Cellular Metabolism
Including Cellular Respiration and
Photosynthesis
Energy
• Energy the capacity to do work.
• Comes in different forms: Chemical, thermal,
light, and mechanical
• Two types of energy:
– Kinetic Energy: is the energy of moving objects; it
is energy in use
– Potential Energy: is stored energy; energy that has
potential to do the work.
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• A molecule stores potential energy until it is
released in the kinetic form of chemical or
thermal (heat) energy.
• Free energy: the sum of potential and kinetic
energy
– The amount of energy that could be used to
power other chemical reactions.
Oxidation-Reduction Reaction
• Transfer energy between molecules in the form of
electrons.
• The molecules that loses an electron is oxidized, while the
molecule that gains an electron is reduced.
• Example: NADH is a common energy carrier within cells. In
the equation below, through a chemical reaction with
hydrogen (2H), NAD+ is oxidized to NADH, while hydrogen
is reduced to hydrogen ions (H+). In the process, the
reverse direction, with NADH being reduced to NAD+ and
hydrogen ion becoming oxidized to hydrogen
• NAD+ +2H NADH + H+
Endergonic Reactions
• Store energy within a molecule because the
reactants have less free energy than the products.
• These reactions require energy input
• Example: The production of glucose from carbon
dioxide (CO2) and water (H2O) is an endergonic
reaction because it requires energy input. Because
energy is expended in the process, this reaction
cannot occur in the reserve direction.
• 6CO2 + 6H2O + energy  C6H12O6 +6O2
Exergonic Reactions
• Release energy, leaving the reactants with
more free energy than the product
• Example: The breakdown of glucose into
carbon dioxide (CO2) and water (H20) is
exergonic reaction because it results in the
release of energy. Because this reaction
releases energy, it cannot occur in the reverse
directions
C6H12O6 (glucose) + 6 O2  6CO2 +6H2O +energy
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• Cells often use the energy released from
exergonic reactions to power endergonic
reactions; these are called coupled reactions.
Endergonic Reactions
Exergonic Reactions
ATP
• Adenosine triphosphate, is referred to as the “the
energy of the cell” (cell energy) because it powers most
of the reactions that take place in a cell.
• ATP consist of
– Ribose, an adenine (a type of nucleotide)
– Chain of three phosphate groups
• The bonds that link the second and third phosphate
group can be broken down to produce ADP (adenosine
diphosphate), a free phosphate group (P), and a
substantial amount of energy used for endergonic
reactions.
ATP  ADP + P + energy
Example: The human body uses, on average, one kilogram
of ATP every hour
Enzymes
• Exergonic reactions require a small initial input
of energy, called activation energy, before the
reaction can proceed.
• Enzyme are proteins that lower the activation
energy of a reaction.
– Active Site of an enzyme binds with the reactants
(substrate) and either changes them in some way
or simply brings them in closer proximity to one
another.
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• Chemical reactions do not use up or change
the enzyme.
• Once the reaction has taken place, the
product is released, and the enzyme is free to
catalyze other reactions.
Enzyme Inhibitors
• The presence of other molecules may inhibit an
enzyme, or prevent it from functioning.
Inhibitions can occur in two ways:
– Competitive inhibition: occurs when the inhibitor
binds with the active site of an enzyme. With the
active site already occupied, the enzyme cannot
bind with the substrate.
– Noncompetitive inhibition: occurs when the
inhibitor binds with an allosteric site (any site other
than the active site) and changes the shape of the
enzyme so that it no longer bonds with the
substrate.
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• Example: Inhibitors are often used as drugs, in
many cases to prevent detrimental reactions
in an organism. Aspirin, for instance, inhibits
the enzymes that causes pain and
inflammation. However, inhibitors can also be
poisonous. Cyanide is a lethal toxin because it
competitively inhibits cytochrome coxidase,
an enzyme involved with cellular respiration.
Cellular Respiration
• Organisms must obtain their own energy from
the environment, usually in the form of food
and solar radiation.
• The process of converting energy into a form
that can be used by cells is called cellular
metabolism.
• Two methods of cellular metabolisms:
– Cellular respiration and Photosynthesis
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• Cellular respiration converts the energy found in
food molecules, especially glucose, to the more
useable form of ATP.
• 36 ATP can be produced from a single molecule of
Glucose
Cellular Respiration Equation
C6H12O6 + 6O2 +ADP + P  6CO2 + 6H20 + ATP
• Energy transfer is not efficient for organisms
Cellular respiration only 40% energy in glucose is
converted to ATP.
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• Cellular respiration occurs in four stages:
1- Glycolysis
2- Oxidation of pyruvate
3- Kreb Cycle
4- Electron Transport Chain
Glycolysis
• Takes place in the cytoplasm (cytosol)
• Converts glucose to two molecules of
pyruvate, the compound from which energy
will be extracted in the Kreb Cycle.
