Metabolism of the Cell Energy Production Metabolism Refers

Metabolism of the Cell Energy Production

Metabolism • Refers to “all chemical reactions necessary to maintain life”. • Anabolic processes (anabolism) build from smaller molecules – for example, building of proteins from amino acids – anabolic processes generally require energy input • Catabolic processes (catabolism) break down larger molecules into smaller ones. – for example, breakdown of glucose into carbon dioxide and water – catabolic processes generally release energy

Cellular Respiration • Describes the series of reactions that break down glucose to release ATP. • Includes glycolysis, Krebs cycle, and oxidative phosphorylation within the electron transport system.

Major Stages of Metabolism I. Glycolysis II. Krebs Cycle (a. k. a. , tricarboxylic acid cycle, TCA cycle, citric acid cycle) III. Oxidative Phosphorylation Electron Transport System and Chemiosmosis • Digestion, the breakdown of food into usable molecules such as glucose, is required before metabolism can occur

Metabolism Terms • ATP – Adenosine triphosphate - simplest storage form of cellular energy • Glucose – main substrate for ATP production; a monosaccharide (simple sugar). • NAD+ – coenzyme that accepts hydrogen; derived from niacin – NADH (more accurately – NADH+H+) reduced form of the coenzyme; includes two additional electrons and one hydrogen. • FAD+ – another coenzyme that accepts hydrogen; derived from riboflavin. – FADH 2 – reduced form including two hydrogens and their electrons

Metabolism Terms(cont. ) • Oxidation - reactions within cellular respiration that occurs due to a loss of electrons (usually seen as hydrogen) or a gain in oxygen. • Oxidation-reduction (redox) reactions – coupled reactions in which one substance donates (loses) electrons (i. e. , is oxidized) and another then gains those electrons (i. e. , is reduced). • Reduction – reaction in which a substrate gains electrons (usually seen as hydrogen)

Metabolism Terms(cont. ) • Phosphorylation – addition of phosphate group to a molecule resulting in addition of energy to the molecule; e. g. , ADP + Pi ATP – substrate-level phosphorylation – direct transfer from one molecule to another • e. g. , bisphophoglycerate + ADP ATP + phosphoglycerate – moves phosphate from bisphoglycerate (which has 2 phosphates) to ADP to make ATP and phosphoglycerate (which has one phosphate) – oxidative phosphorylation – more complex method of ATP production involving electron transport chain (series of redox reactions)

Glycolysis

Glycolysis • Breakdown of glucose into pyruvic acid – Glucose comes from the food that we eat • Occurs in the cytoplasm of the cells • Results in a net production of 2 ATP and 2 reduced electron carriers (NADH)

Glycolysis - Overview 3 Phases: • sugar activation • sugar cleavage • sugar oxidation and formation of ATP Fig. 25. 6, p. 966

Phase 1 – Sugar Activation Glucose (6 -carbon) gains phosphate (and energy) from each of two (2) ATP molecules and becomes an unstable 6 -carbon molecule (fructose-1, 6 -diphosphate) • Adenosine triphosphate (ATP) becomes adenosine diphosphate (ADP) Fig. 25. 6, p. 966

Phase Two – Sugar Cleavage DHAP G 3 P Unstable 6 -carbon molecule, (fructose-1, 6 diphosphate), is broken into 2 3 -carbon molecules (DHAP and G 3 P) • no additional energy required Fig. 25. 6, p. 966

Phase Three – Sugar Oxidation 2 3 -carbon molecules (DHAP and G 3 P) are each oxidized resulting in release of energy used to make 4 ATP (2 from each) and formation of 2 pyruvate (1 from each) • ATP produced by substratelevel phosphorylation Fig. 25. 6, p. 966

Phase Three – Sugar Oxidation (con’t) • oxidation of these 2 3 -carbon molecules (DHAP and G 3 P) also results in formation of 2 molecules of NADH (one from each 3 -carbon molecule), the energy from which will be used in the mitochondria during oxidative phosphorylation • when oxygen is present, pyruvate diffuses into the mitochondria for the next steps of cellular respiration • when oxygen is not present, pyruvate is reduced using the NADH and becomes lactic acid

Glycolysis Energy Summary • Net ATP production = 2 – 4 produced – 2 used • two (2) ATP are utilized in phase 1, so 2 are subtracted from the total produced • 2 NADH produced total (one per G 3 P) – will be used in electron transport chain / oxidative phosphorylation in mitochondria when oxygen is present

