Chapter 9 - Cellular Respiration and Fermentation

Fermentation

A partial degradation of sugars or other organic fuels that occurs without the use of oxygen

Aerobic Respiration

Most efficient catabolic pathway, oxygen is consumed as a reactant along with organic fuel.

Anaerobic Respiration

Prokaryotes use substances other than oxygen as reactions, harvest chemical energy without oxygen.

Cellular Respiration Equation

C6H12O6 + 6O2 —> 6CO2 + 6H2O + Energy (ATP + Heat)

Redox Reactions

The transfer of one or more electrons from one reactant to another.

• Doesn’t need to involve the complete transfer of electrons from one substance to another, it can be the degree (electronegativity) of electron sharing in covalent bonds.

• Redox reactions that move electrons closer to high electronegative atoms release chemical energy.

What products/reactants are oxidized/reduced?

• Oxidized: Glucose & Carbon Dioxide

• Reduced: Oxygen & Water

Oxidation in cellular respiration

Oxidation of glucose transfers electrons to a lower energy state, freeing energy that becomes available for ATP synthesis.

How is glucose broken down?

In a series of steps to allow it to be harnessed efficiently. If only in one step, the cell cannot handle all that energy at once.

Energy Harvest

1. Glucose enters the body.

2. Enzymes called dehydrogenases remove a pair of hydrogen atoms and delivers the 2 electrons and 1 proton to its coenzyme.

• Other proton is released as a hydrogen ion to surroundings.

3. NAD+ , the electron carrier, takes the electrons turning into NADH.

• Another electron carrier: FAD, oxidizing agent.

4. NADH moves electrons to an electron transport chain through various steps.

• Each “downhill” carrier has a greater affinity for electrons than the other and is thus capable of accepting electrons from its “uphill” neighbor.

5. Oxygen catches it at the end, combining it with H+ and forming water.

6. This process creates energy allowing protons to be pumped across the membrane and thus producing ATP.

Stages of Cellular Respiration & their location

1. Glycolysis

• Occurs in cytosol

2. Pyruvate Oxidation & Citric Acid Cycle

• Occurs in matrix

3. Oxidative Phosphorylation: Electron transport and chemiosmosis

• Occurs in inner membrane.

Substrate-level phosphorylation

An enzyme transfers a phosphate group from a substrate molecule to ADP and produces a small amount of ATP. (Glycolysis & Citric acid cycle)

Glycolysis Summarized

• Occurs in cytosol.

1. Energy investment stage: Cell spends 2 ATP to phosphorylate compounds of glucose and splits into two 3-carbon sugars.

2. Energy pay off phase: 4 ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH by electrons released by the oxidation of glucose. Remaining atoms are rearranged to form two molecules pyruvate.

• Occurs whether or not oxygen as is present.

Glycolysis Products

2 pyruvate

2 NADPH

4 ATP (2 ATP used earlier)

2 H2O

Energy Investment Stage

1. Glucose is phosphorylated by ATP, making it more chemically reactive. The charged phosphate also traps the sugar in the cell.

2. Glucose is converted to fructose.

3. Fructose is phosphorylated by ATP.

4. It is now heavily unstable due to the input of 2 ATP so it breaks into two 3-carbon sugar molecules. (G3P & DHAP)

Energy Payoff Stage

• DHAP goes through another process to turn into G3P.

1. G3P is oxidized by NAD+ and a phosphate group is attaches to the oxidized substrate from the energy released in the oxidation.

2. The phosphate group is transferred to ADP.

3. The enzyme relocates the remaining phosphate group.

4. Enolase (enzyme) causes a double bond to form in the substrate by extracting a water mole, making PEP, high potential energy molecule.

5. The last phosphate group is transferred to ADP, forming pyruvate.

• 90% chemical energy from glucose is still unused.

Krebs Cycle Summarized

• Occurs in mitochondrion matrix and when oxygen is present.

Oxidation of Pyruvate to Acetyl CoA

1. Pyruvate enters the mitochondrion via active transport.

2. Pyruvate’s carboxyl group [COO-]  (somewhat oxidized) is fully oxidized and given off as a molecule of CO2. 

3. The two carbon fragments are oxidized and the electrons transferred to NAD+.

Mitochondria Structure

• Matrix (cytosol), intermembrane space (space between two membranes), cristae (inner folds that increase surface area), double membrane, ribosomes, small circular DNA.