The difference in concentration of hydrogen ions means they can leak back into the inner compartment. The only route available is through the middle of the stalked granules called the ATPase enzymes. As the stream of hydrogen ions flows down the concentration gradients, enough energy is released to allow free inorganic phosphate molecules to be added to ADP, forming new molecules of ATP. This is called oxidative phosphorylation. The end product of the electron chain is spare electrons and hydrogen’s which combine with oxygen to form H2O.
Glycolysis
Glycolysis (glyco: - sugar; lyso: - breakdown) describes the breakdown of a hectose sugar usually glucose into two molecules of the three carbon compound pyruvate and it occurs in all cells. In anerobic organisms it is the only stage of respiration.
Initially the glucose is not very reactive and so it is phosphorylated prior to being split into the two-triose sugar molecules. These molecules give up some hydrogen atoms, which can if needed be used to give energy (ATP) before being converted into pyruvate. It is during this formation cycle that the ATP used in the phosphorylating process of the glucose is regenerated. This glycolysis takes place in the cytoplasm of the cell.
A glucose molecule is phosphorylated to make it more reactive. The phosphate molecule charged by 2 ATP and with the help of an enzyme changes the glucose into Fructose-1, 6-phosphate, where the phosphate has become attached to the 1st and 6th carbon in the glucose.
After this step the Fructose-1, the enzyme Triose phosphate isomerase, forming two glyceraldehyde phosphates, splits 6-diophosphate I half. This becomes attached to the third carbon of glyceraldehyde.
The next two stages show energy released from the glyceraldehyde–3 – phosphate, producing NADH and ATP. Since there are 2 glyceraldehyde–3 – phosphates the 2 ATP used earlier to start the reaction have been returned, plus an extra 2NADPH have been made.
There is a change to the glyceraldehyde – 3 – phosphates with the addition of another organic phosphate, which generates ATP.
While the last organic phosphate grouping makes some more ATP.
At the end of the glycolysis cycle 2 ATP were used to start the reaction, though 4 ATP and 2 NADH were made, which gives a net product of 2 ATP and 2 NADH, with the energy being released from the glucose.
Pyruvate Oxidation
This is the “link” between glycolysis and the Krebs cycle; this occurs where with the help of oxygen the pyruvate molecules move from the cytosol to the mitochondrial matrix. With the presence of oxygen the pyruvate molecules are broken-down to form carbon dioxide and hydrogen. The CO2 is removed as a waste product, the hydrogen atoms are oxidised to water thus yielding a huge amount of energy. Before the pyruvate enters the Krebs cycle it combines with a compound called coenzyme A to form acetyl coenzyme A.
In this process a molecule of carbon and a pair of hydrogen atoms are removed. The 2-carbon acetyl coenzyme A now enters the Krebs cycle.
The Krebs cycle
The Krebs cycle (diagram A) also known as the, Tricarboxylic acid cycle (TCA cycle) is the process where living cells breakdown the organic fuel molecules in the presence of oxygen. This process is carried out in the matrix of the mitochondria.
The Krebs cycle plays a central role in the breakdown of organic fuel molecules like glucose. Before the glucose molecule can enter the cycle they must be degraded into a two – carbon compound called acetyl coenzyme A. Once fed into the cycle the acetyl coenzyme A is converted into carbon dioxide and energy.
The cycle is initiated when acetyl coenzyme A reacts with oxaloacetate to form citrate and releases the coenzyme A.
In a succession of reactions the citrate is rearranged to form isocitrate; during this process the isocitrate loses a molecule of carbon dioxide and then undergoes oxidation to form the alpha-ketoglutarate.
NADH+ NADH
Citrate Alpha Ketoglurate (5C)
CO2
Alpha-ketoglutarate in turn loses a molecule of carbon dioxide to form succinyl coenzyme A. This is then enzymatically converted into Succinate.
NADH +NADH ADP ATP
Alpha Ketoglurate Succinate (4C)
CO2
Succinate is then oxidised to form fumarate and is hydrated to form malate.
FAD+ FAD
Succinate Malate (4C)
The cycle is completed when malate is oxidised to form oxaloacetate.
NAD+ NAD
Malate (4C) Oxabacetatate (4C)
Each complete turn of the cycle results in the regeneration of oxaloacetate and the formation of two molecules of carbon dioxide.
