UMass Memorial Health Care, which lost money in the last three months of 2011 amid shrinking patient volume, is telling employees today that it will shed 700 to 900 jobs, about 6 percent of its workforce, through a combination of layoffs at its flagship Worcester hospital and by selling divisions that provide health services. The health care system, which operates UMass Memorial Medical Center and four community hospitals, is the largest employer in central Massachusetts with about 13,500 workers.