Page 122 - The Manga Guide to Biochemistry

Animals preserve fat

Animals have a mechanism for preserving a fixed level of fat in the body. If they
overeat and fat accumulates, that triggers a signal that tells the brain to reduce the
amount that is eaten. Conversely, if the amout of fat is reduced, causing the animal
to get hungry, it will eat until the fat level returns to its original amount.

Interesting! I guess that instinct must come from animals living in the wild, where life
is severe. If it gets too fat, it won’t be able to catch food, and if it gets too skinny, it
could starve and die.

That’s right. For example, the protein hormone insulin causes glucose in the blood to
be absorbed into muscle or adipose tissue and stored as glycogen or fat. The result
is that it lowers the blood sugar level. (Insulin is also used to treat diabetes.)

= a saccharide (glucose)

Blood
vessel

Muscle

High blood Fat
sugar level
Dropping blood
sugar level

Oh, so “blood sugar level” is just the concentration of glucose in the blood! And if
that glucose is absorbed into muscle or fat, the concentration in the blood will drop.

Also, the cell membranes of nerve cells in the hypothalamus (a special part of the
brain) contain a protein called an insulin receptor.*

* Insulin receptors actually exist in the cell membranes of many types of cells in the body, not just
nerve cells.

108 Chapter 3