if you find yourself looking at a diagram like this one of an animal cell and thinking to yourself how am I going to master all of this material if you're feeling overwhelmed and nervous then this video is for you here's what we're going to cover we're going to start with an introduction to cells then we'll look at why cells are small we'll follow that by looking at how cells are internally organized that's about cell compartmentalization and a system of organel that are organized into an end membrane system after we do that we'll be able to walk through cell parts and functions then we'll look at membranes how membranes are made up how membranes transport things from one side to the other and finally we'll look at osmosis my name is Glenn woken Feld also known as Mr W I'm a retired AP biology teacher I love Bol L ogy to help you study I've put together a checklist that you can download at AP Bio sucess / checklist cells and introduction describe the basic structure and fundamental parts of cells cells are the basic unit of life they're the basic unit of structure and function in every organism in terms of their own structure they have a membrane which separates the cytoplasm all the living material inside the cell from the cell exterior they always have genetic information in the form of DNA the double helix and there are systems that maintain and replicate the cell as a whole and the cell's genetic information its DNA one of the most important systems is a system by which the genes in DNA get transcribed into messenger RNA another nucleic acid and that messenger RNA is read by a particle called a ribosome and translated into proteins proteins are all important they're the Workhorse of the cell and one of the most important proteins are enzymes enzymes control the cell's metabolism other proteins might be embedded in the cell's membrane and others might even be exported from the cell compare and contrast procaryotic and eukaryotic cells among living organisms There's a Great Divide between the procaryotes and the ukar here's what it's all about procaryotic cells are small and relatively simple they have no nucleus their chromosome is circular and what that means is that there's a loop the beginning and the end meet up they contain extra chromosomal pieces of DNA that are called plasmids and they're found in two of life's three domains the ARA and the bacteria UK carotic cells are larger and more complex they're found in the domain that has their name the ukaria they have a nucleus they have multiple linear chromosomes so here they're shown over over here and you can see the chromosomes here they have a beginning and an end and those chromosomes have DNA that's associated with proteins which is different from the simpler DNA packaging that is found in the procaryotes ukar have mitochondria and the presence of that organel is the defining feature of the ukar and they have many membranebound organel membrane bound means surrounded by a membrane AP biotopic 2.3 cell size an essential topic use the relationship between surface area and volume to explain why cells are small cells need to have enough membrane surface area to allow for diffusion of substances and molecules in and out everything that gets into a cell gets there through the membrane and it might be counterintuitive but a small cell has much more surface area relative to its volume than a large cell does let's look at the math if you have a cell that's one micrometer one micron on the side that's its length then its surface area if it's cubic would be 1 * 1 * 6 think of the formula for surface area for a cube its volume is 1 * 1 * 1 and therefore its surface area to volume ratio is 6 to 1 six units of surface area to every one unit of volume now if you have a larger Cube one that's 10 microm then its surface area is 600 wow that seems like a lot but its volume is 1,000 because volume is a cubic function and it's going up much faster so if you look at the surface area volume ratio it's 6 to one as an object gets larger its amount of surface area relative to its volume decreases and in this example the larger cell surface area to volume ratio is 1 tenth that of the smaller cell and a large cell can't efficiently use diffusion to get the nutrients it needs in and to get waste carbon dioxide and metabolic waste out why do organisms need to increase surface area in certain tissues how do they do that the why is in order to increase the surface for diffusion of molecules or heat how you can have thin sheets of tissue like these thin sheets that make up the gills of a fish that have been dissected out that's for increasing the intake of oxygen and the diffusion out of carbon dioxide or if you think about the ears of elephants those are big flat sheets of tissue that enable the elephant which has very little surface area relative to its volume and lives in a hot place to get heat out of its body you can also have highly folded surfaces like the internal membrane of the mitochondria or the structures that are called Villi that make up your intestine and there's actually folds within folds explain in terms of surface area why there are no small mouse-sized marine mammals and why some marine mammals like the whales the largest organisms to have