Cell Biology - GRE Subject Test: Biochemistry, Cell, and Molecular Biology

The correct answer is ADP ribosylation factors (ARFs). ARFs are active when bound by GTP, and inactive when bound by GDP. The active GTP-bound form of ARF binds the vesicle coat protein 1 (COPI), to faciliate vesicle budding from the cis-Golgi face.

Eukaryotic flagella are primarily composed of microtubules and the motor protein dynein. Hydrolysis of ATP by dynein produces a sliding movement of the microtubule filaments that produces movement.

Kinesin is a motor protein that associates with microtubules, but it is not present in flagella. Myosin is a motor protein that associates with actin microfilaments.

Microfilaments are the smallest of the three cytoskeletal elements and are made up of actin filaments (myosin is not found in microfilaments). The 9+2 arrangement is found in microtubules, the biggest cytoskeletal elements. The third cytoskeletal element is called the intermediate filament and is made up of related proteins unique to each intermediate filament.

Acid hydrolases are proteins that are specifically designed to function at acidic pH, particularly at levels that would typically denature other proteins. They are commonly found in lysosomes where they aid in the digestion of various cellular wastes and materials.

Though acid hydrolases use water to break apart molecules, acids are not used as reactants, products, or catalysts in these reactions.

An acidic environment is not suitable for the cell as a whole, so the low pH is sequestered in a specific organelle: the lysosome. This low pH in the lysosome activates the hydrolytic enzymes in the lysosome, and allows them to degrade macromolecules that enter the organelle.

Mannose-6-phosphate is a post-translational modification found on proteins important to the functionality of the lysosome (such as acid hydrolases). Ubiquination is a signal for proteins to be brought to the proteosome and degraded. Myristoylation involves the addition of a fatty acid chain, and is often seen in proteins targeted to the plasma membrane. Acetylation is a common modification found on histones that can help make genes transcriptionally active.

An isomerase is an enzyme that catalyzes the rearrangement of bonds in a single molecule. For example glucose-6-phosphate isomerase catalyzes the conversion of glucose-6-phosphate into fructose-6-phosphate during glycolysis.

A hydrolase catalyzes a hydrolytic cleavage reaction, a kinase catalyzes the addition of a phosphate group, and a polymerase catalyzes polymerization reactions.

Clathrin coats are often seen trafficking vesicles from the Golgi apparatus to the plasma membrane. Clathrin protein is used to facilitate membrane invagination and vesicle formation, as well as direct vesicle release.

COPI coats are seen in vesicles headed from the Golgi apparatus back to the endoplasmic reticulum. COPII coats are seen in vesicles headed towards the Golgi apparatus from the endoplasmic reticulum.

Multipotent cell types can give rise to a small number of cell types but have a restricted fate. This is in contrast to totipotent (zygote) or pluripotent cells (germ layers, iPS (induced pluripotent cells)) which can give rise to many different cell types, some that may make up very different parts of the body.

The most abundant molecule in bacterial cell walls is peptidoglycan. Recall that peptidoglycan contains repeating NAG-NAM units, which are made from glucose. In addition, peptidoglycan also has peptides attached to these oligosaccharides. These units form bonds with each other to create a strong and a robust cell wall. Peptidoglycan is synthesized in cytoplasm and on cell membrane before finally being transported to the cell wall.

Recall that phosphate head groups are found on phospholipids, which make up cell membranes (not cell walls).

Gram stain is a laboratory technique used to distinguish bacteria. Gram positive and Gram negative bacteria are distinguished based on their cell walls. Gram positive bacteria have a single thick cell wall (outside its cell membrane). Upon Gram staining, these bacteria will hold the Gram stain because of their thick cell wall. The Gram negative bacteria also have only one cell wall; however, it is a lot thinner and is sandwiched between two cell membranes. The thin nature of the cell wall makes it easier for the Gram stain to leak out of the bacterial cell.

Cell walls can vary in composition based on what organism is being discussed. In the case of plant cells, cellulose is the primary component. Bacteria use peptidolgycan in their cell walls, and fungi use chitin.

The cell wall is composed of cellulose and encloses a plant cell in a rigid framework. Plant cells have both cell membranes and cell walls. Plant cell walls cannot be digested by humans due to the beta glycosidic bond in its structure. The cell membrane forms the outer boundary of a cell and isolates cell contents from the environment. The cell membrane regulates movement of materials into and out of the cell. The cell membrane is also referred to as the plasma membrane. The nuclear envelope regulates movement into and out of the nucleus. The cytoskeleton gives shape and support to cells and is typically involved in cell movement.

Cell walls are robust structures found in most bacteria, fungi, and plants. Gram stain depends on the properties of the cell wall. Some bacteria don’t stain well with Gram stain (such as Mycobacterium that causes tuberculosis) because of unique cell walls (not lack thereof). Fungal and plant cell walls are also made up of polysaccharides. Fungal cell walls contain a special molecule called chitin whereas plant cell walls contain cellulose. Recall that plants obtain energy from sunlight via photosynthesis; therefore, photosynthetic organisms do contain cell walls.

One of the biggest function of a cell wall is to prevent cellular swelling. Recall that a cell placed in a hypoosmolar solution will swell due to the influx of water; however, this cellular swelling will not occur in a cell containing cell wall because the robust cell wall prevents contents of the cell from expanding, thereby preventing cellular lysis.

Cellulose is a polymer of glucose linked by beta (1-4) linkages. Humans lack the enzyme that catalyzes the cleavage of this bond; thus cellulose (dietary fiber) is indigestible and passes through the digestive system unchanged.

The correct answer is nonpolar, small in size, and have a large lipid solubility coefficient. The center of the lipid bilayer is nonpolar and composed of fatty acid chains; therefore, nonpolar molecules diffuse through more easily. Diffusion does not use transport proteins to shuttle molecules through the lipid bilayer, so smaller molecules are able to diffuse more easily between the phospholipids that comprise the bilayer. Finally, a higher lipid solubility coefficient means that the molecule is more soluble in lipids and less soluble in aqueous solutions. A low lipid solubility means that the molecule is more aqueous-soluble than lipid-soluble.

The correct answer is lipid rafts. Lipid rafts are more dense, less fluid domains of the plasma membrane. In addition to clustering cholesterol and sphingolipids, large transmembrane-domain proteins also localize here.

Integrins are transmembrane receptors, micelles are small lipid spheres, and phosphoinositides are a family of lipid molecules.

Integral proteins are proteins that connect the inside of the cell to the outside by traversing the phospholipid bilayer. Carrier proteins, for example, allow substances to pass the membrane by letting them cross through channels.

The correct answer is glycoprotein. HIV and many lentiviruses, which are also retroviruses, bind the CD4 glycoprotein. Lipopolysaccharides are found in bacteria, not vertebrates. Oligosaccharides are sugar polymers and generally function in cell-cell signaling. Cholesterol and phosphatidylcholine are components of the plasma membrane, not necessarily extracellularly exposed. The latter two help define the shape and rigidity of the plasma membrane.