Biochemistry of respiration

Raw materials…

Any organic molecule

Carbohydrate

Lipid

Protein

They contain…

…too much chemical energy

So, respiration involves…

…breaking down large complex organic molecules to produce a molecule that contains a small amount of energy

ATP

ATP is an immediate source of energy because:-

Only one step is necessary to release the energy

The amount of energy released is small enough to be useful.

Aim of respiration

Synthesis of ATP

Carbohydrate is the major substrate for respiration

Soluble carbohydrates exist – so they can easily be transported

Insoluble carbohydrates exist – so they can be easily stored

Overview

Four stages:-

1. Glycolysis2. Link reaction3. Krebs cycle4. Hydrogen / electron carrier system

GLYCOLYSIS

Occurs in cytoplasm

Starts with Glucose

6 carbon compound

GLYCOLYSIS

Ends with Pyruvate

3 carbon compound

Glucose to TP

2 molecules of ATP are invested

TP to Pyruvate

Triose phosphate is converted to pyruvate

(there are a number of intermediate stages)

Energy is recovered…

ATP is made

Hydrogen is released and NAD is reduced

Over all…

Net production of 2 ATP molecules from each molecule of glucose

and

2 reduced NAD molecules

Pyruvate moves into the mitochondria

Precisely…

Into the matrix

LINK REACTION

Pyruvate is converted into

acetyl coenzyme A

Pyruvate

3 carbon compound

Acetyl coenzyme A

2 carbon compound

Oxidative … but no oxygen is involved

Hydrogen is removed Combines with NAD Producing reduced NAD

KREBS CYCLE

Acetyl Co A (2 carbon compound) Combines with a 4 carbon compound Producing a 6 carbon compound

In a number of steps…

6 carbon compound breaks down to form the same 4 carbon compound that combined with acetyl co A

Hence the Krebs CYCLE

What is released?

Carbon – in the form of carbon dioxide

Hydrogen – combining with NAD and another coenzyme, FAD to form

reduced NAD and reduced FAD

Anything else?

There is enough energy released in one of the steps to join ADP and Phosphate to make ATP

ELECTRON / HYDROGEN CARRIER SYSTEM

Carriers are in the inner membrane

Cristae

Hydrogen atoms

From hydrogen carriers –

reduced NAD and reduced FAD

Accepted by the carriers in the membrane

Hydrogen atoms break down to electrons and hydrogen ions (protons)

Protons build up between the two membranes of the mitochondria

Protons move back into the matrix

Through an enzyme – ATP synthetase

ADP and phosphate join forming ATP

The protons rejoin with electrons and oxygen to form water

Oxidative phosphorylation

Phosphate is added to ADP –

phosphorylation

The energy for the process is the result of the removal of hydrogen – oxidation

Role of oxygen

Oxygen is the terminal electron acceptor in the electron carrier system

‘Oxidation’

Oxidation occurs more than once in respiration

BUT In glycolysis the link reaction Krebs cycle

It is the removal of hydrogen

‘Oxidation’

Only in the electron carrier system is oxygen needed

Aerobic and anaerobic respiration

Fermentation

Anaerobic respiration

Occurs when oxygen is limited

NAD is oxidised

Hydrogen combines with pyruvate

Forming :-

lactate in animals

ethanol in plants / fungi

Glycolysis can continue…

Producing some ATP

Forming 2 ATP per glucose molecule

Compared with 38 ATP per glucose molecule with aerobic respiration