Membrane Transport PDF

NFNF1613 BASIC CONCEPT OF CELL FUNCTION Dr Kaisan Mahadi Membrane & Transport By the end of this lecture: Students should be able to: 1. Summarize the components and function of membrane structures. 2. Compare and contrast movement of small and large mol...

NFNF1613 BASIC CONCEPT OF CELL FUNCTION Dr Kaisan Mahadi Membrane & Transport By the end of this lecture: Students should be able to: 1. Summarize the components and function of membrane structures. 2. Compare and contrast movement of small and large molecules across the plasma membrane. 3. Differentiate between cell surface receptors. Fluid Mosaic Model Accepted model for the structure of the plasma membrane. Plasma membrane is a mosaic of components—primarily, phospholipids, cholesterol, and proteins—that move freely and fluidly in the plane of the membrane. Principle components of plasma membrane A phospholipid - lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group. Biological membranes usually involve two layers of phospholipids with their tails pointing inward, an arrangement called a phospholipid bilayer. Cholesterol - another lipid composed of four fused carbon rings, is found alongside phospholipids in the core of the membrane. Membrane proteins - extend partway into the plasma membrane, cross the membrane entirely, or be loosely attached to its inside or outside face. Carbohydrate groups - present only on the outer surface of the plasma membrane and are attached to proteins, forming glycoproteins, or lipids, forming glycolipids. Phospholipids Phospholipids, arranged in a bilayer, make up the basic fabric of the plasma membrane. They are well-suited for this role because they are amphipathic, meaning that they have both hydrophilic and hydrophobic regions. Hydrophilic Water loving Negatively charged phosphate group R group can be polar/charged Facing outward Readily forms electrostatic interactions with water. Hydrophobic Water fearing The fatty acid tails interact with non-polar molecules. Phospholipids Due to amphipathic nature, phospholipids aren’t just well-suited to form a membrane bilayer. They’ll do spontaneously under the right conditions! Eg: In water or aqueous solution, phospholipids tend to arrange themselves with their hydrophobic tails facing each other and their hydrophilic heads facing out. If the phospholipids have small tails, they may form a micelle (a small, single-layered sphere), while if they have bulkier tails, they may form a liposome (a hollow droplet of bilayer membrane). Proteins Second major component of plasma membranes. Integral membrane proteins Peripheral membrane proteins Integrated into the membrane Found on the outside and inside surfaces of membranes Contain hydrophobic regions that Attached to integral proteins or anchor to the hydrophobic core. phospholipids. Some part exposed to outer region of Do not stick into hydrophobic core – the membrane – hydrophilic. loosely attached. Transmembrane proteins – can be one Eg: enzymes on membranes, small (ion channel) or more (GPCR). hydrophobic transporter. TYPEs of cell surface receptors Ion channel-linked receptors G-protein linked receptors Enzyme linked receptors Carbohydrates Third major component of plasma membranes. Found on the outside surface of cells, bound either to proteins or to lipids. Consist of 2-60 monosaccharide units, can be straight or branched. Form distinctive cellular markers together with membrane proteins. (eg: immune cells, blood type). Membrane fluidity Saturated fatty acids have no double bonds (are saturated with hydrogens) - relatively straight. Unsaturated fatty acids, on the other hand, contain one or more double bonds - bend or kink. At cooler temperatures, the straight tails of saturated fatty acids can pack tightly together, making a dense and fairly rigid membrane. Phospholipids with unsaturated fatty acid tails cannot pack together as tightly because of the bent structure of the tails. Because of this, a membrane containing unsaturated phospholipids will stay fluid at lower temperatures than a membrane made of saturated ones.. Membrane fluidity Cholesterol, another type of lipid that is embedded among the phospholipids of the membrane. More cholesterol stiffens membranes by filling in gaps between phospholipids. Cholesterol also helps to minimize the effects of temperature on fluidity. At low temperatures, cholesterol increases fluidity by keeping phospholipids from packing tightly together, while at high temperatures, it reduces fluidity. In this way, cholesterol expands the range of temperatures at which a membrane maintains a functional, healthy fluidity. WATER Movement across membranes Osmosis - net movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. WATER Movement across membranes Osmolarity - total concentration of solutes in a solution. solution with a low osmolarity has fewer solute particles per liter of solution. solution with a high osmolarity has more solute particles per liter of solution. water will move from the side with lower osmolarity to the side with higher osmolarity. Hyperosmotic VS Hypoosmotic VS Isoosmotic. WATER Movement across membranes Tonicity - ability of an extracellular solution to make water move into or out of a cell by osmosis. different from osmolarity because it takes into account both relative solute concentrations and the cell membrane’s permeability to those solutes. Hypertonic VS Isotonic VS Hypotonic Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane. substances will naturally move down their gradients, from an area of higher to an area of lower concentration. some molecules can move down their concentration gradients by crossing the lipid portion of the membrane directly, while others must pass through membrane proteins in a process called facilitated diffusion. Passive transport Selective permeability Diffusion Facilitated diffusion Channels Carrier proteins Selective permeability Phospholipids of plasma membranes are amphipathic. The hydrophobic core helps some material move through the membrane and block others. Polar molecules/charged ions can’t pass through the hydrophobic core easily. diffusion substance tends to move from an area of high concentration to an area of low concentration until its concentration becomes equal throughout a space. Facilitated diffusion Molecules diffuse across the plasma membrane with assistance from membrane proteins. A concentration gradient exists for these molecules, so they have the potential to diffuse into (or out of) the cell by moving down it. For charged or polar molecules, they can't cross the phospholipid part of the membrane without help. Facilitated transport proteins shield these molecules from the hydrophobic core of the membrane, providing a route by which they can cross.  