Membrane Transport Educational Materials

how is the drug pathway via oral administration? drug enter mouth absorbed throguh GIT enters blood stream reaches site of action what does barriers the wall of the GIT to reach bloodstream and the bloodstream to organs have in common? cell membrane Function of biological membranes 1) Define cell boundary 2) Define enclose compartments 3) Control movement of material into and out of cell 4) Allow response to external stimuli 5) Enable interactions between cells 6) Provide scaffold for biochemical activities functions of plasma membrane physical isolation, regulation of exchange with the environment, sensitivity to the environment, structural support - Defines the external boundaries & separates cells from its environment - Holds cell constituents together - Prevents materials from entering serves as a semi-porous barrier to external environment - Allows selective molecules to enter and exit describe the fluid mosaic model? Two layers (Bilayers) of lipid molecules (phospholipids) plus proteins (or glycoproteins) Lipid bilayer: ~5 - 8 nm thick Within monolayer (one layer): there are 2 monolayers forming a bilayer - Polar head (phosphate) groups are hydrophilic and face outwards (in contact with aqueous medium) - Nonpolar lipid tails (hydrophobic) face toward the bilayer interior 1 / 13 Protein arrangement also related to solubility where do cells gain their raw materials for biosynthesis? surroundings Cells must also release byproducts of metabolism to external environment This is achieved by selective permeabilihow are cells able to release byproducts ty of cell membranes: of metabolism, and allow entry of specif- Permeable to specific molecules (nutriic molecules? ents & other essential material) required by cell processes - Non-permeable to unwanted material Lipid molecules form a highly impermeable barrier to most polar and charged how are cell membranes able to display molecules selective permeability? Therefore, water-soluble components of the cell are retained within membranes - Hydrophobic (lipid soluble) molecules and small uncharged molecules display good permeability - However, the hydrophobic core of the what type of molecules have high memlipid bilayer is an almost impenetrable brane permeability? what type of molebarrier to most polar or charged molecules have now permeability? cules - Small ions (e.g. Na+, K+, and Cl) have very low permeability - unable to diffuse across membranes hydrophilic and polar molecules will why do polar or charged molecules struggle to cross the membrane because struggle to cross the membrane via simthe membrane is lipid soluble as it is lipid ple diffusion? (hydrophilic molecules) based how can materials cross the plasma membrane by different routes? 1) Passive diffusion - Involves passage through lipid portion of cell membrane - Does not involve transport proteins - Does not use cellular energy 2 / 13 2) Specialised Mechanisms - Involved 'carrier' (transport proteins) - Carrier mediated transport - May / may not use cellular energy Passive transport does not require enerwhat makes a transport mechanism pas- gy sive? - Driven by concentrations of electrical gradients Active transport requires energy - Works against concentration / electrical what makes a transport mechanism acgradient tive? - Use protein carriers to mediate the transport It is important to note that for a molecule to pass across a cell membrane, regardless of the transport mechanism, it has to be 'in solution' in either the extracellular or intracellular fluid why do drugs need to be in solution for - Molecule needs to be dissolved theretransport? fore needs to have high water solubility to enter and be transported across the membrane 'In solution' - means the molecule is dissolved - i.e. molecularly dispersed what mechanism has an exception to molecules being in solution? Only exception to this is fluid phase pinocytosis - not an important mechanism of drug transport - Entering cell via engulfing rather than dissolving so drug doesn't need to be dissolved in solution what is simple diffusion? .... two aqueous compartments containing unequal concentrations of a soluble compound or ion, separated by a permeable membrane - The solute moves through the membrane from the region of higher concentration to the region of lower concentra3 / 13 tion - Spontaneous process driven by an increase in entropy and therefore a decrease in free energy - No requirement for energy -The rate of movement depends on the difference in concentrations (concentrawhat does passive simple diffusion de- tion gradient) between the two sides -Diffusion will occur until the two compend on? partments have equal solute concentrations Passive has a straight-line relation because the higher concentration gradient the higher the rate of movement. Active transport has a plateau because explain the graph of rate of absorption relies on ATP and there are only a certain and drug concentration at absorption number of protein carriers. Once protein site for both passive and active transport carriers are all occupied, any further increase in material concentration will not increase absorption as no more protein carrier active sites are available for transport Molecules do not cross membrane in straight lines but rather by tortuous paths explain the random of movement of mol- Molecules move under the influence of ecules Brownian motion, however overall motion (i.e., flux) is in the direction of the concentration gradient Small, lipid soluble molecules - Charged molecule cannot be transportwhat type of molecules is simple passive ed across the membrane diffusions applicable to? - Large molecules too will struggle to cross the membrane name the nonpolar gases and which method they cross the cell membrane? 