4. Brain cells depend on glucose for energy and use GLUT3 (a glucose transporter

Question

4. Brain cells depend on glucose for energy and use GLUT3 (a glucose transporter) which has a Km of 1.5 mM, Liver cells use a different glucose transporter, GLUT2 which has a Km of 15 mM =Vmax [S] S] + KM Glucose levels in blood (mM) Starvation conditions Normal level After high carbohydrate meal 5 15 Using the equation and information provided in table above; Compare the rate (as percentage of Vmax) of glucose uptake in both cell types (brain and liver) a. under starvation conditions. b. Normal glucose level c. After high carbohydrate meal Do these calculations fit with physiological functions of brain and liver cells? Explain. A. B.

Explanation / Answer

A.

V = Vmax ([S]/[S]+Km)

Under starvation conditions:

In brain,

V = Vmax (3/3+1.3)

= 0.697 Vmax

In Liver,

V = Vmax (3/3+15)

= 0.166 Vmax

At normal conditions:

In brain,

V = Vmax (5/3+1.3)

= 1.162 Vmax

In Liver,

V = Vmax (5/5+15)

= 0.25 Vmax

After high carbohydrate meal:

In brain,

V = Vmax (15/15+1.3)

= 0.92 Vmax

In Liver,

V = Vmax (15/15+15)

= 0.5 Vmax

B.

Yes.

These calculations fit well with the physiological functions of brain and liver cells.

Liver cells must be active to absorb glucose after a high carbohydrate meal. So, they exhibit maximum Vmax after a high carbohydrate meal.

Brain cells depend exclusively on glucose for their energy requirements. So, Vmax of brain cells is always higher than Vmax of liver cells at all conditions.

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