ever evolved are really huge mammals are warm-blooded ocean water is cold increased size decreases the surface area to volume ratio so there's less heat lost to the environment think about a mouse it's small it has a lot of surface area relative to its volume it would be over here on the graph it couldn't survive because it would experience so much heat stress in the water the smallest marine mammals are the otter and they have a thick coat of fur that helps them maintain their body temperature whales have evolved to be larger and larger for a variety of reasons but one of which is that by being big they lose relatively less heat to the environment than a smaller animal would it's all about the surface area to volume ratio cellular compartmentalization and the endomembrane system how eukaryotic cells are organized what is cell compartmentalization what are its advantages compartmentalization means internal division of a space into sections think about the overhead luggage compartment in an airplane what are the advantages the first is that it allows a cell to have regions with an internal chemistry that are distinct from the cytoplasm a great example of that is the lome shown at G over here and number seven over here lomes contain hydrolytic enzymes those are enzymes that break things down those can't be released into the cytoplasm where they would break down the cytoplasm so they are compartmentalized they're kept in a membranebound space in addition compartmentalization provides a lot of internal surface area and that's important for membranebound enzymes that are found in the Smoothie r or the GGI that can only do their job when they're embedded in a membrane compartmentalization makes that possible and that's also true of the ribosome that are found in the rough ER compare compartmentalization in procaryotic and eukaryotic cells procaryotic cells have few compartments though there are internal regions with specialized structures and functions here's a procaryotic cell you can see that its internal structure is much simpler than that of a eukaryotic cell but one notable compartment is called aoid and it's this over here in a cyanobacterium cyanobacterium evolved into chloroplast we'll talk talk about that in a moment and they have these internal compartments called thids that are essential for the function of photosynthesis UK carotic cells are highly compartmentalized there are many internal membranes that divide the cell into regions with distinct structures distinct chemistry and functions and examples include lomes the endoplasmic reticulum the GOI complex and vacu like this large vacu in a plant cell what is the endomembrane system it's a dynamic connected system of internal membranes and compartments it includes the nuclear membrane the Ruff the Smoothie R the GGI lomes and all the little vesicles that interconnect them membrane and material flow from one compartment to the next so the membrane that's part of the ruar might become the same membrane the same phospholipids that are part of the smooth are and those might become parts of vesicles which flow into the GGI so those phospholipid molecules are the same it's Dynamic there's constant exchange Explain the origin of mitochondria cellular compartmentalization and chloroplast what we're explaining is the origin of the UK carots when did this happen about 1.8 billion years ago before that there was nothing on Earth but procaryotes how did this happen it Rose as a mutualistic endosymbiosis symbiosis means living together two species living closely associated with one another endosymbiosis is when one of those species lives inside the other we normally think of that as parasitism that's a negative kind of endosymbiosis but there's also positive endosymbiosis that's a mutualistic relationship and that is what gave rise to the ukar so what happened is that that here we have representatives of the two procaryotic domains and an AR cell took up a bacterial cell now I want to emphasize that there are many scenarios and many diagrams that represent how this happens and this one is the one that makes most sense to me so the bacterial cell entered inside the arch cell and it evolved into a mitochondrian that mitochondrian still kind of a bacteria secreted vesicles and those vesicles became the nuclear membrane shown over here and it started to develop structures that are part of the endomembrane system like the endoplasmic reticulum which is shown over there so that's how we have ukar then later a second endosymbiotic event led to chloroplasts which were free living CA bacteria entering into into a eukaryotic cell and that set the stage for algae and plants what's the evidence that chloroplast and mondria were once Free Living bacterial cells that arose through endosymbiosis what's the evidence for this idea like bacteria both of these organel mitochondria and chloroplast have their own circular DNA they replicate themselves through binary fision that's how procaryotes replicate themselves they use their own ribosomes to produce some of their own proteins these ribosomes resemble bacterial ribosomes in terms of their R RNA sequence and structure and they have two membranes the outer one so the outer bacterial membrane let's consider it over here