5.0 Introduction

1. All Organisms Are Made of Cells
1. Some Are Small and Single Celled fig 5.1
2. Others Are Composed of Multitudes of Cells

1. Cells
1. All Cells Share Three Major Features fig 5.2
1. Plasma membrane
2. Nucleoid or nucleus
3. Cytoplasm

2. The Plasma Membrane Surrounds the Cell
1. Phospholipid bilayer contains embedded proteins
1. Appear as two dark lines separated by lighter area
2. Appearance due to arrangement of phospholipid molecules fig 3.14
3. Major proteins have large hydrophobic domains

2. Proteins enable cell to interact with environment
1. Transport proteins facilitate passage across membrane
2. Receptors induce cell changes with contact by molecules
3. Markers provide cell identity

3. The Central Portion of the Cell Contains the Genetic Material
1. Genetic material in prokaryotes
1. Single, circular molecule of DNA
2. Is concentrated in the nucleoid, not membrane bound

2. Genetic material in eukaryotes
1. Contained within the nucleus
2. Surrounded by two membranes

4. The Cytoplasm Comprises the Rest of the Cell's Interior
1. Cytoplasm is a semifluid matrix
2. Contains chemicals to carry out growth and reproduction
3. Eukaryotic cells also contain membrane-bound organelles

2. The Cell Theory
1. Most Cells Are Microscopic in Size
1. Exceptions like Acetabularia may be up to 5 cm long
2. Typical eukaryotic cell is 10 to 100 micrometers fig 5.3

2. Cells Were Observed Only With the Invention of the Microscope
1. Robert Hooke
1. First observed honeycomb of empty compartments in cork in 1665
2. Called compartments "cellulae" (cells)

2. Antonie Van Leeuwenhoek
1. First observance of living cells
2. Called organisms "animalcules"

3. Matthias Schleiden
1. Observed plant tissues
2. All plants aggregates of separate cells

4. Theodor Schwann
1. Observed animal tissues
2. All animals composed of individual cells

5. Modern principles of cell theory
1. All organisms composed of one or more cells
2. Cell is smallest living organizational unit
3. Cells arise only from division of other cells

3. Cells Are Small
1. The Resolution Problem
1. Human eye can't see cells because of limited resolution
2. Resolution: Minimum distance between two points to distinguish as separate points
3. Human eye can resolve points if 100 micrometers or more apart

2. Microscopes
1. Magnification increases resolution, objects appear larger than 100 micrometer limit
2. Earliest microscopes used single glass lens between object and eye
3. Modern microscopes are compound microscopes fig 5.4a
1. Use two magnifying and multiple correcting lenses
2. Best resolution distinguishes objects 200 nanometers apart

3. Increasing Resolution
1. Light microscope resolution limited by size of wavelengths of light
2. Wavelength of electrons shorter than light by 1000 fold
3. Transmission electron microscope
1. Electrons transmitted through slice of object
2. Can resolve objects only 0.2 nanometers apart fig 5.4b

4. Scanning electron microscope
1. Electrons directed to surface of object, bounce back
2. Shows a three-dimensional image of the object fig 5.4c

4. Why Aren't Cells Larger?
1. Limitation of communication
2. Limitations of molecular diffusion
1. Faster passage through small cells
2. More efficient communication

3. Limitations of surface-to-volume ratio fig 5.5
1. With increase in size, greater increase in volume than surface area
2. Interaction with outside occurs only at surface
3. Insufficient exchange of materials at plasma membrane for survival
4. Small cells have more surface per unit of volume than large cells

1. Bacteria Are Simple Cells
1. Prokaryotes Are the Simplest Cellular Organisms
1. Great diversity fig 5.7
2. Similar organization, small size fig 5.8
1. Surrounded by membrane, encased in rigid cell wall
2. No internal compartments

3. May adhere in masses, but are function independent of one another

2. Strong Cell Walls
1. Peptidoglycan carbohydrate matrix cross linked with peptide units
2. Gram positive, thick cell wall, retains stain
3. Gram negative, thinner cell wall, releases stain
4. Polysaccharide layer may surround cell wall

3. Simple Interior Organization
1. Lack internal compartmentalization
1. Contain ribosomes, but lack membrane-bound organelles
2. No true nucleus or internal support structures
3. Cell strength due to cell wall fig 5.8

