1. Biosystems maintenance
Homeostasis and Hormonal Action

2. Homeostasis
• What is homeostasis?
• It is maintenance of a constant internal
environment.
• What is your understanding of an internal
environment?
• In the context of homeostasis the internal
environment refers to the medium inside the
body in which the cells function.

3. Homeostasis cont'd
• What is the medium in which your cells
function?
• You’re right, tissue fluid.
• Several factors of the environment (tissue
fluid) affect the functioning of the cell. Let’s
look at three.

4. Factors affecting cell functions
1. Temperature – at low temperatures
metabolic reactions occur very slowly. At
high temperatures proteins, including
enzymes, are denatured.
2. Volume of water – if there is too little water,
cells will lose water by osmosis. If there is an
abundance, the cells will gain water, swell
and burst.

5. Factors affecting cell functions
3. Amount of glucose – because it is the
substrate for respiration, if there is a lack
cells will be deprived of energy. If there is
too much the osmotic balance of cells will be
disrupted and the cells will lose water.
Homeostatic conditions control the composition
of blood and so control the composition of
tissue fluid.

6. Negative feedback mechanism
• This is a technique carried out by the cell to
resist any change from the normal.
• Most homeostatic mechanisms use a negative
feedback control loop.
• The loop includes a receptor (sensor) and an
effector.
• The receptor receives input.
• Input is information about the parameter
being monitored.

7. Negative feedback mechanism cont'd
• A series of events is then triggered which then
allows the effector to perform a corrective
measure.
• As the parameter is continuously monitored,
there is continuous adjustments of the
output.
• The result is an oscillation around the ideal
level or set point.

8. Hormones
• Hormones carry information from one part of
an organism to another with an aim to
achieve coordination.
• The hormones in plants are called plant
growth regulators.
• You have met several hormones during your
course of study. We will focus on two just
now. First let’s look at the system which
produces them.

9. Endocrine system
Position of endocrine glands

10. Hormones cont'd
• A hormone is a chemical produced in one part
of the body, a gland, and carries out its action
in another part of the body, target site.
• The system responsible for producing
hormones is the endocrine system.
• The endocrine system is made up of glands.
• Glands are groups of cells which secrete one
or more than one substances.

11. Hormones cont'd
• Endocrine glands secrete their materials into
the blood.
• In fact, the term endocrine means ‘secreting
to the inside’.
• This means that as hormones are secreted by
these cells they pass into blood capillaries that
are close to them.

12. Hormones cont'd
• There are other glands that secrete materials
into ducts and not the blood.
• These are called exocrine glands.
• Exocrine means ‘secreting to the outside’.
• Examples of exocrine glands include the
salivary glands, liver, sweat glands.
• Endocrine glands include; the pituitary,
thyroid gland, adrenal gland, ovary and testes.

13. Hormones cont'd
• Can you think of a gland that has both
exocrine and endocrine functions?
• Mammalian hormones are relatively small
molecules.
• Some are proteins eg. Insulin.
• Others are steroids eg. Testosterone.

14. Hormones cont'd
• Hormones are secreted in very small
concentrations, usually a few micrograms per
cm3
of blood.
• These concentrations have very large effects
on the body.
• Some endocrine glands secrete hormone very
quickly, others take a little more time.
• The effect can also be short lived or last a long
time.

15. Hormones cont'd
• Even though hormones are carried within the
blood, only target cells respond to each
hormone.
• This is because target cells have receptors
specific to the hormone.
• For protein hormones, the receptors are on
the surface of the cell.
• The hormone attaches and triggers responses
without entering the cell.

16. Hormones cont'd
• For hormones that are steroids, the receptors
are inside the cell in the cytoplasm.
• Because of their small size, they easily pass
through the cell membrane.

17. Hormones cont'd
• What other reasons would allow steroids to
pass in but not proteins?

18. Control of blood sugar
• The pancreas is the endocrine gland that is
responsible for the control of blood sugar.
• The part of the pancreas that carries out this
function is a group of cells called the islets of
Langerhans.
• There are two types of cells in the islets.
These are α cells which secrete glucagon and
β cells which secrete insulin.

19. Control of blood sugar cont'd
• Why is it important that blood glucose levels
be controlled?
• Normal blood glucose levels range between
80 and 120 mg of glucose per 100 cm3
of
blood.
• If the blood glucose level falls below this, the
cells will not have enough respiratory
substrate and will be unable to carry out their
functions.

20. Control of blood sugar cont'd
• This is extremely important for brain cells
which can only use glucose as a respiratory
substrate.
• If glucose levels get too high, the osmotic
balance of cells is affected.
• Before we look at exactly what happens,
remember that the level fluctuates around a
normal mean.

21. Control of blood sugar cont'd
• Okay, so you just ate a meal containing
carbohydrate. What happens?
• The blood glucose level rises as digested food
is absorbed from the intestine.
• As this blood passes through the pancreas, the
α and β cells detect this.
• The α cells respond by stopping the secretion
of glucagon.

22. Control of blood sugar cont'd
• The β cells respond by secreting insulin.
• Let’s look at how insulin affects cells.
1.There is increased absorption of glucose by
muscle cells and adipose tissue. The glucose
enters through special transporter proteins
kept in the cytoplasm. When insulin is
detected by the cells the transporter proteins
move to the plasma membrane.

