The Levels of Organization
in animal complexity
1. Protoplasmic level of organsiation
Protozoans carry out all the metabolic
functions of animals.
Organelles, or intracellular structures,
carry out a variety of functions, such as
digestion, excretion, respiration, and
coordination of movement;
A single cell organism is said to be
operating at the subcellular level of
organisation, i.e. specialisation occurs
within the confines of one cell
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 5 levels of
organization of complexity ranging from
simplest to most complex:
1.
2.
3.
4.
5.
Cellular level of organisation
Tissue level of organisation
Organ level of organisation
System level of organisation
Organ-system level of organisation
LEVEL 1 - The cellular level of organisation
Cells are the basic unit of structure and function in
living things and may serve a specific function within
the organism.
examples Blood cells: Red blood cells (RBC) are responsible
for carrying oxygen around the body and the white
blood cells (WBC) are responsible for fighting against
pathogens. They engulf pathogens.
Nerve cells, bone cells
One multicellular organism still operating at the cellular
level of organisation is the Phylum Porifera,
subkingdom Parazoa
The Phylum Porifera (The Parazoa)
The sponges represent the lineage closest to the
multicellular organisms (protists) that gave rise to
the animal kingdom.
Members of the Phylum Porifera are considered to
be just a loose aggregation of cells.
The cell layers of the sponges are loose association
of cells, not really tissues because the cells are
relatively unspecialized.
The Sponges are sessile (permanently
attached, hence non-motile) sack-like
animals of the Phylum Porifera (full of
pores).
Sponges are the simplest of the
multicellular animals.
They have no organ systems and are
characterized by numerous canals and
chambers that open to the outside by way
of pores.
Many sponges are asymmetrical, but some
exhibit radial symmetry.
Sponges are filter feeders.
Water (carrying suspended plankton and other
potential food) enters numerous small pores called
ostia (singular, ostium).
The ostia are surrounded by donut-shaped cells called
porocytes that open and close to control water flow.
Water flows directly into an open chamber called the
spongocoel (the term "coel" refers to an open space
or body cavity in an animal).
Water leaves the spongocoel by a larger opening (the
osculum).
The interior of the spongocoel is lined with flagellated
cells called choanocytes (or collar cells).
The choanocytes have a tubular collar facing the
spongocoel.
A flagellum extends from the center of this collar, the
movement of which creates currents that force water
through the sponge's "plumbing system".
Suspended food particles (plankton, larvae, etc) in the
water are drawn through the collar from below,
trapped on the outside of the collar, and then
phagocytized.
The collar cells do not digest the captured food by
themselves; instead it is passed to a second cell (an
amoebocyte) waiting in the mesohyl (the acellular
portion of the sponge).
Amoebocytes carry the food to other cells that require
nutrition and thus partly compensate for the sponge's
lack of a circulatory system.
Amoebocytes carry on other jobs.
They can, for example, undergo developmental
changes to turn into any other cell type that may be
required.
This allows for growth, repair, and reproduction of the
sponge.
In addition, they are responsible for producing the
sponge's skeleton (a network of fibers flexible protein
(spongin) and needle-like spicules).
Spicules are usually made of calcium carbonate or
oxides of silicon and the shape of the spicules is
important in classification of the sponges.
Although sponges are classified as multicellular
animals, they demonstrate less integration and
specialisation of function than other animal groups.
They lack tissues organisation, and their cells are the
primary units of structure and function.
The body is two layered and separated by a gelatinous
mesohyl.
LEVEL 2 - Tissue level of organisation
The tissue level of organisation is a much higher level
of organisation than the cell level of organisation.
Tissues are made up of cells that are similar in
structure and function and which work together to
perform a specific activity. Examples - blood, nervous,
bone, etc.
Humans have 4 basic tissues: connective, epithelial,
muscle, and nerve.
There are some multicellular animals that have
attained this level of organisation as their highest
operating level. e.g. the Radiata.
Radiata are group of animals which are characterised
by radial symmetry.
Above the level of sponges, animals can be broadly
divided into two classifications on the basis of body
symmetry: radial and bilateral.
The radially symmetrical Phyla, the Radiata, possess
a central mouth around which the rest of the organs
are radially arranged.
Radiata consist of two phyla Cnidaria (Coelenterates)
and Ctenophora (comb jellies).
Each of these phyla consists of individuals with two
layers of cells, radial symmetry, and an inner
gastrovascular cavity (coelenteron).
The gastrovascular cavity serves as a digestive
structure.
Food is taken in through the mouth, and what is
undigested passes out again through the mouth.
Whereas sponges are filter feeders, the coelenterates
and comb jellies can ingest large food particles and
pass the digested nutrients throughout the body.
The Radiata display a tissue level of organization.
Specialized nerve cells make up a nerve net (nerve
tissue) which permeates the entire body of the
organism and is responsible for sending nerve
impulses all over the body.
Epitheliomuscular cells make up a contractile
tissue (epidermal tissue), which is derived from the
outer ectodermal layer of cells.
Epithelionutrive cells make a gastrodermal tissue
specialised in digestion.
LEVEL 3 – The organ level of organisation
Organs are made up of tissues that work together to
perform a specific activity.
Examples - heart, brain, liver, pancreas etc.
The rest of the animals except the sponges (operating
@ cellular level of organisation) and Radiata
(cnidarians and ctenophores -operating @ the tissue
level) have three germ layers; ectoderm, mesoderm
and endoderm and they have attained the organ level
of organization.
An organ consists of 2 or more tissues that perform a
particular function (e.g., heart, liver, stomach, etc).
Animals with three germ layers are said to be
triploblastic and all triploblastic animals operate at the
organ level of organization.
The simplest triploblastic animal is a flatworm, which
belong to the Phylum Platyhelminthes.
The Phylum Platyhelminthes contains animals that are
multicellular, bilaterally symmetrical, have distinct
tissues, and simple organs for digestion, excretion, and
reproduction.
Flatworms as the name imply are flat.
They exemplify an organ level of complexity compared
with the tissue level shown by the radiata.
An example of a flatworm is Planaria.
There are four classes of flatworms; turbelaria,
Trematodes, Monogenea and Cestodes.
Only Class Turbelaria is free living and the remaining
three are parasitic.
The cnidarians are animals organized mostly on the
tissue level.
From planarians to humans, animals are constructed
on a more complex level of organization.
Not only do cells work together to form tissues, but
tissues of various kinds are closely associated to form
one structure, called an organ, adapted for the
performance of some one function.
E.g. human stomach is an organ composed of
epithelial tissue, connective tissue; muscle tissue and
nervous tissue.
The epithelial tissue layer lines the cavity and contains
the gland cell that secretes gastric juices;
the muscle tissue layers produce the stomach
contractions;
the nervous tissue coordinates muscle contractions
and the connective tissue binds various layers
together.
LEVEL 4 - Organ-system level of organisation)
All triploblastic animals operate at the system level of
organisation.
Organs with a common function associate and they form
the organ-system.
Organs are groups of two or more tissues that work
together to perform a specific function for the organism.
Examples - circulatory system, nervous system, skeletal
system, etc.
The Human body has 11 organ systems - circulatory,
digestive, endocrine, excretory (urinary), immune
(lymphatic), integumentary, muscular, nervous,
reproductive, respiratory, and skeletal.
An organ usually cooperates with other organs in
performance of some life activity and such a group of
structure devoted to one activity is termed an organsystem.
Thus the stomach is part of the digestive system; and
all other parts such as the eosophagus, liver, pancreas
and intestine are necessary for proper performance of
digestion.
Flatworms have the simplest version of these and are
the simplest phylum of animal built on the organsystem level of construction