 This video covers Part 3 of an Introduction to Cytology for Human Anatomy. We will cover the following objective, describe the structure and function of the major organelles and cell junctions. Here we can see an illustration of a prototypical human cell, although this is not indicative of any specific cell. It has most of the organelles found in cells within the human body. We will go through each of these different organelles starting off with the nucleus. That is the control center of the cell. The nucleus contains the genetic instructions. Those genetic instructions are found in chromosomes made of chromatin. The chromatin is made of DNA and proteins. DNA is the genetic material that forms a double helix structure. DNA is a long polymer, a long strand made from nucleotides that are abbreviated A, T, G and C. The DNA helix has two strands, it is a double helix. Those two strands are complementary where there is always an A across from a T and a G across from a C. Each strand of the double helix contains a version of the genetic instructions. This allows DNA to be replicated by splitting the two strands and making a copy of each strand. The DNA is wrapped around proteins called histones to form structures called nucleosomes. The strand of nucleosomes together is wrapped up into the structure called chromatin that is densely wrapped together in a chromosome. The central dogma of genetics is the idea that DNA codes for genes and DNA can be read by a process called transcription where a copy of the gene is produced and that copy of the gene is produced with a different molecule called RNA. RNA is very similar to DNA, but the messenger RNA will then move out of the nucleus and serve as a message for the next step to produce a protein. The messenger RNA is then turned into, is read, the genetic instructions from the DNA that are now in the sequence of the RNA are translated in order to produce a protein. The genetic instructions move from DNA to RNA to protein in the process of transcription and then the process of translation. A protein is a sequence of amino acids. Proteins are the main functional molecules in our cells. There are many different types of proteins that perform specific functions including the membrane proteins that regulate the movement of chemicals in and out and serve as receptors in order to recognize hormones and chemical messages. The protein is made out of amino acids. We can see here the abbreviations for the amino acids, MET for methionine, PRO for proline. These are examples of amino acids and the sequence of the nucleotides that are in the RNA is a code for specific amino acids. A sequence of three different nucleotides in RNA is a codon that codes for an individual amino acid. Each amino acid has a specific codon so that the amino acids don't share any codons. A specific three nucleotide sequence represents a specific amino acid although there could be more than one codon that represents an individual amino acid and it turns out that there are about 20 amino acids, 20 amino acids that are commonly used to produce proteins in translation. Summarized here is the flow of genetic information from the DNA inside the nucleus to messenger RNA, the process of transcription, and then from the messenger RNA moving out into the cytoplasm, into the cytosol to be translated in this process of translation. The messenger RNA is red and a protein is synthesized. Protein synthesis is translation and the organelle that performs translation is called a ribosome. There are ribosomes that are freely floating in the cytosol and these look like small dark bodies that are small dark spots that can be seen in an electron microscope image. They're much too small to see with a light microscope but they're also found in another organelle, a membranous organelle called the rough endoplasmic reticulum. The endoplasmic reticulum is surrounding the nucleus. The nucleus has a double membrane. There are two layers of lipid bilayer forming the nucleus, an inner and outer membrane, and the outer membrane of the nucleus is continuous with a network of fluid filled coiled tubes forming the endoplasmic reticulum. There are two different divisions of the endoplasmic reticulum called the rough and smooth endoplasmic reticulum. Endoplasmic reticulum is commonly just abbreviated ER. The rough ER contains many ribosomes embedded in the plasma membrane. There are many ribosomes embedded in the membrane of the rough endoplasmic reticulum and their function is to translate messenger RNA producing protein. The major function of the rough endoplasmic reticulum is protein synthesis, especially the synthesis of proteins that are destined to be secreted from the cell or destined to join the plasma membrane at the surface of the cell or the plasma membrane in another organelle. The smooth endoplasmic reticulum does not have ribosomes and so the ribosomes are what give the rough appearance, the small dots that stud the rough endoplasmic reticulum and without those dots the endoplasmic reticulum has a smooth appearance and is known as the smooth ER. The functions of the smooth ER are important for lipid metabolism. There are proteins called enzymes inside of the smooth ER that are important for metabolizing lipid chemicals that is nonpolar chemicals, for example cholesterol and steroids which are types of hormones. Steroids are hormones that are produced from cholesterol and so cholesterol metabolism occurs inside of the endoplasmic reticulum in the smooth ER. The Golgi apparatus is a stack of flat membranous discs, another organelle in the cell that's