AS/A-Level Biology - Respiration

In this chapter of AS/A-Level Biology, we will learn about the importance, stages and the significance of Respiration

The Stages of Respiration

• Glycolysis- occurs in the cytoplasm which can take place in aerobic or anaerobic conditions. Glucose is broken down to two molecules of pyruvate.
• The link reaction- occurs in the matrix of the mitochondria. Pyruvate is dehydrogenated and decarboxylated `and converted to acetate
• Krebs cycle- occurs in the matrix of the mitochondria. Acetate is dehydrogenated and decarboxylated. 
• Oxidative phosphorylation- occurs on the folded inner membrane (cristae) of mitochondria. This is where ADP is phosphorylated to ATP.

Glycolysis

1. One ATP molecule is hydrolysed and the phosphate group released is attached to the glucose molecule at carbon 6, called phosphorylation
2. Glucose 6-phosphate is rearranged, using the enzyme isomerase, into fructose 6-phosphate. 
3.  Phosphorylation occurs again forming hexose 1,6-bisphosphate
4. The hexose 1,6-bisphosphate splits into two molecules of triose phosphate. 
5. Each triose phosphate is oxidised, removing hydrogen atoms using dehydrogenaseenzymes.
6. The coenzyme NAD accepts the hydrogen atoms and becomes reduced NAD.
7. Two molecules of ATP are formed, called substrate-level phosphorylation.
8. The triose phosphate molecules are converted to pyruvate, which is actively transported to the mitochondrial matrix. In the process, another two molecules of ADP are phosphorylated to make two molecules of ATP.

Kerb Cycle

1. The acetate is offloaded from coenzyme A and joins with oxaloacetate(4C), to form citrate(6C).
2. Citrate is decarboxylated and dehydrogenated forming a 5-carbon compound
3. . The pair of hydrogen atoms is accepted by the coenzyme NAD (hydrogen acceptor), which becomes reduced NAD.
4. The 4-carbon compound is decarboxylated and dehydrogenated forming a 4-carbon compound and another molecule of reduced NAD.
5. The 4-carbon compound is changed into another 4-carbon compound. During this reaction a molecule of ADP is phosphorylated to produce a molecule of ATP 
6. The second 4-carbon compound is changed into another 4-carbon compound. It's dehydrogenated and the coenzyme FAD accepts the hydrogen atoms, and becomes reduced FAD.
7. The third 4-carbon compound is further dehydrogenated and regenerates oxaloacetate and forms another molecule of reduced NAD.

Oxidative Phosphorlyation

1. NADH is reoxidised to form NAD+ and 2 hydrogen atoms, aided by the enzyme NADH dehydrogenase which is attached to the first electron carrier. The hydrogen atoms split into protons and electrons.  
2. The electrons are passed along electron carriers in the 
3. electron transport chainand lose energy by doing this.
4. The energy that was lost in the electron transport chain is used to pump protons into the intermembranespace creating a proton gradient.
5. The H+ ions diffuse through protein channels that are associated with ATP synthase, which is linked to the synthesis of ATP. The flow of protons is chemiosmosis.
6. The flow of protons through the protein channels creates a proton motive forcewhich drives the rotation of the ATP synthase enzyme attached to the protein channel. The rotation causes the phosphorylation of ADP to make ATP.
7. The electrons are passed from the last electron carrier in the chain to oxygen, which is the final electronacceptor. Hydrogenions also join forming water.

This is the end of the topic

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