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28.1 Levels of Organization
Key Concept The human body has five levels of organization.
Humans, like plants and other animals, are multicellular organisms. This
means our bodies are made up of many cells. However, these cells are
not all the same. Human bodies include many different types of cells.
Each type of cell has a specialized function*. For example, red blood cells
deliver oxygen to all the parts of the body. Muscle cells contract to make
it possible for the body to move. Neurons transmit and receive messages
from other neurons. The body needs all these types of cells to survive.
Specialized cells develop from a single zygote.
Before a human being develops, an egg and sperm unite to form a
zygote. A zygote is a fertilized egg. It is made of just one cell. Then the
zygote begins to divide, and the cells that it forms also divide. This
process continues for a few weeks. The cells that form during this time
are called embryonic stem cells. At this point these cells are all the same.
Every embryonic stem cell contains all the information it needs to
become any kind of specialized cell in the body.
sperm cell (1n)
egg cell (1n)
Fertilization
zygote (2n)
Division
Embryonic stem cells all look alike. Later they can become blood cells, muscle cells, neurons,
skin cells, or any other kind of specialized cell.
Determination
A few weeks after the zygote forms, a process called determination
begins. In this process, each stem cell commits* to becoming one type of
cell, such as a muscle cell or a neuron. Each stem cell still contains all the
genetic information it would need to become any type of specialized cell.
However, the stem cells no longer use all that information. Now, they
will become only one type of specialized cell. For example, a cell that is
going to become a neuron cannot become a blood cell later.
* Academic Vocabulary
function action, role
commit to follow a course of action
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Differentiation
Once a cell has committed to becoming a
particular specialized cell, it must develop
into that cell. This process is called
differentiation, because it is the process
in which cells develop their specialized
shapes and functions. The diagram on
the right shows some cells that have gone
through differentiation.
sperm cells
bone cells
Different types of
cells need different
structures. Sperm
cells need whiplike
tails to help them
swim. Other cells,
such as those in
the stomach, skin,
and bones, do not
need to move.
zygote
epithelial
cells
connective
cells
What is the difference between
determination and differentiation?
Specialized cells function
together in tissues, organs,
organ systems, and the whole
organism.
If you think about your body, you cannot help
but notice that there is more to you than individual cells. You also have tissues and organs. In
fact, there are five levels of organization in a
human body.
1.Cells Each type of specialized cell has its
own job to do. Blood cells carry oxygen to
all the parts of the body. Some lung cells
make mucus that traps particles and protects the lungs from bacteria and viruses.
2.Tissues Groups of cells working together
are called tissues. There are four types of
tissues: epithelial (skin and lining of many
organs); connective (ligaments and tendons);
muscle; and nervous tissues.
Your digestive
system is an
organ system.
It includes
your mouth,
esophagus,
stomach and
intestines.
mouth
esophagus
stomach
intestines
Interactive Reader
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3.Organs Different types of tissue that work together form an organ.
Your heart, lungs, liver, and stomach are only a few of the organs in
your body.
4.Organ systems Two or more organs working together form an
organ system. For example, one organ system allows you to breathe.
This organ system includes your lungs, sinuses, nasal passages,
pharynx, and larynx.
5.Organism Together, all of the organ systems make up an organism.
You are an organism.
Which of the following is a cell, a tissue, an organ, or an organ
system? sperm, heart and blood vessels, liver, lining of the
stomach
28.1 Vocabulary Check
determination
differentiation
tissue
organ
organ system
Mark It Up
Go back and highlight
each sentence that
has a vocabulary
word in bold.
Fill in the blanks with the correct term from the list above.
1. A(An)
work together.
is made up of different types of tissue that
2. The process through which embryonic stem cells commit to becoming
only one type of specialized cell is called
.
3. Groups of cells that work together to perform a specialized function
.
are called
4.
is the process by which committed cells develop
their specialized shapes and functions.
5. Together, your lungs, nasal passages, larynx, pharynx, and sinuses form
a(an)
.
28.1 The Big Picture
6. Put the following five terms in order from simplest to most complex:
tissue, organ system, organ, cell, organism.
7. How do specialized cells differ from each other?
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