Unit 1 Notes

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Department
Biological Sciences
Course
BIO SCI 93
Professor
Diane O' Dowd
Semester
Fall

Description
Unit 1: The Dynamic Cell Comparing Prokaryotic and Eukaryotic Cells  All cells: o Plasma membrane- selective barrier they are bounded by  Allows passage of enough oxygen, nutrients, and wastes to service the entire cell o Cytosol- jellylike, semifluid substance inside all cells in which subcellular components are suspended o Chromosomes- carry genes in the form of DNA o Ribosomes- tiny complexes that make proteins according to instructions from the genes  Eukaryotes: o DNA found in nucleus, which is bounded by a double membrane o Came before prokaryotes in evolutionary history o Cytoplasm = region between the nucleus and the plasma membrane o Much larger than prokaryotes  Prokaryotes: o DNA concentrated in a region that is not membrane-enclosed, called nucleoid A Panoramic View of the Eukaryotic Cell  Many enzymes built right into the membranes  Basic fabric of biological membranes: double layer of phospholipid and other lipids o Embedded in this lipid bilayer are diverse proteins  Animal/Plant cell parts: o Both:  Endoplasmic reticulum- network of membranous sacs and tubes; active in membrane synthesis and other synthetic and metabolic processes; has rough (ribosome-studded) and smooth regions  Nucleus: • Nuclear envelope- double membrane enclosing the nucleus; perforated by pores; continuous with ER • Nucleolus- Nonmembranous structure involved in production of ribosomes; a nucleus has one or more nucleoli • Chromatin- material consisting of DNA or proteins; visible in a dividing cell as individual condensed chromosomes  Plasma membrane- membrane enclosing the cell  Ribosomes- complexes that make proteins; free in cytosol or bound to rough ER or nuclear envelope  Golgi apparatus- organelle active in synthesis, modification, sorting, and secretion of cell products  Mitochondrion- organelle where cellular respiration occurs and ATP is generated  Peroxisome- organelle with various specialized metabolic functions; produces hydrogen peroxide as a by-product, then converts it to water  Microvilli- projections that increase the cell’s surface area  Cytoskeleton- reinforces cell’s shape; functions in cell movement; components are made of protein • Microfilaments 1 • Intermediate filaments • Microtubules o Animal cells, not plants:  Lysosomes- digestive organelle where macromolecules are hydrolyzed  Centrosomes- region where the cell’s microtubules are initiated; contains a pair of centrioles  Flagella- motility structure present in some animal cells, composed of a cluster of microtubules within an extension of the plasma membrane o Plant cells, not animals:  Chloroplasts- photosynthetic organelle; converts energy of sunlight to chemical energy stored in sugar molecules  Central vacuole- prominent organelle in older plant cells; functions include storage, breakdown of waste products, hydrolysis of macromolecules; enlargement of vacuole is a major mechanism of plant growth  Cell wall- outer layer that maintains cell’s shape and protects cell from mechanical damage; made of cellulose, other polysaccharides, and protein  Plasmodesmata- cytoplasmic channels through cell walls that connect the cytoplasms of adjacent cells Cellular membranes = fluid mosaics of lipids and proteins  Lipids and proteins = staple ingredients of membranes o Carbohydrates, important too o Most abundant lipids in membranes = phospholipids  Phospholipids are amphipathic, have a hydrophilic region and a hydrophobic region Membrane Models: Scientific Inquiry  Figure 7.3 The Fluidity of Membranes  Membranes are held together primarily by hydrophobic interactions, which are much weaker than covalent bonds  Some membrane proteins move in a highly directed manner, driven along cytoskeletal fibers by motor proteins connected to the membrane proteins’ cytoplasmic regions o Others are immobile, held by their attachment to the cytoskeleton or to the ECM  Membrane remains fluid as temperature decreases until phospholipids settle into closely packed arrangement and the membrane solidifies o Temperature at which it solidifies depends on types of lipids its made of o Unsaturated hydrocarbon tails  kinking  enhanced membrane fluidity  Cholesterol has different effects on membrane fluidity at different temperatures o High temperatures = cholesterol restrains phospholipid movement  less fluidity  Because cholesterol hinders the close packing of phospholipids, it lowers the temperature required for the membrane to solidify  Cholesterol = “fluidity buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature 2 Evolution of Differences in Membrane Lipid Composition  Increase in unsaturated phospholipids to increase necessary fluidity, etc. Membrane Proteins and Their Functions  Phospholipids = main fabric of the membrane; + proteins, which indicate the membrane’s functions  2 major types of membrane proteins: 1. Integral proteins  Penetrate hydrophobic interior of the lipid bilayer  Majority are transmembrane proteins, which span the membrane 2. Peripheral proteins  Not embedded in the lipid bilayer  Are appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins  On cytoplasmic side of plasma membrane, some membrane proteins held in place by attachment to the cytoskeleton  On extracellular side, certain membrane proteins are attached to fibers of the extracellular matrix  Figure 7.9, The structure of a transmembrane protein  6 functions performed by proteins of the plasma membrane: 1. Transport  L: A protein provides hydrophilic channel across the membrane selective for a particular solute  R: Other transport proteins shuttle the substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane 2. Enzymatic activity  Protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. 3. Signal transduction 3  A membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger (i.e. hormone). The messenger, or signaling molecule, may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein. 4. Cell-cell recognition  Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. This type of cell-cell binding is usually short-lived. 5. Intercellular joining  Membrane proteins of adjacent cells may hook together in various kinds of junctions. 