Cellular Respiration

4 SATGES
1. Glycolysis
  1. END Products
    1. Net gain 2 ATP and 2 Reduced NAD and 2 pyruvate molecules
  2. Description
    1. Glucose is first activated by phosphorylation to make it more reactive and to lower the activation energy for the enzyme-controlled reactions by the addition of two molecules of ATP.
      1. The glucose is converted to hexose phosphate (6-C) molecule which splits to form two molecules of trios phosphate(3-C).
        Hydrogen is removed from each of the TP molecules and transferred to two molecules of NAD forming two molecules of reduced NAD. While two TP molecules are converted into two molecules of pyruvate (3-C).
        The above two steps yield sufficient energy for the synthesis of four molecules of ATP by Substrate-level phosphorylation. There is a net gain of two molecules of ATP since two were used to phosphorylate glucose initially.
  3. Both aerobic and anaerobic
  4. Diagram
2. Link Reaction
  1. END PRODUCTS
    1. Acetyl CoA and Carbon dioxide and reduced NAD.
  2. Description (applies for each one turn of cycle).
    1. Pyruvate diffuses from cytoplasm to mitochondrial matrix.
      1. Pyruvate (3-C) decarboxylated with loss of Co2 and dehydrogenated with loss of hydrogen. These two steps convert pyruvate to acetate (2-C).
        The hydrogen released is accepted by NAD to form reduced NAD.
        Acetate (2-C) combines with coenzyme A to form Acetyl coenzyme A.
3. Krebs Cycle
  1. Description (applies for each one turn of cycle)
    1. Acetyl CoA combines with 4-C compound (Oxaloacetate) to produce a 6-C compound (Citrate). The coenzyme A is regenerated.
      1. Citrate is broken down by a series of oxidative decarboxylation reactions (six of these). Two of the steps involve decarboxylation and four of the steps involve dehydrogenation. During these reactions, tow molecules of carbon dioxide are formed and four hydrogen atoms are removed. The hydrogen atoms lost are collected by two different carriers with the formation of three molecules of reduced NAD and one molecule of reduced FAD. Also, during this one ATP molecule is produced as a result of substrate-level phosphorylation.
        After these reactions, the acetate fragment is broken down and the remaining 4-C residue undergoes conversion to regenerate the 4-C compound (oxaloacetate). It is important that this 4-C compound is regenerated so it can combine with more incoming acetyl CoA otherwise the latter would accumulate and the process will not be able to continue.
  2. Overall
    1. The function of the Krebs is a means of liberating energy from carbon bonds to provide ATP and reduced NAD (and FAD) with the release of carbon dioxide.
      1. Reduced NAD (and FAD) deliver the hydrogen to the ET system so act as triggers for this system.
    2. END PRODUCTS
      1. 2 ATP, 6 Reduced NAD, 2 Reduced FAD and 4 carbon dioxide
      2. (each turn of cycle)
        1 ATP, 3 Reduced NAD, I Reduced FAD and 2 Carbon dioxide.
4. Electron Transport Chain
  1. Description
    1. Hydrogen atoms are carried by the coenzymes NAD and FAD into the Electron Transport Chain. They are termed as hydrogen carriers.
      1. The synthesis of ATP through oxidative Phosphorylation in the electron transport chain can be explained using Chemiosmotic Theory.
        Hydrogen carrier molecules like reduced NAD donate the electrons of the hydrogens they are carrying to the first of the series of three electron carriers in the electron transport chain. The H+ ions remain in solution in the mitochondrial matrix.
        The electrons released provide energy for the first proton pump. The energy is used to pump protons from the matrix across the inner membrane to the inter-membrane space.
        The electrons pass along the chain of electron carrier molecules providing energy for each pump in turn.
        The protons accumulate in the inter-membrane space and the concentration of protons becomes higher in this space than in the matrix hence an electrochemical gradient is set up.
        Protons diffuse back into the matrix through special protein channels in the inner membrane. Associated with each channel is the enzyme, ATP Synthetase.
        As protons diffuse back, ATP Synthetase uses their electrical potential energy to produce ATP.
        At the end of the chain, the electrons and protons combine with oxygen to form water. Oxygen acts as the 'final electron acceptor'. It is essential in the process for the removal of protons and electrons. Without it the protons and electrons would accumulate and cause a 'back-up' along the chain and bring the process to a halt.
        Reduced FAD in the ETC provide energy in the same way however it passes the hydrogens directly to second pump hence the carrier system involving FAD has two pumps and yields two ATP molecules.
        Chemiosmotic Theory
        A model to explain the synthesis of ATP. The theory proposes that the energy for ATP synthesis originates from electrochemical gradient of protons across a membrane
2 Types
1. Aerobic respiration
  1. Requires oxygen and produces carbon dioxide, water and much ATP.
2. Anaerobic Respiration
  1. Takes place in absence of oxygen and produces lactate in animals and ethanol and carbon dioxide in yeast. (together with little energy---2 ATP from glycolysis).
