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Chapter 4
Cellular Metabolism
I.
Aids to Understanding Words
an – without
ana – up
cata – down
mut – change
-zym – causing to ferment
II.
Introduction
A. Life is maintained by a series of chemical reactions.
B. These reactions are controlled by proteins called enzymes.
III.
Metabolic Reactions
A. 1. Anabolism is the combining of smaller molecules to make larger
molecules. It requires energy. It is the reaction that provides the
biochemicals needed by cells for growth and repair. Cells join
monosaccharides, simple sugars, to make the larger molecules of
glycogen using dehydration synthesis. Amino acids are joined by
this process to make protein molecules. Dehydration synthesis is
also used to make fat molecules. In this process small molecules
are combined to make a larger more complex molecule plus water.
2. This figure shows an anabolic reaction in which two
monosaccharides are joined to form a disaccharide and water.
B. 1. Catabolism is a process that breaks down larger, complex
molecules to smaller, simpler molecules. Hydrolysis is an example
of a catabolic process and occurs during digestion.
2. The process illustrated in this figure is hydrolysis of a fat
molecule in which the fat molecule plus water yields one glycerol
molecule and three fatty acid molecules. This kind of fat molecule
is known as a triglyceride.
IV.
Control of Metabolic Reactions
A. Enzymes promote chemical reactions in cells by lowering the amount
of energy needed to initiate a reaction.
B. Enzymes and their substrates can be compared to a lock and key in
that only the correct enzyme can combine with a particular substrate.
This combination of enzyme and substrate specifies the activation of a
particular enzyme. This in turn determines the nature of the reaction
that will follow. It is important to note that while the enzyme controls
the initiation and speed of a reaction, it does not take part in the
reaction and is not consumed. The speed of an enzyme-controlled
reaction depends partly on the amount of the enzyme available as well
as the efficiency of the enzyme.
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C. A coenzyme is an organic substance that must combine with the
enzyme in order for the enzyme to be active. Many coenzymes are
vitamins. A cofactor is a non-protein substance, such as a mineral,
which performs a similar function.
D. The agents that can denature enzymes include heat, radiation,
electricity, certain chemicals or fluids with extreme pH values.
V.
Energy for Metabolic Reactions
A. 1. Energy is the capacity to change or move matter. Energy has the
ability to do work.
2. Common forms of energy include heat, light, sound, electricity,
and mechanical and chemical energy.
3. The form of energy used by cell processes is chemical energy.
That is the energy that is stored in a chemical bond such as ATP.
4. The process of oxidation releases energy.
B. Types of respiration
Type
Anaerobic
Aerobic
Location of reaction
Cytosol of the cell
Mitochondria
within the cell
How released energy is
ATP molecule and NADH ATP molecule, NADH,
captured
FADH2
Number of (ATP)
2
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molecules formed
VI.
Metabolic Pathways
A. A metabolic pathway is a particular sequence of anabolic or catabolic
reactions that is controlled by enzymes. These enzymes must be
positioned in the same sequence as the enzymes they control.
B. A rate-limiting enzyme is usually the first enzyme in the sequence.
The amount of this enzyme regulates the amount and speed of the
reaction.
VII.
Nucleic Acids
A. A gene is that portion of the DNA molecule that contains the genetic
information for making particular proteins.
B. Genetic information tells the cells of the developing body how to
construct structural materials, enzymes, or other vital biochemicals.
They instruct cells to synthesize the enzymes that control the
metabolic pathways.
C. The genome is the total of DNA in a cell. The genome is the subject
of intense research, as we do not understand the functions of most of
the genome.
D. Sugar and phosphate molecules alternate to form a long backbone.
The organic bases project from this backbone and bind weakly to the
bases of the second strand.
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E. The bases of nucleotide pairs in DNA are adenine (A), thymine (T),
cytosine (C), or guanine (G). These bases bind in specific pairs.
Adenine binds only to thymine and cytosine binds only to guanine.
The sequence of these pairs determines the genetic code. The DNA
molecule twists to form a double helix.
F. Drawing to complete.
VIII.
DNA Replication
A. DNA molecules are replicated during Interphase of the cell cycle.
B. Replication begins by breaking the hydrogen bonds that hold the base
pairs together and forming two separate strands. The strands pull
apart and unwind to expose the nitrogenous bases of the nucleotides.
DNA polymerase, an enzyme, brings in new DNA nucleotides to form
complementary pairs with the exposed bases. Another enzyme knits
the strands together with the result that two new DNA strands are
formed each with one old strand and one new strand. These strands
then are incorporated in copies of a chromosome and separate during
mitosis.
IX.
Protein Synthesis
A. Protein synthesis takes place in the ribosomes of the cellular
cytoplasm. The correct amino acids must be present in the cytoplasm
for synthesis to occur.
B. RNA molecules are single rather than double stranded. The
nucleotides contain ribose rather than deoxyribose. The thymine of
DNA is replaced by uracil in RNA.
C. In order for the genetic information to move from the nucleus of the cell
to the ribosome, two types of RNA take part in a process called
translation. The language of nucleic acids must be translated in the
language of amino acids. The amino acids in the cytoplasm must be
aligned correctly on a strand of mRNA. Transfer RNA, tRNA,
synthesized in the nucleus, correctly aligns the amino acids to form
proteins.
There are 20 different types of tRNA because there are some twenty
different amino acids. Each amino acid needs a tRNA guide. Each
type of tRNA has a region at one end that consists of three nucleotides
that bond to a complementary set of nucleotides on the mRNA
molecule that is known as a codon. The corresponding area on the
tRNA molecule is the anticodon.
In protein synthesis, a mRNA molecule is bound to a ribosome. TRNA
molecules move along the mRNA molecule to bind to complementary
codons making the long polypeptide chains of amino acids that are
proteins. A gene that has been transcribed and translated has been
expressed.
D. Proteomics is the study of gene expression – that is which cell types
synthesize which proteins.
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