Chapter 6

Question 1

what is a cell

Answer 1

In the hierarchy of biological organization, the cell is the simplest collection of matter that can be considered a
living entity

Question 2

who first saw cells? what did anotni van leeuwenhoek do?

Answer 2

Cell walls were first seen by Robert Hooke in 1665 as he looked
through a microscope at dead cells from the bark of an oak
tree. But it took the wonderfully crafted lenses of Antoni van
Leeuwenhoek to visualize living cells. Imagine Hooke’s excitement when he visited van Leeuwenhoek in 1674 and the
world of microorganisms—what his host called “very little
animalcules”—was revealed to him

Question 3

who first used microscopes? most likely microscope u will use in the lab?

Answer 3

The microscopes first used by Renaissance scientists, as
well as the microscopes you are likely to use in the laboratory,
are all light microscopes

Question 4

light microscope (LM)

Answer 4

. In a light microscope (LM), visible light is passed through the 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 (see Appendix D

Question 5

Three important parameters in microscopy are

Answer 5

magnification, resolution, and contrast

Question 6

Magnification

Answer 6

is the ratio of

an object’s image size to its real size

Question 7

Resolution d and ex

Answer 7

is a measure of the clarity
of the image; it is the minimum distance two points can be
separated and still be distinguished as separate points. For
example, what appears to the unaided eye as one star in the
sky may be resolved as twin stars with a telescope, which has
a higher resolving ability than the eye

Question 8

The third

parameter, contrast, is

Answer 8

the difference in brightness between
the light and dark areas of an image. Methods for enhancing contrast include staining or labeling cell components
to stand out visually

Question 9

organelles,

Answer 9

the membrane-enclosed structures within

eukaryotic cells

Question 10

em

Answer 10

focuses a beam of electrons through the specimen or onto its surface. there are different types

Question 11

relationship between resolution and wavelentgh. max resolution of modern EM in nm? what the smallest structure tey can see? improvement compared to standard LM?

Answer 11

Resolution is inversely related to the wavelength of the light (or electrons) a microscope uses for imaging, and electron beams have much shorter wavelengths than
visible light. Modern electron microscopes can theoretically
achieve a resolution of about 0.002 nm, though in practice
they usually cannot resolve structures smaller than about
2 nm across. Still, this is a 100-fold improvement over the
standard light microscope.

Question 12

The scanning electron microscope (SEM) is

especially useful for? how does it work?

Answer 12

detailed study of the topography of a specimen (see Figure 6.3). The electron beam scans the
surface of the sample, usually coated with a thin film of
gold. The beam excites electrons on the surface, and these
secondary electrons are detected by a device that translates
the pattern of electrons into an electronic signal sent to a
video screen. The result is an image of the specimen’s
surface that appears three-dimensional.

Question 13

TEM - d and how does it work

Answer 13

The transmission electron microscope (TEM) is used
to study the internal structure of cells (see Figure 6.3). The
TEM aims an electron beam through a very thin section of
the specimen, much as a light microscope aims light through
a sample on a slide. For the TEM, the specimen has been
stained with atoms of heavy metals, which attach to certain
cellular structures, thus enhancing the electron density of
some parts of the cell more than others. The electrons passing through the specimen are scattered more in the denser
regions, so fewer are transmitted. The image displays the
pattern of transmitted electrons.

Question 14

A disadvantage of electron microscopy

Answer 14

is that the methods used to prepare the specimen kill the cells.
Specimen preparation for any type of microscopy can introduce artifacts, structural features seen in micrographs that
do not exist in the living cell.

Question 15

In the past several decades, light microscopy has been

revitalized by major technical advances

Answer 15

(see Figure 6.3).
Labeling individual cellular molecules or structures with
fluorescent markers has made it possible to see such structures with increasing detail. In addition, both confocal and
deconvolution microscopy have produced sharper images of
three-dimensional tissues and cells. Finally, a group of new
techniques and labeling molecules developed in recent years
has allowed researchers to distinguish subcellular structures even as small as
10–20 nm across. this super-resolution microscopy becomes
more widespread.

