Quaestio: How do organisms
obtain the energy stored in food?
Nunc Agenda:
List what foods you have eaten today
and the types of molecules that
compose them.
Energy
• Energy: the ability to do work.
• Can you think of examples?
– In what forms does energy exist?
– How do we use energy on earth?
A cell does 3 main kinds of work:
• Mechanical work: beating cilia, contraction of
muscle cells, movement of chromosomes
during reproduction
• Transport work: moving substances across
membranes
• Chemical work: running chemical reactions,
synthesis of polymers from monomers
The Law of Conservation of Energy
• The Law of Conservation of Energy: Energy can
neither be created nor destroyed; it can only change
forms.
– Remember, the sum of energy in the universe is constant.
• Examples of energy conversions:
– Photosynthesis
• (Light E Electrical E  Chemical E)
– Respiration
• (Chemical E  Kinetic E  Thermal E)
– Internal Combustion Engine
• (Chemical E  Thermal E  Kinetic E).
Energy for Living Things
• All living things need energy to carry out their life
processes.
• Nutrition: the life process in which organisms obtain
energy in food for metabolic processes.
• Energy must exist to run “cellular machinery.”
Examples of Energy Needs
• 1. Locomotion. (Muscle Contractions)
• 2. Building complex molecules from simple ones
(Synthesis).
• 3. Digestion.
• 4. Breathing, Talking, Thinking, Existing!
A Heterotroph’s nutrition must supply the organism with enough chemical energy to fuel its
life’s activities.
Heat
Chemical
energy
CO2
+
H2O
In fireflys, Energy in the form of ATP combines with an enzyme to run a chemical reaction to
produce flashes of lights
Ctenophores (Comb Jellies), like fireflies, have bioluminescence using the power of ATP.
Energy from Food
• Living things rely on the chemical energy stored in
their food to survive.
• Carbohydrates, lipids, and proteins all have chemical
energy and all can be broken down to yield energy
– known as cellular respiration
• Carbohydrates are the foods most commonly broken
down.
– Created during photosynthesis
Introducing the major players and
processes:
ADP
ATP
ATP and ADP
• Cells use chemical energy in the form of ATP
– The energy released during cellular respiration is
“stored” in the form of ADP and ATP.
• ADP: Adenosine diphosphate
– Has two phosphate groups.
• ATP: Adenosine triphosphate
– Has three phosphate groups
Behind the Names
• Adenosine is the combination of a molecule of
the nitrogenous base adenine with a molecule of
the sugar ribose.
– Adenine + Ribose = Adenosine
• Diphosphate = 2 phosphate groups attached to
adenosine.
• Triphosphate = 3 phosphate groups attached to
adenosine.
ATP: C10H16N5O13P3
: Nitrogenous Base
: 5-carbon sugar
Molecular Similarities
• ATP and ADP use the same subunits as the nucleic
acids:
– A nitrogenous base (adenine is present in DNA and RNA).
– A 5-carbon sugar (ribose is present in RNA only).
• Can you remember what DNA has?
– Phosphate groups
What makes ADP and ATP so important?
• ATP has more energy than ADP:
– due to a high-energy bond between the 2nd and 3rd
phosphate group
• When the third phosphate group is removed
from ATP, it forms ADP, and chemical energy is
released.
– ATP + H2O  ADP + P + Energy
Phosphorylation
• Phosphorylation: the transfer of energy when
a phosphate group is transferred among
molecules.
• Phosphorylation is a common way for
chemical energy to be transferred in living
cells.
– ATP loses a phosphate to the molecule that
becomes phosphyorylated.
ATP is recycled
• ATP is used continuously by a cell, but it can
be regenerated by adding a phosphate to ADP.
– It’s a renewable resource!
• If ATP could not be regenerated by the
phosphorylation of ADP, humans would
consume nearly their body weight in ATP each
day
AMP
• AMP stands for adenosine monophosphate. It has
only one phosphate group attached.
• AMP has lower energy than ADP (and ATP).
• ADP is rarely broken down into AMP for energy.
The Role of Glucose.
• Glucose (a simple sugar) is broken down to
supply the energy needed to add a phosphate
group to ADP to form ATP.
