Energy and Metabolism
Chapter 4
Part 1
4.1 Impacts/Issues
A Toast to Alcohol Dehydrogenase
 Metabolic processes break down organic
molecules such as ethanol and other toxins –
binge drinking is currently the most serious drug
problem on college campuses
Video: Alcohol, enzymes, and your liver
 Alcohol dehydrogenase (ADH) converts ethanol
to toxic acetaldehyde, which is then converted to
acetate by ALDH
4.2 Life Runs on Energy
Laws of Thermodynamics
 Energy
• The capacity to do work
Law #1
Energy can be converted from one
form to another, but cannot be created or
destroyed –
Law #2
Energy disperses spontaneously
Material Recycle
 Energy inputs drive a cycling of materials among
producers and consumers
 Producers and then consumers use energy to
assemble, rearrange, and break down organic
molecules that cycle among organisms
throughout ecosystems
ENERGY IN
Light energy radiating from
the sun reaches Earth.
Producers capture some of
it by converting it to
chemical energy. They and
all other organisms use
chemical energy to drive
cellular work.
One way
Flow of energy
PRODUCERS
plants and other selffeeding organisms
nutrient
cycling
CONSUMERS
animals, most fungi,
many protists, bacteria
ENERGY OUT
With each conversion, there is a oneway flow of a bit of energy back to the
environment, mainly in the form of heat.
Fig. 4-2, p. 63
Matter recycling and energy flow
4.3 Energy in the Molecules of Life
 Cells store and retrieve energy by making and
breaking chemical bonds in metabolic reactions
 Some reactions require a net input of energy –
others end with a net release of energy
Chemical Reactions
 Reaction
• Process of chemical change
 Reactant
• Molecule that enters a reaction
 Product
• A molecule remaining at the end of a reaction
A Chemical Reaction
Energy Inputs and Outputs
in Chemical Reactions
 Chemical bonds hold energy – the amount
depends on which elements take part in the bond
 Cells store energy in chemical bonds by running
energy-requiring reactions, and access energy by
running energy-releasing reactions
Energy Inputs and Outputs
in Chemical Reactions
Why the World Doesn’t Go Up in Flames
 Molecules of life release energy when combined
with oxygen – but not spontaneously – energy is
required to start even energy-releasing reactions
 Activation energy
• Minimum amount of energy required to start a
reaction
Activation Energy
Reactants:
2 H 2 + O2
Energy
Activation energy
Difference in
energy between
reactants and products
Products: 2 H2O
Time
Stepped Art
Fig. 4-4, p. 65
Animation: Chemical equilibrium
ATP – The Cell’s Energy Currency
 Energy carriers accept energy from energyreleasing reactions and deliver energy to
energy-requiring reactions
 ATP (Adenosine triphosphate)
• Main energy carrier between reaction sites in
cells
Phosphorylation
 Phosphate-group transfers (phosphorylation)
to and from ATP couple energy-releasing
reactions with energy-requiring ones
ATP: The Energy Currency of Cells
4.4 How Enzymes Work
 Enzymes make chemical reactions proceed
much faster than they would on their own
 Enzyme
• Protein or RNA that speeds a reaction without
being changed by it
Substrates
 An enzyme’s particular substrates bind at its
active site
 Substrate
• A reactant molecule that is specifically acted upon
by an enzyme
Active Sites
 Active site
• Pocket in an enzyme where substrates bind and
a reaction occurs
Factors That Influence Enzyme Activity
 Each enzyme works best within a characteristic
range of temperature, pH, and salt concentration
 When conditions break hydrogen bonds, an
enzyme changes its characteristic shape
(denatures), and stops working
Enzymes, Temperature, and pH
Fig. 4-6a, p. 66
Organized, Enzyme-Mediated Reactions
 Cells concentrate, convert, and dispose of most
substances in enzyme-mediated reaction
sequences
 Metabolic pathway
• Series of enzyme-mediated reactions by which
cells build, remodel, or break down an organic
molecule
Linear and Cyclic Metabolic Pathways
Control of Metabolic Pathways
 Various controls over enzymes allow cells to
conserve energy and resources by producing
only what they require
• Concentrations of reactants and products
• Feedback inhibition
Control of Metabolic Pathways
 Feedback inhibition
• Mechanism by which a change that results from
some activity decreases or stops the activity
Feedback Inhibition
Electron Transfers
 Electron transfer chains allow cells to harvest
energy in manageable increments
 Electron transfer chain
• An array of membrane-bound enzymes and other
molecules that accept and give up electrons in
sequence
Uncontrolled and Controlled
Energy Release
1 Energy input splits glucose
into carbon dioxide, electrons,
and hydrogen ions (H+).
2 Electrons lose energy as
they move through an electron
transfer chain.
carbon dioxide
glucose
+
oxygen
carbon dioxide glucose
+
water
oxygen H+
e–
spark
3 Energy released by electrons
is harnessed for cellular work.
e–
water
A Glucose and oxygen
react (burn) when exposed
to a spark. Energy is
released all at once as light
and heat when CO2 and
water form.
4 Electrons, hydrogen ions, and
oxygen combine to form water.
B The same overall reaction
occurs in small steps with an
electron transfer chain. Energy
is released in amounts that
cells can harness for cellular
work.
Stepped Art
Fig. 4-9, p. 68