exam4

Which of the following channels would not be expected to generate a change in voltage by movement of its substrate across the membrane where it is found? (a) an aquaporin (b) a sodium channel (c) a calcium channel (d) a proton channel

(a) Aquaporin channels are found in the plasma membrane of some cells, where they facilitate diffusion of water across the membrane. Because water is an uncharged molecule, its movement would not be expected to alter the voltage across the membrane.

Proteins that are fully translated in the cytosol and lack a sorting signal will end up in ____. (a) the cytosol. (b) the mitochondria. (c) the interior of the nucleus. (d) the nuclear membrane.

(a) Proteins produced in the cytosol that lack sorting signals remain in the cytosol. Proteins produced in the cytosol and destined for the mitochondria [choice (b)] or the interior of the nucleus [choice (c)] will have a sorting signal to direct the protein to its proper location. Proteins destined for the nuclear membrane [choice (d)] are not translated in the cytosol.

A group of membrane proteins can be extracted from membranes only by using detergents. All the proteins in this group have a similar amino acid sequence at their Cterminus: -KKKKKXXC (where K stands for lysine, X stands for any amino acid, and C stands for cysteine). This sequence is essential for their attachment to the membrane. What is the most likely way in which the C-terminal sequence attaches these proteins to the membrane? (a) The cysteine residue is covalently attached to a membrane lipid. (b) The peptide spans the membrane as an α helix. (c) The peptide spans the membrane as part of a β sheet. (d) The positively charged lysine residues interact with an acidic integral membrane protein.

(a) The cysteine residue is covalently attached to a membrane lipid.

Membrane synthesis in the cell requires the regulation of growth for both halves of the bilayer and the selective retention of certain types of lipids on one side or the other. Which group of enzymes accomplishes both of these tasks? (a) flippases (b) phospholipases (c) convertases (d) glycosylases

(a) flippases

Membrane lipids are capable of many different types of movement. Which of these does not occur spontaneously in biological membranes? (a) switching between lipid layers (b) lateral movement (c) rotation (d) flexing of hydrocarbon chains

(a) switching between lipid layers

Which of the following phospholipid precursors is the most hydrophobic? (a) triacylglycerol (b) diacylglycerol (c) phosphate (d) glycerol

(a) triacylglycerol

Most animal fats form a solid at room temperature, while plant fats remain liquid at room temperature. Which of the following is a feature of lipids in plant membranes that best explains this difference? (a) unsaturated hydrocarbons (b) longer hydrocarbon tails (c) higher levels of sterols (d) larger head groups

(a) unsaturated hydrocarbons

Membrane proteins, like membrane lipids, can move laterally by exchanging positions with other membrane components. Which type of membrane proteins is expected to be the least mobile, based on their function? (a) channels (b) anchors (c) receptors (d) enzymes

(b) anchors

The nucleus, an organelle found in eukaryotic cells, confines the __________, keeping them separated from other components of the cell. (a) lysosomes (b) chromosomes (c) peroxisomes (d) ribosomes

(b) chromosomes

Which of the following choices best describes the role of the lysosome? (a) transport of material to the Golgi (b) clean-up, recycling, and disposal of macromolecules (c) sorting of transport vesicles (d) the storage of excess macromolecules

(b) clean-up, recycling, and disposal of macromolecules

Where does most new membrane synthesis take place in a eukaryotic cell? (a) in the Golgi apparatus (b) in the endoplasmic reticulum (c) in the plasma membrane (d) in the mitochondria (e) on ribosomes

(b) in the endoplasmic reticulum

Which of the following organelles is not part of the endomembrane system? (a) Golgi apparatus (b) the nucleus (c) mitochondria (d) lysosomes

(c) Mitochondria are not part of the endomembrane system, which is thought to have arisen initially through invagination of the plasma membrane. Instead, mitochondria (and chloroplasts) are thought to have evolved from a bacterium that was engulfed by a primitive eukaryotic cell

__________ are fairly small organelles that provide a safe place within the cell to carry out certain biochemical reactions that generate harmful, highly reactive oxygen species. These chemicals are both generated and broken down in the same location. (a) Nucleosomes (b) Lysosomes (c) Peroxisomes (d) Endosomes

(c) Peroxisomes

Most proteins destined to enter the endoplasmic reticulum _________. (a) are transported across the membrane after their synthesis is complete. (b) are synthesized on free ribosomes in the cytosol. (c) begin to cross the membrane while still being synthesized. (d) remain within the endoplasmic reticulum.

