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The Golgi Apparatus: Shipping and Receiving Center
• The Golgi apparatus
– Receives (on the cis-side) many of the
transport vesicles produced in the rough ER
– Consists of flattened membranous sacs called
cisternae
– Exports many substances (from the transside) in transport vesicles
Functions of the Golgi apparatus
- Modification of the
products of the rough ER
Golgi
apparatus
- Manufacture of certain
macromolecules
cis face
(“receiving” side of
Golgi apparatus)
1 Vesicles move
6 Vesicles also
from ER to Golgi
transport certain
proteins back to ER
Figure 6.13
5 Vesicles transport specific
proteins backward to newer
Golgi cisternae
-Probably evolved from
2 Vesicles coalesce to
0.1 0 µm
ER
form new cis Golgi cisternae
Cisternae
3 Cisternal
maturation:
Golgi cisternae
move in a cisto-trans
direction
4 Vesicles form and
leave Golgi, carrying
specific proteins to
other locations or to
the plasma membrane for secretion
trans face
(“shipping” side of
Golgi apparatus)
TEM of Golgi apparatus
Lysosomes: Digestive Compartments
• Lysosomes are
membranous sacs of
hydrolytic enzymes,
and they carry out
intracellular digestion.
• They digest food from
food vacuoles that form
by phagocytosis and
they recycle old cell
parts in autophagy.
Nucleus
1 µm
Lysosome
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
Figure 6.14 A
(a) Phagocytosis: lysosome digesting food
Lysosomes
•
•
•
different lysosomes have
different enzymes for
breaking down different
macromolecules
They have a low pH (around
5); pump H+ ions in from the
cell
Example of a lysosomal
disease: Tay-Sachs
disease, caused by a
missing lysosomal enzyme
for lipid breakdown, leads to
buildup of lipids in the brain,
killing the individual in
infancy.
Lysosome containing
two damaged organelles
1µm
Mitochondrion
fragment
Peroxisome
fragment
Lysosome fuses with
vesicle containing
damaged organelle
Hydrolytic enzymes
digest organelle
components
Lysosome
Vesicle containing
damaged mitochondrion
Figure 6.14 B
Digestion
(b) Autophagy: lysosome breaking down damaged organelle
Vacuoles: Diverse Maintenance Compartments
• Vacuoles are fluid filled and membrane
enclosed.
• A cell may have one or several vacuoles.
– Food vacuoles
• Are formed by phagocytosis
– Contractile vacuoles
• Pump excess water out of protist cells
Vacuoles: Diverse Maintenance Compartments
• Central vacuoles
– Found in plant cells
– Function in cell size and
turgidity
– Store reserves of
important organic
compounds and water
Central vacuole
Cytosol
Tonoplast
Nucleus
Central
vacuole
Cell wall
Chloroplast
Figure 6.15
5 µm
The Endomembrane System: A Review
• Relationships among membranes/organelles of the
endomembrane system
1 Nuclear envelope is
connected to rough ER,
which is also continuous
with smooth ER
Nucleus
Rough ER
2
Membranes and proteins
produced by the ER flow in
the form of transport vesicles
to the Golgi
Smooth ER
cis Golgi
Nuclear envelop
3
Golgi pinches off transport
Vesicles and other vesicles
that give rise to lysosomes and
Vacuoles
Plasma
membrane
trans Golgi
4
Lysosome available
for fusion with another
vesicle for digestion
Figure 6.16
5 Transport vesicle carries 6
proteins to plasma
membrane for secretion
Plasma membrane expands
by fusion of vesicles; proteins
are secreted from cell
Organelles of Endosymbiotic Origin
• Mitochondria and chloroplasts change energy
from one form to another
• Mitochondria
– Are sites of cellular respiration
• Plastids
– Found only in plants, are sites of
photosynthesis
Mitochondria: Chemical Energy Conversion
• Mitochondria (powerhouse of the cell)
– Are found in nearly all eukaryotic cells
–
Have their own DNA- derived from the mother. This DNA
changes very slowly over time because there is no
recombination, only change is due to drift (chance).
