chapter 4: cell structure & function (outline) cell theory cell size prokaryotic cells ...
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Chapter 4: Cell Structure & Function (Outline)
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 (m) 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
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
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
Nutrient recycling – mineralization; nitrogen fixing
Decomposition of dead organisms
Disease (infectious) – tuberculoses; anthrax
Commercial uses – foodstuffs; antibiotics; insulin
Prokaryotes are divided into two domains Domain Bacteria
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: Bacillus (rod)
Coccus (spherical)
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
Consists of double layer of membrane Nuclear pores permit exchange of ribosomal subunits &
mRNA between nucleoplasm & cytoplasm
Contains chromatin in semifluid nucleoplasm Chromatin contains DNA of genes Condenses to form chromosomes
Dark nucleolus composed of ribosomal RNA (rRNA) 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
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 Studded with ribosomes on cytoplasmic side Protein anabolism
Synthesizes proteins 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 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 membrane-
bound 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
However Enzymes synthesized by free ribosomes in cytoplasm
(instead of ER) Active in lipid metabolism Catalyze reactions that produce hydrogen peroxide
H2O2
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
Forms disc-like thylakoids, which are stacked to form grana
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
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
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 non-photosynthetic 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 and 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
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