Topic Biomolecule & cell signalling

Q) Describe the differences between glycogen and glucagon

Main points
Key idea:	Type of molecule + building blocks + bonds
Glycogen ·  Polysaccharide, made up of many α glucose ·Joined by α-1,4 glycosidic bonds ·Highly branched struc ·Ref to α-1,6 glycosidic bonds at branch pts Glucagon ·Protein, made up of aa ·Joined by peptide bonds ·Globular struc due to infolding of polypep chain upon itself ·Ref to interactions (e.g. H, ionic, disulphide & hydrophobic) b/w R gps of faraway aa residues
Key idea:	Role + location
Glycogen ·Energy storage mol · Stored in muscle & liver Glucagon ·Hormone; regulates bl glucose level ·Stored in the α cells, islets of Langerhans, pancreas
Key idea:	Solubility in water
Glycogen ·Soluble in H2O; can be transported via blood to targe cells/tissue ·Infolding of polypep affect distribution of aa ·Ref to polar aa residues on ext surf of mol; non-polar aa residues in the interior Glucagon ·Insoluble in H2O ·Ref to polar OH gps project into mol ·Ref to large size as a contributing factor too
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)

Topic Homeostasis & Cell Signalling Essay - Main Points!

Describe the mechanism of action of glucagon in the homeostatic control of human blood glucose

Main points
Key idea:	Role of glucagon in maintaining homeostatic level of glucose at 100mg/100ml of blood
· Glucagon secreted when blood glucose drops below 100mg/100ml of bl · Ref to α cells (of Islets of Langerhans) detect this change & secrete glucagon directly into bl · Bl transport peptide hormone to target cells (e.g. liver cells)
Key idea:	Glucagon signalling in target cells
· Peptide hormone binds to G protein-linked receptor + activates G protein · Ref to a GTP displaces previously bound GDP in protein · Active G protein moves freely along membr + binds to & activates adenylyl cyclase · Ref to enz catalysing conversion of ATP to cyclic adenosine monophosphate (cyclic AMP or cAMP), a second messenger · cAMP then activates a protein kinase which triggers next step in pathway leading to cell’s response
Key idea:	Cellular responses
· Cellular responses include (i) activation of glycogen phosphorylase for breakdown of glycogen (i.e. glycogenolysis), increase in availability of glucose for cellular resp (ii) Enz (e.g. glycogen synthase) also inhibited (iii) Conversion of aa & fats to glucose (i.e. gluconeogenesis). · Taken together, all these cellular responses restore normal bl glucose conc 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) Insulin and glucagon are peptide hormones. They are unable to pass through the cell memrbane via simple diffusion. They bind to specific cell surface receptors and initiate signal transduction pathways that result in specific cellular responses that will collectively restore the homeostatic level of blood glucose in the body.

Topic Cell Signalling Essay - Main Points!

Q) Using named examples, discuss the main stages of cell signalling

Main points
Key idea:	State 3 main stages of cell signalling
· (i) Ligand-receptor interaction (ii) signal transduction (iii) cellular response
Key idea:	Advantage of signal molecules that bind to surface receptors
· Only target cells respond to signal mol · Ref to non-target cells lacking the specific receptors for the particular signal mol
Key idea:	(i) Ligand-receptor interaction (w examples)
· Certain polar signal mol (ligands) do not enter cell due to hydrophobic core of phospholipid bilayer of membrane · Bind to cell surface receptors · E.g. Insulin – bind to specific receptor tyrosine kinase  · E.g. Glucagon – bind to specific G protein-linked receptor · Interaction of ligand w its specific receptor changes conformation of receptor,  initiate signal transduction
Key idea:	(ii) Signal transduction pathway – component + initiation (example)
· Consists of a series of sequential biochem rx. · E.g. binding of insulin to receptor - phosphorylation of tyrosine residues in the polypep receptor ; signal transduction begins when each phosphorylated tyrosine residue bind to and activates a specific relay protein which in turn triggers a specific transduction pathway
Key idea:	Features of the signal transduction pathway
· Often includes phosphoryation cascade - phosphorylation + activation of a kinase at one level of pathway leads to phosphorylation of several downstream kinases. · Allows for the amplification of the signal
Key idea:	(iii) Cellular response
· A fully activated receptor may trigger diff transduction pathways - result in diff cellular responses from same cell · Due to the amplification of signal along pathway, response will be large despite minute amount of signal molecules (e.g. hormones) released (into the blood)
Key idea:	Cell able to switch off response + mechanisms
· This ensures cells are able to respond appropriately to future signals · E.g. Phosphatases - dephosphorylate + inactivate kinase relay mol · E.g. Intrinsic GTPase activity of G protein - hydrolyzes its bound GTP (i.e. G protein inactivated) · E.g. Phosphodiesterase - converts cAMP to AMP (i.e. 2nd messengers removed)
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)

Topic: Respiration/Photosyn/Cell Signalling Essay - Main Points!

