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Prokaryotic cells have no nuclei with relatively simple internal structures and
include the various types of bacteria
Eukaryotic cells have nuclei, are larger and radically more complex – are
found in higher animals and plants
Comparison of eukaryotic and prokaryotic organisms
Prokaryotes
Eukaryotes
Organism
Cell size
Metabolism
Organelles
DNA
RNA and
protein
Bacteria and cyanobacteria
1 – 10 μm in length
Anaerobic and aerobic
Few or none
Fungi, plants and animals
5 – 100 μm in length
Aerobic
Nucleus, mitochondria
endoplasmic reticulum etc
Circular DNA in cytoplasm
Very long linear DNA molecules
many containing none coding regions;
bounded by nuclear membrane
RNA and protein synthesized
RNA synthesized and processed in
in the same compartment
the nucleus; protein synthesized in
the cytoplasm
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Cytoplasm
No cytoskeleton: cytoplasmic streaming Cytoskeleton composed of
endocytosis and exocytosis
protein filaments: cytoplasmic
are absent
streaming, endocytosis and
exocytosis are present
Cell division
Chromosomes pulled apart
Chromosomes pulled apart
by attachment to cell membrane
by the cytoskeletal spindle
apparatus
Cellular
Mainly unicellular
Mainly multicellular, with
Organization
differentiation
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Major structural features of Eukaryotic cells
The plasma membrane contains the transporters and receptors

The external surface of cells is in contact with other cells, the extracellular fluid,
solutes nutrient molecules, hormones , neurotransmitters and antigens in the fluid

Plasma membranes contain
 many transporters (proteins that span the membrane and carry nutrients into
the cell and various products out)
 Signal receptors (which are surface membrane proteins with highly specific
binding sites for extracellular signaling molecules or receptor ligands)
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 When an extracellular ligand binds to its specific receptor, the receptor
protein transduces the signal carried by that receptor ligand into an
intracellular message
 eg some receptors are associated with ion channels that open when the receptor is
occupied, permitting entry of specific ions
 Others activate or inhibit cellular enzymes on the inner membrane surface
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 Surface receptors act as signal amplifiers; a single ligand
molecule bound to a single receptor may cause the flux of
thousands of ions through an opened channel or synthesis of
thousands of molecules of an intracellular messenger by an
activated enzyme
 Most cells of higher plants have a cell wall outside the
plasma membrane which serves as a rigid protective shell
 The cell wall is composed of cellulose and other
carbohydrate polymers, is thick but porous. Allows water
and small molecules to pass through but swelling of the
cell due to accumulation of water is resisted by the rigidity
of the wall.
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
Exocytosis and endocytosis
 Endocytosis is a mechanism for transporting
components of the surrounding medium deep into
the cytoplasm
 A region of the plasma membrane invaginates,
enclosing a small volume of extracellular fluid
within a bud that pinches off inside the cell by
membrane fussion
 the resulting vesicle (endosome) can move into the
interior of the cell, delivering its contents to another
organelle bounded by a single membrane (eg
lysosomes) by fusion of the two membranes
 The endosome serves as an intracellular extension of
the plasma membrane
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

Phagocytosis is a special type of endocytosis in
which the material carried into the cell is particulate,
such as cell fragments
Exocytosis is the reverse of phagocytosis in which a
vesicle in the cytoplasm moves to the inside surface
of the plasma membrane, fuses with it, then releases
the vesicular contents outside the membrane
 Many proteins destined for secretion into the
extracellular space are packaged into vesicles called
secretory granules then released by exocytosis
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The endoplasmic reticulum (ER)

The endoplasmic reticulum is a highly convoluted , three
dimensional network of membrane-enclosed spaces extending
throughout the cytoplasm and enclosing a subcellular
compartment (lumen) from the cytoplasm

The many flattened branches (cisternae) of this compartment are
continuous with each other and with the nuclear envelope

The ribosomes that synthesize proteins destined for export attach
to the outer surface of the ER, and the secretory proteins are
passed through the membrane into the lumen as they are
synthesized

Proteins that will remain and function within the cytosol are
synthesized on cytoplasmic ribosomes unassociated with the ER