Question 16

microscopes? cytology, biochem?

Answer 16

Microscopes are the most important tools of cytology, the
study of cell structure. Understanding the function of each
structure, however, required the integration of cytology and
biochemistry, the study of the chemical processes (metabolism)
of cells.

Question 17

In early experiments,
researchers used microscopy to. what did these idenfitcations establish a baseline for and what did they let researchers do

Answer 17

identify the organelles in each
pellet and biochemical methods
to determine their metabolic
functions. These identifications
established a baseline for this method, enabling today’s researchers to
know which cell fraction they should collect in order to isolate and study
particular organelles

Question 18

what kinds of lenses do SEM and TEM use

Answer 18

Instead of using glass
lenses, both the SEM and TEM use electromagnets as lenses
to bend the paths of the electrons, ultimately focusing the
image onto a monitor for viewing

Question 19

what is the equipment used for cell frax? how does it work? what happens to the pellet at lower speeds? higher speeds?

Answer 19

The piece of equipment that
is used for this task is the centrifuge, which spins test tubes
holding mixtures of disrupted cells at a series of increasing
speeds. At each speed, the resulting force causes a subset
of the cell components to settle to the bottom of the tube,
forming a pellet. At lower speeds, the pellet consists of
larger components, and higher speeds result in a pellet
with smaller components

Question 20

Cell fractionation enables researchers to

Answer 20

prepare specific
cell components in bulk and identify their functions, a task
not usually possible with intact cells.

Question 21

what are some things all cells have in common? (4x)

Answer 21

s: They are all bounded by a
selective barrier, called the plasma membrane (also referred to as
the cell 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.

Question 22

A major difference between prokaryotic and eukaryotic

cells is

Answer 22

the location of their DNA. In a eukaryotic cell,
most of the DNA is in an organelle called the nucleus, which
is bounded by a double membrane (see Figure 6.8). In a
prokaryotic cell, the DNA is concentrated in a region that
is not membrane-enclosed, called the nucleoid

Question 23

meaning of eukaryotic and prokaryotic

Answer 23

Eukaryotic means “true nucleus” (from the Greek eu, true,
and karyon, kernel, referring to the nucleus), and prokaryotic
means “before nucleus” (from the Greek pro, before), reflecting the earlier evolution of prokaryotic cells

Question 24

The interior of either type of cell is called the cytoplasm; explain it in the euk cells

Answer 24

in eukaryotic cells, this term refers only to the region between
the nucleus and the plasma membrane. Within the cytoplasm
of a eukaryotic cell, suspended in cytosol, are a variety of
organelles of specialized form and function.

Question 25

what do proks not have? describe the cytoplasm of prokaryotic cells

Answer 25

These membranebounded structures are absent in almost all prokaryotic cells,
another distinction between prokaryotic and eukaryotic cells.
In spite of the absence of organelles, though, the prokaryotic
cytoplasm is not a formless soup. For example, some
prokaryotes contain regions surrounded by proteins (not
membranes), within which specific reactions take place.

Question 26

mycoplasmas? how big are typical bacteria and euk cells

Answer 26

d 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, about ten
times the size of mycoplasmas. Eukaryotic cells are typically
10–100 µm in diameter.
Metabolic requiremen

Question 27

plasma membrane

Answer 27

. 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 (Figure 6.6).

Question 28

explain surface area and voume and stuff with cells

Answer 28

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 surface area grows proportionately less than its volume.
(Area is proportional to a linear dimension squared, whereas
volume is proportional to the linear dimension cubed.) Thus,
a smaller object has a greater ratio of surface area to volume

Question 29

why is a A sufficiently high ratio of surface area to volume is especially
important in cells and in what cells in particular

Answer 29

A sufficiently high ratio of surface area to volume is especially
important in cells that exchange a lot of material with their surroundings, such as intestinal cells. Such cells may have many
long, thin projections from their surface called microvilli, which
increase surface area without an appreciable increase in volume.

Question 30

what may be on the outer surfce of the membrane

Answer 30

Carbohydrate side
chains may be attached to proteins or lipids on the outer surface of
the plasma membrane.

