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ATP
Adenosine triphosphate
Has three phosphate groups combined with the nucleotide base adenine and a ribose sugar
The presence of the phosphate groups provide ATP with its energy-releasing properties
Purpose of ATP
Immediate source of energy in a cell, a (very) short-term store
ATP drives metabolism
Glucose can be used to make ATP but it cannot release energy directly
ATP releases energy when hydrolysed
ADP/ATP interaction
ATP is synthesised from ADP (adenosine diphosphate), a molecule with two phosphate groups and inorganic phosphate
Phosphorylation
The addition of phosphate to a molecule
What happens when the terminal phosphate is removed from ATP?
Energy is released
The breakdown of ATP to ADP involves hydrolysis - the splitting of a molecule using water
This reaction is catalysed by the enzyme ATPase
What makes ATP so suitable as an immediate energy store?
The hydrolysis of an ATP molecule releases a relatively small amount of energy, allowing energy to be released in small, manageable steps during energy-requiring reactions
The hydrolysis of ATP is a single reaction involving the breaking of one bond releasing immediate energy, providing the cell with fine control over its immediate energy budget
As a small, soluble molecule, ATP can be transported around the cell easily. This enables it to be transported from mitochondria to any part of the cell
Using ATP in the cell
ATP has a role in active transport and muscle contraction
Anywhere 'work' is required, ATP is used
Provides the energy for many metabolic processes including anabolic reactions
ATP plays a role in the activation of molecules
4 stages of respiration
Glycolysis - splitting of glucose into two 3-carbon pryuvate molecules
Link reaction - conversion of the pyruvate into 2-carbon acetyl coenzyme A (acetyl CoA)
Krebs cycle - The feeding of acetyl CoA into a cycle of oxidation-reduction reactions
Electron transport chain - Use of electrons and hydrogens produced in the Krebs cycle to synthesise ATP
Where does glycolysis occur?
Cytoplasm
Initial stage of glycolysis
The activation of glucose by phosphorylation
This makes the glucose more reactive
The 2 phosphates required come from the hydrolysis of 2 ATP molecules
The phosphorylation of the glucose converts it into fructose bisphosphate
What happens in glycolysis following the phosphorylation of glucose? (Step 2)
The 6C fructose bisphosphate splits into two 3-carbon molecules of triose phosphate
The triose phosphate is oxidised through the loss of hydrogen atoms to form pyruvate
The hydrogen atoms are collected by the hydrogen carrier molecule NAD which becomes reduced to form NADH
The removal of hydrogen involves dehydrogenase enzymes in dehydrogenation
How much ATP does glycolysis provide?
In converting each molecule of triose phosphate into a pyruvate molecule 2 ATP molecules are produced
However as each glucose molecule splits to form 2 triose phosphate molecules this produces 2 ATP molecules for each of the triose phosphate molecules, a gain of 4 ATP
This gives a net gain of 2 ATP for glycolysis as 2 were initially used to activate to the glucose
Glycolysis summary
The initial stage of the cellular respiration of glucose that does not require oxygen and takes place in the cytoplasm
The reduction of NAD between the triose phosphate and pyruvate stages to give two reduced NAD (NADH)
Net gain of 2 ATP
The link reaction
The pyruvate produced in glycolysis is transported into the matrix of a mitochondrion
During the link reaction the pyruvate is converted to acetyl CoA like this:
The pyruvate is decarboxylated with the removal of one molecule of CO2
Dehydrogenation also takes place with the removal of hydrogen leading to the formation of NADH
Following decarboxylation and dehydrogenation, the resulting 2-carbon acetate combines with CoA to form acetyl CoA
Where does the Krebs cycle occur?
mitochondrial matrix
Key stages in the Krebs cycle
The 2-carbon acetyl CoA from the link reaction combines with the 4-carbon acid (oxaloacetate) to form a 6-carbon acid (citrate)
Decarboxylation of the 6-carbon acid (citrate) results in the formation of the 5-carbon acid (oxoglutarate) with the loss of a molecule of CO2
Decarboxylation of the 5-carbon acid (oxoglutarate) results in the formation of the 4-carbon acid oxaloacetate with the loss of a molecule of CO2 and the cycle continues
The reactions in the cycle involve dehydrogenation and dehydrogenase enzymes. At 3 points in the cycle hydrogen is released that subsequently reduces NAD to form NADH. At one point hydrogen is picked up not by NAD but by FAD to form FADH^2
1 molecule of ATP is produced by the transfer of a phosphate group from an intermediate compound to ADP. ATP produced in this way is called substrate-level phosphorylation
Where does the electron transport chain happen?