4 / 13 O2 and CO2 simple diffusion Hydrophobic molecules: - Steroid hormones - Lipid soluble vitamins which hydrophobic molecules cross the - Small, lipid soluble drugs cell membrane by simple diffusion Large polar molecules are repelled by the hydrophobic interior of lipid bilayers, therefore unable to cross membranes by diffusion - In order for a molecule to passively cross a cell membrane it must: - Break any bonds (e.g. hydrogen bonds) with the aqueous medium (step A) - Enter the lipid region (step B) explain the processes involved in pas- Cross the lipid region via diffusion (step sive diffusion C) - Leave the membrane at the other side (step D) - Reform any bonds with the aqueous medium (step E) what is the partitioning step? - depend upon the relative solubilities of the molecule in the aqueous medium and lipid bilayer - Steps A&B and D&E are what is the diffusive process step c Note: both processes govern the rate of what is common to the partitioning steps molecular transport across the cell memand diffusive process brane via passive diffusion When crossing the lipid bilayer, a hydrophilic or charged solute must: 1) Give up interactions with water molecules in its hydration shell explain the energy changes associated 2) Diffuse through lipid in which it is poorwith membrane transport ly soluble This transmembrane passage is a high-energy state and an activation barrier must be overcome: - If drug is very water soluble, the drug 5 / 13 needs to leave its hydrophilic area and enter lipid environment. lots of energy is needed to break all the bonds surrounding the water-soluble drug - hence drug cannot enter by simple passive diffusion Rate of diffusion depends upon several factors, in particular: 1. The diffusion coefficient (D) of the molecule in the membrane - e.g. the size of the molecule and the nature of the membrane (molecular weight cut off ~600 across the intestinal membrane but only ~400 across the skin) what does the rate of diffusion depend - The smaller the molecule the higher on? the rate of diffusion across the cell membrane 2. Concentration gradient across the membrane 3. Thickness of the membrane - thicker means slower the rate of diffusion 4. Surface area - larger = quicker the rate diffusion What is Fick's Law of Diffusion? rate of diffusion = (surface area x diffusion coefficient x concentration gradient) ------------------------------------distance of which diffusion occurs - equation applies only for uncharged molecules 6 / 13 Explain faciliated diffusion Like passive diffusion, involves the movement of molecules in the direction determined by their relative concentrations inside and outside of the cell Molecules travel across the membrane in the direction determined by their concentration gradients and, in the case of charged molecules, by the electric potential across the membrane Facilitated diffusion however differs from passive diffusion in that the transported molecules do not dissolve in the lipid bilayer; instead, their passage is mediated by proteins which enable the transported molecules to cross the membrane without directly interacting with its hydrophobic interior Facilitated diffusion therefore allows polar and charged molecules (e.g. carbohydrates, amino acids, nucleosides, and ions) to cross the plasma membrane Facilitated diffusion however differs from passive diffusion in that the transported molecules do not dissolve in the lipid biwhat is the difference between facilitated layer; instead, their passage is mediated diffusion and passive diffusion by proteins which enable the transported molecules to cross the membrane without directly interacting with its hydrophobic interior Two classes of proteins mediate facilitated diffusion: carrier proteins and channel proteins • Carrier proteins bind specific cargo on explain what are the mediators of facilione side of the membrane then undergo tated diffusion? conformational changes that allow the molecule to pass through the membrane and be released on the other side • Channel proteins form open pores 7 / 13 through the membrane, allowing the free diffusion of the molecule - Most cells are exposed to extracellular glucose concentrations that are higher than those inside the cell - Facilitated diffusion therefore results in the net inward transport of glucose - Once glucose is taken up by these cells it is rapidly metabolised, so intracellular explain the facilitated diffusion of glucose glucose concentrations remain low and glucose continues to be transported into the cell from extracellular fluids - Note, that glucose can be transported in the opposite direction. Such reverse flow occurs, for example, in liver cells, in which glucose is synthesised and released into the circulation what are ion channels Channel proteins simply form open pores in the membrane, allowing small molecules of the appropriate size and charge to pass freely through the lipid bilayer Transport is extremely rapid Ion channels are highly selective because narrow pores in the channel restrict passage to ions of the appropriate size and charge 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 (e.g. ligands or electric potential) what are the different types of active transport based on? Categorised based on whether they use ATP directly and the size of the molecules being transported 8 / 13 what are the different types of active transport? explain them • Primary Active Transport: Active transport of small molecules that directly use ATP as an energy source • Secondary Active Transport: Active transport of small molecules that uses an established electrochemical gradient to power the movement • Endocytosis and Exocytosis: Movement of very large molecules (such as proteins and carbohydrates) across a membrane using vesicles what is primary active transport In primary active transport, ATP directly serves as the energy source driving the transport of the molecules across the membrane All primary transport proteins are therefore also ATPases, enzymes capable