Question 31

how to find surface area or volume? what does a high surface to volume ration mean

Answer 31

Using arbitrary units of length, we can calculate the cell’s surface area
(in square units, or units2
),
volume (in cubic units, or
units3
), and ratio of surface
area to volume. A high
surface-to-volume ratio
facilitates the exchange of
materials between a cell and
its environment.

Question 32

In addition to the plasma membrane at its outer surface, a

eukaryotic cell has

Answer 32

extensive, elaborately arranged internal
membranes that divide the cell into compartments—the
organelles mentioned earlier. T

Question 33

. The plasma membrane and organelle

membranes also participate directly in

Answer 33

the cell’s metabolism

because many enzymes are built right into the membranes

Question 34

what is in mitochondiral membranes and what is their function?

Answer 34

. For example, enzymes

embedded in the membranes of the organelles called mitochondria function in cellular respiration

Question 35

Flagellum:

Answer 35

motility
structure present in
some animal cells,
composed of a cluster of
microtubules within an
extension of the plasma
membrane

Question 36

Centrosome:

Answer 36

region
where the cell’s
microtubules are
initiated; contains a
pair of centrioles

Question 37

cYTOSKELETON:

Answer 37

reinforces cell’s shape;
functions in cell movement;
components are made of
protein. Includes: microfilaments, intermediate filaments, microtubules

Question 38

Microvilli:

Answer 38

projections that
increase the cell’s
surface area

Question 39

Peroxisome:

Answer 39

organelle with
various specialized metabolic
functions; produces hydrogen
peroxide as a by-product and
then converts it to water

Question 40

mitochondrion

Answer 40

organelle where
cellular respiration occurs and
most ATP is generated

Question 41

Lysosome:

Answer 41

digestive
organelle where
macromolecules are
hydrolyzed

Question 42

Golgi apparatus:

Answer 42

organelle active
in synthesis, modification, sorting,
and secretion of cell products

Question 43

Plasma membrane:

Answer 43

membrane

enclosing the cell

Question 44

nucleus and its three parts

Answer 44

s
Nucleolus: nonmembranous
structure involved in production
of ribosomes; a nucleus has
one or more nucleoli
Nuclear envelope: double
membrane enclosing the
nucleus; perforated by
pores; continuous with ER
Chromatin: material consisting
of DNA and proteins; visible in
a dividing cell as individual
condensed chromosomes

Question 45

Central vacuole:(plnts)

Answer 45

prominent organelle
in older plant cells; functions include storage,
breakdown of waste products, and hydrolysis
of macromolecules; enlargement of the
vacuole is a major mechanism of plant growth

Question 46

Cell wall:(plants)

Answer 46

outer layer that maintains
cell’s shape and protects cell from
mechanical damage; made of cellulose,
other polysaccharides, and protein

Question 47

Plasmodesmata: plant

Answer 47

cytoplasmic
channels through cell walls
that connect the cytoplasms
of adjacent cells

Question 48

Chloroplast: plant

Answer 48

photosynthetic
organelle; converts energy of
sunlight to chemical energy
stored in sugar molecules

Question 49

what is the nucleolus? how do u see it? what is made here and how is it made?

Answer 49

A prominent structure within the nondividing nucleus is
the nucleolus (plural, nucleoli), which appears through the
electron microscope as a mass of densely stained granules and
fibers adjoining part of the chromatin. Here a type of RNA
called ribosomal RNA (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 ribosome

Question 50

how does the nucleus direct protein synthesis? what happens then?

Answer 50

As we saw in Figure 5.22, the nucleus
directs protein synthesis by synthesizing messenger RNA (mRNA) according
to instructions provided by the DNA.
The mRNA is then transported to the cytoplasm via the nuclear pores. Once an mRNA
molecule reaches the cytoplasm, ribosomes
translate the mRNA’s genetic message into the
primary structure of a specific polypeptide

Question 51

where is the lamina

Answer 51

. The netlike lamina
lines the inner surface of the nuclear envelope.
(The light circular spots are nuclear pores.)

