Transport Across Cell Membranes PDF 2022-2023

Transport Across Cell Membranes Asst. Prof. Samet UÇAK Dept. of Medical Biology and Genetics 1  Transport of Small Molecules The internal composition of the cell is maintained because the plasma membrane is selectively permeable to sma...

Transport Across Cell Membranes Asst. Prof. Samet UÇAK Dept. of Medical Biology and Genetics 1  Transport of Small Molecules The internal composition of the cell is maintained because the plasma membrane is selectively permeable to small molecules. Most biological molecules are unable to diffuse through the phospholipid bilayer, so the plasma membrane forms a barrier that blocks the free exchange of molecules between the cytoplasm and the external environment of the cell. 2 2  Transport of Small Molecules Specific transport proteins (carrier proteins and channel proteins) then mediate the selective passage of small molecules across the membrane, allowing the cell to control the composition of its cytoplasm. 3 3  Passive Diffusion The simplest mechanism by which molecules can cross the plasma membrane is passive diffusion. During passive diffusion, a molecule simply dissolves in the phospholipid bilayer, diffuses across it, and then dissolves in the aqueous solution at the other side of the membrane. 4 4 5 6 7  Passive Diffusion No membrane proteins are involved. Passive diffusion follows a concentration gradient, which relates to the transport of molecules from a location of higher concentration to a site of lower concentration. Passive diffusion is thus a nonselective process by which any Leukocyte molecule able to dissolve in the phospholipid bilayer is able to cross the plasma membrane and equilibrate between the inside and outside of the cell. 8 8  Passive Diffusion polar duncharged Importantly, only small, relatively hydrophobic -- molecules are able to diffuse across a phospholipid bilayer at significant rates. Thus gases (such as O2 and CO2), hydrophobic => molecules (such as benzene), and small polar but - - uncharged molecules (such as H O and ethanol) 2 are able to passively diffuse across the plasma membrane. Not require energy. 9 9 10  all allow Some leave - - - - - almost leave - - all leave 11  Facilitated Diffusion Facilitated diffusion, like passive diffusion, involves the movement of molecules in the direction determined by their relative concentrations inside - and outside of the cell. Not require energy. However, facilitated diffusion differs from passive diffusion in that the transported molecules do not dissolve in the phospholipid bilayer. 12 * 12  Facilitated Diffusion Instead, their passage is mediated by proteins = that enable the transported molecules to cross the membrane without directly interacting with its hydrophobic interior. Facilitated diffusion therefore allows polar and - - charged molecules, such as carbohydrates, amino acids, nucleosides, and ions, to cross the plasma membrane. 13 13  Facilitated Diffusion Two classes of proteins that mediate facilitated diffusion: Carrier proteins and Channel proteins. 14 14 15  Facilitated Diffusion and Carrier Proteins Carrier proteins bind = specific molecules to be T transported on one side of the membrane. - They then undergo conformational changes - that allow the molecule to pass through the membrane and be released on the other side. - - 16 16  = - - 17  Facilitated Diffusion and Carrier Proteins Carrier proteins are responsible for the facilitated diffusion of sugars, amino acids, and nucleosides across the plasma membranes of most cells. The uptake of glucose, which serves as a primary source of metabolic energy, is one of the most - important - transport functions of the plasma membrane, and the glucose transporter provides a - well-studied example of a carrier protein. 18 18 19  Facilitated Diffusion and Channel Proteins Channel proteins form open pores through the - membrane, allowing the free diffusion of any - -o molecule of the appropriate size and charge. 20 20 21  Facilitated Diffusion and Channel Proteins Channel proteins form open pores through the - membrane, allowing the free diffusion of any - size andE molecule of the appropriateE charge. Channel proteins are either open at all times or - they are “gated,” which controls the opening of - - the channel. 22 22 - 23  Osmosis Water molecules are - small and uncharged, they can diffuse directly across the lipid bilayer— although slowly. - The plasma membranes of many cells also contain water channel proteins (aquaporins) through which water - molecules are able to cross the membrane much more⑤ rapidly than they can diffuse through - the phospholipid bilayer. 