Bioplastics or organic plastics are derived from renewable sources such as starch, vegetable oil and even chicken feathers.
Some plant derived plastics biodegrade, some do not.

The term bio-plastics is used to describe both types of plant derived plastic i.e. biodegradable and biomass derived.

This has led to some CONFUSION as some now think all bioplastics, biodegrade.
Not so.
Some bioplastics, like PLA plastic do biodegrade, indeed are certified compostable. 
Others, like the plant derived PET plastic, do not biodegrade.

Plant Derived PET
Ethane can be derived from plants.This is the same as ethane derived from oil and is used in the same way to make the same PET plastics.
Plant derived PET shares the same long-lasting, non-biodegradable qualities as petroleum derived PET i.e lasts pretty much forever.

Sorts Of Bioplastics

• Cellulose derived plastics such as Cellophane. These plant derived plastics are amongst the first examples of the product and do biodegrade. ­
• Starch based plastics which are compostable such as PLA plastics. They are certified compostable and do biodegrade.
• Polyhydroxyalkanoates or PHAs  are linear polyesters produced in nature by bacterial fermentation of ­sugar or lipids. They are produced by the bacteria to store carbon and energy. They do biodegrade
• chicken feathers bioplastic – biodegrades.
• Ethane based plastics as used Coca-Cola’s PlantBottle which replaces 30 percent of the ethanol in their normal polyethylene terephthalate (PET) plastic bottle with 30 percent plant-derived ethanol. This means the bottle is still considered PET and can be recycled but is NOT biodegradable.

In short, just because a plastic has been made from plants does not mean it is biodegradable.

Useful To Know
The case for and against plant derived PET plastics – great article here
Why most plastics don’t biodegrade
What is Ethane .

Biodegradable/ degradable plastics.

Some conventional plastics are labeled biodegradable which may lead you to think they are, well, biodegradable! They are not. They have an additive that makes the plastic fall apart, degrade, more quickly. And only in certain conditions. You can read more here