Biological Membranes PDF

Unclassified / Non classifié Biological Membranes Membranes have a boundary function. = membrane-bound...

Unclassified / Non classifié Biological Membranes Membranes have a boundary function. = membrane-bound organelle Plasma membrane (PM) A eukaryotic cell has a lot of membranes. There is a plasma membrane, and many membrane-bound organelles. This generic animal cell shows many of the membranes found in eukaryotic cells. Keep in mind that most animal Flagellum cells do not have a flagellum. Modified from https://www.sciencefacts.net/animal-cell.html We will focus on the plasma membrane, but most of what we cover also applies to internal (organelle) membranes.  Unclassified / Non classifié Biological Membranes Fluid Mosaic Model of Biological Membranes The “fabric” of the membrane is a phospholipid bilayer. The phospholipid bilayer is a fluid. There is water on both sides of the membrane; OpenStax Biology 2e. → there is a fluid separating fluids There are proteins associated with biological membranes. OpenStax Biology 2e.  Unclassified / Non classifié Biological Membranes Fluid Mosaic Model of Biological Membranes “Fluid” refers to the phospholipid bilayer, which is truly a fluid. Phospholipids can diffuse from one part of the membrane to another. “Mosaic” refers to the proteins associated with the membrane, and the fact that some of the proteins slowly move around and change location over time. OpenStax Biology 2e.  Unclassified / Non classifié Biological Membranes Fluid Mosaic Model – Phospholipid Diffusion Common. Hydrophilic head groups “Flip-flop”; rare because hydrophilic Hydrophobic head groups must move core past hydrophobic tails for this to occur. Two fatty acid tails attached to each head group Common. Modified from Khan et. al (2013) International Journal of Molecular Sciences 14: 21561-21597  Unclassified / Non classifié Digression about Artistic Abilities Digression: sometimes students worry that the diagrams that they draw on tests are not good enough (not “artistic” enough). In fact, as long as the diagrams make sense, and are labelled appropriately, it’s all good. E.g. the diagrams below, of an individual phospholipid and a phospholipid bilayer, would be fine on a test. “Head and tail” representation of a phospholipid. The head is the head group, and the tails are the fatty acids. Phospholipid bilayer, which is the basis for biological membranes. The hydrophobic fatty acids face each other, while the hydrophilic head groups face outwards.  Unclassified / Non classifié Biological Membranes Phospholipids are amphipathic = having hydrophilic and hydrophobic parts. This is the basis of the phospholipid bilayer, which forms the “fabric” of the membrane. Lots of water on both sides of a membrane. hydrophilic head group hydrophobic fatty acid tails OpenStax Biology 2e.  Unclassified / Non classifié Biological Membranes Dump phospholipids on water: Mess around with conditions (e.g. add → hydrophilic heads are in the ethanol to water & phospholipids): water, and hydrophobic tails → form liposomes, micelles or point into the air bilayer sheets (self-assembly) Shen et al. (2013) International Journal of Molecular Sciences 14: 1589-1607 By Mariana Ruiz Villarreal ,LadyofHats - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3032610  Unclassified / Non classifié Biological Membranes oligosaccharides (on the outside peripheral/extrinsic of the plasma membrane) protein (these proteins are typically on the outside of the plasma membrane) outside of the cell H2O H2O H2O H2O phospholipid bilayer (has a hydrophobic core and hydrophilic surfaces) H2O H2O H2O H2O elements of the integral/intrinsic proteins (these ones cytoskeleton are both membrane-spanning, but not may anchor a all integral proteins are membrane- protein in place spanning; some proteins are anchored cytosol in place and some are not)  Unclassified / Non classifié Biological Membranes Proteins Biological membranes contain proteins. These proteins may be integral/intrinsic or peripheral/extrinsic. Some of the integral proteins are transmembrane (membrane-spanning), while some are not Some of the transmembrane proteins are transport proteins (“trans” means “across”). Integral Proteins Embedded in the phospholipid bilayer; difficult to remove from membranes; some are anchored in place by the cytoskeleton. Peripheral Proteins Not embedded in the phospholipid bilayer, or only loosely embedded. Are much easier to remove from membranes than are integral proteins. Are held place by interactions with phospholipids or with integral proteins; are typically on the outside face of the plasma membrane.  