(c) Proteins destined to enter the endoplasmic reticulum have an N-terminal signal sequence that leads to the docking of the ribosome synthesizing the protein onto the ER and the entry of the protein across the ER membrane as the polypeptide chain is being synthesized.

Signal sequences that direct proteins to the correct compartment are _________. (a) added to proteins through post-translational modification. (b) added to a protein by a protein translocator. (c) encoded in the amino acid sequence and sufficient for targeting a protein to its correct destination. (d) always removed once a protein is at the correct destination.

(c) Signal sequences are found within the amino acid sequence of proteins. They are sometimes removed when the protein is at the correct destination [choice (d)], but not all are removed. For example, nuclear import signals are not removed once a protein is inside the nucleus. A protein translocator resides in the membrane and helps transport soluble proteins across the membrane [choice (b)], but does not add signal sequences to proteins.

Vesicles from the ER enter the Golgi at the ______. (a) medial cisternae. (b) trans Golgi network. (c) cis Golgi network. (d) trans cisternae.

(c) cis Golgi network.

New membrane phospholipids are synthesized by enzymes bound to the _____________ side of the _________________ membrane. (a) cytosolic, mitochondrial (b) luminal, Golgi (c) cytosolic, endoplasmic reticulum (d) extracellular, plasma

(c) cytosolic, endoplasmic reticulum

Mitochondria contain their own genome, are able to duplicate, and actually divide on a different time line from the rest of the cell. Nevertheless, mitochondria cannot function for long when isolated from the cell because they are __________________. (a) viruses. (b) parasites. (c) endosymbionts. (d) anaerobes.

(c) endosymbionts.

Which of the following choices reflects the appropriate order of locations through which a protein destined for the plasma membrane travels? (a) lysosome > endosome > plasma membrane (b) ER > lysosome > plasma membrane (c) Golgi > lysosome > plasma membrane (d) ER > Golgi > plasma membrane

(d) ER > Golgi > plasma membrane

Below is a list of molecules with different chemical characteristics. Knowing that all molecules will eventually diffuse across a phospholipid bilayer, select the option below that most accurately predicts the relative rates of diffusion of these molecules (fastest to slowest). alanine estrogen propanol sodium (a) alanine > propanol > sodium > estrogen (b) sodium > propanol > alanine > estrogen (c) estrogen > propanol > sodium > alanine (d) estrogen > propanol > alanine > sodium

(d) Estrogen is a steroid hormone and will diffuse the fastest across the membrane. Propanol is a small, uncharged molecule with a polar group. Alanine is an amino acid, and although it has a small, nonpolar side group, amino acids are charged molecules. Sodium is an ion and will move the slowest across the bilayer.

Proteins that are fully translated in the cytosol do not end up in _______. (a) the cytosol. (b) the mitochondria. (c) the interior of the nucleus. (d) transport vesicles.

(d) Proteins destined for transport vesicles will be translated on ribosomes associated with the endoplasmic reticulum.

Where are proteins in the chloroplast synthesized? (a) in the cytosol (b) in the chloroplast (c) on the endoplasmic reticulum (d) in both the cytosol and the chloroplast

(d) Proteins in the chloroplast are synthesized in the cytosol and in the chloroplast. The chloroplast proteins that are encoded by the nuclear DNA are synthesized in the cytosol, and the sorting signals on the protein direct them to the chloroplast. The chloroplast proteins encoded by the chloroplast DNA are synthesized on ribosomes inside the chloroplast.

Diversity among the oligosaccharide chains found in the carbohydrate coating of the cell surface can be achieved in which of the following ways? (a) varying the types of sugar monomers used (b) varying the types of linkages between sugars (c) varying the number of branches in the chain (d) all of the above

(d) all of the above

Chloroplasts are found only in eukaryotic cells that carry out photosynthesis: plants and algae. Plants and algae appear green as a result of the presence of chlorophyll. Where is chlorophyll located in the chloroplast? (a) in the first, outer membrane (b) in the space between the first and second membranes (c) in the second, inner membrane (d) in the third, innermost membrane

(d) in the third, innermost membrane

Cholesterol serves several essential functions in mammalian cells. Which of the following is not influenced by cholesterol? (a) membrane permeability (b) membrane fluidity (c) membrane rigidity (d) membrane thickness

(d) membrane thickness

The __________ __________ is made up of two concentric membranes and is continuous with the membrane of the endoplasmic reticulum. (a) plasma membrane (b) Golgi network (c) mitochondrial membrane (d) nuclear envelope

(d) nuclear envelope

Which of the following statements about membrane-enclosed organelles is true? (a) In a typical cell, the area of the endoplasmic reticulum membrane far exceeds the area of plasma membrane. (b) The nucleus is the only organelle that is surrounded by a double membrane. (c) Other than the nucleus, most organelles are small and thus, in a typical cell, only about 10% of a cell's volume is occupied by membrane-enclosed organelles; the other 90% of the cell volume is the cytosol. (d) The nucleus is the only organelle that contains DNA.

