Biology Chapter on Bioenergetics

Cellular Energetics

• Cellular energetics is the study of how cells produce and use energy.
• This is based on the laws of thermodynamics.
• Energy is related to:
  • Capture (respiration, photosynthesis)
  • Storage (ATP, NADH, proton gradient)
  • Conversion (light, chemical, kinetic)

Energy Conservation and Conversion

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form into another.
• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe. Energy transfer increases entropy of the universe.

Energy Conservation and Conversion in Biological Systems

• Not all energy can be converted to biologically relevant work.
• Organisms lose some energy to the environment (primarily heat).
• Some energy is fixed irreversibly (e.g., secondary metabolites in plants).
• Living organisms require a constant input of energy.

Energy Balance

• Ecosystems and individual organisms convert energy and experience energy loss/heat.
• Energy flows through a food chain: Sun → Producers → Primary Consumers → Secondary Consumers → Tertiary Consumers → Decomposers.
• Cellular respiration is a metabolic process where organisms obtain energy by oxidizing nutrients, releasing energy and waste products.

Metabolism and ATP Energy Currency

• Metabolism is the totality of an organism's chemical reactions.
• Anabolism: synthesis of complex molecules using energy.
• Catabolism: breakdown of complex molecules to release energy.
• ATP acts as an energy reservoir in cells, used for chemical work, transport, and movement.

Gibbs Free Energy

• Gibbs free energy (G) is the portion of a system's energy that can perform work.
• The change in Gibbs free energy (ΔG) determines if a reaction is spontaneous.
• ΔG = H - TS (where H is enthalpy, T is temperature, and S is entropy).
  • A negative ΔG indicates a spontaneous reaction.

Chemical Reactions

• Exergonic reactions release energy (ΔG < 0).
• Endergonic reactions require energy input (ΔG > 0).
• Energy released from exergonic reactions can power endergonic reactions.

Enzymes and Metabolic Regulation

• Enzymes catalyze metabolic reactions in cells.
• Enzymes lower activation energy for reactions to proceed more rapidly.
• Enzymes are influenced by pH and regulated by protein modification (e.g., phosphorylation).
• Enzymes can also be inhibited by the end product of a pathway (negative feedback).

ATP as Energy Currency

• ATP (adenosine triphosphate) releases energy when the outermost inorganic phosphate is cleaved off, producing ADP (adenosine diphosphate).
• The stored chemical energy in ATP results from the electrostatic repulsion of the negative charges in the phosphates.
  • ΔG = -7.3 kcal/mol (-30.5 kJ/mol).

Formation of ATP

• Substrate-level phosphorylation: Transfer of a phosphate group from a substrate to ADP to form ATP. (example: Glycolysis).
• Oxidative phosphorylation: ATP synthesis coupled with electron transport (example: inner mitochondrial membrane – chemiosmosis).
• Chemiosmosis: process of generating ATP by moving ions across a semipermeable membrane.

Redox Potential and Electron Transport

• Redox potential (energy stored in redox reactions) reflects the affinity of atoms to release or incorporate electrons.
• Relocating electrons from sugars to oxygen (more electronegative element) releases energy.
• Electron transport chain is a series of redox reactions that transfer electrons.
• NAD+/NADH is a vital electron carrier

NADH

• Nicotinamide adenine dinucleotide (NAD+) is a coenzyme that carries electrons (during redox reactions).
• NADH carries electrons from glycolysis and the Citric acid cycle to the electron transport chain to make ATP.

Energy Harvest: From Electrons to ATP

• Chemiosmosis: a process of ATP synthesis caused by H+ ion moving across membranes.
• Intact membranes are crucial for chemiosmosis.

Energy Harvest: From Electrons to ATP (NADH):

• NADH is a crucial electron carrier involved in the oxidation-reduction processes of electron transport, transferring energy necessary for ATP synthesis.
• H+ gradient across mitochondrial membrane drives ATP synthesis.

Cellular Respiration

• A metabolic process that oxidizes nutrients to produce ATP.
• Involves glycolysis, the Citric Acid Cycle, and Oxidative phosphorylation.
• Different pathways are active at different locations in a cell.

Stages of Cellular Respiration

• Glycolysis: Glucose is broken down to pyruvate. Produces a net of 2 ATP and 2 NADH. Occurrs in the cytosol.
• Pyruvate Oxidation: Pyruvate is transported into the mitochondrial matrix and converted to Acetyl CoA. Produces CO2 and 1 NADH per pyruvate.
• Citric Acid Cycle: Acetyl CoA enters the cycle which generates high energy electrons (carried by NADH and FADH2) and ATP. Two turns of the cycle for each glucose molecule.
• Oxidative Phosphorylation: Oxidizes electron carriers NADH and FADH2, producing a significant amount of ATP via the electron transport chain and chemiosmosis. Occurrs within the inner mitochondrial membrane.

Glycolysis (in detail)

• Glycolysis is the first step of cellular respiration. The steps involved are:
  • Energy investment: glucose becomes fructose-1, 6-bisphosphate, consuming 2 ATP
  • Energy payoff: 2 NAD+ + 4e- + 4H+ → 2 NADH + 2 H+ and 4 ADP + 4P → 4ATP yielding a net of 2 ATP and 2 NADH.
• Glycolysis occurs in the cytosol

Metabolism, Summary

• Metabolism is the totality of an organism's chemical reactions.
• Pathways are interconnected. Glucose breakdown is crucial for ATP capture.
• Respiration occurs via glycolysis, citric acid cycle, and oxidative phosphorylation.
• Fermentation occurs in low oxygen conditions.

Preparing Pyruvate for Citric Acid Cycle

• Pyruvate is converted to acetyl-CoA which is then ready to enter the citric acid cycle.
• This occurs in the mitochondrial matrix and involves the release of CO2 and the transfer of electrons to NAD+.
• This step is also referred to as the link reaction.

Citric Acid Cycle (in detail)

• Cycle breaks down acetyl CoA, generating high-energy molecules.
• 8 NADH, 2 FADH2, and 2 ATP molecules are produced per glucose molecule via two turns of the citric acid cycle.

Electron Transport Chain and ATP Synthase

• The electron transport chain (ETC) is a sequence of electron carriers.
• Energy released pumps H+ across the mitochondrial membrane creating a proton gradient.
• ATP synthase uses the proton gradient to generate ATP (chemiosmosis).

Electron Transport Chain and ATP Synthase Summary

• Electrons are passed along the chain from NADH and FADH2, releasing energy.
• H+ions are pumped into the intermembrane space creating a H+ gradient.
• The gradient drives H+ through ATP synthase, which produces ATP (chemiosmosis).

Fermentation

• Anaerobic alternative to respiration.
• The process regenerates NAD+ from NADH, so glycolysis can continue.
• Two types fermentation pathway: alcohol and lactic acid.

Overall Reaction of Cellular Respiration

• Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP)