Nutrition Science and Everyday Application - beta v 0.1

ORGANIZATION OF LIFE 129
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