CELL STRUCTURE AND FUNCTION CHAPTER 3

Processes of Life  Growth  Reproduction  Responsiveness  Metabolism

Prokaryotes  Do not have membrane surrounding their DNA; no nucleus  Lack various internal structures bound with phospholipid membranes  Small; ~1.0 µm in diameter  Simple structure  Comprised of bacteria and archaea

Eukaryotes  Have membrane surrounding DNA; have nucleus  Have internal membrane-bound organelles  Are larger; µm in diameter  Have more complex structure  Comprised of algae, protozoa, fungi, animals, and plants

Comparing Prokaryotes and Eukaryotes Figure 3.2a

Comparing Prokaryotes and Eukaryotes Figure 3.2b

External Structures of Prokaryotic Cells  Glycocalyces  Flagella  Fimbriae and pili

Glycocalyces  Gelatinous, sticky substance surrounding the outside of the cell  Composed of polysaccharides, polypeptides, or both  Two types  Capsule  Slime layer

Capsule  Composed of organized repeating units of organic chemicals  Firmly attached to cell surface  Protects cells from drying out  May prevent bacteria from being recognized and destroyed by host

Example of Capsule Figure 3.4a

Slime Layer  Loosely attached to cell surface  Water soluble  Protects cells from drying out  Sticky layer that allows prokaryotes to attach to surfaces

Example of Slime Layer Figure 3.4b

Flagella  Are responsible for movement  Have long structures that extend beyond cell surface  Not all prokaryotes have flagella

Bacterial Flagella Structure  Composed of filament, hook, and basal body  Flagellin protein (filament) is deposited in a helix at the lengthening tip  Base of filament inserts into hook  Basal body anchors filament and hook to cell wall by a rod and a series of either two or four rings of integral proteins  Filament capable of rotating 360º

Bacterial Flagella Structure Figure 3.5a

Bacterial Flagella Structure Figure 3.5b

Arrangements of Bacterial Flagella Figure 3.6a

Arrangements of Bacterial Flagella Figure 3.6b

Arrangements of Bacterial Flagella Figure 3.6c

Function of Bacterial Flagella  Rotation propels bacterium through environment  Rotation can be clockwise or counterclockwise; reversible  Bacteria move in response to stimuli (taxis)  Runs – movements of cell in single direction for some time due to counterclockwise flagellar rotation; increase with favorable stimuli (positive chemotaxis, positive phototaxis)  Tumbles – abrupt, random, changes in direction due to clockwise flagellar rotation; increase with unfavorable stimuli (negative chemotaxis, negative phototaxis)

Bacterial Movement

Fimbriae and Pili  Nonmotile extensions  Fimbriae  Sticky, proteinaceous, bristlelike projections  Used by bacteria to adhere to one another, to hosts, and to substances in environment  May be hundreds per cell and are shorter than flagella  Serve an important function in biofilms

Fimbriae Versus Flagella Figure 3.9

Pili  Long hollow tubules composed of pilin  Longer than fimbriae but shorter than flagella  Bacteria typically only have one or two per cell  Join two bacterial cells and mediate the transfer of DNA from one cell to another (conjugation)  Also known as conjugation pili or sex pili

Pilus Versus Fimbriae Figure 3.10

Prokaryotic Cell Wall  Provides structure and shape and protects cell from osmotic forces  Assists some cells in attaching to other cells or in eluding antimicrobial drugs  Animal cells do not have; can target cell wall of bacteria with antibiotics  Bacteria and archaea have different cell wall chemistry

Bacterial Cell Wall  Most have cell wall composed of peptidoglycan; a few lack a cell wall entirely  Peptidoglycan composed of sugars, NAG, and NAM  Chains of NAG and NAM attached to other chains by tetrapeptide crossbridges  Bridges may be covalently bonded to one another  Bridges may be held together by short connecting chains of amino acids  Scientists describe two basic types of bacterial cell walls: gram-positive and gram-negative