Question 52

where is the lamina and what is the lamina and matrix

Answer 52

. a, a netlike array of protein filaments (in animal cells, called intermediate filaments) that
maintains the shape of the nucleus by mechanically supporting
the nuclear envelope. 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. The netlike lamina
lines the inner surface of the nuclear envelope.
(The light circular spots are nuclear pores.)

Question 53

bound v free ribosomes in terms of structure and function

Answer 53

Bound and free ribosomes are structurally identical, and
ribosomes can play either role at different times. Most of the
proteins made on free ribosomes function within the cytosol;
examples are enzymes that catalyze the first steps of sugar
breakdown. Bound ribosomes generally make proteins that
are destined for insertion into membranes, for packaging
within certain organelles such as lysosomes (see Figure 6.8),
or for export from the cell (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.

Question 54

what does endoplasmic and reticulum mean

Answer 54

(The word
endoplasmic means “within the cytoplasm,” and reticulum
is Latin for “little net.”

Question 55

describe ER structure

Answer 55

he ER consists of a network of
membranous tubules and sacs called cisternae (from the
Latin cisterna, a reservoir for a liquid). The ER membrane
separates the internal compartment of the ER, called the
ER lumen (cavity) or cisternal space, from the cytosol. And
because the ER membrane is continuous with the nuclear
envelope, the space between the two membranes of
the envelope is continuous with the lumen of the ER

Question 56

smooth v rough er

Answer 56

There are two distinct, though connected, regions of
the ER that differ in structure and function: smooth ER and
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
through the electron microscope. As already mentioned,
ribosomes are also attached to the cytoplasmic side of the
nuclear envelope’s outer membrane, which is continuous
with rough ER

Question 57

what does the golig do? where do transport vesicles that come to the golgi tend to come from?

Answer 57

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. Not surprisingly, the Golgi apparatus is especially extensive in cells
specialized for secretion.

Question 58

cis and trans face of the golgi. what does te trans face give rise to

Answer 58

. The two
sides of a Golgi stack are referred to as the cis face and the trans
face; these act, respectively, as the receiving and shipping
departments of the Golgi apparatus. The term cis means “on
the same side,” and the cis face is usually located near the ER.
Transport vesicles move material from the ER to the Golgi
apparatus. A vesicle that buds from the ER can add its membrane and the contents of its lumen to the cis face by fusing
with a Golgi membrane on that side. The trans face (“on the
opposite side”) gives rise to vesicles that pinch off and travel
to other sites.

Question 59

where do lysosomal enzymes work best? and what happens if they leak?

Answer 59

. Lysosomal enzymes work best in the acidic environment found in lysosomes. If a lysosome breaks open or leaks
its contents, the released enzymes are not very active because
the cytosol has a near-neutral pH. However, excessive leakage from a large number of lysosomes can destroy a cell by
self-digestion

Question 60

where do some lysosomes prob arise from? How are the proteins of the inner surface of the
lysosomal membrane and the digestive enzymes themselves
spared from destruction?

Answer 60

At least some lysosomes probably arise
by budding from the trans face of the Golgi apparatus (see
Figure 6.12). How are the proteins of the inner surface of the
lysosomal membrane and the digestive enzymes themselves
spared from destruction? Apparently, the three-dimensional
shapes of these proteins protect vulnerable bonds from enzymatic attack.

Question 61

what happens in ppl with an inherited lysosomal storage disease? what is an ex of such an illness?

Answer 61

The cells of people with inherited lysosomal storage
diseases lack a functioning hydrolytic enzyme normally
present in lysosomes. The lysosomes become engorged
with indigestible material, which begins to interfere with
other cellular activities. In Tay-Sachs disease, for example, a
lipid-digesting enzyme is missing or inactive, and the brain
becomes impaired by an accumulation of lipids in the cells.
Fortunately, lysosomal storage diseases are rare in the
general population.