Energy is produced during a number of the cycle reactions, and the reaction between alpha-ketoglutarate and succinate produces a molecule of adenosine triphosphate (ATP), this is the molecule, which powers the cellular functions. Most of the energy obtained from the Krebs cycle is captured by the compounds nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD), which is later, converted into ATP.
The energy transfers occur through a relay of electrons by chemical reaction, this is known as oxidation and reduction, where oxidation involves the loss of an electron and reduction is the addition of an electron.
For each turn of the cycle three molecules of NAD+ are reduced to NADH and one molecule of FAD is reduced to FADH2. These molecules transfer their energy to the electron transport chain, which is the next stage of cell respiration.
The German born British biochemist Sir Hans Adolf Krebs proposed this cycle in 1937. For his work he received the 1953 Nobel Prize for Physiology or Medicine.
Electron Transport System
The electron transport system (diagram B) is the means by which the energy from the Krebs cycle, of the form of hydrogen atoms, is converted to ATP. The hydrogen atoms attached to the hydrogen carriers NAD and FAD are transferred to a chain of other carriers at progressively lower energy levels (diagram C). As the hydrogen’s pass from one carrier to the next, the energy released produces ATP. These series of carriers are called the respiratory chain. The carriers include NAD, flavoprotein, coenzyme Q and iron containing proteins called cytochromes.
The hydrogen atoms are passed along the chain and are split into their protons and electrons, and only the electrons pass from carrier to carrier from which the system’s name is derived. At the end of the chain the protons and electrons recombine, and the hydrogen atoms which are created link with oxygen to form water (H20). This formation of ATP through the oxidation of the hydrogen atoms is called oxidative phosphorylation, which occurs in the mitochondria.
The role of oxygen is to act as the final receptor of the hydrogen atoms. While it only performs this function at the end of the respiration cycle, oxygen is vital because it drives this whole process.
The transfer of hydrogen atoms to oxygen is catalysed by the enzyme cytochrome oxidase; this enzyme is inhibited by cyanide, which in effect prevents the removal of hydrogen atoms at the end of the respiratory chain.
This complex chain of events, the basis of the cells ability to derive ATP from metabolic oxidation, was conceived by the British biochemist Peter Mitchell in 1961. In the years following the announcement of his chemiosmotic theory saw its ample substantiation and revealed its profound implications for cell biology.
ATP
Adenosine triphosphate is the short-term energy store of all cells. It is easily transported and is therefore the universal energy carrier. ATP is formed from the nucleotide adenosine Monophosphate by the addition of two further phosphate molecules (diagram C).
The Importance of ATP
The enzyme ATPase catalyses the hydrolysis of ATP to ADP and the removal of the phosphate produces a quantity of free energy.
The further hydrolysis of ADP to AMP produces a similar amount of energy, though the removal of the last phosphate to give adenosine produces only a small amount of energy. The first two phosphate bonds are usually called the high-energy bonds, due to the large quantity of energy released during the hydrolysis process.
AMP and ADP can be reconverted to ATP by adding phosphate molecules, this is called phosphorylation, and there are two main forms of phosphorylation, photosynthetic and oxidative. The former occurs during photosynthesis while the latter occurs during the cellular reproduction of aerobic cells.
The importance of ATP is as a means of transferring free energy from energy rich compounds to the cellular reaction, which require it. ATP is not the only substance to move energy this way, though because it is the most abundant it is therefore the most important.
ATP is the source of energy for.
- The anerobic process where it provides the energy to build up the macromolecules from their component units.
- Provides energy for the many forms of movement, muscle contraction cillary action and spindle action in cell division.
- As an active transporter it provides the energy required to move the materials against a concentration gradient.
- It is required to form the vesicles necessary in the secretion of cell products.
- It makes chemicals more reactive enabling them to react more readily e.g. the phosphorylation of glucose at the start of the glycolysis cycle.
Diagram A
Stages of Cellular Respiration
Diagram B
The Electron Transport System
NAD = nicotinamide adenine
dinucleotide
FAD = flavin adenine dinucleotide
FP = flavoprotein
CoQ = coenzyme Q
ATP = adenosine triphosphate
- Cytochrome b
- Cytochrome c
- Cytochrome a
Diagram C
The Structure of ATP
O = Oxygen
H = Hydrogen
N = Nitrate
C = Carbon
P - Phosphate
