The term cell was coined by Robert Hooke in 1665. In 1838
matthias schleiden, German botanist proposed the idea that all plants consists
of cells. In 1839, The Eodor Schwann a German zoologist asserted that all plant
and animals are made up of cells. This finding forms the basis of cell theory.
All living organisms on earth are divided in pieces called
cells. Cells are small compartments that hold all of the biolo-gical equipments
necessary to keep organisms. Cells carry out all the basic functions of life:
growth, metabolism and repro-duction. There are some organisms like amoeba
consists of a single cell. This single cell increase in size and when it attains
a certain size, it divides into two separates individuals. In case of multi
cellular organisms, the cell also divides into two parts, but the two parts
remaining joined this process is repeated crore of times so that body mass is
built up. In this process some cells become specialized to perform specific
functions but other retains their capacity for cell division throughout life.
Tissue is a cellular organizational level intermediate
between cells and a complete organism. Hence a tissue is an ensemble of cells
not necessarily identical, but from the same origin, that together carry out a
specific function. Organs are then formed by the functional grouping together of
multiple tissues. The study of tissue is known histology.
The structure of animal tissue is directly related to its
function. Tissue is groups of cells with a basic structure and function. There
are four types of tissues.
Type of Animal Tissue
It is a tissue that is made up of tightly packed cells.
Without much materials with in these cells. The reasons for the tightly packed
cells are to act as a barrier against mechanical injury, invading
micro-organisms and fluid loss. We can define epithetical tissue by considering
two points in mind one is the number of cells layers and two the shape of the
(i) On the basis of cell layers
Muscular and Skeletal System
The single-celled protozoan ancestors of animals had their
weight supported by water and were able to move by cilia or other simple
organelles. The evolution of large and more complex organisms (animals)
necessitated the development of support and locomotion systems. Animals use
their muscular and skeletal systems for support, locomotion, and maintaining
Skeletal Systems of Various Animals
(i) Movement is a major characteristic of animals. This
movement is a result of contraction of muscles. The skeleton helps transmit that
movement. Skeletons are either a fluid-filled body cavity, exoskeletons, or
(ii) Hydrostatic skeletons consist of fluid-filled closed
chambers. Internal pressures generated by muscle contractions cause movement as
well as maintain the shape of the animals, such as the sea anemone and worms.
The sea anemone has one set of longitudinal muscles in the outer layer of the
body, and a layer of circular muscles in the inner layer of the body. The
anemone can elongate or contract its body by contracting one or the other set of
(iii) Exoskeletons are characteristic of the Phylum
Arthropoda. Exoskeletons are hard segments that cover the muscles and visceral
organs. Muscles for movement attach to the inner surface of the exoskeleton
Exoskeletons restrict the growth of the animal, thus it must shed its
exoskeleton (or molt) to form a new one that has room for growth. The bulk and
weight of the exoskeleton and associated mechanical problems limits the size a
animals can attain.
The Nervous System
Multicellular animals must monitor and maintain a constant
internal environment as well as monitor and respond to an external environment.
In many animals, these two functions are coordinated by two integrated and
coordinated organ systems: the nervous system and the endocrine system. Three
basic functions performed by nervous systems are:-
Receive sensory input from internal and external
Integrate the input
Respond to stimuli
Receptors are parts of the nervous system that sense changes
in the internal or external environments. Sensory input can be in many forms,
including pressure, taste, sound, light, blood pH, or hormone levels, that are
converted to a signal and sent to the brain or spinal cord.
Integration and Output
In the sensory centers of the brain or in the spinal cord,
the barrage of input is integrated and a response is generated. The response, a
motor output, is a signal transmitted to organs than can convert the signal into
some form of action, such as movement, changes in heart rate, release of
Some animals have a second control system, the endocrine
system. The nervous system coordinates rapid responses to external stimuli. The
endocrine system controls slower, longer lasting responses to internal stimuli.
Activity of both systems is integrated.
The Endocrine System
The nervous system coordinates rapid and precise responses to
stimuli using action potentials. The endocrine system maintains homeostasis and
long-term control using chemical signals. The endocrine system works in parallel
with the nervous system to control growth and maturation along with homeostasis.
The endocrine system is a collection of glands that secrete
chemical messages we call hormones. These signals are passed through the blood
to arrive at a target organ, which has cells possessing the appropriate
receptor. Exocrine glands (not part of the endocrine system) secrete products
that are passed outside the body. Sweat glands, salivary glands, and digestive
glands are examples of exocrine glands. Hormones are grouped into three classes
based on their structure:
Steroids are lipids derived from cholesterol.
Testosterone is the male sex hormone.
Estradiol, similar in structure to testosterone, is responsible for many female
Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.
Lymphatic System and Immunity
The Lymphatic System
The lymphatic system is composed of lymph vessels, lymph
nodes, and organs. The functions of this system include the absorbtion of
excess fluid and its return to the blood stream, absorption of fat (in the
villi of the small intestine) and the immune system function.
