-- Flashcards
1
Q
cell size
A
- Most cells are between 1 and 100 μm in diameter - most cells are invisible to the naked eye; Light microscopy (LM) can be used to visualize most cells. Electron microscopy (EM) is needed to see sub-cellular structures
2
Q
light microscopy (LM)
A
- In light microscopy (LM), visible light is passed
through a specimen and then through glass lenses. - The lenses refract (bend) the light in such a way that
the image of the specimen is magnified as it is projected into the eye or into a camera.
3
Q
brightfield
A
- Brightfield (unstained specimen).: Light passes directly through the specimen. Unless the cell is naturally
pigmented or artificially stained, the
image has little contrast. - Brightfield (stained specimen).
Staining with various dyes enhances
contrast. Most staining procedures require
that cells be fixed (preserved).
4
Q
phase-contrast
A
Variations in density
within the specimen are amplified to
enhance contrast in unstained cells, which
is especially useful for examining living,
unpigmented cells.
- (for examining cells in a single layer)
5
Q
Differential
Interference
Contrast
(DIC or Nomarski)
A
As in phase-contrast microscopy, optical modifications are used to
exaggerate differences in density, making
the image appear almost 3-D.
- (for examining cells in a single layer)
6
Q
confocal w/ fluorescence
A
- Specimen is stained with a dye that fluoresces - image is “optically sectioned” with a laser, one plane of focus at a time. By capturing sharp images at many different planes, a 3-D reconstruction can be created. - Out of focus layers are excluded with each pass of the laser and then digitally added back later to give a 3D look. - (for examining cells in a big blob of cells)
7
Q
e- microscope (EM)
A
Until recently, the resolution barrier prevented cell biologists from using standard light microscopy to study
organelles.
- Rather than light, the
electron microscope (EM) focuses a beam of electrons
through the specimen or onto its surface.
- Two basic types of electron microscopes (EMs) are used to
study subcellular structures – scanning and transmission
8
Q
Scanning electron microscopes (SEMs)
A
- S for Surface: especially
useful for detailed study of the topography of a specimen - focus a beam of
electrons onto the surface of a specimen. this beam scans the surface. pattern of electrons – The result is an image of
the specimen’s surface that appears three-dimensional.
9
Q
Transmission electron microscopes (TEMs)
A
- T for Through: focus a beam
of electrons through a specimen. - TEMs are used mainly to study the internal structure of cells.
10
Q
cell fractionation
A
A useful technique for studying cell structure and function is
cell fractionation, which takes cells apart and separates
major organelles and other subcellular structures from one another. (used to isolate (fractionate) cell
components based on size and density)
- Cells are ground up in a blender and the homogenate is then subjected to centrifugation. At lower speeds, the pellet consists of larger components, and higher speeds yield a pellet with smaller components. Larger particles will “pellet” to the bottom, smaller pieces remain suspended in the “supernatant”. (each subsequent centrifugation - higher speed, longer time)
11
Q
Cell fractionation enables researchers
A
to prepare specific
cell components in bulk and identify their FUNCTIONS, a task
not usually possible with intact cells. For example, on one of
the cell fractions, biochemical tests showed the presence of
enzymes involved in cellular respiration, while electron
microscopy revealed large numbers of the organelles called
mitochondria. Together, these data helped biologists determine that mitochondria are the sites of cellular respiration.
12
Q
Differential
centrifugation: order of pellets
A
(slowest to fastest)
1. Pellet rich in nuclei and cellular debris
2. “ “ “ mitochondria (and chloroplasts if plant)
3. “ “ “ microsomes (pieces of plasma
membranes and cells’ internal membranes)
4. ribosomes
13
Q
prokaryotic cells
A
Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells
(“pro”, before; “karyon”, kernel or core) These cells lack nucleus and internal
membrane-bound structures.
