Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
There are insulin receptors on many cells, such as those
in the liver, muscle and adipose (fat storage) tissue. Insulin
stimulates cells with these receptors to increase the rate
at which they absorb glucose from the blood, convert it
into glycogen and use it in respiration. This results in a
decrease in the concentration of glucose in the blood.
Glucose can only enter cells through transporter
proteins known as GLUT. There are several different
types of GLUT proteins. Muscle cells have the type
called GLUT4. Normally, the GLUT proteins are kept
in the cytoplasm in the same way as the aquaporins in
collecting duct cells (page 314). When insulin molecules
bind to receptors on muscle cells, the vesicles with GLUT4
proteins are moved to the cell surface membrane and fuse
with it. GLUT4 proteins facilitate the movement of glucose
into the cell (Figure 14.22). Brain cells have GLUT1
proteins and liver cells have GLUT2 proteins, which are
always in the cell surface membrane, and their distribution
is not altered by insulin.
Insulin also stimulates the activation of the enzyme
glucokinase, which phosphorylates glucose. This traps
glucose inside cells, because phosphorylated glucose
cannot pass through the transporters in the cell surface
membrane. Insulin also stimulates the activation of
two other enzymes, phosphofructokinase and glycogen
synthase, which together add glucose molecules to
glycogen. This increases the size of the glycogen granules
inside the cell (Figure 14.23).
Figure 14.23 Transmission electron micrograph of part of a
liver cell (× 22 000). The dark spots are glycogen granules in
the cytoplasm. Mitocondria can also be seen.
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receptor
1
Insulin binds to a receptor in
the cell surface membrane.
insulin
glucose
3 Glucose can now
diffuse into the cell
down its concentration
gradient.
cell surface
membrane
cytoplasm
2 The receptor signals
to the cell and makes
vesicles carrying glucose
transporter proteins
merge with the cell surface
membrane.
glucose transporter
GLUT4
Figure 14.22 Insulin increases the permeability of muscle cells to glucose by stimulating the movement of vesicles with GLUT4 to
the cell surface membrane.