Page 61.3 The Body's Levels of OrganizationScientists group the body's components into an organizational hierarchy of form and function. In thinking about these levels, it is helpful to know the characteristics common to living things and how each level supports these characteristics. For example, the organ system concept allows functions to be considered as an interaction between many organs.1.3a Characteristics That Describe Living ThingsLEARNING OBJECTIVEDNA Probe (DNA Hybridization)1. List the characteristics common to all living things.Several properties are common to all organisms, including humans:- Organization. All organisms exhibit a complex structure and order. In the next section, we observe that the human body has several increasingly complex levels of organization.- Metabolism. All organisms engage in metabolism (mĕ-tab′ō-lizm; metabole = change), which is defined as the sum of all of the chemical reactions that occur within the body. Metabolism consists of both anabolism (ă-nab′ō-lizm, anabole = a raising up), in which small molecules are joined to form larger molecules, and catabolism (kă-tab′ō-lizm; katabole = a casting down), in which large molecules are broken down into smaller molecules. Examples of metabolic reactions include using the cell's own energy to perform certain functions, and contracting muscles to move the body or body organs.WHAT DO YOU THINK?When you digest a meal, what type of metabolic reactions do you think you are utilizing primarily: anabolic or catabolic chemical reactions? Why?Answer- Growth and Development. During their lifetime, organisms assimilate materials from their environment and often exhibit increased size (growth) and increased specialization as related to form and function (development). As the human body grows and develops, structures such as the brain become more complex and sophisticated.- Responsiveness. All organisms exhibit responsiveness, which is the ability to sense and react to stimuli (changes in the external or internal environment). A stimulus to the skin of the hands, such as an extremely hot temperature, causes the human to withdraw the hand from the stimulus so as to prevent injury or damage. Responsiveness occurs at almost all levels of organization.- Regulation. An organism must be able to adjust or direct internal bodily function in the face of environmental changes. Homeostasis (hō-mē-ō-stā′sis; homoios = similar, stasis= standing) refers to the ability of an organism to maintain a consistent internal environment, or “steady state.” When body temperature rises, the body regulates this change by circulating more blood near its surface to facilitate heat loss, and thus return the body to homeostasis. (Homeostasis is discussed in greater depth later in this chapter.)- Reproduction. All organisms produce new cells for growth, maintenance, and repair. Thesomatic (body) cells divide by a process called mitosis, while sex cells (called gametes) are produced by a cell division process called meiosis. The sex cells, under the right conditions, have the ability to develop into a new living organism.WHAT DID YOU LEARN?Mutation by base substitutionWhat does it mean if an organism is ‘responsive’, and how does this characteristic relate to the survival of this organism?1.3b The View from Simplest to Most ComplexLEARNING OBJECTIVE2. Describe the levels of organization in the human body.Anatomists and physiologists recognize several levels of increasingly complex organization in humans, as illustrated in figure 1.2. These levels, from simplest to most complex, are the chemical level, cellular level, tissue level, organ level, organ system level, and organismal level.Figure 1.2Levels of Organization in the Human Body.The most simple level is the chemical level, followedby increasingly more complex levels of organization.LysosomesThe chemical level is the simplest level, and it involves atoms and molecules. Atoms are the smallest units of matter. When two or more atoms combine they form a molecule. Examples of molecules include a sugar, a water molecule, or a vitamin. More complex molecules are called macromolecules and include some proteins and the deoxyribonucleic acid (DNA) molecules. Macromolecules form specialized microscopic subunits in cells called organelles, which are microscopic structures found within cells.The cellular level consists of cells, which are the smallest living structures and serve as the basicunits of structure and function in organisms. Cells and their components are formed from the atoms and molecules from the chemical level. The structures of cells vary widely, reflecting the specializations needed for their different functions. For example, a skeletal muscle cell may be very long and contain numerous organized protein filaments that aid in muscle contraction, whereas a red blood cell is small and has a flattened disc shape that facilitates the quick and effective exchange of respiratory gases.The tissue level consists of tissues, which are groups of similar cells that perform common functions. There are four types of tissues. Epithelial tissue covers exposed surfaces and lines body cavities. Connective tissue protects, supports, and binds structures and organs. Muscle tissue produces movement. Finally, nervous tissue conducts nerve impulses for communication.Page 7The organ level is composed of organs, which contain two or more tissue types that work together to perform specific, complex functions. The small intestine is an example of an organ that is composed of all four tissue types which work together to process and absorb digested nutrients.The organ system level contains related organs that work together to coordinate activities and achieve a common function. For example, the organs of the digestive system (e.g., oral cavity, stomach, small and large intestine, and liver) work together to digest food particles, absorb nutrients, and expel the waste products.The highest level of structural organization in the body is the organismal level. All body systemsfunction interdependently in an organism, which is the living being.WHAT DID YOU LEARN?Does a higher level of organization contain all the levels beneath it? Explain.CONCEPT CONNECTIONThroughout future chapters, boxes like this one will highlight how various organ systems are interconnected and do not work in isolation. For example, the cardiovascular system and respiratory system work together in the transport of