Chapter 14, Problem 33A

Summary Introduction

To analyze and critique:

The endosymbiont theory and also mention its strengths and weaknesses

Introduction: Evidence indicates that a sequence of chemical events preceded the origin of life on Earth and that life has evolved continuously over time. There are many theories that attempted to explain the origin of life from spontaneous generation to theory of biogenesis to the endosymbiont theory.

Answer to Problem 33A

The endosymbiont theory which states that bacteria might have lived inside eukaryotes and eventually became organelles was proposed by Lynn Margulis.

Strengths of the theory- This theory explained the origin of chloroplasts and mitochondria.

Weakness- This theory does not explain the origin of other organelles.

Explanation of Solution

Scientists speculate that prokaryotes were involved in evolution of eukaryotes as prokaryotes are much simple organisms.

The endosymbiont theory which states that bacteria might have lived inside prokaryotes and eventually became organelles was proposed by Lynn Margulis. In 1966, Margulis proposed that the ancestors of eukaryotic cells lived in association with prokaryotic cells. In some cases prokaryotes might have lived inside eukaryotes. Prokaryotes might have entered as undigested prey or could have been internal parasites. Eventually the relationship became mutually beneficial and prokaryotic symbionts became organelles in eukaryotic cells.

Strengths- There is evidence that chloroplasts and mitochondria might have evolved by endosymbiosis. Both the organelles have their own genetic material and ribosomes which resemble the DNA and ribosomes of prokaryotes. Both reproduce independently from the rest of the cell by binary fission just like the prokaryotic cells. Both are enclosed within a double membrane and their inner membrane resembles bacterial membrane. Moreover, the fact that the prokaryotes have often been found to live in close association with eukaryotes also supports this theory.

Weakness- This theory does not explain the origin of other organelles like nucleus, Golgi apparatus and endoplasmic reticulum.

It also does not explain the early steps that led to the emergence of life