Cell Organelles
Assoc.Prof. Dr. Naveed Ahsan
There are two main
categories of cells:
Prokaryote (prokaryotic cells)
 Bacteria
 Have no membrane-bound nucleus
 Nucleic acid is usually found in “loops” in the cytoplasm
 Usually smaller than eukaryotes
 Have fewer organelles than eukaryotes
• Eukaryotes (eukaryotic cells)
• Larger
• Have a membrane-bound nucleus where DNA
is located
• More organelles
• All organisms except bacteria are eukaryotes
• Prokaryotic and eukaryotic cells
• All cells
• surrounded by a plasma membrane.
• have cytosol, containing the organelles.
• contain chromosomes
• have ribosomes
• A major difference
• eukaryotic cell: chromosomes are contained in the
nucleus (within a membranous nuclear envelope)
• prokaryotic cell: the DNA is concentrated in the
nucleoid
Outside of cell
Inside of
cell
0.1
µm
(a)
TEM of a plasma
membrane
Carbohydrate side chain
Hydrophilic
region
Hydrophobic
region
Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
DEFINITION OF CELL ORGANELLES
• Cell Organelle is a specialized subunit within a
cell that has a specific function, and is usually
separately enclosed within its own lipid bilayer.
• Organelles are identified by microscopy.
• There are many types of organelles, particularly
in eukaryotic cells.
Cell organelles are of 2 types.
Membranous Organelles:
Rough Endoplasmic reticulum.
Smooth endoplasmic reticulum.
Mitochondria.
Golgiapparatus.
Lysosomes.
Non membranous Organelles:
centriole
Ribosomes.
Cytoskeletal structures
Nucleus



The nucleus contains most of the cell’s genes
and is usually the most conspicuous organelle.
The nuclear envelope encloses the nucleus,
separating it from the cytoplasm.
The nuclear membrane is a double membrane;
each membrane consists of a lipid bilayer
Fig. 6-10
1 µm
Nucleus
Nucleolus
Chromatin
Inner membrane
Nuclear envelope:
Outer membrane
Nuclear pore
Pore
complex
Surface of
nuclear envelope
Rough
ER
Ribosome
1 µm
0.25 µm
Close-up of nuclear
envelope
Pore complexes (TEM)
Nuclear lamina
(TEM)


Nuclear pore complexs: Pores regulate the
entry and exit of protein molecules and
RNA from the nucleus
The shape of the nucleus is maintained by
the nuclear lamina, which is composed of
protein




In the nucleus, DNA and proteins form genetic
material called chromatin
Chromatin
condenses
chromosomes
to
form
discrete
The nucleolus is located within the nucleus and is
the site of ribosomal RNA (rRNA) synthesis
Nucleoplasm: Nucleoplasm of nucleus contain
various enzymes such as DNA polymerase and
RNA polymerase for m-RNA & t-RNA synthesis.
Each chromosome is a long
molecule of DNA that is
coiled together with several
proteins
Human somatic cells have
46 chromosomes arranged
in 23 pairs.
The various levels of DNA
packing are represented by
nucleosomes, chromatin
fibers, loops, chromatids,
and chromosomes.
Nucleus –Functions
• DNA replication and RNA transcription of DNA occurs
in the nucleus. Transcription is the first step in the
expression of genetic information.
FUNCTION OF NUCLEOLUS
• Synthesis of rRNA and ribosomes
• Contain RNA polymerase, RNAase, ATPase and other
enzymes.
• Nucleolus is also the major site where ribosome
subunits are assembled.
Ribosomes: Protein Factories


Ribosomes are particles made of ribosomal RNA
and protein
Ribosomes carry out protein synthesis in two
locations:
–
–
In the cytosol (free ribosomes)
On the outside of the endoplasmic reticulum or
the nuclear envelope (bound ribosomes)
Fig. 6-11
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large
subunit
0.5
µm
TEM showing ER and ribosomes
Small
subunit
Diagram of a ribosome
The endomembrane system regulates
protein traffic and performs metabolic
functions in the cell

•
Components of the endomembrane system:
–
Nuclear envelope
–
Endoplasmic reticulum
–
Golgi apparatus
–
Lysosomes
–
Vacuoles
–
Plasma membrane
These components are either continuous or
connected via transfer by vesicles
ENDOPLASMIC RETICULUM (ER)



The endoplasmic reticulum (ER) accounts for
more than half of the total membrane in many
eukaryotic cells
The ER membrane is continuous with the nuclear
envelope
There are two distinct regions of ER:
–
Smooth ER, which lacks ribosomes
–
Rough ER, with ribosomes studding its surface
Fig. 6-12
Smooth ER
Rough ER
ER lumen
Cisternae
Ribosomes
Transport vesicle
Smooth ER
Nuclear
envelope
Transitional ER
Rough ER
200
nm
Functions of Smooth ER
The smooth ER is the Tubules arranged in a looping network.
• Modification and transport of protein synthesized in the
rough Endoplasmic reticulum.
• Catalyzes the following reactions in various Organs
In the liver – lipid and cholesterol metabolism --breakdown
of glycogen and along with the kidneys detoxification of
drugs
In the testes-Systhesis of steroid- based hormones.
In the intestinal cells-absorption,systhesis and transport of
fats.
In skeletal and cardiac muscles – storage and release of
calcium.
FUNCTIONS OF ROUGH ENDOPLASMIC
RETICULUM

