Human Body
Human biology is an academic field of biology which focuses on humans; it is closely related to medicine, primate biology, and a number of other fields.
A human being is a multicellular eukaryote consisting of an estimated 100 trillion cells. It should be noted that there is no consensus on the actual number of cells in the human body. Estimates vary WIDELY. See wikipedia article on the cell. As a species, humans are primates and can be distinguished from other primates by their more highly evolved brains. Even though humans are multicellular animals, many of the basic life processes of human cells are basically the same as in simple unicellular eukaryotes such as yeast and even prokaryotes.
One of the major biological differences between humans and yeast is that yeast are unicellular organisms - when a yeast cell divides, it produces a new and independent organism. A human being is initially just one cell, a zygote, but when that cell divides, it forms an embryo. The early embryo consists of totipotent stem cells, which go on to differentiate. Differentiation is the process by which an unspecialized cell becomes specialized into one of the many cells that make up the body, such as a heart, liver, or muscle cells. During differentiation, certain genes are turned on, or become activated, while other genes are switched off, or inactivated. This process is intricately regulated. As a result, a differentiated cell will develop specific structures and perform certain functions. Differentiation can involve changes in numerous aspects of cell physiology; size, shape, polarity, metabolic activity, responsiveness to signals, and gene expression profiles can all change during differentiation.