4.1.1 Respiration

• state that glycolysis takes place in the cytoplasm; • outline the process of glycolysis beginning with

the phosphorylation of glucose to hexose bisphosphate, splitting of hexose bisphosphate into two triose phosphate molecules and further oxidation to pyruvate, producing a small yield of ATP and reduced NAD;

• state that, during aerobic respiration in animals, pyruvate is actively transported into mitochondria;

Respiration: The 4 Parts

Respiration consists of 4 parts:• Glycolysis• Link Reaction• Krebs Cycle• Oxidative Phosphorylation (the electron

transport chain)

Glycolysis

• Glycolysis is the first stage of respiration• Glycolysis splits one molecule of glucose into two

smaller molecules of pyruvate• Glucose is a hexose (6-carbon) molecule• Pyruvate is a triose (3-carbon) molecule.• Pyruvate is also known as pyruvic acid.• Glycolysis takes place in the cytoplasm of cells• It’s the first stage of both aerobic and anaerobic

respiration, and doesn’t need oxygen to take place. It is therefore an anaerobic process.

glucose

2 x triose phosphate

2 x pyruvate

6C

3C

3C

Number of carbons in the molecule

H2O

2ATP

2ADP

4ADP + 4Pi

4ATP

2Pi

2NAD

2NADH

2H

Hexose bisphosphate6C

Phosphorylation2 phosphates areadded

Oxidation(as Hydrogenis lost)

(Hydrolysis)

Stage 1: Phosphorylation

• Glucose is phosphorylated by adding 2 phosphates from 2 molecules of ATP to give a hexose bisphosphate.

• The hexose bisphosphate is split using water (hydrolysis)

• 2 molecules of triose phosphate and 2 molecules of ADP are created

Stage 2: Oxidation

• The triose phosphates are oxidised (lose hydrogen) forming 2 molecules of pyruvate

• Coenzyme NAD+ (a co-enzyme is a helper molecule that carries chemical groups or ions around) collects the hydrogen ions forming 2 reduced NAD (NADH + H+)

• 4ATP are produced, but 2 were used up at the beginning so there is a net gain of 2ATP

• Overall, 2 molecules of pyruvate and a small yield of ATP and reduced NAD are produced

What now?• The pyruvate (pyruvic acid) is now moved by

active transport from the cell cytoplasm into the mitochondrial matrix for the next stage of aerobic respiration

Questions

1. Outline the role of co-enzymes in the glycolysis pathway

2. Explain why the net gain of ATP during glycolyisis is two and not four

3. Explain how oxidation occurs during glycolysis, although no oxygen is involved

Questions1. Outline the role of co-enzymes in the glycolysis

pathway the coenzyme NAD accepts hydrogen atoms from the substrate molecules as triose phosphate is oxidised

2. Explain why the net gain of ATP during glycolyisis is two and not four because two molecules of ATP are used to activate hexose sugar at the beginning of the process, four ATP are made so the net gain is two

3. Explain how oxidation occurs during glycolysis, although no oxygen is involved hydrogen atoms are removed from triose phosphate and combine with NAD which is an oxidation reaction

AC

C

B

Task• Now (from memory only) on A3 paper, draw

the process of glycolysis...• Remember to leave space for the other 3

stages!!!

glucose

2 x triose phosphate

2 x pyruvate

6C

3C

3C

Number of carbons in the molecule

H2O

2ATP

2ADP

4ADP + 4Pi

4ATP

2Pi

2NAD

2NADH

2H

Hexose bisphosphate6C

Phosphorylation2 phosphates areadded

Oxidation(as Hydrogenis lost)

(Hydrolysis)

4.1.1 The Link Reaction

• state that the link reaction takes place in the mitochondrial matrix;

• outline the link reaction, with reference to decarboxylation of pyruvate to acetate and the reduction of NAD;

• explain that acetate is combined with coenzyme A to be carried to the next stage;

Glycolysis: True or Falsea) Glycolysis produces a net gain of 4 ATP moleculesb) The co-enzyme NADH is oxidised during glycolysisc) When coenzymes become reduced, they carry

hydrogen moleculesd) Glycolysis takes place in the matrix of the

mitochondrione) In glycolysis, glucose is converted in this order:

glucose, triose phosphate, hexose bisphosphate then pyruvate

f) Hydrolysis occurs when triose phosphate is converted to pyruvate

If you think they are false, you must write the correct version- and there are false ones!!!

