Lesson 08: Energy from Organic Molecules I

Autotrophs

Organisms that convert the sun's energy into chemical energy (ATP, chemical bonds in inorganic molecules).

Heterotrophs

Organisms that obtain chemical energy from organic molecules produced by autotrophs, representing ~95% of known species.

Cellular Respiration

Process by which cells oxidize organic molecules to extract energy from their chemical bonds, involving oxidation and dehydrogenation reactions.

Oxidation

A chemical reaction involving the loss of electrons from a molecule.

Dehydrogenation

A chemical reaction involving the loss of protons (hydrogen atoms) from a molecule.

Reduction

A chemical reaction involving the gain of electrons by a molecule.

Redox Reactions

Coupled reactions where oxidation (loss of electrons) occurs simultaneously with reduction (gain of electrons).

Electron Carriers

Small chemical cofactors that facilitate electron transfer, easily and reversibly oxidized and reduced.

Nicotinamide Adenine Dinucleotide (NAD+)

A crucial electron carrier in cellular respiration that accepts two electrons and one proton from a substrate to form NADH.

NADH

The reduced form of NAD+, carrying electrons harvested from organic molecules, which can then donate these electrons to reduce other molecules.

Aerobic Respiration

The oxidation of glucose in the presence of molecular oxygen (O_2), which acts as the final electron acceptor.

Glycolysis

The first stage of aerobic respiration, which converts one 6-carbon glucose molecule into two 3-carbon pyruvate molecules, occurring in the cytosol independently of oxygen.

Pyruvate Oxidation

The second stage of aerobic respiration, linking glycolysis to the Krebs cycle, where each pyruvate is converted into Acetyl-CoA, releasing CO_2 and NADH, occurring in the mitochondria (eukaryotes) or plasma membrane (prokaryotes).

Krebs Cycle (Citric Acid Cycle)

The third stage of aerobic respiration, which further oxidizes the acetyl group from Acetyl-CoA, completely oxidizing the carbons from the original glucose molecule, occurring in the mitochondrial matrix.

Electron Transport Chain

The final stage of aerobic respiration where most ATP synthesis occurs, associated with the mitochondrial inner membrane.

Chemiosmosis

The process coupled with the electron transport chain (final stage of aerobic respiration) where most ATP synthesis occurs.

Cytosol

The cellular location where glycolysis occurs in eukaryotes.

Mitochondrial Matrix

The cellular location where pyruvate oxidation and the Krebs cycle occur in eukaryotes.

Mitochondrial Inner Membrane

The cellular location associated with the electron transport chain and chemiosmosis in eukaryotes.

Glyceraldehyde 3-phosphate (G3P)

Two 3-carbon molecules formed from glucose after the energy input phase of glycolysis, which are then oxidized in the energy production phase.

Substrate-level phosphorylation

A mechanism of ATP generation where an enzyme directly transfers a high-energy phosphate from an intermediate molecule to ADP, forming ATP (e.g., in glycolysis and Krebs cycle).

Acetyl-CoA

A two-carbon acetyl group attached to Coenzyme A, formed from pyruvate during pyruvate oxidation, which then enters the Krebs cycle.

Oxaloacetate

A four-carbon molecule that combines with the two-carbon acetyl group from Acetyl-CoA to form a six-carbon citrate molecule in the Krebs cycle, and is regenerated at the end of the cycle.

Citrate

A six-carbon molecule formed at the beginning of the Krebs cycle when Acetyl-CoA combines with oxaloacetate.

Flavin Adenine Dinucleotide (FAD)

An electron carrier similar to NAD+, which accepts electrons in the Krebs cycle to form FADH_2.

FADH_2

The reduced form of FAD, an electron carrier produced during the Krebs cycle.

Net Products of Glycolysis (per Glucose)

2 ATP and 2 NADH molecules.

Products of Pyruvate Oxidation (per Pyruvate)

1 CO_2 molecule, 1 NADH molecule, and 1 Acetyl-CoA molecule.

Products of Krebs Cycle (per Acetyl-CoA)

2 CO2 molecules, 3 NADH molecules, 1 FADH2 molecule, 1 ATP molecule, and 1 oxaloacetate molecule (regenerated).