Cell Biology - GRE Subject Test: Biology

The two most common polysaccharides found in plant cells are starch and cellulose. Starch is the primary source of energy storage, while cellulose is used to construct the plant's cell walls.

When a monosaccharide becomes cyclic in form, the anomeric carbon can have its hydroxyl group pointing in the same direction as the methoxy group, or oriented in the opposite direction. This orientation determines whether the sugar is considered alpha or beta.

Carbohydrates are linked together to form disaccharides and other polysaccharides through glycosidic linkages. A glycosidic linkage is one in where two sugar molecules are bridged by an oxygen atom. Peptide linkages are found between amino acids and phosphodiester bonds are found between nucleic acid monomers. Ionic bonds involve the complete transfer of one or more electrons from one species to another. Hydrogen bonds are weak intermolecular and intramolecular forces that contribute to the stability of many substances such as liquid water.

Glycogen and starch molecules are connected by alpha linkages. Glycogen and starch can be digested by humans because we have an enzyme capable of separating these linkages to produce monosaccharides. Cellulose on the other hand is connected through beta linkages. These beta linkages allow for the polysaccharide to form straight chains which can serve structural purposes such as plant cell walls. Cellulose, however, cannot be digested by humans because we do not have enzymes capable of severing these linkages.

Combrestatin interferes with the formation of microtubules, which make up the cytoskeletal architecture of a cell; therefore, the correct answer choice is involved with some microtubule-based process. DNA and protein synthesis do not involve microtubules, and would not be affected by the lack thereof. Muscle contraction depends on myosin, actin, troponin, etc., and not on microtubules. Membrane depolarization involves sodium/potassium channels, neurotransmitters, etc., and is not directly affected by microtubule inhibition.

The only answer that remains is mitosis, which involves microtubules in chromosomal segregation. The mitotic spindle in this separation is primarily composed of microtubules. The polymerization and depolymerization of microtubules is crucial for mitotic division. Combrestatin therefore prevents proper mitosis.

If the cell cannot progress past metaphase, the cell is most likely having trouble separating the sister chromatids. This process is mediated by attaching the kinetochore microtubules to the kinetochore on the sister chromatids. Our most likely explanation for a problem proceeding past metaphase is that the mutation is affecting the formation of microtubules. The progression from metaphase to anaphase is regulated by the metaphase checkpoint in the cell cycle, which is used to ensure proper attachment of spindle fibers to the centromeres. Problems with spindle fiber formation and binding would cause the cell to be arrested in metaphase.

Actin and myosin are not directly involved in this portion of mitosis, but are very important during cytokinesis. Chromosome condensation has already occurred (during prophase), so our mutation cannot be affecting those proteins.

Cellular division usually takes place in four steps before undergoing cytokinesis. In prophase, the chromosomes condense and become visible, and the spindle apparatus begins to form. In metaphase, the chromosomes line up on the equator of the cell. In anaphase, the chromatids are pulled apart and separated to opposite sides of the cell. Finally, telophase involves the nuclear membrane reforming around the chromosomes, which begin to decondense.

Once the cell has split and transported its sister chromatids to opposite ends of the cell, the nuclear envelopes can begin to regenerate around the genetic material at each pole. This event occurs during the end of mitosis, commonly known as telophase.

In the process of meiosis, crossing over (the exchange of genetic material between homologous chromosomes resulting in recombination) only occurs during prophase I, when pairs of homologous chromosomes recombine during synapsis. This contributes to the increase in diversity due to sexual reproduction.

As the name implies, gap junctions are literally a gap in the plasma membranes of two adjacent cells that connect their cytoplasms. These junctions allow for electrical synapses and rapid cell signaling. While most notably present in cardiac muscle, gap junctions are also present in some neural cells and receptors.

Adherens junctions anchor cells through interactions with the actin cytoskeleton. Desmosomes use cadherin proteins to anchor cells via interactions with intermediate filaments. Tight junctions are used to create barriers that selectively allow molecules through layers of epithelial cells.

