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Transcript
Chapter 6 A Tour of the Cell Cell Biology or Cytology • Cyto = cell ology = study of • Should use observations from several types of microscopes to make a total picture of how a cell is put together. Light Microscope - LM • Uses visible light to illuminate the object. • Relatively inexpensive type of microscope. • Can examine live or dead objects. Light Microscope Occular Lens Objective Lens Stage with specimen Light Source Resolution • Ability to detect two discrete points as separate from each other. • As Magnification increases, Resolution decreases. • LM working limits are100 - 1000X. Limitations - LM • Miss many cell structures that are beyond the magnification of the light microscope. • Need other ways to make the observations. Light Microscope Variations • Fluorescence: uses dyes to make parts of cells “glow”. • Phase-contrast: enhances contrasts in density. • Confocal: uses lasers and special optics to focus only narrow slides of cells. Electron Microscopes • Use beams of electrons instead of light. • Invented in 1939, but not used much until after WWII. TEM SEM Advantages • Much higher magnifications. • Magnifications of 50,000X or higher are possible. • Can get down to atomic level in some cases. Disadvantages • • • • Need a Vacuum. Specimen must stop the electrons. High cost of equipment. Specimen preparation. Transmission Electron Microscope or TEM • Sends electrons through thinly sliced and stained specimens. • Gives high magnification of interior views. Many cells structures are now visible. TEM Limitations • Specimen dead. • Specimen preparation uses extreme chemicals so artifacts are always a concern. Scanning Electron Microscope or SEM • Excellent views of surfaces. • Produces 3-D views. • Live specimens possible. Other Tools for Cytology • Cell Fractionation – break the cell apart and separate out the pieces. • Chromatography – separates mixtures based on their solubility. • Electrophoresis – separates mixtures of protein or DNA using gels and electricity. Cell Fractionation History of Cells • Robert Hooke - Observed cells in cork. • Coined the term "cells” in 1665. History of Cells • 1833 - Robert Brown, discovered the nucleus. • 1838 - M.J. Schleiden, all plants are made of cells. • 1839 - T. Schwann, all animals are made of cells. Cell Theory • All living matter is composed of one or more cells. • The cell is the structural and functional unit of life. • All cells come from cells. Types of Cells • Prokaryotic - lack a nucleus and other membrane bounded structures. • Eukaryotic - have a nucleus and other membrane bounded structures. Both Have: • • • • Membrane Cytosol Ribosomes (but the size is different) DNA Prokaryotic Eukaryotic Nucleus Eukaryotic Prokaryotic Why Are Cells So Small? • Cell volume to surface area ratios favor small size. • Nucleus to cytoplasm consideration (control). • Metabolic requirements. • Speed of diffusion. Tuesday, September 22 • The paramecium and euglena are both single-celled organisms. Identify two pieces of evidence that make them eukaryotic as opposed to prokaryotic. Basic Cell Organization • • • • Membrane Nucleus Cytoplasm Organelles Animal Cell Plant Cell Membrane • Separates the cell from the environment. • Boundary layer for regulating the movement of materials in/out of a cell. Cytoplasm or Cytosol • Cell substance between the cell membrane and the nucleus. • The “fluid” part of a cell. Exists in two forms: • gel - thick • sol - fluid Organelle • Term means "small organ”. Formed body (or compartment) in a cell with a specialized function. • Important in organizational structure of cells. Organelles - function • Way to form compartments in cells to separate chemical reactions. • Keeps various enzymes separated in space. You must be able to: • Identify the major organelles • Give their structure • Give their function Nucleus • Most conspicuous organelle. • Usually spherical, but can be lobed or irregular in shape. Structure • • • • Nuclear membrane Nuclear pores Nucleolus Chromatin Nuclear Membrane • Double membrane separated by a 2040 nm space. • Inner membrane supported by a protein matrix which gives the shape to the nucleus. Nuclear Pores • Regular “holes” through both membranes. • 100 nm in diameter. • Protein complex gives shape. • Allows materials in/out of nucleus. Nucleolus • Dark staining area in the nucleus. • 0 - 4 per nucleus. • Storage area for ribosomes. Chromatin • Chrom: colored • - tin: threads • DNA and Protein in a “loose” format. Will form the cell’s chromosomes. Nucleus - Function • Control center for the cell. • Contains the genetic instructions. Ribosomes • Structure: 2 subunits made of protein and rRNA. No membrane. • Function: protein synthesis. Subunits • Large: • 45 proteins • 3 rRNA molecules • Small: • 23 proteins • 1 rRNA molecule Locations • Free in the cytoplasm - make proteins for use in cytosol. • Membrane bound - make proteins that are exported from the cell. Endomembrane System • Membranes that are related through direct physical continuity or by the transfer of membrane segments called vesicles. Endomembrane System Endoplasmic Reticulum • Often referred to as ER. • Makes up to 1/2 of the total membrane in cells. • Often continuous with the nuclear membrane. Structure of ER • Folded sheets or tubes of membranes. • Very “fluid” in structure with the membranes constantly changing size and shape. Types of ER • Smooth ER: no ribosomes. • Used for lipid synthesis, carbohydrate storage, detoxification of poisons. • Rough ER: with ribosomes. • Makes secretory proteins. Golgi Apparatus or Dictyosomes • Structure: parallel array of flattened cisternae. (looks like a stack of Pita bread) • 3 to 20 per cell. • Likely an outgrowth of the ER system. Function of Golgi Bodies • Processing - modification of ER products. • Distribution - packaging of ER products for transport. Golgi Vesicles • Small sacs of membranes that bud off the Golgi Body. • Transportation vehicle for the modified ER products. Lysosome • Single membrane. • Made from the Golgi apparatus. Function • Breakdown and degradation of cellular materials. • Contains enzymes for fats, proteins, polysaccharides, and nucleic acids. • Over 40 types known. Lysosomes • Important in cell death. • Missing enzymes may cause various genetic enzyme diseases. • Examples: Tay-Sachs, Pompe’s Disease Vacuoles • Structure - single membrane, usually larger than the Golgi vesicles. • Function - depends on the organism. Protists • Contractile vacuoles - pump out excess water. • Food vacuoles - store newly ingested food until the lysosomes can digest it. Plants • Large single vacuole when mature making up to 90% of the cell's volume. • Tonoplast - the name for the vacuole membrane. Function • Water regulation. • Storage of ions. • Storage of hydrophilic pigments. (e.g. red and blues in flower petals). Function: Plant vacuole • Used to enlarge cells and create turgor pressure. • Enzymes (various types). • Store toxins. • Coloration. Microbodies • Structure: single membrane. • Often have a granular or crystalline core of enzymes. Function • Specialized enzymes for specific reactions. • Peroxisomes: use up hydrogen peroxide. • Glyoxysomes: lipid digestion. Enzymes in a crystal Mitochondria • Structure: 2 membranes. The inner membrane has more surface area than the outer membrane. • Matrix: inner space. • Intermembrane space: area between the membranes. Inner Membrane • Folded into cristae. • Amount of folding depends on the level of cell activity. • Contains many enzymes. • ATP generated here. Function • Cell Respiration - the release of energy from food. • Major location of ATP generation. • “Powerhouse” of the cell. Comment – be careful NOT to overuse this phrase. Mitochondria • • • • Have ribosomes (small size). Have their own DNA. Can reproduce themselves. Likely were independent cells at one time. Chloroplasts • Structure - two outer membranes. • Complex internal membrane. • Fluid-like stroma is around the internal membranes. Inner or Thylakoid Membranes • Arranged into flattened sacs called thylakoids. • Some regions stacked into layers called grana. • Contain the green pigment chlorophyll. Function • Photosynthesis - the use of light energy to make food. Chloroplasts • • • • • Contain ribosomes (small size). Contain DNA. Can reproduce themselves. Often contain starch. Likely were independent cells at one time (cyano-bacteria). Plastids • Group of plant organelles. • Structure - single membrane. • Function - store various materials. Examples • Amyloplasts/ Leucoplasts - store starch. • Chromoplasts - store hydrophobic plant pigments such as carotene. Ergastic Materials • General term for other substances produced or stored by plant cells. • Examples: • • • • Crystals Tannins Latex Resins Thursday, September 24 • Which of these statements accurately reflects the relationship between cell size and surface area? • Larger cells are most efficient at transporting