Ch. 6 A Tour of the Cell Dynamic Study Module Flashcards

that mitochondria and chloroplasts have multiple copies of linear DNA molecules associated with their inner membranes

Ex.

Evidence that supports the prokaryotic origins of mitochondria and chloroplasts are all of the following except that mitochondria and chloroplasts have multiple copies of linear DNA molecules associated with their inner membranes.

The endosymbiont theory is consistent with many structural features of mitochondria and chloroplasts. First, rather than being bounded by a single membrane like organelles of the endomembrane system, mitochondria and typical chloroplasts have two membranes surrounding them. Chloroplasts also have an internal system of membranous sacs. There is evidence that the ancestral engulfed prokaryotes had two outer membranes, which became the double membranes of mitochondria and chloroplasts. Second, like prokaryotes, mitochondria and chloroplasts contain ribosomes as well as multiple circular DNA molecules associated with their inner membranes. The DNA in these organelles programs the synthesis of some organelle proteins on ribosomes that have been synthesized and assembled there as well. Third, also consistent with their probable evolutionary origins as cells, mitochondria and chloroplasts are autonomous (somewhat independent) organelles that grow and reproduce within the cell.

“That the ancestral prokaryote had two outer membranes, which became the double membranes of the mitochondria and chloroplasts,” “that mitochondria and chloroplasts have multiple copies of circular DNA molecules associated with their membranes,” “that mitochondria and chloroplasts can grow and reproduce within a cell,” and “that mitochondria and chloroplasts are somewhat independent within a cell” are incorrect because these are all evidence that support the prokaryotic origins of mitochondria and chloroplast. Since this is an except question, none of these answers is correct.

centrosomes; centrioles

Ex.

Animal cells have unique organelles called centrosomes that are composed of structures called centrioles.

In animal cells, microtubules grow out from a centrosome, a region that is often located near the nucleus. These microtubules function as compression-resisting girders of the cytoskeleton. Within the centrosome is a pair of centrioles, each composed of nine 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.

“Centrioles; centrosomes” is incorrect because centrioles are structures found in centrosomes.

“Centromeres; centrioles” is incorrect because a centromere is a region of a duplicated chromosome on each sister chromatid where it is most closely attached to the other chromatid by proteins that bind it to the centromeric DNA.

“Nucleosomes; centrioles” is incorrect because a nucleosome is the basic bead-like unit of DNA packing in eukaryotes.

“Centrosomes; nucleosomes” is incorrect because a nucleosome is the basic bead-like unit of DNA packing in eukaryotes.

mitochondria and chloroplasts

Ex.

The endosymbiont theory explains the origins of mitochondria and chloroplasts.

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, 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 of the origins of mitochondria and chloroplasts.

“Chloroplasts and the plant cell vacuole,” “the Golgi apparatus and mitochondria,” and “mitochondria and ribosomes” are incorrect because the endosymbiont theory explains the origins of mitochondria and chloroplasts.

“There is no correct answer because the endosymbiont theory is no longer supported” is incorrect because the theory is widely accepted as the origins of mitochondria and chloroplasts.

Subunits of ribosomes are assembled in the nucleolus and pass through the nuclear membrane via the nuclear pores.

Ex.

The functional connection between the nucleolus, nuclear pores, and the nuclear membrane is that subunits of ribosomes are assembled in the nucleolus and pass through the nuclear membrane via the nuclear pores.

The nuclear envelope (or nuclear membrane) 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. A prominent structure within the nondividing nucleus is the nucleolus (plural, nucleoli). 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 ribosomes. These subunits then exit the nucleus through the nuclear pores to the cytoplasm, where a large and a small subunit can assemble into a ribosome.

At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous. An 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.

plasmodesmata; gap junctions

Ex.

Cell junctions in plant cells are called plasmodesmata, and communicating junctions in animal cells are called gap junctions.

The nonliving cell walls of plants are perforated with plasmodesmata (singular, plasmodesma; from the Greek desma, bond), channels that connect cells. Cytosol passing through the plasmodesmata joins the internal chemical environments of adjacent cells. These connections unify most of the plant into one living continuum. The plasma membranes of adjacent cells line the channel of each plasmodesma and thus are continuous.

