[Music] do you know the difference between semi-permeability and selective permeability these extremely important concepts describe how the lipid bilayer indiscriminately rejects certain molecules while a living membrane incorporates channel proteins and transporters that actively select which substances can enter and leave a cell with these two forms of permeability cells can protect their dna create the perfect internal conditions to carry out reactions and balance their water content these topics will definitely be on the ap test so follow along as we go over everything you need to know about the basics of membrane permeability this video focuses on section 2.5 of the ap biology curriculum first we'll take a look at what permeability is and define the difference between a semipermeable membrane and a selectively permeable membrane then we'll see how cells use selectively permeable membranes to create a specific environment within the cell to further explore how this is possible we'll see what the fluid mosaic model of cell membranes has to say about permeability after the quiz we'll look at specific molecules and how membranes sort them finally we'll take a look at cell walls and the role that these extracellular structures play in permeability if you only need to review one of these topics feel free to skip forward to the times outlined here let's get started permeability describes how easily a molecule or substance can pass through a membrane you can think of permeability in terms of a coffee filter a coffee filter allows hot water and dissolved coffee particles to pass through the filter while the larger coffee grounds are blocked a lipid bilayer is very similar to a coffee filter the lipid bilayer is made up of phospholipid molecules each of which has a hydrophilic head that is attracted to water and a hydrophobic tail that is attracted to other nonpolar molecules the heads group together and the tails group together to create the lipid bilayer on either side of the membrane is a layer of hydrophilic heads while the middle of the lipid bilayer is a very hydrophobic core while a coffee filter is simply a piece of paper with tiny holes the lipid bilayer can block molecules or let molecules through based on their chemical nature charged molecules ions and large molecules are typically blocked by the lipid bilayer however some nonpolar molecules and gases like oxygen and carbon dioxide can pass right through the lipid bilayer many things affect the permeability of a lipid bilayer starting with the type of phospholipids used phospholipids created with saturated tails tend to squeeze tightly together making a less permeable bilayer phospholipids created with unsaturated fatty acid tails tend to be more fluid making the membrane more permeable to all sorts of substances other factors like temperature and ph also affect the permeability of a plasma membrane for example if we increase the temperature of the water surrounding the cell this will push the phospholipids further apart and make the membrane more permeable this is why individual organisms use different ratios of saturated unsaturated fatty acids within their lipid bilayer membranes to keep their membranes permeable at different temperatures now let's take a look at the difference between a semipermeable membrane versus a selectively permeable membrane by itself a lipid bilayer is a semi-permeable membrane a coffee filter is also a semi-permeable membrane it will always block certain substances and it will always let others through simply based on the membrane's chemical and physical properties substances may also be blocked to partial degrees either mostly allowing a substance to cross the membrane or mostly prohibiting a substance from crossing the membrane but a living cell membrane is made up of much more than just phospholipid molecules most importantly the cell membrane has a number of embedded protein channels and carrier proteins that select specific molecules to let through since these proteins are made from the dna code and embedded into the cell membrane through internal cellular mechanisms it is as if the cell itself is selecting which substances cross the membrane and which substances are prohibited from crossing to sum up lipid bilayers are semi-permeable but cellular membranes are selectively permeable think about this what exactly is the defining characteristic of life on earth some scientists think that life is defined by the ability to control your internal and external cellular environments all organisms on earth from these microscopic nematodes to the enormous blue whale selectively modify the permeability of their cell membrane by embedding it with specific proteins through this ability cells are able to modify their internal and external environments the only real difference between organisms is the various dna codes that produce different membrane proteins and enzymes while these nematodes create proteins that allow them to survive and reproduce a blue whale is essentially doing the exact same thing so everything up to this point has shown how cell membranes are selectively permeable membranes now let's take a look at why cells need selectively permeable membranes the ultimate answer is that cells need to create livable cellular conditions to understand this better let's look at the concepts of diffusion where particles evenly distribute in a solvent and osmosis where water moves through a semipermeable membrane if you were to drop some sugar molecules into a body of water those sugar molecules will eventually become evenly distributed throughout the water imagine an algal cell that is creating sugar the alkal cell creates glucose in the chloroplast and exports that glucose to the cytoplasm here enzymes can process the sugar and mitochondria can process smaller pieces of the sugar to store energy as atp the mitochondria then export this chemical energy to power processes throughout the cell now imagine what would happen without a cellular membrane first of all there is no way to hold all these organelles together so they would just drift apart plus any glucose created by the chloroplast would simply diffuse away into the surrounding environment cell membranes therefore essentially give cells the ability to congregate substances they need to survive now let's consider osmosis water molecules are attracted to polar solutes in the same way that polar solutes are attracted to water so if you divide a body of water with a semi-permeable membrane that blocks large solute particles water itself