Glycolysis: True or Falsea) Glycolysis produces a net gain of 4 ATP molecules- False: the net

gain is 2 ATPs as 2 are used upb) The co-enzyme NADH is oxidised during glycolysis- False: NAD is

reduced during glycolysis to form NADHc) When coenzymes become reduced, they carry hydrogen

molecules- False: they carry hydrogen atoms, not molecules or ions

d) Glycolysis takes place in the matrix of the mitochondrion- False: it takes place in the cytoplasm

e) In glycolysis, glucose is converted in this order: glucose, triose phosphate, hexose bisphosphate then pyruvate- False: the correct order is glucose, hexose bisphosphate, triose phosphate, pyruvate

f) Hydrolysis occurs when triose phosphate is converted to pyruvate- False: hydrolysis occurs when hexose bisphosphate is converted to triose phosphate

Respiration: The 4 Parts

Respiration consists of 4 parts:• Glycolysis• Link Reaction• Krebs Cycle• Oxidative Phosphorylation (the electron

transport chain)

The Link Reaction

• The link reaction happens when oxygen is available…• For each glucose molecule used in glycolysis, two

pyruvate molecules are made• But the link reaction uses only one pyruvate

molecule, so the link reaction and the krebs cycle happen twice for every glucose molecule which goes through glycolysis

• The link reaction happens in the matrix of the mitochondrion

pyruvate

Acetyl CoA

3C

2C

Number of carbons in the molecule

CO2

NADH

1C

NAD

Coenzyme A

(CoA)

acetate2C

Decarboxylation- removal of a carboxyl group (which becomes CO2) by the enzyme pyruvate decarboxylase

Dehydrogenation- the enzyme pyruvate dehydrogenase removes hydrogen atoms from pyruvate and NAD accepts it

The Link Reaction Converts Pyruvate to Acetyl Coenzyme A

• One carbon atom is removed from pyruvate in the form of CO2

• The remaining 2-carbon molecule (acetate) combines with coenzyme A to produce acetyl coenzyme A (acetyl CoA)

• Another oxidation reaction happens when NAD collects more hydrogen ions. This forms reduced NAD (NADH)

• No ATP is produced in this reaction

The Products of the Link Reaction go to the Krebs Cycle and the ETC

• So for each glucose molecule:• Two molecules of acetyl co enzyme A go into

the Krebs cycle• Two carbon dioxide molecules are released as

a waste product of respiration (decarboxylation)

• Two molecules of reduced NAD are formed and go into the electron transport chain (dehydrogenation)

Exam Questions

1. Describe simply how a 6-carbon molecule of glucose can be changed to pyruvate (5)

2. Describe what happens in the link reaction (4)

Answers

1. The 6 –carbon glucose molecule is phosphorylated using phosphate from 2 molecules of ATP (1) and hydrolysed/ split using water (1), to give 2 molecules of the 3-carbon molecule triose phosphate (1). This is then oxidised by removing hydrogen ions (1) to give 2 molecules of 3-carbon pyruvate (1)

Answers

2. The 3-carbon pyruvate is combined with coenzyme A (1) to form a 2-carbon molecule, acetyl coenzyme A (1). The extra carbon is released as carbon dioxide (decarboxylation of pyruvate) (1). The coenzyme NAD is converted into reduced NAD in this reaction by accepting hydrogen ions (dehydrogenation of pyruvate)

The Challenge

• Can you draw glycolysis and the link reaction….?