ANAEROBIC RESPIRATION
1. When there is an absence of oxygen, there is no oxygen to combine with hydrogen to form water. Hence the electron transport chain cannot function due to the back-up along the chain and accumulation and ATP cannot be formed by oxidative phosphorylation. Since the ETC cannot function, reduced NAD cannot be deoxidised and therefore made available to pick up more hydrogen, and so the link reaction and the Krebs cycle cannot take place.
  1. Only the first stage, glycolysis can take place. This is because the reoxidised NAD is made available by anaerobic pathway.
    1. ANAEROBIC PATHWAY
      1. Animals
        Pyruvate accepts hydrogen atoms from reduced NAD and is converted into lactic acid. This reoxidises NAD which is now available for more glycolysis.
        This takes place most commonly in the muscle tissue. During vigorous exercise, the human body cannot get sufficient oxygen t the muscle cells. Hence only glycolysis can occur to produce ATP for cells to function. Pyruvate acts as the hydrogen acceptor and is converted to lactate to continue glycolysis.
      2. Little energy---2 ATP molecules produced each time.
      3. Yeast as well as plant cells (under certain conditions such as root cells in waterlogged soils)
        Alcoholic fermentation
        Pyruvate is decarboxylated to produce ethanal and carbon dioxide. Reduced NAD transfers hydrogen atoms to ethanal which forms ethanol and NAD is reoxidised.
      4. Takes place in the cytoplasm
Other
1. For Reduced NAD, each pair of hydrogen atoms yields enough energy for the synthesis of three molecules of ATP. For Reduced FAD, each pair of hydrogen atoms yields sufficient energy for the synthesis of two molecules of ATP.
  1. This is because in the ETC, reduced NAD passes hydrogen atoms to first pump in the series whereas reduced FAD passes hydrogens toms directly to second pump. Hence the carrier system involving NAD has three pumps yielding three molecules of ATP while the carrier system involving FAD has two pumps yielding two molecules of ATP.
2. Coenzyme
  1. A molecule required by some enzymes in order to function.
3. The carbon dioxide is a waste product of respiration but the hydrogen carrier molecules are a potential for an additional molecules of ATP from the ETC and act as triggers for the system.
4. Facts
  1. Prior to ETC, all ATP produced by Substrate-level phosphorylation.
  2. No direct ATP produced in link reaction.
    1. No carbon dioxide produced in glycolysis or ETC.
      1. Link reaction and Krebs Cycle run twice for each molecule of glucose. (2 pyruvate and thus two acetyl coA)
        Oxygen only needed in ETC.
        ATP only needed in beginning for phosphorylation of glucose molecules.
        Hydrogen carrier molecules like NAD are reduced in the first three stages but oxidised in the last stage of ETC.
5. ATP Synthetase
  1. As the protons move down the concentration gradient through the ATP synthetase, the energy released causes the rotor (F0) and stalk of ATP synthesise to rotate. The mechanical energy form this rotation is converted into chemical energy as Pi is added to ADP to form ATP in the catalytic head (F1 domain).
6. To determine the order of the carrier molecules in the ETC , an experiment was carried out using a series of different inhibitors.
7. Cyanide is a poison that acts as a non-competitive inhibitor of the final acceptor in the ETC (final pump). Cyanide deactivates the enzyme cytochrome c oxidase. This is the last enzyme of the electron transport chain (the final step of cell respiration). This results in the accumulation of protons and electrons thus causing causing a back up along the chain and bringing the process to a halt.
Alternative Respiratory Substrates
1. Lipids
  1. One gram of fat yields twice as much as one gram of carbohydrate.
    1. Fast are used an energy store and are used only when carbohydrate levels are low.
      1. Lipids are first split into constituent molecules of glycerol and fatty acids by hydrolysis.
        Glycerol is phosphorylated with ATP and dehydrogenated with NAD and converted into 3-C triose phosphate, which enters the glycolysis pathway and respires.
        Fatty acid chain molecules are split into 2-C fragments which enter the Krebs cycle as acetyl CoA. The large number of hydrogens released are picked up by the NAD and fed into the ET chains.
2. Proteins
  1. Rarely ever used---only when all reserves of carbohydrate and fats have depleted.
  2. Tissue protein is mobilised to supply energy. The protein of the food is diverted for energy purposes.
    1. The protein is hydrolysed into its constituent amino acids and then it is deaminated in the liver.
      1. The amino group is converted into urea and excreted while the residue is converted to either pyruvate, acetyl CoA or any other Krebs cycle intermediate and oxidised.
3. Most metabolic processes lead to Acetyl CoA which is a kind of crossroads in metabolism. As well as its formation during the oxidation of carbohydrates, it is also formed during the oxidation of fats and proteins.

Next up

Cellular Respiration

Description

Resource summary

Media attachments

Similar

	Created by wardajamshaid990 about 10 years ago