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Metabolic pathways consist of chains and cycles of enzymecatalyzed reactions
Huge range of chemical reactions occur; catalyzed by over 5000 diff
...
Then, it becomes the product
The binding lowers the overall energy level of the transition state making
the activation energy reduced
Net amount of energy released by reaction is unchanged by the
involvement of the enzyme
The rate of reaction is greatly increased, though; by a factor of million or
more

Types of Enzyme Inhibitors

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Enzyme inhibitors can be competitive or non-competitive
Some chemical substances bind to enzymes and reduce the activity of the
enzymes (inhibitors)
The two types are competitive and non competitive
Competitive ones interfere with the active site so that the substrate
cannot bind; non competitive ones bind at a location other than the active
site
Non-competitive inhibition results in a change of shape so that the
enzyme cannot bind to the substrate

Redox Reactions

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Metabolic pathways can be controlled by end product inhibition
Many enzymes are regulated by chemical substances that bind to special
sites other than the active site; these are called allosteric interactions and
the binding site is called allosteric site
Many times, the enzyme that is regulated catalyzes one of the first
reactions in a metabolic pathways and the substance that binds to the
allosteric site is the end product
The end product acts as an inhibitor; enzyme works rapidly when there is
a shortage of end product but can be switched off when there is an excess
EG: The amino acid threonine is converted to isoleucine; as the
concentration builds up, it binds to the allosteric site of the first enzyme
(threonine deaminase), thus acting as a non competitive inhibitor

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Determining the rate of an enzyme controlled reaction involves measuring
rate of disappearance of a substrate OR a rate of appearance of a product
The rate unit should usually include s^-1
Introduction
Cellular Respiration (CR) creates the chemical energy and allows for its
used in the living cell
Much of the energy is lost in heat energy, but a good amount is retained
in ATP
ATP is a small, soluble molecule that can move through facilitated
diffusion to wherever needed
Cell respiration involves oxidation and reduction of compounds
Glucose is made of six carbon atoms, all in a reduced state

During aerobic respiration, glucose goes through enzyme catalyzed
oxidation reactions and decarboxylation reaction
Phases:
Glycolysis: glucose is converted into pyruvate
Link Reaction: pyruvate is converted thru coenzyme A (acetyl CoA); CO2
is given off
Krebs Cycle: acetyl coenzyme A is converted to carbon dioxide
Electron-transport system: the hydrogen that was removed in the
oxidation reactions of glycolysis makes water; most of ATP is made here
Respiration as a series of redox reactions
In CR glucose is oxidized to carbon dioxide, and oxygen is reduced to
water
When one substance is oxidized, another is reduced automatically
In biological oxidation: oxygen atoms can be added, or hydrogen atoms
can be removed; in CR the hydrogen atoms are removed from glucose
and given to hydrogen acceptors (which are reduced)
Since a hydrogen atom consists of a proton and an electron thus gaining
hydrogen (reduction) means gaining one or more electron
OIL RIG= oxidation is loss of electrons; reduction is gaining electrons
Redox reactions occur in the body because of oxidizing agents (compound
that want to take away electrons) or reducing agents (compounds that
want to give or donate electrons)
When reduction occurs, the energy is absorbed (endergonic); when
oxidation occurs, energy is released (exergonic)

An oxidized substance is lower in energy stored within when compared to
reduced substances (eg: CH4 has more energy stored within than CO2)
Overall Oxidation Involves: Adding oxygen, losing electrons, giving off
energy, and removing hydrogen
Overall Reduction Involves: Removing oxygen, gaining electrons, taking
in energy, and gaining hydrogen

Cellular Respiration- Glycolysis

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Glycolysis is a linear series of reactions in which a 6 carbon sugar
molecule is broken down into 2 molecules of the 3 carbon
pyruvate ions
The enzymes involved are located in the cytoplasm outside organelles
Glycolysis occurs by 4 stages:
Phosphorylation
 Glucose reacts with ATP creating glucose phosphate
 The glucose phosphate is now less stable, aka more reactive
 Then fructose phosphate is created
 Then another phosphate group is added at the expense of


another ATP molecule
Overall, 2 ATP molecules are used till this point

Lysis (splitting)
 Fructose biphosphate is split into two molecules of 3 carbon
sugar called triose biphosphate
Oxidation
 Oxidation of the triose biphosphate occurs through the removal
of hydrogens
 The enzyme (a kind of dehydrogenase) works with NAD (a
coenzyme)
 NAD accepts hydrogen ions and electrons
 The NAD is reduced to form NADH and Hplus
ATP Formation
 4 ATP molecules are synthesized as two molecules of triose
phosphate are turned into pyruvate
 Thus, there is a net gain of 2
 The ATP formation till this point is called being at a substrate
level

Once the pyruvate has been formed from glucose in the cytosol, the
remainder of the pathway of aerobic cell respiration is located in the
mitochondria

Link Reactions and Krebs Cycle

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Link Reactions
Pyruvate diffuses into the matrix of the mitochondrion
The 3 carbon pyruvuate is decarboxylated (loses CO2) and also oxidized
(loses hydrogen)
...

