Lesson 3: Cell Differentiation and Specialization (1.1.3)
CELL DIFFERENTIATION AND CELL HEIRACRY
During fertilization, sex cells called gametes fuse to form a new cell the zygote. The
zygote starts out as one cell with half set of
chromosomes from each parent to complete the full
set. The zygote then begins the process of mitosis to
grow in size, becoming an embryo.
The groups of cells produced in the very early stages
of the embryo’s growth are similar to the original
zygote. They are called embryonic stem cells.
Eventually, when the embryo reaches 20-150 cells in
size, this group begins to produce cells that are
different from themselves. This process is called cell
differentiation.
How do cells become different? You learned in middle school that all cells have the
same DNA; however, not all genes that represent specific segments of DNA are activated
in each cell. Cells become different because various segments of DNA, called genes, are
activated or inactivated during embryonic development. For example, liver cells have
genes that are activated to produce bile and cells of the pancreas have a different set of
activated genes that produce insulin. Cells become differentiated and therefore
specialized for a particular job and later form tissues. As cells differentiate, they produce
proteins that are characteristic of the cell’s shape and specific function.
STEM CELLS
Stem cells have the capability to become any type of cell. This is possible because genes
within the cell can be “turned on” or “turned off” at specific times. Every cell of the
organism has the same genetic information that was present in the initial zygote. Thus,
cell differentiation occurs by the selective activation or inactivation of only some of these
genes. For example, some cells could become liver cells while other cells become skin
cells, but both of these cell types contain genes for every other cell type within the
organism.
Some organisms are unicellular meaning they
have one cell. For, example, bacteria and
amoebas are unicellular. These single cells
carry out all life functions. See Table 26.1.
Multicellular organisms are composed of
many cells that work together to carry out life
processes. In multicellular organisms, the
cells group together and divide the labor.
Tissues are groups of similar cells that
perform the same function. Several types of tissues group together to perform particular
functions and are called organs.
An organ system is a group of organs working together for a particular function. The
organ systems of multicellular organisms work together to carry out the life processes of
the organism and each system perform a specific function. The organelles in a cell work
like the organ systems of multicellular organisms.
To recap, individual cells make up tissues. Tissues make up organs and organs make up
organ systems.
CELL COMMUNICATION
Cells are continuously sending messages and molecules to one another in order for the
organism to survive in a process called cell communication. Recall that cells contain a
plasma membrane made of a phospholipid bilayer. Each phospholipid layer consists of
phosphate groups (phosphorous bonded with oxygen) attached to two fatty acid (lipid)
tails. The layers arrange
themselves so that they
phosphate heads are on the
outer edges of the membrane,
and the fatty acid tails
compose the interior of the
membrane. Embedded in the
bilayer are globular proteins.
These proteins are used for
various functions, such as
transporting substance through
the membrane and
communication between cells.
The phospholipids are free to
move around, allowing the
membrane to stretch and
change shape. Receptor proteins, found in the phospholipid bilayer and cytoplasm, are
the proteins responsible for communication. A signal molecule, such as a hormone, binds
to the receptor protein and the cell is able to “read” to communication signal and respond
accordingly. Because receptor proteins are very specific for types of signals, cells have
many different types of receptor proteins. Figure 6.8 shows a phospholipid bilayer with
proteins embedded in it.
Blood sugar levels, for example, are regulated by
hormones. When blood sugar in an organism is
low, the hormone glucagon is released and tells the
pancreas to break down glycogen into individual
glucose molecules. If blood sugar levels are too
high, the hormone insulin is released telling the
pancreas to store glucose as glycogen.
Communication between cells is necessary in order
to maintain a healthy blood sugar level.
Chemical signals are chemicals used to transmit
information to cells for example neurotransmitters
and hormones. Have you ever touched a hot stove
and your hand jerked back immediately? Your cells were communicating with each
other to keep you safe and from burning yourself badly enough that it could be harmful to
your health. How does this work? The nerve cells (see Figure 6.9) send an electrical
signal that stimulates the release of chemical signals to convey a special message to cells
telling the brain you are touching something hot. The brain sent a chemical signal to
your muscle cells causing them to jerk and move your hand off the stove. All of this
occurs in less than 1 second! Cell communication in an organism never stops, because it
is necessary to keep that organism alive.
Lessson 3 Review: Cell Differentiation and Cell Communication
A. Define the following:
unicellular
tissue
receptor protein
organ system
multicellular
eukaryotic
cell differentiation
chemical signal
organs
cell communication
neurotransmitter
B. Select the best answer
1. A student accidentally places her hand on a hot beaker and quickly pulls her hand
away.
What would enable her to jerk away from the hot surface?
A. a chemical signal and nerve cell
B. a blood cell and chemical signal
C. a stem cell and neurotransmitters
D. organ and chemical stimulator
2. Which is the correct order for the organization of living things into a hierarchy system?
A. tissuescellsorgan system organsorganism
B. organ systemcellstissuesorgansorganism
C. organcellstissueorgan systems organism
D. cellstissuesorgan  organs systemsorganism
3. Which would be considered an undifferentiated cell?
A. nerve cell
C. blood cell
C. skin cell
D. stem cell
4. Which is a result of cells communication with other cells?
A. pancreas cells release insulin when blood sugar gets too high
B. blood cells release water because they are placed in salt water
C. a person sweats and fans to cool off
D. skin cells peal as a result of a sun-burn
5. Which is an example of a chemical signal?
A. electric charges within cell
C. protein hormones such as glucose
B. water found within cells
D. protein receptors on the cell surface
C. Complete the following activities.
1. List the hierarchy of living things in order from smallest to greatest.
2. Distinguish between the terms unicellular and multicellular.
3. Describe cell differentiation and how it is controlled?