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7.1 Life at the Edge• Plasma Membrane is a thick film – About 8 nm thick – Controls traffic in and out of the cell• Exhibits selective permeability – Allows some substances into the cell, and others cannot enter – Ability to discriminate between certain materials can be life or death
Fluid Mosaic Model• Lipids and proteins are staple materials in the membrane – Most abundant lipids are phospholipids • Amphipathic molecule – both hydrophilic region and hydrophobic region
Fluid Mosaic Model• Fluid Mosaic Model – Membrane is a fluid structure with a “mosaic” of various proteins
The Fluidity of Membranes• Membranes are held together by hydrophobic interactions – It is possible for the lipids and proteins to shift around within the membrane itself – Adjacent phospholipids switch places about 107 times per second – Proteins are larger and move a lot slower
The Fluidity of Membranes• A membrane remains fluid as temperature decreases – Eventually, the lipids get packed together and the membrane solidifies – Similar to bacon grease turning into lard – If the membrane is rich in unsaturated hydrocarbons, the kinks in their tails make it harder to pack together • So, the membrane will remain fluid at lower temperatures
The Fluidity of Membranes• Cholesterol – Steroid lodged between phospholipid molecules in the membrane – At higher temperatures, it makes the membrane less fluid by restraining phospholipid movement – Lowers temperature for membrane to solidify
The Fluidity of Membranes• Membranes must be fluid in order to function properly – When it solidifies, the permeability changes – Enzymatic proteins may become inactive if they cannot move within the membrane – If too fluid, however, it cannot support protein function
Evolution of Differences inMembrane Lipid Composition • Variations in the cell membrane lipid composition of many species are speculated to be caused by evolutionary adaptation – Fishes that live in extreme cold have a higher amount of unsaturated hydrocarbon tails which allows their membrane to stay fluid at these temperatures – At high temperature extremes, some bacteria and archaea have unusual lipids that may prevent excess fluidity at these temperatures
Evolution of Differences inMembrane Lipid Composition • The ability to change the lipid composition based off the current temperature has also been shown as an adaptation – Some plants that live in cold, such as winter wheat, adjust the amount off unsaturated phospholipids in autumn to keep the membranes from solidifying in the winter
Membrane Proteins and their Functions • A membrane is a collage of different proteins that are often grouped and embedded into the fluid matrix of the lipid bilayer – More than 50 unique proteins have been discovered in the plasma membrane of the red blood cell – Phospholipids make the “material” or structure of the membrane, and proteins determine the function
Membrane Proteins and their Functions • Two major types of proteins – Integral – Peripheral • Integral Proteins – Penetrate the hydrophobic interior of the bilayer – They are “integrated” into the membrane – Mostly transmembrane proteins • Peripheral Proteins – Not embedded; on outside of the membrane – Often exposed to parts of integral proteins
Membrane Proteins and their Functions • On the cytoplasmic side of the cell, some proteins are held in place by being attached to the cytoskeleton • On the extracellular side, certain membrane proteins are attached to fibers of the extracellular matrix – These attachments give a stronger structure than the membrane could provide by itself
Membrane Proteins and their Functions • Proteins on the surface of the cell are important in the medical field – Some can help outside agents invade the cell • HIV (human immunodeficiency virus) infects the cells through the help of a surface protein – Also important in medicine being transferred to the cells, etc.
The Role of Membrane Carbohydrates in Cell-Cell Recognition• Cell-cell recognition – Ability to distinguish one type of neighboring cell from another – Important in defense and making sure everything is working properly• Glycolipids – Short, branched chains of sugars that bond to lipids• Glycoproteins – Short, branched chains of sugars that bond to proteins
Selective Permeability• The fluid mosaic model helps explain S.P. – A steady traffic of small molecules and ions moves across the plasma membrane in both directions – Cell regulates inorganic ions too • Na+, K+, Ca2+, Cl- – Nonpolar molecules such as hydrocarbons, carbon dioxide, and oxygen, are hydrophobic • They can dissolve easily in the lipid bilayer and cross easily – Passive transport
Transport Proteins• Transport Proteins – Proteins that allow hydrophilic substances to pass through without making contact with lipid bilayer – Channel Proteins • Hydrophilic channel that specific ions and materials use to pass into/out of the cell • Aquaporins – Water can pass through these at 3 billion H2O molecules/second
Permeability• Diffusion – The movement of molecules of any substance so that they spread out evenly into the available space • A higher concentration of material X will diffuse into whatever solution has a lower concentration of material X• Concentration Gradient – The region along which the concentration of a chemical substance increases or decreases • No work must be done to make this happen
Passive Transport• Passive transport requires no energy – Like following the flow of a river
Tonicity• Isotonic – The cell and its surrounding solution are the same concentration• Hypertonic – The cell is hypotonic, so the surrounding solution is hypertonic and the contents of the cell will diffuse out of the cell and into the solution• Hypotonic – The cell is hypertonic, so the surrounding solution is hypotonic and the solution around the cell will diffuse into the cell
Water Balance Without Walls • Osmoregulation – Control of solute concentrations and water balance – Paramecium caudatum • Live in hypotonic pond water • To prevent water from rushing in, it has adapted to the water by developing a membrane that is much less permeable to water than other organisms
Water Balance of Cells with Walls • Cells of plants, prokaryotes, fungi, and some protists are surrounded by walls – The walls help to maintain water balance – After being pushed so far, the walls’ inelasticity will cause the wall to rebound, demonstrating turgor pressure • Opposes further water uptake
Water Balance of Cells with Walls • The cell will become Turgid – Very firm, and healthy state for a lot of these cells – This is what allows non-woody plants to stand upright • Flaccid – If the cell is placed in an isotonic solution, there is no pushback of water; so, the cells become limp, or flaccid • Plasmolysis – If placed into a hypertonic solution, the membrane will pull from wall, causing wilting in plants & eventual death
Facilitated Diffusion• Facilitated Diffusion – Passive transport aided by proteins – Polar molecules can diffuse through proteins in the membrane and get into/out of the cell• Ion Channels – Channel proteins that transport ions• Gated Channels – Ion channels that open/close in response to stimuli
Active Transport• Active transport – To pump a solute across a membrane, against its concentration gradient – Proteins that help do this are called carrier proteins
How Ion Pumps Maintain Membrane Potential • Membrane Potential – The voltage across a membrane – Acts like a battery, an energy source that affects the traffic of all charged substances across the membrane • Electrochemical Gradient – Two forces (electrical, chemical) driving ions across the membrane
How Ion Pumps Maintain Membrane Potential• Electrogenic Pump – A transport protein that generates electrical voltage across a membrane• Proton Pump – The main electrogenic pump of plants, fungi, and bacteria
Cotransport• Cotransport – When H+ may enter through into the membrane and “carry” another material with it • Amino acids, sugars, etc
Bulk Transportation• Exocytosis – The transport of a bulk amount of materials through vesicles containing the substance fusing with the membrane• Endocytosis – The transport of a bulk amount of materials by pushing into the membrane, and as it deepens, it eventually pinches off into a vesicle that travels into the cell