produced from membranes and the Golgi apparatus receives vesicles from the rough endoplasmic reticulum and those vesicles get processed through the Golgi apparatus and proteins that have been synthesized in the rough endoplasmic reticulum are sorted inside the Golgi apparatus. Then these vesicles, these small membranous compartments will bud off from the Golgi apparatus and they can follow different pathways to either go join another organelle or to join the plasma membrane at the surface of the cell where they could be a membrane renewal vesicle adding phospholipids and cholesterol and receptor proteins, transporter proteins and channels to the plasma membrane or they could be a secretory vesicle that releases hormones or neurotransmitters or enzymes out of the cell, excretes things out of the cell by exocytosis. Another type of vesicle that could be formed is a organelle called a lysosome or a paroxysome and so these are specialized types of vesicles that have different enzymes inside of them. A lysosome is specialized vesicle containing proteolytic digestive enzymes. Here we can see the example of the function of a lysosome inside of a type of leukocyte called a macrophage that's a cell specialized for phagocytosis. So in phagocytosis the cell wraps its plasma membrane around a particle in the extracellular space and brings that particle in to a vacuole and then that vacuole can merge with a lysosome. The lysosome contains digestive enzymes that will then break down the particle, break down the proteins in the particle and then those, the products of the protein being broken down, the amino acids can then be used to produce new proteins that are important for the cell's function. The paroxysome is a vesicle that's filled with special enzymes, oxidase enzymes and these enzymes are important for detoxifying harmful substances. They also have a role in lipid metabolism in the oxidation of long chain fatty acids but to detoxify harmful substances, for example alcohol and formaldehyde become oxidized by oxidase enzymes that are found in the paroxysome and the production of hydrogen peroxide is a common part of the function of the enzymes in the paroxysome and so because there's hydrogen peroxide inside of these vesicles that's where the name paroxysome comes from and there are enzymes that break down hydrogen peroxide or break down other highly reactive species, highly reactive chemicals that contain oxygen things called reactive oxygen species which can be free radicals meaning they have an unpaired electron and are highly reactive and can damage other chemicals so the enzymes inside the paroxysome can neutralize these free radicals and also break down hydrogen peroxide and produce water and so in the process of neutralizing the oxidants the paroxysome produces water but protects our body from the damaging effects of these highly reactive oxidant chemicals the mitochondria is the powerhouse of the cell the organelle that's responsible for producing ATP the cellular energy currency that's used by proteins here we can see the structure of the mitochondria has a double membrane there's an outer membrane which itself is a phospholipid bilayer with proteins embedded in it and then there's a highly folded inner membrane which is also a phospholipid bilayer with proteins embedded in it and the folds of the inner membrane are known as the cristae of the mitochondria and this folded structure creates a large amount of surface area for proteins that are embedded in the inner membrane that perform the process of oxidative phosphorylation in the mitochondria where ATP is produced this process uses oxygen and produces ATP and so it's known as oxidative phosphorylation mitochondria also perform other metabolic reactions that are important for supporting oxidative phosphorylation the citric acid cycle is a process at the end of the breakdown of glucose where carbon dioxide is being produced and the citric acid cycle supports oxidative phosphorylation by providing high energy electrons to the electron transport chain of the oxidative phosphorylation process the process of fatty acid oxidation also occurs inside of the mitochondria and supports oxidative phosphorylation the mitochondria also contains its own DNA it has a circular structure a DNA with a circular structure unlike the DNA in our nucleus that has a linear structure and this is one of the things that hints at the origin of mitochondria the idea that the idea known as the endosymbiotic theory for the origin of mitochondria is that mitochondria were previously free-living bacteria that became engulfed brought into cells by phagocytosis and then lived inside of those cells that eventually became our cells and the cells of other eukaryotic organisms all the plants and fungi and all the animals the cytoskeleton literally translated as the cells skeleton is an internal framework within the cell of fibrous proteins that helps to support the structure of the cell and support organelles and also facilitate the movement of organelles within the cell and regulate the shape of the cell and can contribute to the movement of cells and the changes of the shape of the cell there are three major classes of cytoskeletal elements microtubules, microfilaments, and intermediate filaments microtubules are hollow tubes they're the largest size you can see 25 nanometer diameter of a large hollow tube called a microtubule which is made out of the protein tubulin which forms dimers and so there's an alpha and beta unit of the tubulin dimer and many of these tubulin dimers are strung together in a long