Control of Blood Glucose (Chapter 14)

Question 1

What happens to carbohydrate in the body?

Answer 1

1) it is transported through the blood stream in the form of glucose in solution in the blood plasma
2) glucose is converted into the polysaccharide glycogen, a short-term energy store found in liver and muscle cells

Question 2

What is the concentration of glucose in a healthy human?

Answer 2

80-120mg of glucose per 100cm3 of blood

Question 3

What happens when there is a low concentration of glucose in the blood?

Answer 3

There is not enough glucose for respiration, ∴ cells can’t carry out normal functions - especially important for brain cells, which can only respire glucose

Question 4

What happens when there is a very high concentration of glucose in the blood?

Answer 4

It can cause major problems, upsetting normal behaviour of cells

Question 5

What is the homeostatic control of blood glucose concentration carried out by?

Answer 5

2 hormones, glucagon and insulin, secreted by endocrine tissue in the pancreas

Question 6

What does the endocrine tissue in the pancreas consist of?

Answer 6

Groups of cells (islets of Langerhans), scattered throughout the pancreas

Question 7

What 2 types of cells does the islets of Langerhans contain and what does each 1 secrete?

Answer 7

1) alpha cells secrete glucagon

2) beta cells secrete insulin

Question 8

What do alpha and beta cells act as?

Answer 8

The receptors and central control for the control of blood glucose concentration

Question 9

What do glucagon and insulin coordinate?

Answer 9

The actions of effectors

Question 10

What happens after a meal when glucose is absorbed into the small intestine and passes into the blood?

Answer 10

1) as this blood flows through the pancreas, alpha and beta cells detect this increase in glucose concentration
2) alpha cells respond by stopping secretion of glucagon, which liver cells respond to and ∴ there is no breakdown of glycogen
3) beta cells respond by secreting insulin into the blood plasma
4) insulin is carried to all parts of the body, in the blood

Question 11

What is insulin?

Answer 11

A signalling molecule

Question 12

Why can’t insulin stimulate mechanisms within the cell directly?

Answer 12

Because it is a protein and can ∴ not pass through CSMs

Question 13

How does the release of insulin result in the decrease in the concentration of blood glucose?

Answer 13

1) insulin binds to a receptor on the CSM and affects the cell indirectly through the mediation of intracellular messengers
2) insulin stimulates cells with insulin receptors e.g. muscle cells, to move vesicles containing GLUT4 proteins from the cytoplasm to the CSM and fuse with it
3) GLUT4 proteins facilitate the diffusion of glucose into the cell, down its concentration gradient ∴ increasing the rate at which they absorb glucose from the blood, convert it to glycogen and use it in respiration
4) this results in the decrease of blood glucose concentration

Question 14

What is the only way that glucose can enter cells?

Answer 14

Through a transporter protein called GLUT

Question 15

Where are GLUT4 proteins normally kept?

Answer 15

In the cytoplasm

Question 16

Which GLUT proteins are always in the CSM and not affected by insulin?

Answer 16

GLUT1 in brain cells and GLUT2 in liver cells

Question 17

How does insulin also trap glucose inside cells?

Answer 17

1) insulin stimulates the activation of the enzyme glucokinase, which phosphorylates glucose
2) phosphorylated glucose cannot pass through the GLUT transporters in the CSM bc it is too big and charged ∴ the glucose is trapped inside the cells

Question 18

What other 2 enzymes does insulin activate and what do they do?

Answer 18

Glucofructokinase and glycogen synthase, which add glucose molecules to glycogen, increasing the size of glycogen granules in the cell

Question 19

What happen when a decrease in blood glucose concentration is detected by alpha and beta cells?

Answer 19

1) alpha cells respond by secreting glucagon

2) beta cells respond by stopping the secretion of insulin

Question 20

What is the effect of decreasing the concentration of insulin in the blood?

Answer 20

The rate of uptake and use of glucose in the liver and muscle cells decreases - uptake still continues, but at a lower rate

Question 21

How does glucagon increase the concentration of blood glucose?

Answer 21

1) glucagon binds to receptor molecules on the CSMs of liver cells, causing the start of cell signalling
2) this activates a G protein that in turn activates and enzyme within the membrane that catalyses the conversion of ATP to cyclic AMP (a second messenger)
3) cyclic AMP binds to kinase enzymes within the cytoplasm that activate other enzymes by adding phosphate groups to them
4) this enzyme cascade amplifies the original signal from glucagon
5) at the end of the enzyme cascade, glycogen phosphorylase is activated and catalyses the breakdown of glycogen to glucose by removing glucose units from the numerous ‘ends’ of glycogen
6) this increases the glucose concentration inside the cell, so that it diffuses out through GLUT2 transporter proteins into the blood ∴ increasing blood glucose concentration

Question 22

How else can glucose be made?

Answer 22

From amino acids and lipids in a process called gluconeogenesis

Question 23

What else happens as a result of glucagon secretion?

Answer 23

The liver releases extra glucose to increase blood glucose concentration

Question 24

Why do muscle cells not respond to glucagon?

Answer 24

Bc they do not have receptors for glucagon

Question 25

What kind of homeostatic mechanism is the control of blood glucose concentration?

Answer 25

Glucagon and insulin work together as part of the negative feedback system in which any deviation of blood glucose concentration from the set point stimulates actions by effectors (liver and muscle cells) to bring it back to normal

Question 26

Why does blood glucose concentration never remain constant?

