PART 3: Cellular Energetics
1. Bioenergetics: the study of how cells accomplish tasks necessary for life; the
study of how cells release energy that is stored in the chemical bonds of energy
rich molecules (ex. glucose, starch, fats) using chemical reactions to change the
type of energy present.
2. Enzymes: Proteins that act as organic catalysts (speed up the rate of a reaction
without altering the reactant(s) or product(s) in any way and without altering
themselves.
 Energy provided by these enzymes is used to catalyse the reactions
necessary in bioenergetics.
 Enzymes are most often named by replacing the suffix of the substrate or
product with –ase.
3. Chemical reaction: to review from Vocab #9 in Part 1, it is the process of
making or taking apart chemical compounds. Reactions require activation
energy; there are two types of reactions – exergonic and endergonic.
 Activation energy (Ea): the energy required to break down chemical bonds
in order to kick start a reaction.
*Enzymes lower the activation energy of reactions, allowing them to
happen more quickly.
 Exergonic reaction: reaction where the products have less energy than the
reactants, meaning that the reaction gave off energy (ex., oxidizing of food
in the mitochondria releases energy that can be used).
 Endergonic reaction: reaction where the products have more energy than
the reactants, meaning that the reaction absorbed (required) energy (ex.,
when plants use carbon dioxide and water to form energy-rich sugars).
Endergonic
Exergonic
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4. Enzyme specificity: the fact that all enzymes are specific to catalyse only one
kind of reaction. This is because specific substrates fit into an active site with
induced fit to form the enzyme-substrate complex. Sometimes, coenzymes and
cofactors contribute to the uniqueness of the enzyme.
 Substrates: the reactants needed to start the reaction.
 Active site: region of the enzyme where the substrate(s) fit like a puzzle
for the reaction to occur.
 Induced fit: the extremely slight changes of shape of the active site to
better fit the substrate(s).
 Enzyme-substrate complex: the complex of an enzyme joined to the
substrate(s) while the reaction is occurring.
 Coenzymes: compounds that accept electrons and pass them along to
other substrates to aid in reaction (ex., vitamins, NAD+, NADP+)
 Cofactors: inorganic elements that help catalyse reactions (ex., Fe+2)
5. Reaction factors: Factors that influence the role of a reaction:
 Temperature: the higher the temperature, the faster the rate of reactions,
until about 42◦ or higher is reached. It is at this point that enzymes
denature (break down). This is beneficial for enzymes in humans, as our
body temperature of 37◦ is high enough to have fast reactions without
denaturing.
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 pH level: different enzymes perform better at specific pH levels. Most
require it around pH 7; however, others prefer it much lower (ex., pH 2 is
ideal for digestive enzymes in the stomach).
 Amounts of enzymes and substrates: if there are more substrates than
enzymes, it will take longer to complete all the reactions. Vice versa, if
there are more enzymes than substrates, it will take much less time to
complete all the reactions.
6. Enzyme regulation: how cells control enzymatic activity by regulating the
conditions that influence the shape of the enzyme. The cells may try to influence
reaction factors (Vocab #5), or the regulation may relate to allosteric sites and/or
feedback inhibition:
o Allosteric sites: a region of an enzyme other than the active site to which
a substance can bind. The two types of allosteric regulators, activators
and inhibitors, bind to these sites to alter the shape of the enzyme.
 Allosteric activator: a substance that binds to the allosteric site
of an enzyme that alters the shape to make it active.
 Allosteric inhibitor: a substance that binds to the allosteric site of
an enzyme that alters the shape to make it inactive.
o Feedback inhibition: when a cell wants to slow down or stop an
enzymatic reaction, it sends inhibitors to render the enzyme inactive.
Two types of these inhibitors are:
 Competitive inhibitors: an inhibitor that binds to the active site,
blocking it from the substrates.
 Non-competitive inhibitors: inhibitors that bind to an enzyme
anywhere but the active site that alters the shape of the enzyme,
making it inactive.
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7. Laws of Thermodynamics: the two fundamental principles of energy.
 First Law of Thermodynamics: the sum of energy in the universe is a
constant; energy cannot be created or destroyed.
 Second Law of Thermodynamics: the energy of the universe tends towards
entropy (disorder) as constant reactions that change energy form
disorganize energy flow.
8. Adenosine triphosphate (ATP): the fuel of the cell; molecule that releases
energy when one phosphate bond is broken (exergonic), forming ADP; when a
phosphate is re-added, energy is absorbed and stored for later use (endergonic).
It is produced from photosynthesis and cellular respiration.
 ATP synthase: enzyme that catalyzes the reaction that forms ATP.
ATP
ADP + P + energy
9. Photosynthesis: the transformation of solar energy into chemical energy in
plants; plants use carbon dioxide, water and solar energy to create glucose (with
oxygen as a by-product) – more detailed in Part 4 of Guide.
6CO2 + 6H2O + sunlight (energy)
C6H12O6 + 6O2
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10. Cellular respiration: the production of ATP through the breakdown of
nutrients. There are two types of cellular respiration: aerobic and anaerobic.
C6H12O6 + 6O2
6CO2 + 6H2O + ATP (energy)
*Notice that it’s backwards of photosynthesis.
o Aerobic respiration: cellular respiration in the presence of oxygen. There
are four stages: glycolysis, formation of acetyl CoA, the Krebs cycle, and
oxidative phosphorylation.
 Glycolysis: the splitting (-lysis) of glucose (glyco-) molecules; a
series of reactions that breaks the 6-carbon glucose molecules into
two 3-carbon pyruvate molecules (pyruvic acid). Throughout the
process, 2 NADH’s are formed (coenzymes carrying energy), 2 ATP’s
are invested and 4 ATP’s created, with a net of 2 ATP’s. This all
occurs in the cytoplasm near the mitochondria
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 Formation of acetyl CoA: each pyruvate is converted to acetyl CoA
by combining with coenzyme A, with by-products of CO2 and 2
NADH. This occurs in the mitochondria.
2 Pyruvic Acid + 2 Coenzyme A + 2NAO+
2 Acetyl CoA + 2CO2 + 2 NADH
 The Krebs Cycle: also known as citric acid cycle; the process where
acetyl CoA is broken down so that energy carbon becomes carbon
dioxide (CO2) and 2 ATP, 6 NAOH and 2 FADH2 are produced.
 Oxaloacetate: a 4-carbon molecule that combines with
pyruvate to form citric acid (6-carbon molecule) at the
beginning of the Krebs Cycle. It is cycled so that we end with
more oxaloacetate in order to repeat the cycle.
 Oxidative phosphorylation: the process of ATP production using
electron carrying coenzymes (FADH2 and NADH) that were charged
in the previous steps of aerobic cellular respiration; this takes place
on the membrane of the cristae in an electron transport chain.
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 Electron transport chain: a series of proteins imbedded in the
inner membrane of the mitochondria that transport electrons
to power chemiosmosis to produce ATP.
o Anaerobic respiration: cellular respiration that does not require oxygen
to produce ATP; it takes place outside of the mitochondria using pyruvate
from glycolysis when oxygen isn’t present. The process post glycolysis is
called fermentation.
 Fermentation: the conversion of pyruvic acid into carbon dioxide
with lactic acid or ethyl alcohol (ethanol). This is why when we
exercise we feel a strain in our muscles; as there is a decline in
oxygen (it is used much faster by demanding muscles), our cells
switch over to anaerobic respiration. Some organisms function this
way (ex., yeast and certain bacteria make ethanol and CO2, other
bacteria make lactic acid).
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