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UNIT 1
Chapter 5
Metabolism, Energy, and Life
I. Metabolism:
a) Definition: the sum of an organism’s chemical
reactions (life functions) which help in maintaining
homeostasis.
b) Life Functions
1. Nutrition: the process in which organic molecules
(food) are obtained (ingested) and used (digested).
2. Transport: the movement or distribution of material
from one location to another. Examples include active and
passive transport.
3. Respiration: the process in which energy (ATP) is produced by
the catabolism of organic compounds through a series of
interconnected chemical reactions. Examples include anaerobic
and aerobic respiration.
4. Excretion: the removal of potentially toxic, metabolic wastes.
Examples of metabolic wastes include: CO2, H2O, and nitrogenous
wastes.
5. Growth: the increase In size of a given organism. This occurs
by the increase in cell size and the increase in cell number.
6. Synthesis: the process in which small simple molecules are
combined to make larger, more complex molecules.
7. Regulation: the coordination and control of the chemical
reactions needed to maintain homeostasis.
8. Reproduction: the process in which the number of
organisms within a population are increased. Reproduction is
only necessary for the survival of the species, not the
individual.
II. Life Functions and Metabolic Pathways
• The metabolism of a cell involves an intricate network of
chemical reactions.
• The metabolic functions are arranged into a series of
interconnected pathways.
• The rate of these pathways is governed by the presence of
biological catalysts called enzymes.
a) Types of Metabolic Pathways
1. Catabolic Pathways:
• release energy by the lysis of complex molecules into
simpler compounds.
Ex: Cellular Respiration
2. Anabolic Pathways:
• consume energy to build complicated molecules from
simpler ones.
Ex: Protein Synthesis from the dehydration
synthesis of amino acids.
FIGURE 1:
Anabolism (Photosynthesis) and
Catabolism (Cellular Respiration)
III. Thermodynamics and Life
Key Concept: An adequate discussion of metabolism is
not possible without an understanding energy, its uses,
its transformations, and its laws.
a) Thermodynamics
• Thermodynamics is the study of energy.
• Energy is the ability to bring about change or to do work.
It exists in many forms, such as heat, light, chemical
energy, and electrical energy.
b. Potential vs. Kinetic energy
1. Potential energy in biochemical systems represents stored
chemical bond energy.
2. Kinetic energy represents the energy of motion of the
chemicals in living systems usually generated by the
breaking of chemical bonds in biochemical systems.
c) Laws of Thermodynamics
1. First Law of Thermodynamics:
•Energy cannot be created or destroyed but it is can be changed
from one form to another. It is always conserved.
•The total amount of energy and matter in the Universe remains
constant, merely changing from one form to another.
2. Second Law of Thermodynamics states that "in all energy
exchanges, if no energy enters or leaves the system, the potential
energy of the state will always be less than that of the initial
state."
• Closed systems will have a tendency to go from a level
low disorder to high disorder. Entropy is a level of
disorder within a given system.
• Energy is naturally converted from a more to a less
usable form.
d) Free Energy and Spontaneity
1. Spontaneous Reactions: chemical reactions that occur
without the addition of an outside source of energy.
2. Non-spontaneous Reactions : chemical reactions that
require an external energy source to occur.
e) Gibbs Free Energy
• Free energy is the portion of a system’s energy that
can perform work when temperature is uniform
throughout the system.
• Free energy is the form of usuable energy needed
to perform work.
• By evaluating the entropy (S) , enthalpy (H) , and
temperature (T) of a system, one has the ability to
determine the amount of free energy (G) available to
perform work.
NOTE:
•Enthalpy (H) indicates whether a reaction is exothermic or
endothermic.
•If DH = (+), the reaction is endothermic and energy is absorbed.
•If DH =(-), the reaction is exothermic and energy is released.
1) Gibbs Free Energy Equation
DG = DH - TD S
If DG = (-), the reaction is spontaneous.
If DG = (+), the reaction is non-spontaneous.
2) Conditions Favoring Spontaneous Reactions
DH = (-) and DS = (+)
F) Exergonic Reactions
Energy releasing processes, ones that "generate" energy, are termed
exergonic reactions.
Figure 1
Time-energy graphs of an exergonic reaction (top) and endergonic
Figure 2: Exergonic Reaction
G)Endergonic Reactions:
•
Reactions that require energy to initiate the reaction are
known as endergonic reactions.
Figure 3: Endergonic
Reaction
Note:
•All natural processes tend to proceed in such a
direction that the disorder or randomness of the
universe increases (the second law of thermodynamics).
• Endergonic = Endothermic
• Exergonic = Exothermic
H) Coupled Reactions
• For non-spontaneous reactions to occur, energy is needed.
• Non-spontaneous reactions must be coupled with a
spontaneous (exothermic or exergonic) reaction to occur.
Ex: Photosynthesis: The spontaneous nuclear fission of
hydrogen on the sun releases solar energy (exergonic
reaction). This energy (light) is absorbed by photosynthetic
organisms to synthesized glucose from carbon dioxide and
water (endergonic reaction).
IV. Energy in Living Systems
a) Adenosine Triphosphate (ATP)
1. Characteristics
• Energy-carrying molecule in living systems.
