Chapter 7, Problem 39A

Summary Introduction

To infer:

About the evolution of bacterial cells from studying their structure

Introduction:

There are two categories of cells; those that have membrane- bound organelles and those without membrane- bound organelles. Cells with membrane -bound organelles are called eukaryotes and those without membrane- bound organelles are called prokaryotes. Most cells are eukaryotes. E.g., multicellular organisms have eukaryotic cells. Unicellular organisms like bacteria are prokaryotes.

Answer to Problem 39A

A bacterial cell is highly evolved as they have survived for millions of years on Earth. It has thick rigid cell wall that not only protects it from harsh environmental conditions but also from host cells. The flagellum helps bacterial cell to move towards nutrients and away from toxic substances. Small circular DNA strands help in replication of DNA. Plasmids provide the bacteria selective advantage of drug resistance.

Explanation of Solution

Most unicellular organisms like bacteria are prokaryotic cells. They lack any membrane bound cell organelles like mitochondria, nucleus, Golgi apparatus, chloroplast etc. Bacteria are simple cells having cell walls and dense structures containing the nuclear materials. Bacteria are found everywhere on Earth. They have survived all types of conditions and are one of the oldest forms of life on this planet. Hence it is highly evolved in structure and function. Several structural features of bacterium have helped in its evolution.

Cell wall- The cell wall of bacteria is thick and rigid. It allows bacterium to tolerate high temperatures. The cell wall is so developed that it protects the bacterium from the immune system of host cells. The appendages on the cell wall like flagella and pili help in providing anchorage. The cell wall also prevents the cell from bursting due to large difference in osmotic pressures between the cytoplasm of bacterium and its outside environment.

Cytoplasm- The cytoplasm of bacterium contains all the necessary structures like ribosomes, chromosome, enzymes, water, gases, and nutrients to carry out functions for growth, metabolism and replication.

Nucleoid- The chromosomal DNA is located in cytoplasm as single, circular strand of DNA. It is not bound by a nuclear membrane. The chromosomes are capable of replication. There are some extra chromosomal genetic materials also found in some bacteria. These are called plasmids and help in transfer of cytoplasmic materials to other bacteria. These provide the bacteria selective advantage against drugs, heavy metals and other factors.