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Cell Membrane Structure
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0:01
chapter 3 the cell membrane despite differences in structure and function all living cells in multicellular organisms have a surrounding membrane the cell membrane also known as the plasma membrane separates the inner contents of a cell from its exterior environment this cell membrane provides a protective barrier around the cell and regulates which material can pass in or out structure and composition of the cell membrane the cell membrane is an
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extremely pliable structure composed primarily of back to back phospholipids or a phospholipid bilayer cholesterol is also present which contributes to the fluidity of the membrane and there are various proteins embedded within the membrane that have a variety of functions a single phospholipid molecule has a phosphate group on one end called the head and two side by side chains of fatty acids that make up the lipid tails the phosphate group
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is negatively charged making the head polar and hydrophilic or water loving a hydrophilic molecule or region of a molecule is one that is attracted to water the phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments the lipid tails on the other hand are uncharged or nonpolar and are hydrophobic or water-fearing a hydrophobic molecule or region of a molecule repels and is repelled by water phospholipids
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are thus ampiphilic molecules an amphiphilic molecule is one that contains both hydrophilic and hydrophobic regions in fact soap works to remove oil and grease stains because it has amphipathic properties the hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that can then be washed away the cell membrane consists of two adjacent layers
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of phospholipids the lipid tails of one layer face the lipid tails of the other layer meeting at the interface of these layers the phospholipid heads paste outward one layer is exposed to the interior of the cell and the other layer is exposed to the exterior because the phosphate groups are polar and hydrophilic these are attracted to water found both in the intracellular and extracellular fluids
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the intracellular fluid is the fluid interior of the cell the extracellular fluid is the fluid environment lying outside the cell this is outside of the enclosure of the cell membrane interstitial fluid is the term given to extracellular fluid that is not contained within blood vessels because the lipid tails are hydrophobic these meet in the inner region of the membrane this region excludes the watery intracellular and extracellular fluids the cell membrane has many proteins
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as well as other lipids such as cholesterol that are associated with the phospholipid bilayer an important feature of the membrane is that it remains fluid the lipids and proteins in the cell membrane are not rigidly locked in place membrane proteins the lipid bilayer forms the basis of the cell membrane but it includes various proteins two different types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral proteins
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as its name suggests an integral protein is a protein that is embedded within a membrane a channel protein is an example of an integral protein that selectively allows particular materials such as certain ions to pass into or out of the cell another important group of integral proteins are called recognition proteins recognition proteins serve to mark a cell's identity so that it can be recognized by other cells a receptor is a type of recognition protein
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that can selectively bind a specific molecule outside of the cell and this binding induces a chemical reaction within the cell a ligand is a specific molecule that binds to and activates a receptor some integral proteins serve dual roles as both receptors and ion channels one example of a receptor ligand interaction is the receptors that are located on nerve cells that bind neurotransmitters such as dopamine when a dopamine molecule binds to a dopamine receptor protein a
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channel within the transmembrane protein opens allowing for certain ions to flow into the cell
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some integral membrane proteins are glycoproteins a glycoprotein is a protein that has carbohydrate molecules attached which extend into the extracellular matrix the attached carbohydrate tags on glycoproteins aid in cellular recognition the carbohydrates that extend from the membrane proteins and even from some membrane lipids collectively form the glycocalyx the glycocalyx is formed from glycoproteins and other carbohydrates that are attached to the cell membrane
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the glycocalyx can have various roles for example it may have molecules that allow the cell to bind to another cell or it may contain receptors for hormones or it may have enzymes that break down nutrients the glycocalyces found in a person's body are products of that person's genetic makeup they give each of the individuals trillions of cells the identity of belonging to a person's body
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this identity is the primary way that a person's immune defense cells know not to attack a person's own body cells but it is also the reason the organs donated by another person may be rejected peripheral proteins are typically found on the inner or outer surfaces of the lipid bilayer but they can also be attached to the internal or external surfaces of an integral protein these proteins typically perform a specific function for the cell some peripheral proteins on the surface
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of intestinal cells for example act as digestive enzymes to break down nutrients to sizes that can pass through the cells into the bloodstream transport across the cell membrane one of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell these substances include ions such as calcium ion sodium ion potassium ion and chloride ion as well as nutrients including sugars fatty acids and amino acids
