Maintaining a Balance
Topic 1: Enzymes and Metabolism
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT
 Identify the role of enzymes in metabolism, describe their
chemical composition and use a simple model to describe
their specificity on substances
 Identify the pH as a way of describing the acidity of a
substance
Introduction
Most organisms are active within
a limited temperature range,
despite the large fluctuations in
temperatures that occur in the
outside environment. Organisms
that live in environments where
they may be subjected to
extremes of temperature have
adaptations that enable them to
keep their internal temperature
within a relatively narrow range.
world.edu
Introduction
Organisms must also maintain a
relatively constant balance of
chemicals within their bodies if
they are to remain functionally
active. One of the main reasons
why the maintenance of a
constant temperature and
chemical balance is important is
to ensure efficient metabolism.
Metabolism is the sum total of
all chemical reactions occurring
within a living organism.
whatdoino-steve.blogspot.com
Introduction
All metabolic reactions in living cells are controlled by enzymes.
Enzymes are protein molecules, present in cells, which act as
catalysts, controlling the rate of each part of the complex
chemical reactions that take place in cells.
weartfestival.com
Introduction
Catalysts are chemical substances that can accelerate chemical
reactions, but they remain unchanged at the end of the reaction
and can be reused. They function very rapidly at low
temperatures, making them ideal for cell functioning.
scienceclarified.com
Introduction
Metabolism is divided into two:
 Anabolic: Reactions that involve building up large organic
compounds from simpler molecules
 Catabolic: Reactions that involve breaking down complex
organic compounds to simpler ones.
en.wikipedia.org
Introduction
An example of anabolic
metabolism is the breakdown of
starch. Starch is a large
polysaccharide molecule made
from small monosaccharide
units such as glucose, a product
of photosynthesis in plants.
Anabolic steroids increase protein
synthesis within cells, which results
in the buildup of cellular tissue
(anabolism), especially in muscles.
ayushveda.com
Introduction
An example of catabolic metabolism is the digestion of food.
Large food molecules such as protein are broken down into small
units called amino acids, which can then be easily absorbed from
the gut into the bloodstream.
old.maxfitmag.com
Introduction
By understanding the chemical
composition, functions and
characteristics of enzymes, we
can better understand their role
in controlling chemical
reactions in cells and therefore
metabolism in living organisms.
simple.wikipedia.org
The Chemical Composition of Enzymes
Enzymes are protein
molecules and are made by
living cells. They are globular
proteins, meaning that they
have long chains or sequences
of amino acids that have been
folded into a specific shape.
Their effective functioning
relies on their shape.
en.wikipedia.org
The Chemical Composition of Enzymes
The molecule on which an enzyme acts is called a substrate. An
enzyme fits together with its substrate molecule at a precise place
on the surface of the much larger enzyme molecule, called the
active site. The shape of this active site must not be altered if
the enzyme is to function. (This is much like a key fitting a
particular lock)
students.cis.uab.edu
The Chemical Composition of Enzymes
Some enzymes have a non-protein group such as a vitamin or
metal ion (zinc, copper or iron) that binds with the protein part
and helps to form the active site. This is termed a co-enzyme or
cofactor. It can easily be separated from the protein part of the
enzyme, but its presence is essential for the enzyme reaction to
occur.
immunesystemimmunity.blogspot.com
The Chemical Composition of Enzymes
A functional enzyme may therefore consist of protein only, or it
may be in the form of an enzyme-cofactor complex, where the
enzyme part of the complex is a protein.
sciencedirect.com
The Chemical Composition of Enzymes
Poisons are substances that have
harmful effects on living
organisms. Some poisons exert
their toxic effect by disabling
cofactors and thereby inhibiting
enzyme functioning. The heavy
metal mercury and cadmium
replace zinc cofactors in some
enzymes and inhibit their
functioning.
biologycorner.com
The Role of Enzymes in Metabolism
Enzyme catalysts are able to speed up or slow down reactions
without a change in temperature. This is extremely important in
cells, since heat damages living tissue. For a chemical reaction to
begin, activation energy is necessary. The role of the enzyme is to
lower the activation energy needed to start a reaction, so that the
reaction can proceed quickly, without a change in temperature.
rtmsd.org
The Role of Enzymes in Metabolism
In chemical reactions that occur
in the non-living world, heat
could provide the necessary
activation energy for a chemical
reaction. It is important to
remember that an enzyme does
not provide activation energy, it
reduces the amount of activation
energy needed by bringing
specific molecules together,
rather than relying on them
colliding randomly.
sciencephoto.com
The Role of Enzymes in Metabolism
For example, oxygen and glucose
may be chemically combined to
release energy. In the lab, we can
activate this by adding heat. We can
burn the glucose, cause it to react
with oxygen in the air and cause it
to release energy as heat and light.
