in most prokaryotic cells morphology is maintained by the cell wall in combination with cytoskeletal elements and we're going to talk about all of that but the cell wall is a structure found in most prokaryotes and some eukaryotes and it envelops the cell membrane protecting the cell from changes in osmotic pressure that's its main function it is there to protect the cell from changes in osmotic pressure so what's osmotic pressure osmotic pressure occurs because of differences in the concentration of solutes on opposing sides of a semipermeable membrane water can pass through a semipermeable membrane but solutes like dissolved molecules like salts and sugars and other compounds cannot pass through a semipermeable membrane when the concentration of solutes is greater on one side of the membrane so wherever it's greater water will diffuse across the membrane from the side with the lower concentration or the side that has more water to the side with the higher concentration or the side that has less water does that make sense because it's going to be higher concentration so think of it as salt water and so higher would mean it's a higher concentration of salt and less water versus the side that has a lower concentration of salt but more water okay so it's going to diffuse from the side with the lower concentration or more water to the side with the higher concentration or less water until the concentrations on both sides become equal this diffusion of water is called osmosis and it can cause extreme osmotica pressure on a cell when it's in external environment changes so the external environment of a cell can be described as isotonic hypertonic and hypotonic in an isotonic medium the solute concentration inside and outside of the cell are approximately equal so there's no net movement of water across the cell membrane so up here is this is showing you just water in two like flasks basically with a semi-permeable membrane in between so here's an isotonic medium we've got a pretty equal on both sides if we've got osmotic pressure say this is our solution so this is where our salt is and this is our just our water so we're going to have more water moving over here if we have more salt over here so that makes sense whereas if let's say this has a lower concentration versus here we're gonna have more does this make sense um but let me backtrack a second uh so this is this is just a way you can visibly see it now let's think about a cell the cell membrane is going to be our semipermeable membrane okay so we're talking about actual movement of water in and out of a cell so think about what that would do to the cell how stressed out the cell will be if water is moving in or out of it right so in an isotonic medium this cell is put into something where the concentrations inside of the cell are about the same as the concentrations of any sort of solute outside of the cell so there's really no net movement of water we're happy there in a hypertonic medium the solute concentration outside of the cell is going to exceed that inside of the cell hypertonic solute outside of the cell is going to be greater than the solute inside of the cell so water is going to diffuse out of the cell okay because water is always going to move to wherever the solute concentration is greatest so water is going to diffuse out of the cell and in turn into the external medium and the cell is going to what shrink because we're having water move out of the cell into the external environment in a hypotonic medium the solute concentration inside of the cell is going to be greater than that outside of the cell so water is going to move to wherever the concentration is greatest so it's greatest inside of the cell so water is going to move by osmosis into the cell and that's going to cause the cell to swell and possibly burst so the degree to which a particular cell is able to withstand those changes in osmotic pressure is called tonicity so cells that have a cell wall are better able to withstand those changes in osmotic pressure and help maintain their shape in hypertonic solutions so if we remember hypertonic solution concentration outside of the cell is greater than that inside of the cell so in hypertonic um environments cells that lack a cell wall so they don't have a cell wall can become dehydrated because they're shrinking and that causes uh crenation and which is basically just shriveling of the cell and the plasma membrane is going to contract and the plasma membrane will appear scalloped or notched okay so this here's our hypertonic solution um water's moving out of the cell we're shrinking and here's our notched plasma membrane chronation okay by contrast cells that possess a cell wall will undergo plasmolysis rather than crenation so this is in a cell that does not have a cell wall osmosis and cells without a cell wall okay here's osmosis and cells with the cell wall so if you have a cell wall and you undergo plasma you're going to undergo plasma lysis instead of crenation when you're in a hypotonic or hypertonic solution so here we are hypertonic without a cell wall hypertonic with a cell wall in plasmolysis the plasma membrane is going to contract and detach from the cell wall not everywhere but most mostly everywhere and there's a decrease in interior volume right because here's our normal cell we have a decrease in interior volume but the cell will remain intact and it will allow the cell to maintain some shape and integrity for a period of time which is what we want we don't want the cell to completely you know this could be very detrimental to a cell whereas if we're still kind of maintaining our outer structure then as soon as water can come back in we can fill that back in so likewise in cells that lack a cell wall here in a hypotonic solution we're going to be more prone to burst if we don't have a cell wall versus if we have our cell wall and water's flowing in that cell wall can still kind of maintain our structure right and we're just gonna bust like we would if we were wearing you know oh if we were bloated and wearing really tight skinny jeans the presence of a cell wall what you need to remember is the presence of a cell wall allows the cell to maintain its shape and integrity all cellular life has dna genomes