Introduction to Prokaryotes
Prokaryotes are usually single-celled organisms, it has been around for billions of years and it can be found in air, water and soil. Some can cause serious diseases. They can thrive in habitats not suitable for any eukaryotes –Extreme heat, cold, acidity, salinity. Prokaryotes have plasma membrane surrounding the cell but no membrane bound organelles such as the mitochondria, nucleus or Golgi bodies.
Bacteria Cell Wall
Bacteria cell wall is a layered structure which surrounds the protoplasm of the cell to protect cells from the environment. The lipid bilayer cell membrane of most of the Gram-positive bacteria is covered by a porous peptidoglycan layer which does not exclude most antimicrobial agents. Gram-negative bacteria are surrounded by two membranes. The outer membrane functions as an efficient permeability barrier because it contains lipopolysaccharides and proteins. Bacteria cell wall is made up of a unique peptidoglycan (a polymer of disaccharide which is cross linked to amino acids) called Murein. Its basic structure is a carbohydrate backbone of alternating units of N-acetyl glucosamine and N-acetyl muramic acid. Bacteria lacking a cell wall are called mycoplasma, which usually inhabit osmotically protected environments and have sterol like compounds in their membranes.
Organelles and Inclusions
Cytoplasm contains chromosomes and ribosomes. A chromosome is usually a circular DNA molecule. Enzymes are attached to the plasma membrane. Often distinct granules are found in cytoplasm for storage of fat, glycogen and enzymes. Ribosomes are the only cytoplasmic organelles in prokaryotes.
Mobility, Response to Stimuli and Reproduction
Bacteria have rotating rings that gives it propeller movement to allow move to different environments. Some bacteria have short hair like structures to help the bacteria to adhere to each other and to surfaces. A special pilli are involved in bacterial reproduction – Sex Pilli.
Prokaryotes have the ability to move toward environmental stimuli. They can also respond to light, oxygen and magnets. Prokaryotes reproduce asexually by Binary fission, or sexually by conjugation.
Classification of Prokaryotes
Classification can be based on oxygen requirement, Nutrition, Photosynthetic Capacity, Chemosynthetic Capacity, Feeding of Organic Matter, Staining and Shape. Based on nutrition, bacteria can be classified as heterotrophs, chemosynthetic and photosynthetic bacteria. Archaea is also called Archaebacteria; they are more closely related to eukaryotes than prokaryotes. In a 3-dimensional system, it contains Archaea, bacteria and eukaryotes.
Protists
Protists are all eukaryotes and therefore all have cell organelles, most of them are single-celled but multi-celled form exists. Protists contain three groups: algae, slime molds (fungi) and protozoa. Algae include three groups: red algae, brown algae and green algae. Protozoa have contractile vacuoles which collect excess water and pump it outside the cell body. Amoeba is a typical protozoa. Protozoa can reproduce via sexual and asexual pathway. They can form cysts during harsh conditions.