Question 62

Vacuoles - def, fxn, solution in a vac v in the cytoplasm

Answer 62

are large vesicles derived from the endoplasmic
reticulum and Golgi apparatus. Thus, vacuoles are an integral
part of a cell’s endomembrane system. Like all cellular membranes, the vacuolar membrane is selective in transporting
solutes; as a result, the solution inside a vacuole differs in
composition from the cytosol.
Vacuoles perform a variety of

Question 63

Many unicellular

eukaryotes living in fresh water have

Answer 63

contractile vacuoles
that pump excess water out of the cell, thereby maintaining
a suitable concentration of ions and molecules inside the
cell (

Question 64

what is a central vcuole, how does it develop, what is inside and what is its function.

Answer 64

Mature plant cells generally contain a large central
vacuole (Figure 6.14), which develops by the coalescence
of smaller vacuoles. The solution inside the central vacuole,
called cell sap, is the plant cell’s main repository of inorganic
ions, including potassium and chloride. The central vacuole
plays a major role in the growth of plant cells, which enlarge as the vacuole absorbs water, enabling the cell to become
larger with a minimal investment in new cytoplasm. The
cytosol often occupies only a thin layer between the central
vacuole and the plasma membrane, so the ratio of plasma
membrane surface to cytosolic volume is sufficient, even
for a large plant cell.

Question 65

what is the biggest compartment in a plant cell. where is the rest of the cytoplasm in the plant cell

Answer 65

The central vacuole is
usually the largest compartment in a plant cell; the rest of the cytoplasm
is often confined to a narrow zone between the vacuolar membrane
and the plasma membrane (TEM).

Question 66

mito and chloroplast- what are they and fxns

Answer 66

Organisms transform the energy they acquire from their surroundings. In eukaryotic cells, mitochondria and chloroplasts
are the organelles that convert energy to forms that cells
can use for work. Mitochondria (singular, mitochondrion)
are the sites of cellular respiration, the metabolic process
that uses oxygen to drive the generation of ATP by extracting energy from sugars, fats, and other fuels. Chloroplasts,
found in plants and algae, are the sites of photosynthesis.
This process in chloroplasts converts solar energy to chemical
energy by absorbing sunlight and using it to drive the synthesis of organic compounds such as sugars from carbon dioxide
and water

Question 67

endosymbiont theory

Answer 67

Mitochondria and chloroplasts display
similarities with bacteria that led to the endosymbiont
theory, illustrated in Figure 6.16. This theory states that an
early ancestor of eukaryotic cells engulfed an oxygen-using
nonphotosynthetic prokaryotic cell. Eventually, the engulfed cell formed a relationship with the host cell in which
it was enclosed, becoming an endosymbiont (a cell living
within another cell). Indeed, over the course of evolution,
the host cell and its endosymbiont merged into a single
organism, a eukaryotic cell with a mitochondrion. At least
one of these cells may have then taken up a photosynthetic
prokaryote, becoming the ancestor of eukaryotic cells that
contain chloroplasts.
This is a widely accepted theory, which we will discuss in
more detail in Concept 25.3. This theory is consistent with
many structural features of mitochondria and chloroplasts

Question 68

what kind of behavior do chloroplasts exhibit

Answer 68

As with mitochondria, the static and rigid appearance of
chloroplasts in micrographs or schematic diagrams cannot
accurately depict their dynamic behavior in the living cell.
Their shape is changeable, and they grow and occasionally
pinch in two, reproducing themselves. They are mobile and,
with mitochondria and other organelles, move around thcell along tracks of the cytoskeleton, a structural networ

Question 69

what is a chloroplast a part of and give other examples of organells in this fmily

Answer 69

The chloroplast is a specialized member of a family of
closely related plant organelles called plastids. One type
of plastid, the amyloplast, is a colorless organelle that stores
starch (amylose), particularly in roots and tubers. Another
is the chromoplast, which has pigments that give fruits and
flowers their orange and yellow hues.

Question 70

The peroxisome is

Answer 70

a specialized metabolic compartment
bounded by a single membrane (Figure 6.19). Peroxisomes
contain enzymes that remove hydrogen atoms from various substrates and transfer them to oxygen (O2), producing
hydrogen peroxide (H2O2) as a by-product (from which the
organelle derives its name

Question 71

what types of peroxisomes are found in fat storing plat seed tissues? what do they do? how are peroxisomes related to other organells? what are some theories about that?

