Levels of organization
In unicellular organisms, all the life functions are performed by the single cell independently. On
the contrary, however the multicellular organisms have various levels of organization within
them. The multicellular organisms have the following 5 levels of organization ranging from
simplest to most complex: cells --> tissues --> organs --> organ systems --> organisms
LEVEL 1 - Cells
Cells are the basic unit of structure and and various functions in the living organisms. Specific
functions are performed by the different cells. Examples: blood cells, nerve cells, bone cells, etc.
LEVEL 2 - Tissues
The cells similar in structure and fucntion combine to make tissues. The cells in the tissues work
together to execute explicit activities. The basic tissues in human body are classified into 4 broad
catagories: connective tissue, epithelial tissue, muscle tissue, and nervous tissue.
LEVEL 3 - Organs
The tissues aggregate to form various organs. Examples: liver, kidneys, eyes, etc.
LEVEL 4 - Organ Systems
Two or more tissues that work together to perform a specific function for the organism comprise
the organ systems. Examples: circulatory system, excretory system, endocrine system, skeletal
system, etc. There are eleven organ systems in the Human body. These are excretory (urinary),
circulatory, digestive, respiratory, endocrine, immune (lymphatic), muscular, nervous,
reproductive, integumentary and skeletal.
LEVEL 5 - Organisms
These are the highest level of organization. These are the complex aggregation of the various
organ systems that interact with each other to carry out all basic life processes. This means that
the organism can ingest materials, release energy from food, excrete wastes, grow and develope,
respond to the stimuli and reproduce. An organism may be made up of only one cell such as
bacteria or protest or multiple cells. Examples: bacteria, amoeba etc.
Cell as the basic unit of life
Cells are considered as the elementary organizational and functional units of living organisms.
The Cell theory was formulated by M J Schleiden in 1838 and Theodore Schwann in 1839. The
principles of cell theory are:
All the living organisms are composed of cells.
The living organisms could be made up of one or more cells.
The new cells arise from pre-existing cells through the process of cell division.
The cells in different organisms vary in the number, shape and size.
The organisms made up of a single cell are called unicellular organisms. These are capable of
independent existence. The single cell carries all the functions like digestion, excretion,
respiration, growth and reproduction. Example: Euglena, Amoeba, Paramecium etc. The
organisms made up of more than one cell are called multicellular organisms. In multicellular
organisms the cells vary in their shape (could be polyhedral, oval, discoidal, cylindrical, spindle
shaped) and size depending on their function. The shape of the cells is decided by the function
they perform.
Parenchyma that perform storage function have Polyhedral shape
Sclerenchyma cells that provide mechanical support have Spindle shape
White blood cells that defend the body against pathogens are Amoeboid shaped
Nerve cells – Long and branched that conduct nerve impulses
Muscle cells – cylindrical or spindle shaped cells concerned with the movement of body
parts
The cell were discovered by English scientist Robert Hooke in the year 1665. He observed
honeycomb like dead cells in a thin slice of cork under microscope. The term ‘cell ‘, which
means a small room or compartment was coined by him.
Anton Von Leeuwenhoek in the year 1667 first saw the living cell
Matthias J Schleiden who was a German botanist based on his studies in different plant
cells in 1838.
Theodore Schwann who was a British zoologist in 1839 based on his studies on different
animal cells.
Staining Technique: It is a method of using dyes like saffranine, eosin, fast green,
methyleneblue etc. to colour the parts of cells.
The size of the cell varies from few micrometers (µm) to few centimeters (cm).
The size of bacteria varies from 0.1 to 0.5 µm.
The smallest cell PPLO (Pleuro pneumonia like organism) is about 0.1 µ in diameter.
The largest cell is an ostrich egg that measures 170 to 180 mm in diameter. Some
Sclerenchyma fibres measure up to 60 cm in length. However the average size of the cell
ranges from 0.5 to 10 µm in diameter.
EUKARYOTIC AND PROKAROTIC CELLS
Prokaryotes (Greek word pro- before and karyon nut or kernel, referring to the cell nucleus).
The organisms without a cell nucleus (karyon) and membrane-bound organelles.
Most are unicellular (just a few prokaryotes are multicellular).
Eukaryotes (Greek word eu- true and karyon nut or kernel, referring to the cell nucleus)
The organisms with well defined nucleus.
The cells are organized into membrane bound complex structures called organelles.
Majority of eukaryots are multicellular.
Features Eukaryotic Cell Prokaryotic Cell
Nucleus Present Absent
Cell Type Usually multicellular Usually
True Membrane bound Present Absent
Nucleus
Example Animals and Plants Bacteria and Archaea
Microtubules Present Absent or rare
Endoplasmic reticulum Present Absent
Mitochondria Present Absent
Cytoskeleton Present May be absent
Ribosomes larger smaller
Golgi apparatus Present Absent
Chloroplasts Present (in plants) Absent; chlorophyll scattered in
the cytoplasm
Permeability of Nuclear Selective not present
Membrane
Plasma membrane with Yes Usually no
steroid
Cell wall Only in plant cells and fungi Usually chemically complexed
(chemically simpler)
Cell size 10-100um 1-10um
UNICELLULAR AND MULTICELLULAR ORGANISMS
The unicellular organisms are made of a single cell while Multicellular organisms are
made of multiple cells.
Levels of organization are found in multicellular organisms. In unicellular organism the
cellular functioning is distributed at the organelle level, whereas in Multicellular it
involves cellular, tissue, organ and organ system levels.
The life of a unicellular organism depends on single cell, however in Multicellular
organism different cells perform discrete cellular functions.
A unicellular organism is exposed to the environment while in Multicellular organism the
outer cells/ epithelial cells face the environment and the inner cells are dedicated to do
the certain basic cell function.
Unicellular organism die with the death of the cell. In multicellular organisms, the cell
death and regeneration is a common phenomenon.
