Hormones (DP IB Biology: HL)

Introduction to Hormones

• A hormone is a chemical substance produced by an endocrine gland and carried by the blood
  • The endocrine glands that produce hormones in animals are known collectively as the endocrine system
  • A gland is a group of cells that produces and releases one or more substances (a process known as secretion)
• Hormones are chemicals which transmit information from one part of the organism to another and that bring about a change
• They alter the activity of one or more specific target organs
  • Hormones only affect cells with receptors that the hormone can bind to
  • These are either found on the cell surface membrane, or inside cells
  • Receptors have to be complementary to hormones for there to be an effect
• Hormones are used to control functions that do not need instant responses

Insulin and Glucagon

• The pancreas is an organ found in the abdomen of mammals
• It functions as both an endocrine gland and an exocrine gland
  • Endocrine glands secrete hormones directly into the blood, whereas exocrine glands secrete substance via a duct
  • The exocrine function of the pancreas is to produce digestive enzymes to be delivered to the small intestine
  • The endocrine function of the pancreas is to produce the hormones glucagon and insulin

• Within the pancreas, these two functions are performed by different tissues
  • Most of the cells of the pancreas secrete digestive enzymes, but throughout the organ, there are small sections of cells known as the islets of Langerhans that produce hormones
  • The islets of Langerhans contain two cell types: alpha cells (α cells), which secrete glucagon, and beta cells (β cells), which secrete insulin

The location and structure of the pancreas

The control of blood glucose by glucagon and insulin

• If the concentration of glucose in the blood decreases below a certain level, cells may not have enough glucose for respiration and so may not be able to function normally
• If the concentration of glucose in the blood increases above a certain level, this can also disrupt the normal function of cells, potentially causing major problems
• The control of blood glucose concentration is a key part of homeostasis
• Blood glucose concentration is controlled by glucagon and insulin:
  • Glucagon is synthesised and secreted by α cells when blood glucose falls and stimulates liver and muscle cells to convert stored glycogen into glucose to be released into the blood, increasing blood glucose concentration
  • Insulin is synthesised and secreted by β cells when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen to be stored, decreasing blood glucose concentration

Control of blood glucose levels

• There are over 3 million people suffering from diabetes in the UK
• Diabetes is a condition in which the homeostatic control of blood glucose has failed or deteriorated
• In individuals with diabetes their insulin function is disrupted which allows the glucose concentration in the blood to rise
• An elevated blood glucose level can lead to noticeable symptoms, some of which are harmful, e.g.
  • The kidneys are unable to filter out this excess glucose in the blood and so it is often present in the urine
  • The increased glucose concentration also causes the kidneys to produce large quantities of urine, making the individual feel thirsty due to dehydration
  • Continuously elevated blood glucose levels can also damage tissues, in particular their proteins
• There are two different types of diabetes: type I and type II

Type I diabetes

• Type 1 diabetes is a condition in which the pancreas fails to produce sufficient insulin to control blood glucose levels
• It normally begins in childhood due to an autoimmune response whereby the body’s immune system attacks the β cells of the islets of Langerhans in the pancreas
  • The β cells produce and  release insulin

• Insulin causes the cells to take up glucose from the blood for respiration and for storage as glycogen; without insulin the glucose remains in the blood, resulting in an individual feeling fatigued
• If the blood glucose concentration reaches a dangerously high level after a meal then organ damage can occur
• Type 1 diabetes is normally treated with regular blood tests to check glucose levels, insulin injections and a diabetes appropriate diet
  • Health authorities encourage type I diabetics to eat a similar diet to the general public. They suggest five portions of fruit and veg a day, minimally processed food and consuming more polysaccharides than monosaccharides or disaccharides

Type II diabetes

• Type II diabetes is more common than type I
• It usually develops in those aged 40 and over, however more and more young people are developing the condition
• In type II diabetes the pancreas still produces insulin but the receptors have reduced in number or no longer respond to it
• The lack of response to insulin means there is a reduced glucose uptake by the cells, which leads to a high blood glucose concentration
  • This can cause the β cells to produce more and more insulin in the attempt to lower blood glucose levels
  • Eventually the β cells can no longer produce enough insulin and blood sugar becomes uncontrollable

Type I Diabetes and Type II Diabetes Table

• Thyroxin is a hormone that is released from the thyroid gland, located in the neck
• Thyroxin's main role is to regulate the basal metabolic rate (BMR); this is the speed at which metabolic reactions occur in the body when it is at rest
  • Thyroxin therefore targets almost all cells in the body, as all cells metabolise
  • However, the most metabolically active cells, such as those of the liver, muscle and brain, are most affected

• Thyroxin plays a role in regulating body temperature
  • If the body becomes cooler, this triggers increased thyroxin secretion by the thyroid gland
  • The increase in thyroxin increases the metabolic rate, which  increases the generation of body heat
  • This causes body temperature to rise
• Thyroxin deficiency, caused by a condition known as hypothyroidism, has the following effects on the body:
  • Lack of energy
  • Low mood
  • Forgetfulness
  • Weight gain
    • Less glucose and fat is broken down by cellular respiration to release energy