Answer 26

• Bc of the time delay between a change in blood glucose concentration and the onset of actions to correct it
• These time delays in control systems result in oscillation, where blood glucose concentration sometimes rises slightly above or drop lightly below the required level

Question 27

What other hormone increases the blood glucose concentration?

Answer 27

Adrenaline

Question 28

How does adrenaline increase the blood glucose concentration?

Answer 28

1) adrenaline binds to receptors on the surface of liver cells that activate the same enzyme cascade as glucagon and leads to the same end result - the breakdown of glycogen by glycogen phosphorylase
2) adrenaline also stimulates the breakdown of glycogen stores in muscle cells during exercise - the glucose produced remains in muscle cells where it is needed for respiration

Question 29

What is 1 of the most common metabolic diseases in humans?

Answer 29

Sugar diabetes (diabetes mellitus)

Question 30

What are the 2 types of sugar diabetes?

Answer 30

1) insulin-dependent (type 1) - the pancreas is incapable of secreting sufficient insulin, due to a deficiency in gene coding for production of insulin or an attack on beta cells by the immune system
2) non insulin-dependent diabetes (type 2) - the pancreas does secrete insulin, but liver and muscle cells do not respond properly to it - associated with diet and obesity

Question 31

What are the symptoms of sugar diabetes?

Answer 31

1) when glucose is reabsorbed into the blood after a carbohydrate meal, glucose concentration increases and stays high
2) some glucose (+salts and water) passes out in the urine because the kidney cannot reabsorb all the glucose∴ person can feel hungry and thirsty
3) slow uptake of glucose into cells, even when there is plenty in the blood
4) between meals, blood glucose concentration may decrease steeply because there is no glycogen to mobilise as it was not stored when there was plenty of glucose

Question 32

How might a coma result in someone with diabetes?

Answer 32

1) as there is a slow uptake of glucose into cells, the cells lack glucose and ∴ metabolise fats (+proteins) as alternative energy sources
2) metabolising fat can lead a build up of veto acids in the blood, which decreases the blood pH, which can denature enzymes
3) the combination of dehydration, salt loss and low blood pH can cause coma in extreme situations
4) if there is not enough glucose for respiration, coma may result

Question 33

What is the treatment for people with type 1 diabetes?

Answer 33

1) they receive regular injections of insulin, made by genetically-engineered cells and blood samples are taken to check that the insulin is effective
2) mini-pump deliver the exact volumes of insulin they need, when they need it
3) carefully-controlled diet can also help to maintain a near-constant blood glucose concentration

Question 34

What is the treatment for people with type 2 diabetes?

Answer 34

They use diet and regular exercise to keep blood glucose within normal limits

Question 35

Why are urine samples often taken?

Answer 35

1) they are easier to collect than blood samples

2) simple tests of urine can give early indications of health problems, which can then be investigated more thoroughly

Question 36

What does the presence of glucose and ketones in the urine indicate?

Answer 36

That are person may have diabetes - if blood glucose concentration increases above a certain value, the renal threshold, not all of the glucose is reabsorbed from the filtrate in the PCT of the kidney ∴ some is present in the urine

Question 37

When is it not unusual for some protein to be present in the urine for short periods of time?

Answer 37

During a high fever, pregnancy or after vigorous exercise

Question 38

What does a large quantity or long term presence of protein in the urine indicate?

Answer 38

That there may be a disease affecting the glomeruli, a kidney infection or high blood pressure, which is a risk factor in heart disease

Question 39

Why is there normally no protein in the urine?

Answer 39

Most protein molecules are too large to be filtered and the ones that are are then reabsorbed by endocytosis in the PCT, broken down and the amino acids are absorbed by the blood

Question 40

What are dipsticks?

Answer 40

Test strips that can be used to test urine for a range of different factors, including pH, glucose, ketones and protein

Question 41

What enzymes to dipsticks for detecting glucose contain?

Answer 41

Glucose oxidase and peroxidase

Question 42

How does a glucose dipstick work?

Answer 42

1) the 2 enzymes are immobilised on a small pad at 1 end of the stick
2) the pad is immersed in urine and if it contains glucose, glucose oxidase catalyses a chemical reaction in which glucose is oxidised into gluconolacctone and hydrogen peroxide is also produced
3) peroxidase catalyses the reaction between hydrogen peroxide and a colourless chemical in the pad to form a brown compound
4) the resulting colour of the pad is matched against a colour chart
5) the colour chart shows the colours that indicate different glucose concentration - the more glucose present, the darker the colour

Question 43

What is the problem with urine tests?

Answer 43

They do not indicate the current blood glucose concentration, but rather whether the concentration was higher than the renal threshold in the period of time while urine was collecting in the bladder

Question 44

What does a biosensor allow?

Answer 44

People with diabetes to check their blood to see how well they are controlling their glucose concentration

Question 45

How does a biosensor work?

Answer 45

1) like the dipstick, the biosensor uses a pad impregnated with glucose oxidase
2) a small sample of blood is placed on the pad is inserted
3) glucose oxidase catalyses the reaction to produce gluconolactate and at the same time, a tiny electric current is generated
4) the current is detected by an electrode, amplified and read by the meter, which produces a reading for blood glucose concentration in seconds
5) the more glucose present, the greater the current and the greater the reading from the biosensor