Topic Cell Signalling Essay - Main Points!

Q) Using named examples, describe the hormonal control of blood glucose level in human. Include the cell signalling mechanisms in the answer

Main points
Key idea:	Homeostatic control of blood glucose in human
· Normal bl glucose level in humans: 100mg/100ml of blood · When bl glucose level increases above norm: β cells (Islets of Langerhans, pancreas) detect change, secrete insulin directly into bl, transported to their target cells (e.g. liver) · When the level decreases below norm: α cells detect change, secrete glucagon directly into blood. · Blood carries hormones to target cells (e.g. liver, muscle cells)
Key idea:	Stages of cell signalling
· Ligand receptor interaction - hormones bind to their specific receptors on the cell surface membrane of target cells · Signal Transduction - upon binding, receptors change conformation resulting in signal transduction during which ext signal carried by hormone is converted into int signal · Specific cellular responses - at the end of transduction pathways
Key idea:	Named example : Cell signalling by insulin
· Insulin binds receptor tyrosine kinase (via its extracellular ligand binding sites of ind polypep subunits of the receptor)   · Subunits dimerize, activates tyrosine kinase region of each subunit, phosphorylates tyrosine residues of other subunit · Each phosphorylated tyrosine residue may bind to + activates a specific relay protein, then triggers a specific transduction pathway · A fully activated receptor may trigger diff transduction pathways, result in different cellular responses · Each transduction pathway includes phosphorylation cascade + amplification of the signal
Key idea:	Effects of insulin (cellular responses)
1) Vesicles containing glucose carrier proteins to move + fuse to the cell membrane, increasing no. of glucose carrier proteins on cell membrane surface, increases uptake of glucose into cell (i.e. increase glucose permeability) 2) Increase in rate of cellular respiration where glucose is oxidized + converted into CO2, water, ATP in presence of O2 (includes activation of glucokinase) 3) Increase conversion of glucose to protein and fat in liver cells 4) Increase in syn of DNA + RNA 5) Activation of glycogen synthetase - increase rate of conversion of glucose to glycogen (i.e. glycogenesis) 6) Inhibition of phosphorylase - glycogen no longer broken down to glucose
Key idea:	Named example : Cell signalling by glucagon
· Glucagon binds to G protein-coupled receptor, activates G protein where GTP displaces previously bound GDP in protein · Activated G protein binds to + activates adenylyl cyclase, enzyme catalyses conversion of ATP to cyclic adenosine monophosphate (cAMP), a second messenger · cAMP activates a protein kinase which triggers next step in the pathway
Key idea:	Effects of glucagon (cellular responses)
1) Activation of glycogen phosphorylase for breakdown of glycogen (i.e. glycogenolysis) - increase availability of glucose for cellular respiration 2) Enzymes (e.g. glycogen synthetase) inhibited 3) Conversion of aa + fats to glucose (i.e. gluconeogenesis) · Taken together, all these cellular responses restore the normal blood glucose concentration level
Comments: (i) Visualize the answer with the aid of a labelled diagram (ii) Verbalize the answer + write down the main points w/o referring to the answer (use abbreviations) (iii) Read again within the next 24h & once more within 72h (do not spend more than 10min) (iv) Glucokinase is involved in the initial stage of glycolysis (v) The information presented takes into consideration the new syllabus