Biochemistry- glycolysis

Question 1

What do catabolic pathways do?

Answer 1

They break down things or oxidise them into small molecules.

Question 2

What are the final products of catabolic pathways in our systems?

Answer 2

Carbon dioxide and water

Question 3

Do catabolic pathways tend to be exergonic or endergonic?

What does this mean that their delta G is?

Answer 3

Exergonic (i.e. they release energy)

It is negative

Question 4

What do anabolic pathways do?

Answer 4

They build things up

Question 5

Do anabolic pathways tend to be exergonic or endergonic?

Answer 5

Endergonic, i.e. they need an input of energy.

Question 6

What happens in oxidation reactions?

Answer 6

Electrons are removed from a substrate

Question 7

What happens in reduction reactions?

Answer 7

Electrons are added onto a substrate

Question 8

What happens to electrons removed from reduced fuel products when they are catabolised to oxidised products?

Answer 8

They are picked up by an electron carrier (e.g NAD+ or FAD, or NADP+).

Question 9

a )Name the two electron carriers which use the electrons they capture to drive oxidative phosphorylation.
How many electrons do they accept, and what do they become when they do so?

b) How can these molecules use the electrons to drive oxidative phosphorylation?

Answer 9

a) NAD+ and FAD.
They can both accept 2 electrons.
NAD+ becomes NADH + H+ and FAD becomes FADH2.

b) Because the electrons on NADH + H+ and FADH2 are high energy electrons and can provide enough energy to ultimately lead to the phosphorylation of ADP to ATP.

Question 10

Name an electron carrier which uses the electrons it captures to reduce oxidised precursors?
How many electrons does it accept?
What does it become when it accepts these electrons?

Answer 10

NADP+
It can accept 2 electrons.
It then becomes NADPH + H+

Question 11

What is the first stage of the catabolism of food molecules?

Answer 11

Macromolecules are degraded to their monomeric units.

This is digestion.

Question 12

What is the second stage of the catabolism of food molecules?

Answer 12

Small molecules from stage 1 enter cells and are converted into a small number of very simply molecules.
(2 carbon acetyl-coA)
Some ATP is reduced.

Question 13

What is the third stage of the catabolism of food molecules?

Answer 13

The acetyl group of acetyl-coA enters the Krebs cycle where it is completely degraded to CO2 and H20. The energy produced is used to drive the synthesis of ATP from ADP and Pi.
Most of the energy derived from foodstuffs is generated in this stage.

Question 14

What is acetyl-coA?

Answer 14

It is a small molecule derived from acetic acid.

The acetyl from the acetic acid is linked to Coenzyme A to form acetyl CoA.

Question 15

What is our primary energy source?

Answer 15

Glucose

Question 16

What determines whether a glucose molecules is an alpha or a beta molecule?

Answer 16

The position of the hydroxyl group on carbon 1.
If it is pointing below the molecule it is an alpha molecule.
If it is pointing above the molecule it is a beta molecule.

Question 17

What is the difference between an alpha-1,4 glycosidic bond and a beta-1,4-glycosidic bond?

Answer 17

In an alpha-1,4-glycosidic bond, the glucose monomer which supplies the carbon 1 which partakes in the bond is in the alpha form.
In a beta-1,4-glycosidic bond, the glucose monomer which supplies the carbon 1 which partakes in the bond is in the beta form.

Question 18

What is lactose?

Answer 18

A disaccharide.

It is formed from a galactose and glucose monomer, joined by a galactose-beta-1,4-glucose bond.

Question 19

What is sucrose?

Answer 19

A disaccharide.

It is formed from a glucose and a fructose monomer, joined by a glucose-alpha-1,2,-fructose bond.

Question 20

What is cellubiose?

Why can we not digest this?

Answer 20

This is the disaccharide which polymerises to form cellulose.
It is formed from 2 glucose monomers joined in a glucose-beta-1,4,-glucose bond (beta-1,4-glycosidic bond).

We cannot digest cellubiose or cellulose because we do not have the enzyme required to break the beta-1,4-glycosidic bond.

Question 21

What is maltose?

Answer 21

This is a disaccharide formed from 2 glucose monomers joined by an alpha-1,4-glycosidic bond.

Question 22

Which substrates can be converted to acetyl coA?

What can they then enter?

Answer 22

Glucose
Fatty acids
Amino acids.
These can all then enter the Krebs cycle.