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waste products are also regulated that must leave the cell one of particular importance is carbon dioxide the membrane's lipid bilayer structure provides the first level of control the phospholipids are tightly packed together and the membrane has a hydrophobic interior this structure allows the membrane to be selectively permeable a membrane that has selective permeability only allows certain substances that meet certain criteria
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to pass through it unaided in the case of the cell membrane only relatively small nonpolar materials can move through the lipid bilayer some examples of these are other lipids alcohol oxygen and carbon dioxide gas however water-soluble materials like glucose amino acids and electrolytes need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer all substances that move through the membrane do so by one of two general
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methods which are categorized based on whether or not energy is required passive transport is the movement of substances across the membrane without expenditure of cellular energy in contrast active transport is the movement of substances across the membrane using energy from adenosine triphosphate
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passive transport in order to understand how substances move passively across cell membranes it is necessary to understand the concentration gradients and diffusion a concentration gradient is the difference in concentration of a substance across a space molecules or ions will spread out or diffuse from where they are more concentrated to where they are less concentrated until they are equal in distribution
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when molecules move in this way they are said to be moving down their own concentration gradient diffusion is the movement of particles from an area of higher concentration to an area of lower concentration imagine being inside a closed room if a bottle of perfume were sprayed the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of that bathroom and this diffusion would go on until no more concentration gradient remains
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if the room temperature is warmer this diffusion occurs even more quickly as the molecules are bumping into each other and spreading out more quickly than they do at cooler temperatures having an internal body temperature of around 98.6 degrees fahrenheit thus also aids in the diffusion of particles within the body whenever a substance exists in greater concentration on one side of the semipermeable membrane any substance that can move down its concentration gradient
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across the membrane will do so consider substances that can easily diffuse through the lipid bilayer such as oxygen and carbon dioxide oxygen generally diffuses into cells because it is more concentrated on the outside rather than the inside carbon dioxide typically diffuses out of cells because it is more concentrated inside than outside neither of these examples requires any energy on the part of the cell and therefore they use passive transport to move across the membrane
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because cells rapidly use up oxygen during metabolism there is typically a lower concentration of oxygen inside the cell than outside as a result oxygen will diffuse from the interstitial fluid directly through the lipid bilayer of the membrane and into the cytoplasm within the cell on the other hand because cells produce carbon dioxide as a byproduct of metabolism carbon dioxide concentrations rise within the cytoplasm therefore
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carbon dioxide will move from the cell through the lipid bilayer and into the interstitial fluid where its concentration is lower this mechanism of molecules moving across the cell membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called simple diffusion large polar or ionic molecules which are hydrophilic cannot easily cross the phospholipid bilayer very small polar molecules such as water
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can cross via simple diffusion due to their small size charged atoms or molecules of any size cannot cross the cell membrane by a simple diffusion because the charges are repelled by the hydrophobic tails that exist in the interior of the phospholipid bilayer solutes dissolve in water on either side of the cell membrane and will tend to diffuse down their concentration gradients but because most substances cannot freely pass through the lipid bilayer their movement is
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restricted to protein channels and specialized transport mechanisms that exist within the membrane facilitated diffusion is the diffusion process used for substances that cannot cross the lipid bilayer due to their size charge or polarity a common example of facilitated diffusion is the movement of glucose into cells where it is used to make atp although glucose can be more concentrated outside of a cell it cannot cross the lipid bilayer by a simple diffusion
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this is because it is both a large molecule and it is polar to resolve this a special carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion even though sodium ions are highly concentrated outside of cells these electrolytes are charged and therefore cannot pass through the nonpolar lipid bilayer of the membrane their diffusion is facilitated by membrane proteins that form sodium channels or
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pores these pores allow sodium ions to move down their concentration gradients from outside the cell to inside of the cell there are many other solutes that must undergo facilitated diffusion in order to move into a cell such as amino acids or to move out of the cell such as waste products because facilitated diffusion is a passive process it does not require energy expenditure by the cell
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water also can move freely across the membrane of all cells either through protein channels or by slipping between the lipid tails of the membrane itself osmosis is the diffusion of water through a semi-permeable membrane the movement of water molecules is not itself regulated by some cells so it is important that these cells are exposed to an environment in which the concentration of solutes outside of the cell is equal to the concentration of solutes inside the cell
