1. 22.7 Gluconeogenesis: Glucose Synthesis
Glucose is synthesized in the tissues of the liver and kidneys. Tissues that use glucose as their main energy source are the brain, skeletal muscles, and red blood cells.
Learning Goal Describe how glucose is synthesized from noncarbohydrate molecules.

2. Utilization of Glucose
Glucose is the primary energy source for the brain, skeletal muscle, and red blood cells. Glucose deficiency can impair the brain and nervous system.
If our glycogen stores are depleted,
liver cells synthesize glucose by gluconeogenesis.
glucose is synthesized in the cytosol of the liver cells, and some is synthesized in the kidneys.

3. Glycogen
Glycogen
can supply us with about one day’s requirement of glucose.
is made from glucose, most of which is synthesized in the cytosol of liver cells.
To begin gluconeogenesis, carbon atoms from noncarbohydrate food sources are converted to pyruvate.

4. Pyruvate to Phosphoenolpyruvate
To start the synthesis of glucose from pyruvate,
two catalyzed reactions are needed to replace reaction 10 in glycolysis.
pyruvate carboxylase uses the energy of ATP hydrolysis to catalyze the addition of CO2 to pyruvate and produce oxaloacetate.
phosphoenolpyruvate carboxykinase converts oxaloacetate to phosphoenolpyruvate.
phosphoenolpyruvate molecules now use enzymes to form fructose-1,6-bisphosphate.

5. Pyruvate to Phosphoenolpyruvate

6. Fructose-1,6-Bisphosphate to Fructose-6-Phosphate
The second irreversible reaction in glycolysis is bypassed when fructose-1,6-bisphosphatase cleaves a phosphate group from fructose-1,6-bisphosphate.
The product fructose-6-phosphate undergoes the reversible reaction 2 of glycolysis to yield glucose-6-phosphate.

7. Glucose-6-Phosphate to Glucose
In the final irreversible reaction, the phosphate group of glucose-6-phosphate is hydrolyzed by a different enzyme, glucose-6-phosphatase, to form glucose.

8. Glycolysis and Gluconeogenesis

9. Energy Cost of Gluconeogenesis
The pathway consists of seven reversible reactions of glycolysis and four new reactions that replace the three irreversible reactions.
Overall, glucose synthesis requires 4 ATPs, 2 GTPs, and 2 NADHs.

10. Lactate and the Cori Cycle
is the flow of lactate and glucose between the muscles and the liver.
occurs when anaerobic conditions occur in active muscle, and glycolysis produces lactate.
operates when lactate moves through the blood stream to the liver, where it is oxidized back to pyruvate.
converts pyruvate to glucose, which is carried back to the muscles.

11. Lactate and the Cori Cycle

12. Regulation of Gluconeogenesis
Gluconeogenesis is
not utilized when the diet is high in carbohydrates.
very active when the diet is low in carbohydrates.
When conditions in a cell favor glycolysis, there is no synthesis of glucose.
When the cell requires the synthesis of glucose, glycolysis is turned off.

13. Regulation, Glycolysis, and Gluconeogenesis

14. Metabolic Reactions of Glucose
Glycolysis involves the breakdown of glucose, and gluconeogenesis involves the synthesis of glucose from noncarbohydrates. The pentose phosphate pathway converts glucose to ribose and provides NADPH. Glycogenolysis degrades glycogen to glucose whereas glycogenesis produces glycogen from glucose.

15. Study Check Identify each process. 1) glycolysis 2) glycogenesis
3) glycogenolysis 4) gluconeogenesis
the synthesis of glucose from noncarbohydrates
the breakdown of glycogen into glucose
the oxidation of glucose to two pyruvate
the synthesis of glycogen from glucose

16. Solution Identify each process. 1) glycolysis 2) glycogenesis
3) glycogenolysis 4) gluconeogenesis
the synthesis of glucose from noncarbohydrates 4) gluconeogenesis
the breakdown of glycogen into glucose 3) glycogenolysis
the oxidation of glucose to two pyruvate 1) glycolysis
the synthesis of glycogen from glucose 2) glycogenesis

17. Concept Map