Chapter 7: How Cells Harvest Energy (Cellular Respiration)

Anaerobic respiration

Some prokaryotes use compounds other than oxygen as reactants

Identify where substrate-level phosphorylation and the reduction of NAD+ occur in glycolysis?

- Substrate level phosphorylation occurs in energy payoff phase. -During phosphoglycerate kinase and pyruvate kinase. -Reduction of NAD+ occurs in energy payoff phase when its reduced to NADH by electrons released by the oxidation of glucose.

Overview

-Cells harvest chemical energy stored in organic molecules and use it to regenerate ATP, the molecule that drives most cellular work. -Respiration has 4 key pathways: Glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

Glycolysis

-Occurs in the cytosol. It begins catabolism by breaking glucose into two molecules of pyruvate.

Pyruvate Oxidation

-Occurs in the mitochondrial matrix of eukaryotic cells. In pyruvate oxidation, pyruvate is converted to Acetyl Co-A (a 2 carbon molecule). In the process, 1 carbon dioxide molecule is released.

Citric Acid Cycle

-Occurs in the mitochondrial matrix of eukaryotic cells. It completes the breakdown of glucose by oxidizing Acetyl Co-A to carbon dioxide.

Main Energy Foods: Carbohydrates & Fats

-Reservoirs of electrons associated with hydrogen. -Stable because of barrier of activation energy.

Electron Transport Chain

Accepts e from the breakdown products of the first two stages (most often via NADH). In eukaryotic cells, the inner membrane of the mitochondrion is the site of electron transport and chemiosmosis, the processes that together constitute oxidative phosphorylation. In prokaryotes, these processes take place in the plasma membrane. -e move from molecule to molecule until they combine with molecular oxygen and hydrogen ions to form water. -as e passed along chain, energy released at each step in the chain is stored in a form the mitochondrion (or prokaryotic cell) can use to make ATP. -This mode of ATP synthesis is called oxidative phosphorylation because it is powered by the redox reactions of the electron transport chain. -oxidative phosphorylation produces almost 90% of the ATP generated by respiration.

Cellular respiration

Although technically it includes both aerobic and anaerobic processes, the term is commonly used to refer only to the aerobic process

Substrate-level phosphorylation

An enzyme transfers a phosphate group from an organic substrate molecule to ADP, forming ATP. -ATP formed directly during glycolysis and citric acid cycle.

Glucose

C6H12O6 + 6O2 -> 6CO2 + 6H20 + energy -catabolism is exergonic, ∆G = -686 Kcal/mol

Catabolic pathways yield energy by oxidizing organic fuels

Carbs, fats, and proteins can all be used as the fuel, most useful to consider glucose.

Aerobic respiration

Consumes oxygen as a reactant to complete the breakdown of a variety of organic molecules. -Most eukaryotic and many prokaryotic organisms can carry out aerobic respiration.

How Does NAD+ trap electrons from glucose?

Dehydrogenase enzymes strip two H atoms from the substrate (glucose), thus oxidizing it. The enzyme passes two electrons and one proton to NAD+. The other proton is released as H+ to the surrounding solution. By receiving 2 e and 1 proton, NAD+ has its charged neutralized when it is reduced to NADH. -NAD+ functions as the oxidizing agent in many of the redox steps during breakdown of glucose. The e carried by NADH lose very litle of their potential energy in process.

Carbon skeleton of glucose changes as it proceeds through glycolysis

Glucose a 6 carbon sugar, is split into 2 3 carbon sugars.

Salt (redox reaction)

Na + Cl -> Na+ + Cl-

Summary equation for cellular respiration.

Organic compounds + O2 -> CO2 + H20 + energy (ATP + heat)

ATP required for preparatory steps of glycolysis?

Phosphorylation of glucose.

Organic fuel molecules are oxidized during cellular respiration

Respiration is a redox process. -In series of reactions, glucose is oxidized and oxygen is reduced. -e lose potential energy along the way, releasing energy that becomes available for ATP synthesis.

Electron transport Chain

Several molecules (primarily proteins) built into the inner membrane of a mitochondrion of eukaryotic cells and the plasma membrane of aerobically respiring prokaryotes. Electrons released from food are shuttled by NADH to the top higher energy end of the chain. At the bottom lower energy end, oxygen captures the electrons along with H+ to form water. -e transfer from NADH to oxygen is an exergonic reaction with a free energy change of -53kcal. -e are passed to increasingly EN molecules in the chain until they reduce oxygen, the most EN receptor.

Redox Reactions Xe- + Y -> X + Ye-

X, the electron doner, is the reducing agent and reduces Y by donating an electron to it. Y, the electron recipient, is the oxidizing agent and oxidizes X by removing its electron. -Energy must be added to pull an electron away from an atom. -The more EN the atom, the more energy is required to take an electron away from it. -e loses potential energy when shifting from less EN atom toward a more EN one.

In summary, during cellular respiration, most electrons travel the following "downhill" route:

glucose -> NADH -> electron transport chain -> oxygen

fermentation

when oxygen is unavailable, an organic molecule, such as acetaldehyde in wine fermentation, can accept electrons instead. This reaction plays an important role in most organism, even though capable of aerobic respiration.