The Cell

3 04 2015

Cells are the smallest unit of life that can replicate independently, and are often called the “building blocks of life”. The study of cells is called cell biology.

The Cell theory holds true for all organisms , whether they be unicellular or multicellular:

  • All living things are composed of cells.
  • The cell is the basic functional unit of life.
  • Cells arise only from pre-existing cells.
  • Cells carry genetic information in the form of DNA. This genetic material is passed on form parent to daughter cell.

One of the primary obstacles that prevented early scientists from being able to study cells was their size.  Today , our primary techniques for examining the organism at the organ, tissue, cellular or subcellular levels are  microscopy, autoradiography, and centrifugation.

Prokaryotes versus Eukaryotes 

The first major biological distinction we can make between living organisms is whether they are prokaryotes or eukaryotes. The prokaryotic/eukaryotic distinction comes from the lack ore presence of a nucleus, respectively. Karyon is Greek for “kernel” or “nucleus”. Pro means “before”, and eu-means “true”. Thus, prokaryotes existed before nuclei, and eukaryotes have true nucley.


Prokaryotes are the simplest of all organisms. They include all bacteria as well as blue green-algae. Their outer cell wall does not enclose any membrane -bound organelles( such as nuclei or endoplasmic reticilum). The gentic material of the organism is contained in a single circular molecule of DNA concentrated in an area of the cell called the nucleoid region.

Prokaryotes also have the interesting ability to carry other pieces of genetic information in small pieces of DNA called plasmids. These are much smaller than the nuclear genome and often contain only a few genes. prokaryotesHowever, these genes  are quite important. The difference between a bacterial strain that is susceptible to antibiotics  and one that isn’t  may be due to a plasmid in the latter strain that confers resistance to a given antibiotic. These plasmids replicate independently of the nuclear genome, and copies of the plasmids can  be transferred from one bacterial cell to another, which helps explain  why bacteria are capable of passing resistance to other bacteria.

Most bacteria exist in one of two shapes. Spherical bacteria , known as cocci, include common pathogens like Staphylococcus aureus. Rod shaped  bacteria like Escherichia coli , are also known as bacilli.

images (1)              Diverse_e_Coli

Staphylococcus aureus                            Escherichia coli

All bacteria contain a cell membrane and cytoplasm , and some have flagella, which can also be found in eukaryotic cells(like sperm), where they give the cell motility . Since these structures are also found in eukaryotic cells, it is difficult to develop antibiotics that may target them. Instead, drugs tend to attack structures found specifically in bacteria.


Eukaryotes can be either unicellular or multicellular. Each cell has a cell membrane enclosing a semifluid  cytosol in which the organelles are suspended. The cytosol is like the city air . It allows for the diffusion of molecules throughout the cell. Genetic material is encoded in DNA organized into linear strands known as chromosomes , which are found within  the nucleus. There are some differences between different types of eukaryotes.; for example plants contain both a cell wall and chloroplasts, which are absent from animal cells.eukaryote

Eukaryotic Organelles

Eukaryotic cells separate their biochemical reactions into distinct membrane -bound organelles, much as cities are divided into different districts and neighborhood. These organelles are suspended within an aqueous cytosol that contains free proteins nutrients, and other solutes.

The cell has a cytoskeleton, which is made up of three types of proteins. From smallest to largest , they are actin filaments, intermediate filaments, and microtubules. These proteins form structures that allow materials to be moved around inside the cell. The cytoskeleton also provides a framework for anchoring other organelles within the cell.

The major organelles are the nucleus, ribosomes, endoplasmatic reticulum, Golgi apparutus, vesicles, vacuoles, lysosomes, microbodies, mitochondria, chloroplasts, and centrioles.

Cell Membrane

The cell membrane encloses the cell and chooses who or what to let in and out the cell. It is a phospholipid fluid mosaic modelbilayer . The theory that underlie this is known as the fluid mosaic model. The  phospholipid bilayer is studded with proteins and lipid rafts that can control the movement of solutes in and out of the cell. These moleculs are usually freely mobile within the membrane.

These phospholipids have a strongly hydrophobic tail(nonpolar) and a hydrophilic (polar)  head. The hydrophilic region face the interior and the exterior of the cell, whereas the hydrophobic tails face each other along the intramembrane space . Cholesterol molecules also are found in the membrane ; these help to regulate  the fluidity of stiffness of the membrane .

Cholesterol sometimes gets a bad reputation , owing to emphasis  on its negative health effects,  but cholesterol is important;  our cells use cholesterol not only to help with membrane fluidity but also to generate all steroid hormones . Animal cells have the ability to produce cholesterol molecules for inclusion in cell membranes and steroid hormone production. We know, however that excessive dietary cholesterol is unhealthy . It has many deleterious health effects such as atherosclerosis.

The proteins that are found in the membranes may be visible on one or both sides of the membrane . Moreover, they play a variety of roles within our cells. Transport proteins, allow polar molecules and ions to move in and out of the cell, whereas cell adhesion molecules(CAMs) are proteins that allow cells to recognize each other and contribute to proper cell differentiation and development.

Source: Biology Encyclopedia




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