Cellular respiration is the process by which cells make ATP by breaking down organic compounds. In the process of photosynthesis
autotrophs (usually plants) convert light energy into chemical energy, which is stored in organic compounds. Then, heterotrophs
( animals and other consumer organiisms) undergo cellular respiration to break these organic compound into simpler molecules
and thus release energy. Some of the energy is used to make ATP. The energy in ATP is then used by cells to do activities.
In order to understand cellular respiration in its totalty, you need to know these key terms:
pyruvic acid: The 3-carbon-compound that is produced during Glycolysis and needed for both aerobic and anaerobic
pathways of cellular respiration that follows glycolysis.
NADH: The reduced form of NAD+; an electron-carrying molecule that functions in cellular respiration.
anaerobic:
Description of the process that does not require oxygen.
aerobic respiration: The process in which pyruvic acid is broken down and NADH is used to make a large amount
of ATP;the part of respiration that is carried out in the presence of oxygen.
Glycolysis: The anaerobic break down of glucose to pyruvic acid, which makes a small amount of energy available
to cells in the form of ATP.
NAD+: An organic molecule that serves as an electron carrier by being oxidized to NAD+ and reduced to NADH.
fermentation:
The breakdown of carbohydrates by enzymes, bacteria, yeast,or mold in the absence of oxygen.
lactic acid fermentation: The chemical breakdown of carbohydrates that produces lactic acid as the main end
product.
alcoholic
fermentation: The anaerobic process by which yeast an other microorganisms breakdown sugars to form carbon dioxide
and ethanol.
kilocalorie: A unit of enery equal to a thousand calories.
Now that you have familiarized yourself with these terms, lets go over the first stage of cellular respiration, glycolysis.
As mentioned earlier is a biochemical pathway in which one six-carbon molecules of glucose is oxidized to produce two three-carbon
molecules of pyruvic acid. First, two phosphate groups are attached to a molecule of glucose to form a six-carbon compond.
Next, this six-carbon compound splits into two three-carbon compound called G3P ( glyceraldehyde 3-phosphate). Then, two G3Ps
recieve a phosphate group through oxidation and become a new three-compound. When the G3Ps are oxidized two molecules of NAD+
are reduced to NADH( an energy carrier). Finally, the phosphate groupsadded in the first and third steps are removed to produce
two molecules of pyruvic acid. These phosphates are added to a molecules of ADP to form ATP. Since there are a total of four
phosphate groups active in this reaction, four molecules of ATP are produced.
Another stage of cellular respiration is known as fermentation. This is when some cells(when oxygen is absent) convert
pyruvic acid into other compounds by means of other biochemical pathways. There are many types of fermentation pathways but
there are two main types.
The first is lactic acid fermentation. This involves the transfer of a hydrogen atom from NADH and the addition
of free proton to pyruvic acid. During this process, NADH is oxidized into NAD+ which is then used in glycolysis. Thus, lactic
acid fermentation is needed to keep glycolysis going.
Lactic acid fermentation is most commonly seen in milk when it spoils and creates cheese, yogurt and other dairy products.
It is also seen when muscles go throw strenuous activity and it starts to give off lactic acid when oxygen is used up.
Another type of fermentation is alchoholic fermentation. In this process, pyruvic acid is converted to ethyl alcohol.
Alcoholic fermentation is seen in the making of wine and beer through yeast cells.
