From Food to the Human Body 

Have you ever heard the saying, “You are what you eat”? There’s a lot of truth behind that simple phrase.

Consider a lemon. It may look like a simple fruit, but it is actually made of billions and billions of tiny chemical building blocks—primarily carbon, hydrogen, and oxygen atoms. These building blocks join together to form molecules, including water, sugars, acids, and vitamin C.

When you eat a lemon, your body breaks these molecules into smaller components that are absorbed and used as raw materials to build, repair, and maintain your own tissues.  The carbon atoms that were once part of a lemon can later become part of a muscle cell, a skin cell, or even a neuron in your brain. Your body doesn’t create these atoms—it simply rearranges them into the molecules it needs. In a very real sense, the food you eat becomes part of you.

To understand how this transformation happens, it helps to understand how the human body is organized. Every part of your body is built from the same chemical building blocks found in food.

The human body is organized into levels of increasing complexity: atoms → molecules → cells → tissues → organs → organ systems → organism. Each level is built from the one before it, with simple structures combining to create increasingly complex levels of organization. Figure 3.1 illustrates how these levels fit together to form the human body.

 From Atoms to Cells: Building the Human Body

Every part of your body is ultimately built from atoms. Although more than 100 types of atoms exist, only a few—primarily carbon, hydrogen, oxygen, and nitrogen—make up most of the nutrients in food and the tissues of your body.

When atoms bond together, they form molecules. Water is a simple molecule made of hydrogen and oxygen atoms, while glucose and vitamin C are larger molecules containing carbon in addition to hydrogen and oxygen.

Some nutrients are especially large molecules called macromolecules.  Carbohydrates, proteins, and fats are the three major dietary macromolecules in nutrition.  In the hamburger shown in Figure 3.4, the bun is rich in carbohydrates, the beef patty provides protein, and the cheese and mayonnaise provide fat. These nutrients provide energy and many of the building materials your cells need to grow, repair themselves, and function normally.

Later in this chapter, you’ll learn how the digestive system breaks these macromolecules into smaller molecules that can be absorbed and used throughout the body. Chapters 4–6 explore carbohydrates, proteins, and fats in much greater detail.

From One Cell to a Complex Body

Think about the last time you exercised, recovered from an illness, or skipped a meal. Every one of those experiences affected cells throughout your body.

Cells are the basic units of life, and all living things are made of them. Cells multiply by dividing, allowing your body to grow, repair damaged tissues, and replace worn-out cells.

You, like every other human being,  began as a single cell.  A sperm from your father and an egg from your mother fused to form a single cell called a zygote. That one cell divided again and again, eventually giving rise to the trillions of specialized cells that make up your body. You’ll learn more about pregnancy in Chapter 11.

Every cell requires nutrients to produce energy, grow, repair itself, and carry out specialized functions. Every bite of food you eat provides raw materials that the trillions of cells in your body use to stay alive and function. Because tissues, organs, and organ systems are all made of cells, good nutrition supports every part of your body.

Although cells are the body’s smallest living units, they are remarkably complex (see Figure 3.5). They contain many specialized structures called organelles. For example, mitochondria produce energy, the nucleus contains the cell’s genes, and ribosomes build proteins. Other organelles transport materials and recycle waste. Together, these structures keep cells alive and functioning.

From Tissues to Organ Systems: Building Body Structure

As cells divide and specialize, they work together to form larger, more complex structures.

A tissue is a group of similar cells that performs a specific function. Two or more tissues combine to form an organ, such as the brain, liver, or heart. Organs then work together as organ systems to carry out major body functions, including digestion, circulation, and reproduction.

Although the human body contains eleven organ systems, several are especially important in nutrition. The digestive system breaks down food and absorbs nutrients, the cardiovascular system transports nutrients throughout the body, the endocrine system produces hormones that regulate metabolism and appetite, the nervous system helps control hunger and food-related behaviors, and the reproductive system has unique nutritional needs during pregnancy and lactation.

Table 3.1 summarizes the organ systems most directly involved in obtaining, transporting, and using nutrients.

The Organism: An Integrated Living System

An organism is a complete living being whose cells, tissues, organs, and organ systems work together to sustain life. From the smallest cell to the entire body, every level depends on nutrients obtained from the foods you eat. In many ways, the saying “you are what you eat” is true—your body is constantly using nutrients to produce energy, repair tissues, support growth, and maintain health.

attributions

This section is an adaptation of “Organization of Life” in Nutrition: Science and Everyday Application, V.1.0 by Alice Callahan, Heather Leonard, and Tamberly Powell under a Creative Commons Attribution-NonCommercial 4.0 International License.