Eukaryotic & Prokaryotic Cells

Eukaryotic & Prokaryotic Cells

Introduction: The Cell:
The cell is a functional unit of all living organisms. Cells have evolved into two fundamentally different types, eukaryotic and prokaryotic, which can be distinguished on the basis of their structure and the complexity of their organization. The simplest organisms which consist of one cell are called prokaryotes. More complex organisms are called eukaryotes and they consist of many cells.
Objectives:
* Define the terms: Eukaryotes & Prokaryotes with examples * Explain the differences between Eukaryotic cells and Prokaryotic cells with particular reference to the characteristics of their: (Nucleus/nucleoid, DNA, Mitotic division, Chromosome number, Cell organelles, Size of Ribosomes, Cell wall structure and composition, Peptidoglycan, Cell membrane, Motility) * List the functions of different cell organelles in eukaryotic and prokaryotic cells
Prokaryotes:
Prokaryotes are single-celled organisms that lack a nucleus and other internal compartments. Early prokaryotes lived at least 3.5 billion years ago and for nearly 2 billion years, prokaryotes were the only organisms on Earth. Bacteria and algae are two examples of prokaryotes.
Characteristics of Prokaryotic Cells:
Figure 1: Prokaryotic Cell
Prokaryotic cells, shown in figure 1, are primarily distinguished by the fact that they lack a membrane-bound nucleus. The genetic material is a single, circular molecule of loosely organized DNA (deoxyribonucleic acid). It is often located in the nucleoid, an irregularly-shaped region within the cell of prokaryotes which has nuclear material without a nuclear envelope and where the genetic material is localized. The cytoplasm of a prokaryotic

cell includes everything inside its membrane. The enzymes and ribosomes are free to move around the cytoplasm because there are no internal structures that divide the cell into compartments. Without separate compartments to isolate materials, prokaryotic cells cannot carry out many specialized functions. Most prokaryotic cells have a rigid external cell wall that contains peptidoglycan, a polymer of amino acids and sugars, as its unique structural component. Some prokaryotic cell walls are surrounded by a structure called a capsule, which is composed of polysaccharides. The capsule enables the prokaryotes to cling almost to everything. Many prokaryotic cells have flagella, which are long, threadlike structures that protrude from the cell’s surface and enable movement. Prokaryotic cells reproduce by a type of cell division called binary fission which is a form of asexual reproduction that produces identical offspring. It occurs in two stages: first, the DNA is copied, and then the cell divides.
Eukaryotes:
A eukaryote is an organism whose cells have a nucleus. Some of them are single-celled and others are multicellular. The first cells with internal compartments were primitive eukaryotic cells, which evolved about 2.5 billion years ago. Human and fungi are two examples of eukaryotes.
Characteristics of Eukaryotic Cells:
Figure 2: Eukaryotic Cell
The most noticeable feature that differentiates these more complex cells from prokaryotes is the presence of a nucleus, a double membrane-bound control center separating the genetic material, DNA, from the rest of the cell. Eukaryotic cells, shown in figure 2, are compartmentalized. A plasma membrane surrounds all cells, and additional intracellular membrane divides eukaryotes into compartments, each with a characteristic structure, biochemical composition and function. The nuclear envelope separates the two major compartments: nucleoplasm and cytoplasm. Eukaryotic cells contain organelles, such as endoplasmic reticulum (the site of protein and phospholipid synthesis), a Golgi apparatus (an organelle that adds sugars to membrane proteins, lysosomal proteins, and secretory proteins), and mitochondria (structures that convert energy stored in the chemical bonds of nutrients into adenosine triphosphate). Eukaryotic cells don’t have a cell wall except plant cells and some fungal cells but their cell walls don’t contain peptidoglycan. A web of protein fibers makes up the cytoskeleton which holds the cell together and keeps the cell’s membrane from collapsing. Many single-celled eukaryotes use flagella and cilia, short hair-like structures, for movement. In other cell, cilia and flagella move substances across the cell’s surface. Cell division in eukaryotic cells is more complex than cell division in prokaryotes. Mitotic division occurs exclusively in eukaryotic somatic cells. In mitotic division, one cell divides to produce two genetically identical cells with the same number of chromosomes. Each organism has a characteristic number of chromosomes and this number is constant within a species.
The differences between eukaryotic and prokaryotic cells:
Figure 3: Eukaryotic and Prokaryotic Cell

Characteristics | Prokaryotic cells | Eukaryotic cells | Average size | 0.2-2 µm in diameter | 10-100 µm in diameter | Nucleus | No nuclear envelope or nucleoli. | Nuclear envelope and nucleoli present. | Chromosomes | Single, circular | Multiple | Cell wall | Usually present, many contain peptidoglycan | Presented in plant cells and some fungal cells, no peptidoglycan | Plasma membrane | No carbohydrates, most lack sterols. | Sterols and carbohydrates present. | Membrane-bound organelles, such as mitochondria and lysosomes | Not presented | Present | Size of ribosome | 70 Svedberg | 80 Svedberg | Mitotic division | No. They divide by binary fission | Yes |

Conclusion:
References:
Books: * Medical Microbiology & Immunology: Examination & Board Review by Warren E. Levinson and Ernest Jawetz * HOLT BIOLOGY by Johanson Raven * Cell Biology by Thomas D. Pollar and William C. Earnshaw * Lippincott 's Illustrated Reviews: Microbiology by Richard A. Harvey PhD, Pamela C. C. Champe, and Bruce D. Fisher.

References: Books: * Medical Microbiology & Immunology: Examination & Board Review by Warren E. Levinson and Ernest Jawetz * HOLT BIOLOGY by Johanson Raven * Cell Biology by Thomas D. Pollar and William C. Earnshaw * Lippincott 's Illustrated Reviews: Microbiology by Richard A. Harvey PhD, Pamela C. C. Champe, and Bruce D. Fisher.