is a vestage of an endocytotic vesicle at learn biology.com we understand why students struggle with AP Bio it's a hard course the material is complex the vocabulary is ridiculous and the pace is withering it's natural to feel overwhelmed and inadequate to get an A or a four or a five you need an easier way to study and that's why we created learn biology.com it has quizzes it has flashcards it has interactive tutorials about every topic in the AP Bio curriculum it has a comprehensive AP Bio exam review system use learn biology.com and you'll gain the skills and confidence Ence that you'll need to Ace your biology course and to crush it on the AP Bio exam so here's your plan go to learnbiology domcom we've got free trials from June through March for both teachers and students you won't believe how much you'll learn you carry out Excel parts and functions describe the structure and function of the nucleus the nucleuses function is to store and protect genetic information in the form of DNA that DNA wrapped around proteins to form chromosomes this representation here is only present during mitosis or meiosis when cells are dividing otherwise the DNA is more diffuse and spread out it's still in chromosomes but we call the form chromatin the nucleolus is this dark part in the center of the nucleus and it assembles ribosomes the entire nucleus is surrounded by a membrane that separates the chromosomes from the cytoplasm and the nuclear membrane has pores that allow molecules to enter and leave the nucleus important among those are messenger RNA which leaves the nucleus goes into the cytoplasm where it's transcribed into proteins and various transcription factors that come into the nucleus to turn genes on or to turn them off describe the structure and function of ribosomes ribosomes are particles composed of ribosomal RNA and protein they consist of a large subunit and a small subunit their function is to read a genetic message encoded in a sequence of messenger RNA and to translate that message into a sequence of amino acids that make up the primary structure of a protein the details of translation protein synthesis will occur in unit 6 what are two locations within a eukariotic cell where ribosomes can be found ribosomes can be free or Bound free ribosomes shown at three float freely within the cytoplasm which is number five bound ribosomes shown at four are connected to the membrane of the rough erer all ribosomes start out as free though through a process called protein targeting they migrate to the ER to become bound they do that if the proteins that the ribosomes are creating are destined to be put inside a vesicle so they can go to the GGI or the membrane or a lome describe the structure and function of mitochondria mitochondria are shown here in the context of an animal cell here's a larger image the function is converting food energy into ATP that's the molecule that cells use to get work done the key structures there's a chromosome that consists of DNA there are also ribosomes which aren't shown in this diagram evidence of endosymbiosis there's an inner membrane it's highly folded that increases surface area area and that's because there are many membrane embedded enzymes and proteins one of which is shown at five that are involved in the process of ATP synthesis the mitochondria as a former independent cell has its own cytoplasm it's called The Matrix it has enzymes for the kreb cycle and enzymes that perform other functions there's an intermembrane space which is an important cellular compartment that enables the mitochondrian to produce at P we'll see how that works in unit 3 and there's an outer membrane that is a vestage of endosymbiosis describe the endoplasmic reticulum the ER and list its two forms the ER is an interconnected series of channels found between the nuclear membrane over here and the GG body in eukariotic cell so here's the ER and it consists of two forms there's the roughy R and the Smoothie R describe the structure and function of the the rough and smooth ER well the rough ER is studded with ribosomes that's what all these little dots are over here and using those ribosomes the ruar synthesizes proteins for inclusion in lomes in other organel in the membrane or for export from the cell the smooth ER is on the outer side of the ER Network always towards the G G and it lacks ribosomes but that doesn't mean that there's nothing in that membrane it has many membrane embedded enzymes and the functions of the Smoothie are vary by tissue so it can include synthesis of lipids converting toxins into soluble forms that can be excreted from the body carbohydrate breakdown and synthesis describe the structure and function of the GGI complex the GG is a series of membranebound flattened sacks and you'll remember that flattening increases surface area the Gogi receives vesicles From the Rough and smooth AR and chemically modified their contents so here's the rough ER here's the smooth ER here's the GGI and here's a vesicle bringing something from the ER system to the GGI the GGI packages those modified proteins into vesicles shown at six that are sent to organel like lomes over here at s to the cell membrane or exported from the cell describe the structure and function of lomes