• One C6H12O6 molecule can be used to form 36
molecules of ATP.
More on Carbohydrates
• Glucose is not usually present in its simple form in
the foods we eat.
• We need to break complex carbohydrates into
glucose first.
• Review: Our digestive system breaks down complex
carbohydrates:
– Starch  Maltose  Glucose
• Can you remember what enzymes are involved and
where?
Question
• If ATP is directly used for energy, why do we
need glucose at all?
Answer:
Glucose contains a lot more energy than ATP, but
is actually a smaller molecule. Glucose is a good
way to store chemical energy, while ATP is more
appropriate for directly supplying immediate
energy for cellular reactions.
More on ATP vs. Glucose
• Glucose Chemical Formula
– C6H12O6
– Smaller Molecule with More Energy.
• ATP Chemical Formula
– C10H16N5O13P3
– Larger Molecule with Less Energy.
Glucose holds more
energy than ATP
Glucose
ATP
Glucose
• Like a gold bar
vs.
ATP
• Cash!
Can you explain the analogy?
Glucose is smaller but holds more energy, and needs to be broken down
or exchanged before you can purchase with it. A suitcase full of money
may be larger, like ATP, but can be used immediately.
Questions
• If ATP is used as the main source of energy in a
cell, then why does a cell only keep a small
amount of ATP present at any time?
– ATP is constantly being recycled from ADP
Ways to transfer energy in the cell
• Transfer phosphate groups
• Transfer electrons
• Transfer hydrogen
Oxidation-Reduction Reactions: the
transfer of electrons
• Oxidation: A chemical change in which an atom or
a molecule loses electrons.
– Example: When sodium combines with chlorine to
form sodium chloride (NaCl), sodium loses an electron
to become a sodium ion (Na+).
• Reduction: A chemical change in which an atom
or a molecule gains electrons.
– Example: Chlorine gains the electron from sodium,
becoming a chloride ion (Cl-).
Questions
• Why did sodium (Na) become Na+?
• Why did chlorine (Cl) become Cl-?
Answer: Sodium lost an electron and became a
positive ion. It now has more protons than
electrons. Sodium was oxidized.
Answer: Chlorine gained an electron and became
a negative ion. It now has more electrons than
protons. Chlorine was reduced.
Remember Oil Rig!
Oxidation
Is
Loss (of electrons)
Reduction
Is
Gain (of electrons)
Another way to remember:
LEO goes GER
• Lose
• Electrons
• Oxidation
• Gain
• Electrons
• Reduction
Oxidation-Reduction Reactions
• When one substance is oxidized, another must
be reduced.
• Redox Reaction: (short for ReductionOxidation Reaction): A reaction that involves
both oxidation and reduction.
Gaining and Losing Hydrogen
• Occasionally, rather than exchanging electrons,
molecules will exchange hydrogen atoms.
– Recall: a hydrogen atom consists of one proton and one
electron. It is the simplest element.
• The molecule that loses the hydrogen is oxidized
– called the oxidant.
• The molecule that gains the hydrogen is reduced
– called the reductant.
Hydrogen Ion =
H+
=
Proton
Hydrogen was Oxidized
Hydrogen
Redox Reactions, Cont’d
• Redox reactions involve a transfer of energy.
• The oxidant (the electron or hydrogen donor)
normally loses energy and the reductant (the
electron or hydrogen acceptor) gains energy.
Biochemical Pathway
• Cellular Respiration follows a biochemical pathway: a
sequence of chemical reactions that leads to a result.
• This pathway is fueled by redox reactions.
• *Remember – if a molecule loses a hydrogen
(oxidation), another molecule must accept that
hydrogen (reduction).
Hydrogen Acceptors
• NAD and FAD are two coenzymes that serve as
hydrogen and electron acceptors.
• NAD = nicotinamide adenine dinucleotide.
• FAD = flavin adenine dinucleotide.
• To be reduced:
• NAD + H  NADH (higher energy)
• FAD + 2H  FADH2 (higher energy)
Hydrogen Acceptors, Cont’d
• The extra energy (electrons) carried by NADH
and FADH2 can be used to make ATP from ADP.