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Biology 2121 – Independent Notes
Metabolism
What the Cell Needs
For any cell to produce sufficient energy to live certain requirements must be met. For one all cells must
have access to a carbon/energy source. Human tissue much like other organisms must obtain either a
carbohydrate, lipid or protein source to produce energy. It is much easier for cells to ‘metabolize’ or
breakdown simple sugars like monosaccharides. Glucose would be an example of a monosaccharide cells
would utilize.
Oxygen gas must be present to produce the maximum amount of energy out of the breakdown of glucose.
However it is important to note that even when low levels of oxygen are present in tissue, the cells are still
able to produce minimal levels of ATP.
Fast and Quick – Creatine Phosphate and ATP Production
(Use figure 9.19 while reading through the next few sections).
Creatine phosphate is a high-energy molecule trapped in muscles. Muscles are able to produce ATP in a
rapid manner using CP. The phosphate from CP is attached or phosphorylated onto the ADP to produce
ATP. This process is mediated by the creatine kinase enzyme.
This process only provides muscles 14-16 seconds worth of ATP. It provides enough ATP so that cells are
able to adjust to longer term ATP production methods listed in the next few sections.
Getting the Most Out of Glucose – Cellular Respiration
The most efficient way cells have of producing ATP is a method referred to as Aerobic Respiration or
Cellular Respiration. 95% of all ATP used for muscle activity is produced via this method.
The first step involves the breakdown of glucose (preferred monosaccharide). This process is called
‘glycolysis’. Glucose a six-carbon compound (C6H12O6), is broken down into (2) 3-Carbon molecules called
pyruvic acid. Oxygen is not needed in this process, but may be present.
Next, pyruvate then is broken down into a 2-Carbon compound that enters a metabolic cycle called the
‘Kreb’s Cycle’. From here it gets very complicated. It is not necessary to understand the steps that are
involved in the Kreb’s Cycle and the subsequent Electron Transport Chain. But you do need to know this:

For every glucose molecule processed, 36 ATP are produced (This includes glycolysis also).
Low Levels of Oxygen – Can APT Still Be Produced?
What happens if oxygen levels are low in tissues? Cells are still able to produce ATP but very low levels.
This process is referred to as Anaerobic Respiration. In humans it is referred to as ‘Lactic Acid
Fermentation’.
The first step involves ‘glycolysis’. Just as in aerobic respiration, glucose is broken down into pyruvic acid.
Again, it is not necessary for oxygen to be present.
Next, if oxygen is not available or in low concentrations, lactic acid is produced as a by-product as ATP is
being synthesized. Lactic acid diffuses out of the muscles and into the bloodstream. The liver can convert
the lactic acid back into glucose. It then can be released back into the blood or stored as glycogen.
This is an inefficient way to produce ATP. For every glucose molecule only 2 ATP (net) are produced.
Muscles depend on this process due to conditions that lead to lower levels of oxygen in the bloodstream.
For example when muscles work hard for long periods of time, blood vessels are compressed and oxygen
levels become very low. Thus oxygen delivery is restricted to the tissues.