Unclassified / Non classifié Biological Membranes Non-transmembrane integral proteins versus peripheral proteins Matter of degree – how easily can they be removed from the membrane? If you Google “peripheral protein”, you will see many different interpretations. Some interpretations suggest that any protein that is not transmembrane is peripheral; → that is not a great interpretation (too simple-minded) Peripheral proteins are easily removed from membranes. Non-transmembrane integral proteins are firmly attached to the membrane. 1. phospholipid 2. cholesterol 3. glycolipid (do not worry about this) 4. sugar 5. integral (transmembrane) protein 6. integral protein (glycoprotein in this case) 7. peripheral protein (anchored to a phospholipid) 8. peripheral protein (anchored to an integral protein) Modified from: Foobar Creative Commons Attribution 2.5  Unclassified / Non classifié Biological Membranes - Integral and Peripheral Proteins transmembrane/membrane-spanning Modified from: Allen et al. (2019) Trends in Biochemical Sciences 44: 7-20. Another view of integral and peripheral proteins. Integral proteins may be membrane-spanning (polytopic or bitopic) or be associated with only one side of the membrane (monotopic). Polytopic integral proteins have several distinct parts of the protein embedded in the phospholipid bilayer. Peripheral proteins are only weakly associated with the membrane. The cylinders in the image represent α-helices. You do not need to know the specific terms such as “polytopic”.  Unclassified / Non classifié Biological Membranes Membrane Fluidity Membranes are fluid. The fluidity is an essential property for proper functioning. Large temperature changes can affect membrane fluidity; → membranes become leaky → cell death In animal membranes, cholesterol is used to adjust membrane fluidity in a complicated manner; cholesterol will increase fluidity at low temperature and decrease fluidity at high temperature. Cholesterol By BorisTM - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6459 94 You do not need to know the structure of cholesterol. Modified from OpenStax 2e.  Unclassified / Non classifié Biological Membranes Proteins in the Membrane Various types and functions, e.g. hormone receptors, enzymes, transport proteins. Some of the Proteins in a Biological Membrane are Transport Proteins Membrane-spanning (= transmembrane), integral proteins. https://www.sciencefacts.net/transport- Substrate-specificity; no general transport proteins. proteins.html Two broad classes: 1) Carrier proteins. 2) Channel proteins (these include aquaporins = water channels). Two types of transport proteins. Traffic of Molecules Across Membranes (there is a lot of traffic) May involve transport proteins. Or simple diffusion (no transport proteins involved). Or endo- and exocytosis.  Unclassified / Non classifié Biological Membranes Various Types of Molecules that Cross Membranes, using Various Mechanisms Gases – O2 and CO2 are the biologically-relevant gases that we are interested in; O2 and CO2 are both small molecules and non-polar. Water – polar; moves across membranes despite being polar. Ions and small polar molecules – repelled by the hydrophobic core of the phospholipid bilayer; small polar molecules include glucose (blood sugar) and many amino acids; → require specialized transport systems Large molecules may also cross a membrane, via specialized transport systems. https://k12.libretexts.org/Bookshelves/Science_an d_Technology/Life_Science_for_Middle_School_(C K- 12)/02%3A_Cell_Biology/2.02%3A_Passive_Transp ort O2, CO2 and water can cross membranes via simple diffusion (= passive transport).  