From Glycolysis to Krebs • Pyruvate created in glycolysis diffuses from the cell cytoplasm into the matrix of the mitochondria to be further broken down • Going from glycolysis to Krebs involves an intermediate step in which pyruvate is reduced and reworked into a 2 -carbon molecule called acetyl-Co. A by removing one CO 2 group and addition coenzyme A; this also produces 1 molecule of NADH for each pyruvate

Krebs Cycle

Krebs Cycle • Begins when the acetyl group of acetyl-Co. A combines with oxaloacetate (4 -carbon molecule) to form citrate (6 -carbon molecule) and release the Co. A. • Remaining reactions involve oxidizing the molecule and regenerating oxaloacetate – reactions also produce carbon dioxide (CO 2), which will be released from the cell; reduced electron carriers, which will be used in the next stage; and GTP, which can be used to produce an equivalent amount of ATP Fig. 25. 7, p. 968

Krebs Cycle (con’t) • Each citrate is rearranged during this cycle to produce two (2) CO 2 molecules, three (3) NADH, one (1) FADH 2, and one (1) ATP. Fig. 25. 7, p. 968

Acetyl-Co. A and Krebs Cycle Energy Summary • ATP Production – One (1) per cycle of Krebs. • NADH Production – one (1) NADH created with the formation of Acetyl Co. A – three (3) NADH created during Krebs cycle • FADH 2 Production – One per cycle of Krebs • REMEMBER: Krebs goes through 2 times for each glucose that started the process

Acetyl-Co. A and Krebs Cycle Energy Summary • REMEMBER: Krebs goes through 2 times for each glucose that started the process, therefore, the total is: – 2 ATP created during the Krebs cycle – 2 NADH created with the formation of Acetyl Co. A – 6 NADH created during Krebs cycle – 2 FADH 2 created during Krebs cycle

Total Energy Summary, Through Krebs Cycle From one (1) molecule of glucose: • 4 ATP – 2 (Glycolysis) + 2 (Krebs) • 10 NADH – 2 (Glycolysis) + 2 (Acetyl Co. A formation) + 6 (Krebs) • 2 FADH 2 – 2 (Krebs)

Oxidative Phosphorylation Electron Transport System (ETS) and Chemiosmosis

Electron Transport System • Utilizes the NADH and FADH 2 produced in Glycolysis and Krebs. • Occurs in the inner membrane of the mitochondria. • Cannot occur without oxygen

Electron Transport System Step 1 Molecules within the inner membrane of the mitochondria take the two (2) electrons from NADH and the two (2) from FADH 2 and pass them from one to another. (i. e. , redox reactions) Fig. 25. 8, p. 969

Electron Transport System Step 2 • The transfer of electrons moves hydrogen atoms (H+) into the intermembrane compartment of the mitochondrion. Fig. 25. 8, p. p. 969

Electron Transport System Step 3 • When enough H+ atoms collect in the compartment, they travel down their concentration gradient into the mitochrondrial matrix in a process called chemiosmosis. • Movement occurs through ATP synthase enzyme that uses the energy of H+ movement to create ATP from ADP Fig. 25. 8, p. 969

Electron Transport System Step 4 • Once the electrons have moved to the end of the molecules in this chain, two(2) electrons combine with one (1) oxygen atom (formed from the breakdown of molecular oxygen, or O 2) and two (2) hydrogen atoms to make water. • Oxygen is a KEY component to this process. – If oxygen is not present to combine and make water, then the electron transport system backs up, no H+ atoms are released and ATP cannot form. – Krebs cycle cannot be run because NAD and FAD are not regenerated

Electron Transport System ATP Totals • Each pair of H+ moved results in formation of 1 ATP • ATP from NADH – each NADH moves 3 pairs of H+ : – 6 NADH from Krebs 18 ATP – 2 NADH from Acetyl Co. A 6 ATP – 2 NADH from Glycolysis 6 ATP • BUT there is a cost to move the NADH in from the cytoplasm at 1 ATP each, so the net total in most cells is 4 ATP. • This loss DOES NOT OCCUR IN HEART CELLS OR LIVER CELLS. In these cells movement is more efficient, and the ATP production is 6 ATP!

Electron Transport System ATP Totals • ATP from FADH 2 – FADH 2 transfers its H+ pairs at a different point, resulting in only 2 ATP per molecule – 2 FADH 2 from Krebs 4 ATP. • Total ATP: – ATP from NADH = 18 + 6 + (4 or 6) = 28 (or 30) – ATP from FADH 2 = 4 – TOTAL from ETS = 32 (or 34) ATP

Total ATP Production through the Entire System 2 ATP (Glycolysis) + 2 ATP (Krebs) + 32 -34 ATP (from ETS) = 36 -38 ATP Fig. 25. 10, p. 972