Electron Transport

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Transcript Electron Transport

Electron Transport
Lecture 24
Chapter 14
Q1
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How many steps constitute the citric acid cycle?
A) 2
B) 4
C) 6
D) 8
E) 10
Q2
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Where in the cell does the citric acid cycle occur?
A) Nucleus
B) Cytoplasm
C) Mitochondria
D) Lysosomes
Answer: both B in bacteria and C in eukaryotes
Q3
• Which high energy molecules are generated by the citric
acid cycle?
• In what numbers do they form?
• ANSWER:
• NADH x 3
• FADH x 1
• GTP x 1
Q4
• Name the two forms of storage molecule used by plant
cells?
Q5
• Why do cells need to store energy locally?
Oxidative Phosporylation
The first cells on the plants were fermentative using simple
organic molecules to generate ATP - not too efficient and left
a lot of energy stll in the food!
Soon after it appears that a much more efficient system
evolved - transport of electrons along the plasma membrane
it was so successful that today, billions of years later, the
very same process is essential to life on Earth!
The same principles are used as the basis of both
photosynthesis (generation of complex molecules using light
energy) and oxidative phosporylation (catabolism of food in
the presence of oxygen)
As you have heard endlessly - the main energy currency of
cells is ATP, and not surprisingly it is the key player here.
Most energy comes from membranebased mechanisms
The process consists of two recognized steps
1) Electron transport chain - Protons (H+) are actively
driven across the membrane as electrons are handed
between a number of membrane embedded proteins.
2) ATP synthase - harnesses the electrochemical H+
gradient of the membrane (generated by step 1 above) to
drive the generation of ATP from ADP and Pi.
Students are given the analogy of a dam and turbines. This is
not a bad analogy as the parts of the ATP synthase protein
do actually spin when making ATP, as protons flow through
the protein, as we shall see in the video.
Chemiosmotic coupling
• The linkage of electron transport, proton pumping, and ATP
synthesis is called the chemiosmotic coupling model.
• This mechanism evolved in bacteria way back in history
• Then when these bacteria became a constituent of
eukaryotic cells first as mitochondrion and then as
chloroplasts, they enhanced those cells with the benefits of
this very efficient process
Mitochondria
• Nearly all eukaryotic cells have mitochondria
• When glucose is converted to pyruvate (glycolysis) only
10% of the energy stored in the chemical bonds of the
glucose is harnessed in the formation of the the 2 ATP
molecules
• In mitochondria the sugar is broken down completely to
CO2 and H2O and about a net total of 30 ATP molecules
are generated from each molecule of glucose.
Mitochondria structure
• Mitochondria have two membranes - outer and inner
• Mitochondria have two internal compartments - the matrix
inside the inner membrane, and the intermembrane space,
which exists between the two membranes
• The outer membrane is peppered with large protein pores
(made by a protein called porin) that let pass all small
molecules and proteins up to 5000 daltons in size. The
inner membrane is very selective and only permits
movement of molecules as per the plasma membrane,
using protein channels.
• The inner membrane is highly folded into cristae to
increase surface area
• The inner membrane of mitochondria is the site of this
oxidative phosphorylation in eukaryotes, and the inner
membrane surface in prokaryotes.
14_03_02_mito_4compart.jpg
Food for Mitochondria
• Pyruvate (from sugars) and fatty acids (from fats) are
transported across to the mitochondria from the cytosol
before conversion to acetyl CoA, by pyruvate
dehydrogenase
• This molecule then enters the citric acid cycle in the matrix
of the mitochondria. It is here that the majority of the highenergy electrons carriers are charged with electrons - the
NADHs and FADHs
• It is these that are then sent over to the membrane for
participation in the electron transport chain.
• SEE SUMMARY…
14_05_generate_energy.jpg
Respiratory Chain
• Oxidative phosphorylation is also known as the electron
transport chain or the respiratory chain.
• This chain contains over 40 proteins of which 15 are
directly involved in electron transport.
• Most of these are integral membrane proteins, some are
transmembrane in nature.
• Most of these exist as three large respiratory enzyme
complexes
– NAHD dehydrogenase complex
– Cytochrome b-c1 complex
– Cytochrome oxidase complex
• Each is complexed with metal ions and other chemical groups
14_10_resp_enzy_comp.jpg
Proton Pumps
• Each of these three complex proteins is really a proton
pump
• They each need energy to work
• This energy comes from the high energy electrons given to
them by NADH or FADH
• The electrons contain so much energy that each complex
takes what it needs to pump protons to the outside of the
membrane and then passes the same electron to the next
complex in the chain - hence the name electron transport
chain…
14_06_Protons_pumped.jpg
Electrochemical gradient
• Once again the difference on the two sides of the
membrane of protons generates two forces;
• 1) Concentration difference - more protons on the outside
then in by a factor of 10.
– Because protons are important factors in determining the pH of the
media - we see that there will be an associated pH difference - the
inside of mitochondria is pH 8 and the intermembrane space is pH
7 - as this is highly permeable to ions because of the ‘leaky’ nature
of the outer membrane - the whole cell is pH 7 also.
• 2) Charge difference - Protons are positively charged. So if
there are more on the out side surface of the membrane
then that will have a positive charge. Therefore, the inside
of the mitochondria will have a negative charge. This
makes the mitochondria a large battery…
14_11_batteries.jpg
The first battery
• Just like a real battery the energy stored in maintaining this
electrochemical difference can be used by the
mitochondria to drive other systems
• One of these is the ATP synthase - to make ATP from ADP
and Pi…
The combination of these two forces is additive
and adds to the potential energy
The structure of the enzyme ATP synthase,
which makes ATP by using the energy of
protons entering the cell
14_15_ATPsynthase2.jpg
The flow of energy
14_21_Redox_potential
.jpg
along
the electron
transport chain - as
you can see the
analogy with a river
and dam system is
appropriate as the
electron can be
considered as the
water in the river,
and each protein
complex as the
turbines in each
dam. Eventually,
the water flows to
the ocean and
cannot flow any
longer
O2 enters the picture here! In the very last step.
Cytochrome oxidase combines it with protons to generate water
What is the purpose of all this?
• The whole process has two tasks
– One one side it is to use the energy of food to supply the energy
build up a difference in the concentration of protons on the two
sides of the inner mitochondrial membrane
– The other is to utilize this difference to allow the flow of protons
back into the matix of the mitochondria and make ATP in the
process
Where does all the O2 I breath go?
• Answer: the vast majority ends up being used in the final
step of the electron transport chain, inside the
mitochondrial inner membrane!
• That O2 combines with electrons and protons to make
water!
• The carbon in the form of CO2 which we breath out comes
from the the carbon in the food that we consume. The O2
comes from cellular water.