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3.5.2 AQA A level Biology Respiration Notes
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Detailed notes covering A Level Biology Specification Reference 3.5.2 - Respiration
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3.5.2 Respiration- Photosynthesis transfers solar energy into chemical energy of carbohydrates such as glucose
- Glucose can’t be used directly by cells as a source of energy, instead they use ATP as
immediate energy source
- Formation of ATP from hydrolysis of glucose takes place in cellular respiration
o Aerobic respiration = requires oxygen and produces carbon dioxide, water and
much ATP and involves complete breakdown of glucose
o Anaerobic respiration = absence of oxygen and produces lactate (in animals) or
ethanol and carbon dioxide (in plants and fungi) but only little ATP and involves
incomplete breakdown of glucose
Process of aerobic respiration:
1. Glycolysis = splitting of 6C glucose into 2 3C pyruvate molecules
2. Link reaction = 3C pyruvate molecule enter series of reactions to form acetyl coenzyme A
(2C molecule)
3. Krebs cycle = introduction of acetyl coenzyme A into oxidation-reduction reactions cycle
yielding ATP and lots of reduced NAD and FAD
4. Oxidative phosphorylation = electrons and reduced NAD and FAD used to synthesise ATP
with water as by-product
, GLYCOLYSIS:
- First stage in aerobic and anaerobic respiration
- Occurs in cytoplasm – as glucose is too big or no transport proteins available
1) Phosphorylation of glucose glucose phosphate
o Glucose made more reactive by addition of two phosphate molecules
(phosphorylation) to be split into two
o Phosphate molecules from hydrolysis of 1 ATP 1 ADP + Pi provide energy to
activate glucose and lowers activation energy for enzyme-controlled reactions
2) Phosphorylation of glucose phosphate hexose bisphosphate
o Using phosphate from hydrolysis of 1 ATP 1 ADP + 1 Pi
3) Splitting of the phosphorylated glucose
o Each glucose molecules split into 2 3C molecules = triose phosphate
4) Oxidation of triose phosphate
o Hydrogen removed from each of 2 triose phosphate molecules and transferred to
hydrogen carrier molecule = NAD reduced NAD
5) Production of ATP
o Enzyme-controlled reactions convert each triose bisphosphate into 3C molecule =
pyruvate whilst 2 molecules of ATP are regenerated from ADP
Overall yield of one glucose molecule in glycolysis:
- +2 ATP molecules
- +2 reduced NAD molecules
- +2 pyruvate molecules
Glycolysis = indirect evolution evidence
- Enzymes for glycolytic pathway found in cytoplasm of
cells so no organelle or membrane required
- Doesn’t require oxygen
- Absence of oxygen = pyruvate converted to lactate or
ethanol in anaerobic respiration to re-oxidise NAD so
glycolysis continues
Anaerobic Respiration:
- In absence of oxygen, Krebs cycle nor electron transfer chain can continue as all FAD and
NAD will be reduced
- For continuation of glycolysis, products (pyruvate and hydrogen) must be constantly
removed
- Hydrogen must be released from reduced NAD to regenerate NAD
- NAD replenished by pyruvate molecule accepting hydrogen from reduced NAD
- Oxidised NAD produced can be used in further glycolysis
- Less ATP produced in anaerobic because lactate and ethanol isn’t completely respired so
energy not used to make ATP so still have chemical energy and there’s no oxygen which is
required in the further steps of respiration
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