State some of the irreversible biological changes that take place in nature.

A change which cannot happen backward, that is, it cannot be reversed is called an irreversible change. When you burn a piece of paper, it turns to ash. It cannot become paper again. Your height cannot decrease. These are irreversible changes. They cannot be reversed at all.

All complex natural processes are irreversible. The phenomenon of irreversibility results from the fact that if a thermodynamic system, which is any system of sufficient complexity, of interacting molecules is brought from one thermodynamic state to another, the configuration or arrangement of the atoms and molecules in the system will change in a way that is not easily predictable. Some "transformation energy" will be used as the molecules of the "working body" do work on each other when they change from one state to another. During this transformation, there will be some heat energy loss or dissipation due to intermolecular friction and collisions. This energy will not be recoverable if the process is reversed.

The following is a list of spontaneous events which contribute to the irreversibility of processes.

• Heat transfer through a finite temperature difference

• Friction

• Plastic deformation

• Flow of electric current through a resistance

• Magnetization or polarization with a hysteresis

• Unrestrained expansion of fluids

• Spontaneous chemical reactions

• Spontaneous mixing of matter of varying composition/states

Examples of the irreversible reactions that take place in nature:

1) Photosynthesis

12H₂O + 6CO₂→ C₆H₁₂O₆ + 6O₂ + 6H₂O

2) Biodegradation of matter

3) Cellular respiration

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

4) Germination of seeds.

5) Flowering of bud.

6) Bio gas production from cow dung.