Download CHAPTER 6 A TOUR OF THE CELL

CHAPTER 6
A TOUR OF THE CELL
6.2: Eukaryotic cells have internal
membranes that compartmentalize their
functions
A Panoramic View of the Eukaryotic Cell
A Panoramic View of the Eukaryotic
Cell
•
In addition to the plasma membrane, a eukaryotic cell
has extensive internal membranes which:
 Partition the cell into compartments.
 Participate in the cell’s metabolic reaction since many enzymes
are built into the membranes.
 Have the basic structure of a
biological membrane (a double layer
of phospholipids) with unique lipid
and protein compositions depending
upon their specific functions.
- For example, those in the membranes of
mitochondria function in cellular respiration
B- Eukaryotic Cell
Eu: True
Karyon: Nucleus
Animal Cell
Plant Cell
Compare between Animal and Plant cell?
Page 100 - 101
Fig. 7.7
An Animal Cell
Fig. 7.8
A Plant Cell
Cell Organelles
The eukaryotic cell’s genetic instructions
are housed in the nucleus and carried out
by the ribosomes
The Nucleus: Genetic Library of the Cell
• The nucleus contains most of the genes in a eukaryotic cell.
• The nucleus averages about 5 µm in diameter.
• The nucleus is separated from the cytoplasm by a double
membrane that is called the “nuclear envelope”:
• The nuclear membrane contains pores that allow large
macromolecules and particles to pass through.
• The nuclear membrane is maintaining the shape of the nucleus
Fig. 6.10
• The nucleus contains DNA organized with
proteins into a complex called chromatin:
• In non-dividing cell chromatin appear as diffuse
mass.
• when the cell prepares to divide, the chromatin fibers
coil up to be seen as separate structures, chromosomes.
• Each eukaryotic species has a characteristic number of
chromosomes.
– A typical human cell has 46 chromosomes, but sex cells (eggs
and sperm) have only 23 chromosomes.
• Nucleolus is a dark region involved in production of
ribosomes.
•The nucleus directs protein synthesis by synthesizing
messenger RNA (mRNA).
• The mRNA travels to the cytoplasm and
combines
with ribosomes to translate its genetic
message into the primary structure of a specific
polypeptide:
The nucleus controls protein synthesis in the cytoplasm:
Messenger RNA (mRNA) synthesized in the nucleus by DNA
instructions
mRNA passes through nuclear pores into cytoplasm
Attaches to ribosomes where the genetic message is translated
into primary structure of specific protein
Ribosomes: Protein Factories in the Cell
• Ribosomes made of rRNA and protein.
• Carry out protein synthesis
• A ribosome is composed of two subunits.
Fig. 7.10
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In the nucleolus, ribosomal RNA (rRNA) is
synthesized and
assembled with proteins from
the cytoplasm to form large and small ribosomal
subunits.
• The subunits pass from the nuclear pores to the
cytoplasm where they combine to form ribosomes.
• Cell types active in proteins synthesis (e.g., pancreas)
have large numbers of ribosomes and prominent
nucleoli.
• Both ribosomes and nucleoli are not enclosed in
membrane.
• Types of Ribosomes:Ribosomes:1) Free ribosomes are suspended in the cytosol and
synthesize proteins that function within the
cytosol.
2) Bound ribosomes are attached to the outside of the
endoplasmic reticulum.
– These synthesize proteins that are either
included into membranes or for secretion
outside the cell.
• Ribosomal subunits are structurally identical and can
shift between the two roles.
CHAPTER 6
A TOUR OF THE CELL
The Endomembrane System
Regulates protein traffic and
Performs metabolic functions in the cell
Many of the internal membranes in a eukaryotic cell are part of the
endomembrane system.
These membranes have diverse functions and structures.
The endomembrane system includes:
a)
b)
c)
d)
endoplasmic reticulum
Golgi apparatus
lysosomes
vacuoles
A-The endoplasmic reticulum:
(Biosynthetic Factory)
 The endoplasmic reticulum
(ER)includes network of
membranous tubules and internal,
fluid-filled spaces, the cisternae
which separates its internal lumen
from the cytosol.
