1. Respiration as Redox
Respiration is a redox process that transfers hydrogen from sugar to oxygen.
Valence electrons of carbon and hydrogen lose potential energy as they shift toward electronegative oxygen.
Released energy is used by cells to produce ATP.
Organic molecules with abundant hydrogen are excellent fuels because they are rich in high-energy electrons. A mole of glucose yields 686 Kcal of heat when burned in air.

2. Stepwise conversion of Energy
Cellular combustion of glucose is not a single explosive step, as the energy would be difficult to harness.
In the cell, glucose is oxidized gradually in a series of enzyme-controlled steps that occur during glycolysis and the citric acid cycle (Krebs cycle).
Hydrogens stripped from glucose are not transferred directly to oxygen, but are first passed to a special electron acceptor - NAD+ or FAD.

3. NAD+ and the Oxidation of Glucose
Coenzyme = Small nonprotein organic molecule required for proper enzyme catalysts.
Nicotinamide adenine dinucleotide (NAD+) = A dinucleotide which functions as a coenzyme in the redox reactions of metabolism. Found in all cells.
Assists enzymes in electron transfer.
Another such coenzyme is FAD (flavin adenine dinucleotide).

4. XH2 + NAD+ --> X + NADH + H+
X = Various substrates oxidized by enzymatic transfer of electrons to NAD+.
NAD+ = Oxidized coenzyme (net positive charge).
NADH = Reduced coenzyme (electrically neutral).
These high energy electrons transferred from substrate to NAD+ are then passed down the electron transport chain to oxygen, powering ATP synthesis (oxidative phosphorylation).

5. Overview of Cellular Respiration
There are three metabolic stages of cellular respiration: a. Glycolysis b. Krebs Cycle c. Electron transport chain and oxidative phosphorylation

6. Overview-cont’d. Glycolysis is a catabolic pathway that:
Occurs in the cytosol.
Partially oxidizes glucose (6C) into two pyruvic acid (3C) molecules.
The Krebs Cycle is a catabolic pathway that:
Is located within the mitochondrial matrix.
Completes glucose oxidation by breaking down a pyruvic acid derivative (acetyl CoA) into carbon dioxide.

7. Overview-cont’d. Glycolysis and the Krebs Cycle:
Directly produce a small amount of ATP.
Supply energized electrons that indirectly drive most ATP production by oxidative phosphorylation

8. Oxidative phosphorylation
accounts for most ATP production during respiration.
includes an electron transport chain made of electron-carrier molecules built into the inner mitochondrial membrane.
Oxygen pulls energized electrons down the electron transport chain to a lower energy state.
This exergonic slide of electrons is coupled to ATP synthesis.
For each molecule of glucose oxidized, the cell makes about 36 to 38 ATP molecules.

9. Energy Coupling Mechanisms
two basic mechanisms that couple the exergonic oxidation of glucose to the endergonic synthesis of ATP
substrate-level phosphorylation
chemiosmosis.

10. Substrate-level phosphorylation
the direct enzymatic transfer of phosphate to ADP from an intermediate substrate in catabolism.
Only a small percentage of ATP is produced this way: most ATP is produced by oxidative phosphorylation.

11. Chemiosmotic Coupling: The Basic Principle
The mechanism for coupling exergonic electron flow from the oxidation of food to endergonic ATP production is chemiosmosis.
The coupling of exergonic electron flow down an electron transport chain to endergonic ATP production by the creation of an electrochemical proton gradient across a membrane. The proton gradient drives ATP synthesis as protons diffuse back across the membrane.

12. Chemiosmosis - cont’d.
Proposed by British biochemist, Peter Mitchell (1961).
The term chemiosmosis emphasizes a coupling between (1) chemical reactions and (2) transport processes.

13. Chemiosmosis - cont’d.
Membrane structure plays a prominent functional role in chemiosmosis:
Integral membrane proteins translocate H+ across a membrane, creating a proton (or pH) gradient.
The membrane's phospholipid bilayer is impermeable to H+, so it counteracts the tendency for protons to leak back across the membrane by diffusion.

14. Transmembrane protein complexes called ATP synthase use the potential energy stored in a proton gradient to make ATP by allowing H+ to diffuse down the gradient, back across the membrane. As protons diffuse through the ATP synthase complex, ATP synthase phosphorylates ADP.
The energy required to create the proton gradient comes from:
Light - during the energy-capturing reactions of photosynthesis.
Oxidation of glucose - during glycolysis and the Krebs Cycle of respiration.

15. Chemiosmosis
During respiration, chemiosmosis occurs across the inner membrane of the mitochondria.
Using energy from the oxidation of glucose, the electron transport chain translocates H+ from the mitochondrial matrix, across the inner membrane to the intermembrane space.
Cristae or infoldings of the inner mitochondrial membrane, increase the surface area available for chemiosmosis to occur.