The shape of the lesson
By the end of the lesson, learners can
- Glucagon and the cAMP cascade: Press Next step to move through all seven stages.
- Insulin and glucagon: negative feedback: Press After a meal (blood glucose jumps to 9 mmol dm −3) and watch insulin act on the muscle cell and liver cell; the green GLUT4 vesicles move to the membrane.
- Measuring glucose: strips and biosensors: Choose test strip, set the glucose slider (0 to 20 mmol dm −3) and press Test: the pad darkens and you match it against the colour chart, which only reads to the nearest 5.
- Glucagon (first messenger) → receptor → G protein → adenylyl cyclase.
- cAMP (second messenger) → protein kinase A → phosphorylation cascade.
- Insulin: GLUT4 in muscle, glycogenesis in liver. Glucagon: liver only.
- Strip: glucose oxidase + peroxidase + chromogen. Biosensor: current.
One picture every idea returns to
Blood glucose is kept near 5 mmol dm−3 by negative feedback. The α and β cells of the islets of Langerhans in the pancreas are both receptors and the source of the hormones. When glucose is high, β cells secrete insulin: in muscle cells vesicles with GLUT4 transporters fuse with the cell surface membrane, so more glucose enters by facilitated diffusion; in liver cells (and muscle) enzymes for glycogenesis are activated, and glucose is respired faster. When glucose is low, α cells secrete glucagon, which acts on liver cells: it binds to a cell surface receptor, causing a conformational change that activates a G protein, which stimulates adenylyl cyclase to make cyclic AMP (the second messenger). cAMP activates protein kinase A, starting an enzyme cascade of phosphorylations that amplifies the signal until glycogen phosphorylase breaks down glycogen to glucose. Test strips and biosensors measure glucose using glucose oxidase (glucose to gluconic acid and hydrogen peroxide) and, on strips, peroxidase with a colourless chromogen that becomes coloured.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, One hormone molecule, thousands of glucose.. After a meal, glucose floods into the blood; during a long run it is used up fast. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Glucagon and the cAMP cascade | Press Next step to move through all seven stages. Project tab 6 of the lab (Homeostasis lab, cell signalling) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Insulin and glucagon: negative feedback | Press After a meal (blood glucose jumps to 9 mmol dm −3) and watch insulin act on the muscle cell and liver cell; the green GLUT4 vesicles move to the membrane. Project tab 7 of the lab (Homeostasis lab, insulin and glucagon) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Measuring glucose: strips and biosensors | Choose test strip, set the glucose slider (0 to 20 mmol dm −3) and press Test: the pad darkens and you match it against the colour chart, which only reads to the nearest 5. Project tab 8 of the lab (Homeostasis lab, test strips and biosensors) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 23 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Measuring glucose: strips and biosensors, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The hormone is the first messenger; it never enters the cell. cAMP is the second messenger that carries the signal through the cytoplasm. | Return to Glucagon and the cAMP cascade: ask a pair to state the correct version and show it on the lab before moving on. |
| Glycogen, glucagon and glycogenesis are easy to confuse. | Glucagon is the hormone; glycogen is the storage polysaccharide; glycogenesis makes glycogen; glycogenolysis breaks it down. |
Cell signalling and blood glucose: worksheet
6 questions, 23 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how glucagon causes a liver cell to release glucose into the blood.
Explain why glucagon affects liver cells but not muscle cells.
Explain how blood glucose concentration is returned to normal after a meal rich in carbohydrate.
A person drinks a glucose solution. Their blood glucose is 5.0 mmol dm−3 at the start, peaks at 8.2 mmol dm−3 after 40 minutes and is 5.4 mmol dm−3 at 120 minutes. Calculate the percentage increase at the peak and explain the fall after 40 minutes.
Explain the principles of a glucose test strip used on a urine sample.
Suggest two advantages of a biosensor over a urine test strip for a person with diabetes who needs to decide on a dose of insulin.
