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Biochemistry Y1 > Cellular Respiration > Flashcards

Cellular Respiration Flashcards

1
Q

What is catabolism and anabolism?

A

Catabolism - energy obtained through oxidation of carbohydrates, fats and proteins to smaller molecules, producing energy stored as ATP.

Anabolism - complex molecules are synthesised from simple precursors. Reactions require energy as ATP.

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2
Q

What is a metabolic pathway?

A

A series of linked enzyme catalysed reactions. Not a distinct linear series, as some important metabolic intermediates are shared by different pathways, so metabolism is a complex web of pathways.

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3
Q

What is the role of ATP in cellular respiration?

A

ATP hydrolysis used to drive energetically unfavourable/positive Gibbs free energy reactions.

ATP > ADP + Pi = -delta G
ADP + Pi > ATP = + delta G
So energy is input

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4
Q

What are coenzymes?

A

Molecules that donate an organic molecules to an enzyme in catalysis and generally act as a carrier for chemical groups driving the reaction.

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5
Q

What is NAD and its role?

A

Nicotine adenine dinucleotide. +NAD = oxidised form and NADH = reduced form.

Is an important coenzyme for many dehydrogenases in catabolic pathways.

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6
Q

What is FAD and its role?

A

Flavin adenine dinucleotide. FAD = oxidised form and FADH2 = reduced form.

Coenzyme used only by some dehydrogenases and is always covalently linked to its dehydrogenases.

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7
Q

What is the role of enzymes as biological catalysts in metabolic pathways?

A

1 or 2 key enzymes that determine flux and catalyse the rate determining steps of the reaction.

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8
Q

What are the basic control mechanisms in metabolism?

A
  • Separation of synthetic and oxidative pathways: coordinated regulation to prevent futile cycling/cell wasting resources and gaining nothing.
  • Control of rate limiting steps: controlled by long term strategies - controlling amounts of enzymes or their subcellular locations. Controlled by short term strategies - phosphorylation, allosteric control and binding to regulatory proteins.
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9
Q

How is glucose taken up by cells?

A

Glucose transporters, GLUT, transmembrane proteins that catalyse facilitative diffusion of glucose down a concentration gradient. Concentration gradient maintained by rapid cellular metabolism of glucose.

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10
Q

What are the locations and characteristics of the 4 GLUT isoforms?

A

GLUT 1 = not cells, including red blood cells. Intermediate Km

GLUT 2 = liver and pancreatic beta cells. High Km

GLUT 3 = mainly brain. Low Km

GLUT 4 = primarily muscle and adipose tissue. Intermediate Km, insulin sensitive

The lower the Km, the higher the affinity.

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11
Q

What is glycolysis?

A

Oxidation of glucose:
C6H12O6 + 2NAD + Pi > 2pyruvate + 2NADH + 2ATP + 2H2O
Consists of 10 enzyme catalysed steps, split into prepatory and payoff phases.
Can be done in aerobic or anaerobic conditions.

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12
Q

Describe the prepatory phase of respiration.

A
  1. Glucose phosphorylated to glucose 6-phosphate by hexokinase using ATP.
  2. Glucose 6-phosphate > fructose 6-phosphate by glucose 6-phosphate isomerase.
  3. Fructose 6-phosphate > fructose 1,6-bisphosphate by phosphofructokinase using an ATP.
  4. Fructose 1,6-bisphosphate > glyceraldehyde 3-phosphate + dihydroxyacetone phosphate by aldolase.
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13
Q

Describe the payoff phase of glycolysis.

A
  1. Dihydroxyacetone phosphate > glyceraldehyde 3-phosphate by triose phosphate isomerase.
  2. 2 x glyceraldehyde 3-phosphate oxidised into 1,3-bisphosphoglycerate by glyceraldehyde 3-dehydrogenase, reducing 2 +NAD.
  3. 2 x 1,3-bisphosphoglycerate reduced to 2 x phosphoglycerate + 2ATP by phosphoglycerate kinase.
  4. 2 x phosphoglycerate > 2 x phosphoenolpyruvate + 2H2O by enolase.
  5. 2 x phosphoenolpyruvate > pyruvate + 2ATP by pyruvate kinase.
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14
Q

Name the 3 rate-determining steps of glycolysis.

A

Hexokinase
Phosphofructokinase
Pyruvate kinase

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15
Q

Describe hexokinase rate determining step.

A

Hexokinase has a relatively high affinity to glucose and is inhibited by glucose 6-phosphate product in allosteric regulation.

Hexokinase also normally controlled by glucokinase, produced by liver and pancreatic beta cells. Glucokinase affinity is lower than hexokinase so glucose phosphorylation is only catalysed in beta cells when blood glucose concentration is relatively high.

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16
Q

Describe phosphofructokinase rate determining step.

A

Complex regulation that allows glycolysis to respond to cellular energy demand. Fructose 1,6-bisphosphate activator and citrate and proton inhibitors.

Insulin activates phosphofructokinase gene expression and glucagon represses phosphofructokinase gene expression.

17
Q

Describe pyruvate kinase rate determining step.

A

ATP and alanine allosterically inhibit. Allosterically activated by fructose 1,6-bisphosphate.

Hormonally controlled: insulin activates and dephosphorylates, glucagon inhibits and phosphorylates.

18
Q

What is the fate of pyruvate in anaerobic conditions?

A

Converted to lactate in order to regenerate NAD+ for glycolysis to continue. This generates 2 ATP molecules per glucose. Lactate must be removed, transported out of the cell and metabolised by the liver, to prevent acidosis.

19
Q

Where is anaerobic glycolysis important?

