Basic structural and functional features of cells and their organelles I & II

Question 1

Light microscope (LM)

Answer 1

In a light microscope, visible light is passed through the specimen then through glass lenses. The lenses refract (bend) in such a way that the image of the specimen is magnified as it is projected into the eye or into the camera.

Question 2

Organelles

Answer 2

Until recently, the resolution barrier prevented cell biologists from using standard light microscopy when studying organelles, the membrane enclosed structures within eukaryotic cells. To see their structures in any detail required the development of a new instrument.

Question 3

Electron microscope (EM)

Answer 3

In the 1950’s, the electron microscope was introduced to biology. Rather than focusing light, the electron microscope focuses a beam of electrons through the specimen or onto its surface. It was a 100 fold improvement from before.

Question 4

Scanning electron microscope (SEM)

Answer 4

The scanning electron microscope is especially useful for detailed study of the topography of a specimen. 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 the electrons into an electronic signal sent to a video screen. The result is an image of the specimen’s surface that appears three-dimensional. It uses electromagnets as a lens.

Question 5

Transmission electron microscope (TEM)

Answer 5

The transmission electron microscope is used to study the internal structure of cells. 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. It uses electromagnets as a lens.

Question 6

Cell fractionation

Answer 6

A useful technique for studying cell structure and function is cell fractionation. Broken-up cells are placed in a tube that is spun in a centrifuge. The largest cell components settle at the bottom as result of the force, forming a pellet. The liquid above the pellet is poured into a new tube and centrifuged at a higher speed for a longer time. This process is repeated several times, resulting in a series of pellets that consist of nuclei, mitochondria (and chloroplasts if the cells are from a photosynthetic organism), pieces of membrane, and ribosomes, the smallest components. This enables researchers to prepare specific cell components in bulk and identify their function, which is not possible with intact cells.

Question 7

Cytosol

Answer 7

Inside all cells is a semifluid, jellylike substance called cytosol, in which subcellular components are suspended.

Question 8

Eukaryotic cell

Answer 8

A major difference in eukaryotic and prokaryotic cells is the location of their DNA. In an eukaryotic cell, most of the DNA is in an organelle called the nucleus, which is bounded by a double membrane. Eukaryotic is from Greek and means true nucleus.

Question 9

Prokaryotic cell

Answer 9

A major difference in eukaryotic and prokaryotic cells is the location of their DNA. In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid. Prokaryotic is from Greek and means before nucleus, reflecting the earlier evolution of prokaryotes.

Question 10

Nucleoid

Answer 10

In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid.

Question 11

Cytoplasm

Answer 11

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. Within the cytoplasm of an eukaryotic cell, suspended in cytosol, are a variety of organelles of specialized form and function. These membrane bounded structures are absent in prokaryotic cells, another distinction between eu and pro. Despite this, the cytoplasm in prokaryotic cells is not formless, and they contain regions surrounded by proteins within which specific reactions take place. The proteins are not membranes.

Question 12

Fimbriae

Answer 12

Attachment structures on the surface of some prokaryotes.

Question 13

Plasma membrane

Answer 13

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.

Question 14

Geometric relationships between surface area and volume

Answer 14

Using arbitrary units of length, we can calculate the cells surface area (in square units, or units^2), volume (in cubic units or unit^3), and ratio of surface area to volume. A high surface-to-volume ratio facilitates the exchange of materials between a cell and its environment. As a cell (or any other object) increases in size, its surface area grows proportionately less than its volume. Thus, a smaller object has a greater ratio of surface area to volume.

Question 15

Animal cell

Answer 15

An eukaryotic cell that contains a nucleus with nuclear envelope, nucleolus and chromatin, plasma membrane, ribosomes, Golgi apparatus, lysosome, mitochondrion, peroxisome, microvilli, cytoskeleton made of microfilaments, intermediate filaments and microtubules, centrosomes, flagellum, and endoplasmatic reticulum, both smooth and rough.

