3.5 Cellular Respiration

Cellular Respiration

cells harvest chemical energy stored in organic molecules and use it to generate ATP

organic molecules + oxygen → CO2 + H2O + energy

starch is the major source of fuel for animals and breaks down into glucose

catabolic breakdown of glucose:

C6H12O6+6O2→6CO2+6H2O+energy(ATP and heat)

the oxidation of glucose transfers electrons to a lower energy state, releasing energy to be used in ATP synthesis

4 stages: Glycolysis, Pyruvate oxidation, Citric acid cycle (Krebs cycle), and oxidative phosphorylation (ETC and chemiosmosis)

total ATP produced: about 30-32 ATP

Glycolysis

starting point of cellular respiration

occurs in the cytosol

splits glucose (6C) into 2 pyruvates (3C)

2 stages: 

energy investment stage- the cell uses ATP to phosphorylate compounds of glucose

energy payoff stage- energy is produced by substrate level phosphorylation

starting materials: glucose, ATP, NAD+, ADP + Pi

end products: pyruvate, 2 ATP, NADH

2 ATP and 2 NADH are produced per 1 glucose

ATP and NADH later used in the ETC

2 pyruvate used in the Citric acid cycle

Pyruvate Oxidation

if oxygen is present, the pyruvate enters a mitochondrion (eukaryotic cells)

pyruvate is oxidized into acetyl coA

acetyl coA is used to make citrate in the citric acid cycle

CO2 is released as waste

electrons are transferred to carriers (NADH)

starting materials: pyruvate, coenzyme A, NAD+

end products: acetyl CoA, CO2, NADH

Citric Acid (Krebs) Cycle

occurs in the mitochondrial matrix

turns acetyl CoA into citrate, which:

releases CO2

ATP synthesized

electrons are transferred by NAD+ and FAD

NADH and FADH2 will then carry high energy electrons to the ETC

starting materials: acetyl CoA, NAD+, FAD, ADP + Pi

end products: CO2, NADH, FADH2, 2 ATP

2 ATP produced

Oxidative Phosphorylation

consists of electron transport chain (ETC) and chemiosmosis

starting materials: NADH, FADH2, O2, ADP + Pi

end products: NAD+, FAD, H2O, 26-28 ATP
26-28 ATP produced

Electron Transport Chain (ETC)

located in the inner membrane of the mitochondria

consists of a collection of electron carriers (proteins) embedded in the membrane

NADH and FADH2 (from the Krebs cycle) bring electrons to the ETC

electrons go down the ETC in a series of redox reactions until they reach the final electron acceptor, oxygen(very important to the process, because if oxygen did not accept the electrons and drive electrons down the ETC, the chain backs up which stops the flow of electrons and prevents the production of large amounts of ATP)

as electrons move through the ETC, they move from a higher to lower energy level

some of the released energy is used to pump H+ into the intermembrane space, forming an electrochemical gradient of protons (H+)

Chemiosmosis

H+ ions flow down their gradient(established due to ETC) back into the matrix through ATP synthase

drives the formation of ATP from ADP + Pi

the cristae increase the surface area for reactions to occur, which allows more ATP to be synthesized 

produces about 26-28 ATP per glucose

Anaerobic Respiration

generates ATP using an ETC in the absence of oxygen

takes place in prokaryotic organisms that live in environments with no oxygen

final electron acceptors are sulfates or nitrates

Fermentation

generates ATP without an ETC

extension of glycolysis

recycles NAD+ 

occurs in the cytosol

NO oxygen

2 types: alcohol fermentation and lactic acid fermentation

Alcohol Fermentation

pyruvate is converted into ethanol

2 pyruvate→2 acetaldehyde→2 ethanol

ex. bacteria and yeast

Lactic Acid Fermentation

pyruvate is reduced directly by NADH to form lactate

ex. muscle cells

when muscles run out of oxygen, they can go through lactic acid fermentation to produce ATP

causes the burning sensation you may feel when performing strenuous exercise

breakdown of lactate:

muscles produce lactate, which goes into the blood, and is broken down back to glucose in the liver

when lactate is in the blood, it lowers the pH

if lactate builds up and is unable to be broken down it can lead to lactic acidosis(excess

After glycolysis with and without oxygen

With oxygen: typical cellular respiration (glycolysis→pyruvate oxidation→krebs cycle→oxidative phosphorylation)

Without oxygen: Fermentation(anaerobic), Lactic acid fermentation in animals, alcohol fermentation in yeast(glycolysis→lactic acid/alcohol fermentation)