The rough ER
–
–
–
Synthesizes membrane lipid and secretory
protein
Distributes transport vesicles, proteins
surrounded by membranes
Is a membrane factory for the cell
GOLGI APPARATUS



Also called dictyosomes.
The Golgi apparatus consists of flattened
membranous sacs called cisternae
Functions of the Golgi apparatus:
–
Modifies products of the ER
–
Manufactures certain macromolecules
–
Sorts and packages materials into transport vesicles
•
They release protein via modified membrane called
secretory vesicles
Fig. 6-13
cis face
(“receiving” side of
Golgi apparatus)
Cisterna
e
trans face
(“shipping” side of
Golgi apparatus)
0.1
µm
TEM of Golgi apparatus
Lysosomes: Digestive Compartments
 A lysosome is a membranous sac of hydrolytic
enzymes that can digest macromolecules
 Lysosomal enzymes can hydrolyze proteins, fats,
polysaccharides, and nucleic acids.
 Vesicles of 0.5µm in diameter is manufactured in
the Golgi apparatus.
 Size and shape of the lysosomes change with the
stage of their activity.
 pH within the lysosome is distinctly acidic.
Enzymes of lysosomes are potent enough to
digest its own cellular contents in which it
inhabits (“suicide bag”)
lysosomes that digest the degenerated
mitochondria are referred to as cytolysosomes,
( “digestive bags” )
Lysosomes hauls away unusable waste and
dumping it outsides the cell.



Some types of cell can engulf another cell by
phagocytosis; this forms a food vacuole
A lysosome fuses with the food vacuole and
digests the molecules
Lysosomes also use enzymes to recycle the
cell’s own organelles and macromolecules, a
process called autophagy
LYSOSOMAL ENZYMES
1. Proteolytic enzymes
Cathepsins, Collagenase, Elastase
2. Nucleic acid Hydrolysing Enzymes
Ribonucleases. Deoxyribonucleases
3. Lipid Hydrolysing enzymes
Lipases, phospholipases, Fatty acyl estrases
4. Carbohydrate splitting enzymes
α-glucosidase, β-glactosidase,Hyaluronidase,
Aryl sulphatase
5. Other Enzymes
Acid Phosphatase,catalase
Nucleus
1
µm
Lysosome
Lysosome
Digestive
enzymes
Plasma
membrane
Digestion
Food vacuole
(a) Phagocytosis
Vesicle containing
two damaged organelles
1 µm
Mitochondrion
fragment
Peroxisome
fragment
Lysosome
Peroxisome
Vesicle
(b) Autophagy
Mitochondrion
Digestion
I- cell Disease & Lysosomal
Hydrolases
• Rare condition in which lysosomes lack all of the
normal lysosomal enzymes.
• Disease is characterized by severe progressive
psychomotor retardation and a variety of physical
signs, with death often occurring in the first decade.
Tay-Sachs Disease
A lysosome storage disease
• Absence of specific lysosomal enzymes
causes accumulation of unwanted cellular
materials in the brain.
• E.g. Mental retardation and blindness
resulted
from
Ganglioside
GM2
accumulation due to the lack of
hexosaminidase A.
• The lysosomes cannot function properly or
even break to release not just digestive
enzymes, but also “acid” to kill the cell.
Gout and Rheumatoid Arthritis
• Gout is deposition of uric acid crystals of the
joints often from over consumption of meat.
• Other Rheumatoid factor complexes in the
leucocytes (white cells) of the joints.
• These rupture lysosome to release enzymes
that degrade the components of the synovial
membrane in the joints, causing great pain
and joint deformation.
Mitochondria
• Mitochondria is the power house of cell
• The size of mammalian mitochondria in diameter is
0.2 to 0.8 µ and a length of 0.5 to 1.0µm
• Shape of mitochondria is not static .
• The mitochondria is bounded by two concentric
membrane that have different properties and
function
MITOCHONDRIAL
MEMBRANES
Outer Mitochondrial membrane & Their
Functions
• The outer mitochondrial membrane consist of
phospholipid and cholesterol.
• The outer membrane also contain many copies
of the protein called porin.
• These protein form channels that permits
substances with molecular weight of less than
< 10,000 to diffuse freely across the outer
mitochondrial membrane.
• Other protein in the outer membrane
carry out various reactions in fatty acid
and phospholipid biosynthesis and are
responsible for some oxidation reaction.
INNER MITOCHONDRIAL
MEMBRANE AND ITS FUNCTION
• The inner mitochondrial membrane is very rich
in proteins.
• It contain high proportion of the phospholipid
cardiolipin.
• The inner mitochondrial membrane is
impermeable to polar and Ionic substances.
• The inner mitochondrial membrane is highly
folded called cristae
INTERMEMBRANE SPACE
• The space between outer and inner membrane
is known as the inter membrane space.
Mitochondrial matrix
The region enclosed by the inner membrane is
known as the mitochondrial matrix
Composition of Mitochondrial matrix
• The enzymes responsible for citric acid cycle
and fatty acid oxidation are located in the
matrix.
• The matrix also contain several strands of
DNA and ribosomes
• The enzymes required for the synthesis
protein coded in the mitochondrial genome.
Fig. 6-17
Intermembrane space
Outer
membrane
Free ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
0.1 µm
FUNCTION
Function of Mitochondria
• Many
enzymes
associated
with
carbohydrates, fatty acid and nitrogen
metabolism are located within the
mitochondria.
Luft’s disease
• In luft’s disease mitochondrial energy
transduction has been reported.
• Mitochondrial DNA can be damage by free
radicals
• Age related disorder like Parkinson disease
and cardiomyopathy is the main cause of
mitochondrial damage.
Peroxisomes: Oxidation
•
•
Peroxisomes are specialized metabolic
compartments bounded by a single membrane,
its perform oxidation of fatty acids and toxicants,
it contains oxidative enzymes.
Peroxisomes contains catalase produce
hydrogen peroxide and convert it to water.
• Oxygen is used to break down different types of
molecules.
•Abundant in hepatocytes (liver cells), where
oxidation of fatty acids (and other organic
matters) takes place and produces hydrogen
peroxide.
• Life span is short and the numbers of
peroxisome vary.
• These subcellular respiratory organelle have no
energy coupled electron transport system and are
probably formed from by budding from smooth
endoplasmic reticulum.
Functions of Peroxisomes
• They carryout oxidation reaction in which
toxic hydrogen peroxide is produced, which
is destroyed by the enzyme catalase.
• Liver peroxisomes have an active βoxidative system capable of oxidizing long
chain fatty acid.
Zellweger syndrome
• Zellweger syndrome, is a rare congenital disorder
characterized by the reduction or absence of
functional peroxisomes in the cells of an individual.
• Zellweger syndrome is an autosomal recessive
disorder caused by mutations in genes required for
the normal assembly of peroxisomes
• In Zellweger syndrome tissues and cells can
accumulate very long chain fatty acids and
branched chain fatty acids that are normally
degraded in peroxisomes
The cytoskeleton is a network of fibers that
organizes structures and activities in the cell



