The Organization of
Living Things
Chapter 4
Section 3
Key Concept
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As multicellular
organisms develop,
their cells differentiate
(change & separate)
and form levels of
organization
Why it Matters: so
Humans (we are
multicellular) can have
different kinds of cells,
tissues, organs, and
organ systems
Unicellular Organisms
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“uni-” = one
Prokaryotes are unicellular organisms
Some algae, some protists, and some
eukaryotes (yeasts), are unicellular
Can still do everything they need to stay alive
Benefits over multicellular organisms:
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Need fewer resources
Can live in harsher conditions
Multicellular Organisms
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“multi-” = more than one
Plants, animals, some protists, and most fungi
are multicellular
Start as a single cell  many cells  cells
differentiate (change) into different types of
cells  cells group together
Multicellular Characteristics
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Larger size = have less predators and have more
options of things to eat
Longer life – organism will continue to live even if
a single cell dies
Specialization – each type of cell has a specific
job, making the organism more efficient
Levels of Organization
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1st Level: Cells
2nd Level: Tissues
3rd Level: Organs
4th Level: Organ Systems
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Cells  Tissues  Organs  Organ Systems
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Level 1 : Cells
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Cells can be specialized (have
a certain function)
Function = job
Function is related to the cell
structure
Structure = how parts of the
cell are put together
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Shape
Material it’s made from
Structure of a brain cell is
different from muscle cell
Level 2: Tissues
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Tissue = group of cells that work together to do a
specific job
Ex: Heart muscle tissue
is made of heart muscle
cells
Animals have 4 types of
tissue: nerve, muscle,
connective, protective
Plants have 3 types of
tissue : transport, protective, ground
Level 3: Organs
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Organ = structure that is made up of 2 or more
tissues working together to get a specific job done
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Ex: Stomach – muscle tissue moves food, special
tissues make chemicals to digest food, connective
tissue holds stomach together, nervous tissue
sends messages back and forth between the
stomach and brain
Level 4: Organ Systems
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Organ system = group of organs working
together to perform a specific function
Each organ system has a specific job
Ex: Digestive system is made of several
organs including the stomach and intestines
Structure of Animals
Cells
-basic unit of an animal’s structure
-become specialized
_______________________________
Tissues
-made of cells that work together to perform a specific function
__________________________________________
Organs
-made of different types of tissues that work together to perform a specific function
____________________________________________________
Organ Systems
-made of organs that work together to perform a specific function
_____________________________________________________________
Organisms