•Energy – •Cells need energy the ability to do work or

•Energy – the ability to do work or
bring about a change
•Cells need energy to maintain their
•Cells need energy to carry out
reactions used to grow, develop,
and reproduce
Forms of energy:
•Kinetic energy – energy of motion
•Ex: you raise your arm
•Potential energy – stored energy;
capable of producing energy, but not
being used yet
•Ex: food we eat has potential energy
•Chemical energy – composed or
organic molecules such as carbohydrates
•Ex: food we eat, ATP
•First law of thermodynamics (the law
of conservation of energy) – energy
cannot be created or destroyed, but it
can be changed from one form to
•Energy flows; it does not cycle
•As materials change from one form
of energy to another, some energy is
given off as heat (a form of energy)
•Second law of thermodynamics –
energy cannot be changed from one
form to another without a loss of usable
•Heat given off through the
conversion of chemical energy to
kinetic energy is not a usable form of
•For this reason, living things are
dependent upon an outside source of
energy – the sun
Metabolic Pathways and Enzymes
• Cellular reactions are usually part of a metabolic
pathway, a series of linked reactions
• Many reactions have molecules in common
• Energy can be released in small amounts
rather than all at once
• Illustrated as follows:
E4 E5 E6
A → B → C → D → E →F → G
• Letters A-F are reactants or substrates, B-G are
the products in the various reactions, and E1-E6
are enzymes.
• Enzyme - a protein molecule that functions as an
organic catalyst to speed a chemical reaction.
• An enzyme brings together particular molecules
and causes them to react.
• The reactants in an enzymatic reaction are called
the substrates for that enzyme.
• For series of reactions below, A is substrate for E1
and B is product. B then becomes substrate for E2
and C is product. Continues to end of pathway.
E5 E6
A → B → C → D → E →F → G
• Energy of activation (Ea) - the energy that
must be added to cause molecules to react
with one another
• Enzyme lowers the amount of energy
required for reaction to occur
• Enzymes allow reactions to take place at
lower temperatures – otherwise,
reactions would not be able to occur at
normal body temperatures
Energy of activation (Ea)
When no enzyme is
present – more
energy required
When an enzyme is
added – less energy
Enzyme-Substrate Complexes
• Every reaction in a cell requires a specific
• Enzymes are named for their substrates:
Ribonucleic acid
• Active site – part of enzyme that attaches to
• Active site may undergo a slight change in
shape in order to accommodate the
• The enzyme and substrate form an enzymesubstrate complex during the reaction.
• The enzyme is not changed by the reaction
(active site returns to its original state), and it
is free to act again.
Enzymatic reaction
Substrate is broken down
into smaller products
Substrates are combined into
a larger product
Induced fit model
•Because the enzyme must undergo a slight change
in shape to fit with the substrate, this is known as
the induced fit model.
Factors Affecting Enzymatic Speed
Substrate concentration
Temperature and pH
Enzyme concentration
Enzyme inhibition
• Competitive inhibitors
• Non-competitive inhibitors
• Enzyme co-factors
•Substrate concentration:
•Enzyme activity increases as substrate
concentration increases because there are
more collisions between substrate
molecules and the enzyme.
•When active sites on enzymes are filled
almost continuously with substrate, rate of
activity cannot increase further.
• Temperature and pH:
• As the temperature rises, enzyme activity
increases because more collisions occur
between enzyme and substrate.
• If the temperature is too high, enzyme activity
levels out and then declines rapidly because the
enzyme is denatured.
• When enzyme is denatured, its shape changes
and it can no longer bind to substrate.
• Each enzyme has an optimal pH and
temperature at which the rate of reaction is
Rate of an enzymatic reaction as a function
of temperature and pH
• Enzyme Concentration:
• A cell regulates which enzymes are present or
active at any one time and the quantity of
enzyme present by turning on of off genes
• Another way to control enzyme activity is to
activate or deactivate the enzyme, such as
through phosphorylation (removal of
phosphate group).
• Enzyme Inhibition:
• Occurs when an active enzyme is prevented
from combining with its substrate.
• When the product of a metabolic pathway is
in abundance, it binds competitively with
the enzyme’s active site, a simple form of
feedback inhibition.
• Other metabolic pathways are regulated by
the end product binding to an allosteric site
(another area of enzyme).
• Poisons such as cyanide are often enzyme
inhibitors; penicillin is an enzyme inhibitor
for bacteria.
