Transcript Cells
Cells The Units of Life Early Discoveries Mid 1600s - Robert Hooke observed and described cells in cork Late 1600s - Antony van Leeuwenhoek observed sperm, microorganisms 1820s - Robert Brown observed and named nucleus in plant cells Cell Theory Schleiden and Schwann Every organism is composed of one or more cells Cell is smallest unit having properties of life Virchow All exisiting cells arise from pre-existing cells. Cell Smallest unit of life Can survive on its own or has potential to do so Is highly organized for metabolism Senses and responds to environment Has potential to reproduce Measuring Cells Vary in Size Why Are Cells So Small? Surface-to-volume ratio The bigger a cell is, the less surface area there is per unit volume Above a certain size, material cannot be moved in or out of cell fast enough Size is Limited Ways to Study Cells: Microscopes Create detailed images of something that is otherwise too small to see Light microscopes Simple or compound Electron microscopes Transmission EM or Scanning EM Different Types of Light Microscopy: A Comparison Limitations of Light Microscopy Wavelengths of light are 400-750 nm If a structure is less than one-half of a wavelength long, it will not be visible Light microscopes can resolve objects down to about 200 nm in size Electron Microscopy Uses streams of accelerated electrons rather than light Electrons are focused by magnets rather than glass lenses Can resolve structures down to 0.5 nm Elecrton Microscopes Electron micrographs Ways to Study Cells: Cell Fractionation Structure of Cells Two types of cells Prokaryotic Eukaryotic All cells have: Plasma membrane Region where DNA is stored Cytoplasm Prokaryotic Cells No nucleus Nucleoid area where DNA resides No membrane bound organelles. 70s ribosomes Cell walls contain petidoglycan or pseudomurein Prokaryotic Organisms Archaeobacteria Eubacteria Cyanobacteria A prokaryotic cell E. coli Eukaryotic Cells Have a nucleus and other organelles Eukaryotic organisms Protistans Fungi Plants Animals Overview of an animal cell The nucleus and its envelope Functions of Nucleus Keeps the DNA molecules of eukaryotic cells separated from metabolic machinery of cytoplasm Makes it easier to organize DNA and to copy it before parent cells divide into daughter cells Cytomembrane System Group of related organelles in which lipids are assembled and new polypeptide chains are modified Products are sorted and shipped to various destinations Components of the cytomembrane system Endoplasmic reticulum Golgi apparatus vesicles Endoplasmic Reticulum In animal cells, continuous with nuclear membrane Extends throughout cytoplasm Two regions - rough and smooth Rough ER Arranged into flattened sacs Ribosomes on surface give it a rough appearance Some polypeptide chains enter rough ER and are modified Cells that specialize in secreting proteins have lots of rough ER Smooth ER A series of interconnected tubules No ribosomes on surface Lipids assembled inside tubules Smooth ER of liver inactivates wastes, drugs Sarcoplasmic reticulum of muscle is a specialized form Endoplasmic reticulum (ER) Golgi Bodies Put finishing touches on proteins and lipids that arrive from ER Package finished material for shipment to final destinations Material arrives and leaves in vesicles The Golgi apparatus Vesicles Membranous sacs that move through the cytoplasm Lysosomes Peroxisomes Lysosomes Review: relationships among organelles of the endomembrane system The mitochondrion, site of cellular respiration Organelles with no Membranes Ribosomes Cytoskeleton Function in protein synthesis Function in maintenance of cell shape and positioning of organelles Centrioles (animals only) Function during cell division Ribosomes Cytoskeleton Present in all eukaryotic cells Basis for cell shape and internal organization Allows organelle movement within cells and, in some cases, cell motility Cytoskeletal Elements intermediate filament microtubule microfilament Microtubules Largest elements Composed of the protein tubulin Arise from microtubule organizing centers (MTOCs) Polar and dynamic Involved in shape, motility, cell division Microtubules Largest elements Composed of the protein tubulin Arise from microtubule organizing centers (MTOCs) Polar and dynamic Involved in shape, motility, cell division Microfilaments Thinnest cytoskeletal elements Composed of the protein actin Polar and dynamic Take part in movement, formation and maintenance of cell shape Intermediate Filaments Present only in animal cells of certain tissues Most stable cytoskeletal elements Six known groups Different cell types usually have 1-2 different kinds Cell Junctions tight junctions adhering junction gap junction Cell Membranes Structure and Function lipid bilayer fluid fluid one layer of lipids one layer of lipids Fig. 4.3, p. 52 Lipid Bilayer Main component of cell membranes Gives the membrane its fluid properties Two layers of phospholipids Figure 8.2 Two generations of membrane models The detailed structure of an animal cell’s plasma membrane, in cross section Functions of Membrane Proteins Transport Enzymatic activity Receptors for signal transduction Figure 3.4.1 Functions of Membrane Proteins Intercellular adhesion Cell-cell recognition Attachment to cytoskeleton and extracellular matrix Figure 3.4.2 oligosaccharide cholesterol groups phospholipid EXTRACELLULAR