cell (biology) - Overview, Anatomy of cells, Subcellular components

The basic unit of plant and animal bodies; it comprises, at least, a nucleus or nuclear material and cytoplasm enclosed within a cell membrane. Some cells, such as the mature red blood corpuscles of mammals, lack a nucleus, but possessed one at an earlier stage of development. Many cells are more complex, and contain other specialized structures (organelles), such as mitochondria, chloroplasts, Golgi bodies, and flagella. Many simple organisms comprise a single cell, and may lack a membrane separating nuclear material from cytoplasm. Advanced organisms consist of a variety of co-operating cells, often specialized to perform particular functions and organized into tissues and organs. Plant cells are typically surrounded by an outer cell wall containing cellulose. Cell division may occur by splitting into two parts (fission), by mitosis, and (in the case of reproductive cells) by meiosis. All nucleated cells contain within their nuclei the entire inherited genetic information of that individual, but in specialized cells, such as a liver or brain cell, only a minute fraction of their genetic database is operational.

Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm, a typical cell mass 1 nanogram). The largest known cell is an ostrich egg.

The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells, and cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. The name was chosen by Robert Hooke when he compared the cork cells he saw to the small rooms monks lived in.

Overview

Properties of cells

Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.

All cells share several abilities:

Reproduction by cell division (binary fission, mitosis or meiosis). Metabolism, including taking in raw materials, building cell components, converting energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. Cell contents are contained within a cell surface membrane that contains proteins and a lipid bilayer.

Some prokaryotic cells contain important internal membrane-bound compartments, but eukaryotic cells have a highly specialized endomembrane system characterized by regulated traffic and transport of vesicles.

Anatomy of cells

There are two types of cells, eukaryotic and prokaryotic. Prokaryotic cells are usually singletons, while eukaryotic cells are usually found in multi-cellular organisms.

Prokaryotic cells

Main Article:Prokaryote

Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack most of the intracellular organelles and structures that are characteristic of eukaryotic cells (an important exception is the ribosomes, which are present in both prokaryotic and eukaryotic cells). Prokaryotic cells have three architectural regions: appendages called flagella and pili — proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:

The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a filter and communications beacon. It also prevents the cell from "exploding" (cytolysis) from osmotic pressure against a hypotonic environment. A cell wall is also present in some eukaryotes like fungi, but has a different chemical composition.

Eukaryotic cells

Eukaryotic cells are about 10 times the size of 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 the presence of a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. Cell walls may or may not be present.

Subcellular components

All cells, whether prokaryotic or eukaryotic, have a membrane, which envelopes 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 will list these primary components of the cell, then briefly describe their function.

Cell membrane: A cell's defining boundary

Main article: Cell membrane

The cytoplasm of a cell is surrounded by a 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 phosphorous molecules. 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. Cell surface membranes also contain receptor proteins that allow cells to detect external signalling molecules such as hormones.

Cytoskeleton: A cell's scaffold

Main article: Cytoskeleton

The cytoskeleton acts to organize and maintain the cell's shape; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. There is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments.

Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm.

A human cell has genetic material in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome).

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Cells also have a set of "little organs," called organelles, that are adapted and/or specialized for carrying out one or more vital functions.

Cell nucleus (a cell's information center) The cell nucleus is the most noticed organelle found in a eukaryotic cell. It contains the cell's chromosomes, and is the place where almost all DNA duplication and RNA synthesis occur. The nuclear envelope human cell nucleus no text.png|150px|Cell nucleus]]

Mitochondria and Chloroplasts (the power generators) Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell, they play a critical role in generating energy in the eukaryotic cell, a organelles that are modified chloroplasts;

Lysosomes and Peroxisomes (of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system. The centrosome produces the microtubules of a cell - a key component of the cytoskeleton. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells. Some cells, most notably Amoeba have contractile vacuoles, which are able to pump water out of the cell if there is too much water.

Cell functions

Cell growth and metabolism

Main articles: Cell growth, Cell metabolism

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, wherein the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a form of energy, via two different pathways. The second pathway, called the Krebs cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.

Creation of new cells

Main article: Cell division

Cell division involves a single cell (called a mother cell) dividing into two daughter cells.

Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.

DNA replication, or the process of duplicating a cell's genome, is required every time a cell divides.

Protein synthesis

Main article: Protein biosynthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This RNA stand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell.

Origins of cells

Main article: Origin of life

The origin of cells has to do with the origin of life, and was one of the most important steps in evolution of life as we know it. The birth of the cell marked the passage from prebiotic chemistry to biological life.

Origin of the first cell

If life is viewed from the point of view of replicators, that is DNA molecules in the organism, cells satisfy two fundamental conditions: protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. If freely-floating DNA molecules that code for enzymes are not enclosed into cells, the enzymes that benefit a given DNA molecule (for example, by producing nucleotides) will automatically benefit the neighbouring DNA molecules. Therefore the selection pressure on DNA molecules will be much lower, since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: All molecules in a given neighbourhood are almost equally advantaged.

If all the DNA molecule is enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. They bear much of the basic features provided by cell membranes.

Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. On the other hand, a lipid bilayer bubble can contain water, and was a likely precursor to the modern cell membrane. This theory is expanded upon in the book, The Cell: Evolution of the First Organism by Joseph Panno Ph.D.

Origin of eukaryotic cells

The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell – a theory termed the endosymbiotic theory.

There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or viceversa: see the hydrogen hypothesis for the origin of eukaryotic cells. 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope. 1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory. Rudolph Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula). 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory.

References

^ Cell Movements and the Shaping of the Vertebrate Body in Chapter 21 of Molecular Biology of the Cell fourth edition, edited by Bruce Alberts (2002) published by Garland Science.
The Alberts text discusses how the "cellular building blocks" move to shape developing embryos. I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular [..] these pores, or cells, [..] were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this. Robert Hooke ^ The Universal Features of Cells on Earth in Chapter 1 of the Alberts textbook (reference #1, above). This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain

External links

Wikimedia Commons has media related to: Cell (biology) Wikibooks has more about this subject: Cell Biology The Virtual Cell Cells Alive! Journal of Cell Biology A comparison of the generational and exponential growth of cell populations High-resolution images of brain cells The Biology Project > Cell Biology The Image & Video Library of The American Society for Cell Biology is a collection of peer-reviewed still images, video clips and digital books that illustrate the structure, function and biology of the cell, the fundamental unit of life. Molecular Cell Biology fourth edition, edited by Harvey Lodish (2000) published by W. The Cell - A Molecular Approach second edition, by Geoffrey M. Landmark Papers in Cell Biology Gall JG, McIntosh JR, eds. Landmark Papers in Cell Biology. Bethesda, MD and Cold Spring Harbor, NY: The American Society for Cell Biology and Cold Spring Harbor Laboratory Press;