membrane proteins are those proteins that are either a part of or interact with biological membranes they make up around one-third of human proteins and give different kinds of membranes their unique properties they help with both facilitated diffusion and active transport connect cells together participate in signal transduction and act as markers for cell identification proteins are what carry out most of the specific functions of membranes so the amount and types of proteins vary between different membranes membranes can be up to 75% protein by mass membrane proteins come in two flavors integral or intrinsic and peripheral or extrinsic integral membrane proteins are a permanent part of the membrane while peripheral proteins are only transiently associated with either the membrane or integral proteins where these associations are hydrophobic electrostatic or other non covalent interactions there are several different kinds of integral proteins integral mono topic proteins are attached to only one of the two leaflets of phospholipids making up the membrane and they don't span across it there are also transmembrane proteins and lipid anchor proteins transmembrane proteins are those that span the lipid bilayer and can be by topic spanning across the membrane once or poly topic spanning across it more than once lipid-anchored proteins are those which are covalently attached to lipids embedded in the lipid bilayer for example GPI or glycosyl phosphatidyl inositol is a glycolipid that gets attached to a protein c terminus during post translational modification it acts as an anchor for proteins to the outer leaflet of the plasma membrane both integral and peripheral proteins can be post-translationally modified there can be addition of fatty acids diacylglycerol parental chains or GPI recall that cellular membranes are made up of a phospholipid bilayer which consists of two leaflets of phospholipids these phospholipids have polar heads which are hydrophilic or water-loving and nonpolar fatty acyl tails that are hydrophobic or water hating polar substances like to interact with other polar substances and nonpolar substances hang out with other nonpolar substances this really attests to the power of hydrogen bonding water molecules want to interact so badly with their polar buddies that anything nonpolar getting in the way of their hydrogen bonding results in decreased entropy the result is what's called the hydrophobic effect this is why phospholipids in water will spontaneously form lipid bilayers these by layers minimize contact between polar and nonpolar molecules maximize hydrogen bonding and maximize entropy this is also why transmembrane proteins are amphipathic which means that they have regions which are hydrophilic and regions which are hydrophobic the hydrophilic regions are exposed to water on either side of the membrane while the hydrophobic bits are happily interacting with the hydrophobic tails of lipid molecules in the interior of the bilayer as a result transmembrane proteins are stuck permanently in the cell membrane and are very hard to isolate to get them out you need to add detergent which is F up a thick and will disrupt the lipid bilayer there are two basic types of transmembrane proteins alpha helical proteins and beta-barrel proteins note that while helix bundle proteins are found in all types of biological membranes beta-barrel proteins are only found in the outer membranes of gram-negative bacteria mitochondria and chloroplasts evidence that contributes to the endosymbiotic theory in which eukaryotic cells acquired these organelles through the ingestion of prokaryotes transmembrane protein structure can be predicted using a hydropathy plot which has hydrophobicity index on the y axis and amino acid number on the x axis the amino acids making up a protein are localized according to polarity within its final structure in such a way that the polar amino acids face the outside aqueous solutions and the nonpolar amino acids are adjacent to the lipid bilayer transmembrane proteins can be classified by topology which is based on the position of N and C termini as well as start transfer and stop transfer sequences for example type 1 is a single transmembrane pass with the N terminus on the extracellular side of the membrane type 2 is also a single transmembrane pass but the enter - is on the cytosol Excite of the membrane how these different topologies come about will be the topic of another video often transmembrane proteins function as gateways allowing specific substances to pass across the membrane they can undergo conformational changes as they do this they might participate in facilitated or active transport facilitated transport is spontaneous passive transport of substances via transmembrane proteins active transport however requires energy active transport may be necessary for instance if a substance is being carried across the membrane against its chemical or electrical gradient as a final note in animal cells most transmembrane proteins are glycosylated these sugar residues are always present on the non cytosol ik leaflet of the membrane as a result the cell surface is covered in carbohydrates which form what's called the cell coat if you enjoyed this video like and subscribe you can also support me by following the link to my patreon if you have any topics you'd like me to cover please leave a comment