Building sugar without going backward

Science · Biochemistry & Molecular Biology · ages 20-21

Name ______________________ Date ____________

Which pair names gluconeogenic bypass enzymes?
- Pyruvate carboxylase and PEP carboxykinase
- Amylase and lactase from digestion
- ATP synthase and rubisco from other pathways
Why does making glucose need different enzymes rather than reversed ones?
- Three irreversible steps cannot reverse, so bypass enzymes are required
- Reversed enzymes would work but scientists dislike them
- Glucose is too large for glycolysis enzymes to touch
Which materials feed gluconeogenesis?
- Acetyl CoA from fat breakdown
- Carbon dioxide breathed in from air
- Lactate, glycerol, and glucogenic amino acids
Fructose 2,6-bisphosphate rises in the cell. What happens?
- Gluconeogenesis speeds up while glycolysis halts
- Glycolysis is stimulated while gluconeogenesis is restrained
- Both pathways shut down together
Both directions of the glucose pathways run at full speed at once. What is the cost?
- A futile cycle that burns ATP for no net product
- Double glucose output at no extra charge
- Instant shutdown of all other metabolism
Muscles pour lactate into the blood during hard exercise. How does the body recover that carbon?
- The liver reconverts lactate to glucose in the Cori cycle
- The lungs breathe the lactate out directly
- Fat cells turn the lactate into new fat only
Fatty acid breakdown supplies acetyl CoA, so fat can replace lactate as a glucose source in humans.
Circle one: True False
A new drug mimics fructose 2,6-bisphosphate. What happens to glucose making?
- It rises, since mimics always boost every pathway
- It stays flat, since signals never affect enzymes
- It falls, since the switch restrains gluconeogenesis

Answer key

For grown-ups. Fold this page away before handing over the rest.

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Pyruvate carboxylase and PEP carboxykinase · Those two bridge the bottom irreversible steps. Digestive and photosynthetic enzymes belong to other stories.
Three irreversible steps cannot reverse, so bypass enzymes are required · Irreversibility under cell conditions blocks simple reversal. Only dedicated bypasses can bridge those steps.
Lactate, glycerol, and glucogenic amino acids · Lactate, glycerol, and certain amino acids enter at their own points. Fat derived acetyl CoA cannot.
Glycolysis is stimulated while gluconeogenesis is restrained · One switch, two opposite effects: that is reciprocal regulation in action.
A futile cycle that burns ATP for no net product · Opposite full speeds cancel production while spending energy. The cell pays for nothing.
The liver reconverts lactate to glucose in the Cori cycle · Lactate returns to the liver and reenters glucose making. The carbon loops instead of being lost.
False · Acetyl CoA cannot become net glucose in humans. Fat is superb fuel but no sugar source.
It falls, since the switch restrains gluconeogenesis · Apply the switch rule to the mimic: glycolysis up means glucose making down. Reciprocal control decides.