• Produces 2 ATP and 2 NADH (energy carrying
molecule). Water is also released in this
reaction.
Glycolysis Diagram
Oxidation of Pyruvate
• The two molecules of pyruvate are oxidized and
transformed into molecules of acetyl CoA.
• Takes place in mitochondria
• Also produces one molecule of NADH
• Releases CO2
Kreb Cycle
• Takes place in matrix of the mitochondria
• Processes each acetyl CoA to produce 3
NADH, 1 FADH2, and 1 ATP for a total of 6
NADH, 2 FADH2, 2 ATP per glucose.
• Carbon dioxide is also released in this
reaction.
Diagram of Kreb Cycle
Mnemonic Device
Kreb Cycle (order)
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Oxidative Phosphorylation
• After the Kreb Cycle, large amount of ATP
produced from NADH (produces 3 ATP) and
FADH2 (produces 2 ATP)
• Requires the presence of oxygen in the
mitochondria
Electron Transport Chain
• Is series of molecules embedded in the inner
membrane of the mitochondria
• The 10 NADH and 2 FADH2 (Produced from
previous stages) power the production of the final
32 ATP
• Chemiosmosis: coupling of the movement of
electrons down the electron transport chain with
the formation of ATP.
– Coupled Reaction: A reaction that uses the product of
one reaction as part of another reaction.
Steps of the Electron Transport Chain
• 1- Electron carriers NADH and FADH2 shuttle
electrons to the inner mitochondrial membrane.
• 2- NADH and FADH2 donate their electrons to the
first in a series of membrane proteins. Each
protein uses the energy in the electron to pump
H+ into the intermembrane space of the
mitochondrion before passing the electron the
next carrier. The final electron receptor is O2,
which combines with two protons, H+ to form
water
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• 3- By pumping H+ into the intermembrane space,
the electron transport chain sets up a high
concentration gradient. H+ flows down gradient
through the ATP synthase, a membrane protein
that catalyzes the production of ATP from ADP.
 Chemiosmosis
Oxidative
phosphorylation
Summary of Cellular Respiration
Stage
Location
Reaction
Glycolysis
Cytosol
Converts 1 molecule of glucose to 2 molecules of
pyruvate
2 ATP and 2 NADH molecules are produced and water
is released
Oxidation of
pyruvate
Mitochondria
Converts 2 molecules of pyruvate to 2 molecules of
acetyl CoA
2 NADH molecules are produced and carbon dioxide is
released
Kreb Cycle
Mitochondrial
Matrix
Converts 2 molecules of acetyl CoA to 6 molecules of
NADH, 2 molecules of FADH2, and molecules of ATP.
Carbon dioxide is released
Electron
Transport Chain
Mitochnondria
10 NADH molecules and 2 FADH2 are converted to 32
ATP molecules
Oxygen is consumed and water is produced
Fermentation
• Eukaryotic cells can produce ATP through
fermentation.
• Fermentation is much less efficient than the
four stages of cellular respiration, but allows
ATP to produce when oxygen is not available
• Begins with glycolysis producing only 2 ATP.
• All other stages cannot be completed without
oxygen.
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• Two types of fermentation:
– 1- Alcoholic Fermentation: Pyruvic acid is converted to
ethanol.
• Used by fungi and some plants
• Used to make beer, wine, and bread
– 2- Lactic Acid Fermentation: Pyruvic acid is converted to
lactate.
• Lactic acid fermentation is used by animals and bacteria
• Muscle Cramps (occurs when over exercise your muscles)
• Sour Cream and buttermilk
Example: The sour taste of sourdough comes from the
lactic acid produced by the fermentation of bacteria
Photosynthesis
• Plants, some protists, and bacteria, create
food molecules (sugars) from carbon dioxide
and solar energy through the process of
photosynthesis.
• Equation for photosynthesis:
6CO2 + 6H2O  C6H12O6 + 6O2
Light
Players of Photosynthesis
• Organelle: Chloroplast
• Chloroplast is divded into inner and outer portion
of the organelle
– Stroma: inner fluid portion
– Thylakoid: Green disk membrane system (first stage of
photosynthesis occurs)
– Grana: Flatten channels and disk (thylakoid) arranged
in stacks
Thylakoid
Grana
Stroma
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• Autotroph: Organisms that is self-nourishing.
• Heterotroph: organisms that must consume food.
• Bundle Sheath cells: Cells that are tightly wrapped
around the veins of a leaf. Site of the Calvin Cycle in
C4 plants
• Mesophyll: interior leaf
• Mesophyll Cells: contains many chloroplast and
host the majority of photosynthesis
• Photolysis: process by which water is broken up by
an enzyme into hydrogen ions and oxygen atoms.