Mitochondrion
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
Figure 6.17
Mitochondrial
DNA
100 µm
Mitochondria: Chemical Energy Conversion
–
Are the site of oxidative metabolism (conversion of glucose to ATP, carbon
dioxide, and water), also known as cellular respiration.
* Which type of cell would you expect to have a lot of mitochondria?
–
Are enclosed in a double membrane. Inner membrane is folded for increased
surface area. This is where the metabolism occurs; enzymes are embedded
in the membrane.
Mitochondrion
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
Figure 6.17
Mitochondrial
DNA
100 µm
Plastids: Capture of Light Energy
• Plastids
– have a double membrane
– have their own DNA
– function in photosynthesis (the chloroplast is an
example)
– contain pigments such as chlorophyll, carotenoids
– can also be for storage (leukoplasts)
Chloroplasts
–
Are found in leaves and other green organs of plants and in algae
–
Their structure includes
• Thylakoids, membranous sacs
• Stroma, the internal fluid
Chloroplast
Ribosomes
Stroma
Chloroplast
DNA
Inner and outer
membranes
Granum
1 µm
Figure 6.18
Thylakoid
Peroxisomes: Oxidation
• Peroxisomes
– Produce hydrogen peroxide and convert it to
water
Chloroplast
Peroxisome
Mitochondrion
Figure 6.19
1 µm
The Cytoskeleton
Cytoplasm – includes all the space inside the
plasma membrane but outside the nucleus (includes
organelles, cytosol, and cytoskeleton)
Cytoskeleton: microlattice of fibers supports the
cell and gives it 3-dimensional shape. Organelles
attach to the fibers.
The cytoskeleton gives the cell spatial information,
which is very important in development
The cytoskeleton is not stationary, it is dynamic.
The Cytoskeleton
– Is a network of fibers extending throughout the
cytoplasm, and it organizes cell structures and
activities.
Microtubule
Figure 6.20
0.25 µm
Microfilaments
Roles of the Cytoskeleton: Support, Motility, and Regulation
–
Gives mechanical support to the cell
–
Is involved in cell motility, which utilizes motor proteins
ATP
Vesicle
Receptor for
motor protein
Motor protein
Microtubule
(ATP powered)
of cytoskeleton
(a) Motor proteins that attach to receptors on organelles can “walk”
the organelles along microtubules or, in some cases, microfilaments.
Vesicles
Microtubule
0.25 µm
Figure 6.21 A, B
(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell
axons via the mechanism in (a). In this SEM of a squid giant axon, two
vesicles can be seen moving along a microtubule. (A separate part of the
experiment provided the evidence that they were in fact moving.)
Components of the Cytoskeleton
• There are three main types of fibers that make
up the cytoskeleton
Table 6.1
Microtubules
• Microtubules
– Shape the cell
– Cilia and flagella for motility
– Guide the movement of organelles
– Help separate the chromosome copies in
dividing cells
Centrosomes and Centrioles
• The centrosome
– Is considered to be a “microtubule-organizing
center” and it organizes the spindle fibers used
to guide the movement of chromosomes
during cell division.
In animal cells, the centrosome:
– Contains a pair of centrioles which are made
of microtubules in a nine-triplets pattern.
Centrosome
Microtubule
Centrioles
0.25 µm
Figure 6.22
Longitudinal section
of one centriole
Microtubules
Cross section
of the other centriole
Cilia and flagella – locomotory organelles
• Cilia and flagella share a common ultrastructure of microtubules
in a 9 + 2 arrangement. The base structure is similar to that of
centrioles (nine triplets).
Outer microtubule
doublet
Dynein arms
0.1 µm
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
(b)
0.5 µm
(a)
0.1 µm
Triplet
(c)
Figure 6.24 A-C
Cross section of basal body
Plasma
membrane
Cilia and Flagella move through the action of motor proteins
• The protein dynein
– Is responsible for the bending movement of
cilia and flagella
Microtubule
doublets
ATP
Dynein arm
(a) Powered by ATP, the dynein arms of one microtubule doublet
grip the adjacent doublet, push it up, release, and then grip again.
