Download Metabolism/Energy

Energy Transfer
1st Law of Thermodynamics The energy of the universe is constant. Energy can be transformed &
transferred, but it cannot be created or destroyed.
2nd Law of Thermodynamics Every energy transfer or transformation makes the universe more
disordered. In other words, every energy transfer or transformation increases the entropy of the
universe
Kinetic energy is the energy of motion. Moving objects perform work by imparting motion to other
matter. Heat is kinetic energy that results from the random movement of molecules. If an object is
not moving, it still has the capacity to do work. This is called potential energy. Chemical energy is a
form of potential energy. It is stored in molecules as a result of the arrangement of atoms in those
molecules. It is released as the molecules are broken down. In most energy transformations, ordered
forms of energy are at least partly converted to heat. Free energy is the portion of a systems energy
that can perform work when temperature is uniform throughout the system; it is represented by G.
Based on free energy
changes, reactions can be
classified as exergonic
(energy outward) or
endergonic (energy
inward). An exergonic
reaction proceeds with a
net release of free
energy. They occur
spontaneously. An
endergonic reaction is one that absorbs free energy from its surrounds; the energy is
stored in the product; they are nonspontaneous.
Energy coupling is the use of an exergonic process to drive an endergonic process. The
hydrolysis of ATP is exothermic; it is coupled with an endothermic reaction to provide:
 Transport work (as in the Na/K pump)
 Mechanical work (as in muscle contraction)
 Chemical work (as in energy requirements to synthesize macromolecules such as
proteins)
ATP is used like an “on-off” switch to
control chemical reactions and to send
messages. ATP + H2O  ADP + Pi
When ATP is hydrolyzed in a test tube, the release of free energy
heats up the surrounding water. In the cell, this would be inefficient and dangerous; with
the help of enzymes the cell is able to couple
the energy of ATP hydrolysis directly to
endergonic processes by transferring a
phosphate group from ATP to some other
molecule. The receipient of the phosphate
group is said to be phosphorylated. Nearly all
cellular work depends on ATPs energizing of
other molecules by transferring phosphate
groups. For instance, when ATP powers the
movement of muscles, it transfers phosphate to
contractile proteins. ATP must constantly be
regenerated in a cell. A working muscle cell
regenerates its entire pool of ATP about once
every minute.
Enzymes
Even a spontaneous chemical reaction may occur
so slowly that it is imperceptible. A catalyst is a chemical agent that changes the rate of a
reaction without being consumed in the reaction. Enzymes are biological catalysts, most
enzymes are proteins.
Every chemical reaction involves bond breaking and bonds forming. The initial investment of
energy for starting a reaction (the energy required for breaking bonds in the reactant
molecules) is called the activation energy. You can think of the activation energy as the
amount of energy needed to push the reactants over an energy barrier or hill, so that the
“downhill” part of the reaction can begin.
Characteristics of Enzymes
 Most are proteins
 Substrate specific
 Have an optimum T and
pH
 Have an active site
 Can exist in active &
inactive form
 Can be denatured
 May require a cofactor
 Subject to mutation
Cellular Respiration
Cellular respiration and
photosynthesis are both examples of oxidation reduction
reactions. Oxidation is a loss of electrons; reduction is a gain
of electrons. Draw arrows to show which is which-
C6H12O6 + 6O2  6CO2 + 6H2O + ATP
What is oxidized? What is reduced?
An overview of the relationship between
cellular respiration and photosynthesisCellular Respiration is a catabolic process. Molecules are
broken down and energy is released. Cellular respiration
is the most prevalent and efficient catabolic pathway. It
requires oxygen and is sometimes called aerobic
respiration. Fermentation is the partial breakdown of
sugars that occurs without the use of oxygen.
