Chapter 3: Functioning cells
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Eukaryotes
•
Eukaryotic cells possess
– membrane-bound organelles

nucleus containing DNA
 mitochondria
 endoplasmic reticulum
– structures not membrane-bound

ribosomes
 microtubules
– cytosol

aqueous solution in which organelles lie
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Fig. 3.3: Animal cell
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Membranes
•
Membranes enclose cell and organelles
• Lipid bilayer of phospholipids
– hydrophilic polar head
– hydrophobic fatty acid tail
•
Membrane proteins
– peripheral proteins attached loosely by non-covalent
interactions
– integral proteins are transmembrane, extending through
membrane
(cont.)
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Fig. 3.6: Plasma membrane
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Membranes (cont.)
•
•
Membranes usually with different molecules on
each side
Carbohydrates may be attached to lipids or
proteins
– glycolipids
– glycoproteins
•
Carbohydrates occur on non-cytosolic side of
membrane
– lumen (inside) of organelles
– outer surface of plasma membrane to form glycocalyx
(cont.)
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Membranes (cont.)
•
Membrane are fluid mosaics
– lipid (and some protein) molecules can move laterally
– proteins embedded in irregular arrangement
•
Membranes are selectively permeable
– H2O, O2, CO2 cross freely
– ions and other polar molecules can only cross at selective
pores formed by transmembrane proteins
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Nucleus
•
Nucleus surrounded by double membrane
– nuclear envelope
•
•
Nuclear envelope continuous with endoplasmic
reticulum
Perforated by nuclear pores
– pores composed of protein complexes
– permit passage of selected molecules, including RNA
(cont.)
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Nucleus (cont.)
•
•
Nucleus contains DNA
DNA molecules winds around histone molecules to
form nucleosomes
– DNA twists into helical chromatin strands
•
When cell is not dividing, chromatin strands
– aggregate to form densely staining heterochromatin
– disperse to form lightly staining euchromatin
•
When cell divides, chromatin strands condense to
form chromosomes
(cont.)
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Nucleus (cont.)
•
Nucleus usually contains one or several nucleoli
(sing. nucleolus)
– densely-staining area of DNA, RNA and protein
•
Nucleolus size depends on level of protein
synthesis in cell
– site of ribosomal RNA synthesis
– site of assembly of ribosomal subunits
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Ribosomes
•
Ribosomes are site of protein synthesis
– composed of two subunits assembled in the nucleolus
– subunits associate with mRNA molecule in cytosol
•
Ribosome moves along mRNA molecule
synthesising polypeptide
– more ribosomes are bound, forming a polyribosome or
polysome
– polysome may remain free in cytosol or attach to
endoplasmic reticulum
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Endomembrane system
•
Cell and nucleus enclosed in membranes
– plasma membrane, nuclear envelope
•
Membranes enclose components inside cell
– endomembrane system
•
Cell components
–
–
–
–
–
endoplasmic reticulum
Golgi apparatus
lysosomes
endosomes
vacuoles
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Endoplasmic reticulum
•
Endoplasmic reticulum (ER) extends through
cytosol
– network of sacs (cisternae)
– continuous with outer membrane of nuclear envelope
•
Cisternae usually flat, sheet-like
– linked by tubular cisternae
– extensive surface area
(cont.)
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Endoplasmic reticulum (cont.)
•
Ribosomes bound to surface of rough ER
– synthesise polypeptides
– polypeptides pass into lumen of ER
– folding assisted by binding protein (BiP)
•
Smooth ER lacks ribosomes
– synthesise lipids (rough ER can also do this)
– enzymes involved in lipid synthesis are on cytosolic face
of membrane
– also possesses enzymes involved in detoxifying lipidsoluble drugs and harmful metabolic products
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Golgi apparatus
•
Golgi apparatus composed of stacks of cisternae
– 4 to 10
– disc-shaped, flat or curved
•
Golgi apparatus has distinct orientation
– cis face towards ER

lacks ribosomes
 cytosol between Golgi apparatus and ER filled with small
vesicles
– trans face outwards

associated with tubular membranes of the trans-Golgi
network
(cont.)
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Golgi apparatus (cont.)
•
•
Golgi apparatus processes and packages
glycoproteins and polysaccharides
cis face
– proteins and glycoproteins enter from ER via vesicles
– modified as they pass through stack of cisternae
•
trans face
– sorting and packaging of products in trans-Golgi network
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Fig. 3.13: Golgi apparatus
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Sorting and transport
•
•
Products of Golgi apparatus transported to target
organelles or exported from cell
Sorting
– products localised by association with ‘cargo’ receptors
on inner face of membrane of cisternae or trans-Golgi
network
•
Transport
– vesicle-marker proteins (v-SNARE) on outer membrane
identify different vesicle types
– v-snare proteins attach to target docking proteins
(t-SNARE) on target membrane
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Lysosomes
•
•
Lysosomes contain hydrolytic enzymes for
breaking down old organelles
Enzymes for lysosomes manufactured in Golgi
apparatus
– enzymes marked in cis cisternae for sorting in trans-Golgi
network
– markers recognised by endolysosome
– enzymes released into endolysosome
– active uptake of H+ decreases pH
– endolysosome matures into lysosome
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Transport vesicles
•
•
Materials can be exported from or imported into
the cell by vesicles fusing with the plasma
membrane
Exocytosis (exportation)
– continual (constitutive secretion) or intermittent (regulated
secretion)
•
Endocytosis (importation)
– vesicles fuse with endosomes
– some materials recycled, others broken down
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Mitochondria
•
•
Mitochondria are thought to have evolved from
engulfed prokaryotes
Mitochondria are the site of cellular respiration
– release energy by oxidation of sugars and fats (oxidative
phosphorylation)
– released energy stored in ATP
– cells with high level of metabolic activity have large
numbers of mitochondria
(cont.)
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Mitochondria (cont.)
•
Double membrane
– outer membrane

