Fungi are typical contaminants in a few products that contain limited water and have low pH values; many of these products are lotions and creams.
Fungi are eukaryotic: they contain membrane-surrounded organelles, par-ticularly membrane-bound nuclei within which is the DNA comprising the chromosomes.
Other membrane-enclosed organelles include the mitochondria (which once were bacteria according to the endosymbiont hypothesis), the Golgi apparatus, endoplasmic reticula, lysosomes, and nucleoli (where ribosomes and ribosomal RNA are made). The term organelle is used to emphasize the parallel between the organs of animals and the structures within cells, each of which performs a very specific function. The extensive membrane systems in eukaryotes are needed because of their large volume and their need for regulation and transport as a result of that large volume.
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Cytoplasmic matrix, microfilaments, and microtubules
Within the eukaryotic cell is also a homogeneous, somewhat bland structure known as the cytoplasmic matrix. It is actually the most important and complex structure of a cell because it is the site where organelles exist and where a majority of the important biochemical processes take place. The cytoplasm is composed of 70 to 85% water — both free and bound to the surfaces of proteins. The pH is about 6.8 to 7.1, except in the digestive vacuoles and lysosomes where it can be as low as 3 to 4.
Within the cytoplasm are microfilaments of about 4 to 7 nm in diameter that provide a structure to aid in cell motion and shape. These filaments contain networks or parallel arrangements and have the same basic structure as actin found in human muscle protein. The other filamentous organelle is a thin cylinder about 25 nm in diameter and known as a microtubule. These structures along with microfilaments help cells maintain their shapes and aid movement, but their major role is in intracellular transport of substances throughout the complex cytoplasm.
Organelles
Endoplasmic reticulum. The endoplasmic reticulum (ER) is an irregular network of branching and fusing membranous tubules around 40 to 70 nm in diameter with flattened sacs called cisternae interspersed along the way.
A large part of the function of the ER is synthesizing protein from the ribosomes located along the surface of the ER. Since the ER is covered with ribosomes, it is known as rough endoplasmic reticulum. The other type known as smooth ER lacks ribosomes and is perhaps involved in lipid synthesis. The ER transports proteins, lipids, and other materials throughout cells and serves as the site of cell membrane synthesis.
Golgi apparatus. This organelle is made of flattened sacs, called cister-nae, that are stacked upon one another. The origin of the Golgi is the ER.
The sac that forms from the ER and faces it is called the cis side; the opposite and already formed maturing face is called the trans side. The membranes composing this organelle lack ribosomes. About eight or more cisternae are contained in a sac; each one is about 15 to 20 nm thick and they are separated from each other by 20 to 30 nm. At the edges of the cisternae are tubules and vesicles.
The main function of the Golgi is to package materials and prepare them for secretion. Apparently, material is transported from the ER to the cis side of the cisternae and then transported to the trans side and on the next cisternae by vesicles that bud off the edges and move to the next sac. Most of the proteins that enter from the ER into the Golgi are glycoproteins that are modified in the Golgi by the addition of specific groups; they are then sent on their way to their proper locations for use.
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Lysosomes. Lysosomes are synthesized by the ER and the Golgi appa-ratus. The digestive enzymes they contain are manufactured by the rough endoplasmic reticulum and then packaged by the Golgi apparatus. Lyso-somes are spherical, single membrane-bound particles about 50 to 500 nm in diameter. They contain all the enzymes (hydrolases) needed to digest the macromolecules on which cells thrive. Oddly enough, the internal pH of lysosomes is acidic (3.5 to 5.0) because the unit membranes around the lysosomes pump hydrogen ions into their interiors for the proper functioning of the hydrolases.
The key beneficiaries of lysosomes are the cells that obtain their nutrients via endocytosis: the cells engulf materials by enclosing them in vacuoles surrounded by “pinched-off” portions of cell membranes. These vacuoles (known as vesicles if they are particularly small) are created by phagocytosis (engulfment of particles) or pinocytosis (ingestion of liquid and the solute molecules). The vacuoles or vesicles are also called phagosomes or pino-somes, depending on the method of creation; collectively they are called endosomes. The membranes of the lysosomes fuse with those of the endo-somes to become food vacuoles as the digestive enzymes of the lysosome mix into the materials carried within the endosomes. Once digestion takes place, the nutrients diffuse into the cytoplasm, leaving behind residual bod-ies containing indigestible materials.
