In-depth summary for content covered in the respiration topic of A-level AQA Biology. This will still be applicable to other exam boards, but take caution when looking at key-words and the order of specific processes.
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
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