Bioenergetics and Thermodynamics

• Bioenergetics is the study of how living organisms acquire and utilize energy to perform biological work.
• It encompasses the metabolic pathways involved in energy production, storage, and consumption.
• The flow of energy in biological systems is governed by the laws of thermodynamics.

Laws of Thermodynamics

• First law: energy cannot be created or destroyed, only converted from one form to another.
  • In biological systems, chemical energy is converted to other forms of energy, such as kinetic, potential, and thermal energy.
• Second law: the entropy (disorder) of a closed system always increases.
  • Living organisms maintain order by constantly inputting energy to counteract the increase in entropy.
  • Some energy conversions are inefficient, and energy is lost as heat, increasing entropy.

Gibbs Free Energy

• Gibbs free energy (G) is a thermodynamic potential that measures the amount of energy available in a chemical or biological system to do useful work at a constant temperature and pressure.
• The change in Gibbs free energy (ΔG) is used to predict the spontaneity of a reaction.
• ΔG = ΔH - TΔS
  • ΔH is the change in enthalpy (heat content)
  • T is the absolute temperature (in Kelvin)
  • ΔS is the change in entropy
• If ΔG < 0, the reaction is spontaneous (exergonic).
• If ΔG > 0, the reaction is non-spontaneous (endergonic) and requires energy input.
• If ΔG = 0, the reaction is at equilibrium.

ATP: The Energy Currency of the Cell

• Adenosine triphosphate (ATP) is the primary energy currency of the cell.
• ATP consists of an adenosine molecule attached to three phosphate groups.
• The bonds between the phosphate groups are high-energy bonds, and their hydrolysis releases a significant amount of energy.
• ATP hydrolysis is often coupled to endergonic reactions to drive them forward.
• ATP is regenerated from ADP (adenosine diphosphate) and inorganic phosphate (Pi) through energy-releasing processes like cellular respiration and photosynthesis.

Metabolic Pathways

• Metabolic pathways are a series of interconnected biochemical reactions that convert specific substrates into products.
• These pathways can be catabolic (breaking down complex molecules) or anabolic (building up complex molecules).
• Catabolic pathways release energy, often by oxidizing molecules.
• Anabolic pathways require energy input, often in the form of ATP.
• Metabolic pathways are tightly regulated to maintain homeostasis and respond to changing energy demands.

Key Catabolic Pathways

• Glycolysis: the breakdown of glucose into pyruvate, producing ATP and NADH.
  • Occurs in the cytoplasm.
  • Can occur with or without oxygen (anaerobic or aerobic).
• Citric Acid Cycle (Krebs Cycle): oxidizes acetyl-CoA, producing ATP, NADH, and FADH2.
  • Occurs in the mitochondrial matrix (in eukaryotes).
• Oxidative Phosphorylation: uses the electron transport chain and chemiosmosis to generate a large amount of ATP from NADH and FADH2.
  • Occurs in the inner mitochondrial membrane (in eukaryotes).
  • Requires oxygen as the final electron acceptor.
• Beta-oxidation: the breakdown of fatty acids into acetyl-CoA, producing NADH and FADH2.
  • Occurs in the mitochondrial matrix (in eukaryotes).

Key Anabolic Pathways

• Photosynthesis: uses light energy to convert carbon dioxide and water into glucose and oxygen.
  • Occurs in chloroplasts (in plants and algae).
• Gluconeogenesis: the synthesis of glucose from non-carbohydrate precursors, such as pyruvate, lactate, and glycerol.
  • Occurs primarily in the liver and kidneys.
• Protein synthesis: the assembly of amino acids into proteins, using information encoded in mRNA.
  • Occurs on ribosomes.
• Fatty acid synthesis: the synthesis of fatty acids from acetyl-CoA.
  • Occurs in the cytoplasm.

Redox Reactions

• Oxidation-reduction 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 are central to energy metabolism, as electrons are transferred from fuel molecules to electron carriers like NAD+ and FAD.
• NADH and FADH2 then donate electrons to the electron transport chain, driving ATP synthesis.

Electron Carriers

• NAD+ (nicotinamide adenine dinucleotide) is a coenzyme that accepts electrons and becomes reduced to NADH.
• FAD (flavin adenine dinucleotide) is a coenzyme that accepts electrons and becomes reduced to FADH2.
• NADH and FADH2 are important electron carriers in catabolic pathways, transferring electrons to the electron transport chain.

Electron Transport Chain

• The electron transport chain (ETC) is a series of protein complexes embedded in the inner mitochondrial membrane (in eukaryotes) or the plasma membrane (in prokaryotes).
• Electrons are passed from NADH and FADH2 through the ETC, releasing energy that is used to pump protons (H+) from the mitochondrial matrix to the intermembrane space.
• This creates an electrochemical gradient of protons across the inner mitochondrial membrane.

Chemiosmosis

• Chemiosmosis is the process by which the electrochemical gradient of protons (proton-motive force) is used to drive ATP synthesis.
• Protons flow down their concentration gradient through a protein complex called ATP synthase, which uses the energy to phosphorylate ADP to ATP.
• Oxidative phosphorylation is the combination of the electron transport chain and chemiosmosis.

Regulation of Metabolic Pathways

• Metabolic pathways are regulated by a variety of mechanisms to maintain homeostasis and respond to changing energy demands.
• Enzyme regulation:
  • Allosteric regulation: enzymes can be activated or inhibited by the binding of small molecules to regulatory sites.
  • Covalent modification: enzymes can be activated or inhibited by the addition or removal of chemical groups, such as phosphate.
• Hormonal regulation: hormones, such as insulin and glucagon, can regulate metabolic pathways by altering enzyme activity or gene expression.
• Substrate availability: the rate of a metabolic pathway can be influenced by the availability of substrates.
• Feedback inhibition: the product of a metabolic pathway can inhibit an enzyme earlier in the pathway, preventing overproduction of the product.

Photosynthesis in Detail

• Light-dependent reactions: capture light energy and convert it into chemical energy in the form of ATP and NADPH.
  • Occur in the thylakoid membranes of chloroplasts.
  • Involve chlorophyll and other pigments that absorb light energy.
  • Water is split, releasing oxygen as a byproduct.
• Calvin cycle (light-independent reactions): uses the ATP and NADPH generated in the light-dependent reactions to fix carbon dioxide and synthesize glucose.
  • Occurs in the stroma of chloroplasts.
  • Involves the enzyme RuBisCO, which catalyzes the first step of carbon fixation.

Different Metabolic Strategies

• Autotrophs: organisms that can synthesize their own organic molecules from inorganic sources, such as carbon dioxide and water (e.g., plants, algae, and some bacteria).
• Heterotrophs: organisms that must obtain organic molecules from other organisms (e.g., animals, fungi, and many bacteria).
• Chemotrophs: organisms that obtain energy from chemical compounds.
• Phototrophs: organisms that obtain energy from light.