23. Control of blood sugar cont'd
The glucose transporter in liver and brain is a
bit different. It is always found in the cell
membrane. Consequently, glucose can
always move into these cells.
2. The rate of use of glucose by the cells for
respiration is increased.

24. Control of blood sugar cont'd
3. The rate at which glucose is converted to
glycogen increases. This occurs in the liver
cells. Several enzymes are involved. They
control the phosphorylation of glucose and
the formation of 1,4 glycosidic bonds between
α glucose molecules.

25. Control of blood sugar cont'd
• As a result of these actions, glucose is taken
out of the blood and the level falls.
• The fall in glucose levels is detected by both α
and β cells.
• The β cells respond by stopping the secretion
of insulin.
• The α cells respond by secreting glucagon.

26. Control of blood sugar cont'd
• When there is little or no insulin, the rate of
uptake of glucose by the cells is reduced.
• Uptake continues at a slower rate.
• Glucagon affects liver cells.
• View the next slides for the role of glucagon.

27. Control of blood sugar cont'd
1. The rate at which glycogen is broken down to
form glucose (glycogenolysis) is increased.
When the glucagon binds to receptors in the
plasma membranes of the liver cells enzymes
are activated which catalyse glycogenolysis.
2. Fatty acids instead of glucose is used as the
main respiratory substrate.

28. Control of blood sugar cont'd
1. Glucose is produced from compounds such
as fat and amino acids (gluconeogenesis)
• As a result of these actions the liver releases
glucose into the blood.

29. Control of blood sugar cont'd
• The increase in blood glucose level is detected
by the cells of the islets of Langerhans which
respond to lower the blood glucose level.
• There is time delay between when the glucose
level rises and falls and the secretion of the
hormones.

30. Diabetes mellitus
• This is a metabolic disease which causes the
blood sugar level to be too high.
• There are two forms:
1.juvenile–onset diabetes or insulin dependent
diabetes. Here the pancreas does not secrete
insulin.
2.Non-insulin dependent diabetes. Here insulin
is secreted but it is not enough to control the
blood glucose level.

31. Diabetes mellitus cont'd
• The symptoms of both types of diabetes are
the same. These include:
1.Glucose in urine
2.Extreme hunger and thirst. The hunger results
from the loss of glucose. The thirst results
from the loss of water that accompanies the
glucose.

32. Diabetes mellitus cont'd
• Coma resulting from a combination of
dehydration, salt loss and low blood pH.
• Dehydration and salt loss result from excess
urination.
• Low blood pH results from the keto-acids
produced when fats are metabolized.
• Fats are metabolized because the rate at
which glucose is taken up by the cells is
reduced.

33. Diabetes mellitus cont'd
• If the blood glucose levels get too low (eg.
Between meals) a person might fall into a
coma.
• This is because there are no glycogen
reserves.
• Insulin-dependent diabetes can be controlled
by regular injections of insulin.

34. Diabetes mellitus cont'd
• The insulin used is obtained from bacteria.
(Remember Genetic Engineering?).
• Formerly, it was obtained from pigs and
cattle.
• The advantages of using the insulin from GE
are many.

35. Diabetes mellitus cont'd
• These include:
1.Faster response
2.Duration of response is shorter
3.Less chance of immune resistance
4.Effective for people who are intolerant of
animal derived insulin
5.Persons who have ethical problems with
insulin from cattle can use this insulin.

36. Diabetes mellitus cont'd
• Non-insulin dependent diabetes can be
controlled by diet.

37. Ethene
• Ethene is a plant growth regulator.
• It is a gas. This is ideal because diffusion can
occur easily. It is also fat soluble and can
therefore pass through cell membranes easily.
• It has several developmental functions in
plants.

38. Ethene cont’d
• The production of ethene is controlled by
positive feedback mechanism. As ethene is
produced, the enzymes that control its
production are stimulated even more to
increase production.
• Ethene controls many regulatory genes and so
result in many changes in a fruit.

39. Structure of ethene

40. Ethene cont'd
Developmental functions include:
1.It causes ripening of fruit – it is able to do this
because
• it causes the conversion of starch to
soluble sugar.
• it causes cell wall softening.
• It also triggers an increase in respiration rate.

41. Ethene cont'd
2. It aids in wound healing – this gas is released
at the site of a wound. It stimulates the
formation of a callus which plugs the wound.
3. It causes etiolation.

42. Ethene cont'd
• It promotes leaf and fruit fall.
• Breaks dormancy of some seeds.
• Causes the formation of aerenchyma tissue in
aquatic plants.

43. Commercial uses of ethene
• Because of their hardiness unripe fruits are
better shipped than ripe fruits.
• When they arrive at their destination, they are
put in ware houses and synthetic ethene
applied.
• They ripen and are supplied to the merchants.

44. Revision questions
1. The control of blood glucose concentration
uses a negative feedback control mechanism.
a.Explain what is meant by negative feedback.
b.What are the receptors in this control
mechanism?
c. What are the effectors?

45. Revision questions cont'd
2. Explain why people with diabetes mellitus
have virtually no glycogen to be mobilised.
3. Suggest how people with non-insulin-
dependent diabetes can control their blood
glucose level.