Lymph vessels are closely associated with the circulatory
system vessels. Larger lymph vessels are similar to veins. Lymph capillaries are
scatted throughout the body. Contraction of skeletal muscle causes movement of
the lymph fluid through valves.
Lymph organs include the bone marrow, lymph nodes, spleen,
Bone marrow contains tissue that produces lymphocytes.
B-lymphocytes (B-cells) mature in the bone marrow.
T-lymphocytes (T-cells) mature in the thymus gland.
Other blood cells such as monocytes and leukocytes are
produced in the bone marrow.
Lymph nodes are areas of concentrated lymphocytes and
macrophages along the lymphatic veins.
The spleen is similar to the lymph node except that it is
larger and filled with blood.
The spleen serves as a reservoir for blood, and filters or
purifies the blood and lymph fluid that flows through it.
The Respiratory System
Cellular respiration involves the breakdown of organic
molecules to produce ATP. A sufficient supply of oxygen is required for the
aerobic respiratory machinery of Kreb’s Cycle and the Electron Transport System
to efficiently convert stored organic energy into energy trapped in ATP. Carbon
dioxide is also generated by cellular metabolism and must be removed from the
cell. There must be an exchange of gases: carbon dioxide leaving the cell,
oxygen entering. Animals have organ systems involved in facilitating this
exchange as well as the transport of gases to and from exchange areas.
Respiration in Single Cell Animals
Single-celled organisms exchange gases directly across their
cell membrane. However, the slow diffusion rate of oxygen relative to carbon
dioxide limits the size of single-celled organisms. Simple animals that lack
specialized exchange surfaces have flattened, tubular, or thin shaped body
plans, which are the most efficient for gas exchange. However, these simple
animals are rather small in size.
Respiration in multicellular animals
Large animals cannot maintain gas exchange by diffusion
across their outer surface. They developed a variety of respiratory surfaces
that all increase the surface area for exchange, thus allowing for larger
bodies. A respiratory surface is covered with thin, moist epithelial cells that
allow oxygen and carbon dioxide to exchange. Those gases can only cross cell
membranes when they are dissolved in water or an aqueous solution, thus
respiratory surfaces must be moist.
The Circulatory System
Living things must be capable of transporting nutrients,
wastes and gases to and from cells. Single-celled organisms use their cell
surface as a point of exchange with the outside environment. Multicellular
organisms have developed transport and circulatory systems to deliver oxygen and
food to cells and remove carbon dioxide and metabolic wastes.
Circulatory Systems in Single-celled Organisms
Single-celled organisms use their cell surface as a point of
exchange with the outside environment. Sponges are the simplest animals, yet
even they have a transport system. Seawater is the medium of transport and is
propelled in and out of the sponge by ciliary action. Simple animals, such as
the hydra and planaria lack specialized organs such as hearts and blood vessels,
instead using their skin as an exchange point for materials. This, however,
limits the size an animal can attain. To become larger, they need specialized
organs and organ systems.
Circulatory Systems in Multicellular Organisms
Multicellular animals do not have most of their cells in
contact with the external environment and so have developed circulatory systems
to transport nutrients, oxygen, carbon dioxide and metabolic wastes. Components
of the circulatory system include
Blood: a connective tissue of liquid plasma and cells
Heart: a muscular pump to move the blood
Blood vessels: arteries, capillaries and veins that deliver
blood to all tissues
The Integumentary System
The integumentary system (From Latin integumentum, from
integere ‘to cover’; from in- + tegere ‘to cover’ is the organ system that
protects the body from damage, comprising the skin and its appendages (including
hair, scales, feathers, and nails).
The integumentary system has a variety of functions; it may
serve to waterproof, cushion, and protect the deeper tissues, excrete wastes,
and regulate temperature, and is the attachment site for sensory receptors to
detect pain, sensation, pressure, and temperature. In humans the integumentary
system also provides vitamin D synthesis.
The skin is the largest organ in the body: 12-15% of body
weight, with a surface area of 1-2 meters. Skin is continuous with, but
structurally distinct from mucous membranes that line the mouth, anus, urethra,
Two distinct layers occur in the skin: the dermis and
epidermis. The basic cell type of the epidermis is the keratinocyte, which
contain keratin, a fibrous protein.
Basal cells are the innermost layer of the epidermis.
Melanocytes produce the pigment melanin, and are also in the inner layer of the
epidermis. The dermis is a connective tissue layer under the epidermis, and
contains nerve endings, sensory receptors, capillaries, and elastic fibers.
The integumentary system has multiple roles in homeostasis,
including protection, temperature regulation, sensory reception, biochemical
synthesis, and absorption. All body systems work in an interconnected manner to
maintain the internal conditions essential to the function of the body.
The Reproductive System
The ability to reproduce is one of the unifying
characteristics of all living things. Sexual reproduction produces offspring
that are genetically different from their parents. Asexual reproduction produces
offspring genetically identical to their parent.
Asexual reproduction allows an organism to rapidly
produce many offspring without the time and resources committed to
courtship, finding a mate, and mating.
