These notes are intended to accompany lectures in Human Physiology by Dr. Peter King at Francis Marion University, Florence, SC 29501.

Why does our body need oxygen?
necessary for aerobic respiration
What is aerobic respiration?
The breakdown of fuels to produce ATP, using oxygen as the final electron acceptor of the electron transfer chain in mitochondria.
Does glycolysis use oxygen?
No
Glycolysis is a precursor to the Krebs cycle forming pyruvate CH₃COCO₂

When carbohydrates are consumed they are broken down into glucose.
Glucose is the energy currency of the body and is stored in our liver as glycogen.
Our liver balances blood glucose levels (homeostasis) through the balance of glycogenesis and glycogenolysis (controlled by enzymes).

Glucose is absorbed into blood cells and broken down. The chemical energy from the breakdown is used to form adenosine triphosphate, ATP.
ATP is the energy currency of the cell.
Chemical energy in the bond between the 2nd and 3rd phosphate is the most commonenergy used.
ATP -> ADP + P + energy

Cellular respiration starts with glycolysis where glucose is broken down to 2 pyruvate molecules yielding 2 ATP.
If oxygen is below a critical level, the lactic acid pathway is used to replace the coenzyme NAD to allow continuation of glycolysis.
No oxygen is used and this is referred to as anaerobic respiration but more correctly lactic acid (lactate) fermentation.

Lactid acid fermentation is common in skeletal muscle. Lactid acid is responsible for the sore feeling or stiffness in muscles that have been overworked.
Lactic acid is converted back to pyruvic acid and then back to glucose-6-phosphate via the Cori cycle in the liver.

If oxygen is available at the cell (via the repiratory and circulatory systems) pyruvic acid enters the mitochondria, is converted to acetyl-CoA and enters the Krebs cycle (citric acid cycle).
8 enzymes control this cyclic pathway.

Coenzymes NADH and FADH₂ deliver H⁺ to proton pumps that create a H⁺ gradient in the intermembrane space.
The H⁺ gradient is used to phosphorylate ADP to form ATP.
Electrons are lost from the NADH and FADH₂ and flow via an electron transfer system to oxygen.
If oxygen is not present no ATP is formed.

Breakdown of glucose by aerobic respiration yields at least 30 ATP are formed, much more than by fermentation pathways.
Other foods can be used to provide energy (ATP) also.
The are broken down and fit into the glucose breakdown pathways.
e.g. fatty acids produce acetyl-CoA
amino acids feed into Krebs cycle.

When amino acids enter the Krebs cycle the amine goups form urea which is eliminated by the kidneys and urinary system.
Blood glucose levels are maintained by a balance of insulin and glucogon, 2 hormones produced by the endocrine system.

So to answer the question we first started with "why does the body need oxygen" we have involved; digestive system, respiratory system, circulatory system, endocrine system, renal and urinary systems, skeleto-muscular system.

For most of the course we will be talking about body systems.
Don't forget that they are all integrated.
Don't forget that they are working from the organismal to the cellular to the molecular levels.

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This webpage was created by Peter King. Please contact the the author with comments at pking@fmarion.edu.
Last edited July 20, 2010.
http://web.fmarion.edu/humanphys/cellmet.html
copyright Peter King.