Energy: Carbohydrates 3

Question 1

TCA cycle

central pathway in catabolism

Answer 1

Mitochondrial (aerobic)

• oxidative (requires NAD+, FAD+)
• some energy is produced via SUBSTRATE LEVEL PHOSPHORYLATION
• produces some precursors for biosynthesis
• 2 cycles for every one glucose entering glycolysis (6NADH, 2FADH2, 2GTP, 4CO2)

Question 2

Regulation of TCA cycle

Answer 2

Regulated by energy availability (ATP/ADP ratio) and (NADPH/NAD+ ratio)

• activated by ADP
• inhibited by NADH, ATP

Question 3

Pyruvate dehydrogenase (PDH)

Answer 3

Pyruvate (3C) is converted to acetyl CoA (2C) which can then enter the TCA cycle
- happens in the mitochondrial matrix (pyruvate is hydrophilic so has to be transported from the cytoplasm across the mitochondrial membrane)
- PDH is a large multi enzyme complex
- is sensitive to vitamin B1 deficiency as the enzymes require various cofactors
- the reaction is irreversible (loss of CO2) so is a key regulatory step (pyruvate cannot be formed from acetyl CoA
Deficiency leads to lactic acidosis

Question 4

TCA supplying Biosynthetic processes

can convert substrates from one form to another to aid in metabolism

Answer 4

Amino acids can enter and leave the TCA cycle in four different places
Fatty acids (from citrate), haem (from succinate) and glucose (from oxaloacetate)

Question 5

Catabolism of glucose up to the end of the TCA

Answer 5

• all C-C and C-H bonds have been broken
• all C atoms have been oxidised to CO2
• all H atoms have been transferred to NAD+ + FAD
• some substrate level phosphorylation (net 2 ATP)
• NADH + FADH2 have large amounts of reducing energy

Question 6

ETC

Answer 6

• mitochondrial (electron transport and ATP synthesis)
• NADH and FADH2 are reoxidised
• oxygen is the final electron acceptor (reduced to H2O)
• large amounts of energy produced by oxidative phosphorylation

Question 7

Uses of reducing power in ATP synthesis

Answer 7

• electrons on NADH and FADH2 are transferred through a series of carrier molecules to oxygen - ELECTRON TRANSPORT
• free energy is used to drive ATP synthesis - OXIDATIVE PHOSPHORYLATION

Question 8

Mitochondrial electron transport

Answer 8

• NADH+H+ gives its (2H+e-) to the proton translocating complex (PTC) 1
• FADH2 gives its (2H+e-) to PTC2
  The (2) hydrogen ions are pumped across the inner membrane into the inner membrane space using the energy (30%) from the (1) electrons by proton pumps creating a high proton motive force (pmf) in the inner membrane space (a proton gradient)
• the final electron acceptor is oxygen - which makes water
• H+ then flows back down its concentration gradient into the matrix via ATP synthase (as the membrane is impermeable to H+) - this generates ATP
  The energy from the dissipation of the pmf is coupled to the synthesis of ATP

Question 9

Reducing energy potential

Answer 9

• electrons in NADH (uses 3 PTCs) have more energy than in FADH2 (uses 2 PTCs)
  The oxidation of 2 moles of NADH = 5 moles of ATP
  the oxidation of 2 moles of FADH2 = 3 moles of ATP

Question 10

Regulation of oxidative phosphorylation

Answer 10

When ATP concentration is high, (ADP concentration is low), there is no substrate for ATP synthase

• inward flow of H+ stops
• concentration of H+ in the mitochondrial space increases
• prevents further H+ pumping as energy available in the electrons is insufficient
• stops the ETC

Question 11

Inhibition of oxidative phosphorylation

Answer 11

• inhibitors block electron transport e.g. Cyanide (CN-) prevents the acceptance of electrons by O2
• ETC backs up and stops
• poisoning of the terminal PTC, electrons have no where to go, H+ are not pumped out, the pmf decreases, not enough energy to make ATP
• results in death as there is insufficient ATP to sustain life

Question 12

Uncoupling of oxidative phosphorylation

Answer 12

UNCOUPLERS increase the permeability of the mitochondrial inner membrane to protons - they no longer have go through ATP synthase

• reduced pmf
• no drive for ATP synthase
• energy lost as heat

Question 13

Brown adipose tissue

Answer 13

• in the back of the neck
  The degree of coupling is controlled by fatty acids - allows for extra heat generation
• contains thermogenin (UCP1)
  In response to cold noradrenaline activates:
  LIPASE –>releases FA from TACs
  FA activate UCP1 (which transports H+ back into the mitochondria
• energy of pmf is released as heat
  Important in NEWBORN INFANTS and HIBERNATING ANIMALS for non-shivering thermogenesis

Question 14

Phosphorylation

Answer 14

SUBSTRATE LEVEL 
- cytoplasmic and mitochondrial matrix
- energy directly through hydrolysis 
- can occur limitedly in the absence of oxygen 
- minor process for ATP synthesis 
OXIDATIVE 
- inner mitochondrial membrane 
- energy indirectly through pmf
- cannot occur in the absence of oxygen

Question 15

ATP synthesis from glucose

Answer 15

GLYCOLYSIS
- 2 ATP
- 2 NADH = 5 ATP
Pyruvate dehydrogenase 
- 2 NADH = 5 ATP
Kreb's cycle 
- 2 GTP = 2 ATP 
- 6 NADH = 15 ATP 
- 2 FADH2 = 3 ATP 

TOTAL = 32 ATP per 1 moles of glucose