Week 15

Question 1

The citric acid cyclic is amphibolic. What does this mean?

Answer 1

It has both anabolic and catabolic roles in metabolism.

Question 2

Why are amino acids degraded?

Answer 2

We cannot store or excrete excess amino acids. We break them down as a source of energy.

Question 3

Where does most amino acid metabolism take place in humans?

Answer 3

The liver.

Question 4

How do aquatic organisms such as fish excrete their nitrogen?

Answer 4

Excrete ammonium ions into the water. Fish are therefore called ammoniotelic. Ammonium ions are very soluble but are quite toxic.

Question 5

How do birds excrete their nitrogen?

Answer 5

They excrete nitrogen as uric acid acid: there are 4 N atoms in each molecule of uric acid. The are called uricotelic. Uric acid is not very soluble and is excreted in bird droppings.

Question 6

Mammals are termed ‘ureotelic’ based on the way they excrete nitrogen from the body. What does this mean?

Answer 6

We excrete nitrogen as urea, which is less toxic than ammonium ions but more soluble than uric acid.

Question 7

Which amino acids are glucogenic only?

Answer 7

Alanine, Arginine, Asparagine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Histidine, Methionine, Proline, Serine, Threonine, Valine.

Question 8

Which amino acids are both glucogenic and ketogenic?

Answer 8

Isoleucine, Phenylalanine, Tryrosine and Tryptophan.

Question 9

Which amino acids are ketogenic?

Answer 9

Leucine and Lysine.

Question 10

What is meant when an amino acid is termed ‘glucogenic’?

Answer 10

Amino acids that can be metabolised to enter carbohydrate metabolism pathways.

Question 11

What is meant when an amino acid is termed ‘ketogenic’?

Answer 11

Amino acids that can only be converted into ketone bodies.

Question 12

Where in the cell does the urea cycle occur?

Answer 12

All reactions occur in the mitochondrial matrix until citrulline is formed - citrulline goes into cytoplasm and the subsequent reactions occur in the cytoplasm.

Question 13

For every molecule of urea produced, how many molecules of ATP are required?

Answer 13

The equivalent of 4 ATPs are required (ATP → AMP counts as 2).

Question 14

How can nitrogen be recycled and transported back to the liver?

Answer 14

Nitrogen group in transferred to amino acid alanine which can enter the bloodstream and enter the liver, transferring its nitrogen group to glutamate.

Question 15

What type of cells in the body don’t contain ANY mitochondria?

Answer 15

Erythrocytes

Question 16

Where in the cell is the site of oxidative phosphorylation and ATP synthesis?

Answer 16

Inner mitochondrial membrane

Question 17

Which molecules are the outer mitochondrial membranes of cells permeable to?

Answer 17

All molecules smaller than 5000Da

Question 18

Which molecules are the inner mitochondrial membranes of cells permeable to?

Answer 18

Only molecules that have specific transporters embedded in the inner mitochondrial membrane.

Question 19

The Endosymbiotic hypothesis proposes that mitochondria were originally bacteria that became entrapped within eukaryotic cells. What evidence is there to support this?

Answer 19

Mitochondrial contain their own genome of circular DNA, RNA and ribosomes (the can make their own proteins), Mitochondria are enclosed in a double membrane, Mitochondria increase their number by dividing in a fission process (similar to bacterial reproduction).

Question 20

What process does the term ‘Oxidative Phosphorylation’ refer to?

Answer 20

The process that uses the high-energy electrons in molecules of NADH and FADH2 to produce more ATP.

Question 21

How many electrons are released by each NADH molecule during oxidative phosphorylation?

Answer 21

2 electrons (a hydride ion is released from NADH to form NAD+, hydride ion dissociates into 1 proton and 2 electrons).

Question 22

In which direction are protons actively pumped across the inner mitochondrial membrane to generate the electrochemical gradient required for ATP synthesis. Which area has a higher pH as a result?

Answer 22

Protons pumped from mitochondrial matrix to intermembrane space. Results in mitochondrial matrix having a higher pH than the intermembrane space.

Question 23

All mitochondria in offspring are from the maternal line, why is this?

Answer 23

Only the sperm tail contains mitochondria because mitochondria are found within the cell near sites of high ATP utilisation (sperm use the tails to migrate - requires energy). When sperm meets egg, tail gets dissolved.

Question 24

Which complex(es) pass electrons to ubiquinone?

Answer 24

Complex I (NADH Dehydrogenase) AND Complex II (Succinate Dehydrogenase).

Question 25

How many protons are pumped per molecule of NADH for each complex in the elctron transport chain?

Answer 25

Complex I: 4 protons, Complex II: 0 protons, Complex III: 4 protons, Complex IV: 2 protons.

Question 26

Define the term 'Standard Redox Potential'.

