unit 2, lesson 4: Energy and Metabolism: ATP and Catabolic Processes

Energy and Metabolism

• Energy is needed to move in the environment, build required molecules like DNA, proteins, and enzymes, transport nutrients, and balance Na/K levels.
• Everything cells do requires energy.

Adenosine Triphosphate (ATP)

• ATP is the universal energy molecule.
• ATP contains high-energy phosphate bonds.
• Energy-rich compounds include guanosine triphosphate (GTP), cytidine triphosphate (CTP), and uridine triphosphate (UTP).

Catabolic Processes Overview

• Stage 1 involves the breakdown of proteins into amino acids, carbohydrates into simple sugars, and fats into fatty acids.
• Stage 2 includes glycolysis and the conversion to pyruvate an acetyl CoA.
• Stage 3 is the citric acid cycle and oxidative phosphorylation.
• Stage 1 does not generate useful energy.
• Stage 2 generates a small amount of ATP.
• Stage 3 produces the majority of ATP.
• Stage 2 operates anaerobically (without oxygen), while Stage 3 is aerobic (with oxygen).

Energy Supply

• Most organisms get their main energy supply from metabolizing glucose and other sugars.
• Fat and protein metabolism becomes more important when glucose and other sugars are limited.
• Glycolysis is the initial stage in glucose metabolism.

Glycolysis

• Glycolysis occurs in stage two of the metabolic process and takes place in the cytoplasm.
• Glycolysis does not require oxygen and is not very efficient.
• The reaction produces two molecules of NADH (Nicotinamide Adenine Dinucleotide).
• Glycolysis comprises 10 different reactions using 10 different enzymes.
• Glycolysis can be divided into reactions with 6 carbon units that require energy (hexose stage) and reactions with 3 carbon units that provide energy (triose stage).
• Six different reaction types in glycolysis include phosphoryl transfer, phosphoryl shift, isomerisation, dehydration, aldol cleavage, and oxidation.
• Consumption and generation of ATP in glycolysis includes -1 ATP for glucose to glucose 6-phosphate, -1 ATP for fructose 6-phosphate to fructose 1,6-bisphosphate, +2 ATP for 2 x 1,3-Bisphosphoglycerate to 2 x 3- Phosphoglycerate, +2 ATP for 2 x Phosphoenol pyruvate to 2 x Pyruvate resulting in an overall +2 ATP.

Fate of Pyruvate

• The end product of glycolysis is pyruvate, which can be converted to acetyl coenzyme A, lactate, or ethanol.
• Acetyl coenzyme A formed from pyruvate is used in the citric acid cycle and the electron transport chain and is in the mitochondria.

Citric Acid Cycle

• The citric acid cycle which takes place in the mitochondria, is also known as the tricarboxylic acid cycle or Krebs' Cycle.
• The citric acid cycle is an aerobic stage that requires oxygen.
• Acetyl CoA enters the citric acid cycle, where it is fully oxidised to carbon dioxide and water.
• he citric acid cycle provides intermediates for biosynthesis.
• Two carbon atoms enter as acetyl CoA, add onto a four-carbon unit to give citric acid, and then leave as carbon dioxide.
• The citric acid cycle has nine steps and can be divided into parts involving 6-Carbon atoms, 5-Carbon atoms, and 4-Carbon atoms.
• The citric acid cycle does not directly produce many ATP molecules, instead it produces 3 molecules of NADH, 1 molecule of FADH2 (Flavin Adenine Dinucleotide), and 1 molecule of GTP, which are converted into ATP during the electron transport chain.
• GTP (guanosine triphosphate) is a high energy molecule.
• Overall stoichiometry involves Acetyl CoA + 3NAD+ + FAD + GDP + Pi + 2H2O becoming 2CO2 + 3NADH + FADH2 + GTP + 2H+ + CoA
• The regeneration of NAD+ and FAD requires molecular oxygen, so the citric acid cycle only works under aerobic conditions.

Summary of Glycolysis and Citric Acid Cycle

• Glycolysis involves converting glucose to 2 molecules of pyruvate, producing 2 ATP and 2 NADH.
• Pyruvate is converted to acetyl coenzyme A, producing 2 NADH per glucose.
• The Citric acid cycle converts acetyl CoA to CO2, producing 2 GTP, 6 NADH, and 2 FADH2.