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How many Acetyl CoA are produced from each glucose molecule under aerobic conditions?
Two; each glucose is oxidised to two pyruvate molecules, which then become one acetyl group each (as acetyl CoA).
Describe the number of ATP that can be released from the aerobic metabolism of glucose, at each stage of the process.
2 ATP released from glycolysis, 2 from the citric acid cycle, and 32 from the Electron Transport Chain.
Where does the process for anaerobic metabolism of glucose start to differ from the process for aerobic metabolism? How many ATP are produced through this pathway, and what are the end products?
After the oxidation of glucose to pyruvate, instead of further oxidation to Acetyl CoA, the pyruvate is instead fermented to one Lactate molecule each. The only ATP produced are from the initial oxidation of glucose to pyruvate (2 ATP).
If anaerobic metabolism produces no additional ATP after the oxidation of glucose, why is it important?
Anaerobic metabolism serves the important function of regenerating NAD+ from NADH through the fermentation of pyruvate to lactate, which is important for glycolysis to continue.
Are metabolic pathways usually catalysed by their own specific enzyme or by a single enzyme which catalyses one or more processes?
Metabolic pathways occur in small steps, with each step catalysed by a specific enzyme.
What kind of reactions are involved in glycolysis pathways, the citric acid cycle and in the electron transport chain?
Redox reactions; much of the energy from the oxidation reaction is captured in the reduction reaction.
What function does NAD+ play in the glycolysis pathway, the citric acid cycle and in the electron transport chain?
NAD+ functions as a key electron carrier in these pathways.
How many steps are involved in glycolysis, and what phases can they be divided into?
There are ten steps in the glycolysis pathway, and can be divided into the preparatory phase and the pay off phase. The preparatory phase is enedergonic, using 2 ATP, while the pay off phase is exergonic, using 2 NAD+ to produce 4 ATP and 2 NADH.
What are the end products of the oxidation of pyruvate to acetyl CoA?
Carbon dioxide (1 per pyruvate) and ATP.
What functions can acetyl CoA play?
Acetyl CoA can enter the citric acid cycle, which produces NADH and FADH2, which in turn enter the electron transport chain. Alternatively, acetyl CoA also synthesises fatty acids, cholesterol and other non-essential amino acids.
Describe the mechanism of the electron transport chain.
The ETC contains four large complexes to contain electron carriers and enzymes - they are integral proteins inside the inner mitochondrial membrane. As the electrons move along the respiratory chain, they lose energy, captured by proton pumps that actively transport H+ out of the mitochondrial matrix and into the intermembrane space. This establishes a gradient of proton concentration and electric charge- the proton-motive force. The ATP synthase enzyme in the inner mitochondrial membrane is driven by hydrogen ions moving down with the concentration gradient through the enzymes (out of the intermembrane space) to form ATP.
In eukaryotes, name the cellular locations for glycolysis, fermentation, the respiratory chain, the citric acid cycle and the oxidation of pyruvate.
Glycolysis and fermentation occur outside mitochondria. The respiratory chain takes place in the inner mitochondrial membrane, while the citric acid cycle and pyruvate oxidation occur in the mitochondrial matrix.
Describe the function and mechanism of the 'uncoupling of mitochondria'.
In brown fat cells, found for example in newborn babies, electron transfer is uncoupled from ATP synthesis. The inner membrane of the mitochondria contains a protein called thermogenin, which provides protons with an alternate pathway back to the matrix, dissipating the energy of the proto-motive force as heat and warming the newborn infant.
In what ways can the respiratory chain be inhibited?
1. Inhibited ATP synthase.
2. No oxygen around to act as a final carrier for electrons at the end of the respiratory chain; the chain gets clogged up and the pathway is no longer viable for production of ATP.
3. Inhibition of electron transfer complexes; e.g. a) Rotenone (insecticide) inhibits step 1.
b) Antimycin A, used by fungi to mess with other fungi, inhibits step 3.
c) Cyanide, azide and carbon monoxide all inhibit step 4.
Discuss the differences in the function and the mechanism of glycogen mobilised in the liver, compared with glycogen mobilised in skeletal muscle.
In the liver, glycogen stores are converted to glucose 6-phosphate, which is then converted to glucose for release into the blood and transport to tissue.
In skeletal muscle, the glucose 6-phosphate is metabolised directly to obtain energy (ATP) during activity, as muscles only mobilise glycogen that they need for themselves.
Liver breaks glycogen down for the whole family, right down to glucose.
Skeletal muscle only breaks down glycogen for itself, and doesn't need to break it down all the way.