14
Q
eukaryotic cells
A
Protists, fungi, animals, and plants all consist of eukaryotic cells (“eu”, true). This
means that the genetic material is physically separated from the rest of the cell by a
membrane
15
Q
All cells share certain basic features:
A
- They are all bounded by
a selective barrier, called the plasma membrane. - Inside all cells
is a semifluid, jellylike substance called cytosol, in which
subcellular components are suspended. - All cells contain
chromosomes, which carry genes in the form of DNA. - And all
cells have ribosomes, tiny complexes that make proteins according to instructions from the genes.
16
Q
Prokaryotic cells are characterized by having…
A
No nucleus (the nucleoid is the region where the
DNA is located but is not membrane bound).
In fact, no membrane-bound organelles.
17
Q
cytoplasm
A
The interior of either type of cell is called the cytoplasm; in
eukaryotic cells, this term refers only to the region between the
nucleus and the plasma membrane.
18
Q
Eukaryotic cells are characterized by having…
A
only about 5% of the
membrane of a eukaryotic cell is the plasma membrane.
- DNA in a nucleus that is bounded by (double membrane) a membranous
nuclear envelope.
- Membrane-bound organelles (internal membranes that
compartmentalize different functions).
- Cytoplasm in the region between the plasma
membrane and nucleus (note: cytosol is the fluid
portion of the cytoplasm).
19
Q
euk prok size numbers
A
- At the lower limit,
the smallest cells known are bacteria called mycoplasmas,
which have diameters between 0.1 and 1.0 μm. These are
perhaps the smallest packages with enough DNA to program
metabolism and enough enzymes and other cellular equipment to carry out the activities necessary for a cell to sustain
itself and reproduce. - Typical bacteria are 1–5 μm in diameter.
- Eukaryotic cells are typically 10–100 μm in diameter.
20
Q
SA/V ratio rationale
A
- At the boundary of
every cell, the plasma membrane functions as a selective
barrier that allows passage of enough oxygen, nutrients, and
wastes to service the entire cell. - For each square micrometer of membrane, only a limited amount of a particular substance can cross per second, so the ratio of surface area to
volume is critical. - As a cell (or any other object) increases in
size, its volume grows proportionately more than its surface area. The center gets farther and farther
away from the surface.
21
Q
Eukaryotes have exploited the SA:V principle in two ways:
increased the surface area of these large cells
A
- Larger organisms do not have larger cells, they just have more cells.
- The internalization of membrane bound structures has effectively increased the surface area of these large cells (imagine each small box as being an internal, membrane-bound organelle).
22
Q
nucleus
A
The nucleus contains most of the genes in the eukaryotic
cell. (Some genes are located in mitochondria and chloroplasts.)
23
Q
nuclear envelope
A
The nuclear envelope is a double membrane. Two membranes, each a lipid bilayer with associated proteins.
- pore complex
- nuclear lamina
- nuclaer matrix
24
Q
pore complex
A
- The envelope is perforated by
pore structures. At the lip of
each pore, the inner and outer membranes of the nuclear envelope are continuous. - Inside each pore is a pore complex that consists of eight very large protein granules
arranged in an octagon. - It plays an important role in the
cell by regulating the entry and exit of proteins and RNAs, as
well as large complexes of macromolecules.
25
nuclaer lamina
Except at the
pores, the nuclear side of the envelope is lined by the nuclear
lamina, a netlike array of protein filaments that maintains
the shape of the nucleus by mechanically supporting the nuclear envelope.
26
nuclaer matrix
There is also much evidence for a nuclear
matrix, a framework of protein fibers extending throughout
the nuclear interior. The nuclear lamina and matrix may help
organize the genetic material so it functions efficiently
27
nucleolus
- A prominent structure within the nondividing nucleus
- Consists of ribosomes and ribosomal RNA (rRNA); site of ribosome synthesis
- rRNA is synthesized from instructions
in the DNA. Also in the nucleolus, proteins imported from
the cytoplasm are assembled with rRNA into large and small
subunits of ribosomes. These subunits then exit the nucleus
through the nuclear pores to the cytoplasm, where a large and
a small subunit can assemble into a ribosome.
28
chromosomes
- condensed chromatin
Within the nucleus, the DNA is organized into discrete units
called chromosomes, structures that carry the genetic information.
