Basic Cellular Processes: Transport Mechanisms & Cell Membranes PDF

BASIC CELLULAR PROCESSES  TRANSPORT MECHANISMS  Plasma membrane has many functions but the most basic one is to define the borders of the cell and keep the cell functional.  It is selectively permeable.  Structure and Components of the plasma membrane Component Function/Feat...

BASIC CELLULAR PROCESSES  TRANSPORT MECHANISMS  Plasma membrane has many functions but the most basic one is to define the borders of the cell and keep the cell functional.  It is selectively permeable.  Structure and Components of the plasma membrane Component Function/Feature Phospholipid Main fabric of the membrane Cholesterol Maintains the integrity and fluidity of the plasma membrane Dampens effects of temperature Integral Proteins Transport of substance through membrane -receptor function -cell adhesion -structural support Peripheral Proteins Cell recognition Carbohydrates *cell recognition *effective interaction with the aqueous environment TRANSPORT MECHANISMS TRANSPORT MECHANISMS Among the most sophisticated functions of the plasma membrane is the ability to transmit signals by means of complex, integral proteins known as receptors.  FLUID MOSAIC MODEL 1890- the existence of plasma membrane was discovered 1915- when its components were determined (lipids and proteins) 1935- Hugh Davson and James Danielli described the first widely accepted model of the membrane as “railroad track”  FLUID MOSAIC MODEL 1950- the usage of Transmission Electron Microscope (TEM) allowed the researchers to see that the core of the plasma membrane consisted of a double, rather than a single layer.  FLUID MOSAIC MODEL  1972- the fluid mosaic model was proposed by Seymour J. Singer and Garth L. Nicolson  The model describes the plasma membrane as a fluid structure with a mosaic of components- phospholipids, cholesterol, proteins, and carbohydrates, that gives the membrane a fluid character. FLUID MOSAIC MODEL Phospholipids The main fabric of the membrane is composed of amphiphilic phospholipid molecules: *hydrophilic- water-loving areas(polar heads) *hydrophobic- water-hating areas (non-polar tails)  Make up the second major component of plasma membrane Proteins Integral proteins- integrated completely into the membrane structure and their hydrophobic membrane- spanning regions interact with the hydrophobic region of the phospholipid bilayer.  The third major component of the plasma membrane Are always found on the exterior Carbohydrates surface of cells and are bound to either to proteins (glycoproteins) or to lipids (glycolipids) Carbohydrate chains may consist of 2- 60 monosaccharide units and can either be straight or branched.   Glycocalyx- collective name of the carbohydrate components of glycoproteins and glycolipids. Carbohydrates  Its highly hydrophilic  Aids in the interaction of the cell with the watery environment and in the cell’s ability to obtain substances dissolved in the water.  The fluidity of the membrane is due to two components: the phospholipids and the cholesterol Membrane Fluidity In their saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms, no double bonds between adjacent carbon atoms resulting to straight tails. Membrane Fluidity  Unsaturated fatty acids do not contain a maximal number of hydrogen atoms, but they do contain double bonds between adjacent carbon atoms resulting to a bending in the string of carbons approximately 30 degrees.  Membrane Fluidity Lies alongside the Cholesterol- an phospholipids in the additional membrane membrane, and tends constituent that to dampen the effects assists in maintaining of temperature on the fluidity in animals membrane  METHODS OF TRANSPORT Transport Method Active/Passive Material Transported Diffusion Passive Small-molecular weight material Osmosis Passive Water Facilitated Diffusion Passive Sodium, Potassium, Calcium, Glucose Primary Active Transport Active Sodium, Potassium, Calcium Secondary Active Active Amino Acids, lactose Transport Phagocytosis Active Large macromolecules Pinocytosis/Potocytosis Active Small molecules (water/liquid) Receptor-mediated Active Large quantities of  PASSIVE TRANSPORT Substances move with A naturally occurring the concentration phenomenon and does gradient, from an area not require the cell to of greater exert any of its energy concentration to an to accomplish the area of lower movement. concentration.   Materials that can easily move through the membrane:  Lipid-soluble material with a PASSIVE low molecular weight TRANSPORT  Substances such as fat- soluble vitamins A,D,E, and K  Fat-soluble drugs and hormones  Molecules of oxygen and carbon dioxide  DIFFUSION  Is a passive process of transport  Where a single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space  This expends no energy  1.Extent of the concentration gradient 2. Mass of the molecules diffusing 3. Temperature Factors 4. Solvent density affecting 5. Solubility diffusion 6. Surface area and thickness of the plasma membrane 7. Distance traveled  Facilitated Diffusion Ions and polar molecules are repelled Materials diffuse by the hydrophobic parts across the plasma of the cell membrane; membrane with the thus, they have to enter through the facilitated help of membrane transport proteins, proteins shielding them from the repulsive force of the membrane.  Facilitated Diffusion  Transport proteins are the collective term for integral proteins that facilitates transport, and they function as:  Channels  Carriers  Facilitated Diffusion Channel proteins have hydrophilic domains exposed to the intracellular and Channels- are specific for the extracellular fluids, they substance that is being additionally have a transported hydrophilic channel through their core that provides a hydrated opening through the membrane layers.  