Water and small solutes can pass freely from cell to cell, and several experiments have shown that in some circumstances, certain 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. Animal cells have gap junctions (also called communicating junctions), which provide cytoplasmic channels from one cell to an adjacent cell and in this way are similar in function to the plasmodesmata in plant cells. 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 animal embryos.

“Tight junctions; gap junctions” is incorrect because tight junctions are not found in plants. Tight junctions form seals around cells that establish a barrier that prevents leakage of extracellular fluid across a layer of epithelial cells.

“Desmosomes; plasmodesmata” is incorrect because desmosomes are not found in plants. Desmosomes (also called anchoring junctions) function like rivets, fastening cells together into strong sheets. Intermediate filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm. Plasmodesmata are cell junctions found in plants.

“Plasmodesmata; tight junctions” is incorrect because tight junctions form seals around cells that establish a barrier that prevents leakage of extracellular fluid across a layer of epithelial cells.

“Gap junctions; plasmodesmata” is incorrect because gap junctions are communicating junctions in animals and plasmodesmata are cell junctions found in plants.

in the rough endoplasmic reticulum

Ex.

A protein that ultimately functions in the plasma membrane of a cell is most likely to have been synthesized in the rough endoplasmic reticulum.

In addition to making secretory proteins, rough ER is a membrane factory for the cell; it grows in place by adding membrane proteins and phospholipids to its own membrane. As polypeptides destined to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane itself and anchored there by their hydrophobic portions. The ER membrane expands and portions of it are transferred in the form of transport vesicles to other components of the endomembrane system.

nucleoid

Ex.

The region of a bacterial cell that contains the genetic material is called the nucleoid.

A major difference between prokaryotic and eukaryotic cells is 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. In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid.

“Nucleus” is incorrect because this is the eukaryotic organelle that contains the genetic material in the form of chromosomes, made up of chromatin.

“Nucleolus” is incorrect because this is a structure in the nucleus of a eukaryotic cell that consists of chromosomal regions containing ribosomal RNA (rRNA) genes along with ribosomal proteins imported from the cytoplasm for ribosomal subunit assembly.

“Nucleosome” is incorrect because this is the basic bead-like unit of DNA packing in eukaryotes.

“Capsule” is incorrect because in prokaryotes, this is a dense and well-defined layer of polysaccharide or protein that surrounds the cell and enables it to adhere to substrates or other cells.

The intestinal cells are bound together by tight junctions.

Ex.

Your intestine is lined with individual cells. No fluids leak between these cells from the gut into your body because the intestinal cells are bound together by tight junctions.

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 prevent leakage of extracellular fluid across a layer of epithelial cells. For example, tight junctions between skin cells make us watertight by preventing leakage between cells in our sweat glands.

Gap junctions (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.

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

have no membrane-bounded organelles in their cytoplasm

Ex.

Bacterial cells are prokaryotic. Unlike a typical eukaryotic cell they have no membrane-bounded organelles in their cytoplasm.

All cells share certain basic features. They are all bounded by a selective barrier, called the plasma membrane. Inside all cells is a semifluid, jellylike substance called cytosol, in which subcellular components are suspended. All cells contain chromosomes, which carry genes in the form of DNA. And all cells have ribosomes, tiny complexes that make proteins according to instructions from the genes. A major difference between prokaryotic and eukaryotic cells is 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. In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid. Within the cytoplasm of a eukaryotic cell, suspended in cytosol, are a variety of organelles of specialized form and function. These membrane-bounded structures are absent in prokaryotic cells.

Thus, the presence or absence of a true nucleus is just one aspect of the disparity in structural complexity between the two types of cells. Lacking a true nucleus and the other membrane-enclosed organelles of the eukaryotic cell, the prokaryotic cell is much simpler in structure.

of the geometric relationships between surface and volume

Ex.

Cells are small because of the geometric relationships between surface and volume.

