1. Control of Metabolic Pathways
Higher Human Biology
Unit 1 – Human Cells

2. Metabolic Pathways Consist of several stages
One metabolite is coverted to another and so on
Each step is controlled by an enzyme
Each enzyme (protein) is coded for by a gene (basic)

3. If the enzymes are present, the pathway proceeds
If one enzyme is absent, the pathway comes to a halt
Gene(s)
Gene(s)
Gene(s)

4. This enzyme action can be regulated at the level of gene expression (as follows) or enzyme action (which we’ll come to later)

5. Control by switching genes on and off
To prevent wasted resources, some genes that code for enzymes are switched on and off as required.

6. E.g. Lactose Metabolism in E.coli
Lactose is a sugar found in milk
Glucose
Galactose

7. Background Glucose is used in respiration by E.coli for energy release
E.coli can only make use of glucose in lactose if it is released from galactose
This is done by ß-galactosidase (enzyme)
E.coli’s chromosome has a gene that codes for ß-galactosidase
E.coli only produces ß-galactosidase in the presence of lactose (gene is switched on)
It fails to produce ß-galactosidase if lactose is absent (or if only glucose is present) (gene is switched off)

8. LAC operon video
This is the same video that is on the website
We call the lactose an inducer as it prevents the repressor binding to the operator gene. It leads to the induction of ß-galactosidase.

9. Jacob Monod Hypothesis
The hypothesis of gene action was first put forward by two scientists – Jacob and Monod.
It has since been supported by experimental evidence from bacterial investigations.