Transcript General Biology I (BIOLS 102)

Chapter 4: Cell Structure &
Function (Outline)
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Cell Theory
Cell Size
Prokaryotic Cells
Eukaryotic Cells
 Organelles
 Nucleus
 Endomembrane System
 Cytoskeleton
 Centrioles, Cilia and Flagella
Development of Cell Theory
 In 1665, English Scientist Robert Hooke discovered cells
while looking at a thin slice of cork
 In 1673, Anton van Leuwenhoek observed pond scum &
discovered single-celled organisms using a handmade
microscope
 In 1831, English botanist Robert Brown described the
nucleus of cells
 In 1838, German Botanist, Matthias Schleiden, stated
that all plant parts are made of cells
 In 1839, German physiologist Theodor Schwann stated
that all animal tissues are composed of cells
 In 1858, Rudolf Virchow German physician concluded
that cells must arise from preexisting cells
Cell Theory
 A unifying concept in biology
 Originated from the work of biologists
Schleiden, Schwann & Virchow
 States that:
 All organisms are composed of cells (Schleiden &
Schwann, 1838-39)
 The cell is the basic unit of structure & function in
organisms (Schleiden & Schwann, 1838-39)
 All cells come only from preexisting cells since cells
are self-reproducing (Virchow, 1858)
Cell Size
 Most much smaller than one millimeter (mm)
 Some as small as one micrometer (mm)
 Size restricted by Surface/Volume (S/V) ratio
 Surface is membrane, across which cell acquires
nutrients and expels wastes
 Volume is living cytoplasm, which demands
nutrients and produces wastes
 As cell grows, volume increases faster than
surface
 Cells specialized in absorption modified to
greatly increase surface area per unit volume
Surface to Volume Ratio
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TotalSurfaceArea
(Height  Width  Number Of Sides  Number Of Cubes)
96 cm2
192 cm2
384 cm2
TotalVolume
(Height  Width  Length x Number Of Cubes)
64 cm3
64 cm3
64 cm3
SurfaceAreaPerCube/VolumePerCube
(Surface Area/ Volume)
1.5/1
3/1
6/1
Sizes of living things and their
component
Prokaryotic Cells
 Prokaryotes – lack a membrane-bounded nucleus and
are structurally less complicated than the eukaryotes
 Prokaryotes are responsible for either all or significant
portions of all of the following
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Nutrient recycling – mineralization; nitrogen fixing
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Decomposition of dead organisms
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Disease (infectious) – tuberculoses; anthrax
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Commercial uses – foodstuffs; antibiotics; insulin
 Prokaryotes are divided into two domains
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Domain Bacteria
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Domain Archaea
Prokaryotic Cells
 Nuclear body is not bounded by a nuclear membrane
 Usually contains one circular chromosome composed
of deoxyribonucleic acid (DNA)
 The nuclear body is called a nucleoid
 Extra chromosomal piece of DNA called plasmid
 Structurally simple
 Three basic shapes:
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Bacillus (rod)
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Coccus (spherical)
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Spirilla (spiral)
Prokaryotic Cells:
The Envelope
 Cell Envelopes include
 Glycocalyx
 Layer of polysaccharides outside cell wall
 May be slimy and easily removed, or
 Well organized and resistant to removal (capsule)
 Cell wall
 Consist of peptidoglycan (amino disaccharide & peptide)
 Maintains shape of the cell
 Plasma membrane
 Like in eukaryotes – a phospholipid bilayer with proteins
 Form internal pouches (mesosomes), why?
Prokaryotic Cells:
Cytoplasm
 Cytoplasm - semifluid solution bounded by a
plasma membrane containing
 Nucleoid – location of the single bacterium
chromosome (coiled)
 Plasmid – extrachromosomal piece of circular DNA
 Inclusion bodies – Stored granules of various
substances
 Ribosomes – tiny particles where protein is
synthesized (contain RNA & protein in 2 subunits)
 Thylakoids – extensive internal membranes found
in cyanobacteria, function?
Prokaryotic Cells:
Appendages
 Appendages are made of protein that include
 Flagella – the most common form of bacterial
motility (made up of a filament, hook & basal
body)
 Fimbriae – small, bristle-like
fibers that sprout from the
cell surface (attach bacteria
to a surface)
 Conjugation pili – rigid tubular
structures used to pass DNA
from cell to cell
Prokaryotic Cells: Visual Summary
Eukaryotic Cells
 Domain Eukarya
 Protists
 Fungi
 Plants
 Animals
 Eukaryotic cells contain:
 a true nucleus, bound by a double membrane
 a complex collection of organelles
 a plasma membrane
Eukaryotic Cells :
Organelles
 Compartmentalization:
 Allows eukaryotic cells to be larger than
prokaryotic cells
 Isolates reactions from others
 Two classes:
 Endomembrane system:
 Organelles that communicate with one another
 via membrane channels and small vesicles
 Energy related organelles
 Mitochondria & chloroplasts
 Basically independent & self-sufficient
Animal and Plant Cells
Nucleus
 Command center of cell, why?
