Cellular Respiration•The second stage of cellular respiration is the Krebs cycle, which operates only when oxygen is available. •Because it depends on oxygen, it is an aerobic process.

•It is named after its discoverer Hans Krebs.•The Krebs cycle takes place in the innermost compartment of the mitochondrion called the matrix

Mitochondrion

Hans KrebsDiscovered “Krebs” cycle in 1937

Won Nobel Prize in 1953

Cellular Respiration•Pyruvic acid molecules, produced by glycolysis, enters the matrix and are broken down through chemical reactions resulting in the production of citric acid. •Because this is the first product made, it is also called the Citric Acid Cycle.•Along with the production of citric acid, it also releases carbon dioxide (CO2).

Cellular Respiration•During the cycle, energy is released by the breaking and rearranging of carbon bonds and is captured in ATP, NADH, and FADH2.

•Once the Krebs cycle is complete, the cell goes on to derive the majority of its ATP from the electron transport chain.

Cellular Respiration: Krebs cycle

Steps in Cellular Respiration

Location What goes in What comes out

ATP Results

(1) Glycolysis Anaerobic process

Cytoplasm (cytosol)

one Glucose2 ATP to start

2 pyruvic acids2 NADH

4 produced for a net gain of 2 ATP

(2) Krebs cycle (Citric Acid cycle) Aerobic process

MitochondrialMatrix

2 pyruvic acids C02, 4 NADH,2 FADH2

2 ATP

(3) Electron Transport Chain 

Inner Mitochondrial Membrane (cristae)

NADH, FADH2,oxygen

WaterNAD+ (recycled)FAD+ (recycled)

32-34 ATP

Total       36-38 ATP

Cellular RespirationElectron Transport Chain

•The electron carriers produced during glycolysis (NADH) and the Krebs cycle (NADH and FADH2) bring high-energy electrons to the electron transport chain. •The passing of electrons through the electron transport chain causes H+ ions to build up in the intermembrane space, making it positively charged relative to the matrix.

Cellular RespirationElectron Transport Chain

•The charge difference across the membrane forces H+ ions through channels in enzymes known as ATP synthases. As the ATP synthases spin, a phosphate group is added to ADP, generating ATP.•At the end of the chain the electrons are taken up by oxygen molecules, along with hydrogen atoms, to make water.•This is why oxygen is known as the final electron acceptor.

Cellular RespirationElectron Transport Chain

•The electron transport chain is the stepwise process of cellular respiration that is responsible for producing:•Water (with the help of oxygen we breathe)•32 to 34 ATP (thanks to the proton, H+ , gradient)•NAD+ and FAD+ (which are recycled to be used again in the Krebs cycle and glycolysis)

Cellular RespirationElectron Transport Chain

•This process happens in the mitochondria along the inner membrane called the cristae.

Cellular RespirationElectron Transport Chain

•To put things in perspective think about how we breathe in oxygen with our lungs, transport it with red blood cells in our arteries to cells, and the oxygen is ultimately used inside the mitochondria of every cell to accept electrons at the end of the electron transport chain.

Cellular RespirationElectron Transport Chain

•For Prokaryotes, which don’t have mitochondria, this process happens in the cell membrane.

Cellular Respiration: Electron Transport

Cellular Respiration

Cellular Respiration

Fermentation•Fermentation is an anaerobic process that converts NADH back to NAD+, thus producing energy.

Eukaryotic cells (1) Lactic Acid Fermentation•Lactic acid is produced in muscles during rapid exercise.

•This is because the body cannot supply enough oxygen to the tissues to produce the needed ATP.

Fermentation•A buildup of lactic acid causes a burning sensation in the muscles.

(2) Alcoholic fermentation•It occurs in yeast. •It produces alcohol and carbon dioxide(CO2).•The CO2 is what causes dough to rise, air spaces in bread, and bubbles in alcoholic drinks.

FermentationProkaryotic cells•Some types of bacteria produce alcohol and CO2.

•Other kinds of bacteria produce lactic acid.

Fermentation