Question 16

Nucleus

Answer 16

Containing the nuclear envelope, nucleolus and chromatin. The nucleus contains most of the genes in the eukaryotic cell, (some genes are located in mitochondria and chloroplasts). It is usually the most conspicuous organelle averaging about 5 (my)m in diameter.

Question 17

Nuclear envelope

Answer 17

Double membrane enclosing the nucleus, perforated by pores, continuous with ER. The nuclear envelope encloses the nucleus, separating its contents from the cytoplasm. The nuclear envelope is a double membrane, the two membranes each a lipid bilayer with associated proteins, are separated at 20-40 (my)m. The envelope is perforated by pore structures that are about 100 nm in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous.

Question 18

Nucleolus

Answer 18

Nonmembranous structure involved in production of ribosomes, a nucleus has one or more nucleoli.

Question 19

Chromatin

Answer 19

Material consisting of DNA and proteins, visible in a dividing cell as individual condensed chromosomes. The complex of DNA and proteins making up chromosomes is called chromatin. When a cell is not dividing, stained chromatin appears as a diffuse mass in micrographs, and the chromosomes cannot be distinguished from one another, even though they are separate.

Question 20

Ribosomes

Answer 20

(Small brown dots), complexes that make proteins, free in cytosol or bound to the rough ER or nuclear envelope. Ribosomes are complexes made of ribosomal RNA and protein, that are cellular components that carry out protein synthesis. They are not membrane bound and are therefore not considered organelles. Cells that have high rates of protein synthesis have particularly large numbers of ribosomes as well as prominent nucleoli, which makes sense given the role of nucleoli in ribosome assembly. Free ribosomes are suspended in the cytosol, and bound ribosomes are attached to the outside of the ER or nuclear envelope.

Question 21

Microvilli

Answer 21

Projections that increase the cells surface area.

Question 22

Endoplasmatic reticulum (ER)

Answer 22

Network of membranous sacs and tubes, active in membrane synthesis and other synthetic and metabolic processes, has rough (ribosome-studded) and smooth regions. The ER is such an extensive network of membranes that it accounts for more than half the total membrane in many eukaryotic cells. Endoplasmatic means within the cytoplasm, and reticulum means little net. It consists of a network of tubules and sacs called cisternae, (cisterna, reservoir for a liquid). It separates the internal compartment called the ER lumen, or cisternal space, from the cytosol. And because the ER membrane is continuous with the nuclear envelope, the space between the two is also continuous with ER lumen.

Question 23

Plant cell

Answer 23

A plant cell contains the same things an animal cell does, except it also has a cell wall, chloroplast and a big central vacuole.

Question 24

Nuclear lamina

Answer 24

A intricate protein structure called a pore complex lines each pore and plays an important role in the cell by regulating the entry and exit of proteins and RNAs as well as large complexes of macromolecules. Except at the pores, the nuclear side of the envelope is lined by nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope.

Question 25

Chromosomes

Answer 25

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.

Question 26

Endomembrane system

Answer 26

Many different membranes of the eukaryotic cell are part of the endomembrane system, which indulges the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, various kinds of vesicles and vacuoles, and the plasma membrane. This system carries out a great variety of tasks in the cell, including synthesis of proteins, transport of proteins into membranes and organelles out of the cell, metabolism and movement of lipids, and detoxification of poisons.

Question 27

Vesicles

Answer 27

The membranes of the endomembrane 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.

Question 28

Smooth ER

Answer 28

There are two distinct, though connected, regions of the ER that differ in structure and function. Smooth ER and rough ER. Smooth ER is named so, because its outer surface lacks ribosomes.

Question 29

Rough ER

Answer 29

There are two distinct, though connected, regions of the ER that differ in structure and function. Smooth ER and rough ER. Rough ER is studded with ribosomes on the outer surface of the membrane and thus appears rough through the electron microscope.

