discovery of the cell and cell theory


human cell

discovery of the cell was made by an English scientist, Robert Hook in 1665. He observed thin slices of cork under his crude microscope.


organization of cells in organisms


Cells are usually grouped together to make tissues, organs, organs system and finally organisms.
A tissue is a group of cells of the same size, shape, and function. Examples of some tissues are muscles tissue and nerve system.

An organ is a structure that contains more than one type of tissue. It is normally big enough to see with the naked eyes. Examples of some organs are the heart and brain of animals, and leaves, roots, the stem of animals.

The organ does not work alone. Usually, a number of organs work together to carry out a certain function. A group of organs works together is called an organ system. Our digestive system is an organ system. The organ taking part in the digestive system are the mouth, the food pipe, the stomach, and the intestine. Each organ has a special function. Together, they digest food.   
  

The shape, size and the number of the cell


 Cells vary in shape, size, and number. Most cells are small and cannot be seen by naked eyes. The smallest cell is that of a bacterium. Mammalian red blood cell is just about 8um in diameter. Amoeba is a relatively larger single-celled organism,(10-100 micro miter across). Some cells as the egg of a hen or an ostrich of the cells in a fiber-yielding plant (like juicy hairs of lemon) are large enough to be seen by the naked eyes. The egg of an ostrich measuring 170mm x 130mm is the largest cell. The longest cell is the never cell in giraffe's neck which can be nearly 3 m long. Some cells in the human body are just about 0.1 micrometers in size. A blood cell is about 7 micrometers in size and the never cell is more than a meter long.

Not all cells are of the same shape either. Some are round; others are spindle-shaped, elongated or even irregular out of its like Amoeba. The projections that protrude out of its body called pseudopodia to appear and disappear. The red blood cells of sheep are nearly spherical and those of camel are elliptical. A white blood cell in the human body can change in shape and never cell is very long and branched.

The number of cells present in an organism also varies. Some small organism has a few cells of the white large organism have millions and billions of cells. Such as Amoeba, Paramecium and most bacteria are made up of just a single cell. Such organism is called unicellular organisms. Another organism, such as humans, are made of many cells. They are called multicellular organisms. In multicellular organisms, the structure of the body is more organized and also complex.

The number of cells does not, in any way, affects the functioning of an organism. The single cell of amoeba carried out the functions necessary for life.

cell/ organism amoeba

How structure related to functions
irregular shaped with a projection of
its help to move easily on any sides
also, help in catching food.

Nerve cell                       


 very long and branched shaped 
with thin thread-like projection
help it to send massage over long
distances in the body.

Red blood cell 

               
Spherical shape provides more 
surface area for absorbing oxygen

Mesophyll


long elongated shape allows for
more chloroplast to be present
inside the cell; this help in
photosynthesis.

Cell structure and function.


You already know that though cells show a lot of variation in their shape and size, they have a number of things of features in common. All plant and animal cells consist of three basic parts or components:

1.  Cell membrane          2. Cytoplasm, and

2, Nucleus
The cytoplasm and nucleus are enclosed within the cell membrane (also called plasma membrane).
Protoplasm (proto= first; plasma= liquid): The living substance of the cell is called the protoplasm. It includes the cytoplasm and nucleus.

Cell membrane


 The Outermost layer, that encloses the cytoplasm and the nucleus within, is called the cell membrane. This is present in all living organisms. It is also known as the plasma membrane. The plasma membrane regulates the entry of substances inside the cell. It selects substances and does not allow everything to enter the cell. It is semipermeable (or semi-porous).

Cell wall:- 


 In-plant cells, there is an additional covering over the living plasma membrane called cell wall. This is because plants are immobile and need extra protection from the variations in temperature, pressure, wind and other environmental stress.

Cytoplasm
The cytoplasm (kytes = hollow; plasma = liquid) is a jelly-like fluid that occupies the space between the cell membrane and the nucleus. A number of tiny components or structure called the cell organelles are present in the cytoplasm.


Nucleus

nucleus is a spherical structure present in the center of the cell and surrounded by the cytoplasm. It is known as the center of the cell. Generally, only one nucleus is present in a cell, but in some cases, more than one nucleus may be present.

nucleus consists of four part:- 

1. Nuclear membrane  2. Nuclear sap
3. Nucleolus 4. Chromosomes

 nuclear membrane is a thin porous membrane that separates the nucleus from the cytoplasm. It is thus the outer most covering layer of the nucleus. Being porous, it allows the movement of material between the cytoplasm and the nucleus.
jelly-like substances called nuclear sap or nucleoplasm.
Inside the nucleus, a small spherical body called the nucleolus.

In addition to the nucleus, a thread-like structure called chromosomes are present in a nucleus. Chromosomes carry a number of genes. The chromosomes and the genes help in the inheritance of characters from the parents to their children.

Organelles

These are extremely small structures present in the cytoplasm. Cytoplasm composed of many living and the non-living parts called cell organelles which are concerned with cell function.


Mitochondria


Mitochondria are oval or rod-shaped organelles commonly called ‘powerhouse of the cell’ because the production of the energy from food takes place here.

Endoplasmic reticulum
It synthesizes stores and transports material within the cell.


Golgi complex


It secrets enzymes, proteins, and hormones.


Lysosome


All cells contain enzymes within breaking down the various material. They are also called suicidal bags. Plant cell does not have lysosomes.


Ribosomes


It is the site of protein synthesis.

Vacuole


The space within the cytoplasm of the plant cell containing a cell sap is called vacuole.


  
 

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