will move through the membrane to make the concentration on both sides of the membrane equal cells need selectively permeable membranes due to osmosis not only do cells need to hold solutes inside but they need to control their water balance ph and the specific concentration of solutes within their cytosol to do so cells load their cell membranes with proteins responsible for importing or exporting the proper substance plus some cells have a contractile vacuole that collects water from the cell and expels the water into the surrounding environment in summary cells use many different proteins within their cell membranes to create a selectively permeable membrane and control their internal environments selective permeability is a direct consequence of the semipermeable membrane and the proteins that are embedded into that membrane also known as the fluid mosaic model there are several components within cell membrane that influence the permeability of a cell the basic semi-permeability of the cell membrane is derived from phospholipid molecules phospholipid molecules stick together through hydrogen bonds and non-polar interactions to form the lipid bilayer which naturally repels molecules of a certain size and chemical composition the membrane becomes selectively permeable thanks to the work of proteins proteins come in many different types and have different functions to preserve the internal cellular environment some proteins allow water to move freely others are like giant powered gates that can select when ions move through the membrane some proteins import and export larger molecules to and from the cell we will cover all of these membrane proteins further in sections 2.6 through 2.9 one final component of the cell membrane that drastically affects permeability is cholesterol cholesterol is a small lipid steroid molecule that embeds directly into the plasma membrane lipid molecules congregate around cholesterol molecules pulling the phospholipids together and creating a less permeable membrane together all of these elements allow the cell to control exactly what enters and exits the cell even in vastly different environmental conditions now that we have covered what permeability is the difference between semi-permeable and selectively permeable membranes and why cells need cell membranes let's see if you are following along pause the video and answer these questions you can find the answers to all of the questions in this video through the quick test prep link in this video's description now that we understand the basics of how selective permeability is created let's see what types of molecules cells allow and reject let's start with small nonpolar molecules molecules like oxygen can easily pass through the cell membrane this is important since most eukaryotic cells need oxygen to complete cellular respiration and store energy in atp likewise small nonpolar molecules of carbon dioxide can easily diffuse through the cell membrane this is important for the process of respiration allowing oxygen to be delivered to the cells from the blood and carbon dioxide to be exported to the bloodstream and out of the lungs many small uncharged polar molecules like water are only blocked to a slight degree these molecules are polar so it is not as easy for them to work through the hydrophobic core of the lipid bilayer the last two groups large polar molecules and ions have a very hard time making it through the lipid bilayer larger molecules are mostly blocked by their size and polarity though they can sometimes slip through the lipid bilayer by contrast ions can be very small but are almost entirely repelled because of their charged nature however cells need still need to move these substances around that's where membrane proteins come into play for instance glucose molecules can be imported to cells via specialized glucose carrier proteins these proteins have an active site that is specific to glucose so only glucose molecules can be imported similarly hydrogen ions play a major role in atp production proton pumps move hydrogen ions to one side of the membrane building up a chemical gradient the protein atp synthase uses this energy stored in the gradient to assemble atp molecules are you hungry for more biological knowledge us too just remember to pace yourself now's a good time to pause the video get a drink of water and stretch your legs when we come back we'll take a look at the last topic in this section of the ap curriculum cell walls and their role in cell permeability and energy storage something this cow knows all about as we have discussed in previous videos cell walls are produced by many different organisms typically cell walls are made of complex carbohydrates like cellulose chitin and peptidoglycans though these molecules have evolved in very diverse forms of life all of these molecules are based on six carbon molecules of glucose in a ring structure this also means that these fibers are storing a lot of energy plants and algae use cellulose to create walls arthropods and fungi use chitin and prokaryotes like bacteria use peptidoglycans as base molecules for their cell walls while we will go into the specifics for these organisms later in this course the cell wall of each of these organisms affects the permeability of the cell membrane in similar ways in general cell walls do not affect the permeability of small molecules small molecules can easily slip through the fibers of the cell wall if they are nonpolar they can also easily enter the cell by contrast cell walls readily block many larger molecules large molecules tend to get caught up in this extra layer of fiber so in addition to the structural support that cell walls provide they also help organisms filter the environment around them and only accept the molecules that they need phew now that we've seen how different types of molecules experience different permeability when trying to pass through the cell membrane and how cell walls affect permeability i wonder if you can answer a few more questions you can now pause the video again and answer this set of questions you can find the answers through the quick test prep link in this video's description don't forget to check out all of our other ap biology resources that can also help you study for the test thanks for watching we hope you found this video helpful and informative if so please like this video leave us any questions or comments you have about membrane permeability and subscribe to the biology dictionary channel for all of our ap biology content good luck