There are many different categories of t-cells that carry out various roles for the immune system. differentiation for most t-cells occurs during the t-cell maturation, but some differentiation occurs after maturation following antigen presentation. one type of t-cell's functioning is dependent on antigen presentation and provides the immune system with continued protection against past pathogens. what is this category of t-cell? a) t b) memory t c) cytotoxic t d) natural killer t

How are the tortoises that darwin observed on the galápagos islands similar to the anoles that losos studied

Rease is an enzyme used by plants to break down urea (a nitrogen-containing compound) into carbon dioxide and ammonia. urease urea > > > carbon dioxide and ammonia ammonia is broken down by plants into a nitrogen source plants need to grow. thus, plants could not use urea as a nitrogen source unless it was first converted to ammonia. in soybean plants there are two different kinds of urease, one produced in the seeds and the other produced in the leaves of the plant. three types of soybean plants were used in a set of experiments: normal soybeans and two mutant strains, one lacking the urease in the seeds only (strain 1) and one lacking urease in the leaves only (strain 2). experiment 1 separate areas in a field were planted with normal, strain 1, and strain 2 soybeans. all types of soybeans appeared to grow, flower, and produce seeds equally well. there were no externally detectable differences among the strains. experiment 2 small pieces of plant leaves of equal weight were obtained from each type of soybean plant and separately placed on media in culture dishes. tissue growing in this way will become an unorganized clump of cells referred to as callus. to provide a controlled nitrogen source, half the tissue samples of each type were placed on media containing urea, and the other half of the samples were placed on media containing ammonia. after 30 days, the weight gain for each of the callus samples was determined. results are shown in the table below.

Which of the organisms listed below performs nitrogen fixation? a. legumes b. plants c. bacteria d. fungi