Lecture # 4

Two Aspects of Cell Metabolism:
1. Bioenergetics
2. Structural Aspects

Energy is the ability to do work.

Energy is the ability to change matter.

Most the energy that is utilized by
organisms is in the form of
chemical energy.

Chemical energy is the energy that
is in the bonds holding atoms
together in molecules.

Other forms of energy:
Heat
Light
Motion (kinetic energy)
Electrical
Mechanical

Organisms do NOT make energy.

Organisms convert energy
from one form to another.

They transduce energy.

Transduction is the conversion
of energy from one form to another.

Burn coal → electricity

Heat energy → electrical
                            energy

Use electricity to run a
motor

Electrical → Mechanical
Energy         Energy

Three major types of energy
transductions are found in organisms:

1. transduction of light energy into
chemical energy in carbohydrates

This is called photosynthesis. 
It occurs in green plants
in chloroplasts.

6 CO₂+ 6 H₂O → C₆H₁₂O₆+ 6 O₂

2. Transduction of chemical energy in
carbohydrates into chemical energy in
phosphate compounds.

The most important phosphate compound
is adenosine triphosphate.
ATP

adenine + ribose + 3 phosphate groups

This energy transduction is called respiration
and involves the mitochondria in cells.

C₆H₁₂O₆+ 6 O₂  → 6 CO₂+ 6 H₂O

ATP is not a direct product of this reaction,
but 38 molecules of ATP are made
from each glucose molecule that goes
through the reaction.

3. transduction of chemical energy in ATP
into energy that is utilized by the cell

Energy may be:
Mechanical energy (muscle contraction)
Electrical energy (nerve impulse)
Chemical energy (making a large molecule)
Heat

Food is a “high quality” energy;
heat is a “low quality” energy.

The transduction and utilization of energy
by biological systems is orderly.

The routes through which energy flows
in organisms are called metabolic pathways.

A metabolic pathway is a series of chemical
reactions, each of which is catalyzed by an enzyme
and which are arranged in such a way that the product
of one reaction is the reactant in the next reaction.

Although metabolic pathways are orderly
but they are not always simple.

Thermodynamics deals with energy and energy
transductions in general.

First Law of Thermodynamics

Energy is neither created nor destroyed but only
transformed from one form to another.

The amount of energy in the universe is constant.

The first law is also known as the Law of
Conservation of Energy.

Entropy is the degree of randomness or
the degree of disorder in a thermodynamic system.
S

Second Law of Thermodynamics:
The entropy of the universe never decreases.

A system with maximum entropy can do no work.

The tendency for entropy to increase is the force
that drives reactions.

As entropy increases, energy, usually in the
form of heat, gets transferred.

Changes in entropy and heat are related to
one another by the concept of free energy.
G

Free energy is the part of the total energy
of a system that is available to do work
under conditions of constant temperature and
constant pressure.

∆G = ∆H - T∆S

If ∆G is negative, the reaction can occur
spontaneously. 
A reaction with a negative ∆G is said
to be exergonic.

If ∆G is positive, the reaction cannot occur
spontaneously. 
A reaction with a positive ∆G is said
to be endergonic.

When energy from an exergonic reaction is used
to drive an endergonic reaction, the reactions
are said to be coupled.

A kilocalorie is a unit of energy, defined as heat,
that will raise the temperature of one
kilogram of water from 14.5° to 15.5° C.

Enzymes are catalysts.
They increase the velocity of a chemical reaction
without being used up in the reaction.

Two ways to study enzymes:
1. in vivo experiments
2. in vitro experiments

The substance that an enzyme acts on is called
the substrate of the enzyme.

Enzymes are named by adding the suffix –ase
to the name of the substrate; for example,
sucrose and sucrase.

Two important properties of enzymes are their
specificity and their activity.