Based in and on the inner mitochondrial membranes (cristae)
What molecules are used in the ETC?
The hydrogen atoms collected by NAD from the dehydrogenation in glycolysis, the link reaction and Krebs cycle are carried
What happens in the electron transport chain?
The energy in the hydrogen is converted into ATP
Revisiting the mitochondrion
The more deeply infolded the cristae, and the more infoldings there are, the more extensive the ultrastructure that exists for ATP production in the mitochondrion
Downstreaming
The NAD, FAD and other coenzymes and carriers in the ETC are highly organised and arranged in a sequence of decreasing potential energy
Each carrier downstream has slightly stronger reducing power than the one before it
Therefore the hydrogens are able to move along the chain with carriers being successively reduced and oxidised as hydrogen/electrons pass along the chain in a series of oxidation-reduction (redox) reactions
The carriers of the electron transport chain
The NAD/FAD operate as hydrogen carriers
The NAD and FAD function by bringing the hydrogen to the chain
Steps of electron transport chain
Hydrogen passes along the carriers NAD, flavoprotein and coenzyme Q
Following this, the hydrogen dissociates into electrons and the ETC subsequently acts as an electron carrier
The electrons pass along the cytochromes in a series of redox reactions
Final hydrogen acceptor
Oxygen
At this stage oxygen is used in respiration
The oxygen combines with hydrogen to form water, a waste product
The final stage in the electron transport chain is catalysed by the enzyme cytochrome oxidase
What benefit does the downstreaming of carriers bring?
Energy becomes available as the redox reaction takes place. At certain points there is enough energy to produce ATP by oxidative phosphorylation
How much energy is produced in respiration?
For each reduced NAD sufficient energy is released to produce 3 ATP molecules in the ETC
Reduced FAD enters the chain further along than reduced NAD and there is only sufficient energy to produce 2 ATP
How many ATP are produced by substrate-level phosphorylation and by oxidative phosphorylation?
Substrate-level phosphorylation: 4
Oxidative phosphorylation: 34
Anaerobic respiration
Glycolysis will only continue if its products are removed and not allowed to accumulate
The pyruvate is converted to lactate in animals and ethanol in plants/yeast
The reduced NAD formed during glycolysis must be oxidised again so that NAD will be available to take up further hydrogen atoms from glycolysis
If this did not happen all the NAD would be reduced and glycolysis would stop as there would be no hydrogen acceptors available
The mopping up of these hydrogen atoms is achieved by the hydrogen being used in the reactions between the pyruvate and lactate/ethanol
Role of glycolysis in anaerobic respiration
Glycolysis is the only energy producing stage of respiration, with a net gain of 2 ATP; inefficient compared to aerobic respiration
It is a fast process; taking place throughout the cytoplasm and substances do not have to diffuse in/out of the mitochondrion in addition to it being only a short part of the normal aerobic pathway
Anaerobic respiration in animals
In animals anaerobic respiration can be advantageous
In mammals, anaerobic respiration is most likely to take place in the skeletal muscles
During strenuous exercise, the muscles will be respiring aerobically and anaerobically, and the anaerobic respiration provides extra energy
This extra energy may be enough to make the crucial difference between escaping from a predator, etc
How is oxygen debt formed?
The lactate produced by anaerobic respiration accumulates in the muscles and can cause muscle fatigue/cramp
It is removed when sufficient oxygen becomes available again and anaerobic respiration is no longer necessary
Lactate can be converted back to glucose or metabolised in other ways, processes that require O2
As the body is dealing with the lactate produced because of an oxygen shortage earlier on, the extra oxygen used to metabolise lactate is called the oxygen debt
The extra oxygen is also used to resynthesise depleted ATP
Anaerobic respiration in plants and fungi
In plants/fungi the end product of anaerobic respiration is ethanol, not lactate
Anaerobic respiration in plants and fungi produces carbon dioxide as a waste product
The ethanol is not reconverted to pyruvate but is eliminated as a waste product
Value of anaerobic respiration in plants/fungi
A significant part of most plants/fungi penetrates through soil or other substrates
Oxygen levels can often be low in these environments and the ability to respire anaerobically allows production of ATP to be maintained
The lower metabolic rate in plants/fungi compared to animals means lower ATP yield from anaerobic respiration is not as significant an issue