The basic structural unit of an organ, which is the cell.
Robert Hooke in 1665 observed slices of cork under a simple magnifying device. Hooke observed as boxes or cells in the cork were actually dead cells.
Cells of living organisms could be observed only after the discovery of improved microscopes. Very little was known about the cell for the next 150 years after Robert Hooke's observations.
Both, bricks in a building and cells in the living organisms, are basic structural units. in the living world, organisms differ from one another but all are made up of cells. Cells in the living organisms are complex living structures unlike non-living bricks.
There are millions of living organisms. They are of different shapes and sizes. Their organs also vary in shape, size and number of cells.
Number of Cells
The number runs into billions and trillions. Human body has trillions of cells which vary in shapes and sizes. Different groups of cells perform a variety of functions.
Organisms made of more than one cell are called multicellular (multi : many; cellular : cell) organisms. The number of cells being less in smaller organisms does not, in any way, affect the functioning of the organisms. You will be surprised to know that an organism with billions of cells begins life as a single cell which is the fertilized egg. The fertilised egg cell multiplies and the number of cells increase as development proceeds.
The organisms are made up of a single cell. The single-celled organisms are called unicellular (uni : one; cellular : cell)
A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows and reproduces. Similar functions in multicellular organisms are carried out by groups of specialised cells forming different tissues. Tissues, in turn, form organs.
Shape of Cells
There are many shapes in the organism. the shape appears irregular. Infact, amoeba has no definite shape, unlike other organisms. It keeps on changing its shape.
Observe the projections of varying lengths protruding out of its body. These are called pseudopodia (pseudo : false; podia : feet).
A white blood cell (WBC) in human blood is another example of a single cell which can change its shape. But while WBC is a cell, amoeba is a full fledged organism capable of independent existence.
Generally, cells are round, spherical or elongated. Some cells are long and pointed at both ends. They exhibit a spindle shape.
Size of Cells
The size of cells in living organisms may be as small as a millionth of a metre (micrometre or micron) or may be as large as a few centimetres.
The smallest cell is 0.1 to 0.5 micrometre in bacteria. The largest cell measuring 170 mm x130 mm, is the egg of an ostrich.
There are the different organs of a plant perform specific/specialized functions. For example, roots help in the absorption of water and minerals.
Each organ is further made up of smaller parts called tissues. A tissue is a group of similar cells performing a specific function.
Cell Membrane
The basic components of a cell are cell membrane, cytoplasm and nucleus. The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane. The membrane separates cells from one another and also the cell from the surrounding medium.
The boundary of the onion cell is the cell membrane covered by another thick covering called the cell wall. The central dense round body in the centre is called the nucleus. The jelly-like substance between the nucleus and the cell membrane is called cytoplasm.
In addition to the cell membrane, there is an outer thick layer in cells of plants called cell wall.
Cytoplasm
Cytoplasm is the jelly-like substance present between the cell membrane and the nucleus. Various other components, or organelles, of cells are present in the cytoplasm. These are mitochondria, golgi bodies, ribosomes, etc.
Nucleus
Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane.
When we see a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus. The nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.
There are several small coloured bodies in the cytoplasm of the cells of Tradescantia leaf. They are scattered in the cytoplasm of the leaf cells. These are called plastids. They are of different colours. Some of them contain green pigment called chlorophyll. Green coloured plastids are called chloroplasts. They provide green colour to the leaves.
Now we should be able to compare plant and animal cells. Observe the plant and animal cell carefully.