What is the flow of current and how does the diode behaves in the circuit attached?

Circuit attached is the equivalent circuit of the photdiode.

 

Photodiode is simply diode.
Difference between diodes and photodiodes is in package.
Diodes usually are shielded from light, but photodiodes have transparent packages or windows,
which allow light access to semiconductor.
Diode in equivalent circuit is photodiode.
Under light semiconductor (more accurately p-n transition of diode) generates electrical current.
In equivalent circuit, current of current source imitates electrical current,
generated in diode under light.
ADDED:
Elements of both photodiodes in your circuit are connected in parallel, like in circuit below.
D1, D2 - diodes without capacitance.
V1, V2, V3, V4 - ammeters.
R1 - input resistance of amplifier.

 

So, In the equivalent circuit of photodiode diode is not needed right ?
only capacitance and current source is sufficient.

 

Current flow I1 and I2 which i have shown is this right ?

 

When D1 and D2 becomes Forward bias and reverse bias?
Photodiode operates in reverse so is that diode in reverse bias condition? How to say theoretically?