12.3 Oxidative phosphorylation

Question 1

What is the purpose of CO2 being produced in krebs cycle, and link reaction

What about the hydrogen ions

Answer 1

There is no use, CO2 is a waste product and is removed during gaseous exchange

Hydrogen atoms (and the electrons they possess) are valuable as potential sources of energy

These hydrogen atoms are carried by FAD and NAD to next stage of process called oxidative phosphorylation

Question 2

What happens in oxidative phosphorylation

Answer 2

Mechanism where some of the energy of the electrons within the hydrogen atoms is conserved in the formation of ATP

Question 3

Where does oxidative phosphorylation occur

Answer 3

. Mitochondrial inner folded membrane which is called cristae

The enzymes and other proteins involved in oxidative phosphorylation are present here so ATP synthesis can occur

This occurs more in metabolically active cells eg muscle and liver cells so the mitochondria in these have a more densely packed cristae so bigger surface area

Question 4

The synthesis of ATP by oxidative phosphorylation involves the transfer of electrons down a series of electron carrier molecules

Process:

Answer 4

. The hydrogen atoms produced during glycolysis and Krebs cycle combine with co-enzymes NAD and FAD

. The reduced NAD and FAD donate the electrons of the hydrogen atoms they are carrying to the first molecule in electron transfer chain

. The electrons pass along a chain of electron transfer carrier molecules in a series of redox reactions
As electrons flow along the chain the energy they release causes the active transport of protons across the inner mitochondrial membrane and into inter-membranal space

. The protons accumulate in inner membranal space before they diffuse back into mitochondrial matrix through ATP synthase channels embedded in inner mitochondrial membrane

. At the end of the chain the electrons combine with these protons and oxygen to form water

So oxygen is terminal acceptor of electrons in electron transfer chain

Question 5

What is the reaction for water being produced

Answer 5

1/2 O2 + 2e- + 2H+ —> H2O

Question 6

What is the importance of oxygen in respiration

Answer 6

It acts as a terminal acceptor of hydrogen atoms produced by glycolysis and krebs cycle

If oxygen wasn’t there to fufill its role of removing hydrogen atoms at the end of the chain, the hydrogen ions (protons) and electrons would back up along that chain and the process of respiration would come to a halt

Question 7

Why is energy released a little bit at a time in this process

Answer 7

If lots of energy was released at once, more of it is released as heat so there is less available for useful purposes

When energy is released a little bit at a time, more of it can be harvested for the benefit of the organism

Question 8

How is energy released in a controlled way

Answer 8

The electrons from NAD and FAD are not transferred in one step, they are passed along a series of transfer carrier molecules

. Each of which has a slightly lower energy level

. So electrons move down an energy gradient

. The transfer of electrons this way, down this gradient allows their energy to be released gradually and therefore more usefully

Question 9

What other substances apart from sugar can be oxidised by cells to release energy

Answer 9

. Lipids and proteins in certain circumstances can be used as respiratory substrates without first being converted to carbohydrate

Question 10

How can lipids be respired

Do they release more energy than carbohydrates?

Answer 10

. Lipid first hydrolysed to glycerol and fatty acids
. The glycerol is then phosphorylated and converted to triose phosphate which enters the glycolysis pathway and subsequently the krebs cycle

The fatty acid component is broken down into 2 carbon fragments which are made into acetyl coenzyme A. This then enters the krebs cycle

Overall, the oxidation of lipids produces 2 carbon fragments of carbohydrate, and many hydrogen atoms.
These atoms produce ATP during oxidative phosphorylation

SO LIPIDS RELEASE WAY MORE ENERGY THAN CARBOHYDRATES

Question 11

How is protein respired

Answer 11

. Protein is hydrolysed into its constituent amino acids

. These have their amino group removed in process called deamination

. Before entering the respiratory pathway at different points depending on the number of carbon atoms they contain

So 3 carbon compounds converted to pyruvate
4 and 5 carbon compounds are converted to intermediates in krebs cycle