Transport Across Cell Membrane PDF

Transport Across Cell Membrane  Terms to Know Concentration – the amount of solute in a solution. Solute – the dissolved substance in a solution. Solution – a mixture in which two or more substances are mixed evenly. Concentration gradient - the gradual difference in the conc...

Transport Across Cell Membrane  Terms to Know Concentration – the amount of solute in a solution. Solute – the dissolved substance in a solution. Solution – a mixture in which two or more substances are mixed evenly. Concentration gradient - the gradual difference in the concentration of solutes in a solution between two regions. What is a cell membrane? The cell membrane, also known as the plasma membrane is a phospholipid bilayer that separates the cell interior from the extracellular space→ semi- permeable or selectively permeable Function of the Cell Membrane: Cell membrane separates the components of a cell from its environment “Gatekeeper” of the cell—regulates the flow of materials into and out of the cell—selectively permeable to ions and organic molecules Cell membrane helps cells maintain homeostasis—stable internal balance It plays a role in cell adhesion, cell signaling, and attachment surface for the cytoskeleton to give the shape to the cells and attached to the extracellular matrix to hold the cells together in tissues. Cell Membrane and Cell Wall: ALL cells have a cell membrane made of proteins and lipids protein channel Layer 1 Cell Membrane Layer 2 lipid bilayer protein pump All cells have cell membrane, but only SOME cells have cell walls – ex: plants, fungi, and bacteria Cell Membrane Cell Wall  Plant cells have a cell wall made of cellulose – that cellulose is fiber in our diet Bacteria and fungi also have cell walls, but they do not contain cellulose but peptidoglycan and chitin, respectively. Cell membranes and cell walls are porous allowing water, carbon dioxide, oxygen, and nutrients to pass through easily Composition of the cell membrane 1. Phospholipid bilayer - has polar heads on the outside in contact with water and nonpolar tails inside the bilayer. Unsaturated fatty acids in the lipid hydrophobic tails increase the membrane fluidity. The more the proportion of unsaturated fatty acids the higher the fluidity. 2. Cholesterol - interspersed between phospholipids gives rigidity to the membrane. The more proportion of cholesterol, the more rigid the membrane. 3. Proteins: a. Integral proteins - span the membrane and serve as transporters of species b. Peripheral proteins - are loosely attached to the outer side of the membrane and act as enzymes to facilitate the interaction with the cell's environment.  Transport through the cell membrane some substances needed by the cell need to enter some products like wastes need to exit the cell. Mechanisms of selective movement of substances in and out of the cell 1. PASSIVE TRANSPORT 2. ACTIVE TRANSPORT Cellular Transport Weeee!!! Passive Transport Cell doesn’t use energy – downhill movement 1. Diffusion high 2. Facilitated Diffusion 3. Osmosis low Active Transport Cell uses energy – uphill movement This is 1. Protein Pumps gonna be hard 2. Endocytosis work!! high 3. Exocytosis low Diffusion Diffusion is the movement of small particles (e.g. ions) across a selectively permeable membrane like the cell membrane until equilibrium is reached. These particles move from an area of high concentration to an area of low concentration. outside of cell inside of cell DIFFUSION Diffusion of Ions through plasma membrane The lipid bilayer of the biological membranes is highly impermeable (meaning “not allowing fluid to pass through) to charged substances including small ions such as Na+, K+, Ca2+ and Cl- However, rapid conductance of these ions across the membrane plays a critical role in multiple cellular activities, including the formation and propagation of nerve impulses, secretion of substances into the extracellular space, muscle contraction, regulation of cell volume, and the opening of stomatal pores on plant leaves. Diffusion of Ions through plasma membrane The ion channels are formed by integral proteins Most ion channels are highly selective in allowing only one particular type of ions to pass through the pore Passage of ions through a channel is always downhill Most ion channels are “gated”. The opening and closing of the gates are subject to complex regulations and can be induced by a variety of factors. Diffusion of Ions through plasma membrane Two major types of gated channels: 1. Voltage-gated channels - open and close upon changes in the transmembrane potential (or the ionic charge on the two sides of the membrane). Example: Na+ channel 2. Ligand-gated channels - form a pore through the plasma membrane that opens when a signaling molecule binds, allowing ions to flow into or out of the cell Facilitated Diffusion Facilitated Diffusion is the movement of larger molecules like glucose through the cell membrane – larger molecules must be “helped” Or simply, it is a type of diffusion in which