Controlling blood glucose
Controlling blood glucose
- Blood glucose is held steady by two hormones working against each other.
- The hormone glucagon also shows how a signal gets into a cell.
- That signalling amplifies a tiny message into a big response.
Insulin and glucagon
- When glucose is high, insulin makes muscle and liver cells take in glucose and store it as glycogen — lowering the level.
- When glucose is low, glucagon makes liver cells break glycogen back into glucose — raising the level.
- Two opposing hormones = negative feedback.
Practice
When blood glucose is high, insulin:
Insulin lowers blood glucose by promoting uptake and storage as glycogen.
Practice
When blood glucose is low, glucagon:
Glucagon raises blood glucose by breaking down glycogen into glucose in the liver.
The glucagon cascade (cell signalling)
- glucagon binds a receptor on the liver cell, changing its shape.
- this activates a G-protein, which switches on adenylyl cyclase.
- adenylyl cyclase makes a second messenger, cyclic AMP (cAMP).
- cAMP activates protein kinase A, starting an enzyme cascade (each enzyme switches on the next).
- because each step activates many, the signal is greatly amplified.
- the final enzyme breaks glycogen into glucose.
Practice
In the glucagon cascade, the second messenger inside the cell is:
Adenylyl cyclase makes cAMP, the second messenger that activates protein kinase A.
Practice
Why does the enzyme cascade amplify the signal?
At each step one enzyme switches on many others, multiplying the effect greatly.
Measuring glucose
- Test strips and biosensors use the enzymes glucose oxidase and peroxidase.
- They react with glucose to give a colour change (or an electrical signal) showing how much is present.
Practice
Glucose test strips and biosensors detect glucose using the enzyme:
Glucose oxidase (with peroxidase) reacts with glucose to give a colour change or electrical signal.
You've got it
Key idea
- insulin (high glucose) → store as glycogen; glucagon (low glucose) → break glycogen → glucose
- glucagon cascade: receptor → G-protein → adenylyl cyclase → cAMP (second messenger) → protein kinase A → enzyme cascade
- the cascade amplifies the signal hugely
- glucose measured with glucose oxidase (test strips / biosensors)