I've been given the circuit diagram below. What it does is, the LED shines light on a fluorescent protein. Then the light released by the protein is measured by a photodiode.

In the circuit diagram, there is one circuit containing an LED (left) and the other one has a photodiode (right). I'm trying to understand how this circuit works. Is the LED and the photodiode connected to each other at all or are they separate?

So, does the circuit actually look like this:

 

The circuit given to you, shows three points label as GND. Unless explicitly indicated I could consider them common.

Is this homework? Have you access to the device?

 

The circuit given to you, shows three points label as GND. Unless explicitly indicated I could consider them common.

Is this homework? Have you access to the device?

Thank you for your reply! This isn't homework, it's a project I did in class and I'm trying to understand for myself how the circuit works. I don't have the device with me right now
Could you please draw a sketch of the circuit for me, in the way I did? It would really help me understand

 

There is no need for a common ground when using light as the signal/transfer medium. A common ground may be harmless, but it can also introduce noise. Same goes for the +5V supply. In some ancient photometers, the detector was powered by a lead acid battery rather than from wall voltage for that reason.

I your drawing, I missed the signal labeled "A0". If it is the center conductor in the 1st schematic, then the two versions look equivalent.

Edit: Missed the orientation of the photodiode as pointed out below.

 

I your drawing, I missed the signal labeled "A0". If it is the center conductor in the 1st schematic, then the two versions look equivalent.

..except that the photodiode is backwards in the hand-drawn version.

@Anya4598 it's a good idea to draw your schematics using standard conventions i.e. power at the top, ground at the bottom and signal flow from left to right. It makes for easier understanding and fewer mistakes.