Cell Respiration Glycloysis and the Krebs Cycle First

Cell Respiration Glycloysis and the Krebs Cycle

First - an Overview of Life

Introduction to Cell Respiration and Fermentation n n Cellular Respiration- is a metabolic process used to obtain energy from organic compounds or food, to replace ATP’s This process can run under aerobic and anaerobic conditions. Aerobic conditions yield more ATP. Both cellular respiration and fermentation are catabolic pathways, breaking down organic molecules like glucose. Cell respiration occurs in the mitochondria of your cells for the most part. 4 main steps: Glycolysis, the Link Reaction, the Krebs Cycle, and the ETC (electron transport chain)

Goal of Cell Respiration is ATP. Why? ? ?

The 3 Steps

Another Brief Introduction n n Glycolysis occurs in the cytoplasm it breaks glucose into two molecules of pyruvate. Krebs cycle occurs in the mitochondrial matrix it degrades pyruvate to carbon dioxide. Both processes transfer electrons from substrates to NAD+, forming NADH passes these electrons to the electron transport chain. The ETC electrons move from molecule to molecule until they combine with oxygen and hydrogen ions to form water.

Introduction continued…. n n n As they move along the energy in these electrons is stored in the mitochondria to be used in the formation of ATP via oxidative phosphorylation, which produces 90% of all ATP. Some ATP is made during glycolysis and the Krebs Cycle by substrate-level phosphorylation, enzyme transfers a phosphate group from an organic molecule (the substrate) to ADP, forming ATP. Ultimately 38 ATP are produced per mole of glucose

Substrate-Level Phosphorylation

Step 1 Glycolysis n n During glycolysis, glucose, 6 carbon-sugar, is split into 2, 3 carbon sugars. Glucose which is formed when carbohydrates break down in digestion is the primary source of fuel for cellular respiration. Other substances can also be used like protein, fats, and nucleic acids, but most of these are used to build parts of the cell. They are only used when energy from carbs. Is not present. All ten steps of glycolysis is catalyzed by a specific enzyme.

2 Phases of Glycolysis n n n energy investment phase, ATP provides activation energy by phosphorylating glucose, 2 ATP per molecule energy investment phase, ATP provides the activation energy by phosphorylating glucose 4 ATP (net) and 2 NADH are produced per glucose.

End of Glycolysis n n net yield of glycolysis is 2 ATP and 2 NADH/glucose. Glycolysis occurs whether O 2 is there or not. So it can run under aerobic or anaerobic conditions.

The Krebs Cycle n n Krebs cycle is named after Hans Krebs who was mainly responsible for discovering its pathways in the 1930’s. Entering the Krebs Cycle 75% of the original energy in glucose is still present in the 2 molecules of pyruvate With oxygen present the pyruvate enter the in the mitochondrion where enzymes of the Krebs cycle oxidize the organic fuel to carbon dioxide. Step 1: When pyruvate enters the mitochondria sveral enzymes modify it into acetyl Co. A, carboxyl group is removed as CO 2 NADH is also made using that are given off by the reaction.

Link reaction

Krebs Cycle

Krebs Cycle n n Each cycle produces one ATP by substrate -level phosphorylation, three NADH + H+, and one FADH 2 Krebs cycle produces large quantities of electron carriers

ETC n n n So far only 4 of the 38 ATP that will be produced have been, all by substrate level phosphorylation. The remaining will be produced by the ETC. The majority of the ATP produced comes from the energy carried in the electrons of NADH (and FADH 2) that were produced by the Krebs Cycle. 6 NADH and 2 FADH 2 The energy in these electrons is used in the ETC to power the synthesis of ATP. There are thousands of ETC’s found in each mitochondria, which can number in the 100’s depending on the cell type. Electrons drop in energy as they pass down the ETC

n n Electrons carried by NADH are transferred to the first molecule of the ETC The electrons carried by FADH 2 have lower free energy and are added later in the chain.

ETC Continued…… n n n For every 2 electron carriers (4 electrons), one O 2 molecule is reduced to 2 molecules of water. ETC makes no ATP directly. Its function is to break the large drop in energy from food molecules down to oxygen into small manageable steps. The actual production of ATP comes from a protein complex, ATP synthase, that adds a phosphate onto ADP making ATP. The ETC used the energy from the moving electrons to pump H protons into the inter-membrane of the mitochondria to create a concentration gradient.

ETC Continued. .

Fermentation Overview n n n n Produces ATP without oxygen. No ETC is present since there is no oxygen NAD+ gets recycled by use of an organic hydrogen acceptor like lactate or ethanol. Common in prokaryotes and very useful to humans. Two type lactic acid and alcohol fermentation. A build up of lactate in your muscles from over exerting yourself and not taking in enough oxygen causes soreness. Alcohol fermentation has a by product of CO 2 and ethanol which is used to make alcoholic beverages. Yeast and fungus go through alcohol fermentation. The release of CO 2 by yeast is what causes bread to rise.

Alcohol Fermentation n n Pyruvate is converted to ethanol in two steps. Alcohol fermentation by yeast is used in brewing and winemaking.

Lactic Acid Fermentation n Pyruvate is reduced directly by NADH to form lactate Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver.

Glycolysis Can Use Many Substances Metabolism is extraordinarily versatile and adaptable

Control n n n Regulated by supply and demand If ATP levels drop, metabolism speeds up to produce more ATP. If there is an excess it slows