Answer key and marking notes
- Glucagon binds to a specific receptor on the cell surface membrane of the liver cell, causing a conformational change in the receptor.
- The receptor activates a G protein, which activates adenylyl cyclase.
- Adenylyl cyclase converts ATP into cyclic AMP, the second messenger.
- cAMP activates protein kinase A, which starts an enzyme cascade.
- Each enzyme activates many molecules of the next by phosphorylation, amplifying the signal.
- The final enzyme, glycogen phosphorylase, is activated and breaks down glycogen to glucose, which diffuses out into the blood.
- Only liver cells have glucagon receptors on their cell surface membranes.
- Glucagon cannot enter cells, so without a receptor there is no conformational change and no cAMP cascade in muscle cells.
- The rise in blood glucose is detected by β cells in the islets of Langerhans, which secrete more insulin (and α cells secrete less glucagon).
- Insulin binds to receptors on muscle cells; vesicles containing GLUT4 fuse with the cell surface membrane.
- More glucose enters muscle cells by facilitated diffusion.
- In liver (and muscle) cells insulin activates enzymes for glycogenesis, converting glucose to glycogen, and increases the respiration of glucose.
- Blood glucose falls; as it returns to the set point insulin secretion decreases: negative feedback.
- Increase = (8.2 − 5.0) ÷ 5.0 × 100 = 64 %.
- The high glucose stimulates β cells to secrete insulin.
- Insulin increases glucose uptake by muscle (GLUT4) and liver cells and its conversion to glycogen, so blood glucose falls back towards 5 mmol dm−3.
- The pad contains immobilised glucose oxidase, peroxidase and a colourless chromogen.
- Glucose oxidase catalyses the oxidation of glucose to gluconic acid, producing hydrogen peroxide.
- Peroxidase catalyses the reaction of hydrogen peroxide with the chromogen, forming a coloured compound.
- The intensity of colour depends on the glucose concentration and is compared with a colour chart after a set time.
- The biosensor gives a quantitative, precise digital reading, not a colour match that is only semi-quantitative / subjective.
- It measures blood glucose now; urine glucose reflects the past few hours and only appears once blood glucose is above the renal threshold.
- So low or only slightly high blood glucose can be detected and the insulin dose adjusted accurately (any two points).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which molecule acts as the second messenger when glucagon stimulates a liver cell?
- Acyclic AMP
- Bglucagon
- Cadenylyl cyclase
- Dglycogen phosphorylase
What is the correct order of events after glucagon binds to its receptor on a liver cell?
- AG protein activated → adenylyl cyclase activated → cAMP made → protein kinase A activated
- Badenylyl cyclase activated → G protein activated → protein kinase A activated → cAMP made
- CcAMP made → G protein activated → adenylyl cyclase activated → protein kinase A activated
- Dprotein kinase A activated → cAMP made → G protein activated → adenylyl cyclase activated
Which effect does insulin have on muscle cells?
- Amore GLUT4 transporters are inserted into the cell surface membrane
- Bglycogen in the muscle is broken down to release glucose into the blood
- Camino acids are converted into glucose by gluconeogenesis
- Dthe cells stop respiring glucose and use fatty acids instead
Why does glucagon cause glycogen breakdown in liver cells but not in muscle cells, although both cells store glycogen?
- Amuscle cells have no glucagon receptors in their cell surface membranes
- Bglucagon cannot travel in the blood to the muscles
- Cmuscle cells contain no enzymes that can break down glycogen
- Dglucagon enters liver cells but is too large to enter muscle cells
On a glucose test strip, what is the role of the enzyme peroxidase?
- Ait catalyses the reaction of hydrogen peroxide with a chromogen to form a coloured product
- Bit catalyses the oxidation of glucose to gluconic acid and hydrogen peroxide
- Cit produces an electric current proportional to glucose concentration
- Dit breaks down any sugars other than glucose so they do not interfere
The graph shows blood glucose after a glucose drink for person A and person B. Which statement is the best explanation of the results for person B?
