CELL BIOLOGY CELL STRUCTURE, ORGANELLE, AND MEMBRANE TRANSPORT

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1 BIO 2401 CELL BIOLOGY page 1 CELL BIOLOGY CELL STRUCTURE, ORGANELLE, AND MEMBRANE TRANSPORT Cells = structural units of all living things; recognize one another by their surface proteins (called glycoproteins); since all the cells of humans arise from one cell (fertilized egg), all the cells of an individual have exactly the same surface proteins and genetic information (except for genetic mistakes); different individuals (you and me) have slightly different cell surface proteins. Principal parts of the cell: cytoplasm consists of cytosol (cell liquid-viscous-and dissolved molecules) and organelles except for the nucleus organelles distinct structures or cell parts with specific functions nucleus has nuclear membrane; is controlling center of cell; has genetic information of cell in form of DNA; nucleoli for ribosomal RNA synthesis and a gel-like fluid called karyolymph all cells start out with a nucleus, but a few, like RBCs, lose them when mature ribosomes (106/cell) small granules consisting of ribosomal RNA and protein; are site of protein synthesis free ribosomes float in cytoplasm and seem to function in protein production for cell use, while ribosomes bound to the endoplasmic reticulum make protein for release or secretion endoplasmic reticulum (ER) consists of pairs of membrane folds (enlcosing fluid-filled cavities called cisternae) running throughout the cell and connecting to the cell and nuclear membranes; serves in the transport and packaging of molecules for secretion in association with the Golgi complex smooth ER has no ribosomes and catalyzes reactions involved in lipid metabolism; synthesizes steroid hormones; absorbs, synthesizes and transports fat; detoxifies drugs, pesticides, and carcinogens; breakdown of stored glycogen rough ER has ribosomes and manufactures cell membranes Golgi complex specialized fold of membranes involved in the packaging of lipids and proteins in vacuoles and lysozymes lysozymes vacuole membrane sacs of hydrolytic enzymes formed by Golgi complex.; function in chemical digestion mitochondria double membrane sausage-shaped organelles; the powerhouse of the cell; number of mitochondria in cells depends on supply and demand (muscle, liver and kidney cells have several thousand, while inactive cells, like skin cells, have only a few); have own DNA (but is circular like that of bacteria), can replicate, and have ribosomes for their own protein synthesis projections on the inner membrane are cristae (infolds) location of Kreb's cycle enzymes and cytochrome system

2 BIO 2401 CELL BIOLOGY page 2 cytoskeleton composed of filaments (micro & intermediate) and microtubules filaments = protein rods of actin that function in support and contractions (actin and myosin) microtubules = hollow protein tubes functioning in support and transport of material through the cell centrioles paired, can replicate; function in cell division where they influence microtubular formation associated with cell division vacuoles - membrane sacs used in storage and transporting secretory material plasma membrane outer, limiting membrane that controls what gets in and out of a cell by being selectively permeable; separates intracellular fluid from extracellular fluid and keeps ions in different proportions in these two compartments (this sets up a membrane potential); affects cellenvironment interactions membrane electric potential allows for excitability, irritability and responsiveness of cells; results from high concentration of sodium outside cell and high concentration of potassium inside the cell which leads to polarization ( + one side other side) of the cell membrane (electric potential of about -70mV inside cell) consists of two rows of phospholipids (60%) and cholesterol (40%) with proteins penetrating into and/or passing through the membrane (integral proteins) and peripheral proteins that are loosely bound to the surfaces phospholipids form basic fabric of membrane; have a polar head that is hydrophylic and a nonpolar tail that is hydrophobic; arrangement of phospholipids encourages membrane to self-assemble and reseal if torn cholesterols are a stabilizing force proteins float in the lipid bilayer to form a constantly changing mosaic pattern; have 2 populations integral = firmly inserted proteins; most are transmembrane proteins but a few protrude from only 1 of the layers peripheral = not embedded in the lipid bilayer; attach loosely to integral proteins and are usually removed; used in changing shape or providing cytoskeleton have specializations, such as microvilli which are minute, fingerlike extensions of membrane; increase surface area for absorption Tranport through membranes: basic concepts/terminology: solutions = homeogenous mixtures of components that may be gases, liquids, or solids solute = substance present in smaller amount

3 BIO 2401 CELL BIOLOGY page 3 solvent = substance present in greater amount; dissolving medium (usually a liquid) concentration = % of solute in solvent; expressed in a variety of ways (%, molarity) concentration gradient = molecules move from areas of high concentration to areas of low concentration; the greater the difference in concentration between the two areas, the faster the movement DOWN the concentration gradient PASSIVE PROCESSES (also called PASSIVE TRANSPORT) 1. diffusion (simple diffusion) molecules will freely diffuse through a membrane IF they are lipid soluble (charged molecules are not), small enough to pass through membrane channels, or assisted by a carrier molecule particles move from a high concentration to a low concentration due to their kinetic energy, and each particle diffuses down its own concentration gradient or from more collisions to less collisions diffusion is a slow process, especially in water; is influenced by size of molecules and temperature (raises kinetic energy) 2. facilitated diffusion diffusion made easier because a carrier molecule (integral protein) in the cell membrane binds molecules like glucose and carries them across the cell membrane with NO expenditure of energy by the cell; different molecules usually have different protein carriers; changes in shape of carrier allow it to envelope and then release transported substance while shielding it from the non-polar regions of the membrane 3. osmosis diffusion of a solvent through a selectively permeable membrane; occurs because water is a small molecule and can squeeze between gaps in tails of phospholipids; water can also move through pores or transmembrane proteins called aquaporins tonicity = the ability of a solution to change the tone or shape of cells by altering their internal water volume; are three possible responses: a. isotonic solutions = solutions where same concentrations of non-penetrating solutes as that in cells SO cells retain their normal shape (no NET loss or gain in water) b. hypotonic solutions = solutions that are more dilute than cells (have fewer nonpenetrating solutes); cells will pump up as water rushes into them and can lyse c. hypertonic solutions = solutions that have higher concentrations of nonpenetrating solutes than cells; cells will lose water and shrivel ACTIVE PROCESSES (require the use of energy to move solutes AGAINST concentration gradients) 1. active transport movement of particles by a carrier protein (integral protein) with the expenditure of energy (via hydrolysis of ATP), usually against an electrochemical or concentration gradient; is rapid and unidirectional, and used to concentrate molecules; results because transport protein is phosphorylated and changes its conformation so that it pumps bound solute across the membrane solute pumps (Na-K pump and Ca pump) = primary active transport

