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Summary Bioenergetics BCH2602
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- University Of South Africa (Unisa)
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Bioenergetics BCH2602 Examinations NotesLearning Unit 1- Introduction to metabolism
Metabolism is the entire network of chemical transformations within the cells of living
organisms. What does this mean? It means that metabolism functions to:
o convert food into energy
o convert food into building blocks
o eliminate nitrogenous wastes
Activity 1.1.
Metabolism is the series or collection of life-sustaining chemical reactions with
purposes/goals of conversion of food to energy to run cellular processes to carry out
everyday functions such as breathing, circulating blood, transport of substances into and
between different cells, repairing and growing cells in organisms, the conversion of food/fuel
to building blocks (anabolism) for proteins, lipids, nucleic acids, and some carbohydrates,
and the elimination of metabolic wastes (catabolism) to allow organisms to develop and
reproduce, maintain their structures, and respond to their environments.
Energy exists in various forms:
o Kinetic energy – energy associated with motion
o Thermal energy – kinetic energy associated with random movements of atoms and
molecules
o Electrical energy – distribution of different amounts of electrical charge across
cellular membranes
o Potential energy – energy possessed within a body because of its location or
structure
o Chemical energy – potential energy which is available, and which can be released in
a chemical reaction
o Light – can be harnessed to perform work
All cells in a living organism require energy to perform various cellular processes. Cellular
processes such as breaking and making of chemical bonds of complex molecules form part
of the exchange and transformation of energy. Some chemical reactions are spontaneous
and release energy (exergonic), whilst other chemical reactions require energy (endergonic)
for the reaction to proceed. Energy must be available to perform cellular work such as
mechanical processes (e.g., muscle contraction), transport (e.g., pumping across
membranes) and chemical synthesis (e.g., making of polymers).
There are two laws of physics that are fundamental to our understanding of biology and that
govern energy transformation:
First law of thermodynamics
It is also called the principle of conservation of energy, and it states that “energy can be
transferred or transformed but cannot be created or destroyed”. The first law applies to
all levels of organisation in the living and non-living world. In the living world such examples
are organisms, cells, organelles, and even individual chemical reactions that characterise
metabolism.
ATP and ADP cycle
,In a cell, energy is constantly escaping into and entering from the surrounding environment.
Because of this, it is very difficult to measure energy in a biological system. According to the
first law of thermodynamics, energy may be interconverted – for example, potential energy
can be converted into electrical or thermal energy but cannot create or destroy energy.
When a cell breaks down a polysaccharide, the by-products will ultimately form CO2 and
H2O; some potential energy is conserved when ADP is phosphorylated to form ATP and can
be used immediately by the cell. However, not all the energy of the carbohydrate is
conserved – some energy is converted to thermal energy and is transferred to the
surrounding environment as heat. It is important to note that all the energy involved in this
process can be accounted for – thus, the energy was not destroyed.
The second law of thermodynamics
This law states that “every energy transfer or transformation increases the entropy of
the universe”, where entropy is defined as a measure of disorder or randomness.
Simply put, during every energy transfer or transformation, some energy is lost as heat – but
the question is, in which direction?
What do you think would happen if we place a small ice cube in a litre of hot water, seal this
container (vacuum) and allow this system (ice and water) to reach equilibrium? The answer
is, the ice would melt and the water would decrease in temperature. What does this
mean in terms of the second law of thermodynamics? Well, the flow of heat (thermal
energy) from the hot water to the ice causes the ice to melt spontaneously. The
energy that is “lost” by the water is “gained” by the melting ice. What have we learnt
here? Energy changes have direction and may be spontaneous.
So, what do these laws have to do with biological systems? Cells use heat to maintain order
in our body. All cellular work is powered by adenosine triphosphate (ATP), the “energy
currency” of the cell. ATP is used to couple exergonic and endergonic reactions together.
Organisms’ metabolic processes use enzymes to break down energy-rich glucose into its
potential energy, which is then trapped and stored in the form of ATP, as well as to break
down ATP into ADP and inorganic phosphate to be utilised in other biochemical processes.