24 24 - - 25 26 26 -- 27 27  Tonicity Tonicity describes the amount of solute in a > - solution. The measure of the tonicity of a solution, or the total amount of solutes dissolved in a specific amount - - of solution, is called its osmolarity. Three terms — Hypertonic, Isotonic, and Hypotonic are used to relate the osmolarity of a cell. 28 28  Hypertonic Solution In a hypertonic solution, the fluid contains less - - water than the cell does, such as seawater. - Because the cell has a lower concentration of - solutes, the- water will leave the cell. > - In effect, the solute is drawing the water out of the - cell. This may cause an animal cell to shrivel, or - crenate. shrink - 29 29  Isotonic Solution In an isotonic solution, the extracellular fluid has = the - same osmolarity as the= cell. If the concentration of solutes of the cell matches - - & that of the extracellular fluid, there will be no net - movement =>> of water into or out of the cell. 30 30  Hypotonic Solution In a hypotonic solution, the extracellular fluid has more water a lower concentration of solutes than the fluid inside the cell, and water enters the cell. - Extracellular fluid has a higher - concentration of e water than does the cell. => & This may cause an animal cell to burst, or lyse. - = = Swell up 31 31 32 33 33  Active Transport Driven by ATP Hydrolysis Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine => triphosphate (ATP). - If a substance must move into the cell (and vice - - versa) against its concentration gradient, that is, if the concentration of the substance inside the cell must - be greater than its concentration in the extracellular - - - - fluid, the cell must use energy to move the substance. 34 34 - 35 high W 10 36 37  Active Transport Electrochemical Gradient The interior of living cells is electrically => negative. Cells have higher concentrations of potassium - - (K+) and lower concentrations of sodium (Na+) - - - than does the extracellular fluid. - 38 38  Active Transport Electrochemical Gradient The concentration gradient of Na+ tends to drive it - = - into the cell, and the electrical gradient of Na + also e tends to drive it inward to the negatively charged interior. The electrical gradient of K+, also tends to drive it - into the cell, but the concentration gradient of K+ e - - tends to drive K+ out of the cell. 39 39 40 40  Active Transport Moving Against a Gradient Active transport mechanisms, collectively called pumps. Active transport maintains concentrations of ions and other substances. Much of a cell’s supply of metabolic energy may be spent maintaining these processes. (Most of a red blood cell’s metabolic energy is used to maintain the imbalance between exterior and interior sodium and potassium levels required by the cell.) 41 41 42 42  Ion Channels The best characterized channel proteins, however, are the ion channels, which mediate the passage of ions across plasma membranes. Although ion channels are present in the membranes of all cells, they have been especially well studied in nerve and muscle, where their regulated opening and closing is responsible for the transmission of electric signals. 43 43  Ion Channels Three properties of ion channels: First, transport through channels is extremely rapid. More than a million ions per second flow through open channels a flow rate approximately a thousand times greater than the rate of transport by carrier proteins. 44 44  Ion Channels Three properties of ion channels: Second, ion channels are highly selective because narrow pores in the channel restrict -- passage to ions of the appropriate size and - - charge. Thus specific channel proteins allow the passage of Na+, K+, Ca2+, and CI- across the - membrane. 45 45 Figure 15.17 Ion gradients and resting membrane potential of the giant squid axon Only the concentrations of Na+ and K+ are shown because these are the ions that function in the transmission of nerve impulses. Na+ is pumped out of the cell while K+ is pumped in, so the concentration of Na+ is higher outside than inside of the axon, whereas the concentration of K+ is higher inside than out. The resting membrane is more permeable to K+ than to Na+ or other ions because it contains open K+ channels. The flow of K+ through these channels makes the major contribution to the resting membrane potential of –60 mV, which is therefore close to the K+ equilibrium potential. 46  Ion Channels Three properties of ion channels: Third, most ion channels are not permanently open. Instead, the opening of ion channels is regulated by "gates" that transiently open in response to specific - stimuli. - O Some channels (called ligand - gated channels) open - = in response to the binding of neurotransmitters or other signaling molecules; others (voltage-gated channels) - open in response to changes in electric potential across the plasma membrane. 