the molecules move from the region of higher concentration to the region of lower concentration assisted by a carrier. Proteins in the cell membrane form channels (pores) for large molecules to pass through Proteins that form channels (pores) are called protein channels Glucose molecules outside of cell File:Scheme facilitated diffusion in cell membrane-en.svg Osmosis Osmosis is the diffusion of water through a selectively permeable membrane like the cell membrane Water diffuses across a membrane from an area of high concentration to an area of low concentration. Semi-permeable membrane is permeable to water, but not to sugar Three Osmotic Solutions Hypertonic solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel or shrink → plasmolysis Hypotonic solutions: contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly burst or explode/lysis. Isotonic solutions: contain the same concentration of solute as another solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic. File:Osmotic pressure on blood cells diagram.svg Osmosis—Elodea Leaf When salt is added, The cell losses water, and the internal pressure drops. The cell shrivels, the cell membrane detaches from the wall and constricts the cytoplasm. https://www.youtube.com/watch?v=GOxouJUtEhE  Effects of Osmosis on Life Osmosis Animations for isotonic, hypertonic, and hypotonic solutions Hypotonic Solution Hypotonic: The solution has a lower concentration of solutes and a higher concentration of water than inside the cell. (Low solute; High water) Result: Water moves from the solution to inside the cell): Cell Swells and bursts open (cytolysis)! - cell will die if burst  Osmosis Animations for isotonic, Hypertonic Solution hypertonic, and hypotonic solutions Hypertonic: The solution has a higher concentration of solutes and a lower concentration of water than inside the cell. (High solute; Low water) shrinks Result: Water moves from inside the cell into the solution: Cell shrinks (Plasmolysis)! – cell may die  Osmosis Animations for isotonic, Isotonic Solution hypertonic, and hypotonic solutions Isotonic: The concentration of solutes in the solution is equal to the concentration of solutes inside the cell. Result: Water moves equally in both directions and the cell remains of the same size! (Dynamic Equilibrium) Aquaporins: o Many cells are much more permeable to water because of the presence of aquaporin. o Aquaporin is a class of integral proteins that allow the passive movement of water from one side of the plasma membrane to the other side. o Billions of water molecules – moving in single file- can pass through each channel every second o Aquaporins are particularly prominent in cells, such as those of a kidney tubule or plant root, where the passage of water plays a crucial role in the tissues’ physiological role.  Aquaporins in kidneys: o The hormone called vasopressin, stimulates water retention by the collecting duct of the kidney, acts by way of these aquaporins particularly called Aquaporin 2 (AQP 2). o these aquaporins mediate rapid water transport across water- permeable epithelia and play critical roles in urinary concentrating and diluting processes. Inherited disorder due to mutations in the aquaporin channel Congenital nephrogenic diabetes insipidus Persons suffering from this disease excrete large quantities of urine because their kidneys do not respond to vasopressin What type of solution are these cells in? A B C Hypertonic Isotonic Hypotonic  How Organisms Deal with Osmotic Pressure? ✓ Bacteria and plants have cell walls that prevent them from over-expanding. In plants, the pressure exerted on the cell wall is called turgor pressure. ✓ A protist like Paramecium has contractile vacuoles that collect water flowing in and pump it out to prevent them from over-expanding. ✓ Salt water fish pump salt out of their specialized gills so they do not dehydrate. ✓ Animal cells are bathed in blood. Kidneys keep the blood isotonic by removing excess salt and water.  Active Transport Active transport is the movement of molecules from LOW to HIGH concentration. Energy is required as molecules must be pumped against the concentration gradient. Proteins that work as pumps are called protein pumps. Example: sodium-potassium pumps ANALOGY: ENERGY NEEDED: Active Transport NO ENERGY NEEDED: Diffusion Osmosis Facilitated Diffusion  Sodium-Potassium Pump One of the most important pumps in animal 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. The Na+-K+ ATPase exists in two forms, depending on its orientation to the cell's interior or exterior and its affinity for either sodium or potassium ions.  