Unclassified / Non classifié Biological Membranes Biological Membranes are Selectively Permeable (= Semipermeable) Not everything easily crosses a membrane. Polar or charged molecules (ions) cross membranes only very slowly in the absence of specific transport proteins. But almost anything will cross a membrane, given sufficient time; → might be more accurate to state that biological membranes are differentially permeable. OpenStax Biology 2e Modification of work by Mariana Ruiz Villareal Mechanisms for the Movement of Molecules Across Membranes 1) diffusion a) simple b) facilitated i) channel protein ii) carrier protein 2) active transport (= 1° active transport); involves transport (carrier) proteins 3) co-transport (= 2° active transport); involves transport (carrier) proteins 4) exo- and endocytosis; involves vesicles Channel proteins allow the diffusion of specific solutes (= down a concentration gradient) across a membrane.  Unclassified / Non classifié Biological Membranes – Digression About “[ ]”, “Δ” and “ΔC” [ ] indicates “concentration” of a solute in a solvent; e.g. [H+] is the concentration of protons, [Cl-] is the concentration of chloride ions Δ is the Greek letter “delta” (upper case, capital letter). Δ can be used to indicate “difference” or “change” (these are similar but not identical concepts). ΔC = concentration gradient/difference; = two different concentrations (a comparison) ΔC means that we are comparing concentrations of some solute between two locations. ΔC often comes up when discussing biological membranes; → comparing the concentration of a particular molecule (solute) on either side of a membrane Not correct to state to that one particular location has a ΔC; → because ΔC implies a comparison  Unclassified / Non classifié Biological Membranes – Digression About “[ ]”, “Δ” and “ΔC” As an example of "ΔC" and "[ ]", diffusion of solutes always occurs from higher [solute] to lower [solute], which is down a ΔC. In the example below (time course of ink diffusion in water), which does not include a membrane, the ink is diffusing from higher [ink] to lower [ink], down a ΔC of [ink], which can also be indicated as down a Δ[ink]: https://www.science-sparks.com/diffusion-demonstration/  Unclassified / Non classifié Biological Membranes – Digression About “[]”, “Δ” and “ΔC” “Δ” always means a comparison, Δ[solute] change or gradient. Modified from: OpenStax Biology 2e lower [solute] higher [solute] synthetic membrane For later in the course: gradients of [H+] come up a lot (but there are also concentration gradients/differences of other molecules).  Unclassified / Non classifié Biological Membranes - Diffusion Diffusion is the random mixing of molecules due to kinetic energy. Diffusion may occur in the liquid phase or the gas phase; → we will focus on the liquid phase Both solutes (= the dissolved stuff) and solvents (what does the dissolving) exhibit diffusion. The only solvent that we care about is water; -there is a lot of solute dissolved in water in a cell Diffusion of solutes is from higher concentration to lower concentration; i.e. down a solute concentration gradient (ΔC). In contrast, diffusion of water occurs from areas of lower [solute] → higher [solute]; -water and solutes diffuse in opposite directions First - Diffusion of Solutes in a Solvent ΔC > 0 ΔC = 0 (equilibrium) Down a concentration gradient (ΔC); → higher [solute] to lower [solute] By JrPol - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=4586487  Unclassified / Non classifié Biological Membranes - Diffusion Diffusion of Solutes Diffusion of solutes is from higher concentration to lower concentration; i.e. down a solute concentration gradient (ΔC). At equilibrium, ΔC = 0 (uniform [solute] throughout the system); → net movement stops → but individual solute and solvent molecules are still moving (bouncing around and off each other) ΔC > 0 (net movement) ΔC = 0 (equilibrium) 3D rendering of diffusion of purple dye in water. At equilibrium (beaker on the far right), water and dye molecules are still moving but there is no net movement. By BruceBlaus - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=29452222  Unclassified / Non classifié Biological Membranes - Diffusion Diffusion of Solutes – Plasma Membrane OpenStax Biology 2e.  