Levels of Organization
M Hanafi Said
In unicellular (single-celled) organisms, the single cell performs all life functions. It functions independently. However, multicellular (many celled) organisms have various levels of organization within them. Individual cells may perform specific functions and also work together for the good of the entire organism. The cells become dependent on one another.
Multicellular organisms have the following 5 levels of organization ranging from simplest to most complex:
Cell
a. Are the basic unit of structure and function in living things and
the smallest living units of structure and function are cells.
the smallest living units of structure and function are cells.
b. May serve a specific function within the organism
Examples- blood cells, nerve cells, bone cells, etc.
Nerve Cells
Blood Cells
c.epithelial tissue - cover or line body surfaces; some are capable of producing secretions with specific functions. the outer layer of the skin and sweat glands are examples of epithelial tissue.
d. connective tissue - connects and supports parts of the body; some transport or store materials. blood, bone, and adipose tissue (fat) are examples e. muscle tissue - specialized for contraction, which brings about movement. our skeleton muscles and the heart are examples
f. nerve tissue - specialized to generate and transmit electrochemical impulses that regulate body functions. the brain and optic nerves are examples. 
Tissues
a. Made up of cells that are similar in structure and function and which work together to perform a specific activity
b. Examples - blood, nervous, bone, etc. Humans have 4 basic tissues: connective, epithelial, muscle, and nerve.
Organs
Made up of tissues that work together to perform a specific activity
Examples - heart, brain, skin, etc.
Heart
Skin
Lungs
Brain
Organ Systems
a. Groups of two or more tissues that work together to perform a specific function for the organism
b. Examples - circulatory system, nervous system, skeletal system, etc
c. The Human body has 11 organ systems - circulatory, digestive, endocrine, excretory (urinary), immune(lymphatic), integumentary, muscular, nervous, reproductive, respiratory, and skeletal.
circulatory
digestive
endocrine
excretory (urinary)
immune(lymphatic)
muscular
Organisations
a. Entire living things that can carry out all basic life processes. Meaning they can take in materials, release energy from food, release wastes, grow, respond to the environment, and reproduce
b. Usually made up of organ systems, but an organism may be made up of only one cell
such as bacteria or protist.
Examples - bacteria, amoeba, mushroom, sunflower, human cells -- tissues -- organs -- organ systems -- organism
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