 There are two, connected, regions of
ER that differ in structure and
function:
• Smooth ER looks smooth
because it lacks ribosomes.
• Rough ER looks rough because
bound ribosomes are attached to
the outside of the nuclear
envelope.
Fig. 6.11
Functions of Smooth ER
1.
Synthesis of lipids including phospholipids and
steroids :

e.g. Vertebrate sex hormones and steroids hormones .
2. Smooth ER in liver cells :
a - Contain an embedded enzyme catalyzing the final
step in the conversion of glycogen to glucose.
b - Contain enzymes which detoxify drugs and poisons.
3. Smooth ER in muscle cells are rich in enzymes that
pump calcium ion from the cytosol to the cisternae: i-e,
store calcium ion necessary for muscle contraction .
Rough ER and synthesis of secretory proteins
1 - Rough ER is continuous with the nuclear envelope.
2 - Manufactures secretory proteins and membrane .
3 - Rough ER makes its own membrane phospholipids.
4- Ribosomes bound to rough ER manufacture the
secretory proteins.
5 - These secretory protein departs in transport vesicles
from the ER membrane to other parts of the cell.
Rough ER and secretory proteins synthesis:
Ribosome bound to rough ER synthesize secretory proteins
Growing polypeptide is passed from ribosomes through ER membrane
into cisternal space
Inside cisternal space, new protein folds into its native conformation
If the new protein is a glycoprotein, it covalently bonds to an
oligosaccharide in the ER
Secretory protein departs in transport vesicles from
transitional ER to another part of the cell
Glycoprotein: = Protein covalently bonded to
carbohydrate.
Oligosaccharide = Small polymer of sugar units
Transport vesicles = Membrane vesicle in transit
from one part of the cell to another.
Transitional ER = Specialized region of the ER where
from transport vesicles are pinched off.
B. The Golgi apparatus
finishes, sorts, and ships cell products
 The Golgi apparatus consists of flattened membranous sacs
(cisternae).
 The Golgi apparatus is abundant in cells specialized for secretion.
 Many transport vesicles from the ER travel to the Golgi apparatus
for the modification of their contents.
The function of Golgi apparatus
•
It is a center of manufacturing, warehousing, sorting, and
shipping materials outside the cell.
•
It alters some membrane phospholipids.
•
It modifies the oligosaccharide portion of the glycoproteins.
Fig. 6.12
C. Lysosomes: Digestive compartments
The lysosome is a membrane-bounded sac of hydrolytic
enzymes that digests all macromolecules.Fig.6.13 .
Hydrolytic enzymes (including; lipases, carbohydrases,
proteases and nucleases ) are synthesized in the rough
ER and processed further in the Golgi apparatus then to
the lysosomes.
The optimal pH of these lysosomal enzymes is about pH
5 (acidic).
Functions of Lysosomes:
1- Intracellular digestion
 Lysosomes may fuse with food-filled vacuoles, and their
hydrolytic enzymes digest the food by a process called
phagocytosis.
Phagocytosis
= Cellular process of ingestion, where the
plasma membrane engulfs particulate substances and pinches off
to form a particle-containing vacuole. Fig. 6.14
2.
Recycle cell's own organic material (autophagy).
Autophagy
= A process by which lysosomes engulf other
cellular organelles or part of the cytosol and digest them with
hydrolytic enzymes. The resulting monomers are released into
the cytosol where they can be recycled into new macromolecules.
Fig. 6.14
Fig. 6.14
D-Vacuoles: Diverse Maintenance Compartments
 Vacuoles are membrane-bound sacs that is larger than a
vesicle (transport vesicle).
 A plant or fungal cell may have one or several vacuoles.
Vacuole Types and Functions:
• Food vacuoles are formed by phagocytosis, fuse with
the lysosomes and it is the site of the intracellular
digestion in some protists .
• Contractile vacuoles are found in freshwater
protozoa, pump excess water out of the cell.
