Download Glycolysis

Introduction to
Metabolism
What is energy?
• the ability to do work – carry out life functions
Energy is used to:
• Break down larger compounds (catabolism)
+
• Build complex substances
(anabolism)
______________
metabolism
Types of Energy




Plants use solar (light ) energy
Glucose bonds store chemical potential
energy
Moving objects possess kinetic energy
Living organisms release thermal (heat)
energy
Laws of Thermodynamics
How energy flows between organisms is
governed by the Laws of Thermodynamics
1st law of Thermodynamics – energy
cannot be created or destroyed. It can only
be converted from one form to another.
Example: photosynthesis: plants absorb
solar energy  chemical potential energy
(in bonds of a sugar molecule)
When energy transformations occur, some
energy is lost as heat and is not available to do
work (not free energy).
When energy is lost, we say the entropy
(disorder) is increasing
2nd law of thermodynamics – all energy
transformations increase the entropy of the
universe
Introduction to Metabolism continued
Most of our available energy is stored as chemical potential
energy in the covalent bonds of the food we eat.
Main source of energy:
Glucose – C6H12O6
C6H12O6 + O2  CO2 + H2O + Energy
energy
When the bonds in a glucose molecule are broken, and atoms
are rearranged to form products - energy is released. Some of
this energy becomes available to do work (called Gibbs free
energy). Some energy is lost as heat.
The change is free energy represented as ΔG.
Oxidation of glucose:
ΔG = - 2870 kJ/mol
Why a negative value?
ΔG = G final – G initial
The negative sign shows that the products have
less chemical potential energy (i.e stronger bonds,
more stable) than the reactants.
This means:
The energy released when the products form is greater than the
energy absorbed when the reactants’ bonds broke. Some of this
difference in energy is now available or “free” to do work –
move, produce light, sound. Some is lost as heat. Reactions that
release energy are called Exergonic. Catabolic reactions are
exergonic.
Compare to photosynthesis:
Energy + CO2 + H2O  C6H12O6 + O2
ΔG = 2870 kJ/mol
The products have more chemical potential energy
energy. There has to be an investment of energy to
make this reaction possible. The energy came
from the sun.
Reactions that require energy to be absorbed
before they proceed are called Endergonic
reactions. Anabolic reactions are endergonic.
Balanced equation:
C6H12O6 + 6O2  6CO2 + 6H2O + 36 ATP
The controlled stepwise oxidation of sugar that occurs in the cell preserves useful energy, unlike
the simple burning of the same fuel molecule. In the cell, enzymes catalyze oxidation via a series of small
steps in which free energy is transferred in conveniently sized packets to carrier molecules — most often ATP
and NADH. At each step, an enzyme controls the reaction by reducing the activation energy barrier that has to
be surmounted before the specific reaction can occur. The total free energy released is exactly the same in (A)
and (B).
Thousands of chemical reactions just
occurred in your body. Some required energy
(anabolic). From where did this energy come?
ATP – adenosine triphosphate
ATP is a universal molecule of energy transfer –
like a cell’s currency.
Any energy made available by some cellular
process (ex: cell respiration) is first transferred
to ATP. If that energy is needed later, it is released
by ATP
ATP –Structure & Synthesis
A
Inorganic phosphate
P
P
Adenine
O
P
ribose
- the high energy phosphate bonds (~) are very unstable & can be
easily hydrolyzed (by adding H2O)
Energy + ADP + Pi  ATP + H2O
ATP + H2O  ADP + Pi + Energy
∆ G = - 31 kJ/mol exergonic
(energy released) – available to
do work (free energy)
∆ G = 31 kJ/mol endergonic
(energy taken in)
The process by which ATP is synthesized
(from ADP) and broken down (to ADP) is the
basis of cell metabolism. ADP and ATP are
shuttled throughout your cells to:
a)
Provide energy for endergonic reactions like
building macromolecules, contracting muscles
b)
Store the energy released when exergonic
reactions occur (ex: glucose broken down)
Note: When ADP binds a Pi, ATP is made. This
reaction is catalyzed by an enzyme. ATP made
in this manner is called substrate-level
phosphorylation. The addition of a Pi group
to any molecule is termed phosphorylation.
The Release of Energy in the Cell
Recall: the explosive burning (oxidation) of glucose
How do you convert the great deal of free energy in
glucose into the small more easily managed energy
molecules called ATP?