permeable to ions and small molecules
 many transport channels
– inner membrane

impermeable
 transport of ions by transport proteins
– generates electrochemical gradient
(cont.)
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Mitochondria (cont.)
•
Inner membrane of mitochondria folded into cristae
– lined with enzyme complexes for ATP synthesis
– enzymes use electrochemical gradient to generate ATP
from ATP and inorganic phosphate
•
Matrix space of mitochondria
–
–
–
–
ribosomes
one or more copies of circular mtDNA
mtDNA codes for rRNA, tRNA and mRNA
proteins required for oxidative reactions and DNA
synthesis
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Plastids
•
Plastids occur in plant and protist cells
– photosynthetic organelles
•
Plastids resemble mitochondria in structure
– double membrane

each membrane with different permeability
– ribosomes
– circular DNA
– RNA
•
Evolved from engulfed prokaryotes
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Chloroplasts
•
Chloroplasts contain light-absorbing pigments
– mainly chlorophyll
•
Well-developed internal membrane system
– stacks (grana) of disc-like sacs (thylakoids)
– thylakoids continuous with each other and those in
adjacent grana
•
Chlorophyll molecules on thylakoid membrane
– light energy used to create electrochemical gradient
– gradient used to generate ATP
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Microbodies
•
•
Microbodies remove unwanted compounds from
cells
Microbodies contain oxidative enzymes
– remove hydrogen from molecules and couple it to oxygen
– generate hydrogen peroxide (H2O2)
– catalase breaks down H2O2 into water and oxygen
•
•
Peroxisomes oxidise amino acids and uric acid
Glyoxysomes convert fatty acids to sugars in
germinating seeds
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Cytoskeleton
•
Cytoskeleton imposes and maintains structure of
cell
– fixes organelles in position
– moves organelles around cell
– maintains and remodels cell shape
•
Elements of cytoskeleton
– microtubules
– microfilaments
– intermediate filaments
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Fig. 3.17: Elements of cytoskeleton
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Microfilaments
•
Structure of microfilaments
– diameter 7–8 nm
– composed of actin (42 kD)
•
Free actin (G-actin) interacts to form chains or
filaments of F-actin
– length of F-actin filaments controlled by actin-binding
proteins
•
Interactions between actin and myosin
microfilaments are the basis of many cytoplasmic,
organelle and cell movements
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Microtubules
•
Structure of microtubules
– diameter 25 nm
– composed of α-tubulin and β-tubulin (both 55 kD)
•
Microtubule-associated proteins (MAPs) control
assembly and disassembly of microtubules
• Microtubule arrays may be radiating, bundled or
parallel
– more rigid than microfilaments
– support projections from cells, movement of organelles
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Intermediate filaments
•
Structure of intermediate filaments
– diameter 8–10 nm
– composed of different proteins (40–130 kD)
•
•
Intermediate filament arrays are stable
Provide mechanical support for cell and nucleus
– keratin
– desmin
– nuclear laminins
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Cilia and flagella
•
Eukaryote cilia and flagella project from surface of
cells
– covered by plasma membrane
•
Flagella
– one to a few on cell surface
– length 20–100 μm
•
Cilia
– many on cell surface
– length 2–20 μm
(cont.)
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Cilia and flagella (cont.)
•
Supported by paired microtubules (doublets)
forming axoneme
– microtubules in each pair linked by fibres
– two short arms of dynein on one side of each doublet
•
Nine doublets surround two central doublets
– movement created by doublets sliding relative to each
another
– dynein attaches to adjacent doublet, undergoes
conformational change, then releases doublet
– energy provided by dynein hydrolysis of ATP
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Fig. 3.21a and b: Cilia and flagella
(a)
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Prokaryotic cells
•
Prokaryotic cells
– semirigid cell wall surrounding plasma membrane
– lack membrane-bound organelles
– circular DNA in cytosol

ribosomes attach directly to mRNA, even while mRNA is
being transcribed
– enzymes on plasma membrane

those enzymes occurring in eukaryotic mitochondria
– light-trapping pigments on plasma membrane

those pigments occurring in eukaryotic chloroplasts
– rotating flagella of flagellin fibrils
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