Maintaining a Balance
Topic 2: Metabolic Efficiency
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
 Describe homeostasis as the process by which organisms
maintain a relatively stable internal environment
 Explain why the maintenance of a constant internal
environment is important for optimal metabolic efficiency
Of all living organisms the mammalian body has best perfected
keeping internal functioning constant, no matter what changes
occur in the environment. We are going to look at the regulatory
systems in both plants and animals that act to maintain a balance
in their internal environments.
The word homeostasis comes from the Greek words homoios,
meaning like or the same and stasis, meaning state. This implies a
state of balance or constancy, where conditions stay the same in
the internal environment of living organisms to allow them to
function efficiently, despite fluctuations in the external
Homeostasis is defined as: the maintenance by an organism of a
constant or almost constant internal state, regardless of external
environmental change. Any organised infrastructure, whether a
living organism or a non-living enterprise, needs careful control
and certain constants if it is to run smoothly and efficiently.
If we consider the smooth
running of a hospital or even a
household, a sudden external
change, for examples a power
cut, could have drastic results if
the organisation cannot
continue to work independently
of the outside changes. If the
hospital does not have a back-up
plan, lives could be lost.
In order to maintain a constant internal environment, the
following two steps are essential:
1. Detect the change
2. Counteract the change
In a similar way, living organisms must have mechanisms in place
to enable them to function independently of external changes to
maintain a relatively constant internal state.
Lets think about our hospital and home analogy. In order to
maintain a constant internal environment in the event of a power
cut, how would people within the hospital or home:
 Detect the changes?
 Counteract the change?
When we consider our analogy in more detail, it becomes clear
that some organisations are better equipped to cope with change
than others. Part of the back-up plan in a hospital is to have its
own emergency generator. Most of us however, would not have
one in our homes.
Certain living organisms have a better ‘back-up plan’ than others
when it comes to maintaining a constant internal environment.
Living organisms have developed mechanisms that ensure that
they are able to maintain a constant internal state regardless of
changes in the external environment.
Optimal Metabolic Efficiency
In order to understand how homeostasis is maintained, we need
to understand this at the cellular level. Living organisms are made
of cells, which must function efficiently to maintain life. All
chemical reactions within cells must occur efficiently and be
effectively co-ordinated to bring about optimal metabolic
Optimal Metabolic Efficiency
Each of your cells is surrounded by a small amount of fluid called
intercellular or interstitial fluid and this, together with cytoplasm
inside cells, makes up their internal environment.
Optimal Metabolic Efficiency
Cells are extremely sensitive to changes in their internal
environment and any imbalance adversely affects their
functioning. The internal environment of an organism must be
maintained within a narrow range of conditions.
Optimal Metabolic Efficiency
Temperature, the amount of
cells or fluid and chemical
content for example must be
kept stable so enzymes can
function effectively and
metabolic efficiency can be
Optimal Metabolic Efficiency
Enzymes are extremely sensitive to the temperature and pH of
the environment and changes in concentrations of these, as well
as nutrients such as glucose and oxygen, affect their activity. Cells
cannot tolerate any build-up in levels of waste products such as
carbon dioxide or other metabolic wastes.
Optimal Metabolic Efficiency
Metabolic efficiency relies on a constant level of the following
variables in the internal environment:
 Temperature and pH
 Concentration of metabolites (reactants)
 Concentration of water and salt (osmotic pressure), which
determines the volume of cells or fluid such as blood.
 Absence of toxins that may inhibit enzyme functioning
All chemical reactions necessary for the cell’s survival and
functioning are controlled by enzymes so it’s vital to maintain a
constant environment.
For any chemical reaction to proceed, reactants must be present.
We call these reactants metabolite. Metabolites are chemicals that
participate in chemical reactions in cells (for example, glucose
and oxygen). Some are taken in from the outside environment,
whereas others are products of other metabolic pathways.
Many metabolic reactions rely
on the availability of ATP energy
in cells. If cells cannot produce
sufficient energy, there is a
ripple effect and other metabolic
activity will be adversely
The production of energy relies on chemical respiration, which in
turn relies on an ample supply of metabolites such as glucose and
oxygen, as well as respiratory enzymes and their cofactors. A lack
of any of these metabolites may therefore slow down or stop
chemical respiration, affecting overall metabolic efficiency.
Osmotic Balance
All chemical reaction in living
organisms take place in water. For
chemical reactions to proceed, the
reactants must be dissolved in water.
Therefore the water concentration of
cells and their surrounding fluid is of
enormous importance. Dissolved
substances such as salt affect the
osmotic balance of fluids and so the
concentration of slats and other
dissolved substances must also be
maintained within a narrow range.
An absence of toxins
A build-up of carbon dioxide and/or other wastes (as a result of
chemical reactions in the cells) may be toxic to cells, affecting
enzymes either directly or indirectly.
An absence of toxins
Some interact directly by
blocking the active site of
enzymes, while others act
indirectly by altering the
optimal conditions for enzyme
functioning. For example,
carbon dioxide alters the pH of
fluid. In either case, enzyme
functioning is inhibited and to
these wastes must be removed
to ensure metabolic efficiency.
-Students to write and answer the following questions in their
1. Explain why organisms devote a lot of energy to
maintaining a constant internal environment
2. Define homeostasis
3. Identify body processes that are controlled by homeostasis

1.2 Metabolic Efficiency