Organization of Life

Before you begin to study the different structures and functions of the human body related to nutrition, it is helpful to consider the basic architecture of the body; that is, how its 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 (Figure 3.1).

The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism. The picture shows a pyramid divided into six levels, showing at the top of the pyramid as the chemical level of structure such as hydrogen atoms and oxygen atoms. The second level shows a water molecule to depict molecular organization. The third level shows a single muscle cell to depict the cellular level. The fourth level shows muscle tissue as an example of cells being organized together in a tissue. The fifth level shows the bladder to depict tissues being organized together to form an organ. The fifth level shows the urinary tract system with the kidneys, ureter, bladder, and urethra, depicting how several organs work together to perform a specific function in the body. The six level shows a person drinking water to depict organ systems coming together to form a complex living organism such as a human.

Figure 3.1. 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

Consider the simplest building blocks of matter: atoms and molecules. In Unit 1, you had an introduction to atoms and molecules. Remember, 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. Four macromolecules—carbohydrates, lipids, proteins, and nucleic acids (e.g., DNA, RNA)—make up all of the structural and functional units of cells.

The Basic Structural and Functional Unit of Life: The Cell

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. 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. While all cells in an individual contain the same DNA, each cell only expresses the genetic codes that relate to that cell’s specific structure and function.

As an adult, you are made up of trillions of cells. Each of your individual cells is a compact and efficient form of life—self-sufficient, yet interdependent upon the other cells within your body to supply its needs. 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, enterocytes (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 3.2). 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

This picture shows a single cell with all of its components. The contents of the cell are all labeled. The cell includes a nucleus, mitochondrion, ribosomes, endoplasmic reticulum, golgi body, and lysosome, all held together by a plasma membrane.

Figure 3.2. 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. There are four basic types of human tissues: connective tissue, which connects tissues; epithelial tissue, which lines and protects organs; muscle, which contracts for movement and support; and nerve, which responds and reacts to signals in the environment.

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 systemis 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 3.3). 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 (Table 3.1). For example, the cardiovascular system plays a role in nutrition by transporting nutrients in the blood to the cells of the body. The endocrine system produces hormones, many of which are involved in regulating appetite, digestive processes, and nutrient levels in the blood. Even the reproductive system plays a role in providing nutrition to a developing fetus or growing baby.

This illustration shows eight silhouettes of a human female, each showing the components of a different organ system. The integumentary system encloses internal body structures and is the site of many sensory receptors. The integumentary system includes the hair, skin, and nails. The skeletal system supports the body and, along with the muscular system, enables movement. The skeletal system includes cartilage, such as that at the tip of the nose, as well as the bones and joints. The muscular system enables movement, along with the skeletal system, but also helps to maintain body temperature. The muscular system includes skeletal muscles, as well as tendons that connect skeletal muscles to bones. The nervous system detects and processes sensory information and activates bodily responses. The nervous system includes the brain, spinal cord, and peripheral nerves, such as those located in the limbs. The endocrine system secretes hormones and regulates bodily processes. The endocrine system includes the pituitary gland in the brain, the thyroid gland in the throat, the pancreas in the abdomen, the adrenal glands on top of the kidneys, and the testes in the scrotum of males as well as the ovaries in the pelvic region of females. The cardiovascular system delivers oxygen and nutrients to the tissues as well as equalizes temperature in the body. The cardiovascular system includes the heart and blood vessels.

The lymphatic system returns fluid to the blood and defends against pathogens. The lymphatic system includes the thymus in the chest, the spleen in the abdomen, the lymphatic vessels that spread throughout the body, and the lymph nodes distributed along the lymphatic vessels. The respiratory system removes carbon dioxide from the body and delivers oxygen to the blood. The respiratory system includes the nasal passages, the trachea, and the lungs. The digestive system processes food for use by the body and removes wastes from undigested food. The digestive system includes the stomach, the liver, the gall bladder (connected to the liver), the large intestine, and the small intestine. The urinary system controls water balance in the body and removes and excretes waste from the blood. The urinary system includes the kidneys and the urinary bladder. The reproductive system of males and females produce sex hormones and gametes. The male reproductive system is specialized to deliver gametes to the female while the female reproductive system is specialized to support the embryo and fetus until birth and produce milk for the infant after birth. The male reproductive system includes the two testes within the scrotum as well as the epididymis which wraps around each testis. The female reproductive system includes the mammary glands within the breasts and the ovaries and uterus within the pelvic cavity.

Figure 3.3. Organ systems of the human body

Organ System

Major Organ Components

Major Functions


Heart, blood vessels, blood

Transport oxygen, nutrients, and waste products


Mouth, esophagus, stomach, intestines, salivary glands, pancreas, liver and gallbladder

Digestion and absorption


Endocrine glands (e.g., thyroid, ovaries, pancreas)

Produce and release hormones, regulate nutrient levels


Lymphatic vessels, lymph nodes, thymus gland, spleen

Fluid balance, defend against pathogens


Skin, nails, hair, sweat glands

Protection, body temperature regulation


Skeletal, smooth, and cardiac muscle

Body movement


Brain, spinal cord, nerves

Interpret and respond to stimuli, appetite control


Gonads, genitals

Reproduction and sexual characteristics


Lungs, nose, mouth, throat, trachea

Gas exchange (oxygen and carbon dioxide)


Bones, tendons, ligaments, cartilage, joints

Structure and support, calcium storage


Kidneys, bladder, ureters

Waste excretion, water balance

Table 3.1. The eleven organ systems in the human body and their major functions

An organism is the highest level of organization—a complete living system capable of conducting all of life’s biological processes. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.






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Nutrition: Science and Everyday Application Copyright © 2020 by Alice Callahan, PhD; Heather Leonard, PhD, RDN; and Tamberly Powell, MS, RDN is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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