ATP is required for the contraction of a sarcomere and thus a muscle fiber. At death, the production of ATP ceases and the ATP stores in a cell are quickly used up. The cell's membranes become permeable to calcium and calcium enters the cytosol of the cell and attaches to troponin, removing tropomyosin from the actin active sites. Myosin attaches to the actin active sites and the muscle contracts for several cycles until the ATP stores are depleted. The muscles are then unable to relax. This postmortem contraction of the muscles is called rigor mortis. Rigor mortis is ultimately caused by a lack of ATP that would, if present, (1) allow calcium to be pumped out of the cytosol and into the sarcoplasmic reticulum and (2) .

2. Break down the myosin-actin bridge and cause relaxation. ATP replaces ADP to cause this relaxation. ADP causes the continuous binding of action to myosin thus preventing relaxation. It's action continues in rigor motifs. But ATP presence helps to breaks this link and enhance Relaxation

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!