Feedback inhibition
When there is a sufficient amount of the end product, some of the
product binds to the allosteric site on the enzyme, the active site
changes shape, the reactant cannot bind, and the end product is no
longer produced. http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter8/animations.html
Competitive inhibitors:
•Have a similar shape to the substrate & fit into
the active site of the enzyme
•Don’t take part in the reaction
•Block active site so substrate can’t enter
Non-competitive inhibitors:
•Do not have the same shape as the substrate & do
not compete for the active site
•Bind at some other point on the enzyme molecule,
which still changes the shape of the active site so
enzyme-substrate complex cannot be formed.
• Enzyme Cofactors
• Presence of enzyme cofactors may be
necessary for some enzymes to carry out
their functions.
• Inorganic metal ions, such as copper, zinc,
or iron function as cofactors for certain
• Organic molecules, termed coenzymes, must
be present for other enzymes to function.
• Some coenzymes are vitamins; certain
vitamin deficiencies result in a lack of
certain enzymatic reactions.
The ATP cycle
ATP (adenosine triphosphate)
The energy currency of cells.
A nucleotide made of the following:
• Adenine
• Ribose (a sugar)
• Three phosphate groups
Constantly regenerated from ADP (adenosine
diphosphate) after energy is expended by the cell.
Pneumonic devices – ATP – a triple phosphate
- ADP – a double phosphate
Advantages of ATP:
1) It can be used in many types of
2) When ATP → ADP + P, energy released
is sufficient for cellular needs and little
energy is wasted.
3) ATP is coupled to endergonic reactions
(requires an input of energy) in such a
way that it minimizes energy loss.
Overview of Cellular Respiration
• Makes ATP molecules
• Releases energy in several reactions
• Glycolysis
• Transition reaction
• Citric acid cycle (Kreb’s cycle)
• Electron transport system
• An aerobic process that requires O2
•Cellular respiration takes the
potential chemical energy in the
bonds of glucose and transforms it
into the potential chemical energy in
the bonds of ATP.
•ATP molecules store usable chemical
energy to drive life processes through
coupled reactions.
•It is an oxidation-reduction reaction, or redox
reaction for short.
•Oxidation is the loss of electrons; hydrogen atoms
are removed from glucose.
•Reduction is the gain of electrons; oxygen atoms
gain electrons.
•Remember OIL RIG (oxidation is loss, reduction
is gain)
Enzymes involved:
• NAD+
• Nicotinamide adenine dinucleotide
• Accepts 2 electrons & 1 H+ to become
• Flavin adenine dinucleotide (sometimes
used instead of NAD+)
• Accepts 2 electrons & 2 H+ to become
The NAD+ cycle
Phases of Cellular Respiration
• Four phases:
• Glycolysis
• Transition reaction
• Citric acid cycle (Kreb’s cycle)
• Electron transport system
(If oxygen is not available, fermentation
occurs in the cytoplasm instead of
proceeding to cellular respiration.)
The four phases of complete glucose
Occurs in the cytoplasm (outside the
Glucose  2 pyruvate molecules.
Universally found in all organisms
Does not require oxygen.
Energy-Investment Steps
• Requires 2 ATP to start process and activate
• Glucose splits into two C3 molecules (PGAL)
• Each C3 molecule undergoes the same series of
Energy-Harvesting Steps
• PGAL is oxidized by the removal of electrons
by NAD+; phosphate group is attached to each
PGAL as well (phosphorylation)
• Removal of phosphate from 2 PGAP by 2 ADP
produces 2 ATP, and 2 PGA molecules
•Removal of water results in 2 PEP molecules
•Removal of phosphate from 2 PEP by 2 ADP
produces 2 ATP molecules and 2 pyruvate molecules
Glycolysis summary
• Inputs:
2 NAD+
4 ADP + 2 P
• Outputs:
2 pyruvate
2 ATP (net gain)
•When oxygen is available, pyruvate enters the
mitochondria, where it is further broken down
•If oxygen is not available, fermentation occurs
Inside the Mitochondria
• Structure of mitochondia:
• Has a double membrane, with an
intermembrane space between the two
• Cristae are folds of inner membrane
• The matrix, the innermost compartment,
which is filled with a gel-like fluid.
• The transition reaction and citric acid cycle
occur in the matrix; the electron transport
system is located in the cristae.
Mitochondrion structure and function
Transition Reaction
• Is the transition between glycolysis and the citric
acid cycle.