 Separated from cytoplasm by nuclear envelope
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Consists of double layer of membrane
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Nuclear pores permit exchange of ribosomal subunits &
mRNA between nucleoplasm & cytoplasm
 Contains chromatin in semifluid nucleoplasm
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Chromatin contains DNA of genes
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Condenses to form chromosomes
 Dark nucleolus composed of ribosomal RNA
(rRNA)
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Produces subunits of ribosomes
Anatomy of the nucleus
 Messenger RNA
(mRNA) carries
information about
a protein
sequence to the
ribosome
 Transfer RNA
(tRNA) assembles
the amino acid to
a growing
polypeptide chain
at the ribosomal
site of protein
synthesis
Ribosomes
 Serve in protein synthesis
 Composed of rRNA
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Consists of a large subunit and a small subunit
Each subunit is composed of protein and rRNA
Subunits made in nucleolus
Number of ribosomes in a cell varies depending on
function (e.g. pancreatic cells)
 May be located:
 On the endoplasmic reticulum (ER), thereby making
it “rough”, or
 Free in the cytoplasm, either singly or in groups
called polyribosomes
Ribosome Function
 Ribosome binding to the endoplasmic reticulum
occurs through a signal peptide on the
synthesized protein
 Signal peptide combines with a signal recognition
particle (SRP)
 SRP attaches to SRP receptor, thus allowing
protein to enter the lumen of the ER
 The signal peptide is removed from the protein
(via signal peptidase) in the lumen of the ER
 Ribosomal subunits & mRNA break away and
protein folds into its final shape
Nucleus, Ribosomes, & ER
Endomembrane System
 Restrict enzymatic reactions to specific
compartments within cell
 Consists of:
 Nuclear envelope
 Membranes of endoplasmic reticulum
 Golgi apparatus
 Vesicles
 Several types
 Transport materials between organelles of the
system
Endomembrane System:
The Endoplasmic Reticulum
 A membrane network within the cytoplasm of cells
involved in the synthesis, modification and transport of
cellular materials
 Rough ER
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Studded with ribosomes on cytoplasmic side
Protein anabolism
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Synthesizes proteins
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Modifies proteins - adds sugar to protein (i.e. glycoproteins)
Forms vesicles - transport of large molecules to other parts
of cell (i.e. Plasma membrane or Golgi apparatus)
 Smooth ER
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Continuous with rough ER; No attached ribosomes
Synthesis of lipids (i.e. phospholipids & steroids)
Endoplasmic Reticulum
Endomembrane System:
The Golgi Apparatus
 Golgi Apparatus
 Consists of 3-20 flattened, curved membranebound saccules called cisternae
 Resembles stack of deflated balloons
 Modifies proteins (i.e. glycosylation) and lipids
 Packages them in vesicles
 Receives vesicles from ER on cis face
 Prepares for “export” in vesicles from trans face
 Within cell
 Export from cell (secretion, exocytosis)
Golgi Apparatus
Endomembrane System:
Lysosomes
 Membrane-bound vesicles (common in animal cells
but rare in plant cells)
 Produced by the Golgi apparatus
 Low pH
 Contain hydrolytic enzymes
 Digestion of large molecules
 Recycling of cellular resources
 Destroying nonfunctional organelles
 Lysosomes participate in apoptosis
 Normal part of development
 Example: tadpole → frog
Peroxisomes
 Similar to lysosomes
 Membrane-bounded vesicles
 Enclose oxidative enzymes
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 Enzymes synthesized by free ribosomes in cytoplasm
(instead of ER)
 Active in lipid metabolism
 Catalyze reactions that produce hydrogen peroxide
H2O2
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Toxic molecule
Broken down to H2O and O2 by catalase enzyme
Alcohol detoxification in liver
Germinating seeds oxidize fatty acids to sugars → growth
Peroxisomes & Vacuoles
Energy-Related Organelles:
Chloroplast Structure
 An organelle found within the cells of green plants &
eukaryotic algae
 Bounded by a double membrane
 Inner membrane infolded
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Forms disc-like thylakoids, which are stacked to form
grana
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Suspended in semi-fluid stroma
 Green due to chlorophyll
 Chlorophyll absorbs light between the red and blue
spectrums and reflects green light, making leaves
appear green
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Found ONLY in inner membranes of chloroplast
Energy-Related Organelles:
Chloroplasts
 Chloroplasts are a type of plastid & are considered
to have originated as endosymbiotic cyanobacteria
 Has its own DNA and reproduces independently of
the cell
 Captures light energy to drive cellular machinery
 Photosynthesis
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Synthesizes carbohydrates from CO2 and H2O
Makes own food using CO2 as only carbon source
Chloroplast
Structure
Other Plastids
 Different types of plastids are classified according
to the kinds of pigments they contain
 Chromoplasts lack chlorophyll but contain
carotenoids
 responsible for the yellow, orange, & red colors of
some flowers and fruits
 Leucoplasts are colorless plastids, which synthesize
and store a variety of energy sources in nonphotosynthetic tissues
 Amyloplasts (starch)
 Elaioplasts (lipids)
Energy-Related Organelles:
Mitochondria
 Mitochondria are rod-shaped organelles that can be
considered the power generators of the cell
 Bounded by double membrane
 Cristae – Infoldings of inner membrane that
encloses matrix, why?
