Mitochondria - Power house of the Cell

The mitochondria is another very important organelle in the cell and is called the power house of the cell. It is so because mitochondria is capable of producing the energy.

Structure: 

Mitochondria is a membrane enclosed organelle. It is double layered i.e.; it has two membranes  as outer membrane and inner membrane.
Between these two membranes is the space called intermembrane space.
The outer membrane covers the organelle.
The inner membrane folds upon itself many-a-times to form convolutions which are called cristae. This folding basically increases the surface are; the more area, the more work can be done and the more ATP (energy) we will get. The inner membrane also contains various types of proteins with different functions.
The space enclosed within the inner membrane is matrix. This matrix plays an important role in the synthesis of ATP as it contains various enzymes required for this. It also contains mitochondrial ribosomes, mitochondrial DNA.

Functions: 
As already mentioned above, this is the most important organelle for providing us the energy in the form of ATP. This is because the enzymes for the important cycles involved in the production of ATP are present in the mitochondria.
Also, mitochondria is a storage site for calcium ions. MItochondria is also the site for apoptosis or programmed cell-death.

Nucleus - Brain of the Cell

Nucleus is also called the “brain of the cell”. Why so? Because it contains the genetic material and is responsible for a large number of functions. Nucleus is spheroid and is most prominent part occupying around 10% of the total cell volume.

Structure:
Inside the nucleus is present a structure called nucleolus which consists of rRNA and proteins but no DNA. It is the site of assembly of ribosomes which are important for the process of protein synthesis. Nucleolus disappears when the cell is dividing and reappears after the cell is formed. 

Then, there is chromatin consisting of long strands of DNA associated with proteins. When the cell is in resting stage, the chromatin is relaxed and when the cell is going to divide, chromatin condenses and forms what is known as  “chromosome” as can be seen in the figure.

Structure of Nucleus

Nucleus is surrounded by nuclear membrane/envelope which keeps the nucleolus and chromatin inside the nucleus. Nuclear membrane is double-layered. the outer layer is connected with another organelle as endoplasmic reticulum. The space between both the layers is  fluid-filled space called perinuclear space. 

Now, if the nucleus is membrane bound, then how do DNA, proteins or macromolecules pass through? For this, there are several opening in the nuclear membrane called the nuclear pores which are the sites for exchange of macromolecules.

Functions:
It stores the genetic information in the form of “chromatin”. The gene expression takes place in nucleus which includes transcription where DNA is translated to mRNA. This mRNA is then transported to cytoplasm as ribosomes (which are present outside the nucleus, described here) are required for translation.
So, basically, nucleus says,  ”Hy Dude! I contain your genetic information and will form proteins for you whenever necessary”

Cell Membrane - Protector of the Cell

Simple diagram showing
lipid bilayer and a protein

Cell membrane is the outermost covering of the cell. The thickness varies from 0.1uM to several microns. Almost all the membranes have a typical fluid mosaic model. According to this model, the membrane is composed primarily of lipids and proteins. The lipids vary from 20 to 80 percent with the remainder being proteins. Why proteins and lipids? The lipids give membrane its flexibility while proteins are responsible for maintaining the environment of the cell and also helps in transportation of molecules.

Structure:
A. Lipid bilayer - The cell membrane has two layers (hence called bilayer). Various components of lipids are: 

i. Phospholipids - It is the major component of cell membrane. Forms a lipid bilayer in which the hydrophilic (water-loving) heads are arranged to face the extracellular fluid while the hydrophobic (water-repelling) tails face the cytosolic fluid. This layer is semi-permeable i.e.; it allows only certain molecules to pass through

ii. Glycolipids: Are present on cell membrane surfaces and they have a carbohydrate sugar chain attached to them. This helps in recognizing other cells of the body. 

iii. Cholesterol: Another lipid component of cell membrane. Helps to stiffen the membrane. However, cholesterol is not found in the cell membrane of the plants.

Fluid Mosaic Model of Cell Membrane

 
B. Proteins - There are several types of proteins present in the cell membrane. Some of them are: 

i. Structural proteins - These gives support and shape to the cell.
ii. Receptor proteins - They help cells communicate with their external environment through the use of hormones, neurotransmitters and other signaling molecules.
iii. Transport proteins (globular proteins) - These proteins transport molecules across cell membranes through facilitated diffusion.
iv. Glycoproteins - They have a carbohydrate chain attached to them. They are embedded in the cell membrane and help in cell to cell communications and also helps transport of molecule across the membrane.
v. Channel Proteins: These proteins allow molecules of certain size to pass through the membrane.

Functions:
a. Maintains cell shape - The cell membrane encloses the cell and defines it. Also, it maintains physical integrity of the cell. It forms a barrier between the interior an exterior of the cell thereby protecting the organelles from the outside environment.

 
b. Selectively permeable - The cell membrane has selective permeability meaning it allows certain molecules to pass through thereby regulating entry and exit of molecules. It just acts like a guard saying “Hey Dude! You are not permitted inside unless you have a license!”.