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for example two solutions that have the same concentration of solutes are said to be isotonic when cells and their extracellular environments are isotonic the concentration of water molecules is the same on the outside as it is on the inside and thus the cells maintain their normal shape and function osmosis occurs when there is an imbalance of solutes outside of the cell versus inside the
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cell cells in a hypertonic solution will shrivel as water leaves the cell via osmosis cells in a hypotonic solution will take on too much water and will swell with the risk of eventually bursting a critical aspect of homeostasis in living things is to create an internal environment in which all of the body's cells are in an isotonic solution various organs particularly the kidneys work to maintain this homeostasis another mechanism besides diffusion to passively transport
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materials between compartments is filtration unlike diffusion of a substance from where it is more concentrated to less concentrated filtration uses a hydrostatic pressure gradient that pushes the fluid and the solutes within it from a higher pressure area to a lower pressure area filtration is an extremely important process in the body for example the circulatory system uses filtration to
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move plasma and substances across the endothelial lining of capillaries and into surrounding tissues supplying the cells with nutrients filtration pressure in the kidneys provides the mechanism to remove waste products from the bloodstream active transport membrane proteins that aid in the passive transport of substances do so without the use of atp during active transport however atp is required to move substances across a membrane this is done with the help of protein carriers
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and is usually done against its concentration gradient the sodium potassium pump also called the sodium potassium atpase transports sodium out of a cell while moving potassium into the cell the sodium potassium pump is an important ion pump found within the membranes of many types of cells these pumps are particularly abundant in nerve cells which are constantly pumping out sodium ions
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and pulling in potassium ions in order to maintain an electrical gradient across the cell membrane an electrical gradient is a difference in electrical charge existing across a space in the case of nerve cells for example the electrical gradient exists between the inside and outside of the cell with the inside being negatively charged at around negative 70 millivolts relative to the outside the negative electrical gradient is maintained because of the sodium potassium pump
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the sodium potassium pump maintains a high concentration of sodium ions outside of the cell therefore if the cell needs sodium ions all it has to do is open a passive sodium channel as the concentration gradient of sodium ions will drive them to diffuse into the cell in this way the action of an active transport pump powers the passive transport of sodium ions by creating a concentration gradient when active transport powers the transport of another substance in
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this way it is called secondary active transport symporters are secondary active transporters that move two substances in the same direction for example the sodium glucose symporter uses sodium ions to pull glucose molecules into the cell because cells store glucose for energy glucose is typically at a higher concentration inside of the cell than outside however due to the action of a sodium-potassium pump sodium ions will easily diffuse into the cell when the symporter is opened
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the flood of sodium ions through this importer provides the energy that will allow glucose to move through the symporter and into the cell against its concentration gradient conversely antiporters are secondary active transport systems that transport substances in opposite directions for example the sodium hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions out of the cell the sodium hydrogen antiporter is used to maintain
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the ph of a cell's interior other forms of active transport do not involve membrane carriers at all endocytosis is the process of cells ingesting material by enveloping it in a portion of its cell membrane once pinched off the portion of the membrane and its contents becomes independent and is referred to as a vesicle a vesicle is a membranous sac which is a spherical and hollow organelle bounded by a lipid bilayer membrane endocytosis often brings materials
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into the cell that must be broken down or digested phagocytosis is the endocytosis of large particles many immune cells engage in phagocytosis of invading pathogens like little pac-men their job is to patrol tissues for unwanted matter such as invading bacterial cells phagocytizing them and digesting them pinocytosis brings fluid containing dissolved substances into the cell through membrane vesicles cells regulate the enzyme cytosis of specific substances
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by a receptor-mediated endocytosis receptor receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance in contrast with endocytosis exocytosis is the process of a cell exporting material using vesicular transport many cells manufacture substances that must be secreted such as a factory producing a product for exportation these substances are typically
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packed into membrane-bound vesicles within the cell when the vesicle membrane fuses with the cell membrane the vesicle releases its contents into the interstitial fluid the vesicle membrane then becomes part of the cell membrane thank you for watching
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