Cellular Respiration and Photosynthesis
Cellular respiration is the process by which the chemical energy
of "food" molecules is released and partially captured
in the form of ATP. Carbohydrates, fats, and proteins can all
be used as fuels in cellular respiration, but glucose is most
commonly used as an example to examine the reactions and pathways
In glycolysis, the 6-carbon sugar, glucose, is
broken down into two molecules of a 3-carbon molecule called
pyruvate. This change is accompanied by a net gain of 2 ATP
molecules and 2 NADH molecules.
The Krebs (or Citric Acid) cycle occurs in the
mitochondria matrix and generates a pool of chemical energy
(ATP, NADH, and FADH 2 ) from the oxidation of pyruvate, the
end product of glycolysis. Pyruvate is transported into the
mitochondria and loses carbon dioxide to form acetyl-CoA, a
2-carbon molecule. When acetyl-CoA is oxidized to carbon dioxide
in the Krebs cycle, chemical energy is released and captured
in the form of NADH, FADH 2 , and ATP.
The electron transport chain allows the release
of the large amount of chemical energy stored in reduced NAD
+ (NADH) and reduced FAD (FADH 2 ). The energy released is captured
in the form of ATP (3 ATP per NADH and 2 ATP per FADH 2 ). The
electron transport chain (ETC) consists of a series of molecules,
mostly proteins, embedded in the inner mitochondrial membrane.
The glucose required for cellular respiration
is produced by plants. Plants go through a process known as
photosynthesis. Photosynthesis can be thought of as the opposite
process of cellular respiration. Through two processes known
as the light reactions and the dark reactions, plants have the
ability to absorb and utilize the energy in sunlight. This energy
is then converted along with water and carbon dioxide from the
atmosphere into glucose and oxygen. Since this is the opposite
process of cellular respiration, plants and animals are said
to have a symbiotic relationship. This means that plants and
animals live together and benefit from each other. When humans
and animals breath, they take in oxygen and give off carbon
dioxide. This carbon dioxide is taken up by plants and oxygen
is given off through photosynthesis. There is an equilibrium
of oxygen and carbon dioxide created between animals and plants.
Without one, the other would not survive for long.