Cell physiology Part B part 2

Ajouté : 2026-05-09

[00:00:00]in this lecture we're going to review the nucleus as well as various cytoskeletal proteins but beginning with the nucleus the nucleus is a relatively small component of the cell which makes up about 10 percent of the cell volume but is a critically important component to the cell most cells contain one nucleus one example is a nerve cell so a neuron will contain one nucleus a skeletal muscle cell however will contain many nuclei and that provides the skeletal muscle cell with an enhanced capability of accommodating to various stressors because the skeletal muscle cell will be in a better position to manufacture proteins to a much greater extent than that single neuron and the major function as I just indicated this a second ago of the nucleus is that it is the site where new protein manufacturing begins in order for the cell to make a new protein it must have a set of very clearly defined instructions and that is housed within What's called the blueprint of the cell and that blueprint is the cells or rather the organism's genome and the DNA molecule is is the molecule that houses those very specific instructions on how the cell is to make a functional protein now in order to make that protein of course the cell requires building materials and so what are the building materials for proteins those are the amino acids but where do these amino acids come from there there is something called the amino acid pool in the body that is a very general term referring to the existence of single amino acids in solution contained throughout the body uh circulating through the cardiovascular system contained outside the cell as well as inside the cell so these are materials that are always on hand another mechanism for obtaining the building materials is that the cell May accelerate the breakdown of existing functional proteins when that happens those proteins get disassembled and we now have amino acids that are that become available the third very important source of those amino acids of course is through the diet and the consumption of dietary protein will provide the body with those necessary building materials the amino acids provided provided that the digestive system is effective at breaking apart those dietary proteins and some dietary proteins are easier to break apart than others by the digestive system so taking a closer look at the nucleus here as imaged in this microscopic photo on the left hand side here we see this large body on the picture once again relatively small compared to the entire cell is the nucleus much like the cell the nucleus is surrounded by what's called a nuclear envelope similar but not the same as the cell membrane it's similar in that the nuclear envelope utilizes this same foundational molecule in the phospholipid to assemble the outer rim of the nucleus but the cell membrane is made up of a single bilayer while the nuclear envelope is made up of two bilayers uh in uh perforated throughout the nuclear envelope are of nuclear pores uh just like the cell membrane it is important for molecules to enter the nucleus molecules to exit the nucleus uh and the pores do provide a part of that transport the nucleolus is this structure contained within the nucleus that functions to assemble ribosomes in addition to other functions and the ribosomes become functional in the cytoplasm where those ribosomes serve as the workbench if you will for the manufacture of a new uh a new protein and the nucleoplasm refers to the aqueous medium inside the nucleus and critical to the nuclear function is chromatin and chromatin is an expanded uh structure of what we typically call chromosomes which are made up of DNA molecules and protein molecules and it's the DNA that houses those specific instructions on how the cell is to make a functional protein so the nuclear envelope as indicated we'll review this here is made up of two lipid bilayers as opposed to the single lipid bilayer in this cell membrane and that sets up what's called the perit nuclear space so the cell membrane the overall cell membrane does not have a space the interior of the membrane is made up of those fatty acid Tails but now since we have two uh bilayers there is a space within the nuclear envelope and the space is important because it enhances the transport of ribosomes from the interior of the nucleus out to the cytoplasm and so once again here the nucleolus is a structure that is critical for the manual factor of those ribosomes which then move out into the cytoplasm and the nuclear envelope is also critical for keeping the cytosol and the nuclear enzymes apart so there are proteins that function as enzymes inside the nucleus there are proteins that function as enzymes inside the cytoplasm and that's where those enzymes belong and it's the nuclear envelope that ensures that those enzymes stay where they are supposed to function the nuclear envelope has direct connections to a structure called the endoplasmic reticulum that will address later in this lecture in this unit and that will enhance the efficiency in which the nucleus contributes to the manufacture of new proteins the ribosomes are found on the surface of the endoplasmic reticulum and that's something that will address a little bit a little bit later as indicated a little bit earlier the nuclear envelope is punctured by nuclear pores and that once again that allows the transport of molecules into the nucleus and out of the nucleus uh and so there are a number of very important nuclear proteins um the first one here are what are generally referred to as histones and histones are proteins that allows the DNA molecules to be packed more efficiently space efficiency uh inside the nucleus in fact the DNA molecules wrap around these histones DNA and RNA polymerases once again we look at this term here and we see the last we see the suffix here this is plural but the suffix here ASC indicates that this is an enzyme which means it's a protein and this is a protein that will initiate the manufacture of a new protein or the copying of the DNA molecule depending upon which polymerase has been activated there are a number of proteins that regulate Gene activation as well as the processing of what are called rnas uh uh proteins so rnas are these nucleic acids that are critical for the manufacture of new proteins but it takes proteins to regulate those rnas when they are manufactured two important rnas that we won't go into much detail here in this course but TRNA is referred