• Pyruvate (made during glycolysis) is converted to
acetyl CoA, and CO2 is released
• NAD+ is converted to NADH + H+
• The transition reaction occurs twice per glucose
Transition reaction inputs and outputs
per glucose molecule
• Inputs:
• 2 pyruvate
• 2 NAD+
• Outputs:
• 2 acetyl groups
• 2 CO2
• 2 NADH
Citric Acid Cycle (aka Kreb’s Cycle)
• Occurs in the matrix of the mitochondria.
• C2 acetyl group (produced during transition
reaction) joins a C4 molecule, and C6 citrate results.
• Each acetyl group gives off 2 CO2 molecules.
• NAD+ accepts electrons in three sites and FAD
accepts electrons once.
• Substrate-level phosphorylation results in a gain of
one ATP per every turn of the cycle; it turns twice
per glucose, so a net of 2 ATP are produced.
• The citric acid cycle produces four CO2 per
molecule of glucose.
Citric acid cycle
Citric acid cycle inputs and outputs per
glucose molecule
• Inputs:
• 2 acetyl groups
• 6 NAD+
• 2 FAD
• 2 ADP + 2 P
• Outputs:
• 4 CO2
• 6 NADH
• 2 FADH2
• 2 ATP
Electron Transport System (ETS)
Located in the cristae of mitochondria
Series of protein carriers pass electrons
from one to the other.
NADH and FADH2 carry electrons
picked up during glycolysis, transition
reaction, & citric acid cycle
NADH and FADH2 enter the ETS.
• As a pair of electrons is passed from carrier to
carrier, energy is released and is used to form
ATP molecules by oxidative phosphorylation
(term used to describe production of ATP as a
result of energy released by ETS).
• Oxygen receives electrons at the end of the
ETS, which combines with hydrogen to form
½ O2 + 2 e- + 2 H+ → H2O
Overview of the electron transport
Organization of Cristae
• The ETS consists of 3 protein complexes and 2
mobile carriers.
– Mobile carriers transport electrons between the
– Energy is released by electrons as they move
down carriers
– H+ are pumped from the matrix into the
intermembrane space of the mitochondrion.
• Produces a very strong electrochemical
gradient - few H+ in the matrix and many H+
in the intermembrane space.
• The cristae also contain an ATP synthase
– Hydrogen ions flow through ATP synthase
complex down their gradient from the
intermembrane space into the matrix.
– Flow of 3 H+ through ATP synthase complex
causes the ATP synthase to synthesize ATP
from ADP + P.
– This process of making ATP is called
chemiosmosis, because ATP production is tied
to an electrochemical gradient (H+ gradient)
– Once formed, ATP molecules are transported
out of the mitochondrial matrix.
Cellular respiration song
Energy Yield from Glucose Metabolism
• Per glucose molecule:
– 10 NADH take electrons to the ETS  3
ATP from each
– 2 FADH2 take electrons to the ETS  2 ATP
from each
• Electrons carried by NADH produced during
glycolysis are shuttled to the electron transport
chain by an organic molecule (mechanism of
delivery may vary # of ATP produced by ETS).
Accounting of energy yield per glucose
molecule breakdown
• Occurs when oxygen is not available.
• During fermentation, the pyruvate formed by
glycolysis is reduced to alcohol and CO2, or one
of several organic acids, such as lactate.
• Fermentation uses NADH and regenerates
NAD+, which are free to pick up more electrons
during early steps of glycolysis; this keeps
glycolysis going.
• Occurs in anaerobic bacteria, fungus, & human
muscle cells.
produces 2
•Then pyruvate is
reduced by NADH
into lactate or
alcohol & CO2.
Advantages and Disadvantages of
• Fermentation can provide a rapid burst of ATP
in muscle cells, even when oxygen is in
limited supply.
• For bacteria, glycolysis and fermentation is
the main energy source
• Lactate, however, is toxic to cells.
• Initially, blood carries away lactate as it forms;
eventually lactate builds up, lowering cell pH,
and causing muscles to fatigue.
• Oxygen debt occurs, and the liver must
reconvert lactate to pyruvate.
Efficiency of Fermentation
• Two ATP produced during fermentation are
equivalent to 14.6 kcal; complete oxidation of
glucose to CO2 and H2O represents a yield of
686 kcal per molecule of glucose.
• Thus, fermentation is only 2.1% efficient
compared to cellular respiration (which is 39%
• (14.6/686) x 100 = 2.1%
Glycolysis and Fermentation inputs
and outputs per glucose molecule
• Inputs (into
• Glucose
• 2 ATP
• 4 ADP + 2 P
• Outputs:
• 2 lactate
(fermentation) or
• 2 alcohol & 2 CO2
• 2 ADP (glycolysis)
• 2 ATP (net gain)