of hydrolysing ATP and releasing its energy what is secondary active transport In secondary active transport, ATP is not used as the primary energy source powering transport Instead, the energy required for the transport of a certain molecule comes from moving a second molecule down its electrochemical gradient Thus, secondary active transport always involves either symport or antiport proteins - Uniport transports one solute at a time - Symport transports the solute and a co-transported solute at the same time in the same direction what is the subcatagories of primary and - Antiport transports the solute in (or out) and the co-transported solute the opsecondary active transport posite direction. One goes in the other goes out or vice-versa e.g., Na+-Ca2+ antiporter that transports Na+ into the cell and Ca2+ out 9 / 13 - Resting potential is -70 maintained by the Na K ion pump - Low sodium concentration and high K inside cell - 2 sodium out for every 3 potassium in what is the Na+ and K+ ATPase (P class) Efflux transporter proteins found in the membrane of cells prevent toxins from entering cells Drug is pumped out by efflux transexplain the role of efflux transporter proporters teins - Once drug enters GI, can have poor absorption because it acts as a substrate for the efflux transporter and is pumped out the cell what is the role of drug transporters> Recognised as key players in the processes of drug absorption, distribution, metabolism, and elimination by controlling drug transport across biological barriers and and drug access to metabolising enzymes and excretory pathways Drug transporters in both the gut and the liver can help control access of drugs to systemic circulation by dictating the amount of drug that enters the body from the gut lumen and influencing how much drug escapes first pass metabolism in both gut and liver. Both uptake and efflux transporters are important in determining oral drug bioavailability what is the selected human transport proteins for drugs and endogenous substances 1. Intestinal epithelia 2. Hepatocytes 3. Kidney proximal tubules 4. Blood brain barrier ATP binding cassette (ABC) transporters ABC transporters convert the energy 10 / 13 what are ABC transporters? gained from ATP hydrolysis into trans membrane movement of substrates either into the cytoplasm (import) or out of the cytoplasm (export) Efflux transporters expressed in the inwhat is the relation between efflux transtestine and liver are all members of the ported and ABD transporters ABC superfamily give examples of ABC transporters - P-glycoprotein (Pgp, ABCB1) - Multidrug resistance proteins (MRP16, ABCC1-6) - d breast cancer resistance protein (BCRP, ABCG2) ATP hydrolysis is catalysed by a pair of cytoplasmic ABCs, whereas the translohow is ATP hydrolysis related to ABCs cation of the substrate is facilitated by a pair of transmembrane domains P-gp (ABC transporter) mediates the ATP-dependent export of drugs from cells Expressed in the luminal membrane of the small intestine and blood-brain barrier, and in the apical membranes of excretory cells such as hepatocytes and kidney proximal tubule epithelia what are p glycoproteins (P-gp), where P-gp has an important role in limiting are they found, and their role entry of various drugs into the central nervous system Also plays a part in the intestinal absorption and in the biliary and urinary excretion of drugs P-gp is responsible for the efflux across biological membranes of a broad range of therapeutic drugs, including anticancer drugs 11 / 13 'Macromolecules' are very large molecules made from a large number of smaller molecule Term is applied to: what are macromolecules? what type of • Three biopolymers, nucleic acids, promolecule is the term applied to teins and carbohydrates • Non-polymeric molecules with large molecular mass (e.g. lipids) The individual constituent molecules of macromolecules are called monomers Very large molecules or macromolecules require more complex mechanisms to enter the cells how do macromolecules enter and exit - Membrane permeability of hydrophilic cells? macromolecules is negligible Macromolecules enter and exit the cells selectively through the processes of endocytosis and exocytosis, respectively what is endocytosis? 'membrane invagination to form a vesicle, moving the enclosed materials inside the cell' Phagocytosis- ('cell eating').The cell engulfs sizable objects (e.g. debris & bacteria). Occurs in specialized cells. Pinocytosis- ('cell drinking'). The cell engulfs extracellular fluid, in which molewhat are the different forms of endocytocules are present. Material enters the cell sis? inside a vesicle, but does not mix with cytoplasm. Receptor-mediated endocytosis allows cells to take up specific macromolecules called ligands. 12 / 13 Required not only for uptake of nutrients, but also to regulate the amount of cell surface receptors Cells utilise multiple endocytic pathways, depending on the material to be transported (cargo) what is endocytosis needed for? Uptake of material by endocytosis requires formation of membrane invaginations - if the vesicle doesn't leave, it is removed by the lysosome Endocytosis important in the area of pharmaceutical science, e.g.: - Drug delivery to cells by nanoparticles transcytosis vs endocytosis s. Transcytosis — What's the Difference? https://www.askdifference.com/endocytosis-vs-transcytosis/ Endocytosis and Transcytosis are two processes that involve the movement of substances across a cell12. The main difference between the two lies in their purpose and end result. Endocytosis is mainly concerned with the intake of substances into a cell, while transcytosis emphasizes the passage of these substances across the cell, eventually releasing them on the opposite side1. Endocytosis involves the formation of vesicles, while transcytosis does not and is used to transport larger molecules2. 13 / 13