• Composed of nitrogen-containing base (adenine), a fivecarbon sugar (ribose), and three phosphate groups.
• The energy in ATP is stored in the high-energy bonds
between terminal and second phosphate group.
Adenine
Phosphate Groups
Ribose
• ATP is generated by the process of cellular
respiration.
V. ENZYMES
Substrate
Active Site
Enzyme
a) Characteristics:
1. Enzymes are biochemical catalysts. They control the rate
of chemical reactions by lowering the amount of energy
needed to initiate the chemical reaction.
2. An important characteristic of catalysts is that they are
NOT USED UP or changed when catalyzing a chemical
reaction, thus they are available to repeat the catalysis
repeatedly
ACTIVATION ENERGY
Figure 1. Activation energy.
The substrate contains a certain amount of trapped, or potential,
energy. To release that energy, activation energy has to be added.
The amount of energy needed to initiate the reaction can be
lessened if an enzyme is present.
3. Enzymes are made of protein. Due to this fact, they
display quite a complex, three-dimensional shape that is
specific for particular substrate molecules.
4. The area of an enzyme that is responsible for interacting
with a substrate is the active site.
5. The active site of an enzyme is complementary to
the substrate molecule.
6. The substrate will temporarily bind to the enzyme at the
active site.
7. The enzyme will cause steric strain on the substrate
causing it to become chemically unstable.
8. The instability of the substrate molecule will cause it to
react with less energy than previously needed.
9. Catalysts cannot cause energetically unfavorable reactions
to occur. They are only effective in catalyzing reactions that
are already spontaneous.
10. The model that describes the interaction between enzyme and
substrate is called the Lock and Key Model.
11. The active site of enzymes can be changed slightly to allow
substrate molecules to bind.
• This change in the enzyme’s active site is caused by the
presence of the substrate.
• This model of enzyme-substrate action is called the
induced fit hypothesis. D:\ICIB.exe
12. The active site dictates whether the enzyme has the ability to
catalyze a chemical reaction.
• There are many factors that can affect the active site.
• This is the key to understanding the regulation of
enzyme activity.
b) Factors Affecting Enzyme Activity
KEY IDEA: Enzymes have evolved to function
optimally at a particular pH, temperature, and salt
concentration. Changes in any/all of these variables
will result in enzyme denaturation. This effectively
changes the enzyme’s ability to catalyze reactions.
1. pH vs. Enzyme Activity
• Indicates the acidity/alkalinity of a solution by measuring the
concentration of hydrogen ions.
• Most enzymes function best at a pH range between 6-8.
• Exceptions include the enzyme pepsin which functions to
hydrolyze protein in the acidic fluids of the stomach. The higher
levels of acidity in the stomach causes a conformational change in
the enzyme’s active site.
• This results in the change of the inactive pepsin to the active
pepsinogen.
Table II
pH for Optimum Activity
Enzyme
pH Optimum
Lipase (pancreas)
8.0
Lipase (stomach)
4.0 - 5.0
Lipase (castor oil)
4.7
Pepsin
1.5 - 1.6
Trypsin
7.8 - 8.7
Urease
7.0
Invertase
4.5
Maltase
6.1 - 6.8
Amylase (pancreas)
6.7 - 7.0
Amylase (malt)
4.6 - 5.2
Catalase
7.0
2. Temperature vs. Enzyme Activity
• Temperature is a measure of the average kinetic energy of
matter.
• Increases in temperature increases the KE and thus increases
the velocity of the particles of matter.
• By increasing temperature, the probability of effective
collisions between reactants and between enzyme and reactants
increases, thus increasing the reaction rate.
Temperature vs. Enzyme Activity
Denaturation
Enzyme
Activity
Temperature
• However, when the temperature of a system exceeds the level to
which the enzyme is adapted to functioning, the enzyme denatures
and the reaction rate decreases.
OVERVIEW: By changing the pH, temperature, and salinity of a
system in which enzymes are functioning, a destabilization of the
tertiary structure of the enzyme ensues. This causes a change in
the enzyme’s active site resulting in denaturation.
3. Substrate Concentration and Enzyme Activity
• An increase in substrate concentration will cause a greater
probability of a reaction with enzymes.
• This will cause an increase in enzyme activity until all of
the enzymes are functioning at which point the activity
remains constant.
4. Enzyme Concentration vs. Enzyme Activity
• An increase in enzyme concentration with an
abundance of substrate will cause the reaction rate
to increase.
Enzyme Concentration vs. Enzyme Activity
c) Metabolism Regulation Via Controlling Enzyme Activity
• Allosteric Regulation: Certain molecules bind to a binding site,
distinct from the active site. These molecules can act as activators
or inhibitors by changing the active site configuration.
• Feedback Inhibition: a method of metabolic control in which
the end-product of a metabolic pathway acts as an inhibitor of an
enzyme within that pathway. D:\ICIB.exe
1. Types of Inhibition
• Competitive Inhibition: the process in which a
molecule similar in shape to the substrate binds to the
active site, effectively blocking the substrate molecule.
Ex: Alcohol metabolism
• Noncompetitive Inhibition: the process in which a
molecule binds to a site different from the active site on the
enzyme. This interaction changes the shape of the active
site blocking the substrate molecule.