Q) Using named examples, explain the advantages of metabolic pathways in cells

Main points
Key idea:	What is a metabolic pathway
· Ref to series of rx where product of one rx becomes substrate for next rx
Key idea:	Adv 1 : Allows a crucial molecule to be made from different sources
· Named example: Acetyl-CoA (of aerobic resp) · Acetyl-CoA as product of link rx which becomes substrate for Krebs cycle · Ref to its synthesis fr breakdown of glucose, fatty acids or aa throu other pathways; hence diff entry points to Krebs cycle
Key idea:	Adv 2 : End product inhibition + channel intermediate products to other pathways to make another product
· Named example: Glycolysis · Glucose mol oxidized throu a series of rxs (each rx catalyzed by an enz), form pyruvate, ATP & NADH · When ATP is sufficient, it acts as an allosteric inhibitor · Ref to it binding to an allosteric site on enz (phosphofructokinase) at initial part of glycolytic pathway, inhibits the enz · Result: Not only prevents formation of excess ATP but allows glucose-6-phosphate to be diverted to another pathway that makes glycogen
Key idea:	Adv 3 : Cell responds appropriately to its environmental conditions
· Named example: (fate of) Pyruvate when O2 absent · Will not enter mitochondrion (to combine w co-enzyme A to form acetyl-CoA) · Pyruvate reduced to lactate (in animals) or ethanol (in plants) in cytosol instead · Result: anaerobic respiration still proceed to yield 2 ATP/glucose
Key idea:	Adv 4 : A cyclical metabolic pathway allows its 1st substrate to be regenerated
· Named example: Krebs cycle & Calvin cycle · Result: (i) Krebs cycle (aerobic resp) - regenerate oxaloacetate, hence NADH produced continuously for ATP syn during oxidative phosphorylation (ii) Calvin cycle (light independent rx, photosyn) - regenerate RuBP for continuation of CO2 fixation · Ref to some phosphates made during reduction of glycerate-3-phosphates channeled to another pathway that synthesizes starch
Key idea:	Adv 5 : Crucial in cell signalling
· Named example: Insulin & glucagon signalling · Ref to binding of a signal mol (e.g. hormone) to its specific receptor, result: activation of signal transduction pathways + eventual cellular responses · Details: (i) Insulin binds to receptor tyrosine kinase, activation of several relay mol, in turn activate diff pathways · Ref to diff cellular responses due to binding of hormone to one target cell · Details: (ii) Glucagon binds to G-protein linked receptor, activation of many G proteins, in turn activate adenylyl cyclases · Ref to enz yield enormous amt of second messager cAMP · Each cAMP activates a protein kinase, in turn activates many other protein kinases · Result: phosphorylation cascade + ‘weak’ signal carried by a hormone greatly amplified (i.e. strong cellular response)
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) A rather challenging question as it requires a deeper understanding of the various concepts from different topics (e.g. respiration, photosynthesis and cell signaling). (v) The answer provided is more than sufficient to gain full credit for this question

Topic Homeostasis Essay - Main Points!

Q) Explain what is meant by an endocrine gland

Main points
Key idea:	What is endocrine gland + features
· Group of cells, secrete small amount of hormones (chemical messengers) · Ductless, secretes hormones directly into bloodstream · Ref to transport of hormones to specific target cells/tissues, affect their activities · Ref to integration, coordination, control of physiological activities of organism
Key idea:	Named example : Islets of Langerhans, pancreas
· Made up of α cells + β cells - secrete glucagon + insulin, respectively · Ref to a rich network of bl capillaries in direct contact w secretory cells · Direct release of hormones into bl · E.g. of target cells of insulin and glucagon are liver cells
Key idea:	Control of secretion of hormones
· Rel of either homones - determined by changes of level of bl glucose (stimulus/input) · If bl glucose level is above norm, receptors on surface of β cells detect change,  insulin released · If bl glucose level is below norm, receptors on surface of α cells detect change, glucagon released · Hence, endocrine gland ensures homeostatic level of blood glucose · Via negative feedback mechanism · When homeostatic level is achieved, the gland stops secreting hormones
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)

Topic Nervous Control / Homeostasis Essay - Main Points!

Q) Distinguish between endocrine system and nervous system

Main points
Key idea:	Nature & mode of transmission
Endocrine sys: · Info transmitted chemically · Hormones secreted directly into bloodstream, transported throughout body, regulate activities of target cells upon arrival Nervous sys: · Info transmitted electrically · Nerve impulses travel along nerve axons to a particular destination in body · But transmission of information a/x a synapse occurs chemically (i.e. neurotransmitter)
Key idea:	Nature of the signal molecule
Endocrine sys: · Many types of hormones present (each affecting specific tissues) Nervous sys: · Fewer types of signal mol (e.g. acetylcholine, noradrenaline)
Key idea:	Nature of response
Endocrine sys: · Graded response as strength of cellular response may be due to amount of hormone secreted by endocrine gland · Strength of cellular response subjected to regulation of activities of relay mol along signal transduction pathways in target cells (i.e. ‘cross-talk’ b/w diff signal transduction pathways) Nervous sys: · Action potential follows all-or-nothing principle · If stimulus is strong enough to achieve threshold potential, an AP of a fixed magnitude will be achieved
Key idea:	Speed and duration of effect
Endocrine sys: · Speed is generally slow but long lasting effect (e.g. growth and development). Nervous sys: · Rapid but generally short-lived (e.g. withdrawal from a hot object).
Key idea:	Location of effect
Endocrine sys: · Generally widespread as blood able to transport hormones to target tissues or organs   Ref to which may be located far fr each other and fr source of the hormone Nervous sys: · As nerve impulses travel along nerve axons to a part destination in body, effect is localized (e.g. contraction of a particular muscle gp in one part of the body)
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 e.g. AP – action potential) (iii) Read again within the next 24h & once more within 72h (do not spend more than 10min)