Unclassified / Non classifié Biological Membranes - Diffusion Consider a small unicell living in a pond Aerobic cellular respiration consumes O2 and produces CO2. O2 higher [O2] plasma membrane lower [CO2] CO2 lower [O2] higher [CO2] plasma Diffusion Rate → linear relationship membrane This is simple diffusion of a solute → down a ΔC → no transport protein involved 0 0 Δ[O2] or Δ[CO2] →  Unclassified / Non classifié Biological Membranes – Diffusion of Water Water Diffuses (so do other solvents) Solvents diffuse from an area of lower [solute] to higher [solute] (opposite direction to diffusion of solutes). Osmosis = the diffusion of water across a selectively permeable membrane (special case of diffusion of water). Water can diffuse across biological membranes; two mechanisms: 1) water is a small molecule, so even though it is polar it is small enough that some water molecules can diffuse through the hydrophobic core of a phospholipid bilayer 2) aquaporins are membrane-spanning channel proteins that act as water pores; water can bypass the hydrophobic core of the membrane Aquaporins are channel proteins that act as water channels. These channel proteins provide a hydrophilic passageway for water across the hydrophobic membrane core. OpenStax Anatomy & Physiology.  Unclassified / Non classifié Biological Membranes – Diffusion of Water higher [solute] Modified from OpenStax Biology 2e lower [solute] analogous to the plasma membrane of a (Water can pass, solutes cannot pass) cell In osmosis, water always moves from an area of lower solute concentration to one of higher solute concentration. In the diagram, the solute cannot pass through the selectively permeable membrane, but the water can.  Unclassified / Non classifié Biological Membranes – Diffusion of Water - Cells higher [solute] on outside equal [solute] higher [solute] on inside LadyofHats, Public Domain. cell dehydration possibility of cell bursting (lysis) Pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. In general, animal cells like to be in an isotonic solution.  Unclassified / Non classifié Biological Membranes – Diffusion of Water & Solutes Summary of the Diffusion of Water and Solutes Solutes diffuse down a ΔC of solutes; -from higher [solute] → lower [solute] Water diffuses in the opposite direction; -from lower [solute] → higher [solute] completely impermeable barrier gently remove the barrier water solute lower [solute] higher [solute] https://www.norchemist.com/product/beak er-borosilicate-glass-250-ml/  Unclassified / Non classifié Biological Membranes – Facilitated Diffusion of Solutes Diffusion of Solutes Down ΔC with the Help of a Transport Protein (= Facilitated Diffusion) Simple diffusion will not work for polar molecules or ions; → have hydration shells → repelled by the hydrophobic interior of membranes (presence of a ΔC is insufficient) → need a transport protein (two types: channel proteins and carrier proteins) By LadyofHats Mariana Ruiz Villarreal - Own work. Image renamed from Image:Facilitated_diffusion_in_cell_membrane.svg, Public Domain, Facilitated Diffusion https://commons.wikimedia.org/w/index.php?curid=3981034  Unclassified / Non classifié Biological Membranes – Facilitated Diffusion of Solutes Channel Proteins Channel proteins allow a specific molecule, or a small range of similar molecules, to diffuse down a ΔC; -selection/specificity -these channels often can be opened and closed (i.e. allow diffusion as needed) Down ΔC (diffusion of solutes is always down ΔC). By LadyofHats Mariana Ruiz Villarreal - Own work. Image renamed from Image:Facilitated_diffusion_in_cell_membrane.svg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3981034  Unclassified / Non classifié Biological Membranes – Facilitated Diffusion of Solutes Carrier Proteins Carrier proteins (uniports/uniporters) change shape during the transport process Have a substrate binding site (binding site shows specificity); the substrate is the transported molecule. Substrate binding causes a conformational change; → substrate released on the other side → another conformational change to the original conformation Down ΔC (diffusion of solutes is always down ΔC). By LadyofHats Mariana Ruiz Villarreal - Own work. Image renamed from Image:Facilitated_diffusion_in_cell_membrane.svg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3981034  Unclassified / Non classifié Biological Membranes – Uniport Sequence of Events Labels are in German. By: Jugrü, Creative Commons Attribution-Share Alike 3.0 Unported  Unclassified / Non classifié Biological Membranes – Facilitated Diffusion (Uniport) By: Jugrü, Creative Commons Attribution-Share Alike 3.0 Unported Binding site for substrate; shows specificity. There are no general transporters. Uniports are carrier proteins that move one molecule in one direction.  