1. CELLULAR RESPIRATION SUMMARY
Cellular respiration is the enzymatic breakdown of glucose (C6H12O6) in the
presence of oxygen (O2) to produce cellular energy (ATP):
C6H12O6 + 6O2 6CO2 + 6H2O + 38 ATP
THREE STAGES OF CELLULAR RESPIRATION
1. Glycolysis:
A ten-step process that occurs in the cytoplasm
Converts each molecule of glucose to two molecules of pyruvic acid (a 3-carbon
molecule)
An anaerobic process - proceeds whether or not O2 is present ; O2 is not required.
Net yield of 2 ATP per glucose molecule
Net yield of 2 NADH per glucose (NADH is nicotine adenine dinucleotide, a co-enzyme
that serves as a carrier for H+ ions liberated as glucose is oxidized.)
The pyruvic acid diffuses into the inner compartment of the mitochondrion where
a transition reaction occurs that serves to prepare pyruvic acid for entry into the
next stage of respiration:
(a) pyruvic acid acetic acid + CO2 (a waste product of cell metabolism) + NADH+
(b) acetic acid + co-enzyme A acetyl CoA

2. 2. Citric Acid or TCA Cycle:
Occurs in the inner mitochondrial matrix
The acetyl group detaches from the co-enzyme A and enters the reaction cycle
an aerobic process; will proceed only in the presence of O2
Net yield of 2 ATP per glucose molecule (per 2 acetyl CoA)
Net yield of 6 NADH and 2 FADH2 (FAD serves the same purpose as NAD)
In this stage of cellular respiration, the oxidation of glucose to CO2 is completed
3. Electron Transport System:
Consists of a series of enzymes on the inner mitochondrial membrane
Electrons are released from NADH and from FADH2 and as they are passed along the
series of enzymes, they give up energy which is used to fuel a process
called chemiosmosis by which H+ ions are actively transported across the inner
mitochondrial membrane into the outer mitochondrial compartment. The H+ ions then
flow back through special pores in the membrane, a process that is thought to drive the
process of ATP synthesis.
Net yield of 34 ATP per glucose molecule
6 H2O are formed when the electrons unite with O2* at the end of electron transport
chain. [Note: This is the function of oxygen in living organisms!]
GLUCOSE AS AN ENERGY SOURCE
The above notes describe the process of carbohydrate (glucose) catabolism for the
production of ATP. When glucose is in adequate supply, such as shortly after
consumption of a meal, the hormone insulin which is secreted from the pancreas,
increases glycogen formation (glycogenesis) in the liver. When glucose levels drop
between meals, the hormone glucagon is released from the pancreas and stimulates
the conversion of glycogen into glucose (by the process of glycogenolysis). If all

3. glycogen supplies are depleted, then other substances in the body are converted into
glucose or intermediate products that can enter the above-outlined cellular respiration
pathway. The conversion of fatty acids (from lipids) or amino acids (from proteins)
into glucose or intermediate products is called gluconeogenesis.
FAT AS AN ENERGY SOURCE
Fats (lipids) are stored in adipose tissue. These stored fat molecules are synthesized
in the body from the breakdown products of fat digestion (glycerol and fatty acids),
in a process known as lipogenesis. When needed as an energy source, the fat
reserves are mobilized, moved out of adipose tissue, and broken down into glycerol
and fatty acids in the liver by the process of lipolysis. Glycerol is changed into one of
the intermediate products of glycolysis, so enters the cell respiration pathway. Fatty
acids are changed in a series of reactions called beta-oxidation into acetyl CoA
molecules, which enter cell metabolism at the Kreb's Cycle. When fats are being
used as the primary energy source such as in starvation, fasting or untreated
diabetes, an excess amount of acetyl CoA is produced, and is converted into acetone
and ketone bodies. This produces the sweet smell of acetone on the breath,
noticeable in a diabetic state.
PROTEIN AS AN ENERGY SOURCE
Proteins are used as an energy source only if protein intake is high, or if glucose and
fat sources are depleted, in which case amino acids from protein breakdown are
converted into molecules that can enter the TCA Cycle. These molecules are
produced by either of two categories of reactions that alter the structure of amino
acids. Transamination transfers an amino group (NH2) from one amino acid to
another, whereas deamination removes an amino group from an amino acid. As
they accumulate, the amino groups removed by the process of deamination are
altered to form a harmful waste product (ammonia), so are converted by the liver
into urea which is excreted by the kidneys.

4. FLOWCHART OF GLYCOLYSIS PATHWAY

5. FLOWCHART OF KREBS CYCLE