• Central vacuoles are found in many mature plant
cells.Fig.6.14
Fig. 6.15
Other membranous Organelles
A – Mitochondria:
 Mitochondria are organelles that convert energy acquired from the
surroundings into forms useable for the cellular work.
 They are the sites of cellular respiration, generating ATP from the
catabolism of sugars, fats, and other fuels in the presence of oxygen.
 Almost all the eukaryotic cells have mitochondria.
Structure of the Mitochondrion:




Enclosed by two membranes that are not part of the endomembrane
system . Their membranes are made by the free ribosomes in the
cytosol and also by their own ribosomes.
The outer membrane is smooth.
The inner membrane is convoluted with infoldings called cristae.
The inner membrane encloses the matrix , a fluid-filled space that
contains mitochondrial DNA , ribosomes and enzymes . Fig. 6.17
Fig. 6.17
B - Peroxisomes:
 They are specialized metabolic compartments bounded
by a single membrane,contain enzymes that transfer
hydrogen from various substrates to oxygen.
 An intermediate product of this process is hydrogen
peroxide (H2O2), a poison, but the peroxisome has
another enzyme that converts H2O2 to water.
 Some peroxisomes break fatty acids down to smaller
molecules that are transported to mitochondria for fuel.
 Peroxisomes in the liver detoxify alcohol and other
harmful compounds.Fig.6.18
Fig. 6.18
The Cytoskeleton
• The cytoskeleton is a network of fibers extending
throughout the cytoplasm.
• The cytoskeleton organizes the
structures and activities of the
cell .
Fig. 6.21
The cytoskeleton
Role: Support, Motility and Regulation
•
The cytoskeleton provides mechanical support and maintains shape of
the cell.
•
The cytoskeleton provides anchorage for many organelles and cytosolic
enzymes.
The cytoskeleton is dynamic, dismantling in one part and reassembling
in another to change cell shape.
The cytoskeleton interacts
with motor proteins.
•
•
Fig. 6.21a
•
•
•
•
In cilia and flagella motor proteins pull components of the cytoskeleton
past each other. This is also true in muscle cells.
Motor protein molecules also carry vesicles or organelles to various
destinations along “monorails’ provided by the cytoskeleton.
Interactions of motor proteins and the cytoskeleton circulates materials
within a cell via streaming.
There are three main types of fibers in the cytoskeleton: microtubules,
microfilaments, and intermediate filaments.
Fig. 6.21 b
Centrosomes and Centrioles
• In animal cells, the centrosome has a pair of centrioles, each with
nine triplets of microtubules arranged in a ring.
• In many cells, microtubules grow out from a centrosome near the
nucleus.
• During cell division the centrioles replicate.
Fig. 6.22
Cilia and Flagella
Locomotor organelles protrude from some eukaryotic cells formed
from microtubules
• Microtubules are the central structural supports in cilia and
flagella.
• Both cilia and flagella can move unicellular(protista) and small
multicellular organisms (sperms) by moving water past the
organism.
• If these structures are anchored in a large structure, they move
fluid over a surface. cilia sweep mucus carrying trapped debris
from the
lungs.
Fig. 6.2
Flagella
•
•
•
There are usually just one or a few flagella per cell.
A flagellum has an undulatory movement.
Flagella are the same width as cilia, but 10-200 microns long.
•
Force is generated parallel to the flagellum’s axis.
Fig. 6.23a
Cilia
•
•
•
•
Cilia usually occur in large numbers on the cell surface.
They are about 0.25 microns in diameter and 2-20 microns long.
Cilia move more like oars with alternating power and recovery strokes.
They generate force perpendicular to the cilia’s axis.
Fig. 6.23b
Ultrastructure of a eukaryotic flagellum or cilium
• Both have similar ultrastructure core of microtubules sheathed by the
plasma membrane.
• Nine doublets of microtubules arranged around a pair at the center, the “9 +
2” pattern.
• The structure of cilium and flagellum is identical to that of centriole
centriole..
Fig. 6.24
Microfilaments
(Actin filaments)
Fig. 6.26 The shape of the
microvilli in this intestinal cell
are supported by microfilaments,
anchored to a network of
intermediate filaments.