2. Plasma membrane carries out functions of organelles in eukaryotes
1. Associated with cell division
2. Location of bacterial photosynthetic pigments fig 5.9

3. Reactions not separated, cell is a single metabolic unit

4. Rotating Flagella
1. Long, threadlike structures that protrude from cell surface
2. Used in locomotion and feeding
3. Cell movement results from screw-like rotation fig 5.10

2. Eukaryotic Cells Have Complex Interiors
1. Eukaryotes Are More Complex Than Prokaryotes fig 5.11,12
2. Hallmark Is Compartmentalization
1. Possess internal membrane-bound organelles tbl 5.1
1. Central vacuole in plants stores protein and wastes
2. Vesicles in animals store and transport many materials
3. Nucleus contains chromosomes made of DNA and proteins

2. Cytoskeleton is an internal scaffold of proteins
3. Cell walls: Cellulose/chitin fibers embedded in polysaccharides, proteins

1. The Nucleus: Information Center for the Cell
1. Largest Organelle in Most Cells, Readily Visible
1. Spherical appearance, centrally located fig 5.13
2. Positioned by filaments
3. Repository of all genetic information
4. Usually singular, lacking in mature red blood cells
5. Dark-staining nucleolus visible in cells synthesizing RNA

2. The Nuclear Envelope: Getting In and Out fig 5.13
1. Double layer of membranes
2. Outer continuous with cytoplasm's internal endoplasmic reticulum
3. Membranes pinched together at nuclear pores
1. Embedded with proteins, serve as molecular channels
2. Restrict passage of molecules to proteins and RNA

3. The Chromosomes: Packaging the DNA
1. DNA contains hereditary information specifying structure and function
2. Divided into linear chromosomes
1. Uncoiled as chromatin except when cell is dividing
2. Uncoiling permits proteins to access DNA to direct cell activities

3. Associated with histone protein
1. Enables condensation during cell division
2. Aggregations called nucleosomes fig 5.14

4. Fully condensed chromosomes appear as dark-staining rods fig 5.15
1. Uncoiling permits RNA polymerase to access DNA
1. Makes RNA copies of the DNA
2. RNA directs synthesis of proteins

2. The Endoplasmic Reticulum: Compartmentalizing the Cell
1. General Characteristics
1. Thin membranes not visible in light microscope
2. Endomembrane system divides interior into compartments
1. Channels passage of molecules through cell
2. Provides surface for protein and lipid synthesis

3. Largest membrane called endoplasmic reticulum, abbreviated ER
1. Lipid bilayer with embedded proteins
2. Creates channels and interconnections between folds fig 5.16

2. Rough ER: Manufacturing Proteins for Export
1. Ribosomes assist manufacture of proteins
1. Aggregates of protein and RNA
2. Translate RNA copies of genes into proteins
3. Through electron microscope look rough, like surface of sandpaper fig 5.16

2. Proteins used in cell or may be exported
1. Contain signal sequences fig 5.17
2. Initial translation by free ribosome
3. Signal sequence attaches recognition factor
4. Aggregation travels to ER docking site
5. Protein directed to Golgi apparatus

3. Smooth ER: Organizing Internal Activities
1. Possess few bound ribosomes
2. Contain embedded enzymes
3. Catalyze synthesis of many proteins and lipids
4. Associated with detoxification in liver
5. Vesicles may form at plasma membrane and travel to smooth ER

3. The Golgi Apparatus: Delivery System of the Cell
1. Golgi Bodies fig 5.18
1. Individual, flattened stacks of membranes
2. Abundant in glandular secretory cells
3. Collectively called the Golgi apparatus

2. Functions in Collection, Packaging, Distribution of Molecules
1. Golgi body has front and back ends fig 5.19
1. Materials from ER move to cis face, front or receiving end
2. Molecules pass through to back or trans face
3. Discharged into secretory vesicles

2. Manufactured products of ER transported into it
3. Bind to polysaccharides forming glycoproteins and glycolipids
4. Molecules collect at flattened, stacked folds of membranous cisternae
5. Liposomes are synthetically-manufactured vesicles
1. Contain variety of beneficial substances
2. Injected into body, effective delivery to cells