- Each chromosome contains one long DNA molecule
associated with many proteins.
- Some of the proteins help coil the DNA molecule of each chromosome, reducing its length
and allowing it to fit into the nucleus.
29
chromatin
Consists of DNA and its associated proteins,
in particular a group of proteins called histones around which
the DNA is wound. In addition, there are many regulatory proteins
that function in controlling gene expression.
30
endomembrane system
- regulates protein traffic and
performs metabolic functions in the cell.
- NEGLVP: nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, various kinds of vesicles and vacuoles, and the plasma membrane
- The membranes of this system are related either
through direct physical continuity or by the transfer of membrane segments as tiny vesicles (sacs made of membrane).
- Despite these relationships, the various membranes are not
identical in structure and function. Moreover, the thickness,
molecular composition, and types of chemical reactions carried out in a given membrane are not fixed, but may be modified several times during the membrane’s life.
31
ER stats
• About half of the cell’s membrane is ER membrane.
• Approximately 15% of the entire fluid volume of the
cell is inside the ER.
32
ER
• The ER is a network of interconnecting membrane
enclosed sacs distributed throughout the cytoplasm.
• The ER is folded into a series of tubules (cisternae).
• The internal compartment (the lumen) is called the
cisternal space and is a separate from the cytosol. It
has a distinct protein and ion composition.
• At certain sites, the ER membrane is continuous with
the outer nuclear envelope membrane; the
space between the two membranes of the nuclear envelope is continuous with the lumen of the ER.
33
smooth vs rough ER
Smooth ER is so named because its outer surface lacks ribosomes. Rough ER is studded with ribosomes on the outer
surface of the membrane and thus appears rough.
- Ribosomes are
also attached to the cytoplasmic side of the nuclear envelope’s
outer membrane, which is continuous with rough ER
34
smooth ER
- The smooth ER functions in diverse metabolic processes,
which vary with cell type
- (4): Synthesize lipids, Metabolize carbohydrates, Detoxify drugs and poisons, Store calcium ions
35
smooth ER: lipid synthesis
Enzymes of the smooth ER are important in the synthesis
of lipids, including oils, phospholipids, and steroids. Among
the steroids produced by the smooth ER in animal cells are
the sex hormones of vertebrates and the various steroid hormones secreted by the adrenal glands. The cells that synthesize and secrete these hormones—in the testes and ovaries,
for example—are rich in smooth ER, a structural feature that
fits the function of these cells
36
smooth ER: detox
- Other enzymes of the smooth ER help detoxify drugs and
poisons, especially in liver cells. Detoxification usually involves adding hydroxyl groups to drug molecules, making
them more soluble and easier to flush from the body
- Many drugs induce
the proliferation of smooth ER and its associated detoxification enzymes, thus increasing the rate of detoxification. This,
in turn, increases tolerance to the drugs.
37
smooth ER: Ca2+
In muscle cells,
for example, the smooth ER membrane pumps calcium ions
from the cytosol into the ER lumen. When a muscle cell is
stimulated by a nerve impulse, calcium ions rush back
across the ER membrane into the cytosol and trigger contraction of the muscle cell. In other cell types, calcium ion release
from the smooth ER triggers different responses, such as secretion of vesicles carrying newly synthesized proteins
38
rough ER function
- functions in protein synthesis.
- Many types of cells secrete proteins produced by ribosomes
attached to rough ER.
- rough ER is a
membrane factory for the cell; it grows in place by adding
membrane proteins and phospholipids to its own membrane.
- As polypeptides destined to be membrane proteins
grow from the ribosomes, they are inserted into the ER
membrane itself and anchored there by their hydrophobic
portions.
- Like the smooth ER, the rough ER also makes
membrane phospholipids; enzymes built into the ER membrane assemble phospholipids from precursors in the cytosol.
- The ER membrane expands and portions of it are
transferred in the form of transport vesicles to other components of the endomembrane system.
39
rough ER: producing the protein
- As a polypeptide chain grows
from a bound ribosome, the chain is threaded into the ER lumen through a pore formed by a protein complex in the ER
membrane.
