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Introduction to Cellular
Metabolism
For cells to maintain their structure and function, chem-ical
reactions must occur inside the cell. These chemical
reactions require an input of biologically -usable energy.
The most common and available form of energy within a
cell is the chemical energy found within the structure of an
ATP (adenosine triphosphate) molecule. We use the term
metabolism (meh-TAB-oh-lizm) to describe the total
chemical changes that occur inside a cell. There are two
subcategories of metabolism: anabolism- (an-AB-oh-
lizm) is an energy-requiring process that builds larger
molecules from combining smaller mol-ecules and
catabolism- (ka-TAB-oh-lizm), which is an energy-
-releasing process that breaks down large mol-ecules into
smaller ones. These cellular metabolic pro-cesses are -often
called cellular- respiration or cellular metabolism.
Molecules of ATP are made within the cell during a
stepwise decomposition (catabolism) of organic mole-cules
(carbohydrates, fats, and proteins). We measure the energy
contained in food as calories. This decomposition releases
the chemical energy (calories) stored in these or-ganic
foodstuffs and this energy is used to synthesize ATP
(another form of chemical energy) from ADP (adenosine
diphosphate) and PO 4 (inorganic phosphate). Thus, ATP is
the energy source available to the cell to be used for all cell
processes: chemical reactions use ATP as an energy source
to maintain cellular structure and function.
Photosynthesis by plant cells is the ultimate source of
the organic molecules (foodstuffs) that will be de-
composed to form ATP. Photosynthesis requires 6CO 2 1
12H 2 O in the presence of light and chlorophyll to produce
C 6 H 12 O 6 (glucose), an organic molecule 1 6O 2 (oxygen) as
a waste product 1 6H 2 O (water) as a waste product. The
formation of ATP is the final step in the transformation of
light energy into the chemical energy of a biologically
usable- form. This explains the significance of our depend-
ence on plants to convert sun or light energy into food or
chemical energy.
The most efficient cellular process by which ATP is
formed during the breakdown of organic molecules
-requires molecular oxygen (O 2 ). This process is called
cellular or biochemical respiration or cellular metabo-lism.
The overall chemical equation is:
C^6 H^12 O^6 1 6O^2 S^ 6CO^2 1 6H^2 O 1^
energyintheformofATP.
Respiration, therefore, requires an exchange of gases
between the cell and its surroundings to allow the inflow
Chapter 4of O 2 to the cell and the outflow of CO 2. Biochemical res-
piration is strictly the oxygen-requiring or aerobic process
of ATP production. This biochemical meaning of respira-
tion should not be confused with the everyday meaning of
breathing. The most common substance decomposed
aerobically in cells to produce ATP is glucose, C 6 H 12 O 6.
The breakdown of a glucose molecule into carbon
dioxide gas and water is a continuous process. However,
we will discuss this process in three steps. The first step is
called glycolysis (gligh-KOL-ih-sis). Because it does not
require oxygen, it is also occasionally called anaerobic-
(without oxygen) respiration. This step occurs in the
cytoplasm of the cell. The next two steps are called the
Krebs citric acid cycle and the electron transfer or
transport system. These two steps require oxygen and
they occur in the matrix and on the folds or cristae of the
mitochondria of the cell.Laboratory Exercise: Cellular Metabolism
Biochemical Respiration
Glycolysis
The first step in the biochemical respiration process is
glycolysis. It is common to the aerobic breakdown of
glucose and to the two different types of anaerobic
breakdown of glucose molecules. One type of anaero-bic
glucose decomposition occurs in yeast cells (a type of
fungus) and is called fermentation. The other type occurs in
our muscle cells when we exercise and expe-rience muscle
fatigue and cannot get enough oxygen to our muscle cells.
In the overall process of glycolysis, the C 6 (backbone chain
of six carbon atoms) sugar glucose is slowly broken down
by various enzymatic steps to two C 3 units of pyruvic
(pye-ROO-vik) acid. Refer to Figure 4-1 as we discussGlycolysis.
The first step in glycolysis (which takes place in the
cytoplasm of the cell) is the addition of a phosphate to the
glucose. This process is called phosphorylation (fos-for-
ih-LAY-shun). The phosphate comes from the break-down
of an ATP molecule into ADP and PO 4 , releasing the
energy required to add the phosphate to the glucose. The
glucose phosphate quickly changes to another C 6 sugar
phosphate called fructose phosphate. In another ATP-
requiring reaction, the fructose phosphate is phos-
phorylated by breaking down another ATP into ADP and
PO 4. This phosphate is added to the fructose phosphate,
creating fructose diphosphate. So far we have not made any
ATP but rather we have used up two ATP and these must
be paid back from our final ATP production at the end of
glycolysis.