Answer 71

Specialized peroxisomes called glyoxysomes are found in
the fat-storing tissues of plant seeds. These organelles contain
enzymes that initiate the conversion of fatty acids to sugar,
which the emerging seedling uses as a source of energy and
carbon until it can produce its own sugar by photosynthesis.
How peroxisomes are related to other organelles is still an
open question. They grow larger by incorporating proteins
made in the cytosol and ER, as well as lipids made in the ER
and within the peroxisome itself. Peroxisomes may increase
in number by splitting in two when they reach a certain size,
sparking the suggestion of an endosymbiotic evolutionary
origin, but others argue against this scenario. Discussion of
this issue is ongoing.

Question 72

The remarkable strength and resilience of the cytoskeleton

as a whole are based on its

Answer 72

architecture. Like a dome tent,
the cytoskeleton is stabilized by a balance between opposing forces exerted by its elements. And just as the skeleton
of an animal helps fix the positions of other body parts, the
cytoskeleton provides anchorage for many organelles and
even cytosolic enzyme molecules. The cytoskeleton is more
dynamic than an animal skeleton, however. It can be quickly
dismantled in one part of the cell and reassembled in a new
location, changing the shape of the cell.

Question 73

describe the ends of a microtubules

Answer 73

Because of the orientation of tubulin
dimers, the two ends of a microtubule are slightly different. One end can accumulate or release tubulin dimers at a
much higher rate than the other, thus growing and shrinking significantly during cellular activities. (This is called
the “plus end,” not because it can only add tubulin proteins
but because it’s the end where both “on” and “off” rates are
much highe

Question 74

When cilia or flagella extend from

cells that are held in place as part of a tissue layer, they can2

Answer 74

Many unicellular eukaryotes
are propelled through water by cilia or flagella that act as
locomotor appendages, and the sperm of animals, algae, and
some plants have flagella.

2move fluid over the surface of the tissue. For example, the ciliated lining of the trachea (windpipe) sweeps mucus containing trapped debris out of the lungs (see the EMs in Figure 6.3).
In a woman’s reproductive tract, the cilia lining the oviducts
help move an egg toward the uterus

Question 75

Membrane proteins on this kind of

cilium transmit

Answer 75

molecular signals from the cell’s environment to its interior, triggering signaling pathways that may
lead to changes in the cell’s activities. Cilium-based signaling appears to be crucial to brain function and to embryonic
development.

Question 76

The microtubule assembly
of a cilium or flagellum is anchored in the cell by a basal
body, which is

Answer 76

structurally very similar to a centriole, with
microtubule triplets in a “9 + 0” pattern (Figure 6.24c).
In fact, in many animals (including humans), the basal
body of the fertilizing sperm’s flagellum enters the egg and
becomes a centriole.

Question 77

what are microfilaments? what are they also called? describe structure?

Answer 77

Microfilaments are thin solid rods. They are also called
actin filaments because they are built from molecules of
actin, a globular protein. A microfilament is a twisted double chain of actin subunits (see Table 6.1). Besides occurring as linear filaments, microfilaments can form structural
networks when certain proteins bind along the side of such
a filament and allow a new filament to extend as a branch.
Like microtubules, microfilaments seem to be present in all
eukaryotic cells.

Question 78

what are intemediate filaments

Answer 78

Intermediate filaments are more permanent fixtures of
cells than are microfilaments and microtubules, which are
often disassembled and reassembled in various parts of a
cell. Even after cells die, intermediate filament networks
often persist; for example, the outer layer of our skin consists of dead skin cells full of keratin filaments

Question 79

describe how the nucleus sits in the cell

Answer 79

. For instance, the nucleus typically sits within a
cage made of intermediate filaments, fixed in location by
branches of the filaments that extend into the cytoplasm.
Other intermediate filaments make up the nuclear lamina,
which lines the interior of the nuclear envelope (