Unclassified / Non classifié Biological Membranes – Diffusion In simple diffusion, there is a linear relationship between ΔC and the diffusion rate. simple diffusion For diffusion mediated by a carrier protein, there is a maximum rate at which the protein can transport molecules (i.e. the carrier protein is working as fast as it can); RATE OF DIFFUSION → saturation curve facilitated diffusion (via a carrier protein) 0 0 ΔC  Unclassified / Non classifié Biological Membranes – Diffusion Review about Diffusion Diffusion of solutes and solvents (water is the only solvent that we care about) are different things. Diffusion of water occurs against the ΔC for solutes. Diffusion of water may occur via: 1) simple diffusion 2) facilitated diffusion via channel proteins --e.g., aquaporins Solutes diffuse down their ΔC. Diffusion of solutes may occur via: 1) simple diffusion 2) facilitated diffusion Facilitated diffusion of solutes may occur via: 1) channel proteins 2) carrier proteins  Unclassified / Non classifié Biological Membranes – Moving Against ΔC Some Molecules are Transported Against ΔC This is not diffusion. This requires cellular energy to accomplish; -moving something against ΔC is the performance of cellular work A transport protein is required (carrier protein). The transport protein has substrate binding sites; → specificity Digression: ATP is the “Energy Currency” of a Cell ATP = adenosine triphosphate Cellular work “spends” ATP. ATP is “spent” via hydrolysis: ATP + H2O ADP + Pi (often shown simply as: ATP ADP + Pi ) Will explain ATP in more detail in the next section of the course. Brief overview on next slide.  Unclassified / Non classifié Biological Membranes – Digression – ATP Hydrolysis ATP + H2O ADP + Pi ATP: the Universal Energy Currency https://chem.libretexts.org/@go/page/178800 ATP is the “energy currency” of a cell. The “spending” of ATP is a hydrolysis reaction → the terminal phosphate (PO43-) is removed. Energy is made available by the hydrolysis reaction. That energy can be used to perform cellular work. ATP is found throughout a cell (cytosol, nucleus, etc.). ATPases are enzymes/proteins that hydrolyze ATP.  Unclassified / Non classifié Biological Membranes – Transport May or May Not Require Energy Two Types of Systems for Transport Against ΔC: 1) active transport (= primary [1°] active transport) – directly uses ATP 2) co-transport (= secondary [2°] active transport) – indirectly uses ATP http://www.artnet.com/artists/jiro- osuga/uphill-downhill-diptych-a- cOD2H3tfY0S9L1FkNYyPNg2 Diffusion of solute happens down a ΔC (higher C → lower C) and does not require energy (happens on its own); like riding a bicycle down a hill. Active transport and co-transport move solute against a ΔC (lower C → higher C); requires energy; like riding a bicycle up a hill.  Unclassified / Non classifié Biological Membranes – Active Transport (= 1° Active Transport) Active Transport is Performed by Transport Proteins Called Ion Pumps Ion pumps are also known as cation-translocating ATPases (perhaps a better name). They pump cations across a membrane against a cation concentration gradient at the expense of ATP (= energy). (Moving anything against a gradient requires energy.) There is usually net export of positive charge (some are “electroneutral”). There are no ATPases for anions or for non-charged molecules. Generic Cell plasma membrane ADP + Pi higher [X+] lower [X+] X+ ATP Ion Pump = X+-ATPase = X+-translocating ATPase ATPases hydrolyze (“spend”) ATP “X+” is a generic cation  Unclassified / Non classifié Biological Membranes – Active Transport (= 1° Active Transport) The most common ion pump on the plasma membranes of plant cells is the H+ pump (also known as the H+-translocating ATPase or the H+-ATPase). The H+-ATPase exports H+s against a H+ concentration gradient (Δ[H+]), i.e. against a ΔC for H+s (Δ[H+]), at the expense of ATP. 1 H+ pumped out per ATP hydrolyzed. Generic Plant Cell plasma membrane ADP + Pi higher [H+] lower [H+] H+ ATP H+ Pump = H+-ATPase = H+-translocating ATPase ATPases hydrolyze (“spend”) ATP  Unclassified / Non classifié Biological Membranes – Active Transport (= 1° Active Transport) The Na+/K+ pump is the most common ion pump on the plasma membranes of animal cells. The Na+/K+ pump (also known as the Na+/K+-ATPase) simultaneously pumps out 3 Na+ and pumps in 2 K+ per ATP hydrolyzed Both cations are moving against their ΔC. Net export of positive charge. There are many other examples of ion