What is Cellular Respiration

What is cellular respiration?

• Cellular respiration is the process by which energy is released from organic matter (glucose or other food molecules). Cellular respiration generally involves the presence of oxygen.

6O₂ + C₆H₁₂O₆ -- > 6CO₂ + 6H₂O + Energy

Aerobic respiration: occurs in the presence of oxygen. It is divided into three general parts:

Glycolysis, Kreb’s cycle and Oxidative Phosphorylation.

Glycolysis: glucose is converted into pyruvic acid in a series of steps

• Takes place in the cytosol (cytoplasm)
• 1 glucose molecule forms:
• 2 pyruvates
• 2 NADH
• A net gain of 2 ATP
• A total of 4 ATP is produced, but 2 ATP were used during the beginning stages of glycolysis

Before Kreb’s Cycle (Pyruvate Oxidation)

• Each of the pyruvate is combined with coenzyme A (CoA), and becomes acetyl CoA.
• This step also yields 1 NADH and 1CO₂

Kreb’s Cycle

• Acetyl CoA combines with oxaloacetic acid to form citric acid
• Produces:
• 3 NADH, 1 FADH₂, 1 ATP, CO₂

Oxidative Phosphorylation

• The purpose of this stage: ATP is produced from NADH and FADH₂ molecules
• ATP is produced when electron carriers are re-oxidized in the presence of oxygen
• Oxygen is the final electron acceptor 
• Oxygen accepts two electrons along with two Hydrogen ions in order to form water.          
• Contains two substages

• Electron transport Chain
• Electrons from the NADH and FADH₂ molecules lose energy as they pass through the electron transport chain.
• Also, as the electron carriers pass through the chain, H⁺ ions move from the matrix and onto the outer compartment of the mitochondria
• Once again, oxygen is the final electron acceptor

 

• Chemiosmosis
• There are H⁺ ions that accumulated in the outer compartment. As a result, a type of proton gradient and electrical charge gradient is formed.
• ATP synthase: allows protons to diffuse back into the mitochondrial matrix, and uses energy from this diffusion to produce ATP.

In aerobic respiration, about 34 to 38 ATP molecules are produced.

Anaerobic Respiration: occurs in the absence of oxygen. Takes place in the cytoplasm

What happens when there is no oxygen?

• This means that an electron acceptor is missing and NADH will accumulate. Eventually, glycolysis will stop because all the NAD⁺ molecules are converted to NADH.

Aerobic Respiration is a process that replenishes NAD⁺ molecules, allowing glycolysis to continue.

Alcoholic Fermentation

• Occurs in yeast, bacteria and some plants
• Steps:
• Pyruvate is converted to acetaldehyde
• Carbon dioxide is released
• Acetaldehyde is then reduced by NADH
• NAD⁺ and ethanol (ethyl alcohol) is produced

Lactic Acid Fermentation

• Occurs in certain microbes and some muscle cells
• One Step:
• Pyruvate is the electron acceptor and is reduced by NADH
• Pyruvate now becomes lactic acid
• NADH is converted to NAD⁺

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