Bio 211 Intro Molecular and Cell Biology
Lecture 6 “Cell Energy”
Reading: Campbell Chap. 6 pp. 83-91
So far in this course, we have been working at the level of molecules. We have
been learning about some of the key macromolecules important for cell function. In
today’s lecture, we will be examining how physical laws govern the behavior of
biological molecules in cells. We will see how the law of conservation of energy applies
throughout the universe, including living things. The topic of today’s lecture is
thermodynamics, and we will focus on the dynamic role that energy plays in cells.
Outline:
1. Metabolism
2. Energy
3. ATP
1. Metabolism is the term used to describe all the chemical processes that occur
in cells. We can divide up metabolism into discrete steps occuring in sequence. Since we
have many different processes that must go on--making and breaking down different
biological molecules, there are many of these sequences or metabolic pathways in cells
and they interconnect (locate chart of metabolism?).
The following sequence symbolizes a metabolic pathway:
A ---> B ---> C ---> D
A is a precursor in the reaction pathway, B and C are reaction intermediates and D
is the product. The arrows indicate the sequence of chemical reactions that occur in cells.
As we will see shortly, the cell uses enzymes to help carry out the chemical changes of A
going to B, B going to C and so forth.
The goal of much of metabolism is to manage the energy and raw materials of the
cell. Metabolic reactions that lead to the release of energy by breaking down compounds
into smaller molecules are called degradative or catabolic pathways.
Examples of catabolic pathways we will discuss shortly in this course are
the pathways of cellular respiration. In these reactions, sugars are broken down in the
presence of oxygen to give carbon dioxide and water. Energy stored in the sugars become
available to do the work of the cell.
Other important pathways of the cell (biosynthetic or anabolic reactions) lead to
the synthesis of larger molecules from smaller ones. These reactions require some of the
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energy released from catabolism to help them move forward. An example is the synthesis
of proteins from amino acids.
2. Energy
Today, we will focus on the common currency of these reactions, the release and
reuse of forms of energy. We will divide up this part of the lecture into several topics:
a. Forms of energy
b. Laws of thermodynamics
c. Free energy changes
a. Forms of energy
Energy is the capacity to do work. Life processes depend on the ability to
transform one type of energy into another.
Kinetic energy is the form of energy expended by objects in motion.
A resting object can still possess energy in the form of potential energy.
Biological objects store potential energy in the form of chemical bonds.
Energy can be converted from one type into another. In cells potential energy is
harnessed in chemical bonds during the process of respiration. The chemical energy can
be used to drive other processes such as movement or synthesis of biological molecules.
b. Laws of thermodynamics
Thermodynamics is the study of energy changes in systems. A system is a
collection of matter under study and can be small or large, closed (isolated from
surroundings) or open (in contact with surroundings).
Organisms are open systems; they absorb light or chemical energy from the
environment and release heat and waste products to the surrounding. Two laws of
thermodynamics govern living systems.
First law of thermodynamics (law of conservation of energy): Energy is neither
created or destroyed; the energy of the universe is constant.
Second law of thermodynamics: Every energy transfer increases the entropy
(disorder) of the universe.
In most energy transformations, ordered forms of energy are converted at least in
part to heat. This explains why energy is conserved (as heat is a form of energy), yet
energy transfer increases randomness.
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From the first and second laws of thermodynamics, we can conclude that the
quantity of energy in the universe is constant, but the quality of energy changes. For
organisms, we would observe that an organism takes in organized forms of matter and
energy from its surroundings and replaces them with less ordered forms. (Find a figure)
animal obtains starch, proteins and complex molecules from food
(stored chemical energy)
catabolic processes break down molecules
release simpler molecules (CO2 and H20) with less chemical energy
remainder of energy released as heat
c. Changes in free energy
In this unit of the course, we would ultimately like to be able to take some of the
metabolic processes that occur in living things and describe them using principles of
thermodynamics and chemistry. In the next few minutes, we will focus on some of the
terms and principles that allow is to describe energy changes in cells.
In chemistry, we use the phrase free energy to describe that part of the energy in a
system able to do work. The symbol for free energy is G. Free energy is related to the
total energy in a system (symbolized as H) and its entropy (randomness, symbolized by S)
in the following way:
G
=
H
free energy total energy
T
S
temperature, K  entropy
From this equation, not all energy is available for work. We also note that
temperature affects the intensity of random motion and thus affects the entropy term of
the equation.
