THE CELL



The cell is the basic structural, functional and biological unit of all known living organisms. Cells are the smallest unit of life that can replicate independently, and are often called the "building blocks of life".

 



The starting point for this discipline might be considered the 1830s. Though scientists had been using microscopes for centuries, they were not always sure what they were looking at. Robert Hooke's initial observation in 1665 of plant-cell walls in slices of cork was followed shortly by Antonie van Leeuwenhoek's first descriptions of live cells with visibly moving parts. In the 1830s two scientists who were colleagues — Schleiden, looking at plant cells, and Schwann, looking first at animal cells — provided the first clearly stated definition of the cell. Their definition stated that that all living creatures, both simple and complex, are made out of one or more cells, and the cell is the structural and functional unit of life — a concept that became known as cell theory.

As microscopes and staining techniques improved over the nineteenth and twentieth centuries, scientists were able to see more and more internal detail within cells. The microscopes used by van Leeuwenhoek probably magnified specimens a few hundredfold. Today high-powered electron microscopes can magnify specimens more than a million times and can reveal the shapes of organelles at the scale of a micrometer and below. With confocal microscopy a series of images can be combined, allowing researchers to generate detailed three-dimensional representations of cells. These improved imaging techniques have helped us better understand the wonderful complexity of cells and the structures they form.


Cells consist of a protoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals). While the number of cells in plants and animals varies from species to species, humans contain about 100 trillion (1014) cells. Most plant and animal cells are visible only under the microscope, with dimensions between 1 and 100 micrometres.

There are two types of cells, eukaryotes, which contain a nucleus, and prokaryotes, which do not. Prokaryotic cells are usually single-celled organisms, while eukaryotic cells can be either single-celled or part of multicellular organisms.
Comparation both  Prokaryotes and Eukaryotes


Prokaryotes
Eukaryotes
Typical organisms
Typical size
~ 1–5 µm[10]
~ 10–100 µm[10]
Type of nucleus
nucleoid region; no true nucleus
true nucleus with double membrane
DNA
circular (usually)
linear molecules (chromosomes) with histone proteins
RNA/protein synthesis
coupled in the cytoplasm
RNA synthesis in the nucleus
protein synthesis in the cytoplasm
50S and 30S
60S and 40S
Cytoplasmic structure
very few structures
highly structured by endomembranes and a cytoskeleton
flagella and cilia containing microtubules; lamellipodia and filopodia containing actin
none
one to several thousand (though some lack mitochondria)
none
in algae and plants
Organization
usually single cells
single cells, colonies, higher multicellular organisms with specialized cells
Binary fission (simple division)
Mitosis (fission or budding)
Meiosis
 
  Prokaryotes Cell

Prokaryotic cells were the first form of life on Earth. They are simpler and smaller than eukaryotic cells, and lack membrane-bound organelles such as the nucleus. Prokaryotes include two of the domains of life, bacteria and archaea. The DNA of a prokaryotic cell consists of a single chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid.

A prokaryotic cell has three architectural regions:


  • On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.
  • Enclosing the cell is the cell envelope – generally consisting of a cell wall covering a plasma membrane though some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea). The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
  • Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions. The prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Though not forming a nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids encode additional genes, such as antibiotic resistance genes.

 Eukaryotes Cell

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound compartments in which specific metabolic activities take place. Most important among these is a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. This nucleus gives the eukaryote its name, which means "true nucleus".



Typical animal cell
Typical plant cell
Organelles
 

Cell Wall
Cell Wall

Structure: This is the outermost covering of a cell, and is present only in plant cells. It is made up of pectin, hemicellulose, cellulose microfibrils, and proteins organized into two layers called primary cell wall and middle lamella. In many plant cells a third layer called secondary cell wall, which is made up of lignin, is present between the primary cell wall and the cell membrane.

Function: The cell wall provides support, protection and rigidity to the plant cells. It is a semi-permeable structure and allows only certain set of molecules to enter and exit the cell. The middle lamella serves as a cementing layer between adjoining cells.

Cell Membrane
Cell/Plasma Membrane

Structure: It is present just below the cell wall in plant cells, and forms the outermost covering of an animal cell. It comprises two layers of phospholipids arranged in such a way that their hydrophobic tails are on the inner side and the hydrophilic heads form the outer side. This arrangement is called a phospholipid bilayer.

Function: The cell membrane provides structure and shape to the cell, and is responsible for holding the organelles together. It regulates the entry and exit of molecules and ions from the cell, and plays a vital role in cell eating (phagocytosis) and cell drinking (pinocytosis). It is also involved in the secretion of hormones, enzymes and other molecules outside the cells, as well as in several processes of the immune system.

Nucleus
Nucleus 
Structure: It is the most important organelle as it contains the genetic material of the cell enclosed inside two concentric phospholipid bilayers collectively called nuclear envelope. Inside the nuclear envelope is present chromatin and nucleolus, which is composed of nucleic acids and certain proteins. The nuclear envelope has tiny pores called nuclear pores which allow the transport of molecules, especially regulatory macromolecules and gene products, from the nucleus to the cytoplasm, and vice versa.

Function: The nucleus is often referred to as the 'brain' of the cell, as it controls all the activities that are carried on within the cell. It carries the information code in the form of DNA, and hence is also known as the information storage organelle. It provides a separation between DNA and the regulatory molecules of the cell, thus facilitating their interaction at the right time and stage.
Endoplasmic Reticulum (ER)
Endoplasmic Reticulum 
Structure: This is an extensive network of membranes which is often seen in continuation with the nuclear membrane. The membranes that are arranged in the form of tubes are collectively termed smooth ER; whereas the ones arranged into flattened disc-like structures with ribosomes attached onto the surface are collectively called rough ER.

