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#1 2021-12-09 17:17:10

Registered: 2005-06-28
Posts: 35,550

Life Science Lessons

1. Prokaryotes and Eukaryotes

All living organisms can be sorted into one of two groups depending on the fundamental structure of their cells: the prokaryotes and the eukaryotes. Prokaryotes are organisms made up of cells that lack a cell nucleus or any membrane-encased organelles. Eukaryotes are organisms made up of cells that possess a membrane-bound nucleus that holds genetic material as well as membrane-bound organelles.

Understanding Cells and Cell Membranes

The cell is a fundamental component of our modern definition of life and living things. Cells are regarded as the basic building blocks of life and are used in the elusive definition of what it means to be "alive."

What Is a Prokaryote?

Cells keep chemical processes tidy and compartmentalized so individual cell processes do not interfere with others and the cell can go about its business of metabolizing, reproducing, etc. To achieve this, cell components are enclosed in a membrane which serves as a barrier between the outside world and the cell's internal chemistry. The cell membrane is a selective barrier, meaning that it lets some chemicals in and others out. In so doing it maintains the chemical balance necessary for the cell to live.

The cell membrane regulates the crossing of chemicals in and out of the cell in three ways including:

* Diffusion (the tendency of solute molecules to minimize concentration and thus move from an area of higher concentration towards an area of lower concentration until concentrations equalize)
* Osmosis (the movement of solvent across a selective boundary in order to equalize the concentration of a solute that is unable to move across the boundary)
* Selective transport (via membrane channels and membrane pumps)


Prokaryotes are organisms made up of cells that lack a cell nucleus or any membrane-encased organelles. This means the genetic material DNA in prokaryotes is not bound within a nucleus. In addition, the DNA is less structured in prokaryotes than in eukaryotes: in prokaryotes, DNA is a single loop while in Eukaryotes DNA is organized into chromosomes. Most prokaryotes are made up of just a single cell (unicellular) but there are a few that are made of collections of cells (multicellular).

Scientists have divided the prokaryotes into two groups, the Bacteria, and the Archaea. Some bacteria, including E Coli, Salmonella, and Listeria, are found in foods and can cause disease;1 others are actually helpful to human digestion and other functions.2 Archaea were discovered to be a unique life form which is capable of living indefinitely in extreme environments such as hydrothermal vents or arctic ice.

A typical prokaryotic cell might contain the following parts:

* Cell wall: the membrane surrounding and protecting the cell
* Cytoplasm: all of the material inside a cell except the nucleus
* Flagella and pili: protein-based filaments found on the outside of some prokaryotic cells
* Nucleoid: a nucleus-like region of the cell where genetic material is kept
* Plasmid: a small molecule of DNA that can reproduce independently


Eukaryotes are organisms made up of cells that possess a membrane-bound nucleus (that holds DNA in the form of chromosomes) as well as membrane-bound organelles. Eukaryotic organisms may be multicellular or single-celled organisms. All animals are eukaryotes. Other eukaryotes include plants, fungi, and protists.

A typical eukaryotic cell is surrounded by a plasma membrane and contains many different structures and organelles with a variety of functions. Examples include the chromosomes (a structure of nucleic acids and protein which carry genetic information in the form of genes), and the mitochondria (often described as the "powerhouse of the cell").

Prokaryotes and Eukaryotes

Cells fall into one of two broad categories: prokaryotic and eukaryotic. The single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro = before; karyon– = nucleus). Animal cells, plant cells, fungi, and protists are eukaryotes (eu = true).

Components of Prokaryotic Cells

All cells share four common components: (1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment; (2) cytoplasm, consisting of a jelly-like region within the cell in which other cellular components are found; (3) DNA, the genetic material of the cell; and (4) ribosomes, particles that synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways.

In this illustration, the prokaryotic cell has an oval shape. The circular chromosome is concentrated in a region called the nucleoid. The fluid inside the cell is called the cytoplasm. Ribosomes, depicted as small circles, float in the cytoplasm. The cytoplasm is encased by a plasma membrane, which in turn is encased by a cell wall. A capsule surrounds the cell wall. The bacterium depicted has a flagellum protruding from one narrow end. Pili are small protrusions that project from the capsule in all directions.
Figure 1. This figure shows the generalized structure of a prokaryotic cell.

A prokaryotic cell is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in the central part of the cell: a darkened region called the nucleoid (Figure 1).

Unlike Archaea and eukaryotes, bacteria have a cell wall made of peptidoglycan, comprised of sugars and amino acids, and many have a polysaccharide capsule (Figure 1). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion, while most pili are used to exchange genetic material during a type of reproduction called conjugation.

Eukaryotic Cells

In nature, the relationship between form and function is apparent at all levels, including the level of the cell, and this will become clear as we explore eukaryotic cells. The principle “form follows function” is found in many contexts. It means that, in general, one can deduce the function of a structure by looking at its form, because the two are matched. For example, birds and fish have streamlined bodies that allow them to move quickly through the medium in which they live, be it air or water.

A eukaryotic cell is a cell that has a membrane-bound nucleus and other membrane-bound compartments or sacs, called organelles, which have specialized functions. The word eukaryotic means “true kernel” or “true nucleus,” alluding to the presence of the membrane-bound nucleus in these cells. The word “organelle” means “little organ,” and, as we learned earlier, organelles have specialized cellular functions, just as the organs of your body have specialized functions.

Cell Size

At 0.1–5.0 µm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10–100 µm (Figure 2). The small size of prokaryotes allows ions and organic molecules that enter them to quickly spread to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly move out. However, larger eukaryotic cells have evolved different structural adaptations to enhance cellular transport. Indeed, the large size of these cells would not be possible without these adaptations. In general, cell size is limited because volume increases much more quickly than does cell surface area. As a cell becomes larger, it becomes more and more difficult for the cell to acquire sufficient materials to support the processes inside the cell, because the relative size of the surface area across which materials must be transported declines.



It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.


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