that are organized into one or more chromosomes prokaryotic chromosomes are typically circular and haploid or unpaired and are not bound by a complex nuclear membrane prokaryotic dna and dna associated proteins are concentrated within the nucleoid region of the cell prokaryotic cells may also contain extra extra chromosomal dna or dna that is not part of the chromosome so this extra chromosomal dna is found in plasmids which are small circular double-stranded dna molecules so prokaryotic chromosomes are typically circular and haploid okay but plasmids are small circular double-stranded dna molecules cells that have plasmids often have hundreds of them within a single cell so here's our bacterial chromosome circular and haploid here's our plasmids circular but they're also double stranded okay plasmids are more commonly found in bacteria but they have been also found in archaea and eukaryo eukaryotic organisms as well plasmids often carry genes that confer advantageous traits like antibiotic resistance thus they are super important to the survival of the organism so if they do have plasmas that usually means they can survive in crazier harsher environments than those that do not have them all cellular life synthesizes proteins so in order to synthesize proteins organisms need ribosomes and organisms in all three domains of life possess ribosomes however ribosomes in each of the three domains are structural structurally different ribosomes themselves are constructed from proteins along with ribosomal rna or rrna prokaryotic ribosomes are found in the cytoplasm they're called 70s ribosomes because they have a size of 70s where eukaryotic ribosomes have a size of 80s so what that means is this s stands for this vedburg unit it's just a measure of sedimentation in an ultra centrifuge which is based on size shape and surface quantities and qualities of the structure being analyzed so it just basically means they take these ribosomes and they can spin them down really really fast in this centrifuge and then they can measure the sediment like the little pellet that forms at the bottom and it they give them a size based off of that so yes i'm aware that uh their 70s so they're composed of these two units one is 30s and one is 50s and 30 and 50 does not equal 70 but you know we're in science and not math so it is important for you to know because you'll get confused and you will be asked on this the small subunit of a of a prokaryotic ribosome is 30s the largest 50 but together our complete ribosome is 70s eukaryotic ribosomes are ads and we are so going to get into that math on our next lecture too so some prokaryotic cells have the ability to store excess nutrients within cytoplasmic structures called inclusions so storing nutrients in a polymerized form is is advantageous to the cell because it reduces the buildup of osmotic pressure that occurs in the cell because of you know the accumulation of solutes so if they can store those nutrients and those solutes that helps with osmotic pressure osmotic pressure so various types of inclusions store glycogen and starches which contain carbon um that the cells can access for energy volatin granules are inclusions that store polymerized inorganic phosphate and they can be used in the metabolism for the cell and they can asses assist in the formation of biofilms sulfur granules another type of inclusion these store elemental sulfur which the bacteria use for metabolism and some prokaryotic cells have other types that serve purposes other than nutrient storage so for example some prokaryotic cells produce gas vacuoles which are accumulations of small protein-lined vesicles of gas and these allow the prokaryotic cells to synthesize them and alter their buoyancy which is super cool can we do that like i wish we could create gas vacuoles and just make us lighter some bacteria contain magnetosomes which are inclusions of magnetic iron oxide or iron sulfide surrounded by a lipid layer these allow cells to align along a magnetic field and it helps aid in their movement so if they don't have flagella and they can't move around they sometimes have these magnetosomes and they can along a line along a magnetic field and then cyanobacteria produce carboxysome inclusions carboxysomes are composed of thousands of protein subunits and two different compounds used for carbon metabolism some prokaryotic cells also possess carboxysomes that sequester functionally related enzymes into one location these structures are considered proto-organelles because they compartmentalize important compounds or chemical reactions a lot like a lot of eukaryotic organisms so these pictures up here are just examples of of a few types of these inclusions so inclusions within cells um a light macrograph of volatile granules so red red volatile granules um a phase contrast micrograph of sulfur granules so here's our sulfur sulfur okay a transmission electron micrograph of magnetosomes these are um magnetosomes are the green and then a transmission electromicrograph of gas vacuoles so here's the gas vacuoles it's cool okay so bacterial cells are generally observed as vegetative cells but some genre of bacteria have the ability to form endospores endospores are structures that protect the bacterial genome in a dormant state when environmental conditions are unfavorable endospores allow some bacteria to survive long periods without food or water as well as exposure to chemicals extreme temperatures and even radiation so here's some characteristics of vegetative cells versus endospores so um like if you're if you have a gram-positive cell then an endospore that this is just a stain that we use in the lab to stain the cell wall kind of the cell membrane of um gram-positive cells okay which means they have a really thick cell membrane so these will stain endospores do not absorb the gram stain they have to be stained with a specific type of endospore stain so this is an endospore stain and the endospores will stain green and the vegetative cells will stay in pink we we also do this in the lab and you will have a lab simulation over this over both the endospore stain and the gram stain um so these are just kind of some characteristics to tell you the difference between a cell and an endospore cells are