4. Vesicles: Enzyme Storehouses
1. Lysosomes: Intracellular Digestion Centers
1. Component of endomembrane system, arise from Golgi apparatus
2. Contain concentrated mix of digestive enzymes
3. Enzymes catalyze breakdown of macromolecules within cell
4. Digest worn-out cell components and recycle material into new structures
5. Alter internal pH to effect control of digestion
1. Primary lysosome has high pH and is inactive
2. Secondary lysosome has low pH and is active

6. Avoiding self digestion is an unknown process that requires energy
1. Metabolically inactive eukaryotes die
2. Lysosome membrane digested by enzymes within
3. Cell destroyed by released enzymes
4. Bacteria lack lysosomes can be metabolically inactive

7. Eliminate particles and foreign cells via phagocytosis fig 5.20
1. Digest pathogens engulfed by white blood cells
2. Release enzymes into food vesicle, degrade material inside

2. Peroxisomes: Detoxifiers of Hydrogen Peroxide
1. Enzyme-bearing, membrane-bound vesicles called microbodies
2. Arise from pre-existing microbodies, grow and divide
3. Glyoxysomes in plants convert fats to carbohydrates
4. Peroxisomes contain enzymes that catalyze removal of electrons and hydrogen atoms
1. Need to be contained or would interfere with many metabolic activities
2. Catalase breaks toxic hydrogen peroxide into water and oxygen

5. Ribosomes: Sites of Protein Synthesis
1. Proteins Are Synthesized in Cytoplasm Not Nucleus
1. Read mRNA copy of DNA gene to direct synthesis of protein
2. Ribosomes composed of ribosomal RNA (rRNA) and proteins
1. Composed of two subunits fig 5.21
2. Subunits combine only when attached to messenger RNA
3. Ribosomes in bacterial are smaller than ones in eukaryotes

3. Greater number of ribosomes metabolically active tissues
1. Cytoplasmic proteins made by free ribosomes
2. Rough ER ribosomes produce proteins used on membranes or to be exported

2. The Nucleolus Manufactures Ribosomal Subunits
1. DNA coding for ribosomal RNA (rRNA) clustered to maximize synthesis
1. Large numbers of ribosomes synthesized rapidly
2. Cell can then make large amounts of protein

2. Dark-staining nucleolus visible in cells assembling ribosomes fig 5.22
1. Location of ribosome synthesis
2. Present when chromosomes are uncoiled and invisible

6. Organelles that Contain DNA
1. Mitochondria: The Cell's Chemical Furnaces fig 5.23
1. Tubular or sausage-shaped organelles bounded by double membrane
2. Occur in all organisms
3. Outer membrane is smooth
4. Inner membrane is folded into contiguous layers called cristae
1. Divides into inner matrix and outer compartment or intermembrane space
2. Associated with proteins of oxidative metabolism

5. Possesses own genome
1. Genes direct production of own RNA and ribosomal components
2. Genes for oxidative metabolism are in nucleus

6. Capable of replication
1. Distributed between halves of dividing cells
2. Replenish numbers by simple fission division
3. Components for division are governed by genes in nucleus
4. Not completely autonomous, cannot be cultured separately

2. Chloroplasts: Where Photosynthesis Takes Place
1. Occur in plants and other photosynthetic organisms
2. Confer advantage to cells: Can make own food
3. Contain chlorophyll, give plants green color
4. Bounded by double membrane fig 5.24
1. Internal membranes form disk-shaped thylakoids
2. Photosynthetic pigments on thylakoid surface
3. Stack of thylakoids called granum
4. Surrounded by stroma, a fluid matrix

5. Possess own genome
1. Genes for chloroplast components located in nucleus
2. RNA and protein components for photosynthesis on chloroplast DNA

6. Become leucoplasts when deprived of light
1. Internal structure lost
2. Specialized amyloplasts store starch

7. Chloroplast, leukoplast, amyloplast collectively called plastids

3. Centrioles: Microtubular Assembly Centers
1. Barrel-shaped organelles present in animal and protist cells
2. Occur in pairs at right angles near nuclear envelope, forms the centrosome fig 5.25
3. Some contain DNA involved in producing their structural proteins
4. Associated with assembly and organization of microtubules
1. Influence cell shape, move chromosomes
2. Produce functional internal structure of flagella and cilia
3. Example of microtubule-organizing centers (MTOCs)