6. Attachment to the cytoskeleton and ECM  Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes. The Role of Membrane Carbohydrates in Cell-Cell Recognition  Cell-cell recognition important for distinguishing different types of cells from another o Basis for rejection of foreign cells by the immune system  Membrane carbohydrates- usually short, branched chains of fewer than 15 sugar units o If bonded to lipid = glycolipids o If bonded to protein = glycoproteins o Four blood cell types reflect variation in carbohydrate part of glycoproteins on the surface of red blood cells  Synthesis and Sidedness of Membranes 4 o Each protein has directional orientation in the membrane o Asymmetrical arrangement of proteins, lipids, carbohydrates- determined as the membrane is built by the endoplasmic reticulum and Golgi apparatus The Permeability of the Lipid Bilayer  Nonpolar molecules (hydrocarbons, CO , 2 ) 2re hydrophobic  dissolve in lipid bilayer of membrane and cross easily without the aid of membrane proteins  Polar molecules (glucose, sugars) pass slowly through lipid bilayer  hydrophobic interior of membrane impedes direct passage of ions and polar molecules  Charged atom/molecule  even more difficult to penetrate the hydrophobic interior Transport Proteins  Hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins o Channel proteins: have a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane  Aquaporins o Carrier proteins: hold onto their passengers and change shape in a way that shuttles them across the membrane  Transport proteins are specific to only one substance Passive transport  Passive transport- diffusion of a substance across a membrane with no energy investment  Molecules have thermal energy (heat) due to their constant motion o Result  diffusion o Diffusion moves randomly, but can be directional  Any substance will diffuse down its concentration gradient (represents potential energy) Effects of Osmosis on Water Balance  Osmosis- diffusion of water across a selectively permeable membrane  Water balance of cells without walls o Tonicity- ability of a surrounding solution to cause a cell to gain/lose water  Depends in part on its concentration of solutes that cannot cross the membrane relative to that inside the cell o Isotonic- no net movement of water across the plasma membrane o Hypertonic- “more”  Solution is hypertonic to cell =solution has more solute than cell  cell will lose water, shrivel, die o Hypotonic- “less”  Solution is hypotonic to cell = water will enter the cell faster than it leaves  cell will swell and burst o Organisms that lack rigid cell walls must have other adaptations for osmoregulation (the control of solute concentrations and water balance)  Water balance of cells with walls o Plant cell swells as water enters by osmosis  expands to maximum  exerts back pressure on the cell to oppose further water uptake (turgor pressure) o Cell is turgid (firm)  healthy; if plant’s cells and their surroundings are isotonic, no net tendency for water to enter  cell becomes flaccid (limp) o Cell is of no advantage if immersed in hypertonic environment  Will lose water and shrink  plasmolysis Facilitated Diffusion: Passive Transport Aided by Proteins 5  Channel proteins that transport ions – ion channels o Function as fated channels (open and close in response to a stimulus) The Need for Energy in Active Transport  Needed to pump a solute against its gradient o All carrier proteins instead of channel proteins  Active transport allows for a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment  ATP supplies the energy needed by transferring its terminal phosphate group directly to the transport protein  can make the protein change shape to translocate the solute o i.e. sodium-potassium pump How Ion Pumps Maintain Membrane Potential  Cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of an unequal distribution of anions and cations on the two sides  Voltage across a membrane = membrane potential o acts like a battery, an energy source that affects the traffic of all charged substances across the membrane  Because inside of cell is negative compared to outside, membrane potential favors the passive transport of cations into the cell and anions out of the cell  Two forces drive the diffusion of ions across a membrane: 1. chemical force (the ion’s concentration gradient) 2. electrical force (the effect of the membrane potential on the ion’s movement) o together called  electrochemical gradient  Sodium-potassium pump o Pumps 3 Na+ out of the cell for every 2 K+ pumped into o Net transfer: +1 from the cytoplasm to the extracellular fluid  Electrogenic pump- transport protein that generates voltage across a membrane o i.e. sodium-potassium pump for animals; proton pump (pumps H+ out of cell from cytoplasm to extracellular solution) for plants, bacteria, etc. o Figure 7.20 Cotransport: Coupled Transport by a Membrane Protein  Cotransport- a single ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes in a mechanism o A transport protein can couple the “downhill” diffusion of this substance to the “uphill” transport of a second substance against its own concentration gradient Exocytosis  Requires energy  Exocytosis- cell secretes certain biological molecules by the fusion of vesicles with the plasma membrane  Process: o Transport vesicle budded from Golgi moves along microtubules of cytoskeleton to plasma membrane 6 o Vesicle membrane and plasma membrane come into contact  proteins rearrange the lipid molecules of the two bilayers so the two membranes fuse o Contents of the vesicle spill to outside of the cell, and vesicle membrane becomes part of plasma membrane  Pancreas cells that make insulin secrete it into the extracellular fluid by exocytosis o Neurons use exocytosis to release neurotransmitters that signal other neurons or muscle cells Endocytosis  Endocytosis- cell takes in biological molecules by forming new vesicles from the plasma membrane  Process: o Small area of the plasma membrane sinks inward to form a pocket o As pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell  Addition of membrane by one process offsets the loss of membrane by the other  3 types: 1. Phagocytosis: a. Cell engulfs particle by wrapping pseudopodia around it and packaging it within a membranous sac called a food vacuole. The particle will be digested after the food vacuole fuses with a l
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