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2. Transport Mechanisms.pdf

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PHGY 209
Erik Cook

Naveen Sooknanan McGill Fall 2011 Transport Mechanisms: Transport mechanisms represent the barriers which solutes much cross in order to more between compartments in the body, and ultimately, between external and internal environments. Membranes are important physiological structures as they perform a variety of functions, the most universally important one being the selective barrier to the passage of various molecules while excluding others. The plasma membrane regulated flow in and out of the cell Membranes between organelles regulate transports between organelles and the cytosol The cell membrane separated the ICF within the cell from the ISF It allows for the flow of ions, small organic molecules such as glucose and lactic acid, gases such as oxygen and carbon dioxide, and waste in and out of the cell The capillary was separates the blood (plasma) from the ISF Exchanges gases and nutrients which can be absorbed by nearby cells Belongs to the circulatory system Allows oxygen to bind to red blood cells when blood enters the lungs Excretes waste into the GI tract where nutrients are absorbed Skin allows evaporation of water from its surface The kidney excreted waste in the form of urine Membranes contain various transport mechanisms which allow for increased/decreased flow of solutes in response to various stimuli. The plasma membrane also plays an important role in detecting chemical signals from adjacent cells and also in anchoring adjacent cells to the extracellular matrix The membrane itself is made of a lipid bilayer. These lipids have a special structure and are called phospholipids. Phospholipids are amphipathic; they have a hydrophilic head which interacts with water and 2 hydrophobic tails which stay away from water The phospholipids are arranged so that the hydrophilic heads face outwards (towards the aqueous body fluid) and the tails are hidden within the bilayer The lipids are not bound to each other, so the membrane is allowed to move freely laterally Some proteins anchor certain parts of the membrane to the cytosol Fatty acid chains can wiggle around in the middle section of the bilayer The lipid bilayer can be folded easily but cannot be stretched, like a piece of fabric The plasma membrane contains cholesterol for structural rigidity, but other intracellular membranes dont contain cholesterol. Cholesterol is slightly amphipathic and can arrange itself the same way a phospholipid would 1 Naveen Sooknanan McGill Fall 2011 They help the plasma membrane pinch off to form vesicles They can associate with certain phospholipids and membrane proteins Two types of membrane proteins exist within the plasma membrane: integral membrane proteins and peripheral membrane proteins. Integral proteins are embedded in the plasma membrane and associate closely with lipids and other membrane components o They are amphipathic and can move laterally like other PM components Some are anchored to the cytosolic surface of PM o Most are transmembrane proteins which can span the entire plasma membrane multiple times Polar regions can extend far out of the extracellular side of the PM to associate with ions/water o Some can be associated with chemical signals across the cell membrane I.e. anchoring extra/intracellular filaments o Cannot be extracted from the PM without disruption of the membrane itself Peripheral proteins are not amphipathic and do not associate with the hydrophobic center of the PM o Located on the surface of the membrane o They associate mostly on the cytosolic surface to associate with cytoskeletal filaments to determine structure and shape of the cell The extracellular surface of the plasma membrane also contains sugars (monosaccharide chains) which can covalently link to membrane lipids and proteins This is called the glycocalyx layer Important for cell identification/interactions This all makes up a fluid mosaic model of the cell membrane Mosaic refers to the different components of the membrane Fluid refers to the free movements of membrane components o Like a layer of oil on water Some isolated regions of the cell membrane form lipid rafts by anchoring membrane proteins to cytoplasmic proteins which are covalently linked to membrane lipids Usually cholesterol rich regions Serve as organizing centers for complex intercellular signals o E.g. hormone secretion, binding of paracrin molecules o Lead to changes in cell activity This structure does not conform to the fluid mosaic model o Other such structures are junctions formed between adjacent cells Transport mechanisms are located in all membranes, but we will talk about the important ones in the plasma membrane, and then in the capillary wall The plasma separates the contents of the cell from the ECF 2 Naveen Sooknanan McGill Fall 2011 Movement across the PM depend on both the properties of the membrane as well as the type of solutes crossing it The cell is able to control the rate of solute flow, known as flux The PM, like other membranes, is permeable to certain types of solutes. This is called selective permeability. The PM is highly permeable to small, nonpolar molecules such as oxygen, carbon dioxide and other uncharged molecules It is less permeable to larger molecules and charged molecules such as ions o Ions are electrically repelled by the nonpolar fatty acid chains of the lipid bilayer The PM is impermeable to large macromolecules which stay either on the inside or the outside of the cell unless there is a breakdown of the PM caused by a disease The membrane is very permeable to water by a process called osmosis Like we said before, membrane proteins are scattered throughout the plasma membrane. Their function can include the following Transport of specific molecules in and out of the cells o Via channels which can open or close, or by conformational changes Act as enzymes which catalyze membrane associated reactions Receptors for sending or receiving chemical signals o To and from cell environment Cell surface identity makers o Helps the immune system know which cell are host cells Useful in intercellular adhesions via junctions Structural rigidity by binding to cytoskeleton One protein can have multiple functions Solutes can cross through the plasma membrane in two ways: Small nonpolar molecules directly cross through the lipid bilayer of the PM All other molecules must associate in some way with a membrane protein, either a channel or a carrier (transporter) o Channels allow an ion to flow through a hole, or pore, in the protein o Carriers undergo a conformational change in order to allow molecules to pass through There are four important factors in determining how a solute moves through the PM Lipid solubility (nonpolar molecules) Particle size (small) Electrical charge (neutral) Availability of carriers and channels Transport mechanisms can be divided into two main categories Passive mechanisms are energy independent o Simple diffusion (called diffusion) 3 Naveen Sooknanan McGill Fall 2011 o Carrier-mediated facilitated diffusion o Osmosis (water) Active transport requires ATP (energy) o Carrier mediates active transport Primary Secondary o Pino/phagocytosis Causes a change in the shape and size of the PM by creating vesicles Molecules are in a state of constant movement, rotation and vibration As the temperature of a molecule increases, its speed increases as well o The speed of a water molecule is around 2000km/h at body temperature o Glucose molecules, which are larger, travel at around 850km/h at the same temperature Despite their great speed, molecules are so tightly packed that they can only travel tiny distances before they collide with other molecules Trillions of collisions happen every second Diffusion involves the principle of random thermal motion, which states that molecules will move around in solution so that they eventually form an even distribution throughout the entire medium Molecules in an area of higher concentration will move gradually move towards an area of lower concentration The rate of movement of molecules in a particular area over a given period of time is known as the flux o At equilibrium, the sum of the fluxes in all directions cancel out, so the net flux is equal to 0 (i.e. same concentration everywhere) In the cell, molecules which are able to cross the PM directly through the lipid bilayer without the aid of proteins will use simple
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