Gram-Positive Cell Wall  Relatively thick layer of peptidoglycan  Contains unique polyalcohols called teichoic acids  Some covalently linked to lipids, forming lipoteichoic acids that anchor peptidoglycan to cell membrane  Retains crystal violet dye in Gram staining procedure; appear purple  Acid-fast bacteria contain up to 60% mycolic acid; helps cells survive desiccation

Gram-Negative Cell Walls  Have only a thin layer of peptidoglycan  Bilayer membrane outside the peptidoglycan contains phospholipids, proteins, and lipopolysaccharide (LPS)  May be impediment to the treatment of disease  Following Gram staining procedure, cells appear pink

LPS  Union of lipid with sugar  Also known as endotoxin  Lipid portion known as lipid A  Dead cells release lipid A when cell wall disintegrates  May trigger fever, vasodilation, inflammation, shock, and blood clotting  Can be released when antimicrobial drugs kill bacteria

Periplasmic Space  Located between outer membrane and cell membrane  Contains peptidoglycan and periplasm  Contains water, nutrients, and substances secreted by the cell, such as digestive enzymes and proteins involved in transport

Bacterial Cell Walls Figure 3.13a

Bacterial Cell Walls Figure 3.13b

Archael Cell Walls  Do not have peptidoglycan  Cell walls contain variety of specialized polysaccharides and proteins  Gram-positive archaea stain purple  Gram-negative archaea stain pink

Prokaryotic Cytoplasmic Membrane  Referred to as phospholipid bilayer; composed of lipids and associated proteins  Approximately half the membrane is composed of proteins that act as recognition proteins, enzymes, receptors, carriers, or channels  Integral proteins  Peripheral proteins  Glycoproteins  Fluid mosaic model describes current understanding of membrane structure

Phospholipid Bilayer of Cytoplasmic Membrane Figure 3.14

Cytoplasmic Membrane Function  Controls passage of substances into and out of the cell; selectively permeable  Harvests light energy in photosynthetic prokaryotes

Control of Substances Across Cytoplasmic Membrane  Naturally impermeable to most substances  Proteins allow substances to cross membrane  Occurs by passive or active processes  Maintains a concentration gradient and electrical gradient  Chemicals concentrated on one side of the membrane or the other  Voltage exists across the membrane

Passive Processes of Transport  Diffusion  Facilitated diffusion  Osmosis  Isotonic solution  Hypertonic solution  Hypotonic solution

Effects of Solutions on Organisms Figure 3.18

Active Processes of Transport  Active Transport  Utilizes permease proteins and expends ATP  Uniport  Antiport  Symport  Group Translocation  Substance chemically modified during transport

Cytoplasm of Prokaryotes  Cytosol – liquid portion of cytoplasm  Inclusions – may include reserve deposits of chemicals  Ribosomes – sites of protein synthesis  Cytoskeleton – plays a role in forming the cell’s basic shape  Some bacterial cells produce dormant form called endospore

External Structure of Eukaryotic Cells  Glycocalyces  Never as organized as prokaryotic capsules  Helps anchor animal cells to each other  Strengthens cell surface  Provides protection against dehydration  Function in cell-to-cell recognition and communication

Eukaryotic Cell Walls  Fungi, algae, plants, and some protozoa have cell walls but no glycocalyx  Composed of various polysaccharides  Cellulose found in plant cell walls  Fungal cell walls composed of cellulose, chitin, and/or glucomannan  Algal cell walls composed of cellulose, proteins, agar, carrageenan, silicates, algin, calcium carbonate, or a combination of these

Eukaryotic Cytoplasmic Membrane  All eukaryotic cells have cytoplasmic membrane  Is a fluid mosaic of phospholipids and proteins  Contains steroid lipids to help maintain fluidity  Controls movement into and out of cell  Uses diffusion, facilitated diffusion, osmosis, and active transport  Performs endocytosis; phagocytosis if solid substance and pinocytosis if liquid substance  Exocytosis enables substances to be exported from cell