5. Absent in plant and fungal cells

7. The Cytoskeleton: Interior Framework of the Cell
1. Network of Protein Fibers fig 5.26
1. Supports shape of cell, anchor organelles to fixed location
2. Formed by polymerization of identical protein subunits
3. Also disassembled subunit by subunit

2. Three Types of Cytoskeleton Fibers fig 5.27
1. Actin filaments fig 5.28a
1. Fibers composed of two chains like two intertwined strands of pearls
2. Actin proteins are the pearl molecules
3. Form spontaneously
4. Cell controls polymerization via other proteins
5. Responsible for cellular movements, formation of cellular extensions

2. Microtubules fig 5.28b
1. Spontaneously form hollow tubes of 13 protein protofilaments
2. Alpha and beta tubulin subunits polymerize to form protofilaments
3. Form from MTOC nucleation centers
4. In constant flux, polymerizing and depolymerizing
1. Stabilized when guanine triphosphate (GTP) binds to ends
2. "1" end is away from the nucleating center
3. "2" end is toward the nucleating center

5. Help move materials within the cell itself
1. Kinesin protein moves organelles to "1" end (periphery)
2. Dynein protein moves organelles to "2" end (center)

3. Intermediate filaments fig 5.28c
1. Fibrous proteins twined together to form overlapping tetrameres
2. Composed of various subunits of intermediate size
3. Fibers very stable do not break down readily
4. Vimentin subunits make filaments that provide structural stability
5. Other examples: Keratin and neurofilaments

8. Cell Movement
1. Cell Movement Associated with Cytoskeletal Fibers
1. Cell motion tied to movement of actin filaments, microtubules or both
2. Intermediate fibers prevent excessive stretching
3. Actin filaments also important in determining cell shape
4. Rapid production of microvilli changes cell shape quickly fig 5.29

2. Some Cells Crawl
1. May have implications in healing and slowing spread of cancer
2. Movement of white blood cells is good example
3. Results in regional changes in gel-sol state
1. Periphery is usually more rigid (gel)
2. Interior is usually very fluid (sol)

4. Formation of pseudopods to move cell
5. Cell motion tied to movement of actin filaments and/or microtubules
6. Provide scaffold for anchoring cell enzymes
1. Metabolic enzymes and ribosomes bind to actin filaments
2. Organize metabolic activities of cell by relocating elements

3. Intracellular Molecular Motors
1. ER transport too slow for movement over long distances
2. Utilize embedded linking proteins that bind to motor proteins
3. Kinectin is found in the endoplasmic reticulum
1. Binds vesicles to kinesin
2. Kenesin uses ATP to power movement toward cell periphery
3. Drags vesicle with it

4. Another or a modified protein directs movement in the opposite direction
1. Binds to dynein
2. Moves vesicle towards center of cell

5. Destination of vesicle and contents related to protein in vesicle's membrane

4. Swimming with Flagella and Cilia
1. Eukaryote and bacteria flagella are completely different in structure
2. Eukaryote 9+2 structure of microtubules fig 5.30
1. Undulating movement results from sliding of filaments
2. Projection enclosed by cell membrane
3. Derived from basal body below cell membrane

3. Cilia also show 9+2 arrangement
1. Numerous, short projections called cilia fig 5.1
2. Have functions other than locomotion
1. 1) Pass fluids over tissue surface
2. 2) Bend in response to sound waves 

1. Endosymbiosis
1. Eukaryotes Have Radically Different Cell Structure
1. Possess organelles that resemble bacteria, endosymbiont theory
2. Endosymbiosis of different species of prokaryotes fig 5.31
3. Mitochondria are like bacteria that carry out oxidative metabolism
4. Chloroplasts are like photosynthetic bacteria

2. Evidence Supporting Theory
1. Mitochondria and chloroplasts surrounded by double membrane
2. Mitochondria and bacteria have similar size
3. Mitochondrial ribosomes resemble bacterial ribosomes
4. Mitochondria and chloroplast DNA circular like bacteria
5. Mitochondria divide by simple fission
6. Centrioles resemble spirochaete bacteria

3. Comparison of Features of Modern Cells tbl 5.2