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This 5 page Class Notes was uploaded by Shira Clements on Thursday February 25, 2016. The Class Notes belongs to BSCI105 at University of Maryland taught by Norma Allewell in Fall 2015. Since its upload, it has received 17 views. For similar materials see Principles of Biology I in Biology at University of Maryland.
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Date Created: 02/25/16
Shira Clements BSCI105 Chapter 7- Membrane Structure and Function Plasma membrane- boundary that separates the living cell from its surroundings - Controls traffic into and out of cell by selective permeability – allows some substances in and others not - Made up of mostly fluid with bilayer of lipids- lipids and proteins, mostly phospholipids, because of the amphipathic structure- has hydrophobic and hydrophilic components - Membrane proteins reside in the phospholipid bilayer with the hydrophilic regions protruding- maximize contact of hydrophilic regions of proteins and phospholipids with water in the cytosol and extracellular fluid, matching up the hydrophobic parts of each- the proteins are embedded into the bilayer - Asymmetrical arrangement of proteins, lipids, and carbohydrates in plasma membrane is determined as membrane is being built by ER and Golgi apparatus Fluidity of Membrane- should be fluid to work properly - Held together by hydrophobic interactions- weak bonds - Lipids can pass each other in the line- like someone pushing through a 7 crowd- super quick- 10 times per second - Rare, but happens when a molecules flips from one side of the bilayer to the other- like a summersault- because hydrophilic must pass hydrophobic - Proteins seem to be immobile attached to cytoskeleton or ECM except for those that are most likely driven by motor proteins along cytoskeletal fibers - Membrane is fluid until temperature decreases and phospholipids solidifies in a closely packed arrangement, depending on types of lipids o Unsaturated hydrocarbons (because of double bonds and kinks) and rich in phospholipids- needs lower temp/harder to solidify - Steroid- wedged into phospholipid, but different effects on the membrane o Fluidity buffer- resists changes in membrane fluidity that can be caused by changes in temperature o at really high temperatures, cholesterol makes membrane less fluid by restraining phospholipid movement- it lowers temperature required for membrane to solidify because it stops the close packing of phospholipids - when membrane solidifies- permeability change and enzymatic proteins in membrane can become inactive if activity requires them to move within membrane, but too fluid, then can’t support protein function. Membrane Proteins and Their Functions - Membrane is a collage of different proteins (like mosaic) that is clustered in grouped embedded into the fluid matrix of lipid bilayer- proteins determine the membranes functions - Integral Proteins- penetrate hydrophobic interior of lipid bilayer o most are transmembrane proteins- span the membrane, but some just span some of the hydrophobic interior. Consists of nonpolar amino acids (clearly) with alpha helices. o Some have hydrophilic channel through the center to allow hydrophilic parts to pass - Peripheral Proteins- additions loosely bounded to surface of membrane, not embedded into the bilayer. o Usually exposed to integral proteins - Inside (cytoplasmic side)- some proteins are held by attachment to cytoskeleton or ECM- giving plasma membrane strong framework - Protein Functions on plasma membrane- transport- channel, or changes shape to help move substance along (ATP sometimes) o Enzymatic activity- active site is exposed and acts like an enzyme, sometimes work as a team with others with same goal o Signal transduction- binds with a substance which can cause the protein to change shape and relay the message inside the cell o Cell cell recognition- acts as an identification tag that helps other membranes of other cells recognize it and binds to cell for short time Cells recognize other cells by binding to molecules (often with carbohydrates-short chains) on extracellular surface of plasma membrane Carbohydrates bounded to lipids form glycolipids (glycol=presence of carbohydrates), but most are covalently bonded to proteins forming glycoproteins o Intercellular joining- help bind cells together such tight or gap junctions (long time) o Attachment to cytoskeleton and ECM- microfilaments or other parts of cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes location of membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes. Permeability of Lipid Bilayer- nonpolar molecules are hydrophobic and can dissolve and pass through the lipid bilayer easily, without help of membrane proteins. - Polar/hydrophilic molecules have an issue- because the lipid bilayer only allows certain things to go through (selective permeability), so proteins help with that - Transport Proteins- help hydrophilic substances get across the lipid bilayer- specific to the substance it helps get across the bilayer o Some, called channel proteins, have hydrophilic channels that some molecules/ions use as a tunnel through the membrane Aquaporins- tunnels that help millions of water molecules get through the lipid bilayer every second- help in kidneys o Some, called carrier proteins, hold onto substance and change the shape that helps them shuttle them across the membrane Passive Transport- diffusion of substance across a membrane with no energy investment - Diffusion- movement of molecules of any substance so that they spread out evenly into the available space because of thermal energy which is caused by motion- no work, spontaneous process. o One side is more concentrated than other, so the pores would be large enough for the solute to pass through until there is equal concentrations (equilibrium) of solute and solvent throughout the solution- substance goes from more