Aquaporins- are channel proteins that allow water to pass through at a very high rate Channel proteins are either Facilitated open at all times or “gated” Diffusion The gate controls the opening of the channels  Carrier proteins- this binds a substance and, in doing so, triggers a change of its own shape, Facilitated moving the bound Diffusion molecule from the outside of the cell to its interior or vice versa.  Facilitated Diffusion Channel and carrier proteins transport material at different rates Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.  OSMOSIS Is the movement Moves from an of water through area where a semi-permeable solvent is high membrane in according to the concentration concentration than solute to gradient of water across the an area where membrane, which solvent is low is inversely in proportional to concentration the concentration than the of solutes. solute.  Tonicity  Describes how an extracellular solution can change the volume of a cell by affecting osmosis.  A solution’s tonicity often directly correlates with the osmolarity of the solution  Osmolarity- describes the total solute concentration of the solution.  Tonicity Hypotonic solutions- The extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. Hypertonic solutions- the extracellular fluid has higher osmolarity than the cell’s cytoplasm, therefore the fluid contains less water than the cell does Isotonic solutions- the extracellular fluid has the same osmolarity as the cell, thus no net movement of water into and out of the cell  ACTIVE TRANSPORT Require the use of the cell’s energy adenosine triphosphate (ATP) This occurs when Moves from lower substance move concentration to a higher against the concentration concentration of gradient substances   Primary Active Transport – moves ions across a membrane and creates a difference in charge across that membrane which is directly dependent on Two ATP mechanisms  Secondary Active Transport - for Active describes the movement of a material against its concentration transport gradient that is due to the electrochemical gradient established by the primary transport and does not directly require ATP CARRIER PROTEINS FOR ACTIVE TRANSPORT CARRIER PROTEINS FOR ACTIVE TRANSPORT  1. Uniporter- carries one specific ion or molecule.  2. Symporter- carries two different ions or molecules, both in the same direction.  3. Antiporter- carries two different ions or molecules, but in different directions. PRIMARY ACTIVE TRANSPORT  Primary Transport  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.  ATP is hydrolyzed by the protein carrier and a low-energy phosphate group attaches to it.  As a result, 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.  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  With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the interior of the cell  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. Secon dary Trans port   Brings sodium ions, and possibly other compounds into the cell. SECONDARY  As sodium ion ACTIVE TRANSPORT concentrations build outside (Co- of the plasma membrane Transport) because of the action of primary active transport process, an electrochemical gradient is created. SECONDARY ACTIVE TRANSPORT (Co- Transport) Quiz time! 1. Sugar crosses the cell membrane from an area of high concentration to an area of low concentration using special proteins embedded in the membrane. Which of the following types of cell transport is involved? A. osmosis B. active transport C. facilitated diffusion D. simple diffusion 2. How does aquaporin facilitate water movement in the cell? It__________________. A. shuttles the water molecules inside the cell B. provides a hydrophilic channel for the water molecules C. provides a hydrophobic channel for the water molecules D. changes the shape of the water molecule to enter the cell 3. Which of the following is an example of facilitated diffusion? A. Sugar is transported into the cell via an energy-consuming process. B. Carbon dioxide exits the cell, passing directly through the cell membrane. C. Hydrogen ions are pumped out of the cell against their concentration gradient. D. Water moves out of the cell, passing through channel proteins in the cell membrane. 4. Which of the following substances is NOT a component of the plasma membrane? A. cholesterol B. phospholipid C. proteins D. ribonucleic acids 5. A cell is placed into an isotonic solution. Which of the following is most likely to occur? The cell will ________. A. swell B. shrink C. not change D. dissolve 6. Which of the following substances below constitute the mosaic model of the cell membrane? A. carbohydrates embedded in a lipid bilayer B. different proteins embedded in a lipid bilayer C. cholesterol attached to the surface of the lipid bilayer D. unsaturated hydrocarbons attached inside the cytosol A. Plant cell A (plasmolyzed) is within an isotonic solution because water is leaving the cell. B. Plant cell B (flaccid) is within a hypotonic solution because water is entering the cell. C. There is a higher concentration of water on the outside of plant cell C (turgid) than on the inside. D. There is a lower concentration of water on the outside of plant cell B (flaccid) than on the inside. 8. Which of the following substances is a component of a phospholipid molecule? A. One hydrophilic phosphate head and two hydrophilic fatty acid tails. B. One hydrophilic phosphate head and two hydrophobic fatty acid tails. C. One hydrophobic phosphate head and two hydrophilic fatty acid tails. D. One hydrophobic phosphate head and one hydrophobic fatty acid tails. 9. Research findings on HIV-AIDS revealed that the virus must bind to CD4 protein as the main receptor and CCR5 as a co- receptor to infect a cell. In the given scenario, what function of the membrane proteins is being demonstrated? A. transport B. enzymatic C. cell signaling D. cell-cell recognition 10. The diagram below represents a cell and the relative concentration of several molecules inside and outside of the cell. Which molecule/s could only enter the cell as a result of active transport? A. Molecule A B. Molecule B C. Molecule C D. Molecules A & C