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