Sodium-Potassium Pump One of the most important pumps in animal 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. The Na+-K+ ATPase exists in two forms, depending on its orientation to the cell's interior or exterior and its affinity for either sodium or potassium ions. Sodium-Potassium Pump The process consists of the following six steps: 1. With the enzyme oriented towards the cell's interior, the Several things have happened as a result carrier has a high affinity for sodium ions. Three ions of this process: bind to the protein. ✓ At this point, there are more sodium 2. The protein carrier hydrolyzes ATP and a low-energy ions outside the cell than inside and phosphate group attaches to it. more potassium ions inside than out. 3. As a result, the carrier changes shape and reorients ✓ For every three sodium ions that move itself towards the membrane's exterior. The protein’s out, two potassium ions move in. affinity for sodium decreases and the three sodium ions ✓ This results in the interior being slightly leave the carrier. more negative relative to the exterior. 4. The shape change increases the carrier’s affinity for ✓ This difference in charge is important potassium ions, and two such ions attach to the protein. in creating the conditions necessary for Subsequently, the low-energy phosphate group the secondary process. detaches from the carrier. ✓ The sodium-potassium pump is, 5. With the phosphate group removed and potassium ions therefore, an electrogenic pump (a attached, the carrier protein repositions itself towards pump that creates a charge the cell's interior. imbalance), creating an electrical 6. The carrier protein, in its new configuration, has a imbalance across the membrane and decreased affinity for potassium, and the two ions move contributing to the membrane into the cytoplasm. The protein now has a higher potential. affinity for sodium ions, and the process starts again. Electrical imbalance across the membrane and contributing to the membrane potential Is important in cellular processes like generation of nerve impulses. Co-transport or Secondary Active Transport If the pH outside the cell decreases, would you expect the amount of amino acids transported into the cell to increase or decrease? An electrochemical gradient (Na+ concentration - green), is generated by primary active transport. The energy stored in the Na+ gradient provides the energy to move other substances against their concentration gradients (Glucose - blue), a process called co-transport or secondary active transport. Credit: Rao, A., Ryan, K., Tag, A. and Fletcher, S. Department of Biology, Texas A&M University. Bulk Transport Endocytosis and Exocytosis is the mechanism by which very large molecules (such as food and wastes) get into and out of the cell Food is moved into the cell by Endocytosis Wastes are moved out of the cell by Exocytosis Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: the plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane.  Pinocytosis Phagocytosis A variation of endocytosis is called pinocytosis. This Phagocytosis (the condition of “cell eating”) is the literally means “cell drinking” and was named at a process by which large particles, such as cells or time when the assumption was that the cell was relatively large particles, are taken in by a cell. For purposefully taking in extracellular fluid. In reality, this example, when microorganisms invade the human is a process that takes in molecules, including water, body, a type of white blood cell called a neutrophil will which the cell needs from the extracellular fluid. remove the invaders through this process, surrounding Pinocytosis results in a much smaller vesicle than does and engulfing the microorganism, which is then phagocytosis, and the vesicle does not need to merge destroyed by the neutrophil. with a lysosome. Receptor-mediated Endocytosis Employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances. In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane. If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by the 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 LDL particles from their blood. Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross- react with normal receptor-binding sites and gain entry into cells. Exocytosis Exocytosis is a form of active transport and bulk transport in which a cell transports molecules out of the cell. As an active transport mechanism, exocytosis requires the use of energy to transport material. 1. membrane surrounding the material fuses with cell membrane 2. Cell changes shape – requires energy Ex. Hormones or wastes released from cell Examples of the three exocytosis pathways: a. The constitutive secretory pathway (orange arrow) directly delivers fresh membrane lipids and proteins to the cell membrane. b. The regulated pathway (green arrows) is used for specific cargo like hormones and neurotransmitters. Secretory vesicles traffic and accumulate at the membrane, where they await a signal for release. c. The lysosomal pathway directly transport cargo to the membrane (red arrow), or digest and eject debris out of the cell (pink arrows). For You to Explore: Below are inherited disorders involving specific ion channels. What channels are involved, and the consequences of the disorders. 1. Cystic fibrosis 2. Cardiac arrhythmias 3. Familial hemiplegic migraine (FHM) 4. Hypokalemic periodic paralysis 5. Benign familial monotonal convulsions