Cell Biology: Mitochondria and Thermodynamics

• Cell Biology is presented by Dr. Sara Mohamed Sayed.

Mitochondria and Cellular Energetics

• Bioenergetics, or thermodynamics, focuses on the study of energy changes that happen during biochemical reactions.
• Biological energy adheres to the same chemical and physical laws governing all natural processes.
• Heat serves as a form of biological energy utilized to sustain body temperature.

Laws of Thermodynamics

• Thermodynamics is a physics branch that studies the relationships among heat, work, temperature, and energy.
• It includes fundamental principles and laws describing how energy is transferred and transformed.
• Thermodynamics is relevant in living systems, not just physical chemistry.
• A system, according to thermodynamics, represents a part of space being studied thermodynamically.
• In living organisms, a cell can function as a system.

First and Second Laws of Thermodynamics

• The first law of thermodynamics states that the total energy of a system, including its surroundings, remains constant, stating energy cannot be created nor destroyed, only transferred or transformed.
• In living systems, chemical energy can transform into heat, electrical, or mechanical energy.
• The second law thermodynamics states that the total entropy of a system must increase if a process occurs spontaneously.
• Entropy, denoted as "S," signifies the amount of energy in a state of randomness or disorder.
• The entropy of a system increases (ΔS is positive) as it becomes more random or disordered and vice versa.

Gibbs Free Energy

• Gibbs free energy, denoted ΔG, describes the change in free energy available for useful work like muscle contraction or nerve impulses.
• AG = ΔH - TΔS is derived by combining the first and second laws of thermodynamics.
  • ΔG = the change in free energy of a reacting system.
  • ΔH = the change in enthalpy (heat content of the reacting system).
  • T = the absolute temperature at which the reaction occurs.
  • ΔS = the change in entropy of the system.
• Units of ΔG are measured in Joules/mole or calories/mole.
• ΔG is a key criterion for determining the spontaneity of a reaction.

Gibbs Free Energy and Spontaneity

• If ΔG is negative, energy is released and the reaction is exergonic, which means the reaction is likely to occur.
• If ΔG is positive, the reaction is endergonic, which means the reaction requires energy input from outside (heat absorbed from surroundings), and the reaction isn't likely to occur.
• If ΔG is zero, the reaction system is at equilibrium, and no reaction takes place.

Exergonic vs Endergonic Reactions

• Exergonic Reaction:

  • Type of reaction releases energy.
  • ΔG is negative (ΔG < 0).
  • Reactant energy is higher than product energy.
  • Entropy increases.
  • Spontaneous reaction.
  • Catabolism (e.g., glycolysis and fatty acid oxidation).

• Example: Creatine~P + ADP → Creatine + ATP (ΔG = -3 Kcal/mol).

• Endergonic Reaction:

  • Type of reaction absorbs energy.
  • ΔG is positive (ΔG > 0).
  • Reactant energy is lower than product energy.
  • Entropy decreases.
  • Non-spontaneous reaction.
  • Anabolism (e.g., glycogen and fatty acid synthesis).

• Example: Creatine + ATP → Creatine ~ P + ADP (ΔG = +1.5 Kcal/mol).

• Change in entropy in an exothermic reaction is positive.

ATP (Adenosine Triphosphate)

• ATP is considered the primary carrier of cellular energy.
• Free energy produced from the catabolism of carbohydrates, lipids, and amino acids is used to synthesize ATP.
• ATP is composed of three parts: a nitrogenous base (adenine), a sugar (ribose), and 3 phosphate groups.
• Phosphate groups are joined to a ribose by a phosphate ester bond and to each other by phospho-anhydride bonds.
• ATP-ADP cycle: ATP is hydrolyzed to ADP through the reaction ATP + H2O → ADP + Pi + free energy.
• ADP is combined with a phosphate to form ATP, ADP + Pi + free energy → ATP + H2O.
• Two phosphoanhydride bonds are the most important parts of an ATP molecule.
• Breakdown of one high energy bond of ATP yields 7.3 Kcal/mol.
• Due to its two high-energy bonds, ATP acts as a link between energy-yielding processes or exergonic reactions (catabolic reactions) and energy-requiring processes or endergonic reactions (anabolic reactions).
• Because of ATP's role as a link, it is often referred to as the "energy currency" of the cell.
• Most ATP synthesis occurs in the mitochondria through glucose metabolism.
• Glycolysis is an example of catabolic exergonic reactions.

Mitochondria Structure & Cellular Energetics

• Outer Mitochondrial Membrane: Characterized by many pores, facilitating passage for small molecules.
• Inter-membrane Space: Does not restrict substances from entering or exiting the mitochondrial matrix.
• Inner Mitochondrial Membrane:
  • Impermeable to most small ions like H+, Na+, and K+, as well as molecules like ATP, ADP, pyruvate, and other key metabolites needed for mitochondrial function.
  • Requires specialized carriers or transport systems to move ions or molecules across it.
  • It's highly convoluted, forming structures called cristae, which increase the membrane surface area.
  • Contains ATP synthase complexes, which are proteins considered inner membrane particles attached to the inner surface and include enzymes of the respiratory (electron transport) chain.

Matrix of Mitochondrion

• Gel-like solution bound by the inner mitochondrial membrane, containing:
  • The enzymes of the tricarboxylic acid cycle (TCA) or glucose metabolism.
  • The enzymes of β-oxidation of fatty acids.
  • Miscellaneous enzyme systems.
• Biological Oxidation is the process of oxidation of food stuffs for production of energy.
• Energy is derived from oxidation of carbohydrates, lipids, and proteins of diet
• In the redox reaction 2 H₂ + O₂ → 2 H₂O + energy, H₂ is oxidized while O₂ is reduced.
• Instead of a massive energy release, hydrogen is transferred to oxygen in gradual steps, liberating small energy fractions stored for later use.
• The inner mitochondrial membrane has 5 separate enzyme complexes (I, II, III, IV, and V), with complex V catalyzing ATP synthesis.
• Each complex either accepts or donates electrons to mobile electron carriers.
• The electron transport chain allows each carrier to receive electrons from a donor and then donate electrons to the next carrier.
• Electrons eventually combine with oxygen and protons to form water.
• Transport of electrons down the respiratory chain gives energy.
• Energy is used to transport H+ from the mitochondrial matrix through the inner mitochondrial membrane to the intermembrane space.
• An electrical gradient forms with more positive charges outside the membrane.
• A pH gradient occurs with a lower pH outside the membrane.
• Energy generated by the proton gradient powers ATP synthase to form ATP from ADP + Pi, while also decreasing the pH and electrical gradients.