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The Cytoplasm

The Cell has three basic parts. A nucleus, plasma membrane, and everything in between: The Cytoplasm.

The Cytoplasm is made up of three components as well.

Organelles, Cytosol, and Inclusions

Cytosol: Mostly water, the cytosol is the fluid that suspends all of the other cytoplasmic elements. Dissolved in this watery fluid are all of the substances necessary for cellular function, salts, proteins, sugars, and other substances.

Inclusions: Inclusions may or may not be present in a cell depending on the cell’s type and function. Inclusions are chemical substances stored in a cell’s cytoplasm. Examples include lipids stored in fat cells, and glycogen stored in liver cells.

Organelles: These small structures are the metabolic machinery of the cell. The two general types of organelles are membranous organelles and non membranous organelles. There are organelles in involved in packaging and sorting proteins for export, digesting invaders, storing genetic material, and organelles involved in many other cellular functions.

Membranous organelles:

Mitochondria: The mitochondria produce the cell’s primary metabolic fuel source, ATP. The Mitochondrion has a double membrane, the outer being smooth and the inner membrane being highly folded and shriveled in order to increase surface area. It is on this membrane that ATP is produced from glucose in a process called kreb’s cycle. Mitochondria are probably the most unique organelles because they contain their own DNA, RNA, and ribosomes. These components allow them to self-replicate and produce some of their own proteins involved in mitochondrial function. Mitochondrial DNA (37 genes) closely resembles DNA of bacteria in the purple bacteria phylum. For this reason it is widely believed that long ago mitochondrial ancestors invaded early plant and animal cells, forming a symbiotic relationship that gave rise to all of the complex cell types we know today.

Endoplasmic reticulum: The endoplasmic reticulum is a network of tubes and flattened fluid filled membranous sacs called cisterns. This membranous network has a smooth and a rough component. The Rough ER is studded with ribosomes giving it its rough appearance. The rough Er is responsible for packaging newly synthesized proteins for export to the golgi apparatus, and eventually out of the cell. The smooth ER has no ribosomes, and is involved in lipid metabolism. Additional functions of the endoplasmic reticulum are the production of plasma membrane and the production of peroxisomes.

Golgi Apparatus: The golgi apparatus is made up of stacks of membranous sacs. One side of the apparatus (the cis face) receives incoming vesicles from the rough ER. After packaging and further proccessing the incoming proteins, they depart in vesicles form the opposite side of the apparatus (the trans face). After departure, vesicles can be excreted from the cell in secretory vesicles or incorporated into other membranes throughout the cell. One last function of the golgi apparatus is to package digestive enzymes into vesicles called lysosomes.

Lysosomes: Lysosomes are the digestive machinery of the cell. Essentially vesicles packed with digestive enzymes, these organelles bind with vesicles containing material destined for digestion (debris, damaged organelles, bacteria, viruses, glycogen and other metabolic materials) and fuse, expelling their digestive contents into the newly formed larger vesicle, thus digesting the material. Additionally,  by self rupturing, lysosomes can also be used to destroy the cells of non useful tissue.

Peroxisomes: Like lysosomes, peroxisomes are spherical membranous vesicles that contain many different enzymes. The most powerful and important of which are oxidases and catalyses. These enzymes neutralize alcohol, formaldehyde, and most importantly, free radicals. Peroxisomes convert free radicals into hydrogen peroxide which is then converted to water. You can expect to find lots of peroxisomes in liver cells which are highly involved in detoxification. Unlike lysosomes, which are produced by the golgi apparatus, peroxisomes are produced by special machinery in the endoplasmic reticulum.

The Nucleus: The largest organelle, the nucleus is essentially a container for our genetic material. Surrounded by the nuclear envelope, which is made up of two membranes separated by fluid, the nucleus contains the nucleoli and chromatin.

  • Nucleoli (technically nonmembranous): There are usually around two of these per nucleus although there may be more depending on the cell. These structures are responsible for combining proteins produced in the cytoplasm with rRNA produced within the nucleus. The result is the production of ribosomal sub units which are exported into the cytoplasm to form ribosomes.
  • Chromatin: Chromatin is a gel like substance made up of our genetic material and is composed of about 10% RNA, 60% histone proteins, and 30% DNA. This material is packaged into the fundamental units of chromatin called nucleosomes. These nucleosomes consist of 8 tightly packed histone proteins each with DNA wrapped tightly around them twice. Then the strand of DNA moves onto the next group of histones and wraps around them twice, and so on and so on. This allows the chromatin to be tightly packed during mitosis into chromosomes. This way, our genetic material isn’t damaged during cellular division.

Non-Membranous Organelles

Ribosomes: Composed of protein and ribosomal RNA, ribosomes are are really made up of two smaller subunits. These sub-units latch onto mRNA molecules and synthesis proteins by combining free floating amino acids in the cytoplasm and assembling them into peptide chains. Ribosomes can either be free floating in the cytoplasm, or bound to the rough ER in order to produce ribosomes for exportation from the cell

Cytoskeleton: Cell’s like the human body have sort of skeleton responsible for providing strength and support and providing a site for protein motors to produce movement. This cytoskeleton is composed of three types of rods, as well as motor proteins, and centrioles.

Cytoskeletal Rods

  • Microfilaments: The thinnest of the cytoskeletal elements, microfilaments form a dense web just deep to the cell’s membrane called the terminal web. This web helps give the cell shape and provide structural support to the cell. It is also involved with cellular movement through the use of motor proteins, one example being the cellular division that takes place during cytokinesis.
  • Intermediate Filaments: These filaments have a diameter somewhere in between that of microfilaments and microtubules. They are composed of tetramer fibrils wound on eachother much like a rope. Like a rope, these filaments resist tensile stresses and are purely structural in nature. You would expect to find a large quantity of these in tissues exposed to a lot of tensile force such as skin cells.
  • Microtubules: The largest of the cytoskeletal rods, microtubules are composed of tubulin proteins and are essentially hollow tubes. They can be quickly assembled and dissambled to accomplish various functions. Organelles hang from the microtubules and are shuttled around the cell by motor proteins much like train cars on a track. All of the cell’s microtubule radiate from the centrosome, an area near the nucleus which is composed of matrix and two centrioles.

              Centrioles: Centrioles are barrel shaped non membranous organelles oriented at a 90 degree angle to eachother and located in the centrosome or cell center. Microtubules are assembled and dissambled here and radiate outward like bicycle spokes from the centrioles. Centrioles play an active role in mitosis which will be discussed later in the cell cycle quick review.

Thats the Cytoplasm!!

Feel free to ask questions, I’s sure you have many!

Trevor

2 comments on “The Cytoplasm

  1. Pingback: The Cell, Tissues, Integumentary system, and Skeletal Tissues Study Guide | A & P Source

  2. Pingback: Topic Summary #2 The Cell: part 2 | A & P Source

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