Cytoplasm of Eukaryotes – Nonmembranous Organelles  Flagella  Cilia  Ribosomes  Cytoskeleton  Centrioles and centrosome

Flagella  Shaft composed of tubulin arranged form microtubules  “9 + 2” arrangement of microtubules in all flagellated eukaryotes  Filaments anchored to cell by basal body; no hook  Basal body has “9 + 0” arrangement of microtubules  May be single or multiple; generally found at one pole of cell  Do not rotate, but undulate rhythmically

Cilia  Shorter and more numerous than flagella  Composed of tubulin in “9 + 2” and “9 + 0” arrangements  Coordinated beating propels cells through their environment  Also used to move substances past the surface of the cell

Eukaryotic Flagella Figure 3.27a

Eukaryotic Cilia Figure 3.27c

Eukaryotic Flagella and Cilia Figure 3.27b

Ribosomes  Larger than prokaryotic ribosomes (80S versus 70S)  Composed of 60S and 40S subunits

Cytoskeleton  Extensive  Functions  Anchor organelles  Cytoplasmic streaming and movement of organelles  Movement during endocytosis and amoeboid action  Produce basic shape of the cell  Made up of tubulin microtubules, actin microfilaments, and intermediate filaments composed of various proteins

Centrioles and Centrosome  Centrioles play a role in mitosis, cytokinesis, and in formation of flagella and cilia  Centrioles composed of “9 + 0” arrangement of microtubules  Centrosome – region of cytoplasm where centrioles are found

Cytoplasm of Eukaryotes – Membranous Organelles  Nucleus  Endoplasmic reticulum  Golgi body  Lysosomes, peroxisomes, vacuoles, and vesicles  Mitochondria  Chloroplasts

Nucleus  Often largest organelle in cell  Contains most of the cell’s DNA  Semiliquid portion called nucleoplasm  One or more nucleoli present in nucleoplasm; RNA synthesized in nucleoli  Nucleoplasm contains chromatin – masses of DNA associated with histones  Surrounded by double membrane composed of two phospholipid bilayers – nuclear envelope  Nuclear envelope contains nuclear pores

Endoplasmic Reticulum  Netlike arrangement of flattened, hollow tubules continuous with nuclear envelope  Functions as transport system  Two forms  Smooth endoplasmic reticulum (SER) – plays role in lipid synthesis  Rough endoplasmic reticulum (RER) – ribosomes attached to its outer surface; transports proteins produced by ribosomes

Rough and Smooth Endoplasmic Reticulum Figure 3.32

Golgi Body  Receives, processes, and packages large molecules for export from cell  Packages molecules in secretory vesicles that fuse with cytoplasmic membrane  Composed of flattened hollow sacs surrounded by phospholipid bilayer  Not all eukaryotic cells contain Golgi bodies

Golgi Body Figure 3.33

Lysosomes, Peroxisomes, Vacuoles, and Vesicles  Store and transfer chemicals within cells  May store nutrients in cell  Lysosomes contain catabolic enzymes  Peroxisomes contain enzymes that degrade poisonous wastes

Mitochondria  Have two membranes composed of phospholipid bilayer  Produce most of cell’s ATP  Interior matrix contains 70S ribosomes and circular molecule of DNA

Chloroplasts  Light-harvesting structures found in photosynthetic eukaryotes  Have two phospholipid bilayer membranes and DNA  Have 70S ribosomes

Endosymbiotic Theory  Eukaryotes formed from union of small aerobic prokaryotes with larger anaerobic prokaryotes; smaller prokaryotes became internal parasites  Parasites lost ability to exist independently; retained portion of DNA, ribosomes, and cytoplasmic membranes  Larger cell became dependent on parasites for aerobic ATP production  Aerobic prokaryotes evolved into mitochondria  Similar scenario for origin of chloroplasts