Channels Channel proteins span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion. Selective – only accept one type of molecule. Allows polar and charged molecule to avoid hydrophobic core. Eg: aquaporin, ion gated channel (Na+, K+, Ca2+). Carrier proteins Carrier proteins can change their shape to move a target molecule from one side of the membrane to the other. Selective The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient. Need to change shape and “reset” each time they move a molecule. Active transport Suppose the sugar glucose is more concentrated inside of a cell than outside. If the cell needs more sugar in to meet its metabolic needs, how can it get that sugar in? Active transport: ü Primary active transport üSecondary active transport - cotransport Primary active transport Sodium-potassium pump --> moves Na+ out of cells, and K+ into cells. Maintain correct concentrations of Na+ and K+ in living cells. Generating voltage across the cell membrane in animal cells à electrogenic pump. Sodium potassium pump To begin, the pump is open to the inside of the cell. In this form, the pump really likes to bind (has a high affinity for) sodium ions and will take up three of them. When the sodium ions bind, they trigger the pump to hydrolyze (break down) ATP. One phosphate group from ATP is attached to the pump, which is then said to be phosphorylated. ADP is released as a by-product. Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space. In this conformation, the pump no longer likes to bind to sodium ions (has a low affinity for them), so the three sodium ions are released outside the cell. In its outward-facing form, the pump switches allegiances and now really likes to bind to (has a high affinity for) potassium ions. It will bind two of them, and this triggers removal of the phosphate group attached to the pump in step 2. With the phosphate group gone, the pump will change back to its original form, opening towards the interior of the cell. In its inward-facing shape, the pump loses its interest in (has a low affinity for) potassium ions, so the two potassium ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again. Sodium potassium pump Sodium potassium pump establish membrane potential via: a. 3 Na that move out; only 2 K move in à negative cell interior. b. Building up K concentration gradient inside the cell. The K gradient is steep enough that force potassium ions to move out of the cell via channels. This process continues until the voltage across the membrane is large enough to counterbalance potassium’s concentration gradient. At this balance point, the inside of the membrane is negative relative to the outside. This voltage will be maintained as long as K concentration in the cell stays high but will disappear if K stops being imported. à neuronal cells. Secondary active transport Secondary active transport uses the energy stored from gradients generated from the primary active transport to move other substances against their own gradients. Eg: High concentration of sodium ions in the extracellular space (thanks to the hard work of the sodium-potassium pump). If a route such as a channel or carrier protein is open, sodium ions will move down their concentration gradient and return to the interior of the cell.  In secondary active transport, the movement of the sodium ions down their gradient is coupled to the uphill transport of other substances by a shared carrier protein (a cotransporter). For instance, a carrier protein lets sodium ions move down their gradient, but simultaneously brings a glucose molecule up its gradient and into the cell. The carrier protein uses the energy of the sodium gradient to drive the transport of glucose molecules. Bulk transport Large particles (or large quantities of smaller particles) are moved across the cell membrane. Eg: macrophages. Phagocytosis Pinocytosis Receptor mediated endocytosis Exocytosis Endocytosis Endocytosis (endo = internal, cytosis = transport mechanism) is a general term for the various types of active transport that move particles into a cell by enclosing them in a vesicle made out of plasma membrane. Plasma membrane of the cell invaginates (folds inward), forming a pocket around the target particle or particles. The pocket then pinches off with the help of specialized proteins, leaving the particle trapped in a newly created vesicle or vacuole inside the cell. Subdivided into phagocytosis, pinocytosis, receptor mediated endocytosis. Phagocytosis Large particles, such as cells or cellular debris, are transported into the cell. Once a cell has successfully engulfed a target particle, the pocket containing the particle will pinch off from the membrane, forming a membrane-bound compartment called a food vacuole. Phagocytosis The food vacuole will later fuse with an organelle called a lysosome, the "recycling center" of the cell. Lysosomes have enzymes that break the engulfed particle down into its basic components (such as amino acids and sugars), which can then be used by the cell. PInocytosis Pinocytosis (literally, “cell drinking”) is a form of endocytosis in which a cell takes in small amounts of extracellular fluid. Pinocytosis occurs in many cell types and takes place continuously, with the cell sampling and re-sampling the surrounding fluid to get whatever nutrients and other molecules happen to be present. Pinocytosed material is held in small vesicles, much smaller than the large food vacuole produced by phagocytosis. Receptor-mediated endocytosis Receptor-mediated endocytosis is a form of endocytosis in which receptor proteins on the cell surface are used to capture a specific target molecule. When the receptors bind to their specific target molecule, endocytosis is triggered, and the receptors and their attached molecules are taken into the cell in a vesicle. The coat proteins participate in this process by giving the vesicle its rounded shape and helping it bud off from the membrane. Receptor-mediated endocytosis allows cells to take up large amounts of molecules that are relatively rare (present in low concentrations) in the extracellular fluid. Although receptor-mediated endocytosis is intended to bring useful substances into the cell, other, less friendly particles may gain entry by the same route. Eg: Viruses Exocytosis Exocytosis (exo = external, cytosis = transport mechanism) is a form of bulk transport in which materials are transported from the inside to the outside of the cell in membrane-bound vesicles that fuse with the plasma membrane. Some of these vesicles come from the Golgi apparatus and contain proteins made specifically by the cell for release outside, such as signaling molecules. Other vesicles contain wastes that the cell needs to dispose of, such as the leftovers that remain after a phagocytosed particle has been digested. Thank you