Let me sum it up

Respiration is the metabolic process in animals and plants in which organic substances are broken down to simpler products with the release of energy, which is incorporated into special energy-carrying molecules, ATP (adenosine triphosphate) and subsequently used for other metabolic processes.

The outline of aerobic respiration in eukaryotes are as follows, with glucose being the respiratory substrate.

First, glycolysis occurs in the cytoplasm. Glycolysis the chemical pathway by which a glucose molecule is first phosphorylated, rearranged, cleaved into 2 separate molecules, oxidized and dephosphorylated, yielding 2 pyruvate molecules, a net gain of 2 ATP molecules and 2NADH molecules per glucose molecule.

Pyruvate and NADH produced enters the mitochondria. On passing through pyruvate dehydrogenase, which is an enzyme complex consisting of either 60 or more than 60 subunits, which I don't really remember, pyruvate is decarboxylated (releasing a CO2 molecule) and oxidized (by NAD+, thereby forming NADH) )to an acetyl group, which is bonded with Coenzyme A forming Acetyl-CoA. The acetyl unit then enters the Krebs cycle, whereas the CoA unit recycled for another oxidation of pyruvate. For STPM purposes, the decarboxylation and oxidation of pyruvate is referred to as the link reaction, from the perceived "linking" of glycolysis and Krebs cycle.

Krebs cycle is a cyclic series of biochemical reactions which can be divided into 3 segments:

Segment A : condensation reaction between oxaloacetate and the acetyl group, forming citrate. The cycle is also called citric acid cycle or less commonly tricarboxylic acid cycle for the first product (citrate) formed in the cycle.

Segment B : Citrate rearrangement and decarboxylation. Two CO2 molecules produced per pyruvate molecule that enters the Krebs cycle. NAD+ is reduced to NADH.

Segment C : Regeneration of oxaloacetate.

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What the hell with all the cycle cycle, pathway pathway thing? It is to extract electrons and hydrogen ions from the substrate molecules. The electrons extracted and carried by NADH and FADH2 are released into the ELECTRON TRANSPORT CHAIN, which is a series of membrane-associated proteins. By passing the electron pair from one protein to the next more electronegative one, the energy harnessed from the electrons are used to actively pump protons (H+) into the intermembraneous space from the matrix, against a concentration gradient.

The electrons is finally passed to OXYGEN, which combines with H+ to form water. Now, this is why you need oxygen! It acts as the final electron carrier. Walau A, all this time before studying up till here, I was like --- why the hell am I breathing in oxygen since it doesn't even show up in glycolysis and the Krebs cycle.

OK, so, the proton gradient created across the inner mitochondrial membrance allow passive transport of H+ ions to occur, that is letting the protons to diffuse into the matrix again. Why so kapsiao? Because the protons mostly pass through the membrane-embedded enzyme - ATP synthase. This enzyme couples the reentry of protons to the phosphorylation of ADP to ATP. The passage of protons alters the conformation (3D-shape) of the ATP synthase, thereby catalyzing the formation of ATP from ADP and Pi.

lalala.

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Biology at form 6 level is pretty much still like Greek Mythology.

Summary written based on and with referrence to:
Biology (Eigth edition) (2008), Raven, Johnson, Losos, Mason, Singer.
Publisher: McGraw Hill