- As the new polypeptide enters the ER lumen, it
folds into its native shape.
- Most secretory proteins are
glycoproteins, proteins that have carbohydrates covalently
bonded to them. The carbohydrates are attached to the proteins in the ER by enzymes built into the ER membrane.
40
rough ER: Polypeptides can be directed one of two
| ways:
1. to the ER’s cisternal space,
2. incorporated into the ER membrane.
• Polypeptides can be further modified to change their folding, and/or have carbohydrate
groups added to begin the process of protein maturation.
41
rough ER: fate of the protein
After secretory proteins are formed, the ER membrane
keeps them separate from proteins that are produced by free
ribosomes and that will remain in the cytosol. Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud like bubbles from a specialized region called
transitional ER.
• Some polypeptides entering the ER have address information that instructs their final
destination (secretion is the default destination).
• Some membrane and lumen proteins remain in the ER to maintain its function; others are
transported to the Golgi apparatus.
42
Where Do Newly Synthesized Proteins Go?
made in rough ER:
1) are released outside the cell.
2) are inserted into the plasma membrane.
3) become part of an organelle.
made in cytoplasm:
4) remain in the cytoplasm as soluble proteins.
To make this happen, all proteins need to be directed to the appropriate
place in the cell.
43
ribosomes
- complexes made of ribosomal RNA
and protein (>50 proteins)
- large + small subunit
- the cellular components that carry out protein synthesis. Cells that have high rates of protein synthesis have particularly large numbers of ribosomes.
For example, a human pancreas cell has a few million ribosomes. Not surprisingly, cells active in protein synthesis also
have prominent nucleoli.
44
ribosomes: 2 locales
Bound and free ribosomes are structurally identical, and ribosomes can alternate between the two roles
- free: suspended in cytosol. synthesize cytoplasmic, soluble proteins - ex: enzymes that catalyze the first steps of sugar breakdown
- bound: attached to the outside of the endoplasmic reticulum or nuclear envelope. Those attached to RER synthesize proteins that
are sometimes insoluble and are destined for insertion into membranes, packaging within certain organelles such as lysosomes, or secretion. Cells that specialize in
protein secretion—for instance, the cells of the pancreas that
secrete digestive enzymes—frequently have a high proportion of bound ribosomes.
- also found on Mitochondria and Chloroplasts
45
mitochondria and chloroplasts
-- ENERGY METABOLISM --
• Both of these organelles can move around the cell and divide.
• They have their own ribosomes and DNA that are used to synthesize
some of their proteins.
• These features suggest that at one time they were independent
prokaryotic organisms that developed a symbiotic relationship with
other cells.
46
endosymbiont theory
1. anaerobic pre-eukaryotic cell (w/ ER, nucleus, nuclear envelope) engulfs an oxygen-using nonphotosynthetic prokaryote, which becomes a mitochondrion w/ outer membrane derived from eukaryotic cell
2. now host cell is a nonphotosynthetic eukaryote w/ mitochondrion. it engulfs a photosynthetic prokaryote, which becomes a chloroplast
3. now host cell is a photosynthetic euk
47
mitochondria
- found in virtually all eukaryotic cells.
- vary in size from 1 -10µm (about the same
size as a bacterium…supporting the
endosymbiont theory).
48
mitochondria membranes
• Made up of a an outer smooth
membrane, and an inner membrane that is
folded into a series of convolutions called
cristae.
- As highly folded
surfaces, the cristae give the inner mitochondrial membrane
a large surface area, thus enhancing the productivity of cellular respiration. This is another example of structure fitting
function.
•Between these is the intermembrane
space.
49
matrix
Inside the inner membrane is a region
called the matrix (in which there are free ribosomes) that contains many of the
enzymes important for cellular respiration
where the energy derived from
carbohydrates and lipids is transferred to
the molecules, particularly ATP, that are
used as energy sources for many cellular
process
50
chloroplasts
• The organelle that is responsible for photosynthesis: transforming
sunlight into forms of energy that plant cells can use.