Cellular respiration has 3 stages1. Glycolysis (breaks down glucose into two
molecules of pyruvate)
2. Citric Acid Cycle aka Krebbs Cycle- (completes
the breakdown of glucose)
3. Oxidative Phosphorylation (accounts for most
of the ATP synthesis)
NOTE- you will see these two terms used to
describe ATP synthesis
Substrate level phosphorylation- glycolysis
and the Krebs cycle decompose glucose and
other organic fuels. The energy derived is
used to phosphorylate a small amount of
ADP.
Oxidative phosphorylation- occurs at the
inner membrane of the mitochondria.
Electrons that were picked up by NAD+
(and FADH2) are passed to oxygen. The
energy derived is used to generate the
majority of the ATP produced in cellular
respiration.
GLYCOLYSIS
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Means “splitting of sugar”
Glucose is split into two
molecules of pyruvate (ionized
form of pyruvic acid)
Occurs in the cytosol
Occurs with or without oxygen
Consists of 10 steps, each
catalyzed by a specific enzyme;
the first 5 steps require energy,
the last 5 steps are the energy
“pay-off”
Energy yield from glycolysis is
2 ATP and 2 NADH
Glycolysis releases less than 25%
of the chemical energy stored
in glucose; most of the energy
remains in pyruvate
IF O2 IS PRESENT IN THE CELL, PYRUVATE ENTERS
THE MITOCHONDRIA.
 Pyruvate is converted into a compound called
acetyl coenzyme A (acetyl CoA)
 This step is the junction between glycolysis and
the Krebs cycle
 One molecule of NADH results from this step
and a molecule of CO2 is given off
KREBS CYCLE (CITRIC ACID CYCLE)
 Occurs in the matrix of the mitochondria
 Has eight steps, each catalyzed by a specific
enzyme
 Most of the energy harvested in the Krebs cycle is conserved in NADH (3 molecules)
 One step transfers energy to FAD, resulting in FADH2 (1 molecule)
 Only one molecule of ATP is produced for each “turn” of the Krebs cycle
 Two molecules of CO2 are produced with each “turn”
So far, only 4 molecules of ATP have been generated: 2 from glycolysis and 1 from each molecule of
acetyl CoA that entered the Krebs cycle. The majority of the ATP that results from cellular respiration
occurs during oxidative phosphorylation which takes place on the inner mitochondrial membrane . At
this point, most of the energy “harvested” during substrate level phosphorylation is in NADH and
FADH2.
The electron transport chain is a collection of molecules embedded in the inner membrane of the
mitochondrion. The folding of the inner membrane to form cristae increases the surface area,
providing space for thousands of copies of the electron transport chain in each mitochondrion. Most
components of the electron transport chain are proteins. Other molecules associated with the proteins
are oxidized and reduced as they accept and donate
electrons. The electrons that travel along the electron
transport chain come from NADH and FADH2.
This figure shows the sequence of electron carriers in the
electron transport chain. Notice the molecules labeled
“cyt”. These are proteins called cytochromes. They have
a heme group and are very similar to hemoglobin.
The electrons released by NADH and FADH2 are passed
along, releasing energy as they go. Ultimately they are
“accepted” by oxygen.
The electron transport chain produces no ATP directly.
The energy derived from the “fall” of electrons is linked
to a mechanism called
chemiosmosis.
All along the inner membrane of the mitochondria are
protein complexes called ATP synthase. This is the
enzyme that actually catalyzed the formation of ATP from ADP and inorganic phosphate. Hydrogen
ions “fuel” ATP synthase. For ATP synthase to function, there has to be a concentration gradient of
H+. H+ will only flow down their concentration gradient if the
concentration is greater in the intermembrane space than it is in
the matrix.
How is the H+ concentration gradient maintained? It is
maintained by the electron transport chain. The chain uses the
energy released as electrons move along, to pump H+ into the
intermembrane space. This “coupling” of H+ flow and ATP
synthesis is called chemiosmosis.
