IB DP Biology: SL复习笔记6.6.1 Hormones

Insulin & 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

Pancreas-location-and-microscopic-structure

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

Negative-feedback-regulation-of-blood-glucose-levels

Control of blood glucose levels

Exam Tip

The terms glucagon and glycogen are very often mixed up by students as they sound similar. Remember:

  • Glucagon is the hormone
  • Glycogen is the polysaccharide that glucose is stored as

Learn the differences between the spellings and what each one does so you do not get confused in the exam!

Diabetes

  • 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
  • The insulin used by diabetics can be fast-acting or slow-acting; each allowing for a different level of control

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
  • For type II diabetes treatment involves a sugar and fat controlled diet and an exercise regime
    • Any food that is rapidly digested into sugar will cause a sudden, dangerous spike in blood sugar
  • Obesity is a major risk factor for type II diabetes

Type I Diabetes and Type II Diabetes TableComparing-Type-1-Type-2-diabete

Thyroxin

  • 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
    • Constantly feeling cold
      • Less heat is generated by respiration
    • Constipation
      • Muscular contractions in the gut wall slow down due to reduced energy from respiration
    • Impaired brain development in children

 

 

 

 

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