pumps. Moving molecules against their ΔC requires energy. Generic Animal Cell plasma membrane higher [Na+] lower [K+] 2K+ ADP + Pi lower [Na+] higher [K+] ATP 3Na+ Na+/K+ Pump = Na+/K+-ATPase =Na+/K+-translocating ATPase  Unclassified / Non classifié Biological Membranes – Active Transport Another Example of Active Transport Acidification (= ↑[H+]) of the Cell in the Stomach Lining stomach (part of the digestive process) occurs via the H+/K+- higher [H+] ATPase in the stomach lining. lower [K+] This is an electroneutral ion pump. K+ ADP + Pi Both H+ and K+ are moving against lower [H+] their ΔC. higher [K+] ATP H+ H+/K+ Pump = H+/K+-ATPase Overall, there are a LOT of different ion pumps (cation-translocating plasma membrane ATPases) in biology. They all transport cations. They all use/spend ATP. Most cause a net export of positive Active transport is also known as charge, some are electroneutral. primary (1°) active transport.  Unclassified / Non classifié Biological Membranes – Co-Transport (= 2° Active Transport) Co-Transport Also known as secondary (2°) active transport. Anions or polar molecules can also be transported against ΔC. Indirectly uses ATP. The transport protein is known as a symporter (symport) or antiporter (antiport); = transport proteins that transport two molecules simultaneously One molecule is transported down the ΔC for that molecule. The other molecule is transported against the ΔC. The molecule moving down the ΔC provides the energy for moving the other molecule against the ΔC. co-transport (= secondary (2°) active transport) facilitated diffusion OpenStax Biology 2e  Unclassified / Non classifié Biological Membranes – ATP Dependence of Co-Transport Generic Plant Cell The H+/sucrose symport (transport protein) of plant cells moves sucrose against a Δ[sucrose] and H+s down a ADP + Pi higher [H+] Δ[H+]. lower [H+] H+ These two molecules move ATP H+ Pump simultaneously in the process of co- [sucrose] = H+-ATPase ~ 50 mM transport. H+ H+ The H+ pump (a transport protein) pumps H+s against a H+ concentration gradient (Δ[H+]) at the expense of ATP Sucrose Sucrose (this is active transport). [sucrose] < 10 mM This results in a higher [H+] on the H+/sucrose symport outside of the cell compared to the inside. plasma membrane Sucrose is moving from lower [sucrose] The Δ[H+] powers the transport of to higher [sucrose]. This requires energy. sucrose against the Δ[sucrose].  Unclassified / Non classifié Biological Membranes – ATP Dependence of Co-Transport Generic Plant Cell The H+/sucrose symport is just one example of co-transport; there are many ADP + Pi higher [H+] other examples. lower [H+] H+ The H+/sucrose symport is indirectly H+ Pump ATP-dependent because it requires the ATP [sucrose] = H+-ATPase Δ[H+] set up by the H+ pump (which is ~ 50 mM why the alternative name for the process is secondary active transport). H+ H+ Sucrose Sucrose [sucrose] < 10 mM H+/sucrose symport plasma membrane  Unclassified / Non classifié Biological Membranes – ATP Dependence of Co-Transport The Na+/Ca2+ Antiport is Another Example of Co-Transport (= 2° Active Transport) This antiport is common in the plasma membrane of many types of animal cells. It exports Ca2+ from a cell against a Δ[Ca2+]. The energy required to move Ca2+ against Δ[Ca2+] is supplied by Na+ moving down Δ[Na+]. There is a Δ[Na+] because of the Na+/K+ pump, which needs energy (ATP). Therefore, Ca2+ export is indirectly ATP-dependent. Ca2+ is moving from lower [Ca2+-] to higher [Ca2+]. Animal Cell higher [Na+] lower [K+] plasma membrane higher [Ca2+] lower [Na+] + ADP higher [K+] 2K + Pi lower [Ca2+] 3Na+ ATP 3Na+ Ca2+ Na+/K+ Pump = Na+/K+-ATPase Na+/Ca2+ Antiport  Unclassified / Non classifié Biological Membranes – Summary of Solute Transport (so far) ΔC Moving down ΔC (not work) Moving against ΔC (work) = diffusion = active transport or co-transport no transport transport directly ATP- indirectly ATP- protein protein dependent dependent = simple diffusion = facilitated diffusion = active transport = co-transport (1° active transport) (2° active transport) channel protein carrier protein (uniport/uniporter) not work work  Unclassified / Non classifié Biological Membranes – Exocytosis and Endocytosis (Bulk Transport) Movement of Some Large Molecules & Other Ways of Crossing a Membrane Exocytosis and endocytosis move larger molecules across a plasma