Metabolic requirements also impose theoretical upper limits on the size that is practical for a single cell. At the boundary of every cell, the plasma membrane functions as a selective barrier that allows passage of enough oxygen, nutrients, and wastes to service the entire cell. For each square micrometer of membrane, only a limited amount of a particular substance can cross per second, so the ratio of surface area to volume is critical. As a cell (or any other object) increases in size, its surface area grows proportionately less than its volume. (Area is proportional to a linear dimension squared, whereas volume is proportional to a linear dimension cubed.) Thus, a smaller object has a greater ratio of surface area to volume than a larger object has.

“Small cells are easier to back together” is incorrect because cells are small because of the surface-to-volume ratio.

“Smaller objects have a smaller ratio of surface to volume” is incorrect because smaller objects have a greater surface-to-volume ratio.

“Area is proportional to a dimension cubed (X³), whereas volume is proportional to a dimension squared (X²)” is incorrect because area is proportional to a dimension squared (X²) while volume is proportional to a dimension cubed (X³).

with motor proteins

Ex.

Cell motility, which includes changes both in cell location and in the movement of cell parts, requires interactions of the cytoskeleton with motor proteins.

Some types of cell motility (movement) also involve the cytoskeleton. There are many such examples: Cytoskeletal elements and motor proteins work together with plasma membrane molecules to allow whole cells to move along fibers outside the cell. Inside the cell, vesicles and other organelles often use motor protein “feet” to “walk” to their destinations along a track provided by the cytoskeleton. For example, this is how vesicles containing neurotransmitter molecules migrate to the tips of axons, the long extensions of nerve cells that release these molecules as chemical signals to adjacent nerve cells. The cytoskeleton also manipulates the plasma membrane, bending it inward to form food vacuoles or other phagocytic vesicles.

“With ‘feet’ to ‘walk’” is incorrect because this is an expression that describes the function of motor proteins as they track along the cytoskeleton.

“With glycosomes” is incorrect because glycosomes are fat-storing tissues of plant seeds.

“Without ATP” is incorrect because cell motility requires a motor protein that is powered by ATP.

“With the endoplasmic reticulum” is incorrect because the endoplasmic reticulum is not associated with cell motility.

central vacuole

Ex.

The organelle that is a plant cell’s compartment for the storage of inorganic ions such as potassium and chloride is the central vacuole.

Vacuoles 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 solution inside a cytosol. 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 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.

“Food vacuole” is incorrect because food vacuoles are membranous sacs formed by phagocytosis of microorganisms or particles to be used as food for the cells.

“Contractile vacuole” is incorrect because this is a membranous sac that helps move excess water out of certain freshwater protists.

“Lysosome” is incorrect because this is a membrane-enclosed sac of hydrolytic enzymes found in the cytoplasm of animal cells and some protists.

“Plastid” is incorrect because this is one of a family of closely related organelles that includes chloroplasts, chromoplasts, and amyloplasts. They are found in cells of photosynthetic eukaryotes.

the plasma membrane

Ex.

A substance moving from outside the cell into the cytoplasm must pass through the plasma membrane.

All cells share certain basic features: They are all bounded by a selective barrier, called the plasma membrane, which is a component of the endomembrane system. 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. The plasma membrane also preserves the cell’s environment, which may include other cellular structures, such as ribosomes, the nucleus, and microtubules.

Ribosomes

Ex.

Ribosomes are present in a prokaryotic cell.

Comparing prokaryotic and eukaryotic cells, a major difference between prokaryotic and eukaryotic cells is 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. In a prokaryotic cell, the DNA is concentrated in a region called the nucleoid, which is not membrane-enclosed. Within the cytoplasm of a eukaryotic cell, suspended in cytosol, are a variety of organelles of specialized form and function. These membrane-bounded structures are absent in prokaryotic cells. Thus, the presence or absence of a true nucleus is just one aspect of the disparity in structural complexity between the two types of cells.

Lacking a true nucleus and the other membrane-enclosed organelles of the eukaryotic cell, the prokaryotic cell is much simpler in structure. The DNA in these organelles programs the synthesis of some of their own proteins, which are made on the ribosomes inside the organelles.

