Cells, Tissues, Organs and Systems

Cells are the basic unit of our body. Cells group together to form tissues. Many tissues in turn form organs. Each system consists of organs. So, our body is built on this basis. Cells > Tissues > Organs > Systems.

In this chapter we learnt about the different type of cells, tissues and organs.


The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. Humans have about 100 trillion or 10¹⁴ cells; a typical cell size is 10 µm and a typical cell mass is 1 nanogram. The longest human cells are about 135 µm in the anterior horn in the spinal cord while granule cells in the cerebellum, the smallest, can be some 4 µm and the longest cell can reach from the toe to the lower brain stem . The largest known cells are unfertilised ostrich egg cells, which weigh 3.3 pounds.

In 1835, before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

The word cell comes from the Latin cellula, meaning "a small room". The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.

There are different cell parts with different functions. The cell parts below are those found in 

animal cells.

Nucleus: Contains genetic material. Directs activities of the cell. Controls cell division and 
growth.

Cytoplasm: Fills the cell and contains substances needed for reactions in the cell.

Mitochondrion: Cellular respiration.

Cell membrane: It is selectively pemeable to control movement of substances into and out of the 


cell.

Vacuole: Storage compartment for water, sugars and pigments.
Ribosome: Produces protein from a set of genetic instructions.
Cell wall: Provides support for plant cell.
However, animal cells do not have chloroplasts nor do they have cell walls.

The two types of cells are animal cells and plant cells. Animal cells are different from plant cells in their 


structure. 

The picture above is what an animal cell looks like.

This is what a plant cell looks like. 

See the difference? 

Yes, a plant cell has cell wall and chloroplasts while the animal cell doesn't. Also, the plant cell's vacuole is 

much larger than an animal cell's and there is only one vacuole in a plant cell.

Now, tissues.

Tissues are really cells grouped together. There are a few kinds of tissues which perform different functions.


Plants and animals' tissues are different too. Plants have Epidermal, Xylem, Phloem, Photosynthetic and palisade tissues.


Animal tissues include epithelial, muscle, connective, nerve and glandular tissues.
All these tissues form organs.
Examples of plant organs are roots, stems, leaves. While examples of animal organs are the stomach, liver and   
small intestines.


These form systems such as respiratory system & digestive system.