The synthesis of ATP that results from the electron transport
chain and chemiosmosis is referred to as oxidative
phosphorylation. Oxidative phosphorylation yields a much
greater amount of ATP- either 26 or 28 molecules of ATP for
each molecule of glucose.
Oxygen serves as the final electron acceptor in oxidative
phosphorylation (oxygen is extremely electronegative). Food
can be oxidized without oxygen. NAD+ serves as the oxidizing
agent in glycolysis; if oxygen is not available, pyruvate cannot
enter the mitochondrion, and undergoes fermentation. This is a
way for NADH to “unload” its H and be recycled.
**Note- the number of ATP molecules produced by oxidative phosphorylation may actually be less.,
possibly between 26-28 molecules. Regardless of the actual number, most of the ATP is produced in
oxidative phosphorylation.
In plants, certain fungi and bacteria, ethanol and carbon dioxide are by-products of fermentation.
Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce. The lactate that
accumulates as a waste product may cause muscle fatigue and pain, but lactate is gradually carried away
by the blood to the liver. Lactate is converted back to pyruvate by liver cells.
Glycolysis is common to fermentation and respiration. The
end product of glycolysis, pyruvate, represents a fork in the
catabolic pathways of glucose oxidation. In a cell capable of
both respiration and fermentation, pyruvate is committed to
one of those two pathways, usually depending on whether or
not oxygen is present.
Glycolysis can
accept a wide
range of
carbohydrates for
catabolism. In the
digestive tract,
starch is
hydrolyzed to
glucose, which can be broken down in the cells by
glycolysis and the Kreb’s cycle. Glycogen and sucrose are
also broken down to glucose.
Proteins can be used for fuel, but they must be broken
down into amino acids. Fats are broken down into fatty
acids and glycerol.
Photosynthesis
6CO2 + 6H2O C6H12O6 + 6O2
Like cellular respiration,
photosynthesis is an oxidationreduction reaction. The energy
flow is reversed in photosynthesis;
water molecules are split and
electrons are transferred to carbon
dioxide, reducing it to sugar.
(Carbon dioxide gains electrons
(and H+); this is reduction.)
Describe how the structure of a
leaf facilitates photosynthesis.
Explain the process of
transpiration. Review the structure
of the chloroplast and chlorophyll.
Be able to relate each structure to the process that occurs there.
Photosynthesis consists of two complex stages, each with multiple steps. The first stage (the light
reactions) occur on the thylakoid membranes and the second stage (the Calvin cycle ) occurs in the
stroma. In a thylakoid membrane,
chlorophyll is clustered with proteins and
other smaller organic molecules into
photosystems. Most photosystems contain
chlorophyll a, chlorophyll b and
carotenoids. This allows each photosystem
to “harvest” energy from a wide array of
light wavelengths. No matter which
pigment absorbs the energy, it must
eventually be passed to a particular
chlorophyll a in the photosystem called the
reaction center chlorophyll.
There are two types of photosystems in the thylakoid membrane- photosystem I and photosystem II
(named in order of their discovery). Each photosystem has a particular type of chlorophyll as its
reaction center chlorophyll. The reaction center chlorophyll of Photosystem I is called P700 (because
it is best at absorbing light having a wavelength of 700 nm,) The reaction center chlorophyll at the
center of Photosystem II is called P680.
Noncyclic Electron Flow aka linear electron flow
There are two different ways that electrons might flow in the light reactions. Noncyclic (linear)
electron flow is the predominant route. Follow along with the picture-
1.
Light energy excites the electrons in Photosytem II. The reaction center chlorophyll is
oxidized (now it needs an electron)
2. An enzyme splits water; the electrons are supplied to the chlorophyll molecules; oxygen
combines with another oxygen forming O2
3. The original excited electron passes along an electron transport chain (goes from PSII to PSI).
4. The energy from the transfer of electrons is used to pump protons (H+) creating a
concentration gradient. This gradient will be used in chemiosmosis to phosphorylate ATP.