here's a lome over here here's a lome over here and one thing that I rarely say but I'll let you know that you will almost certainly never have to identify aosome on a diagram on the AP Bio exam it's just a big vesicle lomes are membranebound organel that contain hydrolytic enzymes they're only found in animal cells they carry out intracellular digestion so like for example here you see a particle it actually looks like a cell that's being engulfed by the membrane it'll be packaged into a vesicle and that vesicle will be sent to a lome where hydrolytic enzymes will break this down the lome also recycles worn out damaged or excess organel and molecules and it plays a key role in a process called apotosis the PS silent that's programmed cell death and we'll talk about that later in the course the cytoskeleton is a dynamic network of protein fibers and those fibers make up the bones the skeleton of the cell in the same way as our skeleton our bones working with muscles enables us to move the cytoskeleton is a dynamic Network that enables the cell to move both internally so it enables cells to move materials and organel within themselves and it also enables the cell to move their membrane so that they can do things like endocytosis where cells can engulf things by moving their membrane or also moving their entire bodies this is an organism called an amiba and it moves by extending um parts of the membrane over the surface enabling it to crawl how does it do that through the cytoskeleton what are centrosomes and centrioles centrosomes are the organel they contain two centrioles and the function is creating spindle fibers that's what these are over here that separate chromosomes during mitosis and meiosis the parts of we've talked about so far have all been represented within animal cells but most of those parts are also found in plant cells the one exception is the lome unique to animal cells let's describe the function of the central vacu this large structure right here it's only in plant cells its functions include water storage storing and releasing macromolecules sequestering waste products and maintaining turer pressure we'll talk about this in the context of Osmosis the movement of water but as water flows into a cell it creates outward pressure that keeps plant cells full firm upright they're not wilting describe the evolutionary origin in function of chloroplasts here a chloroplast over here here's a single chloroplast chloroplasts are the endosymbiotic descendants of Freel living photosynthetic bacteria and that's why they have their own DNA their own ribosome that's why they have two membranes their function is creating carbohydrates through photosynthesis we're going to leave their detailed structure and how they work for unit three when we discuss photosynthesis describe the chemical composition and the function of the plant cell wall the cell wall is composed primarily of cellulose which is a polysaccharide it's that polysaccharide that we can't digest but which ruminis cows goats can digest because they have these symbiotic relationships that enable them to break that down and release food energy the major function of the cell wall is it acts as a pressure vessel that prevents overexpansion in response to inward flow of water osmotic pressure so here we see as water flows into a plant cell the central vacu will expand and it'll look like this the cell will be full of water and that water will push against the cell wall keeping the plant cell full and firm avoiding wilting so that's a good thing for the plant and the cell wall is the primary component of wood and the water conducting tubes in plant stems membrane structure and function what is the function of the cell membrane the cell membrane separates the highly organized contents of the cell from the cell's environment cells are open systems they need to let things like food in they need to let things like waste out the membrane is how that's controlled that's because the membrane is a selectively permeable boundary it allows the passage of some substances and particles through but not others that selectivity is really the key to life describe the structure of phospholipids and their role in cell membranes phospholipid structure there's a hydrophobic non-polar tail region here's the structural formula here's a representation there's a hydrophilic polar head notice that the head has this phosphate group over here with its negative charge the whole thing is held together by this three carbon molecule called glycerol in solution when phospholipids are mixed with water the heads of the phospholipids bond with water molecules while the Tails form a waterfree zone so interaction interacting with water interacting with water waterfree Zone over here that creates a Bayer that's the basic framework of the membrane to which proteins and other molecules are added we'll see that further on and the phospholipid bilayer is stabilized by weak bonds Vander wal's bonds that occur between the Tails describe the fluid mosaic model of cell membranes this model posits that the membrane is composed of phospholipids plus proteins plus cholesterol in motion so why is it called fluid MO well it's fluid because the components are moving around laterally