The attachment of thousands of ribosomes gives the rough ER its
granular appearance
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
The smooth ER is free of ribosomes
 Is physically continuous with the rough ER
 Is the site of lipid biosynthesis,
 Contain cytochrome p450 class of enzymes that
 Catalyze hydroxylation of endogenous and exogenous
compounds (metabolism drugs and toxic compounds)
 Are important in biosynthesis of steroid hormones
 Is generally tubular in contrast to the long flattened cisternae of
rough ER
In some tissues eg muscle the ER is specialized for storage and
rapid release of calcium. Release of calcium is a trigger for many
cellular events eg muscle contractions
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The Golgi complex
 Processes and sorts proteins
 Golgi complexes are systems of membranous sacs
arranged as flattened stacks
 The Golgi complex is asymmetric, structurally and
functionally
 The cis side faces the rough ER (and the nucleus) and
the trans side faces the plasma membrane
 Proteins during their synthesis on ribosomes bound
to the rough ER are inserted into the interior of the
ER cisternae
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Small membrane vesicles containing the newly synthesized
proteins bud from the ER and move to the Golgi complex,
fusing with the cis side
As proteins pass from the cis to the trans side enzymes within
the complex modify the protein molecules by adding sulfate,
carbohydrate or lipid moieties to side chains of certain amino
acids. Modification of protein helps in directing it to the right
destination as it leaves the Golgi complex in a transport vesicle
budding from the trans side
 Some proteins are released from the cells by exocytosis
 Other proteins are targeted to intracellular organelles such as
lysosomes or for incorporation into the plasma membrane
during cell growth
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Lysosomes

Are sites of degradative reactions

Are found only in animal cells

Are spherical vesicles bounded by a single membrane bilayer

Contain enzymes capable of digesting proteins, polysaccharides,
nucleic acids and lipids

They function as recycling centres, breaking down complex
molecules brought into the cell by endocytosis, fragments of
foreign cells brought in by phagocytosis, or worn-out organelles
from the cell’s own cytoplasm

These material enter the lysosomes by fusion of the lysosomal
membrane with endosomes, phagosomes or defective organelles ,
and are then degraded to their simple components (amino acids,
monosaccharides, fatty acids etc) which are released for recycling
into new cellular components or further catabolized
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Degradative enzymes within lysosomes are not free to act on all
cellular components because
 The enzymes are confined to the lysosmes
 Lysosomal enzymes are more active at acid pH (pH≤ 5). The
pH in the cytosol is about 7. The difference in pH is another
safety mechanism that prevents lysosomal enzymes from acting
on cytosolic molecules
An H+ pump in the lysosomal membrane utilizes energy of ATPhydrolysis to pump H+ into the lysosome
Under controlled conditions some lysosomal enzymes are
normally secreted from the cell for digestion of extracellular
material in connective tissue and the prostate
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Examples of lysosomal enzymes
Type of enzyme
Polysaccharide-hydrolyzing enzymes
β-glucuronidase
Hyaluronidase
Lysozyme
Protein-hydrolyzing enzymes
Collagenase
Elastase
Peptidases
Nucleic acid-hydrolyzing enzymes
Ribonuclease
Deoxyribonuclease
Lipid- hydrolyzing enzymes
Lipases
Esterase
Phospholipase
Phosphatases
Phosphatase
Phosphodiesterase
Specific substrate
β-glucuronides
Hyluronic acid and chondroitin sulfate
Bacterial cell walls
Collagen
Elastin
Peptides
RNA
DNA
Triacylglycerol and cholesterol esters
Fatty acid esters
Phospholipids
Phosphomonoesters
Phosphodiesters
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In a number of genetic diseases individual lysosomal enzymes are
missing and lead to the accumulation of the substrate of the
missing enzyme
 Lysosomes of affected cells become enlarged with undigested
material which interferes with cellular function – referred to as
‘ lysosomal storage diseases’
Peroxisomes

Membrane-bounded vesicles containing oxidative enzymes that
generate and destroy hydrogen peroxide
 Peroxisomes are specialized for carrying out oxidative
reactions using molecular oxygen
 Contain enzymes that use molecular oxygen to remove
hydrogen atoms from specific organic molecules in an
oxidative reaction that produces hydrogen peroxide
 Catalase utilizes hydrogen peroxide to oxidize other substances
and
detoxify them eg phenols, about a quarter of the
ethanol taken
etc. Important in the liver and kidney
 When excess hydrogen peroxide accumulates catalase converts
it into water