Question 23

Why do carbohydrates require transporters to get across the cell membrane?

Answer 23

Because they are polar molecules due to the OH groups.

Question 24

How is glucose transported into cells?

Answer 24

Via Na+/glucose symporters, and via passive facilitated diffusion transporters (GLUT transporters).

Question 25

What does Km of transporters indicate?

Answer 25

The substrate concentration at which the transporter works at maximum velocity.

Question 26

How do GLUT transporters work?

Answer 26

They have 2 conformations. The first is with the binding site facing the extracellular environment. The binding of glucose to the binding site triggers a conformational change, and now the binding site faces inwards. The glucose unbinds and is released into the cell because there is a lower glucose concentration inside the cell. There is a conformational change again once the glucose has unbound and the binding site now faces the outside.

Question 27

What is glycolysis?

Answer 27

The initial pathway for the conversion of glucose to pyruvate, via 3 stages.

Question 28

Describe glycolysis.

Answer 28

There are 3 stages of glycolysis. 1. Glucose is converted to fructose-1,6-bisphosphate, via 3 reactions in which it has two phosphates added to it (by the conversion of 2 ATPs to 2 ADPs) and its structure is slightly modified. 2. Fructose-1,6-bisphosphate is then split into 2 triose phosphates. 3. 2 triose phosphates are then converted into 2 pyruvate molecules in a series of 5 reactions. This section generates energy, because 2 ADP molecules for each triose phosphate/pyruvate (therefore 4 ADP in total) are converted into ATP. There is therefore a net gain of 2 ATP from glycolysis. This section is also oxidative, and releases electrons, which are picked up by 2 NAD+ molecules to form 2 NADH + 2H+.

Question 29

What is the effect of stage 1 of glycolysis on the glucose molecule?

Answer 29

It is trapped, because it phosphorylated and therefore is negative and it is even more difficult for it to cross the cell membrane. It is also destabilised.

Question 30

What is the result of stage 2 of glycolysis?

Answer 30

Two interconvertible 3 carbon molecules are formed.

Question 31

What is the result of stage 3 of glycolysis?

Answer 31

ATP is generated.

Question 32

What is particular about the third stage of glycolysis?

Answer 32

It has to happen twice for each glucose molecule.

Question 33

What are potential control points in a metabolic pathway?

Answer 33

Enzymes which catalyse irreversible reactions.

Question 34

Where are the control points in glycolysis? | Why can these reactions be control points?

Answer 34

The first third and the last reactions. These reactions can be control points because they have very negative delta Gs, therefore are very exergonic and are irreversible.

Question 35

What do the control points in glycolysis do, and what catalyses them?

Answer 35

1. (1st reaction in the pathway) Converts glucose to glucose-6-phosphate thereby trapping it in the cell. This is catalysed by hexokinase. 2. (3rd reaction in the pathway). This phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate. It is catalysed by phosphofructokinase. 3. (last reaction in the pathway). This is catalysed by Pyruvate kinase and produces ATP.

Question 36

What is the key enzyme in the control of glycolysis? | Which part of the pathway does this catalyse?

Answer 36

Phosphofructokinase. This catalyses the 3rd reaction in the pathway.

Question 37

What are negative modulators for phosphofructokinase?

Answer 37

ATP (end product inhibition) Citrate (an early intermediate in the citric acid cycle. If there are large amounts of this it means there is lots of material or biosynthetic precursors available in the Krebs cycle). H+- since this is a negative modulator, it prevents the build up of lactic acid in the cell which is bad for the cell.

Question 38

What are the positive modulators of phosphofructokinase?

Answer 38

AMP- this is a nucleotide with no high energy phosphate in it, so a high concentration of AMP signals that the cell is low in energy, so triggers glycolysis to make more energy. Fructose-2,6-bisphosphate

Question 39

What is the energy charge of a cell? | When is a cell "charged"?

Answer 39

The energy charge is the ATP/AMP ratio. | A cell is charged if it has a high ATP/AMP ratio.

Question 40

Why is AMP the positive regulator for phosphofructokinase and not ADP?

Answer 40

If ATP is rapidly used up, there is ADP produced, and adenylate kinase can salvage some of the energy from ADP to produce ATP and AMP from 2 ADP molecules.

Question 41

Where does glycolysis occur?

Answer 41

In the cytoplasm