How Cells Work
Chapter 5
Learning Objectives
1. Physics tells us that in any energy transformation: a) energy is
neither created nor destroyed, and b) there is always some
energy lost in an unusable form such as heat. Explain what this
means to living systems.
2. What is ATP and what is its role in cell metabolism. Describe
the ATP cycle.
3. What are enzymes and what is their role in cell metabolism?
What factors affect the efficiency of enzyme function? Give
some examples of conditions that might interfere with normal
metabolism (i.e. enzyme function).
4. Explain what is involved in redox reactions, and why this
process is important in living systems.
5. Describe the relationship between photosynthesis and cellular
respiration in the biosphere.
What Is Energy?
• Total E =
usable E + unusable E
(heat)
• Capacity to do work
• Forms of energy
– Potential energy
– Kinetic energy
How would you describe the energy
in a chemical bond?
What type of energy drives osmosis?
Energy Transformations
• Energy release is coupled •
•
to energy-requiring
•
processes
•
• Cells use energy for:
•
– Chemical work
– Mechanical work
– Electrochemical work
Reactants (describe)
Energy
Enzymes
Coenzymes/Cofactors
Products
What molecule is the only one
directly used by cells to perform
work?
Can some cellular processes be
done without this molecule?
Explain.
Energy Transformations
• First Law of Thermodynamics
– The total amount of energy in the universe
remains constant
– Energy can undergo conversions from one form to
another, but it cannot be created or destroyed
• Second Law of Thermodynamics
– No energy conversion is ever 100 percent efficient
– The total amount of energy is flowing from highenergy forms to forms lower in energy
p. 74
One-Way Flow of Energy
• The sun is life’s primary
energy source
• Producers trap energy
from the sun and
convert it into chemical
bond energy
• All organisms use the
energy stored in the
bonds of organic
compounds to do work
There is a tendency for
increasing disorder
(entropy) in the
universe.
How is it possible that
living systems can resist
this move toward
disorder?
How would you describe the molecules
that contain food energy for other
organisms (stable/unstable; how many
bonds, etc)?
p. 74
Activation Energy
• For a chemical reaction
to occur, some energy
must be “invested” in the
reactant(s)
– Some reactions have a
net release of energy
– Some reactions have a
net storage energy
• Enzymes make
conditions more
favorable, so less
energy is needed to
activate the reaction
p. 78
activation energy
without enzyme
starting
substance
activation energy
with enzyme
energy
released
by the
reaction
products
Enzyme Function (review)
• Highly specific
• Very efficient
– speed up reaction up to
10 billion times faster
– have a maximum rate
– organize metabolic
pathways
– Can be re-used (are
unchanged by the
reaction)
• Temperature & pH can
affect the natural shape of
any enzyme
p. 78
Endergonic Reactions
What does “potential energy
of molecules” mean?
p. 75
Product has more energy than reactants,
and potential energy is stored.
glucose, a
high energy
product
+
6O2
low energy
starting
substances
ENERGY IN
6
6
6
Anabolic (molecule building)
reactions, e.g. ADP + P
6
Name the reactants. What metabolic
process does this describe?
Exergonic Reactions
p. 75
• Products have less energy than starting substance, and energy
is released to fuel cell processes.
• Catabolic (breakdown) reactions (e.g. digestion, ATP use)
glucose, a high
energy starting
substance
+ 6O2
ENERGY OUT
low energy products
6
Name the products. What metabolic
process does this describe?
6
Red-Ox Reactions
p. 76
• Oxidation is the loss of electrons from a molecule
– decreases its potential energy
– acceptor of the electron (often associated with H) is
often oxygen (thus, described as “oxidation”)
• Reduction is the gain of electrons by a molecule
– increases its potential energy
• In the body, oxidation-reduction reactions are coupled &
occur simultaneously; tightly controlled by enzymes
LEO
the Lion says
GER
Loss of
Electrons is
Oxidation
Gain of
Electrons is
Reduction
Main Metabolic Pathways in
Ecosystems
CO2 is
reduced, to
produce
glucose
(where
does the
energy
come from
to fuel
these
reactions?)
p. 77
glucose is
oxidized, to
produce
ATP
(what is the
other form
of energy
released?)
The Role of ATP
p. 75
Cells “earn” ATP in exergonic reactions
Cells “spend” ATP in endergonic reactions
Name some examples of ‘cellular work”. Are these energy
transformations completely efficient?
The Role of Coenzymes
pp. 78, 80, 97
• Co = “with”
• Shuttle molecules/ions
from one reaction to the
next (esp. H+ & e-)
• Unchanged
• Reusable
• Often made from vitamins
– Niacin  NAD (NADP)
– Riboflavin  FAD