 Matrix – Inner semifluid containing respiratory
enzymes
 Involved in cellular respiration – process by which
chemical energy of sugar is converted to ATP
 Produce most of ATP utilized by the cell
 Has its own DNA and reproduces independently of
the cell
Mitochondrial Structure
The Cytoskeleton
 Maintains cell shape
 Assists in movement of cell and organelles
 Three types of macromolecular fibers
 Actin Filaments
 Intermediate Filaments
 Microtubules
 Dynamic, assemble and disassemble as needed
 Protein phosphorylation (e.g. protein kinases)
 Phosphorylation → disassembly
 Dephosphorylation → assembly
Cytoskeleton Protein Fibers
The Cytoskeleton:
Actin Filaments
 Extremely thin filaments like a twisted pearl
necklace
 Dense web just under plasma membrane maintains
cell shape
 Support for microvilli in intestinal cells
 Intracellular traffic control
 For moving stuff around within cell
 Cytoplasmic streaming in plant cells
 Function in pseudopods of amoeboid cells
 Pinches off dividing animal cells apart during mitosis
 Important component in muscle contraction (other is
myosin)
Actin Filaments
Actin Filament Operation
 Actin filaments interact with motor molecules
(proteins that can attach, detach and reattach to
the actin filament)
 Myosin pulls actin filaments in the presence of ATP
 In muscle cells, cytoplasmic myosin tails are bound
to membranes, while heads interact with actin
The Cytoskeleton:
Intermediate Filaments
 Intermediate in size between actin filaments
and microtubules
 Rope-like assembly of fibrous polypeptides
 Vary in nature (i.e. from tissue to tissue and
from time to time)
 Functions:
 Mechanical stability of the plasma- and the
nucleus-membranes
 Cell-cell interaction, like those holding skin cells
tightly together (keratin)
The Cytoskeleton:
Microtubules
 Hollow cylinders made of two globular proteins
called a and b tubulin giving rise to structures
called dimers
 Dimers then arrange themselves into tubular
spirals of 13 dimers around
 Assembly:
 Under control of Microtubule Organizing Center
(MTOC)
 Most important MTOC is centrosome
 Interacts with proteins kinesin and dynein to
cause movement of organelles
Microtubule Operation
Microtubules
 Microtubules disassemble and then
reassemble into a spindle during cellular
division
 Colchicine - a plant toxic (defense
mechanism) that inhibits
polymerization by binding
to tubulin and preventing
microtubule assembly
The Cytoskeleton (Summary)
 Microfilaments regulate:
 Cell shape
 Cell movement
 Intermediate filaments effect:
 The mechanical stability of the plasma- & the
nucleus-membranes
 Cell-cell interaction
 Microtubules effect:
 Localization and transport of organelles
 Cell division
Microtubular Arrays:
Centrioles
 Short, hollow cylinders
 Composed of 27 microtubules
 Microtubules are arranged in 9 sets of 3 each
(9 + 0) pattern
 One pair per animal cell
 Located on centrosome of animal cells
 Oriented at right angles to each other
 Separate during mitosis (cell division)
 May give rise to basal bodies of cilia & flagella
 Plant cells do not have centrioles
Centrioles
Microtubular arrays:
Cilia and Flagella
 Hair-like projections from cell surface that aid in
cell movement
 Very different from prokaryote flagella
 Outer covering of plasma membrane
 Inside is a cylinder of 18 microtubules arranged in
9 pairs
 Two single microtubules run down the centre of
the shaft (9 + 2) pattern found in cilia and flagella
 In eukaryotes, cilia are much shorter and
numerous than flagella
 Cilia move in coordinated waves like oars
 Flagella move like a propeller or cork screw
Cilia and Flagella
 The pairs of microtubules are connected by short
arms of protein dymein
 Movement of the cilia or flagella is the result of
sliding movements between microtubule pairs
 Beneath each cilium of flagellum in the cytoplasm
of the cell is a basal body
 The two central microtubules of the cilia/flagellum
do not extend into the basal boy.
 The nine pairs of microtubule do and they are
joined by a third microtubule.
 Centrioles are needed to create basal bodies in
order to produce cilia and/or flagella