Question 30

Glycoproteins

Answer 30

Most secretory proteins are glycoproteins, proteins with carbohydrates covalently bonded to them. The carbohydrates are attached to the proteins in the ER lumen by enzymes built into the ER membrane.

Question 31

Transport vesicles

Answer 31

Vesicles in transit from one part of the cell to another are called transport vesicles.

Question 32

Golgi apparatus

Answer 32

Organelle active in synthesis, modification, sorting, and secretion of cell products. 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. It has a cis face that receives and a trans face that ships out. Some vesicles transport some proteins backward to less mature Golgi cisternae, where they function. The Golgi apparatus consists of flattened membranous sacs, cisternae, looking like stacked pita bread. Each cisternae in the stack separates its internal space from the cytosol. Each cisternae has its own unique team of enzymes. It manufactures some macromolecules, like many polysaccharides. There are molecular identification tags like phosphate groups added to the products, so that they go to the right place when leaving.

Question 33

Lysosome

Answer 33

Digestive organelle where macromolecules are hydrolyzed. Lysosomes carry out intracellular digestion in a variety of circumstances. A lysosome has a membranous sac of hydrolytic enzymes that many eukaryotic cells use to digest (hydrolyze) macromolecules. Lysosomal enzymes work best in the acidic environment found in lysosomes. If a lysosome breaks open or leaks, its contents, the released enzymes will not be very active since the cytosol has a near neutral pH. Excessive leakage can cause the cell to self digest. Lysosomes are made in the ER and Golgi. The 3D shapes of the lysosomal proteins protect vulnerable bonds from enzymatic attack, which keeps the enzymes within the lysosome.

Question 34

Phagocytosis

Answer 34

Amoebas and many other unicellular eukaryotes eat by engulfing smaller organisms or food particles, a process called phagocytosis. The food vacuole formed in this way 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.

Question 35

Food vacuoles

Answer 35

Vacuoles perform a variety of functions in different kinds of cells. Food vacuoles formed by phagocytosis is an example.

Question 36

Contractile vacuoles

Answer 36

Many unicellular eukaryotes living in fresh water have contractile vacuoles that pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell.

Question 37

Central vacuoles

Answer 37

Mature plant cells generally contain a large central vacuole, which develops by the coalescence of smaller vacuoles. The solution inside the central vacuole, called cell sap, is the plants main repository of inorganic ions, including potassium and chloride. It also 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 38

Mitochondria

Answer 38

Organelle where cellular respiration occurs and most ATP is generated. Mitochondria 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. It is generally 1-10 (my)m long.

Question 39

Chloroplasts

Answer 39

Photosynthetic organelle, converts energy of sunlight to chemical energy stored in sugar molecules. Chloroplasts, found in plants and algae, are the site 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 like sugars from carbon dioxide and water. The membranes of the chloroplast divide the chloroplast space into 3 compartments. The intermembrane space, the stroma and the thylakoid space. Their shape is changeable and they grow and occasionally pinch in two, reproducing themselves. They are mobile and move around the cell on tracks of the cytoskeleton.

Question 40

Endosymbiont theory

Answer 40

Mitochondria and chloroplasts display similarities with bacteria that led to the endosymbiont theory. This theory states that an early ancestor of eukaryotic cells engulfed an oxygen using non photosynthetic 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). Then one of these may have taken up a photosynthetic prokaryote that formed a relationship also.

Question 41

Cristae

Answer 41

Each of the two membranes inclosing the mitochondrion is a phospholipid bilayer with a unique collection of imbedded proteins. The outer membrane is smooth, but the inner membrane is convoluted , with infoldings called cristae. The inner membrane divides the mitochondrion into two internal compartments. The first compartment is the intermembrane space, the narrow region between inner and outer membranes. The second is the mitochondrial matrix. As a highly folded surface the cristae give the inner mitochondrial membrane a large surface thus enhancing the productivity of cellular respiration.