5. Light energy has also excited the PSI chlorophyll, resulting in the donation of electrons. These
electrons are replaced by the ones coming from PSII.
6. The electrons donated by PSI move along another electron transport chain and eventually to
NADP+. NADP+ accepts the electrons and is reduced to NADPH.
.Cyclic Electron Flow
Occasionally electrons take an alternative route
which uses Photosystem I, but not
Photosystem II. This is called cyclic electron
flow. This route produces ATP, but no
NADPH. The production of ATP by this
system is called cyclic photophosphorylation.
CHEMIOSMOSIS IN THE LIGHT
REACTIONS: As water is split in
noncyclic photophosphorylation,
protons (H+) are stored in the
thylakoid compartment. As a
proton gradient is established, H+
are pumped through ATP synthase
embedded in the thylakoid
membrane. ADP is phosphorylated.
THE CALVIN CYCLE: ATP and
NADPH produced in the light
reactions are used to convert CO2
to sugar. As CO2 enters the Calvin cycle it is attached to a five carbon sugar called ribulose biphosphate
(RuBP). The enzyme that catalyzes this step is RuBP carboxylase, better known as rubisco. This six
carbon product is rearranged in a series of steps that requires the phosphate from ATP and the H+ and
electrons from NADPH. The product is a 3 carbon sugar called G3P and a 5 carbon sugar which is
recycled to generate more RuBP. The molecule of G3P can combine with another molecule of G3P to
form glucose. G3P molecules are also used for biosynthesis or the energy needs of the cell.
For the net synthesis of one G3P molecule this cycle must occur 3 times (carbon dioxide molecules
enter one at a time),The Calvin cycle consumes 9 molecules of ATP and six molecules of NADPH for
each molecule of G3P produced.
ALTERNATIVE METHODS OF CARBON FIXATION
Sometimes there must be a compromise between photosynthesis and water loss by a plant. What
structural feature is responsible for this? The type of photosynthesis described so far occurs in plants
called C3 plants (the first organic product of carbon fixation is a 3 carbon compound called 3phosphoglycerate). Important commercial C3 plants include rice, wheat and soybeans. These plants
produce less food when their stomata close on a dry hot day. Less CO2 enters the Calvin cycle, so
rubisco accepts O2 instead and adds it to ribulose biphosphate. The product is eventually broken down
in the mitochondria and perioxisome into CO2 . This is called photorespiration. It is a wasteful process
which generates no ATP and no food.
In other plants, alternate modes of carbon fixation have evolved that minimize photorespiration even
in hot, arid climates. The two most important photosynthetic adaptations are C 4 photosynthesis and
CAM photosynthesis.
C4 Plants have two kinds of photosynthetic cells- bundle sheath cells and mesophyll cells. The two
stages of photosynthesis are separated structurally. Notice in the picture, that CO2 fixation occurs in
the cytoplasm of the mesophyll cells. The enzyme involved is PEP carboxylase and the product is
oxaloacetate (a four carbon sugar). Oxaloacetate is exported into the bundle sheath cells, which break it
down, releasing carbon dioxide. The carbon dioxide in the bundle sheath cell is converted into
carbohydrates through the normal Calvin cycle. This modification keeps the carbon dioxide levels
high enough to supply rubisco; this minimizes photorespiration.
CAM Plants have
adaptations that
separate the stages of
photosynthesis to day
and night (temporal).
This photosynthetic
adaptation evolved in
succulents, cacti,
pineapples and several
other plant families.
These plants open their stomata at night and close them during the day (the reverse of other plants).
At night these plants take up CO2 and incorporate it into a variety of organic acids. This mode of
carbon fixation is called crassulacean acid metabolism or CAM (named after the plant family
Crassulaceae). The mesophyll cells of CAM plants store the organic acids they make at night in their
vacuoles until morning when the stomata close. During the day, when the light reactions can supply
ATP and NADPH for the Calvin cycle, CO2 is released from the organic acids made the night before
and incorporated into sugar.