Need the help of:
• Enzymes
• Coenzymes – not proteins, much smaller, assist
enzymes, act as electron carriers – involved in redox
reactions. Include:
NAD+ – nicotinamide adenine dinucleotide
NADP - nicotinamide adenine dinucleotide phosphate
FAD – flavin adenine dinucleotide
The transfer of glucose energy to ATP
can be accomplished by:
i) aerobic respiration
a) glycolysis
b) kreb’s cycle
c) electron transport system
ii) Anaerobic respiration- a) ethanol fermentation
b) lactic acid fermentation
Both aerobic and anaerobic respiration begin with the
same set of reactions called glycolysis – “sugar splitting”
- occurs in the cytoplasm
- No oxygen is required
- involves 9 enzyme mediated reactions
Glycolysis – occurs in cytoplasm
glucose
ATP
outside cell membrane
ADP
Glucose – 6 – phosphate
Hexokinase – breaks down ATP and 1 Pi attaches to
glucose (phosphorylation of glucose) & ADP is released
Phosphoglucoisomerase – takes molecule and rearranges
it into a fructose
Fructose – 6 – phosphate
Phosphofructokinase (PFK) – takes Pi from ATP and
attaches it; ADP is release
ATP
ADP
- it is the rate limiting step aka the flux generating step
Fructose – 1, 6 - bisphosphate
Aldolase – splits molecule in 2
PGAL (phosphoglyceraldehyde) -aka glyceraldehyde 3-phosphate (G3P)
PGAL (phosphoglyceraldehyde)
NAD+
Pi
NADH +H+
Same as right side
1, 3 - bisphosphoglycerate
ADP
ATP
3 - phosphoglycerate
Phosphate dehydrogenase – Pi (present in
cytoplasm) attaches to PGAL , 2H+ & 2 e- are taken away
(from PGAL) and given to NAD+ to make NADH + H+
Phosphoglycerokinase – 1 Pi group removed and
added to ADP to form ATP (substrate level
phosphorylation)
Phosphoglyceromutase – changes location of Pi
group onto 2nd C so it is more balanced
2 - phosphoglycerate
H2O
Enolase – takes out H2O and makes molecule
symmetrical
PEP (phosphophenol pyruvic acid)
Same as right side
PEP (phosphophenol pyruvic acid)
ADP
ATP
Pyruvate kinase – P group removed and
joined with ADP to make ATP (substrate
level phosphorylation)
pyruvate
w/ O2 – aerobic
respiration in
Kreb’s Cycle
2 ATP net gain
2 NADH + 2 H+
w/out O2 – fermentation
Net Glycolytic Equation
C6H12O6 + 2 ATP + 2NAD+  2 PYRUVATE + 4 ATP + 2 NADH + 2 H2O
 there is a net gain of 2 ATP from glycolysis
recall: glucose = – 2870 kJ/mol
(total possible released)
glycolysis = 2 x ( – 31 kJ/mol)
(1 ATP = – 31 kJ/mol)
= – 62 kJ/mol
efficiency = – 62 kJ/mol x 100%
– 2870 kJ/mol
= 2% there is only 2% of the possible energy released
by glycolysis
glycolysis is extremely inefficient @ harvesting energy
- very small cells, like yeast, bacteria can live like this but our cells
cannot
...if O2 is present, the pyruvate enters the inner Matrix of the
mitochondria for the Krebs cycle & ETS
Structure of Mitochondrion (pl: mitochondria)
Preparatory stage
(aka Transition Stage)
Multiply all products by 2. Why?
Decarboxylation – CO2
removed from pyruvate
Acetyl – CoA
Net products:
6 NADH
2 ATP
2 FADH2
4 CO2
CoA
x 2 everything
oxaloacetate
NADH + H+
NAD+
malate
Coenzyme A
(aka Citric Acid Cycle)
NAD+
NADH + H+
succinate
CO2
 - ketoglutarate
fumarate
FAD
H2 O
H2 O
isocitrate
Krebs Cycle
H2 O
FADH2
citrate
ATP
Pi + ADP
GTP (guanosine
Pi + GDP triphosphate)
NAD+
NADH + H+
Coenzyme A
CO2
Succinyl – CoA
Inner mitochondrial membrane
Kreb
cycle
Proton
cyt b
Proton
pump
e- e -
Q
FMN
NADH
pump
H+
H+
O2
O O
2eO2e + 2H+
H2O
ATP to make ADP + Pi
(+)
H+
FAD
NAD+
-)
ETS
A
T
P
a
s
e
cyt a
cyt a3
Proton pump
INNER
MATRIX
H+
cyt c1
FADH2
(6 NADH,
2 FADH2, 2 ATP)
(
H+
H+
Space b/w inner
& outer m/b
H+
H+ H+
H+
+
H
H+ H+ H+
+
+
+
H
+
H H H
H+ H+ H+
So, how many ATP are made?