Answer 26

A measure of the tendency of the redox couple to donate electrons. Couples with a more negative standard redox potential have a greater tendency to donate electrons.

Question 27

How does the standard redox potential of redox couples chains as we progress through the electron transport chain?

Answer 27

Standard redox potentials get more positive (better at accepting electrons), ensure that electrons pass through the chain in the correct order.

Question 28

Describe the structure of Complex I (NADH Dehydrogenase) and the pathway of the electrons through this molecule.

Answer 28

Very large protein complex composed of many subunits. Contains non-protein components (iron-sulphur clusters) embedded into the matrix arm. Clusters held in place by the amino acid side chains of the protein. Electrons first accepted by FMN and then elctrons are passed along the chain of iron-sulphur clusters eventually passing electrons to ubiquinone. As electrons pass through the complex, this causes proton pumps to be activated (conformational changes driven by electron transport).

Question 29

What are the 2 roles of succinate dehydrogenase?

Answer 29

Convert succinate to fumerate in the Citric Acid Cycle, Capture and donate electrons from FADH2 in ETC.

Question 30

Describe the structure of ubiquinone.

Answer 30

Small, lipid-soluble molecule that is bound into the inner mitochondrial membrane. It mobile within the membrane. It is often abbreviated to Q.

Question 31

Describe the structure of Cytochrome C.

Answer 31

Small protein that sits on top of the inner mitochondrial membrane, contains a haeme group in the centre of the molecule that can bind and release electrons. As this changes the oxidation state of the Fe ion, this can be viewed by a colour change. Mobile elctron carrier moving within the intermembrane space. Highly evolutionarily conserved.

Question 32

How many electrons are passed from Cytochrome C to Complex IV that combine with O2 and 4 protons to form water?

Answer 32

4 electrons

Question 33

How many protons pumps does Cytochrome C Oxidase (Complex IV) contain?

Answer 33

Only 2 proton pumps.

Question 34

The most common metabolic sign of mitochondrial disease is lactic acidosis.  Why?

Answer 34

In the absence of functional mitochondria the only way cells can generate ATP is through glycolysis.  Pyruvate is converted to lactate as pyruvate dehydrogenase is inhibited by the high NADH/NAD+ ratio and lactate is formed. Lactate is exported from the cells and causes a decrease in blood pH.

Question 35

Why might mutations in some of the genes for proteins involved in the electron transport chain be associated with higher rates of cancer?

Answer 35

Defects in the electron transport chain can lead to the production of reactive oxygen species (ROS)., which can damage lipids, proteins and DNA. Damage to DNA can lead to the generation of oncogenes and unregulated cell division. ROS are produced at low levels all the time, but defects in the electron transport chain can increase their production.

Question 36

What is the difference between synthases and synthetases?

Answer 36

Synthetases catalyse reactions that use ATP/GTP whilst synthases do not.

Question 37

How many molecules of ATP does each fully oxidised glucose molecule produce?

Answer 37

30 ATPs

Question 38

What is special about the reducing power of the NADHs formed in glycolysis and why is this?

Answer 38

They only have the reducing power to form 1.5ATPs (rather than the usual 2.5ATPs). This is because energy is required to transport NADH from the cytosol (site of glycolysis) to the mitochondrial matrix (sites of oxidative phosphorylation).

Question 39

Which type of fat cell contains more mitochondria, brown fat cells or white fat cells?

Answer 39

Brown fat cells contain more mitochondria than white fat cells.

Question 40

What protein do brown fat cells use to generate heat?

Answer 40

Brown fat cells use a protein called thermogenin to generate heat. It is embedded in the inner mitochondrial membrane.

Question 41

What is the function of thermogenin and how does it contribute to heat generation?

Answer 41

It acts as a proton transporter across the inner mitochondrial membrane, uncoupling the proton gradient from ATP synthesis. Instead of being used to generate ATP, energy n proton gradient is released as heat.

Question 42

Why has 2,4-DNP previously been prescribed as a slimming aid but is now banned?

Answer 42

Inhibits oxidative phosphorylation by de-coupling the proton gradient. It is a lipid-soluble weak acid that accepts protons to enter the matrix where it releases protons. Cells are starved of ATP because cells are dying (muscle wastage). So much heat generated.

Question 43

How many molecules of ATP does each NADH and each FADH2 make?

Answer 43

Each NADH makes 2.5 ATPs, each FADH2 makes 1.5 ATPs

Question 44

What are the effects of inhibitors on oxidative phosphorylation?

Answer 44

Rotenone inhibits complex I, so stops NADH donating electrons, however FADH2 can still donate electrons in complex II, so ATP still produced but less of it. Antimycin A inhibits complex III, blocking the electron transport chain completely. Complex IV is inhibited by cyanide. and oligomycin inhibits ATP Synthase itself.