Diagram showing Passive Transport - Simple and Facilitated Diffusion

 
c. Passive transport - It involves moving molecules through membranes without the expenditure of energy. the movement is down the gradient. It involves the process of diffusion.

i. Simple Diffusion - It is a form of passive transport where only some substances like small ions     and molecules like carbon dioxide, oxygen and water can easily move across the plasma membrane. 

ii. Facilitated Diffusion - In this process, the molecules move across the membrane but with the help of membrane transport proteins which temporarily binds the molecule which is to be moved. Again, as no energy (ATP) is required, it is a passive process.

Diagram showing Active Transport

 
d. Active Transport - It involves moving the substances across the membrane against concentration gradient. Energy (ATP) is required for this process. Generally requires two carrier proteins - one to recognize the substance to be carried and one to release ATP to provide energy for protein carriers (pumps).

 
e. Exocytosis - Certain substances can be transported out of the cell by fusing vesicles with the membrane by the process called exocytosis.

Diagram showing the process of Exocytosis

f. Endocytosis - This is exactly opposite of exocytosis where the substances are moved into the cell via fusing with the cell membrane. Endocytosis involves pinocytosis (internalizing liquid substances), phagocytosis (internalizing solid particles) and receptor-mediated endocytosis

 
g. Markers and Signaling: Surface protein markers are embedded in the cell membrane that identify the cell, enabling nearby cells to communicate with each other.

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.	Characteristics	Prokaryotes	Eukaryotes
1.	Nucleus	Absent	Present
2.	Cellularity	Unicellular	Unicellular or multicellular
3.	Membrane bound organelles	Absent	Present
4.	Genetic material	Unorganized in nucleoid	Organized in nucleus as chromosomes
5.	Size of the cell	Very small (<5uM)	Relatively large (>10uM)
6.	Cell wall	Present	May or may not be present
7.	DNA	Circular without proteins	Linear with proteins
8.	Ribosomes	Smaller	Larger and more complex
9.	Cytoskeleton	Absent	Present
10,	Cell division	By binary fission	Generally by mitosis or meiosis
11	Reproduction	Always asexual	Sexual or asexual

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

Unit of life - The Cell

“What is a cell?” Since childhood, we are taught, “Cell is the structural and functional unit of life”; we just learn it, write in examinations and its all over then! But have we ever wondered  what does it actually mean? I am sure everyone will agree with me that human body is one of the most amazing machinery on this planet. But have you ever wondered what it runs on? Yes..It runs on cells; rather we can say it is made of millions and millions of cells.

Frankly speaking, even I didn’t ponder about it much until I went to high school, where I started gaining interest in Biology; thoughts started popping up my mind....A cell which we can’t be seen by naked eyes can perform thousands of functions without taking rest; really? How is it possible? I was amazed.! Its really hard to believe, what all wonders a cell do but believe me, its fun to know.! In my next few articles, I will discuss about cell - the structure and its functions.

To start with, any living organism on earth is composed of cell. The organism can have one cell, called unicellular (uni meaning 'one' and cellular meaning 'consisting of cell') like bacteria is unicellular or the organism can have millions of cells, called multicellular (multi meaning 'many' and cellular meaning 'consisting of cell') as in case of humans. Be it in bacteria or human, a single cell can perform thousands of biochemical reactions every minute and moreover, also, reproduce itself.

Lets just start with knowing the size of cell. Cells come in many different sizes. The smallest of all cells can be as small as Mycoplasma, a bacterium, which is just a micron short while the big cells can be as long nerve cell in human which can exceed 3 meters in length. The average size of normal human cell varies from 10 to 100 micrometers (i.e; 0.000001 meters).

By now, you might be wondering what about the shapes of the cells? Do all cells look alike? The answer is No. Again, just like the sizes varying, the shapes of the cells also vary as can be seen in the figure below. The shapes vary in bacteria and some bacteria are even classified and given a name depending on the shapes. For example, E.coli, the most common bacteria is rod shaped, amoeba (what students say, the easiest organism to draw ;-)) is irregularly shaped and it changes its shape as it moves. The plant cell are somewhat like cuboid.  In humans, also, there are variety of shapes, like, the cells on the skin are flat; whereas the muscle cells are long and thin and nerve cells are elongated with some projections at its one end.

Different shapes of cell

Why different shapes? The shape of these cells are according to their jobs. Like, the cells of the skin are tightly packed to form a layer so as to protect the underlying tissues. The muscle cells are long and connected to bones for bone’s movements. and the projections of nerve cells helps it to connect to various other cells and transmit some information.

The word “Cell” itself can be an organism (like bacteria) while the cell can be a part of an organism. Before we understand the functions, it is necessary to understand the organization of cells. How the cells are organized to form a multicellular living organism?

So, my next topic will be organization of cell followed by functions of components of cells.

Hope you like this post. Comments or doubts are welcome!