12.2 Link reaction and krebs cycle

Question 1

What is the purpose of the Krebs cycle

Answer 1

The pyruvate released from glycolysis possesses potential energy that can only be released in the krebs cycle

Question 2

What is purpose of link reaction

Answer 2

Before pyruvate can enter krebs cycle, they must be oxidised in this link reaction

Question 3

Where do link reactions occur

Answer 3

Mitochondria

Question 4

What are the steps of Link reaction

Answer 4

. Pyruvate is produced by glycolysis in cell cytoplasm
. Then they are actively transported into the matrix of mitochondria

. The pyruvate is oxidised to acetate.
Here, the 3 carbon pyruvate loses a molecule of CO2 and two hydrogens

These hydrogens are accepted by NAD to form NADH (aka reduced NAD) which is later used to produce ATP

. The 2 carbon acetate combines with coenzyme A to produce a compound called acetylcoenzymeA

Question 5

What is overall reaction for link reaction

Answer 5

Pyruvate + NAD + CoA —>
Acetyl coenzyme A + NADH + CO2

Question 6

What does the Krebs cycle consist of

Answer 6

It involves a series of oxidation-reduction reactions that occur in matrix of mitochondria

Question 7

Describe steps of krebs cycle

Answer 7

. Acetyl coenzyme A has two carbons, and is combined with a 4 carbon molecule
. This makes a 6 carbon molecule
. In a series of reactions this 6C molecule loses carbon dioxide and hydrogen
. Eventually there is a 4C molecule and a single molecule of ATP produced as a result of substrate level phosphorylation

. The 4C molecule can now combine with more acetyl coenzyme A so cycle begins again

Question 8

For each molecule of pyruvate, what is produced in the link reaction and krebs cycle

Answer 8

. Reduced coenzymes such as two NADH and one FADH
. These have the energy to produce ATP molecules by oxidative phosphorylation
. Therefore they are important products of krebs cycle

. One molecule of ATP is produced per pyruvate, so two in total

. 3 molecules of CO2 per pyruvate as one is from link reaction and two are from krebs cycle.
So 6 molecules in total

Question 9

What are coenzymes

Answer 9

. They are molecules that some enzymes require in order to function

. Co-enzymes play a major role in photosynthesis and respiration where they carry hydrogen atoms from one molecule to another

Question 10

What are 3 examples of coenzymes

Answer 10

NAD is important throughout respiration: It works with dehydrogenase enzymes that catalyse removal of hydrogen atoms from substrates to transfer them to other molecules involved in oxidative phosphorylation

FAD is important in krebs cycle

NADP which is important in photosynthesis

Question 11

Why is the krebs cycle important for organisms

4 reasons

Answer 11

. It breaks down macromolecules into smaller ones, eg pyruvate is broken down into CO2

. It produces hydrogen atoms that are carried by NAD to the electron transfer chain, and provide energy for oxidative phosphorylation. This makes ATP

. It regenerates the 4C molecule that combines with acetyl coenzyme A which would otherwise accumulate

. It is a source of intermediate compounds used by cells in the manufacture of other important substances eg fatty acids, amino acids and chlorophyll