CELL STRUCTURE AND FUNCTION

CHAPTER 3

Processes of Life

GrowthReproductionResponsivenessMetabolism

Prokaryotes

Do not have membrane surrounding their DNA; no nucleus

Lack various internal structures bound with phospholipid membranes

Small; ~1.0 µm in diameterSimple structureComprised of bacteria and archaea

Eukaryotes

Have membrane surrounding DNA; have nucleusHave internal membrane-bound organellesAre larger; 10-100 µm in diameterHave more complex structureComprised 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

GlycocalycesFlagellaFimbriae and pili

Glycocalyces

Gelatinous, sticky substance surrounding the outside of the cell

Composed of polysaccharides, polypeptides, or bothTwo types

Capsule Slime layer

Capsule

Composed of organized repeating units of organic chemicals

Firmly attached to cell surfaceProtects cells from drying outMay prevent bacteria from being recognized and

destroyed by host

Example of Capsule

Figure 3.4a

Slime Layer

Loosely attached to cell surfaceWater solubleProtects cells from drying outSticky layer that allows prokaryotes to attach to

surfaces

Example of Slime Layer

Figure 3.4b

Flagella

Are responsible for movementHave long structures that extend beyond cell surfaceNot all prokaryotes have flagella

Bacterial Flagella Structure

Composed of filament, hook, and basal bodyFlagellin protein (filament) is deposited in a helix at

the lengthening tipBase of filament inserts into hookBasal 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 environmentRotation can be clockwise or counterclockwise;

reversibleBacteria 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 extensionsFimbriae

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 pilinLonger than fimbriae but shorter than flagellaBacteria 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 NAMChains 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 acidsScientists describe two basic types of bacterial cell

walls: gram-positive and gram-negative

Gram-Positive Cell Wall

Relatively thick layer of peptidoglycanContains 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 peptidoglycanBilayer membrane outside the peptidoglycan contains

phospholipids, proteins, and lipopolysaccharide (LPS) May be impediment to the treatment of diseaseFollowing Gram staining procedure, cells appear pink

LPS

Union of lipid with sugarAlso known as endotoxinLipid 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 proteinsGram-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 substancesProteins allow substances to cross membraneOccurs by passive or active processesMaintains 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

DiffusionFacilitated 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 chemicalsRibosomes – sites of protein synthesisCytoskeleton – plays a role in forming the cell’s basic

shapeSome bacterial cells produce dormant form called

endospore

External Structure of Eukaryotic Cells

GlycocalycesNever as organized as prokaryotic capsulesHelps anchor animal cells to each otherStrengthens cell surfaceProvides protection against dehydrationFunction 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 proteinsContains steroid lipids to help maintain fluidityControls 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

FlagellaCiliaRibosomesCytoskeletonCentrioles and centrosome

Flagella

Shaft composed of tubulin arranged form microtubules “9 + 2” arrangement of microtubules in all flagellated

eukaryotesFilaments anchored to cell by basal body; no hookBasal body has “9 + 0” arrangement of microtubulesMay be single or multiple; generally found at one pole

of cellDo not rotate, but undulate rhythmically

Cilia

Shorter and more numerous than flagellaComposed of tubulin in “9 + 2” and “9 + 0”

arrangementsCoordinated beating propels cells through their

environmentAlso 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

NucleusEndoplasmic reticulumGolgi bodyLysosomes, peroxisomes, vacuoles, and vesiclesMitochondriaChloroplasts

Nucleus

Often largest organelle in cellContains most of the cell’s DNASemiliquid portion called nucleoplasmOne or more nucleoli present in nucleoplasm; RNA

synthesized in nucleoliNucleoplasm contains chromatin – masses of DNA

associated with histonesSurrounded by double membrane composed of two

phospholipid bilayers – nuclear envelopeNuclear envelope contains nuclear pores

Endoplasmic Reticulum

Netlike arrangement of flattened, hollow tubules continuous with nuclear envelope

Functions as transport systemTwo 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 cellsMay store nutrients in cellLysosomes 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 DNAHave 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