All cells share certain basic features: They are all bounded by a selective barrier, called the plasma membrane. Inside all cells is a semifluid, jellylike substance called cytosol, in which subcellular components are suspended. All cells contain chromosomes, which carry genes in the form of DNA. In addition, all cells have ribosomes, which are tiny complexes that make proteins according to instructions from the genes.

Ribosomes, rough endoplasmic reticulum, and smooth endoplasmic reticulum

Ex.

Ribosomes, rough endoplasmic reticulum, and smooth endoplasmic reticulum are primarily involved in synthesizing molecules needed by the cell.

Ribosomes, which are complexes made of ribosomal RNA and protein, are the cellular components that carry out protein synthesis. The endoplasmic reticulum (ER) consists of a network of membranous tubules and sacs called cisternae. The smooth ER functions in diverse metabolic processes, which vary with cell type. These processes include synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions. In addition to making secretory proteins, rough ER is a membrane factory for the cell; it grows in place by adding membrane proteins and phospholipids to its own membrane.

As polypeptides destined to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane itself and anchored there by their hydrophobic portions. Like the smooth ER, the rough ER also makes membrane phospholipids; enzymes built into the ER membrane assemble phospholipids from precursors in the cytosol. The ER membrane expands and portions of it are transferred in the form of transport vesicles to other components of the endomembrane system.

A lysosome is a membranous sac of hydrolytic enzymes that an animal cell uses to digest (hydrolyze) macromolecules.

Vacuoles are large vesicles derived from the endoplasmic reticulum and Golgi apparatus. Thus, vacuoles are an integral part of a cell’s endomembrane system but are not directly involved with the synthesis of molecules.

nuclear envelope, the Golgi apparatus, lysosomes, and vesicles

Ex.

The endoplasmic reticulum is part of the endomembrane system, which also includes the nuclear envelope, the Golgi apparatus, lysosomes, and vesicles.

Any of the different membranes of the eukaryotic cell are part of the endomembrane system, which includes 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 variety of tasks in the cell, including synthesis of proteins, transport of proteins into membranes and organelles or out of the cell, metabolism and movement of lipids, and detoxification of poisons. The membranes of this system are related either through direct physical continuity or through the transfer of membrane segments as tiny vesicles or sacs made of membranes.

“Mitochondria, the Golgi apparatus, lysosomes, and vesicles” and the “nuclear envelope, the Golgi apparatus, and mitochondria” are incorrect because mitochondria are not part of the endomembrane system.

“Nuclear envelope, the Golgi apparatus, mitochondria, and chloroplasts” is incorrect because mitochondria and chloroplast are not part of the endomembrane system.

“Nuclear envelope, the Golgi apparatus, and ribosomes” is incorrect because although ribosomes can be found on endoplasmic reticulum forming the studded RER, they are not membrane-bound organelles.

Peroxisomes

Ex.

Peroxisomes are membrane-bound metabolic compartments that specialize in the production of hydrogen peroxide (H₂O₂) and its conversion to water.

The peroxisome is a specialized metabolic compartment bounded by a single membrane. 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).

These reactions have many different functions. Some peroxisomes use oxygen to break fatty acids down into smaller molecules that can be transported to mitochondria and used as fuel for cellular respiration. Peroxisomes in the liver detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to oxygen. The H₂O₂ formed by peroxisomes is itself toxic, but the organelle also contains an enzyme that converts H₂O₂ to water. This is an excellent example of how a cell’s compartmental structure is crucial to its functions: The enzymes that produce H₂O₂ and those that dispose of this toxic compound are sequestered away from other cellular components, which could be damaged.

“Lysosomes” is incorrect because they are membrane-enclosed sacs of hydrolytic enzymes found in the cytoplasm of animal cells and some protists.

“Ribosomes” is incorrect because ribosomes are organelles that are involved in protein synthesis.

“Centrosomes” is incorrect because centrosomes are animal organelles composed of a cylinder of microtubule triplets in a “9 + 0” arrangement.

“Nucleosomes” is incorrect because this is the basic bead-like unit of DNA packing in eukaryotes.

muscle contraction, amoeboid movement, and cytoplasmic streaming in plants

Ex.

Microfilaments function in cell motility including muscle contraction, amoeboid movement, and cytoplasmic streaming in plants.