The Fate of Photosynthetic Products
The products of photosynthesis supply the chloroplasts with chemical energy and carbon skeletons to
synthesize all the major organic molecules of cells. About 50% of the organic material made by
photosynthesis is consumed as fuel for cellular respiration in the mitochondria of plant cells.
Carbohydrates can be transported out of the leaf cells as sucrose. Sucrose provides raw material for
cellular respiration and anabolic pathways that synthesize proteins, lipids and other products. A
considerable amount of sugar is used to make cellulose, the most abundant molecule in the plant.
Plants stockpile extra sugar by synthesizing starch and storing it in the cells of roots, tubers, seeds and
fruits.
Here is a food pyramid that begins with producers and ends with tertiary consumers. If the producer
level contains 25,000 kJ of energy and this pyramid follows the 10% rule, then how much energy gets
transmitted to the tertiary consumers?
Carbon Flow in a Grassland Ecosystem
How much carbon (g/m2) is released into the atmosphere as a result of the metabolic activity of the
herbivores?
ΔG = ΔH – TΔS
G = Free Energy
H = Enthalpy
S = Entropy
T = Temperature in Kelvin (K = C + 273)
Δ represents change in value over time
An experiment determined that when a protein unfolds to its denatured (D) state from the
original folded (F) state, the change in Enthalpy is ΔH = H(D) – H(F) = 56,000 joules/mol.
Also the change in Entropy is ΔS = S(D) – S(F) = 178 joules/mol. At a temperature of 20⁰C,
calculate the change in Free Energy ΔG, in j/mol, when the protein unfolds from its folded
state.
A + B + energy → AB
There are many types of biochemical reactions taking place in any living system. Which of
the following best characterizes the reaction represented above?
A) Catabolism
B) Oxidation-reduction
C) Exergonic reaction
D) Endergonic reaction
A molecule that is phosphorylated:
a. Has an increased chemical reactivity; it is primed to do cellular work
b. Has a decreased chemical reactivity; it is less likely to provide energy for cellular
work
c. Has been oxidized as a result of a redox reaction involving the gain of inorganic
phosphate
d. Has been reduced as a result of a redox reaction involving the loss of an inorganic
phosphate
Which of the following statements describes the results of this reaction?
C6H12O6 + 6O2 6CO2 + 6H20 + energy
a. C6H12O6 is oxidized and O2 is reduced
b. O2 is oxidized and H20 is reduced
c. CO2 is reduced and O2 is oxidized
d. C6H12O6 is reduced and CO2 is oxidized
Which step shows a split of one molecule into two smaller
molecules?
Which step involves an endergonic reaction?
Starting with one molecule of glucose, the
“net” products of glycolysis are:
a.
b.
c.
d.
2
2
2
6
NAD+, 2 H+, 2 pyruvate, 2 ATP and 2 H20
NADH, 2 H+, 2 pyruvate, 2 ATP, and 2 H20
FADH2, 2 pyruvate, 4 ATP and 2 H20
CO2, 6 H20, 2 ATP and 2 pyruvate
The oxygen consumed during cellular respiration is involved directly in which process or
event?
a. Glycolysis
b. Accepting electrons at the end of the end of the electron transport chain
c.The citric acid cycle
d.The oxidation of pyruvate to acetyl CoA
Which process in eukaryotic cells will proceed normally whether O2 is present or not
and therefore probably evolved first?
a. Electron transport
b. Glycolysis
c. The citric acid cycle
d. Oxidative phosphorylation
You have a friend who lost 7 kg (about 15 pounds) of fat on a low carb diet. How did
the fat leave her body?
a. It was released as carbon dioxide and water
b. Chemical energy was converted to heat and released
c. It was converted to ATP, which weighs less than fat
d. It was converted to urine and eliminated from the body