4 BIO 2401 CELL BIOLOGY page 4 2. vesicular transport energized by ATP or GTP exocytosis = moves substances from cell interior to extracellular space; substance is enclosed in a membranous sac (vesicle) which migrates to the cell membrane, fuses with it and ruptures, spilling sac s content out of cell ex: hormone secretion, neurotransmitter release, mucus secretion, waste ejection endocytosis = substance is enclosed by an infolding of plasma membrane and then moves into the cytoplasm where its contents are digested three types of endocytosis: a. phagocytosis = extensions called pseudopods flow around some large or solid material and engulf it; vesicle is formed (phagosome) and fuses with a lysosome so that its contents can be digested b. pinocytosis (bulk phase endocytosis) = routine engulfing of extracellular fluid (with its solutes) by cells to sample the extracellular fluid; important for cells that absorb nutrients c. receptor-mediated endocytosis = main mechanism for specific uptake of most macromolecules by body cells; receptors (plasma membrane proteins) bind only with certain substances and this bound structure is internalized in a vesicle called a coated pit (fuzzy coat); once inside cell the vesicle loses the fuzzy coat and the vesicle fuses with a processing vesicle (endosome); endosome can be recycled back to plasma membrane, combined with a lysosome and ingested, or transported across cell and released by exocytosis Attachments between Cells cell junctions/connections between cells may be due to a) glycoproteins, b) waxy surfaces that fold and fit together in a tongue and groove pattern, and c) three types of cell membrane junctions 1. tight junctions proteins on cell surface of adjacent plasma membranes fuse together forming an impermeable junction that encircles the cell; no room in intercellular space for molecules to pass (ex. epithelial cells & small intestine) 2. desmosomes anchoring junctions in which proteins attach between cells in spot or belt-like pattern and are found where mechanical stress occurs; linkers proteins extend from a buttonlike thickening of membrane and interdigitate in intracellular space with filaments extending across width of cell to anchor buttons on opposite side (ex. skin, heart muscle, uterus) 3. gap junctions protein tubules that are hollow cylinders (connexons) connect adjacent cells and allow passage of nutrients (via water through the cylinders) between cells; important in cell membranes involved in lots of electric activity (ex. heart and smooth muscle) exist in adult electrically excitable cells and in embryonic cells for nutrient distribution prior to development of circulatory system

5 BIO 2401 CELL BIOLOGY page 5 CELL CYCLE: What is the cell cycle? series of changes a cell goes through from the time it is formed until it reproduces itself; 2 stages: interphase and cell division/mitosis; controlled by system of external & internal factors Stages of the cell cycle: 1. interphase = total period from cell formation to division cell prepares for next division; has 3 phases G1 1 st portion; cells metabolically active; synthesize proteins rapidly & grow; variable in terms of length; at end, centrioles start to replicate S DNA replicates; new histones are made & assembled into chromatin G2 enzymes & other proteins needed for division are synthesized & transported to proper sites; centriole replication is complete 2. mitosis = parceling out of replicated DNA of mother cell to 2 daughter cells prophase = chromatin coils & condenses to form chromosomes (2 identical chromatids held together by a centromere); centrioles separate and mitotic spindle forms and lengthens pushing centrioles apart; nuclear membrane fragments; spindle fibers attach to the centromeres and pull on each chromosomes metaphase = chromosomes cluster at center of cell with centromeres at equator anaphase = centromeres of chromosomes split and each chromatid becomes a separated chromosome; these are pulled toward poles as cell elongates telophase = chromosomes at opposite poles uncoil; nuclear membrane forms around chromatin; spindle breaks down and disappears 3. cytokinesis = division of cytoplasm plasma membrane over center of cell is drawn inward to form a cleavage furrow furrow deepens until original cytoplasm is pinched into 2 parts Extracellular Materials body fluids 1. plasma = nonliving fluid component of blood 2. interstitial fluid = fluid between cells 3. cerebrospinal fluid = plasma-like fluid that fills the cavities of the CNS and surrounds the CNS externally; protects brain & spinal cord cellular secretions = substances that aid in digestion & act as lubricants (eg. intestinal and gastric fluids, saliva, mucus, serous fluid) extracellular matrix = jelly-like substance composed of proteins & polysaccharides that are secreted

6 BIO 2401 CELL BIOLOGY page 6 by cell and self-assemble into organized mesh in extracellular space; universal cell glue