They also carry out beta oxidation of fatty acids
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The nucleus

The nucleus contain nearly all of the cell’s DNA; a small amount is
also present in mitochondria

The nucleus is surrounded by a nuclear envelope, composed of
two membrane bilayers separated by a narrow space and
continuous with the rough ER
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At intervals the inner and outer nuclear membranes are pinched
together around opening pores (nuclear pores)
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Associated with the pores are transporters that allow certain
macromolecules to pass between the cytoplasm and the aqueous
phase of the nucleus (nucleoplasm)
 Through these pores enzymes and other proteins synthesized
in the cytoplasm and required in the nucleoplasm for DNA
replication and repair, transcription, and RNA processing enter
the nucleus
 Coming out through the pores are mRNA with associated
proteins which will be translated on ribosomes in the
cytoplasm
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The DNA binding proteins fall into two groups ie the histones and
nonhistone chromosomal proteins. The complex of both classes of
proteins with the nuclear DNA is what is known as chromatin
Histones are relatively small proteins with a high proportion of
positively charged amino acids (lysine and arginine); the positive
charge helps the histones bind tightly to DNA (which is highly
negatively charged)
The nucleolus is a specific region of the nucleus in which DNA
contains many copies of the genes encoding ribosomal RNA
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About half the mass of chromatin is DNA and half is histones
When DNA replicates prior to cell division, large quantities of
histones are also produced to maintain the 1:1 ratio
The histones and DNA associate in complexes called
nucleosomes, in which a DNA strand winds around a core of
histone molecules
The nucleosomes associate to form very regular and compact
supra molecular complexes
The resulting chromatin fibers condense further by forming a
series of looped regions which cluster with adjacent loops to form
the chromosomes visible during cell division
The tight packing of DNA into nucleosomes achieves a
remarkable condensation of the DNA molecule
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Mitochondria (mitochondrion singular)
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Are power plants of eukaryotic cells

Membrane bound with a diameter of about 1 μm

Each mitochondrion has two membranes
 The outer membrane is unwrinkled and completely surrounds
the organelle
 The inner membrane has infolding called cristae, which give it
a large surface area

Enclosed by the inner membrane is the matrix, a very
concentrated aqueous solution of enzymes and chemical
intermediates involved in energy-yielding metabolism
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Mitochondrial enzymes catalyze the oxidation of organic nutrients
by molecular oxygen
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Biochemical anatomy of the mitochondria
Outer membrane.
Freely permeable to small
molecules and ions
Inner membrane.
Impermeable to most small
molecules and ions, including
H+. Contains:Respiratory
electron carriers (complexes
I-IV), ADP-ATP translocases,
ATP synthase and other
membrane transporters
Matrix. Contains: pyruvate
dehydrogenase complex, citric acid
cycle enzymes, fatty acid β–oxidation
enzymes, amino acid oxidation
enzymes, many other enzymes,
many soluble metabolic
intermediates, ATP, ADP, Pi, Mg2+,
Ca2+, K+, DNA, etc
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The chemical energy released in mitochondrial oxidations is used to generate ATP
Each mitochondrion contains its own DNA, RNA, and ribosomes
Cytoskeleton

In the cytosol arrays of protein filaments form networks that give the cell its shape,
provides a basis for its movements, arrange organelles and transport them from
one part of the cell to another

Three types -microtubules 25 nm in diameter
-actin filaments 8 nm in diameter
- intermediate 10 nm diameter
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Actin filaments enable individual eukaryotic cells to crawl about
and participate in the contraction of muscle
Microtubules - are the main structural and force-generating
elements
in cilia.
- in the form of mitotic spindle play a role in partitioning
DNA
One function of intermediate filaments is to provide internal
mechanical support for the cell and to position its organelles
Most organelles appear to be attached directly or indirectly to the
cytoskeleton - propelled along cytoskeletal tracks
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Cytosol
 Is the organelle-free cell sap
 Many reactions and pathways of metabolism
occur here
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Glycolysis
Fatty acid synthesis
Glycogenesis and glycogenolysis
etc
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Centriole
Plasma membrane
Mitochondria
Cytosol
Golgi
apparatus
Filamentous
cytoskeleton
Endoplasmic
reticulum
Ribosomes
Lysosomes
Peroxisomes
Nucleus
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