47 47 48  Active Transport Carrier Proteins for Active Transport There are three types of these proteins or transporters. A uniporter carries- one specific ion or molecule. - A symporter carries two different ions or molecules, both in the same direction. => e An antiporter also carries two different ions or molecules, but in different directions. - 49 49 50  Active Transport One of the most important pumps in animals cells is the sodium-potassium pump (Na+-K+ ATPase), - - which maintains the electrochemical gradient (and the correct concentrations of Na+ and K+) in living cells. The sodium-potassium pump moves K+ into the cell - while moving Na+ out at the same time, at a ratio of - three Na+ for every two K+ ions moved in. - 51 51  Active Transport The process consists of the following six steps. 1. With the enzyme oriented towards the interior of - & the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein. = - 2. ATP is hydrolyzed by the protein carrier and a - low-energy phosphate group attaches to it. - 52 52  Active Transport - 3. The carrier changes shape and re-orients itself towards the exterior of the membrane. The protein’s - affinity for sodium decreases and the three sodium - ions leave the carrier. - 4. The shape change increases the carrier’s affinity -- for potassium - ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate = group detaches from the carrier. - 53 53  Active Transport 5. With the phosphate group removed and potassium - - ions attached, the carrier protein repositions itself - towards the interior of the cell. - 6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions - are released into the cytoplasm. - The protein now has a higher affinity for sodium ions, and the process starts again. 54 54 55  Active Transport (Co-transport) An electrochemical gradient, created by active = transport, can move other substances against - their concentration gradients, a process called co-transport. Many amino acids, as well as glucose, enter a cell this way. 56 56 symporter 57  Active Transport: Energy is required. - Active transport (ATP is the “driving force”). = Active transport (Co-transport) (the energy is = provided by an electrochemical gradient). 58 58  Endocytosis Endocytosis is a type of active transport that - moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. The plasma membrane of the cell invaginates, forming a pocket around the target particle. Newly created vacuole that is formed from the plasma membrane. 59 59  Endocytosis There are three variations of endocytosis. Phagocytosis Pinocytosis Receptor-Mediated Endocytosis 60 60  Phagocytosis Phagocytosis is the process by which large particles, such as cells, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil removes the invader through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil. 61 61 62 63  Pinocytosis This literally means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. This process takes in solutes that the cell needs from the extracellular fluid. 64 64 (credit: modification of work by Mariana Ruiz Villarreal) 65  Receptor-Mediated Endocytosis or receptors & The particles bind to the proteins and the plasma folds membrane invaginates, bringing the substance and the proteins into the cell. - If passage across the membrane of the target of - receptor-mediated endocytosis is ineffective, it will - not be removed from the tissue fluids or blood. - O Instead, it will stay in those fluids and increase in - concentration. 66 66  67 (credit: modification of work by Mariana Ruiz Villarreal)  Receptor-Mediated Endocytosis Some human diseases are caused by a failure of receptor-mediated endocytosis. - For example, the form of cholesterol termed low- - density lipoprotein or LDL (also referred to as “bad” - cholesterol) is removed from the blood by receptor- - mediated endocytosis. - In the human genetic disease familial hypercholesterolemia, the LDL receptors are - defective or missing entirely. People with this - - condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear the - - - chemical from their blood. - 68 68  Exocytosis Exocytosis is the opposite of the processes discussed above in that its purpose is to expel material from the cell into the extracellular fluid. A particle enveloped in membrane fuses with the interior of the plasma membrane. This fusion opens the membranous envelope - to the exterior of the cell, and the particle is expelled into the extracellular space. > 69 69 70