ENVIRONMENT (cytoskeletal proteins beneatch ADHESION the plasma PROTEIN membrane) open gated channel channel protein proten (open) gated channel proten (closed) active transport protein (area of enlargment) TRANSPORT PROTEINS RECEPTOR PROTEIN LIPID BILAYER RECOGNITION PROTEIN CYTOPLASM PLASMA MEMBRANE Fig. 4.4, p. 53 The fluidity of membranes Cell Membranes Show Selective Permeability O2, CO2, and other small nonpolar molecules;and H2O C6H12O6, and other large, polar (water-soluble) molecules; ions such as H+, Na+, CI-, Ca++; plus H2O hydrogen-bonded to them X Membrane Crossing Mechanisms Diffusion across lipid bilayer Passive transport Active transport Endocytosis Exocytosis Diffusion The net movement of like molecules or ions down a concentration gradient Although molecules collide randomly, the net movement is away from the place with the most collisions (down gradient) Concentration Gradient Means the number of molecules or ions in one region is different than the number in another region In the absence of other forces, a substance moves from a region where it is more concentrated to one one where it’s less concentrated - “down” gradient Factors Affecting Diffusion Rate Steepness of concentration gradient Molecular size Smaller molecules, faster diffusion Temperature Steeper gradient, faster diffusion Higher temperature, faster diffusion Electrical or pressure gradients The diffusion of solutes across membranes Osmosis A Special Case of Simple Diffusion Diffusion of water molecules across a selectively permeable membrane Direction of net flow is determined by water concentration gradient Side with the most solute molecules has the lowest water concentration Tonicity Refers to relative solute concentration of two fluids Hypertonic - having more solutes Isotonic - having same amount Hypotonic - having fewer solutes Tonicity and Osmosis 2% sucrose water 10% sucrose 2% sucrose The water balance of living cells The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation Increase in Fluid Volume compartment 1 compartment 2 HYPOTONIC SOLUTION HYPERTONIC SOLUTION membrane permeable to water but not to solutes fluid volume increases In compartment 2 Passive Transport Faciltiated Diffusion Flow of solutes through the interior of passive transport proteins down their concentration gradients Passive transport proteins allow solutes to move both ways Does not require any energy input Transport Proteins Span the lipid bilayer Interior is able to open to both sides Change shape when they interact with solute Play roles in active and passive transport Two models for facilitated diffusion Facilitated Diffusion solute Active Transport Net diffusion of solute is against concentration gradient Transport protein must be activated ATP gives up phosphate to activate protein Binding of ATP changes protein shape and affinity for solute Active Transport High solute concentration Low solute concentration P ATP ADP P P P ATP gives up phosphate to activate protein Binding of ATP changes protein shape and affinity for solute The sodium-potassium pump: a specific case of active transport Review: passive and active transport compared Bulk Transport – Another form of Active Transport Exocytosis Endocytosis The three types of endocytosis in animal cells Mitosis How to clone a nucleus Roles of Mitosis Multicelled organisms Growth Cell replacement Some protistans, fungi, plants, animals Asexual reproduction Chromosome A DNA molecule & attached proteins Duplicated in preparation for mitosis one chromosome (unduplicated) one chromosome (duplicated) Cell Cycle Cycle starts when a new cell forms During cycle, cell increases in mass and duplicates its chromosomes Cycle ends when the new cell divides Interphase Usually longest part of the cycle Cell increases in mass Number of cytoplasmic components doubles DNA is duplicated Mitosis Period of nuclear division Usually followed by cytoplasmic division Four stages: Prophase Metaphase Anaphase Telophase Control of the Cycle Once S begins, the cycle automatically runs through G2 and mitosis The cycle has a built-in molecular brake in G1 Cancer involves a loss of control over the cycle, malfunction of the “brakes” Stopping the Cycle Some cells normally stop in interphase Neurons in human brain Arrested cells do not divide Adverse conditions can stop cycle Nutrient-deprived amoebas get stuck in interphase Stages of Mitosis Prophase Metaphase Anaphase Telophase Early Prophase Mitosis Begins Duplicated chromosomes begin to condense Late Prophase New microtubules are assembled One centriole pair is moved toward opposite pole of spindle Nuclear envelope starts to break up Transition to Metaphase Spindle forms Spindle microtubules become attached to the two sister chromatids of each chromosome Metaphase All chromosomes are lined up at the spindle equator Chromosomes are maximally condensed Anaphase Sister chromatids of each chromosome are pulled apart Once separated, each chromatid is a chromosome Telophase Chromosomes decondense Two nuclear membranes form, one around each set of unduplicated chromosomes Results of Mitosis Two daughter nuclei Each with same chromosome number as parent cell Chromosomes in unduplicated form Cytoplasmic Division Usually occurs between late anaphase and end of telophase Two mechanisms Cell plate formation (plants) Cleavage (animals) Animal Cell Division