4: Mitochondria and Oxidative Phosphorylation

Question 1

endosymbiosis

Answer 1

a symbiotic relationship where one organism lives inside the other - mitochondria
means mitochondria have their own genome and biosynthetic machinery for making RNA and proteins

Question 2

fusion and fission

Answer 2

fusion = two mitochondria fuse together
fission = mitochondria come apart
^ changes the number of mtDNA
before fission, mtDNA localise to the mitochondria polarities to ensure they are incorporated into the new mitochondria

Question 3

mitochondria biogenesis

Answer 3

= the increase in abundance of mitochondrial proteins, mtDNA and mitochondrial number

exercise increases mitochondrial biogenesis

Question 4

mitochondria ribosomes

Answer 4

there are 2 rRNA genes in mitochondrial genome
rRNA is ribosomal RNA that forms subunits of ribosomes
therefore mitochondria have their own ribosomes
mitochondrial ribosomes (55S) made up of 39S large and 28S small subunits
ribosomes = read mRNA and synthesise proteins

Question 5

mitochondrial proteins

Answer 5

mitochondrial ribosomes synthesise proteins encoded by mtDNA
mitochondria can transcribe 13 protein encoding genes and translate them on the ribosomes
but over 1000 proteins are required to make and maintain mitochondria so these are encoded by nuclear DNA and synthesised on cytosolic ribosomes

Question 6

how proteins enter mitochondria

Answer 6

signal sequence on end of protein binds to receptor protein in outer membrane of mitochondria (TOM complex)
protein inserted and enters inter membrane space
if protein is destined to live in matrix, TIM23 complex takes the protein and enters it into the matrix
protein gets folded and signal sequence cut off, creating mature mitochondrial protein

Question 7

how pyruvate enters mitochondria

Answer 7

small channel proteins live in outer mitochondria membrane called porins - allow small molecules into intermembrane space including pyruvate
then enters matrix through mitochondrial pyruvate carriers
in matrix, pyruvate decarboxylated to form acetyl CoA

Question 8

how fatty acids enter mitochondria

Answer 8

fatty acyl-CoA combines with carnitine with help of enzyme CPTI to move through outer mitochondrial membrane (permeable)
translocase protein helps acyl-carnitine get through inner membrane which isn’t very permeable
CPTII does reverse reaction to remove carnitine from acyl-CoA
carnitine leaves via translocase protein

Question 9

how NADH enters mitochondria

Answer 9

malate-aspartate shuttle:
NADH can’t pass inner mitochondrial membrane so it is oxidised to form NAD at the same time oxaloacetate is forming malate via malate dehydrogenase
electrons from NADH held by malate
malate passes through membrane in return for an alpha ketoglutarate
malate dehydrogenase now works in other direction to catalyse malate into oxaloacetate, in turn NAD is reduced to NADH

Question 10

electron transport chain ETC

Answer 10

4 different complexes
high energy electrons carried by NADH and FADH2 are passed through the ETC and lose energy as they travel through
energy released is used to pump protons (H+) across the inner mitochondrial membrane (creates proton gradient for ATP synthesis)

Question 11

complex I = NADH dehyrogenase

Answer 11

NADH releases 2 high energy electrons
NAD is recycled
electrons accepted by iron sulphur clusters
series of redox reactions
electrons passed along with 2 H+ to CoQ to from CoQH2
4 protons pumped into intermembrane space from matrix

Question 12

complex II = succinate dehydrogenase

Answer 12

FADH2 passes high energy electrons in
iron sulphur clusters = redox reactions
CoQ forms CoQH2
FAD recycled into TCA cycle
not enough energy for any protons to be pumped

Question 13

complex III = cytochrome c reductase

Answer 13

CoQH2 from complex I or II gives electrons to iron sulphur clusters
electrons passed to cytochrome c
2 cytochrome c molecules reduced
2 protons released into intermembrane space
further 2 protons pumped across membrane

Question 14

complex IV = cytochrome c oxidase

Answer 14

2 electrons from NADH carried by 2 cytochrome C carriers
electrons combine with protons and oxygen to produce water
1/2 oxygen + 2H + 2 electrons = 1 water molecule
however electrons are held until 2 molecules of water are made at once
results in 4 protons pumped into intermembrane space but from 2 NADH

Question 15

why are protons pumped into intermembrane space

Answer 15

creates significant electrical charge across the membrane
creates pH different across membrane as protons are very acidic
this is called the electromechanical proton gradient or proton motive force
forces ATP synthase

Question 16

ATP synthase

Answer 16

protons enter the alpha portion of the F0 subunit
protons move into ‘c ring’ which causes it to rotate
protons exit into matrix
central shaft rotates very quickly (150 per second)
F1 portion stays static
ADP and Pi bind to specific sites between alpha and beta subunits
rotation of shaft causes conformational change to the F1 region and catalyses the formation of ATP from ADP and Pi