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