If the two microtubule doublets were not attached, they would slide
relative to each other.
Figure 6.25 A
Ciliary/flagellar motion
ATP
Outer doublets
cross-linking
proteins
Anchorage
in cell
(b) In a cilium or flagellum, two adjacent doublets cannot slide far because
they are physically restrained by proteins, so they bend. (Only two of
the nine outer doublets in Figure 6.24b are shown here.)
Figure 6.25 B
Microfilaments (Actin Filaments)
– Are built from molecules of the protein actin
– Are found in microvilli
Microvillus
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
Figure 6.26
0.25 µm
Microfilaments of muscle
• Microfilaments that function in cellular motility
– Contain the protein myosin in addition to actin
Muscle cell
Actin filament
Myosin filament
Myosin arm
Figure 6.27 A
(a) Myosin motors in muscle cell contraction.
Amoeboid motion
– Involves the contraction of actin and myosin
filaments
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
Figure 6.27 B
(b) Amoeboid movement
Cytoplasmic streaming
– Is another form of locomotion created by
microfilaments
Nonmoving
cytoplasm (gel)
Chloroplast
Streaming
cytoplasm
(sol)
Parallel actin
filaments
Figure 6.27 C
(b) Cytoplasmic streaming in plant cells
Cell wall
Intermediate Filaments
– Support cell shape
– Fix organelles in place
– Are fixed and do not disassemble.
Extracellular components and connections
between cells
help coordinate cellular activities
Cell Walls of Plants
– Extracellular structures of plant cells that distinguish
them from animal cells
– Are made of cellulose fibers embedded in other
polysaccharides and protein
– May have multiple layers
Central
vacuole
of cell
Plasma
membrane
Secondary
cell wall
Primary
cell wall
Central
vacuole
of cell
Middle
lamella
1 µm
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Figure 6.28
Plasmodesmata
The Extracellular Matrix (ECM) of Animal Cells
• Animal cells
– Lack cell walls
– Are covered by an elaborate matrix, the ECM
The ECM
– Is made up of glycoproteins and other
macromolecules. Some of these molecules can be
part of self-recognition or membrane-membrane
interactions (e.g. tissue glue that holds cells together).
EXTRACELLULAR FLUID
Collagen
A proteoglycan
complex
Polysaccharide
molecule
Carbohydrates
Core
protein
Fibronectin
Plasma
membrane
Integrin
Figure 6.29
Integrins
Microfilaments
CYTOPLASM
Proteoglycan
molecule
Functions of the ECM include
– Support
– Adhesion
– Movement
– Regulation
Intercellular Junctions in Plants
• Plasmodesmata are channels that perforate plant cell
walls. The cell membranes of neighboring cells are
continuous through these pores in the cell walls. This
allows cells to share molecules and communicate.
Cell walls
Interior
of cell
Interior
of cell
Figure 6.30
0.5 µm
Plasmodesmata
Plasma membranes
Animal Cell Junctions
• In animals, there are three types of intercellular
junctions
– Tight junctions
– Desmosomes
– Gap junctions
Animal Cell Junctions
• Types of intercellular junctions in animals
TIGHT JUNCTIONS
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continuous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
DESMOSOMES
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
Tight junctions
Intermediate
filaments
Desmosome
Gap
junctions
Space
between Plasma membranes
cells
of adjacent cells
Figure 6.31
1 µm
Extracellular
matrix
Gap junction
0.1 µm
GAP JUNCTIONS
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one cell to an adjacent cell. Gap junctions
consist of special membrane proteins that
surround a pore through which ions, sugars,
amino acids, and other small molecules may
pass rapidly. Gap junctions are necessary for communication between cells in many types of tissues,
including heart muscle and animal embryos.