1. Cell-Single celled organisms: In these organisms, a single cell usually carries out all the body processes. Examples of organisms that exist as single cell are Amoeba, Euglena and Paramecium.

2. Tissue: Some organisms exist as a tissue which performs all the life functions example is the Hydra. Details about hydra are explained further down in the tutorial.
3. Organ: Some organisms exist as an organ like the organ of storage in the onion bulb,. Another example of plant organ can be found in rhizome. Heart is an animal organ which is part of a system in the body of multicellular organisms.
4. System: Mammalian system and system in flowing plants, reproductive system, excretory system etc.
5. Complexity of organization in higher organisms, the advantages and disadvantages
In unicellular or single-celled organisms, the cell carries out the whole of the life functions on its own while in multicellular or many celled organisms, there are various levels of organization that exist among them.
Individual cell in addition to performing a particular function as well work jointly with other cells for the wellbeing of the entire organism. Thus there is division of labour among all the cells that the organism is composed of and each of the cells is dependent on the other to function effectively.

The five levels of organization in multicellular organisms are:

1.The Cells: The cell is the basic unit of life. It is the fundamental unit of structure and function of life. It may perform a specific function in the body of a living organism. Examples of cells are- blood cells, nerve cells, bone cells, etc.
Cells are made up of organelles which take care of everything starting from housing the cell’s DNA, to the manufacture of energy. Processes that take place inside the body are carried out on a cellular level.
For instance during the movement of the leg, it is the function of the nerve cells to transmit this signal from your brain to the muscle cells in your leg.
2.Tissues: The tissue is composed of cells that have similar structure and function and which work together to carry out a particular function. Examples of tissues are blood, nervous, bone, etc.
Human beings have 4 fundamental tissues: connective tissue, epithelial tissue, muscle tissue, and nerve tissue. Animal tissue can be subdivided into 4: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
3.Organs: Organs are composed of tissues that work together to perform a specific function. Examples of organs are the heart, brain, skin and so on. For instance, the brain is made up of a lot of different types of tissues which include the nervous and connective tissues.
4.Systems: The system is a group of tissues to work together to achieve a particular function in an organism. Examples of systems are the circulatory system, nervous system, skeletal system, etc.
The Human body is made up of 11 systems which include- circulatory, digestive, endocrine, excretory or urinary, immune or lymphatic, integumentary, muscular, nervous, reproductive, respiratory, and skeletal system.
All these systems work together to keep the body functioning optimally. For example nutrients gotten through the digestive system are transported throughout the body by the circulatory system. In the same way, the circulatory system circulates the oxygen that is assimilated by the respiratory system.
5.Organisms: This is the entire living things that can perform all basic life processes. This means that such living thing can take in materials, discharge energy from food, free wastes, grow, respond to the stimuli in the surroundings, and reproduce.
The are normally made up of systems which combine together to form the organism but an organism can as well be composed of only a single cell like in the case of bacteria or protists such as bacteria, amoeba, mushroom, sunflower.
Living organisms are extremely ordered and possesses the ability to grow, develop, and reproduce. Multi-cellular organisms in addition to human beings depend on the collaboration between organs, tissues, cells, and system to exist.
The levels of organization of life in the right order is therefore
cells – tissues – organs – organ systems – organisms.
The pyramid of life is a hierarchical structure for the organization of life.

Examples of Plant and animal Cells

Cells are extremely small. They are the basic building blocks of every animal and plant. The pictures below show example of plant and animal cell as seen from a microscope.
Animal cell: The Cheek cells
These are cheek cells, seen through a microscope:
Plant cells: Onion cells
Onion cells as seen through a microscope:
The Kingdom Protista are made up of single-celled organisms that posses a true nucleus. This means that they are eukaryotic. Protista may be either autotrophic or heterotrophic.
This means that some of them manufacture their own food while the rest depend on already manufactured food. The mode of movement by protists is depends on their physical features
A few protozoa have pseudopodia which has the ability to extend the cell membrane and thrust forward to surround a particle of food like you would obtain in amoeba. A protist that has one tail-like structure is refered to as a flagellate.
Such an organism would use its flagellum to beat back and forth and push itself through the water like you would obtain in trypanosome and trichosomes. Some Protozoa are enclosed with minute hair-like structures known as cilia which move back and forth swiftly pushing the organisms through the water.
A paramecium is an example of a ciliated organism. Some Protozoa have axopodia, or pencil-like structures, that assist them to be planktonic or floaters in the water. Radiolaria are marine examples of protozoa bearing this structure
There are a lot of debates regarding if protozoa are all one-celled organisms or if they are all one-celled organisms that are heterotrophs. Scientists, who researched on these groups, argue on how to classify a few of these like euglena and dinoflagellate.
The majority of protozoa are useful in that they are significant in lower levels of the food chain. They make available food for living things like snails, clams, and sponges. A few protozoa have the ability of causing diseases in humans and other animals.
Some diseases caused by protozoa in human beings are malaria, black fever, sleeping sickness, and a number of forms of diarrhea.
Hydra is a genus of small, plain, fresh-water animals that is radial symmetrical. They are predators and belong to the phylum Cnidaria and the class Hydrozoa.
They are mostly found in the majority of unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical climates and can be seen by softly sweeping an assembling net through weedy areas.
They are multicellular organisms which are frequently a few millimetres long and can be studied effectively through the help of a microscope. Biologists are particularly interested in Hydra as a result of their ability to regenerate. They also seem not to be old or die out of age.