membrane. Molecules that are too large to be moved by transport proteins, or for which there are no specific transport proteins. Mediated by the cytoskeleton; requires ATP. Exocytosis A mechanism used by cells to export molecules from a cell; -e.g. proteins that are produced on rough ER that are targeted to the outside of a cell -e.g. #2 removal of cellular wastes from an amoeba -e.g. #3 release of neurotransmitters into a synapse between neurons In exocytosis, intracellular vesicles fuse with the plasma membrane. The contents of the vesicles are then released to the exterior of the cell. Modified from OpenStax Biology 2e  Unclassified / Non classifié Biological Membranes – Exocytosis and Endocytosis Endocytosis A mechanism to import molecules into a cell; there are three sub-types of endocytosis: pinocytosis, phagocytosis and receptor-mediated endocytosis. 1) Pinocytosis The least selective of the three sub-types of endocytosis. The plasma membrane bulges inwards and forms a vesicle containing extracellular fluid; → whatever molecules were in the extracellular fluid are now inside the cell. Pinocytosis is also sometimes referred to as “cell drinking”. Modified from: Jacek FH, derivative work from Mariana Ruiz Villarreal "LadyofHats“, Public domain  Unclassified / Non classifié Biological Membranes – Exocytosis and Endocytosis 2) Phagocytosis Is almost the opposite of pinocytosis. The plasma membrane bulges outwards and engulfs one or more particles; → forming a vesicle (vacuole) This type of endocytosis is more specific than pinocytosis; the cell has a particle(s) in mind that it wants to engulf. Mediated by the cytoskeleton. E.g. amoeba feeding, or mammalian macrophage (part of the immune system). PM reaches outwards cytosol Phagocytosis involves the plasma membrane reaching outwards a particle(s). vesicle (vacuole) Laboratoires Servier Creative Commons Attribution- Share Alike 3.0 outside plasma membrane  Unclassified / Non classifié Biological Membranes – Exocytosis and Endocytosis 3) Receptor-Mediated Endocytosis Involves a “coated pit” in the plasma membrane. The pit is coated on the cytosolic side by a layer of protein (protein = “clathrin”). Within the pit itself are located a group of receptors that are specific for a target molecule. Image by Nimrat Gill Binding of the target molecules to at least some of the receptors causes the pit to close and form a vesicle. The molecules are the released from the vesicle to the cytosol. The receptors recycle back to the plasma membrane, re-forming the coated pit. E.g. iron uptake by mammalian cells.  Unclassified / Non classifié Bonus Images – Exocytosis and Endocytosis In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. OpenStax Biology 2e In phagocytosis, the cell membrane surrounds the particle and engulfs it.  Unclassified / Non classifié Bonus Images – Exocytosis and Endocytosis In receptor-mediated endocytosis, the cell targets a specific type of molecule that binds to receptors on the plasma membrane's external surface. The receptors are aggregated in a “pit” lined with the protein clathrin on the cytoplasmic side. Binding of a sufficient number of molecules to the receptors triggers endocytosis. The bound molecules are removed from the receptors, and the receptors are recycled back to the plasma membrane. OpenStax Biology 2e  Unclassified / Non classifié Biological Membranes – Bulk Transport Review about Bulk Transport Endocytosis and exocytosis – for moving large molecules into and out of the cell Exocytosis To transport particles out of the cell Packaged into vesicle and fuses with plasma membrane and contents release outside the cell Endocytosis 1. Pinocytosis Bulges inward taking in molecules/water with it until it fuses off from plasma membrane and becomes a vesicle; “cell drinking” 2. Phagocytosis Bulges outward from plasma membrane, capturing specific particles and forms a vesicle 3. Receptor-mediated endocytosis Bulges inward; cytosolic side coated with clathrin and inside contains molecule- specific receptors; once some molecules bind to receptors, the pit closes and forms a vesicle Study Tip: One technique to help you remember the different types of transport is to create a table with one column that contains each type of transport, a second column stating whether the transport is active/passive, and a third column containing the material transport.