Describe the breakdown of triglycerides and the mobilisation of fatty acids. Where does this occur?
Triglycerides are broken down into glycerol and fatty acids; fatty acids are cleaved from the glycerol by triacylglycerol lipase in adipocytes (fat cells). The fatty acids then bind to albumin in the blood before disassociating from it and entering the cytosol of cells where they are needed.
Is glucose the only monosaccharide that can enter the pathway for glycolysis?
No. Other monosaccharides can enter the glycolysis pathway, but at different stages and with different preparatory processes. Glycerol can likewise also have energy released from it, converted to an intermediate in glycolysis, and then continuing down the glycolytic pathway.
Describe the location and mechanism of the beta-oxidation of fatty acids.
1. Removal of 2-carbon units from the fatty acid as acetyl CoA.
2. Acetyl CoA formed enters the citric acid cycle.
3. NADH and FADH2 transfer electrons to the electron transport chain to yield energy.
This occurs in the mitochondria.
Describe the mechanism of cholesterol metabolism.
Trick question. Cholesterol can't be digested in the gastrointestinal tract or metabolised by our cells. It's removed by transfer to the gastrointestinal tract and excreted through faeces.
Describe the formation and catabolism of ketone bodies.
Acetyl CoA formed in the liver during oxidation of fatty acids can either enter the citric acid cycle or be converted to 'ketone bodies' for export to other tissues.
Ketones are an efficient source of energy, and are oxidised through the citric acid cycle and the electron transport chain to provide energy. They are useful in that they can be used by the brain during starvation (unlike some other energy forms), as can other tissues, such as skeletal and heart muscle.
Describe the difference between catabolism and anabolism.
Catabolism is breakdown. Anabolism is synthesis. Easy enough.
What are the body's three main means of maintaining blood glucose levels?
Dietary sources, gluconeogenesis (synthesis of glucose) and glycogenolysis (breakdown of glycogen stores).
When is glycogenesis activated, what is its regulatory enzyme and what function does it serve? Where does it take place?
Glycogenesis occurs in the liver, takes pace after a carbohydrate rich meal, and functions to synthesise glycogen for energy storage. It is regulated by the glycogen synthase enzyme.
What are the advantages of glycogen being branched?
The glycogen is more soluble, and there are more non-reducing ends, thus increasing the number of sites accessible to glycogen synthase and glycogen phosphorylase, which only act on non-reducing ends. There is a fun diagram for this, ask me through facebook for fig4 in BCH008; it's also in the lecture notes.
Glycogenolysis. What does it do, where does it happen, and why is it important?
Liver glycogen breakdown, activated to release glucose between meals to maintain blood glucose concentration. Catalysed by glycogen phosphorylase.
Okay, now gluconeogenesis. How does it work, and when is it activated?
Gluconeogenesis is essentially the synthesis of glucose, used during fasting and starvation. It is essential for maintenance of blood glucose, but a slower response than glycogen breakdown. It's conceptually the reverse of glycolysis, but takes some different pathways around irreversible steps. It needs a source of energy and carbon atoms; energy comes from fatty acid metabolism, and carbon skeletons come from amino acids, lactate and glycerol.
How can lactate be used in gluconeogenesis?
Lactate can function as a carbon contributor to the synthesis of glucose; this is the only metabolic pathway which uses lactate, and is known as the Cori cycle. It allows hepatic recycling of lactate (produced in anaerobic glycolysis) back to glucose.
When would amino acids be metabolised in the liver?
Only when you're pretty desperate for energy. Amino acids serve other important functions and are a bit of a last resort- it's a bit like chopping your house down to use firewood. Amino acids (like your house) aren't intended to be used for energy (firewood), and serve other important functions (keeping you and your ABBA CDs dry), but it'll do the job if you really need it to.
How does amino acid catabolism occur, and where does this take place?
Amino acid catabolism in the liver involves two major processes: removal and excretion of the amino group through transamination and deamination, and oxidation of the carbon skeleton. In short,the first step is to react the amino acid with alpha-ketoglutarate, which takes on the amino group and becomes L-glutamate (transamination), which donates the amino group to excretion pathways for nitrogenous waste, e.g. as ammonia (deamination). The carbon skeleton can be used as a source of energy, e.g. in the citric acid cycle.
Generally speaking, how are drugs and toxic compounds metabolised in the liver?
Liver is the major site of metabolism of drugs and toxic compound, which are converted from fat soluble to water soluble substances and excreted in urine or bile. Takes place in two phases; modification and conjugation, both with the aim of making the compound more hydrophilic, reactive, soluble in water and easy to excrete.
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