concentrated to less concentrated, or diffuse down own electrochemical concentration gradient- region along which the density of chemical substance increases or decreases, represents potential energy and drives diffusion o Facilitated diffusion- speeds transport of solute- transport proteins help bring substances down concentration gradient Ion channels- channel proteins that transport ions, some function as gated channels- open and close in response to stimuli (some it is electrical, or when substance other than the one that is being transported binds to the channel) Carrier proteins undergo subtle change in shape that somehow translocate the solute binding site across the membrane- change can be triggered by binding/release of transported molecule. Cystinuria- disease that is caused because of absence of carrier protein - Osmosis- diffusion of water across a selectively permeable membrane o Movement of water across cell membrane, and balance of water between cell and environment are crucial to organisms Tonicity- the ability of a surrounding solution to cause a cell to gain or lose water- depends concentration of solutes that cannot cross the membrane relative to that inside the cell Higher concentration of nonpenetrating solutes in the surrounding solution, then water will tend to leave the cell and vice versa Cell with no cell wall is in isotonic environment, then no net movement of water across plasma membrane- water diffuses at same rate across the membrane. Solution that is hypertonic to cell- more solutes outside- so cell will lose water, shrivel up and die. Hypotonic- less solute in solution than inside- too much water enters the cell and cell lyse/bursts. o Osmoregulation- control of solute concentrations and water balance, organisms that lack cell walls need it- certain vacuole or something to help control/adapt o With cell walls, when the cell is placed in a hypotonic solution, the cell will swell and then the walls will exert pressure on cell- turgor pressure- that opposed water uptake. The cell is turgid- very firm- which is healthy for plant cells. However, in isotonic solutions, the plant cells become flaccid- weak- because there is no net tendency for water to enter. In hypertonic solution, the cell will still shrink since water will go out of cell. As it shrivels, the plasma membrane pulls away from the wall- plasmolysis Active Transport- uses energy to move solutes against their gradients, bring solutes to more concentrated- all carrier proteins - Enables cell to maintain internal concentrations of small solutes that differ from concentrations in its environment o An animal cell ahsd much higher concentration of potassium ions and lower concentration of sodium, and plasma membrane helps maintain this by pumping sodium out and potassium in. - ATP supplies energy for most active transport- transfers terminal phosphate group to transport protein, which will change the protein’s shape and allow a different substance to be transported- like the sodium potassium pump- pumps three sodium ions out and two potassium ions in- net transfer of one positive charge to extracellular fluid- stores energy as voltage. - Membrane potential- voltage across membrane- ranges from -50 to 200 mV. Cytoplasmic side’s charge is negative relative to extracellular side because of unequal distributions of anions and cations on the two sides. o acts like a battery, energy source that affects traffic of all charged substances across the membrane. o Electrochemical gradient- Cell likes passive transport of cations into cell and anions out of cell; therefore, two forces drive diffusion of ions across membrane- chemical force (ion’s concentration gradient) and electrical force (effect of the membrane potential on ion’s movement) Electrogenic pump- transport protein that generates voltage across membrane- sodium potassium pump is main Help store energy that can be tapped for cellular work Proton pump- main in plants, fungi, and bacteria- transports protons out of cell Important for ATP synthesis during cellular respiration Cotransport- coupled transport by membrane protein - A single ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes - Substance that has been pumped across membrane can do work as it diffuses back- one thing causes the other to happen Bulk Transport- too big for diffusion or transports - Exocytosis- cell secretes molecules by fusion of vesicles with plasma membrane o Transport vesicle from golgi moves along microtubules of cytoskeleton to plasma membrane and then when they are in contact, specific proteins rearrange the lipid molecules of two bilayers so two membranes fuse and content of vesicles spill out of cell while vesicle becomes part of membrane. (insulin from pancreas, neurotransmitters) - Endocytosis- cell takes in molecules and particulate matter and form new vesicles o Small part of membrane sinks inwards and forms a pocket and pinches in forming a vesicle with content that was outside. o Phagocytosis- cell engulfs a particle by wrapping pseudopodia around it and packing it in a food vacuole and then is digested by lysosome o Pinocytosis- cell gulps droplets of extracellular fluid into tiny vesicles- molecules dissolved in the droplets are needed by cell- not specific to substances it transports o Receptor mediated endocytosis- enables cell to get bulk quantities of specific substance- embedded in membrane are prteins with specific receptor sites exposed to extracellular fliud to which ligands (specific substances) bind. Receptor proteins cluster in regions of membrane called coated pits, which are lined on cytoplasmic side by fuzzy layer of coat proteins. Then, each coated pit forms a vesicle contained the ligand molecule- more bound molecules inside vesicle. After ingested material is freed from vesicle, the emptied receptors are recylcled to the plasma membrane by same vessicel
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