to as Transfer RNA and mRNA is messenger RNA these are two nucleic acids that are very important in the manufacture of new proteins inside the cell but both nucleic acids are manufactured in the nucleus we now move on to the cytoskeleton and the cytoskeleton is a general term referring to the scaffolding neck Network that provides that structural support to the cell and these are made up of a wide arrange of a wide range of different proteins and so these not only provide strength and stability but it also allows flexibility so that the cytoskeleton does have the ability to bend uh but ideally not break and that's what allows cells to be able to withstand mechanical forces up to a point cells will fracture uh depending upon the magnitude of an of an imposed Force but for small forces cells will function by deforming and it's the cytoskeleton that allows the deformation while keeping the structure together the cytoskeleton also plays an important role for those cells that undergo cell division and the cytoskeleton provides a track or a a conveyor belt if you will that helps to pull chromosomes apart after chromosomes have been replicated prior to cell division in some cells such as the neuron the cytoskeleton allows for the cell to move materials inside the cell from one location to another there are three types of cytoskeletal filaments the first that we'll discuss is the intermediate filament and the main function of the intermediate filament is to provide mechanical strength microtubules as the term suggests these are these take the shape of of uh of small tubes and these tubes help to direct intracellular transport and helps to position organelles inside the cell and then the third type of cytoskeletal filament that we'll discuss are what are called actin filaments and these are filaments that player role in forming the shape of a cell surface as well as playing a role in cell movement or simply locomotion all three of these cytoskeletons are made up these are uh are proteins made up of different proteins but all three have to be supported by other proteins as well uh that we just simply refer to that General class as accessory proteins and these include what are called the motor proteins the first uh type of skeletal cytoskeletal filament here is the intermediate filament and the intermediate filament forms what's described as a rope-like fiber it has a diameter on average of about 10 nanometers as you can see in the upper right hand portion of the slide uh we have a range of anywhere from 8 to 12 but you can see these different components that when expanded are two fibrous individual proteins that are wound up in this helical type of manner and then these two fibrous subunits are combined with numerous other fibrous subunits to make one functional intermediate filament and the intermediate filament as just indicated are made up of intermediate proteins and so what we see here in the bottom right hand portion of the slide is an enhanced uh image of under a microscope of these fibrous types of proteins the intermediate filaments carry out a wide range of functions one location is within What's called the nuclear lamina and the nuclear lamina refers to this complex array of molecules including a wide range of different proteins on the immediate interior of the nuclear membrane so what we see here in the gray is the cytoplasm and we have two layers of phospholipid bilayers two of those forming the paranuclear space and on the most immediate interior is that lamina and the intermediate filament it helps to provide that strength and stability to the nuclear envelope as you can see as you can see here so once again the major function of the intermediate filament uh throughout the cell and in this example is to provide that mechanical strength uh the second one here as as mentioned earlier are these microtubules and so this forms what is basically Hollow cylinders at the at the microscopic level and these are made up of proteins that are just simply referred to as tubulin monomers the term monomer refers to an individual functional unit of some Macro Molecule so here we see a large protein that we call a microtubule but that microtubule is made up of these smaller monomers that we'll address here shortly the outer diameter of the entire tube is estimated to be approximately 25 nanometers and these form relatively rigid structures and these rigid structures will basically form a track or a conveyor belt or a pathway for the trafficking of various organelles or other molecules around a cell one example is neurotransmitter transport that takes place inside neurons so for a neuron to move a neurotransmitter from one interior aspect of the neuron to another location inside that neuron that neurotransmitter is transported along a microtubule and so as indicated just a few seconds ago the microtubule is made up of these individual monomers just simply referred to as beta tubulin monomers and Alpha tubulin monomers they are theirs they have similar uh chemical structures not the same but being similar they're able to form what's called a tubulin dimer a connection of these two similar monomers that that when assembled and bound to various other dimers will make this very symmetrical shape type of microtubule as we can see here on this lower right hand microscopic image the third type here are what are referred to as actin filaments uh the term that is used interchangeably with actin filament are is the term microfilament uh and the foundation of the actin filament is the actin monomer and if we take a look at the right hand side of the slide we see that these are called microfilaments because these are the the smallest diameter of the psychoskeletal elements having a diameter of approximately seven nanometers so uh smaller than the other two uh the actin monomers uh when combined together so these globular units uh refer to the actin monomers will make up a single actin polymer Strand and that's a single connection of these uh globular proteins and then when we take one strand and we wind it around a second similar strand we now have an actin filament and the actin filament as indicated has a diameter anywhere from five to nine nanometers and plays a very important role in a wide range of functions inside the cell one such function is skeletal muscle Force production so this protein plays a very important role in the production of muscle Force but also plays an important role in other aspects of the cell so this ends our second lecture on Part B of cell physiology