Blood glucose
Brain cells depend on direct supplies of glucose from the blood, so brain cells are
starved of energy if glucose levels fall too low. Excess glucose is normally removed
from the blood and converted to glycogen or fat for storage. Our body constantly
monitors our blood glucose level internally. However, in certain situations it is
necessary for individuals to manage directly their own blood glucose level – for
example people with diabetes. Diabetics need to be very careful in relation to their
diet and their use of insulin. They need to monitor their blood glucose concentration
regularly and ensure that this stays within safe limits.
The control of glucose concentration in the blood
It is essential that the blood provides the cells with a constant supply of glucose for
respiration. Brain cells are especially sensitive to lowered glucose levels as glucose is
the only respiratory substrate which they can use.
The normal blood glucose concentration in humans is about 90 mg of glucose in 100
cm of blood. If the level falls much below this concentration, then the cells are not
supplied with sufficient glucose for respiration. However, if the level rises above this
level then the high concentration of glucose, as it is a solute, lowers the water
potential of the blood and affects the water balance between cells and tissue fluid.
The level of glucose is regulated around this optimum homeostatically by negative
feedback. Communication is hormonal. The main hormones involved are insulin and
glucagon.
The production of insulin and glucagon
Insulin and glucagon are produced in a gland called the pancreas. Most of the cells of
the pancreas secrete the enzymes and other components of pancreatic juice and the
secretions are poured directly down the pancreatic duct into the duodenum. In this
respect, the pancreas acts as an exocrine gland. However, small clusters of cells,
called Islets of Langerhans, secreted hormones which pass directly into the blood. In
this way the pancreas acts as an endocrine gland. There are two types of cell within
the Islets of Langerhans, alpha cells ( -cells) which secrete glucagon and beta cells (
cells) which secrete insulin.
The function of insulin
Insulin has several effects, all of which help to reduce the level of glucose in the
blood. When insulin binds to a receptor molecule in the surface membrane of a cell, it
triggers responses in the cell. Examples of such responses are given below:
-There are protein molecules in cell membranes which transport glucose from the
blood into the cells by facilitated diffusion. Insulin causes more of these protein
molecules to be sent into the membrane from vesicles in the cytoplasm of the cells.
This results in cells taking in glucose more rapidly from the blood.
-In some cells, especially liver and muscle cells, there are enzymes which speed up
the conversion of glucose into glycogen. This process is known as glycogenesis. The
insoluble glycogen is then stored as granules in the cytoplasm – a human liver can
store up to 75 g of glycogen. Insulin activates these enzymes. This also results in the
cells taking up more glucose from the blood, as the conversion to glycogen lowers the
concentration of glucose within the cell which increases the diffusion gradient of
glucose.
-Insulin also causes other intracellular enzymes to become active. These speed up the
synthesis of proteins and lipids. Although glucose is the main substrate for
respiration, some cells also respire the components of proteins and lipids. If fewer of
these are available, then cells use more glucose and take up more from the blood.
The function of glucagon
The effects of glucagon raise of the level glucose in the blood. The main effects are
as follows:
-Glucagon activates enzymes in the liver cells which speed up the conversion of the
insoluble stored glycogen molecules into glucose. This process is known as
glycogenolysis (=glycogen splitting). The glucose molecules then diffuse out of the
cells into the blood because of the high concentration now present in the liver cells.
-Glucagon increases the production of glucose from other nutrients such as amino
acids or fatty acids in liver cells. This process is known as gluconeogeneis (=new
glucose formation). Glucose produced in this way also enters the blood.
The diagram below shows how glucose enters and leaves the blood:
The mechanism of control of blood glucose concentration
Following a meal, glucose produced by the digestion of carbohydrates is absorbed
into the blood capillaries in the intestine wall, raising the level of glucose in the blood.
The high concentration of glucose stimulates the -cells of the Islets of Langerhans in
the pancreas to secrete insulin. Insulin circulates in the blood and binds to the
receptor sites on cell surface membranes, causing the effects listed above. As a result
the blood glucose level falls back towards the norm and so the secretion of insulin
also falls, in turn stopping these corrective processes.
The body cells are constantly taking up glucose form the blood and respiring it. If a
long period has elapsed since the last meal and no more glucose is absorbed from the
intestines, then the blood glucose level begins to fall below the norm. The lowering
of the level of glucose stimulates the -cells of the Islets of Langerhans to secrete
glucagon. This passes into the blood and binds to the receptor sites on the surface
membranes of liver cells, causing the cells to carry out the functions listed above.
This raises the glucose level back towards the norm. So control is by negative
feedback systems with separate mechanisms for controlling the return to the norm
from different directions.
Activity
1)Describe a situation in which blood glucose levels need to be monitored.
2)What is the normal range of values for blood glucose concentration?
3)What is the normal range of values for blood glucose concentration if a person is
fasting?
Powerpoint presentation including
Questions & Answers!