09 September 2015

#114 Control of blood glucose - glucagon

Insulin and glucagon work together as part of a negative feedback system. As a result of glucagon secretion, the liver releases extra glucose to increase the concentration in the blood. Muscle cells do not have receptors for glucagon and so do not respond to it.

This is question 9, taken directly from the specimen paper for summer 2016 so you won't have to worry about the phrasing :) It is for glucagon but will work fine with adrenaline.

(the numbers marked are according to the mark scheme and not the diagram below)
1. glucagon binds to receptors in cell surface membrane (of liver cell)
2. receptor changes conformation
3. G-protein activated
4. adenylate cyclase activated
5. ATP converted to cyclic AMP
6. (cyclic AMP is) second messenger
7. (cyclic AMP) activates kinase protein 
8. enzyme cascade: amplifies original signal of glucagon
9. ref. phosphorylase enzyme(s) / glycogen phosphorylase
10. glycogen broken to glucose 
11. glucose, diffuses / passes out, of (liver) cell (into the blood)
12. through GLUT2 transporter proteins
13. AVP ; e.g. ref. to stimulating gluconeogenesis


 Syllabus 2016-2018

 14.1  Homeostasis in mammals

Homeostasis in mammals requires complex systems to maintain  internal conditions near  constant.

The kidneys  remove wastes from the blood and are the effectors for controlling the water potential of the blood.

a)   discuss the importance of homeostasis in mammals and explain the principles  of homeostasis in terms of internal and external stimuli, receptors, central  control,  co-ordination systems, effectors (muscles and glands)

b)   define  the term  negative feedback and explain how it is involved in homeostatic mechanisms

c)   outline  the roles of the nervous system and endocrine system in co-ordinating homeostatic mechanisms, including thermoregulation, osmoregulation and the control of blood glucose concentration

d)   describe the deamination of amino  acids  and outline  the formation of urea  in the urea  cycle (biochemical detail of the urea  cycle is not required)

e)   describe the gross structure of the kidney and the detailed structure of the nephron with its associated blood vessels using photomicrographs and electron micrographs

f) describe how the processes of ultrafiltration and selective reabsorption are involved with the formation of urine in the nephron

g)   describe the roles of the hypothalamus, posterior pituitary, ADH and collecting  ducts in osmoregulation

h)   explain how the blood glucose concentration is regulated by negative feedback control mechanisms, with reference to insulin and glucagon

i) outline  the role of cyclic AMP as a second messenger with reference to the stimulation of liver cells by adrenaline and glucagon

j) describe the three main stages of cell signalling in the control of blood glucose by adrenaline as follows:

•   hormone-receptor interaction at the cell surface

•   formation of cyclic AMP which binds to kinase  proteins
an enzyme cascade involving activation  of enzymes by phosphorylation to amplify the signal

k)   explain the principles  of operation of dip sticks  containing glucose oxidase and peroxidase enzymes, and biosensors that can be used for quantitative measurements of glucose in blood and urine

l) explain how urine analysis  is used in diagnosis with reference to glucose, protein  and ketones

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