Biology 12

122 MHR • Unit 2 Homeostasis

When a high number of receptors are activated,

the cell’s metabolism increases the production of

insulin. This process becomes the integrator, and

the release of insulin from the cell is the effector.

This system has one potential weakness — left

on its own, it could deal well with an over-

abundance of sugar, but not with a shortage. To

prevent this problem from occurring, a similar

system within the alpha cells of the pancreas

controls the release of glucagon to raise blood sugar

levels when necessary (see Figure 4.15). Glucagon

raises blood glucose by stimulating the breakdown

of glycogen. Rather than changing the chemistry of

many of the body cells, this hormone targets a group

of cells (found primarily in the liver) that store

excess glucose in the form of glycogen. Glycogen is

a polysaccharide that acts as a reserve of glucose

for the body. Unlike fats (the other reserve of

energy), glycogen can be broken down rapidly and

released as glucose into the bloodstream. While

insulin and glucagon work together to maintain

blood sugar at normal levels, they have opposite

effects and are therefore antagonistic.

Unfortunately, complications with this regulating

system can arise. One major complication is

diabetes. The World Health Organization has

declared that a diabetes epidemic is underway. In

1985 it was estimated that there were 30 million

cases of diabetes in the world; in 1995 there were

135 million cases; by 2025 it is predicted the

number of cases will reach 300 million. Diabetes

has many different forms, but the two main ones

are Type 1 (insulin-dependent) diabetes and Type 2

(non-insulin-dependent) diabetes.

Homeostasis

Blood glucose is normal at 75 mg –110 mg/100 mL

high blood glucose

pancreas secretes

insulin

adipose (fat-storing)

tissue uses glucose

to form fat

adipose tissue

breaks down fat to glucose

liver stores glucose

as glycogen

liver breaks down

glycogen to glucose

muscle cells store

glycogen and build

protein

in

between

eating

glucagon pancreas secretes

glucagon

low blood glucose

insulin

after

eating

Figure 4.15When blood

glucose levels are high, the

pancreas secretes insulin.

Insulin promotes the storage

of glucose as glycogen and

the synthesis of proteins and

fats (as opposed to their use

as energy sources). Therefore,

insulin lowers blood glucose

levels to normal. When blood

glucose levels are low, the

pancreas secretes glucagon.

Glucagon acts in a way

opposite to insulin; therefore,

glucagon raises blood glucose

levels to normal.