Occurs in the light dependent reaction
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• Photophosphorylation: process by which ATP is
produced during light-dependent reaction of
photosynthesis
• Photorespiration: process by which oxygen
competes with carbon dioxide and attaches to
RuBP. Plants that experience this has a lower
capacity of growth.
• Photosystem: cluster of light-trapping pigments
involved in photosynthesis. Photosystem I and
Photosystem II are two most important.
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• Pigment: a molecule that absorbs light of a
particular wavelength. Pigments include
carotenoids (orange), phycobilins, and chlorophyll
• Rubisco: an enzyme that catalyzes the first step of
the Calvin Cycle in C3 plants
• Stomata: Structure through which CO2 enters a
plant and water vapor and O2 leave
• Transpiration: natural process by which plants
lose H2O via evaporation of leaves
Light Dependent Reactions
• Convert solar energy into ATP and NADPH, the
reduced form of the electron receptor,
NADP+.
• During these reactions, water is split, leaving
oxygen as a waste product.
– Why is oxygen considered to be a waste product?
• These reactions take place in photosystem in
the choloroplast.
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• Photosystems comprise cluster of molecules
composed of light-absorbing pigments and a
reaction center, which includes a primary
electron acceptor and two chlorophyll a
pigment molecules.
• There are two photosystems work
sequentially, with light first being absorbed by
photosytem II and later by photosystem I
Steps to light dependent reactions
• 1- Photosystem II absorbs solar energy in the form
of light.
• 2- The solar energy excites electrons in the reaction
center of photosystem II, which the n enter an
electron transport chain. These electrons originate
from the splitting of water, which produces free
electrons and O2
• 3- As electrons pass down the electron transport
chain, protons are pumped into the thylakoid
membrane space of the chloroplast. Protons diffuse
out of the thylakoid membrane space through an
ATP synthase, creating ATP.
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ATP and NADPH
made in the light
dependent reaction
• 4- Photosystem I accepts electrons from the
electron transport chain and uses light energy
to excite the electrons further.
Cellular Respiration and Light
Dependent Reaction
• Cellular respiration and light dependent
reactions of photosynthesis use similar
processes to produce ATP.
• Scientist believe that the electron transport
chain used in cellular respiration may have
evolved from the transport system used in
photosynthesis.
Calvin Cycle
• Uses ATP and NADPH from the light-dependent
reaction to convert CO2 into sugar that the plant
can use.
• CO2 is obtained through the stomata.
• Carbon fixation: incorporates the CO2 into organic
molecules
• The incorporation is completed by the energy rich
enzyme rubisco (ribulose biphosphate carboxylase
(RuBP)), a protein made during light-dependent
reaction of photosynthesis . Abundant in leaves
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• CO2 is split into:
– 3 carbon molecule PGA (3-phosphoglycerate)
– Converts PGA into 3-carbon sugar molecule
glyceraldehyde 3-phosphate
– Used to make glucose and sucrose
• The production of a single 3-carbon sugar
molecule require 3 CO2, 9 ATP, 6 NADPH
Calvin
Cycle
Diagram
Photorespiration
• When the enzyme rubisco incorporates oxygen,
rather than CO2, into organic molecules, plants
create energy through the process of
photorespiration.
• Occurs most in arid regions where plants must
close their stomata to prevent water loss to the
air.
• The results in a buildup of oxygen levels in the leaf,
which makes rubisco more likely to bind with the
oxygen.
• Detrimental to plants because it consumes more
ATP to produce each 3-carbon sugar.
• Three different categories this type of method: C3
pathway, CAM pathway, and C4 pathway
C3 Plants
• Found in areas with moderate temperature
and above amount of rainfall
• Exacerbated in Hot arid climates, where the
rate of photosrespiration increases as the
temperature goes up.
• Consequently C3 plants are rarely found in
these climates
• Located in the temperate zones
• Examples: Wheat, barley, and sugar beets
C4 Plants
• Use the enzyme PEP carboxylase to fix CO2 in the
mesophyll cells of their chloroplast.
• The fixed CO2 is then shuttled to specialized
structures known as bundle-sheath cells, where it is
released and incorporated into the Calvin Cycle.
• Energetically expensive, but limits photorespiration
by allowing high concentration CO2 to build up in
the bundle-sheath cells
• Examples: Corn and sugar cane are common in
warm environments
CAM Plants
(Crassulacean acid metabolism)
• Plants reduce photorespiration and conserve
water by opening their stomata only at night.
• CO2 enters through the stomata and is fixed into
organic acids, which are then stored in the cell’s
vacuole .
• During the day, the acids break down to yield
high levels of CO2 for use in the Calvin cycle
• Common in dry environments
• Examples: Pineapples and cacti