Before learning about the structures and functions of the human body, it’s useful to understand how the body is organized from its simplest parts to its most complex. The body can be described in terms of levels of organization, each building upon the one before it: atoms, molecules, cells, tissues, organs, organ systems, and, finally, the organism.

At the smallest level, atoms join together to form molecules. These molecules combine to create cells, the basic units of life. Groups of similar cells form tissues, and different tissues work together to make organs. Multiple organs that perform related functions form organ systems, and all of the body’s organ systems together make up a complete organism—the human body.

(See the figure below for a visual of how each level builds upon the previous one.)

 From Atoms to Cells: Building the Human Body

All matter—everything that takes up space—is made up of one or more elements, such as hydrogen, oxygen, carbon, and nitrogen. The smallest particle of an element is an atom. When atoms join together, they form molecules, and when molecules connect, they create larger macromolecules.

Three key types of macromolecules—carbohydrates, lipids, and proteins—make up most of the structures and functions of our cells. They are also the main nutrients found in the foods we eat.

To understand how nutrients work in the body, it helps to first understand their chemical makeup. Nutrients are chemical molecules in food that the body needs for growth, energy, and overall health.

On this page, we’ll take a quick look at how atoms bond to form molecules. Then, we’ll explore how these molecules come together to build the structures of the human body.

 Atoms, Elements, and MOlecules in the Human Body

You may remember from science class that matter is anything that has mass and takes up space. Everything—living or nonliving—is made of matter.

The smallest unit of matter is an atom. The chair you’re sitting on, the food you ate for breakfast, and even the air you breathe are all made of atoms. An atom is the smallest particle of an element, much like a single blade is the smallest part of grass.

An element is made up of only one kind of atom. There are more than 100 known elements in the universe, but just a few—hydrogen, carbon, nitrogen, and oxygen—make up most of all living things.

Many of these elements are also found in the foods we eat. You can find all elements listed on the Periodic Table of Elements, shown in Figure 3.3.

Atoms are unimaginably small. Even a single microscopic cell contains trillions—sometimes hundreds of trillions—of atoms. Atoms combine to form larger, more complex structures called molecules. A molecule is made of two or more atoms chemically bonded together.

Macromolecules: Building Blocks of the Human Body

Biological macromolecules are large, complex compounds formed when atoms and smaller molecules bond together in unique ways. These macromolecules serve as the raw materials that build and maintain all living organisms.

Three major types of macromolecules are carbohydrates, proteins, and lipids. Each plays an important role in the body’s structure and function.

Later in this unit, you’ll learn how the macromolecules in food are digested (broken down) into smaller molecules that the body can use. These small molecules then become the building blocks for muscles, organs, and other body tissues. (We’ll explore each group—carbohydrates, proteins, and lipids—in more detail in upcoming units.)

From One Cell to a Complex Body

Cells are the most basic units of life, and all living things—plants, animals, and humans—are made up of them. New cells arise from preexisting cells through cell division.

You began as two separate cells—a sperm cell from your father and an egg cell from your mother—that joined together inside your mother’s womb. Each of these cells contained genetic information (DNA), and when they fused, they formed a single new cell called a zygote.

This single cell then divided many times and developed into specialized cells that formed your body’s organs, systems, blood, bones, tissues, and skin. (We’ll learn more about this in the unit on pregnancy.)

As a teenager or adult, your body is made up of trillions of cells. There are hundreds of different types, including red blood cells, nerve cells, and skin cells. Each cell carries out the basic functions necessary for life: taking in nutrients, removing wastes, sensing and responding to the environment, moving, growing, and reproducing.

Many cells have short lifespans and are constantly replaced. For example, cells lining the intestines are replaced every 2–4 days, while skin cells are replaced every few weeks.

Although the cell is considered the most basic unit of life, it is highly complex (see Figure 3.5). A typical human cell has a flexible outer layer called the cell membrane (or plasma membrane), which surrounds the cytoplasm—a gel-like fluid that holds the cell’s internal structures. Within the cytoplasm are many organelles—tiny “organs” made from macromolecules—that perform specific functions to keep the cell alive.

A typical animal cell contains the following organelles:

• Nucleus: Contains the cell’s genetic material (DNA) and controls cell activities.

• Mitochondria: Known as the powerhouse of the cell; it produces usable energy from nutrients.

• Ribosomes: Build proteins based on instructions from DNA.

• Endoplasmic reticulum (ER): Processes and packages proteins and lipids.

• Golgi apparatus (Golgi body): Modifies and distributes macromolecules like proteins and lipids.

• Lysosomes: Act as the cell’s recycling and cleanup centers; they digest macromolecules and destroy harmful invaders.

Together, these organelles keep the cell functioning smoothly—allowing your body to grow, repair itself, and stay alive.

From Tissues to Organ Systems: Building Body Structure

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 and the central nervous systems.

The human body comprises eleven distinct organ systems (Figure 3.6). Assigning organs to organ systems can be imprecise, since organs that “belong” to one system can also have functions integral to another. In fact, many organs contribute to more than one system. And most of these organ systems are involved in some way in nutrition-related functions within the body. (Table 3.1). The four 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 digestive system breaks down food into smaller molecules, allowing nutrients to be absorbed into the bloodstream and used by the body.
   3. The endocrine system produces hormones, many of which are involved in regulating appetite, digestive processes, and nutrient levels in the blood.
   4. The reproductive system plays a role in providing nutrition to a developing fetus or growing baby.
      

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