CELLULAR RESPIRATION- is the most efficient way for the cells to harvest energy stored in food.
It is a
catabolic pathway for the production of ATP, a high energy molecule.
Cellular respiration may occur in the presence or absence of oxygen. Oxygen is present in the aerobic
respiration, but absent in anaerobic respiration.
Stages of cellular respiration: glycolysis, pyruvate oxidation, Krebs cycle (citric acid cycle), and oxidative
phosphorylation (electron transport chain and chemiosmosis)
Most of the process of cellular respiration takes place in mitochondria which is the power house of the
cell.
I. GLYCOLYSIS- “splitting of sugar”
It is the process wherein the six-carbon sugar which is glucose is broken down into two molecules of
pyruvate or pyruvic acid.
C6H12O6→ C3H4O6
Glycolysis takes place in the cytoplasm.
Proceeds even in the absence of oxygen.
STEPS:
1. Glucose is phosphorylated using two ATP molecules.
Phosphorylation is the process of adding a phosphate group to a molecule.
2. After the phosphorylation of glucose, it is converted into two 3- Carbon molecule called PGAL
(phosphoglyceraldehyde) as an intermediate compound.
3. A phosphate group is added to each PGAL molecule.
4. NAD in the cytoplasm with the help of the enzyme will strip off the Hydrogen. Same thing happens in
another NAD, forming two molecules of NADH.
5. The phosphate bonds of PGAL are broken forming two molecules of ATP from each molecule of PGAL
and converting them to molecules of pyruvic acid or pyruvate.
Products of glycolysis: 2 pyruvate or pyruvic acid, 2 NADH, 4 ATP or net gain of 2 ATP
Pyruvic acid oxidation – transition stage that occurs in the mitochondrial matrix
- stage between glycolysis and Krebs cycle,
1. Pyruvate is broken down into acetic acid. In the process of doing this, NAD removes H to create NADH
and Carbon dioxide is created as waste.
2. Coenzyme- A bonds to acetic acid creating Acetyl- CoA .
These two molecules will now enter the Krebs cycle to begin the aerobic phase of cellular respiration.
Products: 2 NADH, 2 CO2, 2 Acetyl -CoA
II. Krebs Cycle (Citric Acid cycle)
• It is the series of chemical reactions that take place in the presence of oxygen inside the matrix of
mitochondrion.
• The process was discovered by Hans Krebs. It is called citric acid cycle due its intermediate product,
citric acid.
• It is coupled with the release of carbon dioxide and hydrogen ions that result in the production of
ATP.
STEPS:
1. The 2 Carbon acetyl group of acetyl coA combines with 4 Carbon (oxaloacetic acid) releasing CoA and
forming a 6 Carbon molecule called citric acid.
2. The 6-Carbon molecule or citric acid is broken down to a 5- Carbon compound ( ketoglutaric acid), NAD
removes H to make NADH and CO2 is released as waste product.
3. The 5 Carbon compound is broken down by an enzyme producing a 4 -Carbon compound called (succinic
acid) forming NADH and ATP. The waste product of this process is CO2.
4. The 4-carbon molecules is rearranged forming malic acid and high- energy electrons are formed
including the electron carriers NADH and FADH2.
FAD- flavine adenine dinucleotide.
5. Acetyl CoA bonds to 4 Carbon molecules, CoA breaks away resulting to 6 carbon molecules recreating
citric acid.
Products: 6 NADH, 2 FADH2, 2 ATP, 4 CO2
The electrons in the electron carriers, NADH and FADH2 are used to generate ATP molecules in the
processes that take place in the electron transport chain.
III. Oxidative phosphorylation : Electron Transport Chain and Chemiosmosis
Enzymes and ion carriers line the inner membrane of the mitochondria, where the electron transport
chain takes place.
STEPS:
1. NADH and FADH2 deliver Hydrogen ions and donates 2 electrons to the electron transport chain with
the help of enzyme.
2. The electrons are passed down from one acceptor to another and start the chain of reaction.
3. Electrons activate the protein channels to pump Hydrogen ions out of the matrix.
4. Hydrogen ions activate the ATP synthase.
5. Hydrogen diffuses through ATP synthase. The energy generated from this movement bonds ADP to P
creating ATP. The process repeats until it creates 34 ATP.
6. At the end of the ETC, oxygen accepts hydrogen and one electron to form water.
Products: 34 ATP and 6 water, 4 FAD, 8 NAD
Chemiosmosis describes the movement of electrons down their electrochemical gradient.
Coenzyme- binds with an enzyme to catalyze a reaction.
ATPs Produced in Cellular Respiration
Pathways Coenzymes Reduced or Produced ATPs Used
Glycolysis 4 ATP -2 ATP
Pyruvic acid oxidation
Krebs cycle 2 ATP
Electron Transport Chain
3 ATPs per NADH, 2 ATPs per
FADH2
2 NADH ( from Glycolysis) 4-6 ATP
2 NADH ( from pyruvic oxidation) 6 ATP
6 NADH ( from Krebs cycle) 18 ATP
2 FADH2 ( from Krebs cycle) 4 ATP
Gross ATPs produced 38 ATP
ATPs used 2
Net ATP produced 36- 38 ATP
Fermentation is the process that provides ATP molecules in the absence of oxygen.
- 2 ATP per glucose molecule.
Two types:
1. Lactic acid fermentation
2. Alcohol fermentation
Difference between lactic acid and alcohol fermentation:
Lactic Acid Fermentation:
Occurs in: Animal muscle cells during intense exercise and in some bacteria.
End Product: Lactic acid (3- carbon compound)
Carbon Dioxide Production: No carbon dioxide is produced.
Process: Glucose is converted into two molecules of lactic acid and yields 2 ATP.
Uses: Important in making dairy products like yogurt and cheese.
Alcohol Fermentation:
Occurs in: Yeast and some types of bacteria.
End Products: Ethanol/ alcohol (2 -carbon compound) and carbon dioxide.
Carbon Dioxide Production: Carbon dioxide is produced as a byproduct.
Process: Glucose is broken down into ethanol and carbon dioxide, producing 2 ATP.
Uses: Used in baking, brewing, and wine-making industries.
Key Differences:
End Products: Lactic acid vs. ethanol and carbon dioxide.
Carbon Dioxide: Only produced in alcohol fermentation.
Applications: Lactic acid fermentation is common in muscles and dairy production, while alcohol
fermentation is used in food and beverage production.