Question 42

Mitochondrial matrix

Answer 42

The inner membrane divides the mitochondrion into two internal compartments. The second is the mitochondrial matrix, which is enclosed by the inner membrane. The matrix contains many different enzymes as well as the mitochondrial DNA and ribosomes. Enzymes in the matrix catalyze some of the steps of cellular respiration. Other proteins that function in respiration, including the enzyme that makes ATP, are built into the inner membrane.

Question 43

Thylakoids

Answer 43

Inside the chloroplast is another membranous system in the form of flattened, interconnected sacs called thylakoids. In some regions, thylakoids are stacked like poker chips, each stack is called a granum.

Question 44

Granum

Answer 44

In some regions, thylakoids are stacked like poker chips, each stack is called a granum.

Question 45

Stroma

Answer 45

The fluid outside the thylakoids is the stroma, which contains the chloroplast DNA and ribosomes as well as many enzymes.

Question 46

Plastids

Answer 46

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.

Question 47

Peroxisome

Answer 47

Organelle with various specialized metabolic functions, produces hydrogen peroxide as a by-product and then converts it to water. The peroxisome is a specialized metabolic compartment bounded by a single membrane. Peroxisomes contain enzymes that remove hydrogen atoms from certain molecules and transfer them to oxygen (O2) producing hydrogen peroxide (H2O2). These reactions have many different functions. For instance in the liver they help detoxify alcohol and other harmful compounds by transferring a hydrogen from the toxins to oxygen. Now, H2O2 is toxic itself, but peroxisome also contains an enzyme that converts H2O2 into water. This is an example of how structure is crucial to function, since all of this is hidden away from places where it can do harm. Peroxisomes grow larger by incorporating proteins made in the cytosol and ER and within the peroxisome itself. Its evolution is still unclear.

Question 48

Cytoskeleton

Answer 48

Consists of microtubules, intermediate filaments and microfilaments. It reinforces the cell’s shape, functions in cell movement, components are made of protein. The cytoskeleton is a network of fibers extending throughout the cytoplasm. It plays a major role in organizing the structures and activities of the cell.

Question 49

Motor proteins

Answer 49

Some types of cell motility also involve the cytoskeleton. The term cell motility includes both the changes in the cell location and more limited movement of cell parts. Cell motility generally requires the interaction of the cytoskeleton with motor proteins. A motor protein has two feet that walk along the cytoskeleton, it is ATP powered and it sticks to a receptor for the motor protein on e.g. a vesicle. It can walk along the microtubules or in some cases, microfilaments.

Question 50

Microtubules

Answer 50

Tubulin polymers, they are hollow tubes, of a diameter 25nm with a 15nm lumen. The protein subunits are tubulin, a dimer consisting of alfa-tubulin and beta-tubulin. Microtubules do (maintenance of the cell shape), cell motility, chromosome movements in cell division and organelle movements. It looks like a close-knit spiral, one spiral is the alfa-tubulin(yellow) and it aligns with a spiral of beta-tubulin(beige). All eukaryotic cells have microtubules, hollow rods constructed from a globular protein called tubulin. Each tubulin is a dimer, a molecule made up of two subunits. Microtubules grow in length by adding tubulin dimers, they can also be disassembled and their tubulin used to build microtubules elsewhere in the cell. They are tracks throughout the cell for the motor proteins to use. Microtubules also take part in cell division.

Question 51

Intermediate filaments

Answer 51

Fibrous proteins coiled into cables, a lot like yarn. Diameter is 8-12nm and it’s comprised of several different proteins, one of which being keratins. It plays a role in the maintenance of cell shape, anchorage of nucleus and certain other organelles, and the formation of nuclear lamina. They are named after their size, and while the other two are found in all eukaryotic cells, intermediate filaments are only found in the cells of some animals, including vertebrates. Specialized for bearing tension, they are a diverse class of cytoskeletal elements, each type is constructed from a particular molecular subunit belonging to a family of proteins whose members include the keratin in hair and nails. They are more permanent fixtures and are often assembled and disassembled in different parts of the cell.