sensitive to extreme temperatures endospores are going to be resistant cells have normal water content and activity enzymatic activity whereas endospores don't have water content and there's no metabolic activity they're basically just sitting there waiting for the cell to get put into an environment where it's more favorable where they can start to the cell can become a more active cell again and start to multiply vegetative cells are capable of active growth whereas if you're in an endospore they're dormant and there's no growth the process by which vegetative cells transform into endospores is called sporulation and it generally begins when nutrients become depleted or environmental conditions become unfavorable the process begins with the formation of a septum in the vegetative bacterial cell the septum will divide the cell asymmetrically and it will separate the dna for spore from the mother cell so the four spore which will form the core of the endospore is just essentially a copy of the cell's chromosomes and is separated from the mother cell by a second membrane then a cortex will form around the forespore and it will lay down layers of calcium and diplicalenic acid between membranes a protein spore for spore coat will then form around the cortex while the dna in the of the mother cell disintegrates and further maturation of the endospore occurs with the formation of the outermost exosporium the endospore will then be a release and it that's the completion of sporulation endospores of certain species have been shown to persist in a dormant state for extended periods of time up to 50 to 60 years however when living conditions improve endospores undergo germination which is the re-entering of a vegetative state after germination the cell becomes metabolically active again and it's able to carry out all of its normal functions including growth and cell division so we want to be in a vegetative state because that causes us to be metabolically active so if we're not in that we we enter that dormant state and we go and we create an endospore and we you know basically just sit there and wait but not all bacteria have the ability to form into spores there are a number of clinically significant endospore forming gram-positive bacteria to the primarily in the the genre of bacillus and clostridium so those are bacillus anthracis um this is the causative agent of anthrax these can produce endospores that are capable of surviving for many decades and we have a lecture on this um clostridium tetani causes tetanus clostridium difficile causes c diff and clostridium botulinum causes botulism pathogens like these are particularly difficult to treat because their endospores are so hard to kill because that's the whole purpose of an end spore right is to protect us in an unfavorable environment okay structures that enclose the cytoplasm and internal structures or the of the cell are known collectively as the cell envelope in prokaryotic cells the structures of the cell envelope vary depending on the type of cell and organism so we already know that all cells prokaryotic and eukaryotic have a plasma membrane also called the cytoplasmic membrane or cell membrane that exhibits selective permeability so that means that it allows some molecules to enter and leave the cell while it restricts the passage of others the structure of the plasma membrane is often described in terms of the fluid mosaic model which refers to the ability of the membrane components to move fluidly within the plane of the membrane so the plasma membrane structure of most bacterial and eukaryotic cell types is a bilayer composed mainly of phospholipids bilayer means two layers so we have a layer of phospholipids here and a layer here so by layer okay think of it like as a sheet of paper whereas the top of the paper is one layer and the bottom of the paper is another layer these phospholipids and the proteins there's different proteins that are embedded in them have the ability to move laterally within the plane of the membrane as well as between the two layers so these fossil lipids can move along this membrane they could also switch and flip from the top membrane to the bottom membrane and it's all fluid the proteins can move all of it just can move along the membrane so these proteins that are on the membrane are important for a bunch of different reasons but they um they are important for cell to cell communication they can also sense environmental conditions and pathogenic virulence factors which we're going to talk about once we get into diseases but basically that just means specific things to bacteria or viruses that that are important for the bacteria to cause disease membrane proteins and phospholipids may have carbohydrates associated with them and those are called glycoproteins or glycolipids okay so these are glycoproteins glycolipids these are membrane proteins or phosphopro proteins that have carbohydrates on them these glycoprotein and glycolipid complexes will extend out from the surface of the cell and it allows the cell to interact with the external environment so here's our protein and here's our carbohydrate okay so here's our gly it's a glycoprotein okay these are just a normal this is just a normal protein normal protein but this is going to be a this has a carbohydrate on it so this will be a glycoprotein this could be a glycolipid so um glycoproteins and glycolipids in the plasma membrane can vary pretty significantly in chemical composition among different archaea bacteria and eukaryotes so that allows scientists to use them to characterize unique species plasma membranes from different cell types also contain unique phospholipids which contain fatty acids that can be used to identify unique types of cells based on their differences in fatty acids and some prokaryotic cells have membrane structures that enable them to perform photosynthesis which is the process by which green plants and some other organisms use sunlight to synthesize food from carbon dioxide and water these structures consist of an enfolding of the plasma membrane that enclose the photosynthetic pigments and then in cyanobacteria these membrane structures are called filcolloids and photosynthetic bacteria they're called chromatophores or chlorosomes