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Chapter 14
Energy Generation in Mitochondria and Chloroplasts
Generation of Energy
• Millions of years ago there was no O2 available for oxidative phosphorylation to occur
• Organisms produced energy from fermentation, still see this today
• As O2 became available, a more efficient method of energy production developed– Based on the transfer of e- along the membrane
Organism’s Energy Source
• Small amount of ATP from glycolysis in the cytosol of cells
• Majority made by a membrane based process in 2 stages– Stage 1 – e- transport chain
• e- transferred along e- carriers in the membrane
– Stage 2 – flow of H+ down an electrochemical gradient to produce ATP
• Use a complex called ATP synthase
Stage 1
• NADH (from the Kreb’s cycle) brings in the e- and transfers them to the carrier molecules
• The e- moves down the chain and looses energy at each step – as this happens, H+ are pumped across the membrane
• This creates an electro-chemical gradient across the membrane
Stage 2
• The electrochemical gradient is a form of stored energy – it has the potential to do work
• The H+ can now move down the gradient and return to the other side of the membrane thru ATP synthase – in this process, generates ATP from ADP and Pi
Chemiosmotic Coupling
• Once called the chemiosmotic hypothesis– Chemi from making ATP, osmotic because of crossing the
membrane
• Now known as chemiosmotic coupling
Mitochondria
• Produce most of a cells ATP – acetyl groups in the Kreb’s cycle producing CO2 and NADH
• NADH donates the e- to the electron transport chain and becomes oxidized to NAD+
• e- transfer promotes proton pump and ATP synthesis in process called oxidative phosphorylation
• Cells that require large amounts of energy such as the heart have large numbers of mitochondria
Mitochondria
• Contain their own copies of DNA and RNA along with transcription and translation system (ribosomes)
• Are able to regenerate themselves without the whole cell undergoing division
• Shape and size dependent on what the cell’s function is
Mitochondria
• Double membrane creates 2 spaces
Matrix: large internal space
Intermembrane space: between the membranes
Outer membrane
Inner membrane
Mitochondria
Inner Membrane
• Inner membrane is the site of the e- transport chain, across which the proton pump occurs and contains ATP synthase
• Inner membrane is highly folded – called cristae – increasing the surface area on which the above reactions can take place
High Energy e-
• Mitochondria use pyruvate and fatty acids and convert it to acetyl CoA in the matrix
• Citric acid cycle generates NADH and FADH2 which carry the e- to the electron transport chain
Summary – MUST KNOW
Proton Pumping
• Many molecules can supply the e- - carbohydrates and fatty acids
• O2 ultimate e- acceptor producing H2O as waste
Movement of Electrons
Oxidative Phosphorylation
Electron Transport Chain
• Resides in the inner mitochondrial membrane – also called respiratory chain
• 15 proteins involved in the chain – grouped in 3 large respiratory enzyme complexes– NADH dehydrogenase complex– Cytochrome b-c1 complex– Cytochrome oxidase complex
• Pumps protons across the membrane as e- are transferred thru them
Respiratory Enzyme Complexes
Proton Gradient
• e- transfer is an oxidation/reduction reaction
• NADH has high-energy e- has a low electron affinity so the e- is readily passed to NADH dehydrogenase and so on down the chain
• Each transfer couples the energy released with the uptake of a H+ from the matrix to the intermembrane space setting up the electrochemical gradient
Proton Gradient
• Gradient of proton (H+) concentration across the inner mitochondrial membrane – a pH gradient with the pH in the matrix higher than in the intermembrane space
• Proton pumping also generates a membrane potential – matrix side is negative and intermembrane space is positive
4 Complexes in Membrane
Location of H+
Electrochemical Gradient
Oxidative Phosphorylation
• ATP synthase is the protein complex responsible for making ATP by creating a path for H+ thru the membrane
• ATP synthase is an enzyme
ATP Synthase
• Multisubunit protein responsible for making ATP
Summary
Bidirectional Pump
Coupled Transport Can Move Other Molecules
Oxidation of Sugar and Fats