There is a remarkable similarity, moreover, in the molecular composition and metabolism[2] of all living things. This similarity has been taken to mean that life could have originated only once in the past and had a specific chemical composition on which its metabolic processes depended. This structure and metabolism were handed down to subsequent living things by reproduction, and all variations thereafter resulted from occasional mutation, or changes in the nature of the heredity-transmitting units. One of the most extraordinary of all the attributes of life is its ordered complexity, both in function and structure. It is agreed among biologists that the functional manifestations of life include movement, respiration, growth, irritability (reaction to environmental changes), and reproduction and that these phenomena are therefore possessed by all cells. The first four of these can be grouped under a single word: metabolism. We can therefore say that living things have two common properties: metabolism and reproduction. Therefore, when we say we are studying life processes, we actually are studying the metabolism and reproduction of cells. Since metabolism is the sum of the biochemical reactions taking place in a living organism, it properly belongs to the field of investigation of biochemists. Cell reproduction is the concern of both biochemists and morphologists[3] since it can be studied by either biochemical or morphological techniques. Cell Structure [Illustration: Figure 2 _Generalized diagram of a cell, showing the organelles, or “little organs”, of its internal structure. The organelles that are labeled are important for this booklet._] Cell membrane Cytoplasm Chromatin Mitochondrion Nucleolus Endoplasmic reticulum Nucleus Nuclear membrane The basic structure of a cell is shown in Figure 2. Each cell consists of a dense inner structure called the nucleus, which is surrounded by a less dense mass of cytoplasm. The nucleus is separated from the cytoplasm by a double envelope, called the nuclear membrane, which is peppered with perforations. The cytoplasm contains a network of membranes, which form the boundaries of countless canals and vesicles (or pouches), and is laden with small bodies called ribosomes. This membranous network is called the endoplasmic reticulum and is distinct from the mitochondria, which are membranous organelles (little organs) structurally independent of other components of the cytoplasm. The outer coat of the cell is called the cell membrane, or plasma membrane, and forms the cell boundary. [Illustration: Figure 3 _Electron micrograph of a primary spermatocyte cell of a grasshopper, showing the nucleus (N), endoplasmic reticulum (ER), mitochondrion (M), chromatin (C), nuclear membrane or nuclear envelope (NE), cell membrane (CM), and intercellular space (I). The magnification is about 25,000 times the actual size._]