Question 80

By supporting a cell’s shape, intermediate

filaments help …

Answer 80

the cell carry out its specific function. For
example, the network of intermediate filaments shown
in Figure 6.25 anchors the microfilaments supporting the
intestinal microvilli. Thus, the various kinds of intermediate filaments may function together as the permanent
framework of the entire cell

Question 81

how do actin and myosin fxn in muscle contraction

Answer 81

Myosin motors in muscle cell contraction. The ”walking” of
myosin projections (the so-called heads) drives the parallel myosin and
actin filaments past each other so that the actin filaments approach each
other in the middle (red arrows). This shortens the muscle cell. Muscle
contraction involves the shortening of many muscle cells at the same
time (TEM).

Question 82

how do ameoba move

Answer 82

Amoeboid movement. Interaction of actin filaments with myosin
causes contraction of the cell, pulling the cell’s trailing end (at left)
forward (to the right) (LM).

Question 83

what is cytoplasmic streaming

Answer 83

) Cytoplasmic streaming in plant cells. A layer of cytoplasm cycles
around the cell, moving over tracks of actin filaments. Myosin motors
attached to some organelles drive the streaming by interacting with
the actin (LM).

Question 84

The plasma membrane

is usually regarded as the boundary of the living cell, but …

Answer 84

most
cells synthesize and secrete materials extracellularly (to the
outside of the cell). Although these materials and the structures they form are outside the cell, their study is important
to cell biology because they are involved in a great many
essential cellular functions.

Question 85

how are microfibrils made from cellulose? what happens to them after being made?

Answer 85

Microfibrils made
of the polysaccharide cellulose (see Figure 5.6) are synthesized
by an enzyme called cellulose synthase and secreted to the
extracellular space, where they become embedded in a matrix
of other polysaccharides and proteins. This combination of
materials, strong fibers in a “ground substance” (matrix), is
the same basic architectural design found in steel-reinforced
concrete and in fiberglass.

Question 86

what is a secondary cell wall? what type of plant has a lot of these

Answer 86

Other cells
add a secondary cell wall between the plasma membrane
and the primary wall. The secondary wall, often deposited in
several laminated layers, has a strong and durable matrix that
affords the cell protection and support. Wood, for example,
consists mainly of secondary walls. Plant cell walls are usually
perforated by channels between adjacent cells called plasmodesmata, which will be discussed shortly

Question 87

what is the ecm? what kinds of cells have it? what are the main ingredients? most abundant glycoprotein in it?

Answer 87

Although animal cells lack walls akin to those of plant cells,
they do have an elaborate extracellular matrix (ECM).
The main ingredients of the ECM are glycoproteins and
other carbohydrate-containing molecules secreted by the
cells. (Recall that glycoproteins are proteins with covalently
bonded carbohydrates, usually short chains of sugars.) The
most abundant glycoprotein in the ECM of most animal
cells is collagen, which forms strong fibers outside the cells
(see Figure 5.18). In fact, collagen accounts for about 40%
of the total protein in the human body.

Question 88

A proteoglycan molecule

consists of

Answer 88

a small core protein with many carbohydrate
chains covalently attached, so that it may be up to 95%
carbohydrate. Large proteoglycan complexes can form when
hundreds of proteoglycan molecules become noncovalently attached to a single long polysaccharide molecule

Question 89

how are some cells attached to the ecm? what do these things do? what do integrins do? what are they? where did their namecome from

Answer 89

Some cells are attached to the ECM by
ECM glycoproteins such as fibronectin. Fibronectin and
other ECM proteins bind to cell-surface receptor proteins
called integrins that are built into the plasma membrane.
Integrins span the membrane and bind on their cytoplasmic
side to associated proteins attached to microfilaments of the
cytoskeleton. The name integrin is based on the word integrate:
Integrins are in a position to transmit signals between the
ECM and the cytoskeleton and thus to integrate changes
occurring outside and inside the cell.

Question 90

ecm and nucleus

Answer 90

Researchers have also learned
that the extracellular matrix around a cell can influence the
activity of genes in the nucleus. Information about the ECM
probably reaches the nucleus by a combination of mechanical and chemical signaling pathways.