Structure of the chloroplast
Chloroplasts are to photosynthesis, as mitochondria are to cellular
respiration
There is a double membrane around the chloroplast, the inner membrane
folds extensively to form an interconnected membrane called the
thylakoid
Thylakoid membranes are organized into flat, compact, circular discs
called grana (singular granum) and between the grana are tubular

membranes that are suspended in the stroma (like the matrix of the
mitochondria)
Chlorophyll is found in the grana; the stroma also has lipid droplets,
ribosomes, and starch grains
Reactions of the process
Light Dependent




use light energy to split water (photolysis);
hydrogen is removed and retained by the photosynthesis specific
hydrogen acceptor known as NADP+
ATP is formed from ADP and a phosphate group using light
energy (photophosphorylation)
Oxygen is the waste product of that; occurs in the grana

Light Independent Build up sugars using carbon dioxide




The reduced NADP+ and ATP are used in sugar production
This occurs in the stroma of the chloroplast
Requires continuous supply of the products of the light
dependent reaction but doesn’t require light itself

Light Dependent Reactions

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Chlorophyll taps the light energy in this reaction
The chlorophyll don’t occur randomly, but rather they are grouped
together in a system called photosystems which are in the thylakoid
membranes
Each photosystem has several hundred chlorophyll molecules with other
accessory pigments (carotene and xanthophylls)
The pigment molecules harvest light of different wavelengths and send all
of them to the reaction centre (single chlorophyll molecule), which is then
photoactivated
Photosystem 1(p1)- has a reaction centre that is activated by light of
wavelength 700nm; the centre is called P700
Photosystem 2 (p2)- has a reaction centre that is activated by light of
wavelength 680nm; the centre is called P680
These photosystems are grouped in the thylakoid membranes along with
proteins that have functions:



Enzymes that adjoin the ADP and P; and also reduce the NADP+
molecules
Electron carrier molecules

Transfer of light energy
When the light energy reaches the reaction centre, the ground state
electrons present there are sent into an excited state; the spaces they
vacate are filled by more ground state electrons
First the electrons from p2 are sent along electron carriers
(plastoquinone), some of the energy causes the pumping of the hydrogen
ions into the thylakoid spaces from the stroma; this is where the protons

accumulate and cause a drop in pH leading to a proton gradient ready for
chemiosis
As the electrons allow for this to occur, when they reach p1, they’re back
to their ground state
The holes in the p2 are filled by electrons from water molecules (in
ground state); the positive ‘vacancies’ can break the water molecules and

release hydrogen ions and oxygen atoms along with ground state
electrons
The oxygen molecules combine to form O2 and the hydrogen is used for
reduction
Photophosphorylation
The hydrogen ions in the thylakoid space flow out the ATP synthase
enzymes to create ATP from ADP and P
The electrons after passing through p1 are finally used to reduce NADP+
Since the pathway is linear, the photophosphorylation reaction in which
they are involved in is described as noncyclic photophosphorylation
ATP and reduced NADP+ do not accumulate since they are immediately
used in the fixation of CO2, after which the ADP and NADP+ diffuse back
into the grana for reuse
Studying Light Dependent Reactions With Isolated Cholorplasts
You can isolate chloroplasts and suspend them in a buffer solution with
the same concentration as cytosol
They function properly in such conditions
In light dependent reactions the oxygen given off is derived exclusively
from water
In labs, the oxygen electrode can detect the oxygen given off by isolated
chloroplasts
DCPIP can replaced the role of NADP+ in lab conditions because it
becomes colorless from blue when it is reduced

Mezzanine of Light Independent

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In Light independent reactions, the carbon dioxide is transformed into the
carbohydrates, they occur in the stroma
The pathways of this occurring was investigated by a method called
feeding experiments, in this experiment, the radioactively labeled carbon
dioxide was fed to cells
It is taken up the same way and fixed the same way to produce products
A brief pulse of radioactive CO2 was introduced and its progress
monitored; the intermediates and products contained radioactive markers
Chromatography was used to isolate these compounds from the sampled
cells

End Product Inhibition

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Steps






The first product is glycerate 3 phosphate (GP); it is called the
fixation step
The initial product is reduced to 3 carbon sugar phosphate using
NADPH + H+ and ATP; this is the reduction step
The triose phosphate is further metabolized to make
carbohydrates such as sugars, sugar phosphates, and starches,
and lipids, amino acids, and organic acids; this is the product
synthesis step
Some of the triose phosphate is metabolized to make the
molecule that first reacts with CO2; this the regeneration of
acceptor step; the reactions of this process are called the Calvin
cycle

Initially, the acceptor molecule was thought to be a 2 carbon compound
(as the C from carbon dioxide could adjoin to create a 3 carbon
compound/GP, but it turned out to be a 5 carbon compound to create two
GPs)
The carbon compound that is the acceptor molecule is called ribulose
biphosphate (RuBP)
The enzyme involved is RuBisCo – ribulose biphosphate carboxylase



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