Cells and Cell parts

The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life.[1] Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals). Humans contain about 100 trillion cells; a typical cell size is 10 µm and a typical cell mass is 1 nanogram. The longest human cells are about 135 µm in the anterior horn in the spinal cord while granule cells in the cerebellum, the smallest, can be some 4 µm and the longest cell can reach from the toe to the lower brain stem (Pseudounipolar cells).[2] The largest known cells are unfertilised ostrich egg cells, which weigh 3.3 pounds.[3][4]

In 1835, before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.[5]

The word cell comes from the Latin cellula, meaning "a small room". The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.[6]

Anatomy



The cells of eukaryotes (left) and prokaryotes (right)

There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.

Table 1: Comparison of features of prokaryotic and eukaryotic cells

 

Prokaryotes

Eukaryotes

Typical organisms

bacteria, archaea

protists, fungi, plants, animals

Typical size

~ 1–10 µm

~ 10–100 µm (sperm cells, apart from the tail, are smaller)

Type of nucleus

nucleoid region; no real nucleus

real nucleus with double membrane

DNA

circular (usually)

linear molecules (chromosomes) with histone proteins

RNA-/protein-synthesis

coupled in cytoplasm

RNA-synthesis inside the nucleus
protein synthesis in cytoplasm

Ribosomes

50S+30S

60S+40S

Cytoplasmatic structure

very few structures

highly structured by endomembranes and a cytoskeleton

Cell movement

flagella made of flagellin

flagella and cilia containing microtubules; lamellipodia and filopodia containing actin

Mitochondria

none

one to several thousand (though some lack mitochondria)

Chloroplasts

none

in algae and plants

Organization

usually single cells

single cells, colonies, higher multicellular organisms with specialized cells

Cell division

Binary fission (simple division)

Mitosis (fission or budding)
Meiosis

Prokaryotic cells

Main article: Prokaryote



Diagram of a typical prokaryotic cell

The prokaryote cell is simpler, and therefore smaller, than a eukaryote cell, lacking a nucleus and most of the other organelles of eukaryotes. There are two kinds of prokaryotes: bacteria and archaea; these share a similar structure.

Nuclear material of prokaryotic cell consist of a single chromosome that is in direct contact with cytoplasm. Here, the undefined nuclear region in the cytoplasm is called 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 finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Some eukaryote cells (plant cells and fungi cells) also have a cell wall;

Inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. A 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 enable additional functions, such as antibiotic resistance.

Eukaryotic cells

Main article: Eukaryote

Plants, animals, fungi, slime moulds, protozoa, & algae are all Eukaryotic. These cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain 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." Other differences include:

The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.

The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also contain some DNA.

Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."[7]

Eukaryotes can move using motile cilia or flagella. The flagella are more complex than those of prokaryotes.



Structure of a typical animal cell



Structure of a typical plant cell

Table 2: Comparison of structures between animal and plant cells

Typical animal cell

Typical plant cell

Organelles

Nucleus

Nucleus

Nucleolus (within nucleus)

Nucleolus (within nucleus)

Rough endoplasmic reticulum (ER)

Rough ER

Smooth ER

Ribosomes

Smooth ER

Cytoskeleton

Ribosomes

Golgi apparatus (dictiosomes)

Cytoskeleton

Golgi apparatus

Cytoplasm

Cytoplasm

Mitochondria

Mitochondria

Plastids and its derivatives

Vesicles

Lysosomes

Vacuole(s)

Centrosome

Cell wall

Centrioles

Subcellular components

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary components of the cell, then briefly describe their function.

Membrane

Main article: Cell membrane

The cytoplasm of a cell is surrounded by a cell membrane or plasma membrane. The plasma membrane in plants and prokaryotes is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (hydrophobic fat-like molecules) and hydrophilic phosphorus molecules. Hence, the layer is called a phospholipid bilayer. It may also be called a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.