All of the choices describe functions of different types of cell junctions. Anchoring junctions (such as adherens junctions and desmosomes) use the cytoskeletons of each cell, as well as certain transmembrane proteins, to anchor the cells together. Gap junctions create gaps in the plasma membrane between two adjacent cells that connect their cytoplasms. Tight junctions are capable of forming barriers that are nearly impermeable to fluid flow.

All of the choices presented describe functions of cell junctions. Adherens junctions and desmosomes are responsible for anchoring neighboring cells to one another. Hemidesmosomes help anchor the extracellular matrix in place, binding the cell membrane to proteins in the basal lamina or matrix. Gap junctions are an example of junctions that connect the cytoplasm of two neighboring cells and allow for communication via ions and other small molecules.

Cellular junctions allow cells to block materials from moving between cells, or for providing communication between cells. Tight junctions are junctions between cells that form a tight seal. This prevents water and other fluids from moving past the cells. Tight junctions are essential for maintaining concentration gradients, preventing osmosis from equilibrating ion concentrations between two regions.

Desmosomes serve to anchor the cytoskeletons of two adjacent cells, helping with force transduction. Gap junctions allow for cellular communication by creating perforations between cells through which ions and small molecules can flow. Villi are not a type of cell junction, and are structures that serve to increase cell surface area.

In order to have the action potential spread evenly and completely throughout the cardiac muscle cells, the cells need junctions that allow for ions to move between them. This action is accomplished by gap junctions between the cells. The intercalated discs between cardiac muscle cells are composed of gap junctions, and allow for electrical stimulation to travel from one cell to the next. This feature means that cardiac muscle can be depolarized and contract simultaneously.

Tight junctions are used to prevent fluid flow between cells. Desmosomes help with force transduction by linking the cytoskeletons of adjacent cells. Valves are not a type of cellular junction, and are macrostructures that prevent the backflow of fluids in vessels of the body.

Cells progress through the cell cycle by controlled expression and degradation of cyclin proteins. Cyclin-dependent kinases (CDKs) are always present in the cell and are not degraded after progression to a new stage of the cell cycle. Cyclins bind their respective CDKs to activate them. This activation causes a chain of events that allow the cell to progress to the next phase of the cell cycle. Afterwards, cyclins are ubiquinated and degraded until they are needed again.

During anaphase, sister chromatids are pulled apart towards opposite poles. The cell becomes longer and the cleavage furrow (contractile ring) forms. This marks the beginning of cytokinesis. The process completes during telophase, producing two new daughter cells. Cytokinesis must be preceeded by karyokinesis (physical movement of the chromosomes).

When oxygen is not readily available in the blood, cells use fermentation as a means of anaerobic respiration. The process is used to generate NAD+, which can be used as a reactant in glycolysis to produce small amounts of ATP. Glycolysis still occurs in this environment, breaking glucose into pyruvate and producing two ATP per cycle.

Fermentation in humans converts pyruvate to lactate (lactic acid) and NADH to NAD+.

Under anaerobic conditions, fermentation follows the process of glycolysis. While glycolysis is responsible for creating ATP, fermentation allows the body to regenerate the NAD+ that is reduced during glycolytic processes. This key step allows glycolysis to continue, and more ATP to be made.

Fermentation is a form of anaerobic respiration where there is no oxygen available as the final electron acceptor to the electron transport chain. As such, pyruvate is reduced, yielding lactic acid and, in the presence of lactate decarboxylase, ethanol. However, the products of fermentation do not undergo the Krebs cycle nor electron transport chain. The purpose of fermentation is to regenerate NAD+ so that glycolysis can continue. The cell does this by transferring the electron from NADH to pyruvate. Fermentation is less efficient per glucose than is aerobic oxidation, generating a fraction of the ATP. Lactic acid and ethanol are actually quite toxic to the cell (in humans they are immediately sent to the liver to be detoxified).