Before studying the various structures and functions of the human body, it’s helpful to consider the basic architecture of the body and how the smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: atoms, molecules, cells, tissues, organs, organ systems, and organisms. Higher levels of organization are built from lower levels. Therefore, atoms combine to form molecules, molecules combine to form cells, cells combine to form tissues, tissues combine to form organs, organs combine to form organ systems, and organ systems combine to form organisms as shown in  the figure below.

Figure 3.8 Levels of structural organization of the human body. The organization of the body often is discussed in terms of distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism.

The Levels of Organization

All matter in the universe is composed of one or more unique elements, such as hydrogen, oxygen, carbon, and nitrogen. The smallest unit of any of these elements is an atom. Atoms of individual elements combine to make molecules, and molecules bond together to make bigger macromolecules. Three macromolecules—carbohydrates, lipids, and proteins—make up all of the structural and functional units of cells, and are components of foods that we consume.

To understand the structure of the nutrients in the body and in foods, and how these nutrients function within the body, you must first understand the basic chemical structure of molecules. Nutrients are chemical molecules that are found in foods and required for  growth and to maintain health. On this page,  we’ll take a quick look at how atoms bond together to form these chemical molecules then move on to other structures of the human body.

Atoms and Molecules 

You may recall from your science class that matter is anything that has mass and takes up space. All living and nonliving things are composed of matter. Atoms are the smallest unit of matter. The chair you are sitting in is made of atoms. The food you ate for breakfast was built from atoms. Even the air you breathe is made of atoms. An atom is the smallest unit of an element, just like a blade is the smallest unit of grass. An element is made entirely from one specific type of atom. There are more than 100 elements that make up the world we live in, however hydrogen, carbon, nitrogen, and oxygen make up the bulk of all living things. Many elements are found in the foods we eat and all of them are found on the Periodic Table of Elements as noted in Figure 3.2.

Figure 3.9. The Periodic Table of Elements. Note the four elements circled in blue (hydrogen, carbon, nitrogen, and oxygen). These four elements make up the bulk of all living things, especially the nutrients.

Atoms are unimaginably small. Even within a single microscopic cell, there is room for not just billions, but trillions or even hundreds of trillions of atoms.  Atoms combine to form a larger and more complex entity called a molecule. Molecules are composed of two or more atoms held together by chemical bonds.

Biological Macromolecules

Biological macromolecules are formed when atoms two or more moledules  bond with each other in unique and varied ways. Macromolecules are the raw materials used to build living organisms.

Carbohydrates, proteins, and lipids  are examples of  macromolecules.  Later in the unit, you’ll see how the macromolecules in foods are digested (broken down) to molecules, then your body uses the molecules as the building materials to support muscles and other body tissues. (More on this when we discuss carbs, proteins, and lipids in future units.)

Cells: Basic Structural and Functional Parts of LIfe

Cells are the most basic building blocks of life. All living things are composed of cells. New cells are made from preexisting cells, which divide in two. Who you are has been determined because of two cells that came together inside your mother’s womb.  (We will discuss this more in the unit on pregnancy.) The two cells containing all of your genetic information (DNA) fused to begin the development of a new organism. Cells divided and differentiated into other cells with specific roles that led to the formation of the body’s numerous organs, systems, blood, blood vessels, bones, tissues, and skin.

As a teen or an adult, you are made up of trillions of cells.  There are hundreds of types of cells (e.g., red blood cells, nerve cells, skin cells). Each individual cell conducts all the basic processes of life. It must take in nutrients, excrete wastes, detect and respond to its environment, move, breathe, grow, and reproduce. Many cells have a short life span and have to be replaced continually. For example, cells that line the intestines are replaced every 2-4 days, and skin cells are replaced every few weeks.

Although a cell is defined as the “most basic” unit of life, it is structurally and functionally complex (Figure 3.3). A human cell typically consists of a flexible outer cell membrane (also called a plasma membrane) that encloses cytoplasm, a water-based cellular fluid, together with a variety of functioning units called organelles. The organelles are like tiny organs constructed from several macromolecules bonded together. A typical animal cell contains the following organelles:

• Nucleus: houses genetic material (DNA)
• Mitochondria: often called the powerhouse of the cell, generates usable energy for the cell from energy-yielding nutrients
• Ribosomes: assemble proteins based on genetic code
• Endoplasmic reticulum: processes and packages proteins and lipids
• Golgi apparatus (golgi body): distributes macromolecules like proteins and lipids around the cell
• Lysosomes: digestive pouches which break down macromolecules and destroy foreign invaders

                                            Figure 3.10 The cell structure

Tissues, Organs, Organ Systems, and Organisms

A tissue is a group of many similar cells that share a common structure and work together to perform a specific function.

An organ is a group of similar tissues arranged in a specific manner to perform a specific physiological function. Examples include the brain, liver, and heart. An organ system is a group of two or more organs that work together to perform a specific physiological function. Examples include the digestive system and central nervous system.

There are eleven distinct organ systems in the human body (Figure 3.4). Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, many organs contribute to more than one system. And most of these organ systems are involved in nutrition-related functions within the body in some way. (Table 3.1). The three that are most important when studying nutrition include:

1. 1. The cardiovascular system: plays a role in nutrition by transporting nutrients in the blood to the cells of the body.
   2. The endocrine system:  produces hormones, many of which are involved in regulating appetite, digestive processes, and nutrient levels in the blood.
   3. The reproductive system: plays a role in providing nutrition to a developing fetus or growing baby.

Figure 3.11 Organ systems of the human body