For every H+ ion that is pumped out, 1 re-enters and
the energy it releases (as it moves down the
electrochemical gradient) is used to make 1 ATP
So, 34 H+ pumped out, 34 re-enter = 34 ATP made
by ETC
34 ATP + 2 ATP + 2 ATP = 38 ATP
ETS
glycolysis
Kreb’s
Efficiency of aerobic respiration : 38 x (31kJ/mol) ÷ 2870 kJ/mol =
41%
Note: One small problem- the 2 NADHs produced in glycolysis (in the cytoplasm) must
be brought into the mitochondrion at a cost of some energy, usually estimated to be
1 ATP per NADH. So, final ATP count is 36.
How many O2 molecules are required as
the final electron acceptors (for each
glucose molecule)?
10 NADH  pass on 2e- each  20 e2 FADH2  pass on 2e- each  4 eEach oxygen atom has room for 2 electrons in outer
shell
2
6
O2 = 2 oxygen atoms, each accept 2e- = 4e-
24e- ÷ 4e-(per O2 molecule) = 6O2
Recall: Oxygen atom accepts 2e-, 6O2 + 24e- + 24 H+ = 12 H2O
molecules
6 H2O molecules get used up in previous reactions, net gain
of 6H2O
Alleluia, alleluia,
alleluia……….
Overall equation for cell respiration (aerobic
respiration), the process by which the energy
stored in glucose is released and stored in
ATP is:
C6H12O6 + O2  6CO2 + 6H2O + 36 ATP
Why is this chemical equation somewhat deceiving?
Cell Respiration song
http://www.youtube.com/w
atch?v=3aZrkdzrd04
Fermentation
Recall: NAD functions in the cell as an energy transport
compound. The cell has a limited supply of this compound. In
glycolysis, 2 molecules of NAD+ are reduced to NADH + H+.
Under aerobic conditions, the NADH transfer their H and 2e- to
the ETC (where O2 is the final electron acceptor). But, without O2,
how would NADH unload the electrons it picked up? If NADH
doesn’t get oxidized the cell’s supply of NAD+ would run out. The
result ……
glycolysis would stop (no NAD+ as a reactant) and the cell would
die from lack of ATP. All single celled organisms (like bacteria)
that can currently live in areas without oxygen (anaerobic) would
cease to exist.
So...how to keep glycolysis going? NADH must find another
acceptor for H & its electron(s). That acceptor is……
….Pyruvate.
The process that enables a cell to continue synthesizing
ATP by the breakdown of glucose under anaerobic
conditions is FERMENTATION.
2 types:
1.
Lactic Acid (lactate) fermentation
2.
Ethanol Fermentation (aka alcoholic fermentation)
Lactic Acid (lactate) Fermentation
•
•
•
•
•
•
Occurs in fungi (cheese making),
bacteria (in yogurt) and muscles
depleted of O2
pyruvate becomes the acceptor of H
atoms and e- from NADH. NAD+ is
shuttled back to the glycolytic
pathways so ATP can continue to be
made (rate ↑). Pyruvate becomes
lactate
Enzyme LDH – lactate
dehydrogenase mediates this
process.
During strenuous exercise, muscles
cells have greater demand for ATP,
not enough O2. Lactic acid produced
– causes muscle soreness
O2 debt “paid back” by deep
breathing
Lactic acid removed to the liver and
converted to glucose
Ethanol Fermentation
•
•
•
•
Involve yeast – single celled
fungi
Occurs in bread making (CO2
forms bubbles in dough and
alcohol evaporates during
baking) beer, wine,
champagne (CO2 does not
escape)
CO2 is removed from pyruvate
to become acetaldehyde, then
acetaldehyde accepts H to
become ethanol (aka ethyl
alcohol, grain alcohol)
Final products= ethanol +
CO2
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