• Structure is similar to a mitochondrion in that chloroplasts have a
double membrane. Inside the inner membrane is a compartment
called the stroma (analogous to the matrix of mitochondria). stroma is the fluid outside the thylakoids
• Chloroplasts have another compartment: a series of discs called
thylakoids, which in some cases are stacked into structures called
grana. The thylakoids are where chlorophyll is located and is the site
of photosynthesis.
51
cytoskeleton
-- Structural and transport functions --
• Maintains cell shape, and provides support and anchor points for
organelles.
• Provides the mechanisms for cell movement (flagellae and
pseudopodia) and force generation (e.g. muscle contraction).
• Serves as tracks for motor proteins that move materials and
organelles within cells.
There are three major types of
cytoskeletal components:
• Microfilaments
• Intermediate filaments
• Microtubules
52
motor proteins
Motor proteins that attach to receptors on vesicles (or organelles) can “walk”
the vesicles along microtubules or, in some cases, microfilaments
53
microtubules
- tubulin polymers
- hollow tubes
- 25 nm diameter w/ 15 nm lumen
- protein subunits: tubulin, a dimer of alpha- and beta-tubulin
functions:
- maintenance of cell shape (compression-resisting "girders")
- cell motility (cilia/flagella)
- chromosome movements in cell div
- organelle mvmnts
54
microfilaments
- actin filaments
- Two intertwined strands of actin, each
a polymer of actin subunits
- 7 nm diameter
functions:
- maintenance of cell shape (tension (pulling forces)-bearing elements)
- changes in cell shape
- muscle contraction
- cytoplasmic streaming in plant cells
- cell motility (amoeboid mvmnt (pseudopodia))
- division of animal cells (cleavage furrow formation)
55
intermediate filaments
- Fibrous proteins supercoiled into thicker cables
- 8-12 nm diameter
- subunits: One of several different proteins (such as
keratins) , depending on cell type
functions:
- maintenance of cell shape (tension-bearing elements)
- anchorage of nucleus and certain other organelles
- formation of nuclear lamina
56
centrosome
```
- Most animal cells have a centrosome, a region near the nucleus where
the cell’s microtubules grow out from. Within the centrosome is a pair of
centrioles, each about 250 nm (0.25 μm) in diameter. Each is made up of nine
sets of three microtubules.
•Centrosomes are important for cell
division (mitosis) in animal cells.
• They are made from microtubules
and other proteins
• The 2 centrioles are oriented at right
angles to each other and are
microtubule organizing centers
(MTOCs)
```
57
cilia vs flagella
• Microtubules are the structural components of cilia and flagella.
• Cilia and flagella are covered by the cell’s plasma membrane.
• Similar to microvilli but moveable.
• Flagella are usually longer than cilia, and cells that have them usually
only have one or two; not on all cells…only on sperm in mammals
• Cilia are usually present in great numbers.
58
cell junctions
- Neighboring cells often adhere, interact,
and communicate via sites of direct physical contact.
- In animals, there are three main types of cell junctions: tight
junctions, desmosomes, and gap junctions.
59
tight junctions
```
- At tight junctions, the plasma
membranes of neighboring cells are
very tightly pressed against each
other. Forming continuous seals around the cells, tight
junctions prevent leakage of extracellular fluid across a layer of
epithelial cells.
- As if the
cells were sewn together.
```
60
desmosomes
- At desmosomes, membranes of neighboring cells are pressed together -- function like rivets, fastening cells together into strong sheets.
- Akin to a shirt button.
- Desmosomes are linked to intermediate filaments (these filaments anchor desmosomes in the cytoplasm), providing shear and tensile strength between cells.
- attach muscle cells to each other in a muscle
61
gap junctions
```
- (communicating junctions)
provide cytoplasmic
channels between adjacent
cells.
- necessary for communication between cells in many
types of tissues, such as heart muscle, and in animal embryos.
- (Analogous to
plasmodesmata of plant
cells.)
```
62
ECM
- Animal cells lack cell walls but are covered by an elaborate
extracellular matrix (ECM).