materials across the membrane since their surface area is increased. • Smaller cells must have more phospholipids per area in order to adequately transport materials into the cell. • Cells must maximize their surface area to volume ratio in order to maintain homeostasis. • Cells must minimize their surface area exposure to the extracellular matrix in order to retain cytosol. Cytoskeleton • Network of rods and filaments in the cytoplasm. Functions • Cell structure and shape. • Cell movement. • Cell division - helps build cell walls and move the chromosomes apart. Cytoskeleton Components • Microtubules • Microfilaments • Intermediate Filaments Microtubules • Structure - small hollow tubes made of repeating units of a protein dimer. • Size - 25 nm diameter with a 15 nm lumen. Can be 200 nm to 25 mm in length. Tubulin • Protein in microtubules. • Dimer - a and b tubulin. Microtubules • Regulate cell shape. • Coordinate direction of cellulose fibers in cell wall formation. • Tracks for motor molecules. Microtubules • Form cilia and flagella. • Internal cellular movement. • Make up centioles, basal bodies and spindle fibers. Cilia vs. Flagella • Cilia - short, but numerous. • Flagella - long, but few. • Function - to move cells or to sweep materials past a cell. Cilia and Flagella • Structure - 9+2 arrangement of microtubules, covered by the cell membrane. • Dynein - motor protein that connects the tubules. Dynein Protein • A contractile protein. • Uses ATP. • Creates a twisting motion between the microtubules causing the structure to bend or move. Centrioles • Usually one pair per cell, located close to the nucleus. • Found in animal cells. • 9 sets of triplet microtubules. • Help in cell division. Basal Bodies • Same structure as a centriole. • Anchor cilia and flagella. Basal Body Microfilaments • 5 to 7 nm in diameter. • Structure - two intertwined strands of actin protein. Microfilaments are stained green. Functions • • • • • Muscle contraction. Cytoplasmic streaming. Pseudopodia. Cleavage furrow formation. Maintenance and changes in cell shape. Intermediate Filaments • Fibrous proteins that are super coiled into thicker cables and filaments 8 - 12 nm in diameter. • Made from several different types of protein. Functions • Maintenance of cell shape. • Hold organelles in place. Cytoskeleton • Very dynamic; changing in composition and shape frequently. • Cell is not just a "bag" of cytoplasm within a cell membrane. Cell Wall • Nonliving jacket that surrounds some cells. • Found in: • • • • Plants Prokaryotes Fungi Some Protists Plant Cell Walls • All plant cells have a Primary Cell Wall. • Some cells will develop a Secondary Cell Wall. Primary Wall • Thin and flexible. • Cellulose fibers placed at right angles to expansion. • Placement of fibers guided by microtubules. Secondary Wall • Thick and rigid. • Added between the cell membrane and the primary cell wall in laminated layers. • May cover only part of the cell; giving spirals. • Makes up "wood”. Middle Lamella • Thin layer rich in pectin found between adjacent plant cells. • Glues cells together. Cell Walls • May be made of other types of polysaccharides and/or silica. • Function as the cell's exoskeleton for support and protection. Extracellular Matrix - ECM • • • • Fuzzy coat on animal cells. Helps glue cells together. Made of glycoproteins and collagen. Evidence suggests ECM is involved with cell behavior and cell communication. Intercellular Junctions • Plants-Plasmodesmata Plasmodesmata • Channels between cells through adjacent cell walls. • Allows communication between cells. • Also allows viruses to travel rapidly between cells. Intercellular Juctions • Animals: • Tight junctions • Desmosomes • Gap junctions Tight Junctions • Very tight fusion of the membranes of adjacent cells. • Seals off areas between the cells. • Prevents movement of materials around cells. Desmosomes • Bundles of filaments which anchor junctions between cells. • Does not close off the area between adjacent cells. • Coordination of movement between groups of cells. Gap Junctions • Open channels between cells, similar to plasmodesmata. • Allows “communication” between cells. Chapter Summary • Answer: Why is Life cellular and what are the factors that affect cell size? • Be able to identify cellular parts, their structure, and their functions. Cell Animation Link • http://multimedia.mcb.harvard.edu/anim _innerlife_hi.html • You may need to replay this several times to catch all of the parts.