Which statement best describes a cause and its effect that occurs during the process of muscle contraction? A.
The presence of calcium ions within nerve cells causes the sarcomere to shorten.
B.
The shortening of the sarcomere causes the release of calcium around the myofibrils.
C.
The release of calcium ions causes myosin and actin to attach to each other.
D.
The pull of myosin on actin causes the motor neuron to send a nerve impulse.

Astructure that is found in a plant cell and not in an animal cell is a ? central vacuole mitochondrion nucleus ribosome

The trp operon in e.coli regulates genes that code for enzymes required for synthesis of the repressible operon. when tryptophan is not available amino acid tryptophan. the trp operon is a in the environment, the trp repressor protein activeinactiveregulated. therefore, rna polymerase can bind to the genes to begin transcription.

Mouse liver cells were homogenized and the homogenate subjected to equilibrium density-gradient centrifugation with sucrose gradients. fractions obtained from these gradients were assayed for marker molecules (i.e., molecules that are limited to specific organelles). the results of these assays are shown in the figure. the marker molecules have the following functions: cytochrome oxidase is an enzyme involved in the process by which atp is formed in the complete aerobic degradation of glucose or fatty acids; ribosomal rna forms part of the protein-synthesizing ribosomes; catalase catalyzes decomposition of hydrogen peroxide; acid phosphatase hydrolysis monophosphoric esters at acid ph; cytidylyltransferase is involved in phospholipid biosynthesis; and amino acid permease aids in transport of amino acids across membranes. a) name the marker molecule and give the number of the fraction that is most enriched for each of the following cell components: lysosomes; peroxisomes; mitochondria; plasma membrane; rough endoplasmic reticulum; smooth endoplasmic reticulum.