The cytoskeleton is a network of fibers extending
throughout the cytoplasm
It organizes the cell’s structures and activities,
anchoring many organelles
It is composed of three types of molecular
structures:
–
–
–
Microtubules
Microfilaments
Intermediate filaments
Fig. 6-20
Microtubule
0.25 µm
Microfilaments
Roles of the Cytoskeleton: Support,
Motility, and Regulation




The cytoskeleton helps to support the cell and
maintain its shape
It interacts with motor proteins to produce motility
Inside the cell, vesicles can travel along
“monorails” provided by the cytoskeleton
Recent evidence suggests that the cytoskeleton may
help regulate biochemical activities
ATP
Vesicle
Receptor for
motor protein
Motor protein (ATP
powered)
Microtubule
(
b
Vesicles
Microtubule
of cytoskeleton
0.25 µm
Components of the Cytoskeleton

Three main types of fibers make up the
cytoskeleton:
–
–
–
Microtubules are the thickest of the three
components of the cytoskeleton
Microfilaments, also called actin filaments,
are the thinnest components
Intermediate filaments are fibers with
diameters in a middle range
Table 6-1
10 µm
10 µm
10 µm
Column of tubulin dimers
Keratin proteins
Actin subunit
Fibrous subunit (keratins
coiled together)
25 nm
7 nm
Tubulin dimer
8–12 nm
Table 6-1a
10
µm
Column of tubulin dimers
25
nm
Tubulin
dimer
Table 6-1b
10
µm
Actin
subunit
7
nm
Table 6-1c
5
µm
Keratin
proteins
Fibrous subunit (keratins
coiled together)
8–12
nm
Extracellular components and connections
between cells help coordinate cellular activities


Most cells synthesize and secrete materials that are
external to the plasma membrane
These extracellular structures include:
–
Cell walls of plants
–
The extracellular matrix (ECM) of animal cells
–
Intercellular junctions
Cell Walls of Plants




The cell wall is an extracellular structure that
distinguishes plant cells from animal cells
Prokaryotes, fungi, and some protists also have cell
walls
The cell wall protects the plant cell, maintains its
shape, and prevents excessive uptake of water
Plant cell walls are made of cellulose fibers
embedded in other polysaccharides and protein
Animal Cells



Animal cells lack cell walls but are covered by
an elaborate extracellular matrix (ECM)
The ECM is made up of glycoproteins such as
collagen, proteoglycans, and fibronectin
ECM proteins bind to receptor proteins in the
plasma membrane called integrins
Fig. 6-30a
Collagen
Proteoglycan
complex
EXTRACELLULAR
FLUID
Fibronectin
Integrins
Plasma
membrane
Microfilaments
CYTOPLASM
Functions of the Extracellular matrix :
- Support
–
–
–
Adhesion
Movement
Regulation