permeability and active transport
• The sodium-potassium pump (Na+/K+ ATPase) uses ATP to transport Na+ out of the cell and K+ into the cell
  • Establishes a resting membrane potential (typically -60 to -90 mV) with the cell interior negative relative to the exterior
  • Crucial for generating action potentials in neurons and muscle cells
• Electrochemical gradients store potential energy that can be harnessed for various cellular processes
  • Cotransport of glucose and amino acids with Na+ in intestinal and kidney epithelial cells
  • Calcium signaling in muscle contraction, neurotransmitter release, and intracellular communication
• Ion channels allow the selective passage of specific ions across membranes in response to stimuli (voltage, ligands, mechanical stress)
• Proton gradients generated by electron transport chains drive ATP synthesis in mitochondria and chloroplasts through chemiosmosis

Applications in Bioenergetics Research

• Mitochondrial dysfunction is implicated in various diseases (Parkinson's, Alzheimer's, cancer) and the aging process
  • Studying mitochondrial bioenergetics may lead to new therapies and interventions
  • Mitochondrial DNA mutations and oxidative stress are key areas of investigation
• Manipulating photosynthetic pathways has the potential to increase crop yields and enhance carbon sequestration
  • Introducing C4 photosynthesis into C3 crops (rice, wheat) could improve water and nitrogen use efficiency
  • Engineering more efficient RuBisCO enzymes may boost CO2 fixation rates
• Biofuel production relies on understanding energy conversion processes in microorganisms
  • Optimizing fermentation pathways in yeast and bacteria to produce ethanol, butanol, and other fuel molecules
  • Developing algal systems for combined wastewater treatment and biodiesel production
• Bioenergetic principles are applied in the design of biosensors and bioelectronic devices
  • Glucose fuel cells use enzymes to convert blood sugar into electrical energy
  • Microbial fuel cells harness electron transfer from bacterial metabolism to generate electricity
• Comparative bioenergetics explores energy metabolism adaptations in diverse organisms
  • Identifying unique metabolic strategies in extremophiles (high temperature, pressure, salinity) may have biotechnological applications
  • Investigating energy allocation trade-offs between growth, reproduction, and maintenance in different life history strategies