Ribosomes. The basic structure of the eukaryotic ribosomes is similar to that of prokaryotes, except that the subunits are larger in the eukaryotes.
A ribosome is composed of a large subunit of size 60S and a smaller unit of size 40S for a total size of 80S (S refers to a Svedberg unit, a measure of how quickly a particle sediments in a centrifuged gradient). The primary role of a ribosome is translation of the messenger RNA (mRNA) transcribed from the gene within the nucleus of a eukaryotic organism. This translation pro-cess is protein synthesis. Ribosomes are manufactured within the nucleoli of cells.
Within the cytoplasm of a cell, the ribosomes appear as tiny particles that give the cytoplasm a stippled appearance. However, ribosomes are also intimately associated with the rough endoplasmic reticulum, studding its membranes, somewhat like a biker’s black leather jacket is covered with silver studs.
Mitochondria. Mitochondria are currently thought by some scientists to be derived from endosymbionts that were once prokaryotic invaders of large progenitor cells. A symbiosis developed with the large cells that could not manage energy distribution very well. The symbiotic invaders could supply the cells with energy in the form of adenosine triphosphate (ATP).
There is considerable support for this theory that, during the early formation of life, mitochondria were simply bacterial invaders turned good. Mitochon-dria contain circular genome-like bacteria; mitochonMitochon-dria replicate indepen-dently of the cell and even have prokaryotic-like 70S ribosomes.
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Nuclei. The nucleus of a eukaryotic cell, as opposed to the nucleoid of a prokaryotic cell, is bound by a membrane called the nuclear envelope.
This double-layered membrane is very porous in order to allow macromol-ecules easy access in and out of the cell. The nucleus contains several linear chromosomes compared to the single circular chromosome of a prokaryote.
It also contains the nucleoplasm and the nucleolus that manufactures RNA for synthesizing ribosomes.
The nucleoplasm term is used to describe all the enzymes and proteins in the nucleus that are involved in replication of the genome. The key item within the nucleus is the chromatin that makes up the chromosomes, which are composed of DNA and proteins called histones. The nucleus is not visible until the cell undergoes mitosis and the chromosomes are duplicated and condensed by forming coils and supercoils around the histones just before being separated into the daughter cells.
External cell organelles
Motility and protection are useful characteristics for any organism. Eukary-otic cells enjoy the benefits of motility by means of flagella or cilia. The presence of a glycocalyx (described below) provides the benefit of protection from the external environment.
Cilia and flagella. The eukaryotic flagellum is composed of microtu-bules arranged in what is referred to as a 9 + 2 arrangement (nine pairs of hollow tubules surrounding a single pair of tubules in the center). The whole arrangement is surrounded by an extension of the cell membrane. The move-ment can either be a back-and-forth, whip-like motion or a twirling motion resulting when the tubules slide past each other in an almost muscle-like fashion; all movement is coordinated by the cell membranes. In contrast, bacterial flagella are simple. Each flagellum consists of a protein strand and undergoes a spinning motion due to a basal body connected into the cell wall. Cilia are composed of the same basic architectures as flagella, but they are shorter. Usually they cover a large portion of a cell and beat in a regular fashion
External cell coverings. A eukaryotic cell may have a structure known as a glycocalyx that serves as an external boundary layer beyond the mem-brane (in protozoa and a very few algae) or the cell wall (in most algae and fungi) that comes in direct contact with the environment much like the glycocalyx of a prokaryote. The structure is composed of polysaccharides that comprise a slime layer for protection, provide for attachment to surfaces, or even serve as a mechanism for communications with other cells.
The cell walls of fungi and algae provide shape, support, and protection.
A fungal cell is composed of a thick inner layer of chitin or cellulose fibers and a thin outer layer of mixed glycans covered by the glycocalyx. The cytoplasmic membrane is the typical bilayer of lipids into which proteins are free to move. The key difference between eukaryotes and prokaryotes is
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66 Cosmetic Microbiology: A Practical Approach the presence of sterols in the membranes of eukaryotes in addition to phos-pholipids. The sterols add some stability to the membrane by making it less flexible.