Question 52

Microfilaments (Actin filaments)

Answer 52

Two intertwined strands of actin. The diameter is 7nm, and it is only made up of actin (protein). It maintains cell shape, has functions for changes in cell shape, muscle contractions, cytoplasmic streaming (in plant cells), cell motility, cell division (in animal cells). Microfilaments are thin solid rods. They are also called actin filaments because they are built from the molecules of actin, a globular protein. They 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. In some kinds of animals cells, such as nutrient absorbing intestinal cells, bundles of microfilaments make up the core of microvilli, delicate projections that increase the cells surface area.

Question 53

Centrosomes

Answer 53

Region where the cell’s microtubules are initiated, contains a pair of centrioles. In animal cells, microtubules grow out from a centrosome, a region that is often located near the nucleus and is considered a “microtubule-organizing center”. These microtubules function as compression resisting girders around the cytoskeleton.

Question 54

Centrioles

Answer 54

Within the centrosome is a pair of centrioles, each composed of 9 sets of triplet microtubules arranged in a ring. Although centrosomes with centrioles may help organize microtubule assembly in animal cells, many other eukaryotic cells, lack centrosomes with centrioles, and instead organize microtubules by other means.

Question 55

Flagella

Answer 55

Motility structure present in some animal cells, composed of a cluster of microtubules within an extension of the plasma membrane. In eukaryotes, a specialized arrangement of microtubules is responsible for the beating of flagella and cilia, microtubule containing extensions that project from some cells. Many unicellular eukaryotes are propelled through water by cilia or flagella that act as locomotor appendages, and the sperm of animals. When cilia or flagella extend from the cells that are held in place as part of a tissue layer, they can move fluid over the surface of the tissue. Flagella are usually limited to just one or a few per cell. They differ in beating patterns. Flagellum has an undulating motion like the tail of a fish. Though different in multiple ways, flagella and cilia have a structure in common, which is a group of microtubules sheathed in an extension of the plasma membrane, 9 doublets of microtubules are arranged in a ring with 2 single microtubules in its center. It is called the 9+2 pattern.

Question 56

Cilia

Answer 56

In eukaryotes, a specialized arrangement of microtubules is responsible for the beating of flagella and cilia, microtubule containing extensions that project from some cells. E.g. the cilianated lining of the trachea (windpipe) sweeps mucus containing trapped debris out of the lungs. In a woman’s reproductive tract, the cilia lining the oviducts help move the egg towards the uterus. Motile cilia usually occur in large numbers on the cell surface. Cilia have alternating power and recovery strokes, like the oars of a racing boat crew. Cilia can also act as a signal receiving antenna for the cell, in that case, there is generally only one per cell, and it is non motile. It is called primary cilium and in vertebrates, it seems almost all cells have one. It transmits information from the outside of the cell to the inside that can lead to changes. This kind of cilia is crucial to brain function and embryonic development. Nonmotile cilia has a 9+0 pattern of microtubules.

Question 57

Basal body

Answer 57

The microtubule assembly of a cilium or flagellum is anchored in the cell by a basal body, which is structurally similar to a centriole, with a microtubule triplets in a 9+0 pattern. In fact, in many animals (including humans), the basal body of the fertilizing sperm’s flagellum enters the egg and becomes a centriole.

Question 58

Dyneins

Answer 58

Bending involves large motor proteins called dynein’s that are attached to along each outer microtubule doublet. A typical dynein protein has two “feet” that “walk” along the microtubule of the adjacent doublet, using ATP for energy. One foot maintains contact while the other releases and reattaches one step farther along the microtubules. The outer doublets and two central microtubules are held together by cross-linking proteins. If the doublets were not held in place, the walking action would make them slide past each other, but instead the movement of the dynein feet cause the microtubules and the organelle as a whole, to bend.

Question 59

Actin

Answer 59

Microfilaments are thin solid rods. They are also called actin filaments because they are built from the molecules of actin, a globular protein.

