Homeostasis and hormonal action

Biosystems maintenance

Homeostasis and Hormonal Action

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.

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.

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.

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 compositionof blood and so control the composition of tissue fluid.

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.

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.

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.

Endocrine systemPosition of endocrine glands

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.

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.

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.

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.

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.

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.

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.

Hormones cont'd

• What other reasons would allow steroids to pass in but not proteins?

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.

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.


• 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.


• 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.


• 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.


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.


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.


• 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.


• 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.


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.


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.


• 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.

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.

Diabetes mellitus cont'd

• The symptoms of both types of diabetes are the same. These include:

1.Glucose in urine2.Extreme hunger and thirst. The hunger results

from the loss of glucose. The thirst results from the loss of water that accompanies the glucose.


• 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.


• 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.


• 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.


• These include:1.Faster response2.Duration of response is shorter3.Less chance of immune resistance4.Effective for people who are intolerant of

animal derived insulin5.Persons who have ethical problems with

insulin from cattle can use this insulin.


• Non-insulin dependent diabetes can be controlled by diet.

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.

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.

Structure of ethene

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.

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.

Ethene cont'd

• It promotes leaf and fruit fall.• Breaks dormancy of some seeds. • Causes the formation of aerenchyma tissue in

aquatic plants.

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.

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?

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.

Technology

Homeostasis and hormonal action