- AB secretes little or no insulin, so glucose uptake by muscle and liver cells stays low
- BB secretes too much glucagon after the drink, so glycogen is made more quickly
- CB absorbs glucose from the gut more slowly than A, so the peak is reached later
- DB secretes more insulin than A, so glucose stays high for longer
In a liver cell, one glucagon receptor activates 10 molecules of adenylyl cyclase, each of which makes 100 cAMP. Every 2 cAMP activate one protein kinase A, and each protein kinase A activates 10 molecules of the next enzyme. How many molecules of that next enzyme are activated?
- A5000
- B10 000
- C1000
- D500
A biosensor for blood glucose uses glucose oxidase immobilised on an electrode. Why does it give a more reliable result than a urine test strip for adjusting an insulin dose?
- Ait gives a precise numerical reading of the blood glucose concentration at that moment
- Bit measures glucose in the blood using a different enzyme from the strips
- Cit detects glucose only after the kidneys have stopped reabsorbing it
- Dit shows a colour change that can be matched against a colour chart
Quiz key
Glucagon (the first messenger) binds to a cell surface receptor; via a G protein, adenylyl cyclase is activated and converts ATP into cyclic AMP, the second messenger that activates protein kinase A. Adenylyl cyclase is the enzyme that makes the second messenger. Glycogen phosphorylase is the final enzyme in the cascade, which breaks down glycogen.
The receptor changes shape and activates a G protein, which activates adenylyl cyclase; this converts ATP into cAMP, and cAMP activates protein kinase A to start the cascade. cAMP cannot be made before adenylyl cyclase is active, and protein kinase A is activated by cAMP, so it cannot come first.
Insulin causes vesicles containing GLUT4 to fuse with the cell surface membrane of muscle cells, so more glucose enters by facilitated diffusion; it also stimulates glycogenesis. Breaking down glycogen and gluconeogenesis raise blood glucose, which is the job of glucagon in the liver. Insulin increases, not stops, the use of glucose in respiration.
Target cells need specific receptors. Liver cells have glucagon receptors; muscle cells do not, so glucagon has no effect on them. Glucagon is carried in the blood to all tissues. Muscle cells can break down glycogen (for their own use, for example when stimulated by adrenaline). Glucagon does not enter any cell; it binds to receptors on the outside.
Glucose oxidase converts glucose and oxygen into gluconic acid and hydrogen peroxide; peroxidase then uses the hydrogen peroxide to convert a colourless chromogen into a coloured compound. The electric current is produced in a biosensor electrode, not by peroxidase. Specificity comes from glucose oxidase acting only on glucose, not from any enzyme destroying other sugars.
B starts higher (7 mmol dm−3 compared with 5), rises to over 13 mmol dm−3 and is still near 12 after 4 hours, so glucose is not being removed: this fits little or no insulin (type 1 diabetes), meaning few GLUT4 transporters in muscle membranes and little glycogenesis. Glucagon raises blood glucose by glycogen breakdown, not synthesis. Slow absorption would give a lower, later peak followed by a fall, not a sustained high level. More insulin would lower glucose faster, not keep it high.
cAMP = 10 × 100 = 1000. Protein kinase A activated = 1000 ÷ 2 = 500. Next enzyme = 500 × 10 = 5000. 10 000 forgets to halve at the protein kinase A step; 1000 is the number of cAMP molecules; 500 is the number of protein kinase A molecules. The calculation shows how each step of the cascade amplifies the signal.
The current produced at the electrode is proportional to glucose concentration and is converted to a digital value, so the reading is quantitative and reflects blood glucose now. Both methods use glucose oxidase, so the enzyme is not the difference. Glucose appears in urine only once blood glucose is above the level the PCT can reabsorb; that is a weakness of urine strips, not a feature of the biosensor. Colour matching is the semi-quantitative method used by strips.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.