Fission, budding, fragmentation, and the formation of
rhizomes and stolons are some of the mechanisms that allow organisms to
The hydra produces buds;
Starfish can regenerate an entire body from a fragment of the
Animal life cycles have meiosis followed immediately by
gametogenesis. Gametes are produced directly by meiosis. Male gametes are sperm.
Female gametes are eggs or ova.
The plant life cycle has mitosis occurring in spores, produced by meiosis, that
germinate into the gametophyte phase. Gametophyte size ranges from three cells
(in pollen) to several million (in a “lower plant” such as moss). Alternation of
generations occurs in plants, where the sporophyte phase is succeeded by the
The sporophyte phase produces spores by meiosis within a
sporangium. The gametophyte phase produces gametes by mitosis within an
antheridium (producing sperm) and/or archegonium (producing eggs). Within the
plant kingdom the dominance of phases varies. Nonvascular plants, the mosses and
liverworts, have the gametophyte phase dominant. Vascular plants show a
progression of increasing sporophyte dominance from the ferns and “fern allies”
Flowering plants, the angiosperms, were the last of the seed
plant groups to evolve, appearing over 100 million years ago during the middle
of the Age of Dinosaurs (late Jurassic). All flowering plants produce flowers
and if they are sexually reproductive, they produce a diploid zygote and
The Digestive System
Animals, for the most part, ingest their food as large,
complex molecules that must be broken down into smaller molecules (monomers)
that can then be distributed throughout the body of every cell. This vital
function is accpomplished by a series of specialized organs that comprise the
Digestive system in various organism:
Single-celled organisms can directly take in nutrients from
their outside environment. Multicellular animals, with most of their cells
removed from direct contact with the outside environment, have developed
specialized structures for obtaining and breaking down their food.
Animals Depend on Two Processes: Feeding and Digestion
Animals are heterotrophs, they must absorb nutrients or
ingest food sources.
Ingestive eaters, majority of animals, use a mouth to ingest
Absorptive feeders, such as tapeworms, live in a digestive
system of another animal and absorb nutrients from that animal directly through
their body wall.
Filter feeders, such as oysters and mussels, collect small
organisms and particles from the surrounding water
Substrate feeders, such as earthworms and termites, eat the
material (dirt or wood) they burrow through.
Fluid feeders, such as aphids, pierce the body of a plant or
animal and withdraw fluids.
The Excretory System
Cells produce water and carbon dioxide as by-products of
metabolic breakdown of sugars, fats, and proteins. Chemical groups such as
nitrogen, sulfur, and phosphorous must be stripped, from the large molecules to
which they were formerly attached, as part of preparing them for energy
conversion. The continuous production of metabolic wastes establishes a steep
concentration gradient across the plasma membrane, causing wastes to diffuse out
of cells and into the extracellular fluid.
Single-celled organisms have most of their wastes diffuse out
into the outside environment. Multicellular organisms, and animals in
particular, must have a specialized organ system to concentrate and remove
wastes from the interstinal fluid into the blood capillaries and eventually
deposit that material at a collection point for removal entirely from the body.
Excretory systems in various animals
Excretory systems regulate the chemical composition of body
fluids by removing metabolic wastes and retaining the proper amounts of water,
salts, and nutrients. Components of this system in vertebrates include the
kidneys, liver, lungs, and skin.
Not all animals use the same routes or excrete their wastes
the same way as humans do. Excretion applies to metabolic waste products that
cross a plasma membrane. Elimination is the removal of feces.
Photosynthesis is the process by which plants, some bacteria,
and some protistans use the energy from sunlight to produce sugar, which
cellular respiration converts into ATP, the “fuel” used by all living things.
The conversion of unusable sunlight energy (solar energy) into usable chemical
energy, is associated with the actions of the green pigment chlorophyll. Most of
the time, the photosynthetic process uses water and releases the oxygen .We can
write the overall reaction of this process as :
The above chemical equation translates as:
Six molecules of water plus six molecules of carbon dioxide
produce one molecule of sugar plus six molecules of oxygen.
Structure of leaf
Plants are the only photosynthetic organisms to have
leaves (and not all plants have leaves). A leaf may be viewed as a solar
collector crammed full of photosynthetic cells.
The raw materials of photosynthesis, water and carbon
dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar
and oxygen, leave the leaf.
Water enters the root and is transported up to the leaves
through specialized plant cells known as xylem.
Diversity in Living Organisms
Biologists such as Ernst Haeckel, Robert Whittaker and Carl
woese have tried to classify all living organism into broad categories called
kingdom. The classification which Whittaker proposed has five kingdoms: monera,
protista, fungi, plantae and animals.
Monera : These organisms do not have a defined nucleus
nor do any of them show multi- cellular body designs. This group includes
bacteria, blue green algae, or cyanobacteria and mycoplasma.
Protista : These groups include many kinds of unicellular
eukaryotic organisms some of these organisms use appendages, such as hair like
cilia or whip like flagella for moving around. e.g. : unicellular algae,
Fungi : there are heterotrophic eukaryotic organisms. They
are decaying organic materials as food and are therefore called saprophytes.
They have a cell wall made up of a tough complex sugar called chitin.