Question 91

At tight junctions,

Answer 91

the plasma
membranes of neighboring cells are
very tightly pressed against each
other, bound together by specific
proteins. Forming continuous seals
around the cells, tight junctions
establish a barrier that prevents
leakage of extracellular fluid across
a layer of epithelial cells (see red
dashed arrow). For example, tight
junctions between skin cells make
us watertight.

Question 92

Desmosomes (what type of junction and def)

Answer 92

(one type of anchoring junction) function like rivets,
fastening cells together into strong
sheets. Intermediate filaments
made of sturdy keratin proteins anchor desmosomes in the cytoplasm.
Desmosomes attach muscle cells
to each other in a muscle. Some
“muscle tears” involve the rupture
of desmosomes.

Question 93

Gap junctions (what type of junction and def)

Answer 93

(also called communicating junctions) provide
cytoplasmic channels from one
cell to an adjacent cell and in this
way are similar in their function to
the plasmodesmata in plants. Gap
junctions consist of membrane proteins that surround a pore through
which ions, sugars, amino acids,
and other small molecules may
pass. Gap junctions are necessary
for communication between cells in
many types of tissues, such as heart
muscle, and in animal embryos.

Question 94

what does mechanical signaling invovle? what may cytoskeleton changes trigger? in this way what can the ecm help with

Answer 94

Mechanical signaling
involves fibronectin, integrins, and microfilaments of the
cytoskeleton. Changes in the cytoskeleton may in turn trigger
signaling pathways inside the cell, leading to changes in the set of proteins being made by the cell and therefore changes in the
cell’s function. In this way, the extracellular matrix of a particular tissue may help coordinate the behavior of all the cells
of that tissue.

Question 95

Membrane proteins:

Answer 95

Proteins
embedded in the plasma membrane or
other cellular membranes help
transport substances across membranes,
conduct signals from one side of the
membrane to the other, and participate
in other crucial cellular functions. Many
proteins are able to move within the
membrane.

Question 96

Cellular respiration:

Answer 96

Cellular respiration includes many steps, some carried out
by individual proteins or protein complexes in the cytoplasm and the mitochondrial matrix. Other proteins and protein complexes, involved in generating ATP
from food molecules, form a “chain” in the inner mitochondrial membrane

Question 97

Photosynthesis:

Answer 97

Large complexes of proteins with associated nonprotein
molecules are embedded in chloroplast membranes. Together, they can trap
light energy in molecules that are later used by other proteins inside the
chloroplast to make sugars. This is the basis for all life on the planet.

Question 98

Transcription:

Answer 98

In the nucleus, the information
contained in a DNA sequence is transferred to
messenger RNA (mRNA) by an enzyme called
RNA polymerase. After their synthesis, mRNA
molecules leave the nucleus via nuclear pores.

Question 99

Nuclear pore:

Answer 99

The nuclear pore
complex regulates molecular
traffic in and out of the nucleus,
which is bounded by a double
membrane. Among the largest
structures that pass through the
pore are the ribosomal subunits,
which are built in the nucleus

Question 100

Translation:

Answer 100

In the cytoplasm, the information in mRNA is used to
assemble a polypeptide with a specific sequence of amino acids. Both
transfer RNA (tRNA) molecules and a ribosome play a role. The
eukaryotic ribosome, which includes a large subunit and a small
subunit, is a colossal complex composed of four large ribosomal RNA
(rRNA) molecules and more than 80 proteins. Through transcription
and translation, the nucleotide sequence of DNA determines the
amino acid sequence of a polypeptide, via the intermediary mRNA.

Question 101

Cytoskeleton:

Answer 101

Cytoskeletal structures are polymers of protein
subunits. Microtubules are hollow structural rods made of
tubulin protein subunits, while microfilaments are cables that
have two chains of actin proteins wound around each other.
.

Question 102

Motor proteins:

Answer 102

Responsible for
transport of vesicles and movement of
organelles within the cell. This requires
energy, often provided by ATP hydrolysis