Biology Today — December 2017

Eukaryotes

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Cells with distinct nucleus and cell organelles. It is assumed that eukaryotic cells

evolved from prokaryotic cells either by symbiotic origin or origin by invagination.

(i) Symbiotic origin : According to Margulis some anaerobic predator host

cells engulfed primitive aerobic bacteria but did not digest them. These aerobic

bacteria established themselves inside the host cells as symbionts. Such predator

host cells became the first eukaryotic cells. The predator host cells that engulfed

aerobic bacteria evolved into animal cells while those that captured both aerobic

bacteria and blue-green algae became eukaryotic plant cells. The aerobic bacteria

established themselves as mitochondria and blue-green algae as chloroplasts. (ii)

Origin by invagination : According to this view, cell organelles of eukaryotic

cells might have originated by invagination of surface membrane of primitive

prokaryotic cells.

Origin of multicellular organisms Either by failure of separation of daughter cells after cell division or by aggregation

of cells.

(i) Photosynthetic, e.g., plants. (ii) Heterotrophic, e.g., animals.

EXPERIMENTAL PROOF OF ABIOGENIC MOLECULAR EVOLUTION OF LIFE

• Formation of simple organic compounds from simpler compounds under
  reducing conditions were experimentally proved by Stanley Miller
  and Harold Urey.


• Mixture of methane, ammonia, hydrogen and water vapour were
  subjected to electric spark of about 75,000 volts and temperature
  of about 800°C was provided.


• Methane, ammonia, hydrogen were taken in ratio of 2 : 2 : 1. Mixture
  was passed through condenser and gases were continuously
  circulated for few days.


• On analysing the composition of liquid inside the apparatus, amino
  acids like alanine, glycine and aspartic acid were found.

Fig.: Miller’s experiment

When the hot gases in the

spark chamber are cooled,

water vapour condenses and

any soluble molecules present

are dissolved.

Boiling chamber

Water vapour

Condenser

Electric spark

chamber

CH 4 , NH 3 , H 2 , H 2 O

Cold water flow in

Hot water out

Water

An electric spark simulates

a lightning storm.

To vacuum

Energy from the spark powers

reactions among molecules

thought to be present in

Earth’s early atmosphere.

Boiling water adds

water vapour to the

artificial atmosphere.

Organic molecules (amino acids)

appear after a few days.