A
  • Red blood cells, as they do not have mitochondria
  • Anaerobically respiring skeletal muscle in exercise
  • Tumour cells
  • Hypoxic conditions
20
Q

What is the fate of pyruvate in aerobic conditions?

A

Pyruvate is oxidised to acetyl-CoA by pyruvate dehydrogenase, a decarboxylation reaction in mitochondria.

21
Q

How is pyruvate dehydrogenase controlled?

A

Inhibition:

  • Product pyruvate in allosteric inhibition to reduce amount wasted in cells
  • Phosphorylation by kinases stimulated by product alpha-CoA and NADH
  • When ATP levels of high

Activation:
- Phosphatase phosphorylates and activates, and is stimulated by calcium ions and insulin.

22
Q

What is the importance of acetyl-coenzyme A as a metabolic intermediate?

A

Acetyl CoA is oxidised in Krebs/citric acid/tricarboxylic acid cycle. It regenerates NADH and FADH2.

23
Q

Describe the complete oxidation of acetyl-coenzyme A.

A
  1. ACoA + oxaloacetate > citrate by citrate synthase in a condensation reaction.
  2. Citrate > isocitrate by aconitase in molecular rearrangement.
  3. Isocitrate > alpha-ketoglutarate + CO2 + NADH by isocitrate dehydrogenase in decarboxylation.
  4. Alpha-ketoglutarate > succinyl CoA + CO2 + NADH by alpha-ketoglutarate dehydrogenase in decarboxylation.
  5. Succinyl CoA > succinate by succinyl thiokinase. CoA group is hydrolysed and this energy drives GDP + Pi > GTP, where GTP is energetically equivalent to ATP.
  6. Succinate > fumarate + FADH2 by succinate dehydrogenase.
  7. Fumarate > malate by fumarase in molecular rearrangement.
  8. Malate > oxaloacetate +NADH by malate dehydrogenase.
24
Q

How is the Krebs cycle regulated to meet cellular demands?

A

3 rate determining steps:

  • Citrate synthase: allosteric inhibition by NADH, succinyl CoA, citrate and ATP. Allosteric activation by ADP.
  • Isocitrate dehydrogenase: allosteric inhibition by ATP. Allosteric activation by ADP and calcium.
  • Alpha ketoglutarate dehydrogenase: allosteric inhibition by its products and is sensitive to calcium levels.
25
Q

What is the importance of the Krebs cycle in regenerating reactive intermediates?

A

Regenerates oxaloacetate and reduced cofactors to drive ATP synthesis by oxidative phosphorylation.

26
Q

What is the role of creatine phosphate/phosphocreatine in short term energy generation?

A

Creatine phosphate + ADP > creatine + ATP by creatine kinase in muscles.

Blood creatine levels used as a measure of kidney function.

27
Q

Describe the structure of mitochondria.

A

Outer membrane - has protein pores so is permeable.

Inner membrane - highly impermeable, high folded (cristae) to increase surface area. Contains many transport proteins and other transporters part of electron transport chain.

Intermembrane space with a matrix containing enzymes and ribosomes.

28
Q

What is the function of mitochondria?

A

Synthesise ATP by oxidative phosphorylation, involving oxidation of NADH and FADH2 in electron transport chain.

29
Q

Describe electron transport chain.

A
  1. NADH oxidised by complex I/NADH dehydrogenase, producing NAD+. Electrons from NADH move through complex I.
  2. This movement drives activate transport of protons from matrix to intermembrane space.
  3. The electrons from complex I are transported by electron carrier, ubiquinone, to complex III/cytochrome C reductase. Ubiquinone can also transport electrons from FADH2 oxidation at complex II/succinate.
  4. Electrons move through complex III and drive active transport of protons.
  5. Complex III donates electrons to small inner membranous protein, cytochrome C.
  6. Cytochrome C transports electrons to complex IV/cytochrome C oxidase.
  7. Electrons move through complex IV and are donated to 2 oxygens (terminal electron acceptor) to form water.
  8. Movement of electrons drives active transport of protons.
30
Q

How is ATP synthesised using proton motile force?

A

PMF due to electrical (150mV) and pH gradient (0.5pH).

  1. Protons moving through ATPase to matrix.
  2. Protein ‘stalk’ rotates, changing conformation of beta subunits, driving ATP synthesis. Requires 4 protons for 1 ATP.
31
Q

How many ATP molecules can NADH and FADH2 produce?

A

NADH can produce 10 protons so 2.5 ATP.

FADH2 can produce 6 protons so 1.5 ATP.

32
Q

Why are fats important as an alternate fuel source?

A

Fats primarily stored as triacylglycerol in adipose tissue. TAG is an important energy source during exercise, fasting, hibernation and in new-borns.

33
Q

Describe fat oxidation.

A
  1. Triglycerol is hydrolysed by lipase in ;lipolysis to produce glycerol and 3 fatty acids.
  2. Fatty acids are activated by CoA attachment, catalysed by acyl CoA synthase using ATP, producing acyl CoA + AMP + PPi.
  3. Pyrophosphate rapidly broken down > 2 free pyruvates. 2 high energy phosphate bonds are hydrolysed, equivalent to 2 ATP, ensuring acyl CoA production is energetically favourable.
  4. Fatty acids oxidised in beta pxidation to produce many acetyl CoA molecules.
  5. Acetyl CoA goes to citric acid cycle to produce many NADH and FADH2.
34
Q

How is beta oxidation regulated?

A

Occurs largely at level of CPTI, carnitine acyl transferase I. CPTI inhibited by malonyl-CoA. Pathway is aerobic entirely. Requires a supply of NAD+ and FAD.

Biochemistry Y1 (10 decks)

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