5.5.3 The Evolution of Photosynthesis

Question 1

notes

Answer 1

• Photosynthesis evolved by adapting an existing system into the photosynthetic machinery.
  • The evolution of photosynthesis required an antenna for absorbing light, a living cell, and the constraints of the Earth’s ancient environment.
  • The evolution of photosynthesis likely included the following: bacteriorhodopsin as the pigment; a member of Archaea as the organism; and H2S as the proton source.
• A good candidate for the relative of the first photosynthetic cells is a member of the Archaea. Some Archaea, contain a light absorbing pigment called bacteriorhodopsin. This pigment could have acted as an antenna for light.
• In a photosynthetic cell, a gradient of hydrogen ions builds inside the thylakoid membrane. This gradient drives the production of ATP. The cell that first evolved photosynthetic capabilities needed to develop a battery to drive ATP production.
• The ancient photosynthetic unit was probably similar to that shown in the illustration on the left. Bacteriorhodopsin acted as the antenna and a gradient of hydrogen ions drove the synthesis of ATP. But what was the proton source?
• The ancient seas contained a lot of hydrogen sulfide (H2S), which could be split to form hydrogen ions and elemental sulfur. This proton source is currently exploited by purple sulfur bacteria. Cyanobacteria were the first organisms to use water as a proton source.

Question 2

The waste byproduct produced by photosynthesis in cyanobacteria is

Answer 2

• oxygen gas

Question 3

In a hypothetical scenario, the first photosynthetic unit used bacteriorhodopsin as an antennae to capture light and generate a gradient of ____________________ drove the synthesis of ATP.

Answer 3

• hydrogen ions

Question 4

The most likely candidate for the first photosynthetic organisms are the

Answer 4

• Archaea

Question 5

The extant Archaean group known as the extreme halophiles use a pigment called ___________________ to harness light energy.

Answer 5

• bacteriorhodopsin

Question 6

The first photosynthetic organisms were likely archaeans because

Answer 6

• they evolved later than bacteria, the first heterotrophs