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Lesson 10.3 Levels of Organization
Life's Organization
All matter is made of atoms. Atoms combine and form molecules.
Molecules make up cells. A large animal, such as a Komodo dragon,
is not made of one cell. Instead, it is made of trillions of cells
working together. The skin of the Komodo dragon, shown in Figure 1
in your text, is made of many cells that are specialized for protection.
The Komodo dragon has other types of cells, such as blood cells and
nerve cells, which perform other functions. Cells work together in
the Komodo dragon and enable the whole organism to function.
This is the same way that cells work together in you and in other
multicellular organisms.
Copyright © McGraw-Hill Education
Recall that some organisms are made of only one cell. These
unicellular organisms carry out all the activities necessary to
survive, such as absorbing nutrients and getting rid of wastes. No
matter their sizes, all organisms are made of cells.
Lesson 10.3
Goes with Student Page 418
Unicellular Organisms
Unicellular organisms have only one cell. These organisms do all
the things needed for their survival within that one cell. The
amoeba in Figure 2 in your text is a unicellular organism. It takes in,
or ingests, other unicellular organisms for food to get energy.
Unicellular organisms also respond to their environment, get rid of
waste, grow, and reproduce. Unicellular organisms include both
prokaryotes and some eukaryotes.
A cell without a membrane-bound nucleus is a prokaryotic cell.
In general, prokaryotic cells are smaller than eukaryotic cells. As
shown below on the left, prokaryotic cells also have fewer cell
structures. A unicellular organism made of one prokaryotic cell is
called a prokaryote. Some prokaryotes live in groups called colonies.
Some can also live in extreme environments, as shown in Figure 2 in
your text. The heat-loving bacteria that live in hot springs get their
energy from sulfur instead of light.
A eukaryotic cell has a nucleus surrounded by a membrane and
many specialized organelles. The amoeba in Figure 2 in your text
has an organelle called a contractile vacuole. The contractile vacuole
collects extra water from the amoeba’s cytoplasm and pumps it out.
The contractile vacuole keeps the amoeba from swelling and
Copyright © McGraw-Hill Education
A unicellular organism that is made of one eukaryotic cell is called
a eukaryote. There are thousands of different unicellular eukaryotes.
The alga that grows on the inside of an aquarium and the fungus that
causes athlete’s foot are unicellular eukaryotes.
Lesson 10.3
Goes with Student Page 419
Multicellular Organisms
A multicellular organism is made of many eukaryotic cells working
together. Each type of cell in a multicellular organism has a specific
job that is important to the survival of the organism.
Cell Differentiation
Remember that all cells in a multicellular organism come from
one cell, a fertilized egg. Cell division starts quickly after fertilization.
The first cells made can become any type of cell, such as a muscle
cell, a nerve cell, or a blood cell. The process by which cells become
different types of cells is called cell differentiation (dihf uh ren shee
AY shun).
Copyright © McGraw-Hill Education
A cell’s instructions are contained in its chromosomes. Nearly all
the cells in an organism have identical sets of chromosomes. If an
organism’s cells have identical sets of instructions, how can the
cells be different? Different cell types use different parts of the
instructions on the chromosomes. A few of the many different types
of cells that can result from cell differentiation are shown in Figure 3
in your text.
Lesson 10.3
Goes with Student Page 421
Animal Stem Cells Not all cells in a developing animal differentiate.
Stem cells are unspecified cells that are able to develop into many
different cell types. There are many stem cells in embryos but fewer
in adult organisms. Adult stem cells are important for cell repair and
replacement. For example, stem cells in your blood marrow can
produce more than a dozen different types of blood cells. These
replace the cells that are damaged or worn out. Stem cells in your
muscles can produce new muscle cells. These can replace torn
muscle fibers.
Copyright © McGraw-Hill Education
Plant Cells Plants also have unspecialized cells, similar to the
stem cells of animals. These cells are grouped in areas called
meristems (MER uh stemz). Meristems are in different areas of
a plant, including the tips of roots and stems. Cell division in
meristems produces different types of plant cells with specialized
structures and functions. These functions include transporting
materials, making and storing food, or protecting the plant.
Meristem cells might become part of stems, leaves, flowers,
or roots.
Lesson 10.3
Goes with Student Page 422
In multicellular organisms, similar types of cells are organized
into groups. Tissues are groups of similar types of cells that work
together to carry out specific tasks. Most animals, including humans,
have four main types of tissues. These are muscle tissue, connective
tissue, nervous tissue, and epithelial (eh puh THEE lee ul) tissue.
Muscle tissue, shown in Figure 4 in your text, makes movement
possible. Connective tissue provides structure and support. Nervous
tissue carries messages to and from the brain. Epithelial tissue forms
the protective outer layer of skin and the lining of major organs and
internal body cavities.
Copyright © McGraw-Hill Education
Plants also have different types of tissues. The three main types
of plant tissue are dermal tissue, vascular (VAS kyuh lur) tissue,
and ground tissue. Dermal tissue provides protection and helps
reduce water loss. Vascular tissue, shown in Figure 4 in your text,
transports water and nutrients from one part of a plant to another.
Ground tissue provides storage and support. Photosynthesis takes
place in ground tissue.
Lesson 10.3
Goes with Student Page 423
Complex jobs in organisms require more than one type of tissue.
Organs are groups of different tissues working together to perform a
particular job. Your stomach is an organ that breaks down food. It is
made of all four types of tissue: muscle, epithelial, nervous, and
connective. Each type of tissue performs a specific function
necessary for the stomach to work properly and break down food.
Muscle tissue contracts and breaks up food. Epithelial tissue lines the
stomach. Nervous tissue signals when the stomach is full. Connective
tissue supports the stomach wall.
Copyright © McGraw-Hill Education
Plants also have organs. The leaves shown in Figure 5 in your text
are specialized for photosynthesis. Each leaf is made of dermal
tissue, ground tissue, and vascular tissue. Dermal tissue covers the
outer surface of a leaf. The leaf is an important organ because it
contains ground tissue that produces food for the rest of the plant.
Ground tissue is where photosynthesis takes place. The ground tissue
is tightly packed on the top half of the leaf. The vascular tissue moves
both the food produced by photosynthesis and water throughout the
leaf and plant.
Lesson 10.3
Goes with Student Page 424
Organ Systems
Most organs do not function alone. Instead, organ systems are
groups of different organs that work together to complete a series of
tasks. Human organ systems can be made of many different organs
working together. For example, the digestive system is made of the
stomach, the small intestine, the liver, and the large intestine. These
organs all work together to break down food. Blood absorbs and
transports nutrients from food to cells throughout the body.
Copyright © McGraw-Hill Education
Plants have two main organ systems—the shoot system and the
root system. The shoot system includes leaves, stems, and flowers.
The shoot system transports food and water throughout the plant.
The root system anchors the plant and takes in water and nutrients.
Lesson 10.3
Goes with Student Page 425
Multicellular organisms usually have many organ systems. The
cells of these systems work together and carry out all the jobs
needed for the organism to survive. For example, the cells in the
leaves and the stems of a plant need water to live. They cannot
absorb water directly. Water enters through the roots and is carried
through the stem to the leaves by the transport system.
Copyright © McGraw-Hill Education
There are many organ systems in the human body. Each organ
system depends on the others and cannot work alone. For example,
the respiratory system and circulatory system carry oxygen to the
cells of the muscle tissue of the stomach. The oxygen aids in the
survival of muscle tissue cells. Figure 6 in your text will help you
review how organisms are organized.
Lesson 10.3
Goes with Student Page 426