- The ECM is made up of glycoproteins such as collagen,
fibronectin and proteoglycans.
63
proteoglycan
- A proteoglycan molecule consists of a small core protein w/ attached carbohydrates
- A proteoglycan
complex consists
of hundreds of
proteoglycan
molecules attached
noncovalently to a
single long polysaccharide molecule.
64
collagen
```
Collagen fibers
are embedded
in a web of
proteoglycan
complexes
```
65
fibronectin
```
attaches the
ECM to
integrins
embedded in
the plasma
membrane.
```
66
integrins
```
- ECM proteins bind to receptor proteins in the plasma membrane
called integrins. On the side of the integrin that faces the interior of the cell, it is bound to associated proteins
attached to
microfilaments.
- This linkage can
transmit signals
between the cell’s
external environment
and its interior and can
result in changes in cell
behavior.
```
67
where'd all the cell's internal membranes come from?
```
- Imagine a prehistoric procaryotic cell with
DNA and ribosomes attached to the plasma
membrane.
- Invaginations of surface membrane
of ancient procaryotic cell led to
internalization of membranes creating
rough and smooth ER.
- In addition, the DNA became bound
with a double membrane.
```
68
microvilli
- Increase surface area of the cell.
| - Helps to increase rate of uptake of molecules from extracellular space.
69
4 structures in plant cells not in animal cells
- central vacuole (Storage, hydrolysis of macromolecules)
- chloroplast
- cell wall (maintains cell's shape)
- plasmodesmata (Cytoplasmic channels between adjoining cells)
70
3 structures in animal cells not in plant cells
- flagellum
- centrosome
- lysosome
71
cell organelles
- Important for compartmentalization within the cell - the physical
separation of cell functions.
- Allows different chemical environments to exist within the cell.
1. pH; allows enzymes to function in an isolated environment.
Some hydrolytic enzymes require a low pH to function. These enzymes are
sequestered in organelles. If they “escape” to the cytosol, they are largely
ineffective.
2. Ionic gradients; generation of ATP in mitochondria requires an ionic
gradient that is established across the mitochondrial inner membrane.
72
Golgi apparatus
- responsible for the functional
maturation of proteins.
- After leaving the ER, many transport vesicles travel to the Golgi
apparatus. We can think of the Golgi as a warehouse for receiving, sorting, shipping, and even some manufacturing. Here,
products of the ER, such as proteins, are modified and stored
and then sent to other destinations.
- especially extensive in cells specialized for secretion.
73
Golgi structure
• Similar in structure to the smooth ER, and is
usually seen as a series of stacked discs or flattened membranous
sacs (cisternae) within the cell.
• There is a definite direction or polarity to the
structure, with
the membranes of cisternae on opposite sides of the stack differing in thickness and molecular composition.
• Transport vesicles are budded off the
RER and fuse with the Golgi apparatus at
its cis (receiving) face. Processed material leaves at
the trans (shipping) face, where vesicles pinch off and travel to other sites.
74
within the golgi
• Material is transferred from one disc to
another by transport vesicles that are guided by
special signaling proteins or tags attached to
the outside of the vesicles.
•As proteins travel through the Golgi apparatus they are modified until they become the final product that
can either be used by the cell or released from it by secretion.
•This modification process often occurs using specialized enzymes that are located at different points
between the cis and the trans part of the Golgi. Modifications include glycosylation, phosphorylation and
sulfation.
75
products of the golgi
•The main products of the Golgi are sets of small vesicles that bud off from the trans side of the Golgi.
Three main groups:
1. Lysosomes
2. Secretory vesicles—membrane bound organelles that release their contents to the outside of the cell.
3. Vesicles containing proteins destined to be part of the plasma membrane.
- In addition to its finishing work, the Golgi apparatus also
manufactures some macromolecules. Many polysaccharides
secreted by cells are Golgi products.
76
lysosome
- Contain enzymes
important for the digestion
of all the major types of
macromolecules.