Hydra

Scientific classification
Kingdom – Animalia
Subkingdom – Eumetazoa
Phylum – Cnidaria
Subphylum -Medusozoa
Class -Hydrozoa
Subclass – Leptolinae
Order -Anthomedusae
Suborder – Capitata
Family – Hydridae
Genus -Hydra Linnaeus

Motion and locomotion of Hydra

Whenever Hydra are startled or attacked, they retract their tentacles to form small buds. They can as well retract their entire body column into a small gelatinous sphere. Hydra usually reacts in the same manner irrespective of the direction of the stimulus, and this might be as a result of the simplicity of the nerve net.

The sessile or sedentary behavior of Hydra

Hydra is usually sedentary or sessile but can on occasional basis transport itself swiftly particularly when hunting for food. They normally do this by bending over and fastening themselves to the substrate with the mouth and tentacles and after that let go of their feet, which makes available the typical attachment.
This process in hydra is referred to as looping. The body subsequently bends over and makes a fresh position of attachment with the foot.
Through this process of “looping” or “somersaulting”, a Hydra can be in motion quite a lot of inches (c. 100 mm) on a daily basis. Hydra may also move about through amoeboid movement of their bases or by merely coming off the substrate and hovering away through the water current.

Reproduction and life cycle of Hydra

When there is a lot of food, are plentiful, loads of Hydra reproduce asexually by generating buds in the body wall. These buds grow to be small adults and merely breaking away when they are full-grown.
When conditions are unsympathetic, habitually before winter or in poor feeding and nutritional situations, some Hydra undergo sexual reproduction. Inflammations in the body wall expand into either a straightforward ovary testes.
The testes discharge free-swimming gametes into the water, and these possibly may fertilize the egg in the ovary of another individual hydra.
The fertilized eggs ooze a hard outer coating, and, as the fully developed hydra dies, these dormant eggs get discharged to the bottom of the lake or pond to wait for favorable conditions, at which point they hatch into nymph Hydra. Some type of Hydra like Hydra circumcincta and Hydra viridissima, are hermaphrodites and may at the same time bring into being both testes and an ovary.
Various members of the Hydrozoa pass through a body alteration from a polyp to an adult form known as a medusa. Nevertheless, all Hydra, regardless of being hydrozoans, hang about as polyps all the way through their lives.

Feeding of Hydra

The feeding in Hydra is majorly on minute aquatic invertebrates like as Daphnia and Cyclops.
When feeding, Hydra extends their body to their highest length and after that little by little extends their tentacles. In spite of their plain construction, the tentacles of Hydra are amazingly extensible and can be extended up to four to five times the length of the entire body.
Just immediately they are completely extended, the tentacles are bit by bit maneuvered more or less waiting to make contact with an appropriate prey animal. Once they come in contact, nematocysts on the tentacle shoot into the prey, and the tentacle then coils over the prey.
Just within a space of 30 seconds, the majority of the rest tentacles would have already united in the attack to suppress and hold back the besieged prey. Just about two minutes, the tentacles will have bordered the prey and stirred it into the opened mouth opening.
In a space of about ten minutes, the prey will have been completely engulfed into the body cavity, and digestion will have been on track. Hydra is able to elongate its body wall by a long way in order to digest prey that is two times its size.
After two or three days, the hard to digest parts of the prey will be released through the contractions of the aperture in the mouth.
The feeding attitude of Hydra illustrates the cleverness of what seem to be just a mere nervous system.
A few species of Hydra occur in a joint relationship with a combination of types of unicellular algae. The algae are sheltered from predators by Hydra and, in return, photosynthetic products from the algae are valuable as a source of food to Hydra.