Lecture Six - Energy transformations in cells (respiration) Flashcards
What are the two major types of metabolism?
Autotrophic: In which complex materials are built up from simple, inorganic materials using energy from sunlight or reduced inorganic forms of elements. Heterotrophic: Metabolism in which the energy inherent in organic matter is released as organic compounds such as sugars and fats are broken down to inorganic matter (respiration).
What is the relationship between heterotrophic metabolism and catabolism?
The breakdown of food by heterotrophic metabolism takes place in a number of stages know collectively as catabolism.
What is the simple sequence of reactions that occurs during respiration?
Glucose -> glycolysis -> Krebs cycle -> respiratory e- transport chain -> 6CO2 + energy.
Describe the sequence that takes place during aerobic glycolysis.
Takes place in the cytosol of cells. The first five reactions are priming reactions used to raise the energy level of the intermediates involved. ATP is expended during conversion of glucose to glucose-6-P and fructose-6-P and fructose-6-P to fructose-1,6-P subscript 2. By the end of the priming sequence, one glucose molecule has been converted to two molecules of glyceraldehyde-3-P (G3P). The breakdown of the sugars Glyceraldehyde-3-P to 1,3 bis-phosphoglyerate sees energy being produced (energy pay off stage). Substrate level phosphorylation involves the energy of the exothermic reaction (e.g. 1,3-biphosphoglycerate to 3-phosphoglycerate) being sufficient to directly transfer the phosphate from the substrate to ADP. A second substate level phosphorylation completes glycolysis (PEP -> Pyruvate). Net result of glycolysis: 1 glucose -> 2 pyruvate. Involves the expenditure of 2 ATP on the priming reactions and the formation of 4 ATP and 2 NADH in the energy return phase. Net energy output of 2 ATP and 2 NADH.
Describe the sequence of reactions that takes place during anaerobic glycolysis.
If oxygen is not present, conditions are anaerobic and glycolysis halts as there would be no NAD+ to support further conversion of G3P to BPG. When there is no O2, cells can regenerate NAD+ through to mechanisms: 1) The conversion of pyruvate to lactate which allows NADH to be oxidised and NAD+. 2) The conversion of pyruvate to ethanol. Also allows NADH to be oxidised to NAD+. In both cases, the energy yield of glycolysis is only 2ATP/glucose molecule (no NADH).
Describe the sequence of reactions that take place during the Krebs cycle.
If oxygen is present, NAD+ will be available and pyruvate can enter the mitochondria for further metabolism. During the Krebs cycle, pyruvate is completely oxidised into carbon dioxide. For every pyruvate entering the cycle, the cycle regenerates 4 NADH, 1 FADH2 and 1 ATP. The reactions involving the release of carbon dioxide are known as oxidative decarboxylations. The ATP in the Krebs cycle comes from substrate level phosphorylation.
Describe the sequence of reactions that occur during the electron transport chain.
The reductant NADH and FADH, generated by the Krebs cycle, can be oxidised through the electron transport chain in a series of reactions leading to the consumption of oxygen. Transport, down the electron transport chain of electrons from NADH and FADH releases energy and results in a proton gradient. The energy from the e- transport is used to create a proton gradient (proton movie force) across the inner mitochondrial membrane. The proton motive force drive the phosphorylation of ADP to ATP. The coupling for the proton gradient and its use in the chemical reaction of ATP formation is known as chemiosmosis. The hydrogen ions flow back through the ATP synthase complex causing the cylinder to rotate. This activates the catalytic site causing ADP and Pi to combine to form ATP.
sufficient protons are moved during the oxidaton of NADH to allow the formation of three ATP.
Note that in the case of FADH, oxidation is coupled to the formation of 2 ATP.
For each pyruvate entering the Krebs cycle
4NADH -> 4x3 = 12ATP
1 FADH -> 2 ATP
1 ATP -> 1 ATP
2 pyruvate are formed per glucose entering glycolysis
Therefore the yield per glucose is twice this i.e. 30 ATP.
Summerise the inputs and outputs of aerobic respiration.
Glycolysis produces 2 ATP (substrate level phosphorylation) and 2 NADH in the cytosol.
The NADH can enter the mitochondria where it is oxidised to 2 x 3 = 6 ATP.
The transport process (of NADH) consumes 2 ATP.
(in some cases 1 ATP – depends on shuttle mechanisms)
The net gain is 4 ATP + 2 ATP from substrate level phosphorylation.
Total yield is 36 ATP per glucose.