within the plane of the phospholipid Bayer which you can see here it's a mosaic because it's composed of a variety of pieces in the same way as a mosaic tile is composed of different tiles with different colors that get put together to form the art within the phospholipid bilayer you have phospholipids that are at four and N you have proteins that are at 1 3 3 five and six and you have molecules like cholesterol which are shown at seven and then inside and outside the phospholipid bilayer you might have additional molecules attached to proteins or phospholipids and those might be glycoproteins which are not shown but at eight you have a glycol lipid glyco means it's a carbohydrate and it's attached to these fatty acids right over here describe how proteins fit into the cell membrane there are three ways that proteins can fit in and if you think about hydrophobic hydrophilic what binds with what it makes a lot of sense so there are transmembrane proteins that's shown over here and over here here notice that beautiful Alpha Helix that you should remember from our unit one review there's a hydrophobic core that fits into the non-polar inner portion of the membrane so these would be hydrophobic amino acids over here and over here and there'd be hydrop philic regions that extend into the cytoplasm and the membrane exterior there are integral proteins such as the one at five and these proteins would have a non-polar region that extends into the hydrophobic membrane middle and a single hydrophilic region that juts into the cytoplasm as this one does or into the cell exterior then finally you have peripheral proteins like this one that's shown at three and they attach to phospholipid heads that are either on the cytoplasmic side of the membrane or in the cell exterior these terms transmembrane integral peripheral they're not important in and of themselves but you should be able to understand the concept of how things fit into the membrane membrane transport how things get across the cell membrane diffusion is the tendency of molecules to spread out from where they're more concentrated to where they're less concentrated so here you have some substance more concentrated over here less concentrated over here here's ink going into a beaker more concentrated and then less concentrated this happens spontaneously it's based on the kinetic energy of the molecules and a good way to Envision this is to think of molecules as flowing down a concentration gradient from higher concentration to lower concentration explain how cells control what diffuses across their membrane diffusion is called passive transport and that's because it requires no cellular energy there are two forms the first is simple diffusion across the phospholipid Bayer that's what's shown up here there are two things that can do simple diffusion the first are any small nonpolar molecules such as oxygen nitrogen carbon dioxide the second are non-polar substances such as steroid hormones or fats facilitated diffusion is for polar molecules and ions that can't diffuse across the phospholipid bilayer so they require protein channels and these are transmembrane proteins they go all the way through that only let specific molecules or ions pass so over here you see one for glucose over here you see one for amuno acids those are both polar molecules and here you see one for an ion such as calcium they're be others for ions such as sodium or pottassium compare and contrast active and passive transport as you do explain what powers each process passive transport is what we just discussed it's when molecules or ions flow down a diffusion gradient here more concentrated to less concentrated and that's what it's all about it's diffusion from high concentration to low concentration it relies on the kinetic energy of the diffusing molecules or ions it doesn't require cell energy as you can see here molecules are flowing down their concentration gradient that's different from active transport where we're pumping a molecular ion up its concentration gradient from lower concentration to higher concentration over here lower concentration to higher concentration that requires an energy expenditure on the part of the cell usually that's ATP that's being converted into ADP and phosphate to power the pumping process but also the flow of electrons can power the pumping of protons and that's important in the creation of ATP itself compare and contrast endocytosis and exocytosis these are both forms of bulk transport in endocytosis the membrane pinches in to surround a particle or some extracellular fluid creating a cavity that can become a vesicle in exocytosis a vesicle containing something fuses with the membrane and then that contents is dumped outside the cell both of these processes require energy on the part of the cell and they both require the involvement of the cytoskeleton which is changing the shape of the membrane what is membrane potential how do cells create membrane potential how is membrane potential used membrane potential is an electrical charge across a membrane that creates a voltage difference how is it created through cells expending energy to pump ions across their membranes a key example and this is a preview of a topic that we'll get into in