- Lysosomal enzymes work best in the acidic environment (pH 5)
found in lysosomes. If a lysosome breaks open or leaks its contents, the released enzymes are not very active because the cytosol has a neutral pH. However, excessive leakage from a large
number of lysosomes can destroy a cell by self-digestion.
77
creation of lysosomes
Hydrolytic enzymes and lysosomal membrane are made
by rough ER and then transferred to the Golgi apparatus for
further processing. At least some lysosomes probably arise by
budding from the trans face of the Golgi apparatus
78
How are the proteins of the inner surface of the
lysosomal membrane and the digestive enzymes themselves
spared from destruction?
the three-dimensional
| shapes of these proteins protect vulnerable bonds from enzymatic attack.
79
phagocytosis
- Some types of cells can ingest food in vacuoles in a process called phagocytosis. Amoebas and many other protists eat by engulfing smaller organisms or food particles, a process called phagocytosis.
- The food vacuole formed in this way then fuses with a lysosome, whose enzymes digest the food.
- Digestion products, including simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell.
80
autophagy
- Lysosomes also use their hydrolytic enzymes to recycle the cell’s own organelles and macromolecules, a process called autophagy.
- During autophagy, a damaged organelle or small amount of cytosol becomes surrounded by a double membrane (creating a vesicle), and a lysosome fuses with the outer membrane of this vesicle. The lysosomal enzymes dismantle the enclosed material, and the organic monomers are returned to the cytosol for reuse.
- With the help of lysosomes, the cell continually renews itself.
81
Ultrastructure of a MOTILE eukaryotic flagellum or cilium
- 9(2) + 2(1): Nine doublets of microtubules are arranged in a ring; in the center of the ring are two single microtubules; sheathed in an extension of the plasma membrane
- The microtubule assembly of a cilium or
flagellum is anchored in the cell by a basal body, which is
structurally very similar to a centriole, with microtubule
triplets in a “9 + 0” pattern.
- flexible cross-linking proteins,
evenly spaced along the length of the cilium or flagellum,
connect the outer doublets to each other and to the two central microtubules
- radial spokes
82
dyneins
Each outer doublet also has pairs of protruding proteins spaced along its length and reaching toward
the neighboring doublet; these are large motor proteins called
dyneins, each composed of several polypeptides. Dyneins
are responsible for the bending movements of the organelle.
- A typical dynein protein has two “feet”
that “walk” along the microtubule of the adjacent doublet
- this walking bends cilia and flagella
83
beating of flagella
```
- Flagella move in
an undulating pattern
like a snake.
- generates force in the
SAME direction as the flagellum’s axis
```
84
beating of cilia
```
Cilia beat in a wave
pattern with a
power and recovery
stroke.
- generating force in a direction PERPENDICULAR to
the cilium’s axis
```
85
cortex
A three-dimensional
network formed by microfilaments just inside the plasma membrane (cortical microfilaments) helps support the cell’s
shape. This network gives the outer cytoplasmic layer of a cell, called the cortex, the semisolid consistency of a gel, in contrast with the more fluid (sol) state of the
interior cytoplasm.
[ cortex: gel w/ actin network; sol: w/ actin subunits ]
86
Microfilaments are well known for their role in cell motility
- particularly as part of the contractile apparatus of muscle cells.
- Thousands of actin filaments are arranged parallel to one
another along the length of a muscle cell, interdigitated
with thicker filaments made of a protein called myosin. Myosin acts as a motor protein using "walking mechanism" simialr to dynein. Contraction of the
muscle cell results from the actin and myosin filaments sliding
past one another in this way, shortening the cell.
- Localized contraction brought about by actin and myosin
also plays a role in amoeboid movement. A
cell such as an amoeba crawls along a surface by extending cellular extensions called pseudopodia. Actin moves forward and extends amoeba.
87
cytoplasmic streaming
In plant cells, both actin-myosin interactions and sol-gel
transformations brought about by actin may be involved in
cytoplasmic streaming, a circular flow of cytoplasm
within cells (Figure 6.27c). This movement, which is especially common in large plant cells, speeds the distribution of
materials within the cell.
