Chapter 5-7, 10
Read P. 75-82, 91-100, 107-117
Introduction to Metabolism and Enzymes
Catabolic reactions (also called “catabolism”)
break down larger, more complex molecules
into smaller molecules and release energy in
the process. The smaller end products of a
catabolic reaction may be released as waste
or they may be fed into other reactions.
Metabolic pathways are catabolic (break-down molecules)
or anabolic (synthesize or combine molecules) and result
in molecular products which can be used by the cell
immediately, used to initiate another chemical reaction, or
stored in the cell. Uses energy.
Requirements for Metabolism
How Do Enzymes Function?
Factors Affecting Enzyme Function
Like living organisms, enzymes function within an optimum range that is specific to the organism.
Outside their range enzymes become denatured, which means, the active sites lose their shape
and therefore are unable to fit the reactants. Not being able to bring the reactants together means
that the activation energy will not be able to be lowered. This has a drastic affect on the
metabolism and the overall well-being of the organism. Without the enzymes, the necessary
metabolic reactions may slow down or possibly not occur at all resulting in death.
Temperature: Enzymes will also work at optimum
temperatures depending on the environment and species.
Thermophilic bacteria, for example will have enzymes that
function best for their metabolic reactions at very high
temperatures (70°C). Outside this range they are not
operational. Humans have an internal temperature range of
about 37°C. Obviously thermophilic bacterial enzymes would
not operate in our bodies or vice versa.
ATP – The reaction of ATP (energy)
The reactions that occur are as
1. Catabolically (breaking of large
into small, breaking bonds,
A-P-P-P -----------> A-P-P + P
2. Anabolically (forming large from small, building
bonds, storing energy):
A-P-P + P -------------> A-P-P-P
When these reactions occur what happens to the P and
the energy (stored or released)? The answers explain how
ATP drives cellular work.
Energy and Phosphate Transfer
ATP, as a molecule, is a form of stored energy.
In order for cells to perform work this energy must be released. Like a shelled
nut, the energy is within and to obtain this energy, the shell must be broken.
Likewise, ATP must catabolically break into ADP and P. In this chemical
reaction energy is released and a P (phosphate) is made available.
As the Laws of Thermodynamics state: energy is never lost but transferred.
Therefore, the energy and the P from this reaction must be transferred
The energy can go into forming new bonds between different reactants,
for an example. This would be an anabolic reaction where energy is
required and stored to build new bonds. This energy would come from
the breaking apart of ATP. The P is typically transferred to a protein.
As stated in previous units, proteins are unique to their function and
enzymes unique to their reactions. Transferring a P to a protein allows a
unique shape required for some metabolic reactions.
When the P is removed from the protein, the original shape is retained
and the reaction stops, permitting the original reaction to start again.
ATP is continuously being used by cells, which means the catabolic
breakdown of ATP is occurring always.
However, ATP is also a renewable resource, which means that the
opposite reaction, the anabolic formation of ATP is also always
occurring. Because these two can occur in both directions it is called an
equilibrium and is known as the ATP cycle.
4. Examples of Cellular Metabolic
1. Protein Digestion
Proteins are a polymer composed of amino acids (aa) as monomers joined
together by peptide bonds in a chain. Important in the cell, they are also a
nutrient often acquired through the diet. These proteins are too large to
be used by cells and therefore must be digested or broken down.
How would this look in the form of an equation (it is after all a metabolic
This reaction shows a protein of 3 amino acids in length,
catabolically being broken into its individual monomers. The
individual monomers are now small enough to be transported
into and used by the cell. As a result of this catabolic reaction,
energy is released.
This energy can now be transferred and stored in the
2. Protein Synthesis
Protein Synthesis is usually achieved in a two-step process of transcription and translation.
Transcription occurs in the nucleus of the cell and involves DNA giving the initial code for the
particular protein that is needed to be built. DNA, as a transcript, delivers the protein code in the form
of a mRNA (m=messenger).
In translation, this mRNA moves out of the nucleus and finds its way to a ribosome. The ribosome is
able to interpret the mRNA's message and thus the DNA's code. It is the nitrogen bases (A,U,C,G) of
the mRNA that is read and the message is read in groups of threes, known as a codon. Because the
entire message is so long it is easier for the message to be read in groups of threes, after each reading
the particular amino acid found and brought to the ribosome site. It is the tRNA (t=transfer) that will
match the read codon with its opposite bases known as an anticodon. With the anticodon the specific
amino acid is also transferred by the tRNA to the ribosome. The building of protein has now begun.
Photosynthesis is a metabolic pathway that most autotrophs, such as
green plants, perform.
The chloroplast is the organelle required; therefore it can be assumed
that any cell containing chloroplasts will perform photosynthesis (the
taking of inorganic carbon dioxide and water to form organic glucose
It is a very complicated process, much more complicated than the intent
of this course. It involves several reactions, both anabolic and catabolic,
and also involves ATP.
There are two steps involved in the process of photosynthesis:
Light Reactions and Dark Reactions (AKA – Calvin Cycle)
The light reactions involve taking water (H2O) and catabolically
breaking it apart into oxygen (O2). This reaction involves light and
produces ATP. It occurs in the stroma or fluid portion of the
Step 1 (catabolic): H2O -------> O2 + energy
Step 2 (anabolic): ADP + P + energy -------> ATP
The dark reactions however are not dependent on light because the ATP
produced in the previous step will now be the energy source to
anabolically produce glucose (C6H12O6) from carbon dioxide (CO2).
This step occurs in the thylkoid sacs of the chloroplasts:
Step 3 (catabolic): ATP -------> ADP + P + energy
Step 4 (anabolic): energy + CO2 ---------> C6H12O6
Combining these 4 steps produce the overall equation:
Overall: CO2 + H2O + light energy ---------------> C6H12O6 + O2
4. Aerobic Cellular Respiration
Like photosynthesis, aerobic cellular respiration is a complicated
metabolic pathway that involves several reactions, both anabolic and
The difference is that cellular respiration occurs in all cells as it is a
characteristic of living organisms. Also, it is commonly described as an
overall catabolic process.
The reaction that we will observe will be aerobic cellular respiration
which involves O2, and occurs in the mitochondria.
The entire process involves three steps: Glycolysis, Kreb's Cycle and
Electron Transport chain. These steps are much more complicated than
the steps in photosynthesis. There are also many more molecules
involved in the process.
What we will outline here will be in its simplest form. It will leave out a
number of reactions that in reality occur and are extremely important to
the process. First, let's see the overall reaction:
C6H12O6 + O2 --------------> CO2 + H2O + energy
As you probably observed, the reaction is the reverse of
Students make the mistake of thinking that the primary function of
respiration is to break down glucose. This occurs but the reason why all
cells do some form of respiration is for the production of energy.
Aerobic and anaerobic respiration connections between cellular respiration and other
pathways fermentation is another anaerobic (non oxygen requiring) pathway for breaking
down glucose, fermentation doesn't require oxygen eithe