“Muscle contraction, amoeboid movement, and flagella motion” is incorrect because the flagellum contains a 9 + 2 pattern of microtubules and not microfilaments.

“Muscle contraction, amoeboid movement, and cilia motion” is incorrect because cilia contain a 9 + 2 pattern of microtubules and not microfilaments.

“Muscle contraction, amoeboid movement, and centriole function” is incorrect because the centriole contains a 9 + 0 pattern of microtubules and not microfilaments.

“Flagella motion, cilia motion, and centriole function” is incorrect because all of these structures are composed of microtubules.

Chromosomes are only visible as a cell is about to divide.

Ex.

Chromosomes are only visible as a cell is about to divide is false.

In all cells, 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. 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 discrete chromosomes are present. As a cell prepares to divide, however, the chromosomes coil (condense) further, becoming thick enough to be distinguished as separate structures.

in the rough endoplasmic reticulum and in the Golgi apparatus

Ex.

The walls of plant cells are largely composed of polysaccharides and proteins that are synthesized in the rough ER and in the Golgi apparatus.

As a polypeptide chain grows from a bound ribosome, the chain is threaded into the ER lumen through a pore formed by a protein complex in the ER membrane. As the new polypeptide enters the ER lumen, it folds into its native shape. Most secretory proteins are glycoproteins, proteins that have carbohydrates covalently bonded to them. The carbohydrates are attached to the proteins in the ER by enzymes built into the ER membrane. Products of the endoplasmic reticulum are usually modified during their transit from the cis region to the trans region of the Golgi apparatus. For example, glycoproteins formed in the ER have their carbohydrates modified, first in the ER itself, then as they pass through the Golgi. The Golgi removes some sugar monomers and substitutes others, producing a large variety of carbohydrates. Membrane phospholipids may also be altered in the Golgi.

human skin cell

Ex.

Observing a fluorescent micrograph cell with intermediate filaments would help you identify the cell as a human skin cell.

Intermediate filaments are named for their diameter, which is larger than the diameter of microfilaments but smaller than that of microtubules. Unlike microtubules and microfilaments, which are found in all eukaryotic cells, intermediate filaments are found only in the cells of some animals, including vertebrates. Specialized for bearing tension (like microfilaments), intermediate filaments 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 keratins. 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. Also, 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. Chemical treatments that remove microfilaments and microtubules from the cytoplasm of living cells leave a web of intermediate filaments that retains its original shape. Such procedures suggest that intermediate filaments are especially sturdy and that they play an important role in reinforcing the shape of a cell and fixing the position of certain organelles.

“Prokaryotic cell” is incorrect because intermediate filaments are found only in the cells of some animals, including vertebrates.

“Eukaryotic cell” is incorrect because unlike microtubules and microfilaments, which are found in all eukaryotic cells, intermediate filaments are found only in the cells of some animals, including vertebrates.

“Plant cell” and “fungal cell” are incorrect because intermediate filaments are found only in the cells of some animals, including vertebrates.

to synthesize proteins that are secreted as glycoproteins

Ex.

The function of the rough endoplasmic reticulum (RER) is to synthesize proteins that are secreted as glycoproteins.

The endoplasmic reticulum (ER) is such an extensive network of membranes that it accounts for more than half of the total membranes in many eukaryotic cells. (The word endoplasmic means “within the cytoplasm,” and reticulum is Latin for “little net.”) The 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. Many cells secrete proteins that are produced by ribosomes attached to rough ER. For example, certain pancreatic cells synthesize the protein insulin in the ER and secrete this hormone into the bloodstream. As a polypeptide chain grows from a bound ribosome, the chain is threaded into the ER lumen through a pore formed by a protein complex in the ER membrane. The new polypeptide folds into its functional shape as it enters the ER lumen. 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.

“To synthesize lipids,” “to detox drugs and alcohol,” “to synthesize sex hormones,” and “to store calcium ions” are incorrect because these are all functions of the smooth endoplasmic reticulum (SER).

a middle lamella

Ex.

The extracellular matrix of the animal cell has all of the following molecular components except a middle lamella.

