2.2 Cellular Respiration: The Details

Energy Carriers

NAD+ and FAD+ are low energy, oxidized coenzymes that act as electron acceptors.

When an electron(s) are added to these molecules, they become reduced to NADH and FADH2.

In this case, reducing a molecule gives it more energy.

Aerobic Respiration: Overview

Occurs in Four Distinct Stages:1. Glycolysis: 10-step process in the cytoplasm.2. Pyruvate Oxidation: 1-step process in the

mitochondrial matrix.3. Krebs Cycle: 8-step cyclical process in the

mitochondrial matrix.4. Electron Transport Chain & Chemiosmosis:

Multi-step process in the inner mitochondrial membrane.

Energy Transfer Terminology

Substrate-level Phosphorylation: ATP forms directly in an enzyme-catalyzed

reaction.

Oxidative Phosphorylation: ATP forms indirectly through a series of enzyme-

catalyzed redox reactions involving oxygen as the final electron acceptor.

Glycolysis 2 ATPs are used in

steps 1 & 3 to prepare glucose for splitting.

F 1,6-BP splits into DHAP and G3P.

DHAP converts to G3P. 2 NADH are formed in

step 6. 2 ATP are formed by

substrate-level phosphorylation in both steps 7 and 10.

2 pyruvates are produced in step 10.

Glycolysis

Energy Yield & Products:4 ATP produced – 2 ATP used = 2 net ATP

2 NADH

2 pyruvates

Further processing in aerobic cellular respiration

(if oxygen is available)

Mitochondria

Smooth

Highly folded

Folds of the inner membrane

Protein-rich liquid

Fluid-filled intermembrane

space

Pyruvate Oxidation(if oxygen is present…)

The following occurs for each pyruvate:

1. CO2 removed.2. NAD+ reduced to NADH and the 2-

carbon compound becomes acetic acid.3. Coenzyme A (CoA) attaches to acetic

acid to form acetyl-CoA.

Pyruvate Oxidation

Pyruvate Oxidation

Energy Yield & Products:2 NADH

2 acetyl-CoA

2 CO2 (released as waste)

The Krebs Cycle

Occurs twice for each molecule of glucose, 1 for each acetyl-CoA.

The Krebs Cycle1. In step 1, acetyl-CoA combines with oxaloacetate to

form citrate.2. In step 2, citrate is rearranged to isocitrate.3. NAD+ is reduced to NADH in steps 3, 4 and 8.4. FAD is reduced to FADH2 in step 6.5. ATP if formed in step 5 by substrate-level

phosphorylation. The phosphate group from succinyl-CoA is transferred to GDP, forming GTP, which then forms ATP.

6. In step 8, oxaloacetate is formed from malate, which is used as a reactant in step 1.

7. CO2 is released in steps 3 and 4.

The Krebs Cycle

Energy Yield & Products:2 ATP

6 NADH

2 FADH2

4 CO2 (released as waste)

NADH and FADH2 carry electrons to the electron transport chain for further production of ATP by oxidative

phosphorylation.

Electron Transport Chain (ETC)

A series of electron acceptors (proteins) are embedded in the cristae.

These proteins are arranged in order of increasing electronegativity.

The weakest attractor of electrons (NADH dehydrogenase) is at the start of the chain and the strongest (cytochrome oxidase) is at the end.

Electron Transport Chain (ETC)

These proteins pass electrons from NADH and FADH2 to one another through a series of redox reactions.

ETC protein complexes are alternately reduced and oxidized as they accept and donate electrons.

Electron Transport Chain (ETC)

As the electrons pass from one molecule to the next, it occupies a more stable position.

The free energy released is used to pump protons (H+) to the intermembrane space. 3 for every NADH and 2 for every FADH2.

This creates an electrochemical gradient, creating potential difference (voltage) similar to a battery.

Electron Transport Chain (ETC)

Protons are forced to pass back into the matrix through special proton channels associated with ATP synthase (ATPase).

For every H+ that passes through, enough free energy is released to create 1 ATP from the phosphorylation of ADP.

Conditions must be aerobic because oxygen acts as the final electron and H+ acceptor (water is formed as a byproduct).

Electron Transport Chain (ETC)

FADH2 FAD+

NADH dehydrogenase

Cytochrome b-c1 complex

ATP Yield from Aerobic Respiration

Controlling Aerobic Respiration

Regulated by feedback inhibition and product activation loops.

Phosphofructokinase is an allosteric enzyme that catalyzes the third reaction in glycolysis and is inhibited by ATP and stimulated by ADP.

If citrate (first product of Krebs cycle) accumulates, some will pass into the cytoplasm and inhibit phosphofructokinase and slow down glycolysis.

As citrate is used up, its concentration will decrease and the rate of glycolysis will increase.

Controlling Aerobic Respiration

A high concentration of NADH indicates that the ETCs are full of electrons and ATP production is high.

NADH allosterically inhibits an enzyme that reduces the amount of acetyl-CoA that is shuttled to the Krebs cycle, reducing the amount of NADH produced.