Explain how an action potential is generated. Sodium ion channels on one end of an axon suddenly open to allow the accumulation of positive sodium ions, making the voltage on that end positive, creating a length‑wise imbalance in the potential with the opposite, negative end. A group of positive potassium ions collects next to an axon, inducing a positive potential on the axon surface. The positive potassium ions suddenly disperse, making the potential on the axon surface suddenly flip sign. When the negative potential inside an axon becomes slightly less negative, sodium ion channels open, allowing positive sodium ions to rush into the axon, flipping the internal potential to a positive value. Potassium channels of an axon suddenly open to allow positive potassium ions to rush out of the axon. When enough leave, the potential inside the axon becomes negative, and the potassium ions rush back in, flipping the potential.

Experiments in environmental toxicology can sometimes be manipulative experiments in which the researcher actively chooses and manipulates the independent variable. in hunt's study, for example, dosages of bpa were manipulated and the effects were measured. in manipulative studies, the researcher controls all the other variables in the experiment, so any health effects observed in the test subjects can be attributed to differences in the independent variable. in other cases, researchers use natural experiments in which the dependent variable (typically a measure of organism health) is measured under differing contexts that are not manipulated. say, for example, that an accidental chemical spill contaminates five ponds. to determine the possible effects of the toxic chemical on frogs, a researcher could compare the hatching rate of frog eggs laid in those five ponds to the hatching rate of eggs laid in five uncontaminated ponds nearby. this would be an example of a natural experiment because concentrations of the toxic chemical in the ponds were not controlled by the experimenter, but rather resulted from the chemical spill. drag type of experiment on the left to the example of experiment on the right. blood concentrations of bpa in college students are compared to their recent manipulative consumption of canned food items 2. the feeding behavior of fish in streams that receive acidic runoff from strip mines is compared to the feeding behavior of fish in unaffected streams. the deformity rate in baby birds from nests in pesticide-sprayed fields is compared to the deformity rate in birds from nests in unsprayed fields 4 tumor development is compared in mice exposed to five dosages of a known carcinogen in the laboratory foraging activity levels are compared in tadpoles exposed to four concentrations of toxic metals in the laboratory. growth of corn plants is compared in field plots sprayed with three different dosage: s of weed killer 7 bpa concentrations in the urine of people with diabetes are compared to bpa concentrations in the urine of people without diabetes - natural; manipulative

Environmental impacts on evolution : mastery test

The idea of spontaneous generation was disproved by in a experiment involving jars of meat