Function: The rough ER is the site for protein synthesis from the attached ribosomes, and is responsible for the transport of these proteins and other molecules along with the smooth ER. The smooth ER plays an important role in carbohydrate metabolism, drug detoxification and lipid biosynthesis.


Ribosome
Ribosome 
Structure: Ribosomes are thousands of tiny spherical structures that are made of RNA and proteins, and are present in prokaryotes as well as eukaryotes. Although ribosome is not a membrane-bound structure it is considered to be an organelle owing to its size as well as functional importance. It is composed of two subunits which collectively form distinct binding and functional sites for tRNA molecules.

Function: It is the molecular machine that reads information on mRNA obtained from nucleus, and synthesizes polypeptide chains for the cell. This process is called translation, and also involves the participation of tRNA molecules which serve as the carriers for amino acids that need to be assembled to form a protein. The tRNA attaches at the aminoacyl (A) site, travels through the peptidyl (P) site and exits via the exit (E) site, generating a polypeptide chain in the process.
Golgi Apparatus
Golgi Apparatus 
Structure: The Golgi apparatus is a huge network of membranous stacks called cisternae. These are divided into four structural and functional components called cis-Golgi, medial-Golgi, endo-Golgi and trans-Golgi. Each component carries a specialized set of enzymes and proteins.

Function: The Golgi apparatus is responsible for modifying the polypeptide chains synthesized in the ER by ribosomes to get the final, effective protein molecules. These modifications include the addition of sugar molecules, lipid moieties, functional groups etc. It is also the site for breakdown of proteins to get functionally active forms of the protein. Through its ability to form and fuse with membrane-bound vesicles, it serves to package and distribute macromolecules to other parts of the cell, and also facilitates the release of molecules, especially enzymes and hormones outside the cells.
Lysosome
Lysosome 
Structure:: These are spherical organelles with a highly acidic interior that contains degradative or lytic enzymes called hydrolases. Lysosomes are polymorphic and exist as primary, secondary, autophagic and secretory lysosomes.

Function: Lysosomes serve as the waste disposal system of the cell, and the lytic enzymes present inside them are capable of digesting any type of macromolecule including proteins, lipids, carbohydrates and nucleic acids. They can digest unwanted molecules, aged or damaged organelles as well as foreign bodies like bacteria, viruses and other pathogens. They play a vital role in processes central to protection against pathogens as well as in cell membrane repair, fertilization and self-destruction.
Mitochondria
Mitochondria 
Structure: This essential organelle consists of two phospholipid bilayers that form an outer membrane which encloses all the contents of the organelle; and an inner membrane which folds to form several compartments called cristae. The space between the two membranes is termed as the inter-membrane space.

Function: Mitochondria are the powerhouses of a cell, and are responsible for the breakdown of sugar molecules to release ATP (adenosine triphosphate), which is used to transport energy within the cell for metabolism. The outer membrane contains specialized proteins that allow molecular transport across mitochondria. The inner membrane contains numerous enzymes, and is the site for electron transport chain and ATP synthesis. These organelles are also known to harbor a set of proteins which when released into the cytoplasm lead to activation of the self-destructive processes of a cell. Hence the survival of a cell depends on the integrity of its mitochondria.
Centrosome
Centrosome 
Structure: Located just outside the nucleus, this organelle consists of a pair of centrioles surrounded by a protein network called pericentriolar material (PCM). Centrioles are cylinder-shaped structures made up of microtubules, and are arranged orthogonal to each other. They are absent in fungi and most higher plants.

Function: Centrioles are the centers for microtubule nucleation during cell division, and form an important structural component of the mitotic spindle. These, along with the attached microtubules, are involved in assorting the chromosomes into the resultant daughter cells.
Vacuole
Vacuole 
Structure: Present in all plant cells and only a few animal cells, these organelles consist of a membrane called tonoplast, within which water and other molecules including organic molecules are stored. It has no particular shape or size, and reduces or enlarges according to its contents and cellular needs.

Function: Vacuoles function as the storehouse of a cell and store food and water, as well as waste material before it is transported outside the cell. They also provide the necessary turgor pressure against cell walls in plant cells.
Chloroplast
Chloroplast 
Structure: A type of plant plastid, chloroplast is made up of a liquid matrix called stroma that is enclosed within two membranes termed outer and inner membranes. Dispersed into the stroma, a specialized network of membranes called thylakoids are organized into stacks called grana. These membranes contain chlorophyll and other photosynthetic pigments.

Function: Plants differ integrally from animals in their ability to prepare food within their cells through the process of photosynthesis. Here solar energy is harnessed by converting it into chemical energy in the form of ATP, which is then used for starch synthesis. This process of photosynthesis occurs through a set of light-dependent reactions that take place in the grana, and a set of dark (light-independent) reactions that occur in the stroma. In addition, chloroplasts are also the site of photorespiration, that involves light-dependent oxygen fixation.
Peroxisome
Peroxisome 
Structure: These are small spherical organelles morphologically similar to lysosomes. They consist of a central crystalloid core that is enclosed within a phospholipid bilayer. The central crystalline core consists of a variety of enzymes that are essential for several metabolic activities of the cell.

Function: Peroxisomes are the site for breakdown/oxidation of fatty acids to hydrogen peroxide, which is then decomposed by catalase. They also play an important role in seed germination by helping carbohydrate formation from the lipid stores of cells. They are also a site for some of the reactions of photorespiration.


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