Cells convert energy from ATP into:
o mechanical energy – to move organelles
o electrical energy – to move ions against a concentration gradient
, o thermal energy – to release heat
Photosynthesis is a process whereby light energy from the sun is converted into food
(carbohydrates) using chemical energy. This process occurs in most plants, algae, and
some bacteria. The plants store carbohydrates in starch (long polysaccharides) whilst
animals store carbohydrates in glycogen. These molecules contain a lot of chemical bonds
and therefore they have a lot of chemical energy. When these bonds are broken down
during metabolism, the chemical bonds release energy which may be used for cellular
processes. The synthesis of food (glucose) by photosynthesis is depicted by the following
equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Indirectly, we can say that photosynthesis also provides nutrients to animals. The starch that
stores glucose (in plants) can be broken down further into glucose by cellular respiration (in
animals) to produce ATP. They utilise carbon and hydrogen compounds in combination with
oxygen in the environment to produce carbon dioxide (CO2) and water (H2O). This process
occurs through a number of biochemical reactions (cells’ metabolism), and this can be
depicted by the following equation: 6CO2 + 6H2O + energy → C6H12O6 + 6O2
Activity 1.2.
First law of thermodynamics
Digestion
Mechanical and chemical digestion of food uses ATP energy. The bonds that occur between
the three phosphates in ATP hold the energy of the molecule. When those bonds are
broken, the energy is released. The chemical energy found in our food can be converted to a
variety of various forms, another form of chemical energy when it is stored as glucose or fat,
thermal energy because our body produces heat when digesting food or into kinetic energy
when walking, breathing, and so on.
Second law of thermodynamics
When you are going for a walk, the energy exchanges that occur in your body when your leg
muscles contract to drive your body forward taking chemical energy from glucose and
converting it to kinetic energy are inefficient. A large percentage of the energy from your fuel
sources is transformed into heat which dissipates into the surrounding environment while the
remainder is used for homeostasis and other purposes.
The transfer of heat increases the entropy of the surroundings, as does the fact that you emit
carbon dioxide and water when you metabolize fuel to power your walk. Even when at rest,
any organism retains some level of metabolic activity, resulting in the breakdown of complex
, molecules into simpler and more abundant ones, as well as the production/ generation of
heat, thus increasing the entropy of the environment.
Metabolism can be broken down into two opposing reactions:
1. Catabolism: Comprises reactions that degrade or break down complex substances into
simpler molecules to eventually harvest energy
2. Anabolism: Uses energy and comprises all processes that involve synthesis or building
of complex substances from simple molecules such as proteins and nucleic acids.
Anabolic and catabolic pathways generate the order that drives life. During the metabolic
process, initial reactants/substrates will go through a series of reactions that lead to the
formation of product(s). This process whereby a product of one reaction becomes a
substrate of the next is referred to as a metabolic pathway.
Gibbs free energy (G)
Gibbs free energy (G) of a system is a measure of the amount of usable energy (energy that
can do work) in a system at constant temperature and pressure. The change in Gibbs free
energy (ΔG) during a reaction allows us to understand a reaction's energetics and
spontaneity (whether it can happen with or without added energy). Consider the following
equation regarding the change in Gibbs free energy:
ΔG = G final – G initial
The free energy is stored in the bonds present in the reactants and products of a reaction. In
a thermodynamic reaction, the total free energy of the products (G final) must be subtracted
from the total free energy of the reactants (G initial) to obtain the change in free energy (ΔG).
This value tells us the maximum usable energy released (or absorbed) in going from the
initial to the final state of a reaction – thus, an exergonic or endergonic reaction. In addition,
its sign (positive or negative) tells us whether a reaction will occur spontaneously, that is,
without added energy.
o ∆G = 0, the reaction will be in equilibrium – which means that the concentration of
products and reactants does not change.
o ∆G < 0, or a decrease in free energy means that energy is released during the
reaction; the reaction is spontaneous.
o ∆G > 0, or an increase in free energy means that energy is used up in the reaction;
the reaction is not spontaneous.
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