unit three is how mitochondria pump protons from the mitochondrial Matrix to the intermembrane space that gives the intermembrane space a positive charge relative to The Matrix it has all these prot phons inside of it and that creates an electrochemical gradient of voltage difference and that pulls protons through this channel it's called ATP synthes back into the Matrix and that powers the creation of ATP some other examples chloroplasts do pretty much exactly the same thing and there's also a 70 molt charge across the membranes of nerve cells that makes nerve puls as possible and that's how I am able to think and teach biology and how you are able to learn it is biology mind-blowing or what I want to acknowledge how difficult and complex some of these Concepts can be and I want to encourage you to go to learn dm.com and with a free trial you can do the tutorials and you can use our unit reviews and it's going to really help you to get on top of this material setting you up for Success on your unit test or the AP Bio exam time icity and osmo regulation that's the college board's term I wish that was just called osmosis and its consequences so let's start by defining what osmosis is it's the diffusion of water water diffuses like everything else and that means that water is going to move from higher to lower concentration water is the solvent of life so we have to pay attention to how it moves water flows down its diffusion gradient just like everything else from hypotonic to hypertonic those are essential terms to know hypotonic means relatively more water and relatively less salute and these terms are not absolute they're always relative to another solution so in this diagram this side is hypotonic it has a higher percentage of water molecules it has a lower percentage of solute molecules this side is hyperonic that means that it has relatively less water a lower percentage of water molecules and a higher percentage of solute molecules but that's this compared to this now what is that going to do that's going to have water move down its concentration gradient which means it'll move from here to here after osmosis takes place it'll look like this over here this level will go down this level will go up the water actually is getting pushed up on this side that's called asmatic pressure it has enormous consequence and the thing for you to memorize is that water diffuses from hypotonic to hypertonic here's a fabulous and fun example a student places a gummy bear in a cup of water overnight the next day the gummy bear has expanded explain well the gummy bear is mostly sugar the water in the cup to is 100% water water always flows from hypotonic to hypertonic so water's going to flow from the hypotonic solution in the cup into the gummy bear the gummy bear will expand because of osmotic pressure osmosis implants use the principles of Osmosis to explain each of the images Below on the left side the cell is hypotonic to the environment water leaves the cell the membrane peels away from the wall that's also known as plasmolysis the vacu shrinks and the plant wilts how could the environment be hypertonic to the cell how could the cell be hypotonic to its environment this environment has to have for example a lot of solutes dissolved in it so that it has a lower water concentration than is in the cytoplasm that'll cause water to go from the hypotonic cell to the hypertonic environment in the middle the cell is isotonic to its environment that's a new term isotonic means that the solute concentration and the water concentration are the same on both sides of the membrane so water enters and leaves leaves and enters at the same rate and on the right side the cell is hypertonic to its environment so water flows into the cell that osmotic pressure in the case of plants is called turer pressure it causes the vacu to expand it pushes the membrane against the cell wall and this is a good situation for plants to be in it makes them full firm happy they're not wilted osmosis and animal cells use the principles of Osmosis to explain each of the images below cell number one the cell is hypotonic to its environment water leaves the cell water always flows from hypotonic to hypertonic so the cell shrivels up in Situation Number Two the cell is isotonic to its environment water enters and leaves the Cell at the same rate this is an important condition for animal cells because they have a membrane but not a wall cells that are kept in a tissue culture need to be kept in an isotonic solution and in situation three the cell is hypertonic to its environment and therefore water is flowing into the cells and because there 's no cell wall the membrane can't really stop the inward force and the cell will ultimately burst explain the function of the contractile vacu in freshwater protest such as paramia a protest is a ukar that's not a plant an animal or a fungus protests in freshwat are hypertonic to their freshwater environment that means that the cytoplasm has more solutes dissolved in it than the lake or stream that they're swimming in and as a result water moves into these cells by osmosis flowing from hypotonic to hypertonic to osmo regulate that's a new term osmo regulate means to regulate your osmotic balance the contractile