An antigen can induce an immune response in a host organism. Antigens are targeted by antibodies that are produced by the organism's immune system in response to contact with the antigen. Antibodies are specific to antigens. Many different cell types are
Chemicals that are secreted from antigen-presenting cells then activate helper T cells.
Adjacent plant cells have narrow channels called plasmodesmata that pass through the cell walls of the connected cells and allow a cytoplasmic connection between the cells.
Which of the following statements best describes a primary function of plasmodesma
They allow the movement of molecules from one cell to another, enabling communication between cells.
A hydrophilic peptide hormone is produced in the anterior pituitary gland located at the base of the brain. The hormone targets specific cells in many parts of the body.
Which of the following best explains a possible mechanism that would enable the hormo
The hormone is released into the bloodstream where it can be transported to all cells with the correct receptors.
G proteins are a family of receptor proteins that are involved in transmitting signals from outside a cell to inside a cell. When a signaling molecule binds to a G protein, the G protein is activated. The G protein then activates an enzyme that produces a
cAMP results in the activation of an enzyme that amplifies the signal by acting on many substrate molecules.
Metformin is a drug used to treat type 2 diabetes by decreasing glucose production in the liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of glucose metabolism. Metformin activates AMPK in liver cells but cannot cross the plasma m
It is a ligand that activates the signal transduction pathway of the activation of AMPK.
Which of the following steps in a signaling pathway typically occurs first once a chemical messenger reaches a target cell?
A ligand binds to a receptor.
During a fight-or-flight response, epinephrine is released into the body's circulatory system and transported throughout the body. Some cells exhibit a response to the epinephrine while other cells do not.
Which of the following justifies the claim that d
Cell signaling depends on the ability to detect a signal molecule. Not all cells have receptors for epinephrine. Only cells with such receptors are capable of responding.
Signal transduction may result in changes in gene expression and cell function, which may alter phenotype in an embryo. An example is the expression of the SRY gene, which triggers the male sexual development pathway in mammals. This gene is found on the
If the SRY gene is absent or nonfunctional, the embryo will exhibit female sexual development.
Researchers have discovered details about apoptosis (programmed cell death) by studying embryologic development of a nematode worm, Caenorhabditis elegans. Apoptosis is a normal developmental process in C. elegans. They found several genes involved in apo
An experiment showed that a mutation in the ced?9 gene led to excessive cell death in C. elegans.
The insulin receptor is a transmembrane protein that plays a role in the regulation of glucose homeostasis. The receptor's extracellular domain binds specifically to the peptide hormone insulin. The receptor's intracellular domain interacts with cellular
The storage of GLUT4 in vesicles inside the cell will increase.
The beta-2 adrenergic receptor is a membrane-bound protein that regulates several cellular processes, including the synthesis and breakdown of glycogen. The receptor binds specifically to the hormone epinephrine. The binding of epinephrine to the beta-2 a
The rate of glycogen synthesis in the cell will increase
Fibroblast growth factor receptors (FGFRs) are transmembrane proteins that regulate cellular processes such as cell proliferation and differentiation. The extracellular domains of FGFR proteins bind specifically to signaling molecules called fibroblast gr
The irreversible association of FGFR proteins
Phosphofructokinase (PFK) is a key enzyme in glycolysis. ATP is one of the two substrates for the reaction catalyzed by PFK. ATP is also an allosteric regulator of PFK. Figure 1 shows the enzyme-substrate interactions of PFK.
A researcher found a mutation
Negative feedback regulation does not occur, so the enzyme will be active when glycolysis is not needed.
A person's blood glucose level fluctuates during the day, as represented in Figure 1. Two hormones, insulin and glucagon, are directly involved in regulating the blood glucose level to maintain a healthy level. Insulin acts to lower the blood glucose leve
Immediately after the meal, the blood glucose level will increase, and then insulin will be secreted to counter the increase.
Blood clots are formed by a positive feedback loop. Two pathways exist, the extrinsic and intrinsic pathways, which converge during clot formation. There are many clotting factors involved, most of which are proteins. Vitamin K is required for the formati
Factor X will not be activated, which will prevent thrombin from forming.
A model of the typical life cycle of a cell is shown in Figure 1.
Scientists have estimated that it takes yeast cells approximately 20 hours to complete the entire cycle. Table 1 shows the amount of time in each phase of the life cycle for yeast cells.
Researchers grew seedlings of corn, Zea mays, in loose and compact sand. The researchers measured the amount of time required for the cells in the growing root tips of the seedlings to double in number. The mean cell doubling times for the two groups of s
More cells are produced per unit of time in the root tips growing in compact sand than in the root tips growing in loose sand.
The relative amounts of DNA present in the nucleus of a cell at four different stages of the life cycle are shown in Figure 1.
Based on Figure 1, which of the following statements correctly links a stage of the cell cycle with the event occurring at that
Synthesis of sufficient DNA for two daughter cells occurs in stage II.
Researchers tracked the amount of DNA (measured in picograms) over time beginning with a single cell and continuing through several rounds of cell division. The researchers observed threadlike chromosomes prior to cell division. The threadlike chromosomes
There is a change from 3 to 6 picograms of DNA because DNA is replicated before each round of cell division.
A cell culture commonly used in research was selected to study the effect of a specific virus on the timing of cell cycle phases. Two separate cultures were started, one untreated and one inoculated with the virus. Both cultures were incubated under ident
Seventy-five percent of the virus-infected cells are found in mitosis. The virus stimulates frequent cell division.
A student used microscopy to investigate the relative lengths of the different stages of mitosis. The student prepared slides of cells isolated from a growing onion root tip and viewed the slides under a dissecting microscope. The student then made diagra
The epidermal growth factor receptor EGFR is a cell surface receptor. When a growth factor binds to EGFR, the receptor is activated. The activated EGFR triggers a signal transduction pathway, which leads to increased frequency of cell division.
Which of t
Increased cell division will lead to the formation of a tumor.
The tumor suppressor protein p53 binds to DNA and activates target genes, which results in the synthesis of p21, CD95, and other proteins. The p21 protein promotes cell-cycle arrest, whereas the CD95 protein promotes apoptosis.
Which of the following will
Uncontrolled cell proliferation
Figure 1 represents the relative time and sequence of the phases of the cell cycle.
Which statement best predicts why a cell's progression through the cell cycle might be halted at the G1/S checkpoint?
There are not enough nucleotides available to construct new DNADNA.