Thursday, December 20, 2012

Cell Organization

So, in the last post, we discussed about the cell size and shape. Now, whether this cell is a part of an organism or an organism by itself, it has several components in common. Depending on this, the cells can fall into two categories as Prokaryotic and Eukaryotic Cells. Is it difficult to remember these words? Make it easy by knowing the meanings of these words. So, Pro means 'before' and karyotic comes from Greek word (karyos) meaning 'in a kernel', which refers to the nucleus of the cell. So, 'prokaryotic' means “before nucleus” and “eu” as in eukaryotic cell means “true” thereby 'eukaryotic' meaning “True nucleus”.

So, is there only the difference of nucleus being there or not in these two types of cells? No. There are many other differences as follows.. 

Lets start with Prokaryotes. Prokaryotes are unicellular and simplest kind of cells to evolve. The two forms of prokaryotic cells are bacteria and archaebacteria. The size of prokaryotic cell ranges from 0.0001 to 0.003 mm. They lack a nucleus (description of nucleus) and surrounding nuclear membrane. Apart from nucleus, it also lacks several organelles like mitochondria, endoplasmic reticulum, chloroplasts (in case of plants and some algae), golgi apparatus. You might wonder how do prokaryotes manage without these organelles and  what about their functions? Interestingly, the functions are taken up by prokaryotic plasma membrane. The prokaryotic cells can be distinguished into three regions as:
Diagram of a Prokaryotic Cell

1. Outside: Flagella and Pili - These are the proteins which are attached to the cell surface and used for movement and/or communication amongst themselves.
2. Cell Envelope: Consisting of cell wall, plasma membrane, and some have capsule which separates them from the exterior region and helps in intake of nutrients from the outside world.
3. Inside: Cytoplasmic Region - Consisting of the genetic material; the genome (DNA), ribosomes and various other inclusions. Also, there is an additional extra-chromosomal body present in bacteria as plasmids which confer additional functions like antibiotic resistance etc. As mentioned, prokaryotes do not have nucleus, instead they have nucleoid (suffix -oid meaning 'similar') because it is almost at the same place where DNA is present. Remember that nucleoid has no physical boundaries, it is just an imaginary structure.

Now, coming to the structure of eukaryotic cell. Eukaryotes include fungi, plants, animals and also some unicellular organisms (like yeast). As against prokaryotes, the eukaryotes have a well defined nucleus with nuclear membrane and also various membrane bound organelles like mitochondria, golgi apparatus etc. Other features are as follows:
Diagram of a Eukaryotic Cell
(Note: this is the diagram of an animal cell with no cell wall)

a. Cell/plasma membrane - Just like prokaryotes, all eukaryotic cells have an outer cell membrane. Cell walls may (in plants) or may not (animals) be present.
b. The cytoplasm has various membrane bound organelles. Inside the nucleus, there is more organized DNA (in the form of chromosome) which is associated with proteins called histones.
c. Many of the cells have cilia which helps in locomotion and movement and also functions as “antennae” for various cell signalling pathways.
d. Some cells do have flagella but with a more complex structure than prokaryotes.

Below is the table form of the above mentioned differences between prokaryotes and eukaryotes.
Don’t hesitate to give more differences if you know.! :)

Table 1: Differences between Prokaryotes and Eukaryotes.
Sr. No.
1. NucleusAbsentPresent
2. CellularityUnicellularUnicellular or multicellular
3. Membrane bound organellesAbsent Present
4.Genetic materialUnorganized in nucleoidOrganized in nucleus as chromosomes
5. Size of the cellVery small (<5uM)Relatively large (>10uM)
6. Cell wallPresentMay or may not be present
7. DNACircular without proteinsLinear with proteins
8. RibosomesSmallerLarger and more complex
10,Cell divisionBy binary fissionGenerally by mitosis or meiosis
11ReproductionAlways asexualSexual or asexual

Next, we will discuss the organelles in detail one by one with their functions.