Choice (a) is true; the area of the endoplasmic reticulum membrane is 20-30 times that of the plasma membrane in a typical cell. Chloroplasts and mitochondria are also surrounded by a double membrane [choice (b)]. The cytosol is about half the volume of a typical eukaryotic cell, with membrane-enclosed organelles making up the other half of the volume [choice (c)]. Chloroplasts and mitochondria also carry their own genome, whereas the nucleus carries the genome of the organism [choice (d)].

Which of the following statements is true? (a) Lysosomes are believed to have originated from the engulfment of bacteria specialized for digestion. (b) The nuclear membrane is thought to have arisen from the plasma membrane invaginating around the DNA. (c) Because bacteria do not have mitochondria, they cannot produce ATP in a membrane-dependent fashion. (d) Chloroplasts and mitochondria share their DNA.

Choice (b) is correct. Lysosomes are part of the endomembrane system and are not thought to have come from the engulfment of an ancient prokaryotic cell [choice (a)]. Bacteria use their plasma membrane for ATP production [choice (c)]. Chloroplasts and mitochondria have their own DNA and do not share [choice (d)].

The amino acid sequences below represent the sequences of transmembrane helices. The characteristics of α helices that form a channel are different from those that form a single transmembrane domain. Select the helix that forms a single transmembrane domain. (a) VGHSLSIFTLVISLGIFVFF (b) IMIVLVMLLNIGLAILFVHF (c) ILHFFHQYMMACNYFWMLCE (d) VTLHKNMFLTYILNSMIIII

Choice (b) is correct. The peptide sequences for options (a), (c), and (d) are amphipathic—that is, they contain a hydrophilic amino acid every 3-4 amino acids in the sequence. This indicates they may be part of a protein that makes a channel in the membrane, and they require hydrophobic amino acids on the surface that interacts with the lipid bilayer and a hydrophilic surface on the inside of the channel. Option (b) is the only sequence that is completely hydrophobic, indicating that it forms a helix in which all the side chains interact with the lipid bilayer.

Which of the following statements about the endoplasmic reticulum (ER) is false? (a) The ER is the major site for new membrane synthesis in the cell. (b) Proteins to be delivered to the ER lumen are synthesized on smooth ER. (c) Steroid hormones are synthesized on the smooth ER. (d) The ER membrane is contiguous with the outer nuclear membrane.

Choice (b) is false. Proteins to be delivered to the ER lumen are synthesized on rough ER; these areas appear "rough" because ribosomes are attached to the cytosolic surface of these ER regions.

A protein can be embedded on the cytosolic side of the membrane bilayer by employing a hydrophobic α helix.

False. An embedded protein employs an amphipathic helix. The hydrophobic side interacts with the fatty acid tails of the membrane lipids, and the hydrophilic portion interacts with the aqueous components of the cytosol.

Lipid-linked proteins are classified as peripheral membrane proteins because the polypeptide chain does not pass through the bilayer.

False. Lipid-linked proteins are classified as integral membrane proteins because although they are not transmembrane proteins, they are covalently bound to membrane lipids and cannot be dissociated without disrupting the membrane's integrity.

Lysosomes are small organelles where fatty acid synthesis occurs.

False. Lysosomes house enzymes that break down nutrients for use by the cell and help recycle materials that cannot be used, which will later be excreted from the cell.

The first eukaryotic cells on Earth must have been aerobic; otherwise, they would not have been able to survive when the planet's atmosphere became oxygen-rich.

False. The first eukaryotic cells likely contained a nucleus but no mitochondria. These ancestral eukaryotes subsequently adapted to survive in a world filled with oxygen by engulfing primitive aerobic prokaryotic cells.

Primitive plant, animal, and fungal cells probably acquired mitochondria after they diverged from a common ancestor.

False. The mitochondria in modern plant, animal, and fungal cells are very similar, implying that these lines diverged after the mitochondrion was acquired by the ancestral eukaryote.

The differences in permeability between artificial lipid bilayers and cell membranes arise from variations in phospholipid content.

False. The primary difference between cell membranes and artificial membranes is that cell membranes have proteins responsible for creating a selective permeability, which varies with the location and function of the membrane.

Transporters are similar to channels, except that they are larger, allowing folded proteins as well as smaller organic molecules to pass through them.

