ATP & Cellular Respiration Stages

Role of ATP in Linking Reactions

• ATP links anabolic reactions (building complex molecules) and catabolic reactions (breaking down complex molecules).
• Catabolic reactions transfer energy from complex molecules to ATP.
• Anabolic reactions transfer energy from ATP to complex molecules.

Electron Carriers

• NADH and FADH2 transport energy.

Four Stages of Cellular Respiration

• Glycolysis breaks down a 6-carbon glucose molecule into two 3-carbon pyruvate molecules.
• Substrate-level phosphorylation synthesizes some ATP during glycolysis.
• Substrate-level phosphorylation involves an enzyme-catalyzed reaction that transfers a phosphate group from a substrate to ADP.
• Pyruvate Oxidation converts the 3-carbon pyruvate into a 2-carbon acetyl group.
• The Citric Acid Cycle completely oxidizes the acetyl group to carbon dioxide.
• Some ATP is synthesized during the Citric Acid Cycle.
• The Electron Transfer Chain delivers high-energy electrons to oxygen through a sequence of electron carriers.
• Free energy from electron flow generates an H+ gradient by chemiosmosis.
• ATP synthase uses the H+ gradient to make ATP.

Glycolysis

• Occurs in the cytosol.
• Is anaerobic.
• No CO2 is released.
• Requires 2 ATP for activation energy.
• 4 ATP are produced, resulting in a net gain of 2 ATP.
• 2 NADH molecules are formed.
• 2 pyruvate molecules are produced.

Pyruvate Oxidation and the Citric Acid Cycle

• Active transport moves pyruvate into the mitochondrial matrix, where pyruvate oxidation and the citric acid cycle occur
• Oxidation of pyruvate yields CO2, acetyl-coenzyme A (acetyl-CoA), and NADH.
• The acetyl group of acetyl-CoA enters the citric acid cycle.

Mitochondria

• Are found inside all eukaryotes.
• Contain a membrane within a membrane.
• Site of the Citric Acid Cycle (inner matrix) and oxidative phosphorylation (inner membrane).
• The Citric Acid Cycle (tricarboxylic acid cycle or Krebs cycle) oxidizes acetyl groups completely to CO2.
• The Citric Acid Cycle generates 3 NADH and 1 FADH2.
• The Citric Acid Cycle synthesizes 1 ATP by substrate-level phosphorylation.

Inside the Mitochondria

• Pyruvic acid converts to Acetyl CoA
• 1 CO2 is released during pyruvic acid conversion
• Electrons transfer to NADH during pyruvic acid conversion
• Acetyl CoA enters Citric Acid cycle
• 2 CO2 molecules are released during the citric acid cycle
• 1 ATP molecule is generated during the citric acid cycle
• Electrons are transferred to 3 NADH and 1 FADH2

Electron Transport Chain (ETC)

• NADH and FADH2 carry electrons to the Electron Transport Chain.
• The ETC moves protons into the space between the inner and outer mitochondrial membranes.
• Buildup of protons creates a high ion gradient.
• Oxygen accepts electrons and forms water.

Making ATP

• Protons are packed between inner and outer mitochondrial membranes.
• Protons move into the inner matrix through protein channels (ATP synthase).
• The H+ gradient powers ATP synthesis by ATP synthase via chemiosmosis.
• ATP synthase binds ADP with inorganic phosphate (Pi).

Total Energy Summary

• In Glycolysis, the ATP is 2 (net gain), and NADH is 2.
• In Pyruvate Oxidation, the NADH is 2.
• In the Krebs Cycle the ATP is 2 (1 in each cycle), NADH is 6 (3 in each cycle), and FADH2 is 2 (1 in each cycle)
• Substrate phosphorylation occurs.
• The total number of ATP produced from one glucose molecule is 38.
• Sometimes NADH are produced in the cytosol during glycolysis, resulting in 2 ATP per NADH instead of 3.
• The total ATP in those cases can be 34

Anaerobic Respiration

• The Electron Transport Chain cannot function, the Citric Acid Cycle stops, but pyruvate is still produced from glycolysis.

Fermentation

• When oxygen is absent or limited, electrons from 2 NADH produced by glycolysis may be used.
• Electrons carried by NADH transfer to an organic acceptor molecule, converting NADH to NAD+.
• Glycolysis continues to supply ATP via substrate-level phosphorylation.

Lactate Fermentation

• Pyruvate is converted into lactate.
• Occurs in some bacteria, plant tissues, and skeletal muscle.
• Used to make buttermilk, yogurt, and dill pickles.

Alcoholic Fermentation

• Pyruvate is converted into ethyl alcohol and CO2.
• Occurs in some plant tissues, invertebrates, protists, bacteria, and single-celled fungi like yeasts.
• Used to make bread and alcoholic beverages.

Lipid Metabolism

• Lipid molecules contain carbon, hydrogen, and oxygen, but in different proportions than carbohydrates.
• Triglycerides are the most abundant lipid.
• Lipid Catabolism (lipolysis) breaks lipids down into pieces that can be converted to pyruvic acid.
• Some pieces are channeled into the TCA cycle.
• Enzymes in the cytosol convert glycerol to pyruvic acid.
• Pyruvic acid enters the TCA cycle.
• Enzymes convert fatty acids to acetyl-CoA (beta-oxidation).
• Beta-Oxidation is a series of reactions.
• Breaks fatty acid molecules into 2-carbon fragments inside mitochondria.
• Each step generates acetyl-CoA and NADH and leaves a shorter carbon chain bound to coenzyme A.
• For each 2-carbon fragment removed from fatty acid, the cell gains 12 ATP from acetyl-CoA in the TCA cycle, and 5 ATP from NADH.
• A single 18-carbon fatty acid molecule yields 144 ATP upon breakdown.
• Fatty acid breakdown yields about 1.5 times the energy from glucose breakdown.
• Free Fatty Acids (FFAs) are lipids that can diffuse across plasma membranes.
• FFAs circulate in the blood, bound to albumin mostly.
• Sources of FFAs in the blood include the diffusion of fatty acids that cannot be used in the synthesis of triglycerides.
• Sources of FFAs are the lipid stores (in liver and adipose tissue) from broken down triglycerides.

Protein Metabolism

• Amino Acid Catabolism is the removal of the amino group via transamination or deamination.
• Transamination attaches an amino group of amino acid to keto-acid.
• Keto acid converts to amino-acid.
• That keto acid leaves the mitochondrion, enters the cytosol, and becomes available for protein synthesis.
• Deamination prepares an amino acid for breakdown in the TCA cycle by removing the amino group and hydrogen atom.
• This generates ammonium ions, which are highly toxic even in low concentrations.
• Liver cells (primary sites of deamination) have enzymes that use ammonium ions to synthesize urea.
• Urea is the water-soluble compound excreted in urine.
• If glucose and lipid reserves are inadequate, liver cells break down internal proteins and absorb amino acids from blood.
• Amino acids are deaminated and the remaining carbon chains are broken down to provide ATP.
• Proteins are more difficult to break apart than complex carbohydrates or lipids.
• The process creates ammonium ion, which is toxic to cells.
• Proteins form the structural and functional components of cells.