How do we know whether a chemical reaction will be spontaneous (occur on its
own)? In a spontaneous process the free energy (G) of a system decreases.
The change in free energy as a system goes from a starting state to a final state is
represented by G.
G = G final state - G starting state
It can also be represented by:
G = H - T S
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For a process to occur spontaneously, the system must either give up energy
(decrease H) or give up order (increase in S) or both. For the process to be spontaneous,
G must be negative. The more negative the G value, the greater the amount of work
the process can perform.
In metabolism, exergonic reactions proceed with a net release of free energy; G
is negative and the reaction is spontaneous.
Endergonic reactions require an input of free energy; G is positive and the
reaction is non-spontaneous.
Cellular respiration is an example of a highly exergonic process:
C6H12O6
+
6O2
----->
6 CO2
+
6H20
G = -686 kcal/mol (-2870 kJ/mol)
For each mole of glucose broken down, 686 kcal of energy are made available for
work.
Photosynthesis, on the other hand, where sugars are made from carbon dioxide
and water, is a highly endergonic process. It requires a large input of energy, which is
ultimately derived from energy from the sun.
Staying alive: metabolic disequilibrium. Because chemical systems at equilibrium
have a G = 0, they can do no work. A cell that reached equilibrium would be dead;
therefore many processes are driven forward by having them out of equilibrium, meaning
they are forced in one direction only by the concentrations of reactants and products or by
having the products be immediately used in a subsequent reaction.
Figure 6.4 summarizes with three parallel examples the relationship between free
energy, spontaneous change and work.
3. ATP
A cell does three main kinds of work:
(1) Mechanical work, such as beating of cilia, muscle contraction
(2) Transport work, moving substances across membranes
(3) Chemical work, enabling non-spontaneous reactions to occur spontaneously,
such as protein synthesis.
The molecule that powers most kinds of work in the cell is ATP: adenosine
triphosphate.
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ATP is a nucleotide and consists of sugar, 3 phosphate groups and the nitrogenous
base, adenine. Fig. 6.6 ATP structure.
Energy is released when one or more phosphate groups are hydrolyzed. When
water hydrolyzes the terminal phosphate bond, a molecule of inorganic phosphate is
removed from ATP, which then becomes ADP, adenosine diphosphate (Fig. 6.6b). The
reaction is exergonic and causes the release of 7.3 kcal of energy per mole of ATP
hydrolyzed (under standard conditions):
ATP -----> ADP + Pi
G = - 7.3 kcal/mol
Hydrolysis of the phosphates from ATP leads to a more stable molecule as not as
many negatively charged atoms are crowded into one place.
ATP performs work in the cell by linking up (or chemically coupling) ATP
hydrolysis to otherwise energetically unfavorable cellular reactions. We will soon be
looking at process called glycolysis which cells use to derive energy. The first reaction of
glycolysis is energetically unfavorable:
glucose + Pi -----> glucose-6-phosphate
G = +3.3 kcal/mol
When coupled to ATP hydrolysis, the reaction becomes energetically favorable:
glucose + Pi -----> glucose-6-phosphate
ATP-----> ADP + Pi
overall:
G = +3.3 kcal/mol
G = -7.3 kcal/mol
glucose + ATP -----> glucose-6-phosphate + ADP G = -4.0 kcal/mol
Transfer of a phosphate group from ATP causes this energetically unfavorable
reaction to occur spontaneously.
As we shall see, ATP is the major energy currency of the cell. When cells need
energy they use ATP and hydrolyze one of the phosphate bonds. When energy is
plentiful in the cell, the stores of ADP are phosphorylated to make ATP.
Summary: In today’s lecture, we studied how cells use energy, otherwise known as
thermodynamics. Much of the energy of cells is tied up in chemical bonds, particularly
those of the molecule, ATP. Linking up energetically unfavorable reactions with the
breakdown of ATP is a key way cells drive their metabolic processes. In the next lecture,
we will see how enzymes assist cells in metabolism by accelerating the rate of chemical
reactions.
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