Question 60

Myosin

Answer 60

Microfilaments are well known for their role in cell motility. Thousands of actin filaments and thicker filaments of a motor protein called myosin interact to cause contraction of muscle cells. In the unicellular eukaryote Amoeba and some of our white blood cells, localized contractions brought about by actin and myosin are involved in movement of the cells. In plant cells, actin-myosin interaction contributes to cytoplasmic streaming, a circular flow of cytoplasm within cells. This movement, which is especially common in large plant cells, speeds the distribution of materials within the cell.

Question 61

Cell wall

Answer 61

Outer layer that maintains cell’s shape and protects cell from mechanical damage, made of cellulose, other polysaccharides and protein. The cell wall is an extracellular structure of plant cells. The wall protects the plant cell, maintains shape and prevents excessive uptake of water. The strong cell walls of specialized cells hold up the plant against gravity. This wall is much thicker than the plasma membrane, ranging from 0.1 (my)m to several micrometers. Composition varies from species to species and even from cell to cell in the same plant. Some basic things are the same. Microfibrils made of the polysaccharide cellulose are synthesized by an enzyme called cellulose synthase and secreted to the extracellular space, where they become imbedded in a matrix of other polysaccharides and proteins. This combination of materials, strong fibers in a ground substance (matrix), is the same basic design found in steel-reinforced concrete and in fiberglass.

Question 62

Primary cell wall

Answer 62

A young plant cell first secretes a relatively thin and flexible wall called the primary cell wall.

Question 63

Middle lamina

Answer 63

Between primary cell walls of adjacent cells is the middle lamina, a thin layer rich in sticky polysaccharides called pectin. It glues adjacent cells together. Pectin is also used as a thickening agent in jams and jellies.

Question 64

Secondary cell wall

Answer 64

When the cell matures and stops growing, it strengthens the its wall. Some plant cells do this by secreting hardening substances (like lignin) into the primary wall. 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 durable matrix that affords the cell protection and support. Wood for example, consists mainly of secondary walls.

Question 65

Extracellular matrix (ECM)

Answer 65

Although animal cells lack walls akin to those of plant cells, they do have an elaborate extracellular matrix. The main ingredients of the ECM are glycoproteins and other carbohydrate-containing molecules secreted by the cells. (Glycoproteins are proteins with covalently bonded carbohydrates.)

Question 66

Collagen

Answer 66

The most abundant glycoprotein in the ECM of most animal cells is collagen. Collagen accounts for about 40% of the total protein in the human body. The collagen fibers are embedded in a network woven of secreted proteoglycans.

Question 67

Proteoglycans

Answer 67

The collagen fibers are embedded in a network woven of secreted proteoglycans. A proteoglycan molecule consists of a small core protein with many carbohydrate chains covalently attached; 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 68

Fibronectin

Answer 68

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.

Question 69

Integrins

Answer 69

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 70

Plasmodesmata

Answer 70

Cytoplasmic channels through cell walls that connect the cytoplasm’s of adjacent cells. It might seem that the nonliving cell walls of plant cells from one another, but actually cell walls are perforated with plasmodesmata, which are membrane lined channels filled with cytosol. By joining adjacent cells, plasmodesmata unify most of a plant into one living continuum. The plasma membranes of adjacent cells line the channel of each plasmodesmata and thus are continuous. Water and solutes can pass freely from cell to cell, and some proteins and RNA molecules can do this as well. The macromolecules transported to neighboring cells appear to reach the plasmodesmata by moving along fibers of the cytoskeleton.

Question 71

Tight junctions

Answer 71

At tight junctions, 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. For example, tight junctions between skin cells make us watertight.

Question 72

Desmosomes

Answer 72

One type of anchoring junction, desmosomes 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 73

Gap junctions

Answer 73

Also called communicating junctions, provide cytoplasmic channels form one cell to 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 a necessary for communication between cells in many types of tissues, such as heart muscle, and in animal embryos.