False. Transporters work by changing conformation after specific binding of the solute to be transported. Channels exclude molecules on the basis of size and charge, but do not depend on specific recognition of the molecules moving through.

On the other hand, membrane-associated proteins do not span the bilayer and instead associate with the membrane through an α helix that is __________________.

amphipathic

Which of the following organelles has both an outer and an inner membrane? (a) endoplasmic reticulum (b) mitochondrion (c) lysosome (d) peroxisome

b) mitochondrion

which of the following membrane lipids does not contain a fatty acid tail phosphatidylcholine a glycolipid phosphatiserine cholesterol

cholesterol

Other proteins are __________________ attached to lipid molecules that are inserted in the membrane. __________________ membrane proteins are linked to the membrane through noncovalent interactions with other membrane-bound proteins.

covalent, peripheral

the highly ordered structure of the bilayer makes its generation and maintenance energetically unfavorable

false membrane is energetically favorable

phosphatidylserine is the most abundant phosolipid found in cell membranes

false the most abundant phospholipid depends on the cell

Glycolipids are found on the surface of healthy cells, and contribute to the cell's defense against chemical damage and infectious agents. A. In which organelle are sugar groups added to membrane lipids?

golgi aparatus

which of the following phenomenon will be observed if a cells membrane is pierced membrane reseals membrane collapses a tear is formed membrane expands

membrane reseals

The specialized functions of different membranes are largely determined by the __________________ they contain. Membrane lipids are __________________ molecules, composed of a hydrophilic portion and a hydrophobic portion. All cell membranes have the same __________________ structure, with the __________________ of the phospholipids facing into the interior of the membrane and the __________________ on the outside. The most common lipids in most cell membranes are the __________________. The head group of a glycolipid is composed of __________________.

proteins, amphipathic, bilayer, fatty acid tails, hydrophilic head groups phospholipids, sugar

The plasma membrane sewrves many functions, many of which depend on the presence of specialized membrane proteins. which of the following roles would still occur if the bilayer were lacking these proteins intercellular communication selestive permeability cellular movement import/export of molecules

selective permeability

although cholesterol is hydrophobic it has a hydrophilic head group

true

glycolipids lack the glycerol component found in phospholipids

true

The Golgi apparatus is made up of a series of membrane-enclosed compartments through which materials destined for secretion must pass.

true

while many prokaryotic cells have single membrane bilayer, all eukaryotic complex system of internal membrane bound compartments. how might it be advantagous for the cell to have these additional compartments

Compartmentalization using intracellular membranes allows eukaryotic cells to separate a variety of cell processes. Although this requires a higher degree of coordination, the cell also gains a more stringent degree of control over these processes (examples include: the separation of transcription and translation; the separation of enzymes involved in protein modifications for secreted versus cytosolic substrates; the separation of proteolytic events in the lysosomes versus the cytosol; the separation of anaerobic metabolism in the cytosol and aerobic metabolism in the mitochondria).

Although membrane proteins contribute roughly 50% of the total mass of the membrane, there are about 50 times more lipid molecules than there are protein molecules in cellular membranes. Explain this apparent discrepancy.

Membrane proteins are much larger molecules than the membrane lipids. Thus, fewer are required to represent the same total mass contributed by the lipid components of the membrane. By this estimation, the molecular weight of the average membrane protein is 50 times that of the average membrane lipid.

Glycolipids are found on the surface of healthy cells, and contribute to the cell's defense against chemical damage and infectious agents. . By what mechanism are glycolipids transported to the plasma membrane and presented to the extracellular environment?

Membranes that contain newly synthesized glycolipids bud from the Golgi apparatus to form vesicles. These vesicles then fuse with the plasma membrane. The glycolipids that were facing the lumen of the Golgi will now face the extracellular environment/

A protein that relies on protein-protein interactions to stabilize its membrane association is classified as a peripheral membrane protein because it can be dissociated without the use of detergents.

true

Cells expend energy in the form of ATP hydrolysis so as to maintain ion concentrations that differ from those found outside the cell.

true

. CO2 and O2 are water-soluble molecules that diffuse freely across cell membranes.

true

which type of lipids are the most abundant in the plasma membrane

phospholipids

a bacterium is suddenly expelled from a warm human intestine into the cold world outside. which of the following adjustments might the bacterium make to maintain the same level of membrane fluidity

produce lipids with hydro carbon tails that are shorter and have more double bonds

Membrane components in the cell are made in the endoplasmic reticulum.

true

Membrane proteins that extend through the lipid bilayer are called __________________ proteins and have __________________ regions that are exposed to the interior of the bilayer.

transmembrane, hydrophobic