(A-level) AQA Biology Respiration Topic Summary

Oliver Dyson


Respiration Notes (12)

Respiration is the name of the process performed by organisms to produce ATP

-> and so provides energy for metabolism

 Aerobic respiration – requires oxygen and produces carbon dioxide, water and a lot of ATP
 Anaerobic respiration – takes place without oxygen

Anaerobic produces lactic acid in animals

-> but ethanol and carbon dioxide in fungi, yeast and plants

This can be split up into 4 stages:

 Glycolysis – the splitting of the 6C glucose into two 3C pyruvate molecules
 Link reaction – 3C pyruvate molecules react to for acetyl coenzyme A (a 2C molecule)
 Krebs cycle – introduction of this product into a cycle of oxidation and reduction

This yields some ATP and a large amount of NADH and FADH

 Oxidative phosphorylation – using electrons of NAD and FAD synthesises ATP and water



Glycolysis:

This is the initial stage in aerobic and anaerobic respiration

-> and occurs within the cytoplasm of all living cells




Figure 1 – reproduced from [1]

This process breaks down a hexose sugar (usually glucose) is split up

-> into two molecules of the 3C molecule pyruvate

 Glucose is first phosphorylated to glucose phosphate – this makes it more reactive by adding
two phosphate molecules

These phosphate molecules come from the hydrolysis of two ATP molecules – the energy is used to
activate glucose, and lower the activation of the energy for the enzyme-controlled reactions later

 The phosphorylated glucose is split up into two triosephosphate molecules (TP)

, Oliver Dyson


 The TP is the oxidised – this removes the hydrogens
to a molecule called NAD to form reduced NAD
 ATP is the produced by enzyme – controlled
reactions which convert each TP molecule into
another 3C pyruvate molecules

This reforms two molecules of ATP from ADP

Energy yields from glycolysis:

The overall yield from one glucose molecule is therefore:

 Two molecules of ATP
 Two molecules of reduced NAD (NADH)
 Two molecules of pyruvate

Glycolysis is present within every living organism so is an
indirect piece of evidence for evolution

-> requires no membranes / oxygen

 Only enzymes / cytoplasm
Figure 2 – reproduced from [1]


Link Reaction:

The pyruvate molecules produced by glycolysis possess potential energy which can be utilised in the
Krebs cycle

-> however they must first be oxidised via the link reaction

 This involves the active transport of the pyruvate into the matrix of the mitochondria
 Once here, the pyruvate is oxidised to acetate

This involves the pyruvate losing a molecule of CO 2 and two hydrogens

 This forms NADH as products (and CO2 as waste)
 The two-carbon acetate combines with a molecule called coenzyme A (CoA)

This produces acetylcoenzyme A (2C compound)

Pyruvate + NAD + CoA -> acetyl CoA + NADH + CO 2

This reaction occurs twice for each glucose

-> since two pyruvate molecules form via glycolysis



Krebs Cycle:

Involves a series of oxidation and reduction reactions in the matrix of the mitochondria

-> since the enzymes necessary are present

 The 2C acetylcoenzyme A combines with a 4C molecule called oxaloacetate
 This produces a 6C compound – this loses CO 2 and causes NADH, FADH and ATP to form