Question

Although the fossil evidence is sketchy, the rise of oxygen in Earth's atmosphere is typically linked to the activity of ____ ?

A

Glycolysis

B

Anammox

C

Photosystems 1 and 2

D

Photosystem 1

E

Rhodopsin

Answer 1

Answer : (C) Photosystems I and II

Explanation :

Oxygen in the Earth's atmosphere may increase due to a number of reasons like organic matter decomposition, evaporation of soil Oxygen, splitting of water etc. Of these the most amount of oxygen that help rise the Oxygen content of Earth's atmosphere come from water splitting. This splitting of water is achieved through photosynthesis.

Photosynthesis may be classified into two parts viz. The light reaction and the dark reaction. In the light reaction, solar energy is utilized for production of energy rich assimilatory powers viz. ATP and NADPH. These assimilatory powers are used in the dark reaction to reduce CO2 to carbohydrates.

Light reaction may occur in two ways. These two types are called cyclic photophosphorylation and non-cyclic photophosphorylation (Z-Scheme). In cyclic photophosphorylation, only ATP is produced. In non cyclic photophosphorylation, both ATP and NADPH is produced.

Only Photosystem I (PS I) is present in cyclic photophosphorylation, where as both Photosystems viz. PS I and PS II are present in non cyclic photophosphorylation.

In cyclic photophosphorylation, electron is excited from photosystem I by energy from sunlight and is again accepted back by photosystem I. But, in non cyclic photophosphorylation, electron from PS II goes to NADP. The loss of electron from PS II is compensated by accepting an electron from PS I. Thus, PS I loses an electron. To make up this loss, electron is obtained from splitting of water. The reaction of splitting of water is:

2 H2O ---> 4Protons (H+) + 4 electrons + O2

The protons are combined with NADP along with electrons from PSII to form NADPH.

A greater part of this Oxygen release into the atmosphere is by Ocean Phytoplankton.