In plants, the middle lamella is a thin layer of adhesive extracellular material, primarily pectins found between the primary walls of adjacent young cells. Animal cells lack walls akin to those of plant cells, but animal cells 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. In fact, collagen accounts for about 40% of the total protein in the human body. The collagen fibers are embedded in a network woven out of proteoglycans secreted by cells. A proteoglycan molecule consists of a small core protein with many carbohydrate chains covalently attached, so 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. 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, which 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.

“Collagen,” “proteoglycans,” “fibronectin," and integrins” are all components of the extracellular matrix of animal cells. Since this is an except question, they are incorrect answers.

Eukaryotic cells are compartmentalized, which allows for specialization.

Ex.

In terms of cellular function, the most important difference between prokaryotic and eukaryotic cells is that eukaryotic cells are compartmentalized, which allows for specialization.

A major difference between prokaryotic and eukaryotic cells is 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. In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid. In addition, within the cytoplasm of a eukaryotic cell, suspended in cytosol, are a variety of organelles of specialized form and function. These membrane-bounded structures are absent in prokaryotic cells. Thus, the presence or absence of a true nucleus is just one aspect of the disparity in structural complexity between the two types of cells.

Motor proteins

Ex.

Cilia and flagella move due to the interaction of the cytoskeleton with motor proteins.

In flagella and motile cilia, flexible cross-linking proteins, evenly spaced along the length of the cilium or flagellum, connect the outer doublets to each other and to the two central microtubules. Each outer doublet also has pairs of protruding proteins spaced along its length and reaching toward the neighboring doublet; these are large motor proteins called dyneins, each composed of several polypeptides. Dyneins are responsible for the bending movements of the organelle. A dynein molecule performs a complex cycle of movements caused by changes in the shape of the protein, with ATP providing the energy for these changes.

plasmodesmata

Ex.

Dye injected into a plant cell might be able to enter an adjacent cell through plasmodesmata.

Plant cell walls are usually perforated by channels between adjacent cells called plasmodesmata. The cytoplasm of one plant cell is continuous with the cytoplasm of its neighbors via plasmodesmata, which are cytoplasmic channels through the cell walls.

Cilia

Ex.

Basal bodies are most closely associated with cilia.

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

is the cytoskeleton

Ex.

The network of fibers that organizes structures and activities in a cell is the cytoskeleton.

The cytoskeleton is a network of fibers extending throughout the cytoplasm. The most obvious function of the cytoskeleton is to give mechanical support to the cell and maintain its shape. This is especially important for animal cells, which lack walls. The remarkable strength and resilience of the cytoskeleton as a whole are based on its architecture. Like a dome tent, the cytoskeleton is stabilized by a balance among 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. The three main types of fibers that make up the cytoskeleton are: microtubules, which are the thickest of the three types; microfilaments (also called actin filaments), which are the thinnest; and intermediate filaments, which are fibers with diameters in a middle range.

“Are microtubules,” “are microfilaments,” and “are intermediate filaments” are incorrect because these are the three main types of fibers that make up the cytoskeleton.

“Are centrioles” is incorrect because centrioles are structures in the centrosome of animal cells and are composed of a cylinder of microtubule triplets in a “9 + 0” arrangement.

secrete a lot of protein

Ex.

You would expect a cell with an extensive Golgi apparatus to secrete a lot of protein.

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.

Components of the cytoskeleton often mediate the movement of organelles within the cytoplasm.

Ex.

Components of the cytoskeleton often mediate the movement of organelles within the cytoplasm is true.

The cytoskeleton, which plays a major role in organizing the structures and activities of the cell, is composed of microtubules, microfilaments, and intermediate filaments. Several types of cell motility (movement) involve the cytoskeleton. The term cell motility encompasses both changes in cell location and more limited movements of parts of the cell.

Intermediate filaments are specialized for bearing tension (like microfilaments) and are a diverse class of cytoskeletal elements. Microtubules shape the cell, guide organelle movement, and separate chromosomes in dividing cells. Cilia and flagella are motile appendages containing microtubules. Microfilaments are thin rods functioning in muscle contraction, amoeboid movement, cytoplasmic streaming, and microvillus support.