polymer forming the tube of tubulin and so the tube of the microtubule is a cytoskeletal element that radiates out around the nucleus and it's important for anchoring organelles in place and providing a framework for organelles to move around on so there are motor proteins that bind to the microtubules and can move organelles throughout the cell and during cell division the microtubules are a framework that allows the chromosomes to be moved into the two daughter cells and so that's a special structure made of microtubules called the mitotic spindles that form during cell division microfilaments are the smallest of the cytoskeletal elements they're formed from the protein actin and so the actin subunits are strung together in polymers forming long chains that have a small diameter approximately 7 nanometers is the diameter of one of these fibers of a microfilament microfilaments are concentrated underneath the plasma membrane and they are important for regulating the shape of the cell and proteins can pull on actin in order to change the shape of the cell and this is how muscle cells contract when proteins called myosin pull on actin are able to change the shape of muscle cells during contraction Intermediate filaments are a large number of different protein fibers and they're sort of a catch-all, a medium group they're larger than the microfilaments and smaller than the microtubules here we can see an example of an intermediate filament called keratin actin provides the structure of our skin and hair and nails and intermediate filaments inside the cell help to provide a framework, a structure in the cell that attach to cell adhesion proteins proteins that are embedded in the plasma membrane and so the intermediate filaments that contribute to the cytoskeleton attach to the proteins the plasma membrane that help anchor cells to one another or to proteins in the extracellular environment the centrosome is an organelle formed from microtubules centrosomes are located near the nucleus and are consist of two units called centrioles so a pair of centrioles is called a centrosome and each centriole is made from short cylinders of microtubules and so you can see here that triplets of microtubules are bound together with other proteins and nine triplets arrange together form a cylinder called a centriole and two centrioles together form a centrosome the function of the centrosome is to direct the formation of microtubules cilia are extensions of the cell membrane on the surface of an epithelium we can see cilia in the respiratory tract they are important for moving mucus along the respiratory tract so cilia can beat back and forth in order to move mucus along the respiratory tract when we inhale particles they get trapped in the mucus lining the respiratory tract and the cilia of that mucus membrane are able to beat back and forth in order to move those particles out of the airway the flagella is in structure similar to a really large cilium so it has the same basic structure of a cilia where there are microtubules that extend out through cilia and flagella that are important for the movement and motor proteins that can pull on those microtubules in order to move the cilia or in this case the flagella the only example of a cell with a flagellum in the human body is the sperm cell the male gamete the sperm cell has a tail which is a flagellum that functions in order to allow the sperm to swim in order to repel itself microvilli are smaller extensions of the plasma membrane on the surface of epithelial cells the microvilli function to increase the surface area of a cell and an example of where you'd find microvilli are lining the digestive tract there's microvilli in the small intestine where nutrient absorption occurs to increase the surface area of the small intestine so that we can have a maximal amount of surface area to absorb nutrients across tight junctions are a type of intercellular junction where two cells are held together forming an impermeable barrier so there are interlocking proteins that encircle the cells and prevent chemicals from moving in the space in between the two cells Desmosomes are a type of cell junction that anchors the intermediate filaments of the cytoskeleton of two adjacent cells together and so there are proteins that cross the plasma membrane of two adjacent cells and interconnect these are called cedherins and the two cedherins bind to one another and then on the intracellular part the desmosome binds to the intermediate filaments and in this way the cytoskeleton of two adjacent cells is anchored together in order to create a strong linkage between adjacent cells gap junctions are a type of cell junction that creates a hollow space, a pore connecting the cytosol of two adjacent cells the gap junction has a hollow cylindrical shape that is created by proteins that span the plasma membrane of both of the two adjacent cells these proteins are called the connexon complex of proteins that span the plasma membrane of both of the two adjacent cells and are open to allow the cytosol to flow from one cell to the next this is important for chemical communication between cells and gap junctions are found in cardiomyocytes the muscle cells of the heart where they allow electrical signals to rapidly spread through the heart muscle they're also found in some smooth muscle tissues for example in the smooth muscle tissue lining the intestines there are gap junctions connecting the smooth muscle cells together so that they can have a coordinated contraction pattern as electrical activity spreads from one cell to another