Aerobic Respiration

 Respiration is an exothermic reaction taken place in almost all living organisms as a way of producing ATP (adenine triphosphate) which is used in metabolic processes. There are two types of respiration; aerobic (with plentiful oxygen) and anaerobic (lack of/ no oxygen) respiration. So first of all, where does this happen?

As they say, the mitochondria is the
 power house of the cell(Roya Ann Miller)

Respiration occurs in the mitochondria of the cell. Infamously known as the power house of the cell, the mitochondria has a double membraned structure where, as studied in cell structure, it provides a number of roles. The mitochondria consists of an outer membrane, an intermembrane space, an inner membrane (folded to form cristae) and a mitochondrial matrix. 

- The outer membrane acts a protective and selective permeable membrane to allow substances into the mitochondria 

-The mitochondrial matrix acts as the site of the link reaction and the Krebs cycle

- The inner membrane is the site of the electron transport chain and it impermeable to protons therefore allows the proton gradient to be established (part of oxidative phosphorylation) 

- The intermembrane space is used as the site for protons to be stored in order to maintain the proton gradient for oxidative phosphorylation

Aerobic respiration is the most common and effective way of ATP synthesis, with 4 main stages.

1) Glycolysis 

- 1 molecule of glucose is combined with 1 molecule of ATP to form hexose monophosphate and ADP (adenine diphosphate) 

- Another molecule of ATP combines with the hexose monophosphate to form hexose bisphosphate and another molecule of ADP

- This molecule of hexose bisphosphate then splits into two molecules of triose phosphate which are oxidised through the reduction of NAD (nicotinamide adenine dinucleotide) forming two molecules of pyruvate, 2 molecule of red. NAD and 4 molecules of ATP (but 2 are used up therefore the net ATP production is 2)

2) Link Reaction

- After glycolysis, the two molecules of pyruvate enter the link reaction where they combine (separately) with Co-enzyme A (CoA) to form Acetyl CoA and releasing one molecule of reduced NAD and CO2 (per molecule of pyruvate)

3) Krebs Cycle 

- From the link reaction, the Acetyl group from the Acetyl CoA combines with a molecule of oxaloacetate to form citrate (a 6 carbon compound)

- This Citrate is decarboxylated and dehydrogenated to form a 5C compound (most of the compounds in this cycle aren't names at this level of understanding) releasing a molecule of red.NAD and CO2 

-This 5C compound is decarboxylated and dehydrogenated to form a 4C compound, again releasing CO2  and another molecule of red.NAD

- This 4C compound then releases a molecule of red.NAD, also forming another 4C compound 

-The 4C compound then forms a new 4C compound, releasing a molecule of red.FAD (flavin adenine dinucleotide) 

-The final unnamed 4C compound is then oxidised releasing another red.NAD and forming oxaloacetate (the original combined compound; hence it's called The Krebs Cycle)

4) Oxidative Phosphorylation 

-  At the inner membrane lies the Electron transport chain where the reduced NAD and FAD molecules from the previous reactions release a hydrogen atom, which in turn will split into an electron and a H+ ion. 

-The electron enters the electron transport chain (from Fe2+ ions) and travels down it, releasing energy as it travels.

- The Hydrogen ions are actively transported across the inner membrane and into the intermembrane space where a hydrogen gradient (also known as a proton gradient) is established. 

-Due to the proton impermeable, the only way of the H+ ions to travel across the membrane is via active transport by transporter/carrier proteins. This is where the enzyme ATP synthase actively pumps the hydrogen ions across, releasing energy which drives the phosphorylation of ADP into ATP. 

-The electrons from the electron transport chain will then combine with an oxygen molecule to form two O^2- ions which will ultimately combine will the transported H+ ions to from water. In this last step, oxygen is known as the final electron acceptor (accepts the final electrons for the whole process of respiration)