In the human body, an enzyme is
necessary to lower the required
activation energy so that glucose can
react with oxygen to release energy.
sciencephoto.com
The Role of Enzymes in Metabolism
Enzymes are substrate specific, meaning that one particular
enzyme can work on only one particular substrate molecule. This
is because the active site is reciprocally shaped to bind with that
molecule.
students.cis.uab.edu
The Role of Enzymes in Metabolism
The enzyme itself is not chemically changed in the reaction so it
can be reused in subsequent reactions. Enzyme controlled
reactions are always reversible.
students.cis.uab.edu
How Enzymes Work
Enzymes are large, globular protein molecules with one or more
indentations on their surface called active sites. For an enzyme to
catalyse a reaction, the small substrate molecules must
temporarily bind to these active sites.
alevelnotes.com
How Enzymes Work
At first a lock-and-key model was proposed. It was thought
that the active site is rigid and the small substrate molecule is
reciprocally shaped and fits into the active site, like a lock fits a
key. Once this enzyme-substrate complex has formed, the close
proximity of the molecules allows the reaction to be rapidly
catalysed and the products of the reaction are released.
acrazychicken.blogspot.com
How Enzymes Work
To validate this model, predictions were made and tested. The
results led to the proposal of the currently accepted amended
version of the model, known as the induced-fit model. This
model is based on the realisation that proteins are not rigid.
Evidence suggests that the binding of a substrate to the active site
of an enzyme induces the enzyme to alter its shape slightly, to fit
more tightly around the substrate.
intranet.canacad.ac.jp
How Enzymes Work
Enzymes are highly efficient. They work
rapidly, having a high rate of reaction or
turnover number (the number of
substrate molecules that one enzyme can
act on in 1 minute). Catalase is the
fastest acting of all enzymes, having a
turnover number of 5 million substrate
molecules per minute. Enzymes are also
highly effective. Only minute traces are
needed to bring about reactions and they
can be reused.
bioweb.wku.edu
How Enzymes Work
The rate of an enzyme
controlled reaction is affected
by the concentration of the
substrate. If an enzyme and
substrate have a high affinity for
each other, the reaction will
proceed rapidly. The higher the
substrate concentration, the
greater the rate of enzyme
reaction, until all available
enzymes are being used to
catalyse reactions. This is
known as the saturation point.
schools-wikipedia.org
How Enzymes Work
Increasing the substrate
concentration beyond the
saturation point will not
increase the rate of reaction,
since all of the enzymes are
working at their maximum
turnover rate and will have to
be reused to act on the
additional substrate. The only
way to increase the reaction rate
would be to increase the
enzyme concentration.
schools-wikipedia.org
Characteristics of Enzymes
Enzymes, due to their protein nature, are sensitive to
temperature and to pH. Enzymes within cells function best at
the body temperature of the living organism. In most living
things, they function normally at temperatures up to 40C.
Above this, their efficiency (rate of reaction) decreases.
smh.com.au
Characteristics of Enzymes
At temperatures above 60C, most enzymes stop functioning
altogether. This is because heat causes the hydrogen bonds that
maintain the form of the enzyme to break. This in turn changes
the shape and structure of the molecule which is said to
denature.
chemguide.co.uk
Characteristics of Enzymes
Any change in shape that affects the active site will alter the
functioning of the enzyme because the active site is no longer
reciprocally shaped to the substrate molecule.
livingscience.co.uk
Characteristics of Enzymes
Excessive cold also causes the enzyme to change shape and its
functioning to slow down or stop. However, the change in shape
due to extreme cold is often reversible.
students.cis.uab.edu
Characteristics of Enzymes
Each enzyme also has its own narrow range of pH within which it
functions most efficiently. Levels of alkalinity or acidity outside of
the optimum pH have a similar effect to that of temperature
changes. They alter the shape of the enzyme and slow down or
stop its functioning. Extremes of pH, like temperature, cause the
enzymes to denature.
chemistryinyourcupboard.org
Characteristics of Enzymes
Within cells, most enzymes
function at or near neutral, but
enzymes in the digestive tract
function in an acidic or alkaline
medium. For example, the
protein-digesting enzymes
pepsin and rennin, found in
gastric juice in the stomach,
function best in strong acid.
paraibaparadise.com
What is pH
pH is a way of describing the acidity of a substance. The pH scale
is used to measure the acidity or alkalinity of a substance. pH is a
logarithmic value of the concentration of hydrogen ions (𝐻+ ) in
solution. The greater the hydrogen ion concentration, the lower
the pH.
abundanthealthcenter.com
What is pH
The pH scale runs from 0 to 14,
where 7 is neutral. The
presence of hydrogen ions in a
solution makes it more acidic
and so solutions with a pH
below 7 are acidic and those
with a pH above 7 are alkaline.
The further away from the
neutral value of 7, the stronger
the respective acid or base.
adrianasassoon.wordpress.com
Activity
-Students to complete DOT Point Question 1.1 (see HSC DOT
Point book)