Cell Respiration

Cell Respiration:
     Cellular respiration is the conversion of energy into a form usable by the cell. This energy is used for:

•      Synthesizing molecules such as proteins, DNA and RNA
•      Channels that require ATP in order to pump molecules or ion across the membrane
•      Transport within the cell of chromosomes, vesicles, fibres in muscle cells 

Each cell creates its own ATP (adenosine triphosphate) by carefully converting the energy within organic compounds to form ATP from ADP (adenosine diphosphate)and Phosphate. ATP then provides energy for the cell when it is split into ADP and phosphate.These two processes form a cycle.

Anaerobic Cell Respiration (Glycolysis)
     The energy released by cell respiration is usually from organic compounds such as carbohydrates and lipids. Carbohydrates like glucose can be converted into pyruvate in the cytoplasm of the cell to start the chain reaction that forms ATP. This reaction is named glycolysis. The glucose gives a phosphate group to an ADP to form ATP and the resulting pyruvate.
     Glycolysis is an anaerobic process meaning it requires no oxygen. This results in a small amount of ATP being made by glycolysis. Glycolysis continues with the conversion of pyruvate into lactate (lactic acid) in humans and while yeast cells can convert pyruvate into ethanol and carbon dioxide. Lactate, ethanol and carbon dioxide are all toxic and will need to be removed. This is partly why anaerobic cell respiration works at a limited rate so that the toxic substances are not overwhelming the body. .

Aerobic Cell Respiration:
     With oxygen present, pyruvate can be oxidised to release up to ten times more ATP per glucose molecule. This occurs at the mitochondria. This process also produces carbon dioxide as a waste produce, but the other by product of aerobic cellular respiration is water. Water can be used within the body and even provide enough water to hydrate small animals.

Oxidation and Reduction:
     Simply put, oxidation is the loss of electrons while reduction is the gain of electrons. These two processes work hand in hand. Where one happens the other usually follows. (Oxidation can also occur with the loss of hydrogen or oxygen atoms as shown below)

NAD (Nicotinamide adenine dinucleotide):
     NAD works as one of various electron carriers in the body that accepts and gives up electrons between the oxidizing and reducing agents. A phosphorylated version of NAD is used in photosynthesis called NADP (Nicotinamide adenine dinucleotide phosphate)

          NAD + 2 electrons ---> reduced NAD

     Chemically, NAD originally has a positive charge so it exists as NAD+. Two hydrogen atoms are then removed from the reduced substance and one of the hydrogen atoms is split into a proton and an electron. The NAD+ accepts the electron while the other proton (H+) is released. The NAD now accepts the remaining hydrogen atom, an electron and proton, to form NADH

          (NAD+)  +  (2H+)  +  2 Electrons  --> NADH   +   (H+)
or
          (NAD+)  + 2H --> NADH  +  (H+)

Similarly, the addition of oxygen atoms is called oxidation and losing oxygen atoms is reduction

How to memorize: LEO the lion has a ferocious roar 'GER'
          Lose                                 Gain
          Electrons                          Electrons
          Oxidation                          Reduction

Glycolysis:
     The conversion of a sugar molecule into pyruvate is established through a metabolic pathway.


Glucose is first phosphorylated with an enzyme called hexokinase that uses a phosphate group from each of the 2 ATP molecules. Each time it is phosphoyrlated, it gains a phosphate group from the splitting of the ATP molecules into ADP plus phosphate. The splitting of the ATP molecules also provides energy for the process. ATP turns into 2 ADP groups. The phosphorylated fructose is now called fructose diphosphate (or hexose biphosphate becuase it's a 6 carbon molecule with 2 phosphate groups) because of its two additional phosphate groups.
The fructose biphosphate goes through lysis to become two molecules of triose phosphate (3 carbon group plus a phosphate group). Each triose phosphate is is oxidized (loses 2 electron and 2 hydrogen) to become glycerate-3-phosphate. The energy released by the oxidation is used to add another phosphate group to the triose phosphate to become a triose biphosphate molecule capable of creating ATP molecules.The hydrogen is picked up by NAD+ to become NADH + (H+). (Remember this equation from above?). The two triose biphosphate molecules lose all their phosphate groups to ADP to form 4 ATP molecules and two pyruvate molecules. There is a net gain of 2 ATP since ATP is used during phosphorylation at the start of glycolysis.
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Mitochondria:
     Mitochondria are known as the energy power plants of the cell. Cellular respiration mostly occurs here and as a result most of the ATP is also created here. They have an outer smooth membrane followed by the inter membrane space and then an inner membrane. Within the inner membrane is a large space called the matrix which contains enzymes, ribosomes and some DNA (mDNA). The inner membrane has many folds that become the cristae.














Transition Reaction:
     As stated in its name, the transition reaction is the bridge between glycolysis and the Kreb's cycle. Each pyruvate created in the glycolysis process, gives off CO2 and is also oxidized (lose electrons). The oxidation reaction passes on electrons to NAD+ to become NADH + (H+). The remaining molecule is a two-carbon acetyl group. This acetyl group is attached to a coenzyme A called CoA to form Acetyl-CoA.



Kreb's Cycle:
     The Kreb's cycle is a cyclical metabolic pathway that occurs in the matrix of the mitochondria. It is also called the citric acid cycle because it begins with a citrate molecule. The cycle occurs twice per glucose molecule.
     When acetyl-CoA enters the Krebs cycle, it immediately joins with a C4 molecule to form a citrate. Oxidation then occurs three times in each cycle and three NAD+ molecules pick up the released hydrogen and electrons and a FAD+ picks up the remaining electrons and hydrogens. When two NAD+ molecules are reduced a CO2 molecule is also produced by removing a carbon from the 6 carbon citrate. This results in a 4 carbon acetyl group.  Substrate level phosphorylation also occurs by using a high energy metabolite to transfer a phosphate group to ADP to form ATP.

Electron Transport System:
     Electrons are carried through a system of carrier proteins which provide the energy need to combine hydrogen with oxygen to form water. This is also where a large quantity of ATP is produced. The ETS takes place in the cristae of the mitochondria. The carrier proteins begin by removing the electrons carried from other processes of cellular respiration by NADH and FADH. As electrons pass from a high to low energy state, more energy is released and used to build a yield of 36 or 38 ATP.
     NADH, as an electron carrier from within the matrix, release their electrons and hydrogen ions.  The electron enters the first carrier of the ETS and the carrier becomes reduced and immediately oxidized as it passes the electron to the next carrier. In contrast, the hydrogen ion is passed through the channel to the inter-membrane space. Similarly, FADH2 deposits two hydrogen ions and electrons at the second carrier. When NADH delivers electrons there is a yield of 3 ATP molecules while FADH only produces 2 ATP molecules. After releasing their hydrogen ions and electrons, the NAD+ and FAD are recycled and return to previous processes of cellular respiration to pick up more electrons. Once again, the hydrogen ions pass through the channel and the electrons enter the ETS. As the electrons move from one carrier to the next, they gradually lose energy. As a large concentration of hydrogen ions buildup in the inter-membrane space, they begin to move through the last carrier which is called ATP synthase. The hydrogen ions move from the inter-membrane space back into the matrix through ATP synthase to fuel the production of ATP. The process of hydrogen ions moving is called chemiosmosis where ions travel through a selectively permeable membrane following their concentration gradient of high to low. Any excess hydrogen ions and low energy electrons are accepted by oxygen molecules at the last carrier to form water.
Just like NAD+ and FAD, ATP can also be recycled. After the cell uses the ATP energy it returns as ADP+P. The ATP synthase binds the ADP+P together to form ATP