Scanning electron microscope images indicate that G-actin has a globular structure; however, X-ray crystallography shows that each of these globules consists of two lobes separated by a cleft. This structure represents the “ATPase fold”, which is a centre of enzymatic catalysis that binds ATP and Mg 2+ and hydrolyzes the former to ADP plus phosphate . This fold is a conserved structural motif that is also found in other proteins that interact with triphosphate nucleotides such as hexokinase (an enzyme used in energy metabolism ) or in Hsp70 proteins (a protein family that play an important part in protein folding).  G-actin is only functional when it contains either ADP or ATP in its cleft but the form that is bound to ATP predominates in cells when actin is present in its free state. 
One of the main functions of the circulatory system is to pump blood throughout the body so that oxygen and nutrients can reach the tissues. The heart works as the pump, and is it actually considered a “double pump”, since the right side distributes blood towards the lungs for oxygenation, while the left side forces blood through the aorta so that oxygenated blood can circulate to the tissues. Blood vessels within the circulatory system include an arterial system and a venous system. Arteries carry blood away from the heart and towards tissues, while the venous system carries deoxygenated blood back to the heart.
1. Discuss the process that occurs in signal transduction cascades. In the animation illustrating T-cell proliferation, signal transduction begins when a foreign antigen carried by the antigen-presenting cell (APC) is recognized by the T-cell receptor. Several molecules located in or near the cell membrane mediate the transmission of signals, resulting in the production of inositol triphosphate (IP 3 ), which travels into the cell's cytoplasm. IP 3 initiates the release of calcium, which, through several intermediate steps, allows a transcription factor to enter the nucleus. Within the nucleus, this transcription factor and other essential factors trigger the expression of the gene interleukin-2 (IL-2). IL-2 protein is produced and released extracellularly, where IL-2 binds to receptors on the T cell. This receptor/IL-2 protein complex signals the T cell to proliferate.