5.1.4 Hormonal Communication

Question 1

What is the endocrine system?

Answer 1

• a communication system using hormones as signalling molecules
• uses the blood circulatory system to transport its signals
• signals released by the endocrine system are hormones

Question 2

What are the two types of hormone?

Answer 2

• protein and peptide hormones and derivates of amino acids: e.g. adrenaline, insulin and glucagon
• steroid hormones: e.g. oestrogen and testosterone

Question 3

How do protein, peptide hormones work differently from steroid hormones?

Answer 3

• proteins are not soluble in the phospholipid membrane and do not enter the cell
• protein hormones need to bind to the cell surface membrane and release a second messenger inside the cell
• steroid hormones can pass through the membrane and enter the cell and the nucleus, to have a direct effect on the DNA in the nucleus

Question 4

Where are hormones released from?

Answer 4

• the endocrine glands
• ductless glands, consisting of groups of cells that manufacture and release the hormone directly into the blood in capillaries running through the gland

Question 5

How are hormones detected?

Answer 5

• they are transported all over the body but the cells receiving an endocrine signal are called target cells
• these cells may be grouped together in a target tissue or more wildly dispersed in a number of tissues
• for non-steroid hormones, the target cells must possess a specific receptor on their plasma membrane that is complementary in shape to the shape of the hormone
• hormone binds to this receptor and initiates changes in the cell

Question 6

What are first messengers?

Answer 6

• non-steroid hormones
• they are signalling molecules outside the cell that bind to the cell surface membrane and initiate an effect inside the cell
• usually cause the release of another signalling molecule in the cell, which is called the second messenger
• the second messenger stimulates a change in the activity of the cell

Question 7

Describe how some non-steroid hormones act via a G protein in the membrane?

Answer 7

• the G protein is activated when the hormone binds to the receptor
• G protein activates an effector molecule, usually an enzyme that converts an inactive molecule into the active second messenger
• in many cells, the effector molecule is the enzyme adenyl cyclase, which converts ATP to cycle AMP (cAMP)
• this second messenger may act directly on another protein, or may initiate a cascade of enzyme-controlled reactions that alter the activity of the cell

Question 8

What is the structure of the adrenal glands?

Answer 8

• found lying anterior to the kidneys
• each gland is divided into the outer adrenal cortex and the inner adrenal medulla
• both regions are well supplied with blood vessels and produces hormones which are secreted directly into blood vessels

Question 9

Describe the structure of the adrenal cortex

Answer 9

has three distinct layers of cells:

• zona glomerulosa: the outermost layer, which secretes mineralocorticoids such as aldosterone
• zona fasciculate: the middle layer, which secretes glucocorticoids such as cortisol
• zona reticularis: the innermost layer, which is through to secrete precursor molecules that are used to make sex hormones

Question 10

Describe the adrenal medulla

Answer 10

• found at the centre of the adrenal gland
• secretes adrenaline and noradrenaline

Question 11

Describe the action of steroid hormones secreted from the adrenal cortex

Answer 11

• adrenal cortex uses cholesterol to produce hormones, so they are steroid based
• able to enter cells directly by dissolving into the cell surface membrane
  1. steroid hormone passes through the cell membranes of the target cell
  2. steroid hormone binds with a specific receptor (with a complementary shape) in the cytoplasm
  3. receptor-steroid hormone complex enters the nucleus of the target cell and binds to another specific receptor on the chromosomal material
  4. binding stimulates the production of mRNA molecules, which code for the production of proteins

Question 12

What roles do hormones from the adrenal cortex play in the body?

Answer 12

• mineralocorticoids: e.g. aldosterone from the zona glomerulosa help to control the concentrations of sodium and potassium in the blood so maintains blood pressure
• glucocorticoids: e.g. cortisol from zona fasciculata help to control the metabolism of carbs, fats and proteins in the liver. also, is released in response to stress or for low blood glucose concentration

Question 13

Where is adrenaline released and describe its structure

Answer 13

• released from the adrenal medulla into the blood
• is a polar molecule derived from the amino acid tyrosine
• cannot enter through plasma membrane
• must be detected by special receptors on the plasma membrane of target cells
• many cells and tissues have adrenaline receptors, so effects are widespread

Question 14

What is the role of adrenaline?

Answer 14

• preparing the body for activity:
• relaxing smooth muscles in bronchioles
• increasing stroke volume of the heart
• increasing heart rate
• causing general vasoconstriction to raise blood pressure
• stimulating conversion of glycogen to glucose
• dilating pupils
• increasing mental awareness
• inhibiting action of the gut
• causing body hair to stand erect

Question 15

Why is the pancreas unusual in its functions?

Answer 15

• it has both exocrine and endocrine functions

Question 16

What are the two main secretions of the pancreas?

Answer 16

• pancreatic juices containing enzymes which are secreted into the small intestine
• hormones which are secreted from the islets of Langerhans into the blood

Question 17

What do exocrine glands do?

Answer 17

• they secrete substances into a duct

Question 18

What is the exocrine function of the pancreas?

Answer 18

• the synthesis and release of digestive enzymes in the pancreas

Question 19

Describe the exocrine function in the pancreas

Answer 19

• exocrine cells are in small groups surrounding tiny tubules
• each group of cells is called an acinus
• the acini are grouped together into small lobules separated by connective tissue
• the cells of the acini secrete the enzymes they synthesis into the tubule at the centre of the group
• the tubules from the acini join to form intralobular ducts that eventually combine to make up the pancreatic duct
• the pancreatic duct carries the fluid containing the enzymes into the first part of the small intestine (duodenum)

Question 20

What enzymes are present in the fluid from the pancreatic duct?