vacu that's this organel over here fills with water and then contracts to expel water from the cell if the environment becomes more hypertonic then the cell can ad apt by decreasing its rate of contractile vacu contraction and do the reverse in more hypotonic environments describe the structure of leaf stomata stomata are pores that are on the underside of a leaf if you took a leaf and you did microscopic cross-section you'd see layers of cells the top layer and the bottom layer are responsible for waterproofing the leaf and they're covered with wax but nevertheless the leaf needs to be open to its environment in order to exchange gases and other material um each stom which is singular stomata is plural is formed by two guard cells so here's one guard cell here's the other guard cell and when there's sufficient water these guard cells Buckle outward like that and that creates a pore that allows carbon dioxide to enter the leaf for photosynthesis but also allows water vapor to escape these stomata can be regulated they can close in response to environmental cues including water stress and then open when water is abundant explain how guard cells are regulated in order to open and close the stomata when water is available cells adjacent to the stomata pump pottassium ions into the guard cells so we're talking about these cells and these cells the ones that are adjacent to the guard cell and they're represented like this over here the pumping of potassium makes these gar cells hypertonic to the adjacent cells water follows by osmosis causing the guard cells to buckle and open up when water is scarce the pumping stops pottassium ions flow out of the guard cells and water follows by osmosis causing the stomata to close down to revert back to this form what is water potential water potential is a concept that it's really closely related to osmosis and osmotic pressure but it's more quantitative the symbol for water potential is this Greek letter sigh so water potential is a measurement of water's tendency to move from where it is to where it's not adding solute decreases a body of water's water potential adding pressure increases water potential and water will flow from areas of higher water potential to areas of lower potential so if you look at this device over here it's called a Ube and um these two parts are separated by a membrane that's permeable to water but not to the solute so here we are we're adding solute to this side of the Ube that makes this side hypotonic but that's another way that you can say it has higher water potential and water's going to move from higher water potential to lower water potential potential that's going to force up the level of water over here that's going to lower it over here could you have explained this entire thing with just hypotonic and hypertonic well I just did but you'll see that water potential is more quantitative and it allows for other factors to be included such as pressure explain the formula for water potential here it is over here in plain English it's water potential equals solute potential plus pressure potential so s is water potential S Subs is solute potential and remember that adding solute to water decreases its water potential s p is pressure potential and adding pressure like if you pressed on a syringe that increases water potential so let's look at these two examples over here so we've added salute to this side of the Ube pressure potential is zero there's nothing pressing down on either side but adding solute low lowered the solute potential over here to 0.23 megap pascals and that's the unit that's used for pressure so the total water potential over here is minus 0.23 megapascals the water potential over here is just zero there's no pressure there's no solute so water moves from higher water potential to lower water potential that's why it's moving from this side of the Ube to this side in this example we have a potato cell or potato tissue that's been placed in water and so again there's no pressure on either side it's just an open container but the cell has ative 1.0 megap Pascal solute potential so the total water potential is 1.0 this is just water so the water potential is zero over here water moves from the be into the cell which has a lower water potential that's causing water to flow into the cell it's causing the cell to expand all of these things related to the cell wall in a typical lab in AP biology you put potato tissue into water or Solutions of various concentration and you can see what happens and you can actually quantify it based on water potential that's what water potential is used for you are a hero for reviewing this much of unque unit 2 of AP biology that is phenomenal now what I want you to do is continue your hero's journey by going up to learn biology.com and doing the unit 2 revieww that I've set up for you it's got flashcards quizzes multiple choice F frqs and tap challenges you will learn so much you'll feel so confident for your next test or for the AP Bio exam unit 2 is a unit for which I have a lot of great music and I want to encourage you to check out the cell song My membrane song and my osmosis song and now head on over to the next video in this series where we'll review all of unit 3 see you