Answer 20

• pancreatic amylase
• trypsinogen
• lipase

Question 21

Where are islets of Langerhans located?

Answer 21

• dispersed in small patches among the lobules of acini

Question 22

What is in the islets of Langerhans and what is secreted?

Answer 22

• contains alpha cells and beta cells that make up the endocrine tissue in the pancreas
• alpha cells secrete glucagon
• beta cells secrete insulin

Question 23

Describe how insulin is released from the beta cells

Answer 23

• the cell membranes of the beta cells contain both calcium ion channels and potassium ion channels
• potassium ion channels are normally open and the calcium ion channels are normally closed
• potassium ions diffuse out of the cell making the inside of the cell more negative
• when glucose concentrations outside the cell are high, glucose molecules move into the cell
• glucose is quickly used in metabolism to produce ATP, involving the enzyme glucokinase
• the extra ATP caused the potassium channels to close
• potassium can no longer diffuse out and this alters the p.d. across the cell membrane as it becomes less negative inside
• this change in p.d, open the calcium ion channels
• calcium ions enter the cell and cause the secretion of insulin by making the vesicles containing insulin move to the cell surface membrane and fuse with it, releasing insulin by exocytosis

Question 24

What is the normal blood concentration of glucose?

Answer 24

• between 4 - 6 mmoldm^-3

Question 25

What is hypoglycaemia?

Answer 25

• inadequate delivery of glucose to the body tissues and in particular, the brain
• due to blood glucose concentration dropping below 4 mmol dm^-3
• may cause tiredness, confusion, seizures, death

Question 26

What is hyperglycaemia?

Answer 26

• when blood glucose concentration is allowed to rise too high for long periods
• consistently higher than 7mmoldm^-3 is a sign of diabetes mellitus

Question 27

How is the blood glucose concentration in the blood monitored?

Answer 27

• cells in the islets of Langerhans monitor blood glucose concentration and release relevant hormone: insulin if blood glucose is too high, glucagon if too low
• these hormones act on hepatocytes which can store glucose in the form of glycogen

Question 28

Describe what happens when blood glucose rises too high

Answer 28

• high blood glucose concentration is detected by the beta cells in the islets of Langerhans
• beta cells secrete insulin into the blood which travels through the body in the circulatory system
• target cells are liver cells, muscle cells and some other body cells including those in the brain
• target cells possess the complementary membrane-bound receptors for insulin
• when insulin binds to the insulin receptor, this activates the enzyme tyrosine kinase which is associated with the receptor on the inside of the membrane
• this causes phosphorylation of inactive enzymes in the cell, causing a cascade of enzyme-controlled reactions inside the cell

Question 29

What effects does insulin have on the target cells?

Answer 29

• more transporter proteins specific to glucose are placed into the cell surface membrane
• achieved by causing vesicles containing these transporter proteins to fuse with the membrane
• more glucose enters the cell
• glucose in the cell is converted to glycogen for storage (glycogenesis)
• more glucose is converted into fats
• more glucose is used in respiration

Question 30

What happens when blood glucose drops too low?

Answer 30

• low blood glucose is detected by the alpha cells in the islets of Langerhans
• alpha cells then secrete the hormone glucagon into the blood
• the target cells are the hepatocytes, which possesses a specific receptor for glucagon
• glucagon binds to these receptors
• this stimulates a G protein inside the membrane, activating the adenyl cyclase inside each cell
• this converts ATP to cAMP, activating a series of enzyme-controlled reactions

Question 31

What are the effects of glucagon on the target cells?

Answer 31

• glycogen is converted to glucose (glycogenolysis) by phosphorylase A, one of the enzymes activated in the cascade
• more fatty acids used in respiration
• amino acids and fats are converted into additional glucose by gluconeogenesis
• overall increases blood glucose concentration

Question 32

Draw the negative feedback loops of regulating blood glucose

Answer 32

Question 33

What is diabetes mellitus?

Answer 33

• a condition in which the body is no longer able to produce sufficient insulin to control its blood glucose concentration
• can lead to prolonged very high concentration of glucose, hyperglycaemia after a rich carb meal
• can also lead to hyperglycaemia after exercising

Question 34

Describe type 1 diabetes

Answer 34

• a result of an autoimmune response
• the body’s immune system attacks and destroys the beta cells
• no longer able to synthesise sufficient insulin and cannot store excess glucose as glycogen
• however, when the blood glucose finally falls, there is nos tore of glycogen that can be used to release glucose
• therefore, can lead to hypoglycaemia

Question 35

Describe type 2 diabetes

Answer 35

• can produce insulin but not enough or responsiveness to insulin declines
• probably specific receptors on the surface of the liver and muscle cells lose their ability to respond to insulin in the blood
• increase blood glucose concentration can damage organs and circulation
• onset factors include: obesity, lack of exercise, diet high in sugars, Asian, Afro, family

Question 36

How to treat type 1 diabetes

Answer 36

• correct dose of insulin administered
• insulin pump therapy
• islet cell transplantation
• a complete pancreas transplant
• growing new islet of Langerhans using stem cells

Question 37

How to treat type 2 diabetes

Answer 37

• lifestyle changes
• medication to reduce amount of glucose liver released to bloodstream
• maybe insulin injections

Question 38

Where is the source of insulin for treating diabetes from?

Answer 38

• used to be from pigs
• now E.coli bacteria produces insulin that has undergone genetic modification to manufacture human insulin
• exact copy of human insulin
• less chance of developing tolerance
• lower risk of infection
• cheaper to manufacture