class -8 bio chapter- Cell structure and function

                           Class-8 

                Cell structure and function 

Cell definition:- 

A cell is defined as the structural and functional unit of life a cell can replicate itself independently hence they are known as building blocks of life.

Types of organisms on the basis of number of cells:- 

 there are two kinds of organisms on the basis of their cell.

1). unicellular organism:-  the organism that are made up of single cell are called unicellular organisms. example Amoeba ,Paramecium ,bacteria ,etc.

2). multicellular organisms:-  the organisms which are composed of a cell that assume function in coordinated manner as name that multicellular organisms . example plants ,human beings animals etc. 

• Robert hooke discovered the cell in 1665.

•Robert hooke observe the theme slice of cork under self designed microscope and noticed a honeycomb structure.

• he noticed that each box was separated from the other by partition  or a wall.

• he named these structures as cell which is derived from the Latin word called cella meaning small room.

"Cork is a part of the bark of a tree ".

Cell structure and function:- 

Though cells may differ in shape and size all of them may have the same basic structure referred to as the generalized cell structure for generalized cell a cell  has both living and nonliving parts the living parts have a definite shape structure and functional are called organelles.  the main parts of a generalized cell are cell membrane ,cytoplasm nuclear and mitochondria.

Cell membrane :- 

It is outer covering of a cell 

Function :-

A). It protects the cell from outside environment.

B).it gives the shape.

C).  allows water mineral and other selected substance to pass through it

2). Cytoplasm 

It is a transparent Jelly like material with fills the cell between nucleus and cell membrane most of the chemical reaction which keep the cell active take place in the cytoplasm and several organelles are embedded in cytoplasm.

3). Nucleus 

It is a large spherical organelle present in all the cells it is considered to be the brain of the cell nucleus is surrounded by a membrane called the nuclear membrane and it is filled with Semi solid substance called the nucleoplasm thread like structures called chromosomes are present in the nucleoplasm chromosomes carry  genes which transfer the characteristics of a cell to the new cell that are formed during cell division also present inside the nucleus is around granule called nucleus which contains a network of fibrous material called chromatin fibre.

Function of nucleus 

It is responsible for cell division 

It controls all metabolic activities of the cell. 

Transmission of all the characters forl one generation to another.

Mitochondria :-)

Are rod-shaped round organelles mitochondria perform the function of respiration as they generate energy for the cell they are known as powerhouse of the cell.

Vacuoles:-)

It is a fluid filled space enclosed in the membrane vacuoles store excess water useful mineral pigments and many other substance they also help in digestion and excretion in the unicellular organisms.

Golgi complex:-)

Golgi Complex are flat baggies tax placed in parallel positions on one another They Secrete  enzymes  which help in chemical reaction in living bodies.

Ribosomes:-) 

Ribosomes are small granules found freely in the cytoplasm or are located on endoplasmic reticulum they help in protein synthesis protein factories.

Lysosomes:)-

Lysosomes are small round bodies having enzymes to break down harmful bacteria.

Centrosome) -):

Centrosomes  are present only in animal cells that facilitates cell division.

Endoplasmic reticulum:)- 

It is a network of tube and channels and is involved in the synthesis storage and transport of cell products.

Plastids:)-

 they are present only in plant cells plastids contain certain pigments that have a specific role to play in the functioning of the plant depending on the pigment colour plastids are of three different kind

A. Chloroplast contain the green pigment chlorophyll which helps in the photosynthesis.

B. Chromoplasts:)- contain non green pigment That Give colours to flowers and fruit

C.  Leucoplasts :)- are colourless and store food in the form of Carbohydrates fats and proteins

Cell wall :-) 

It is present only in plant cells it is the nonliving part of the cell.

Function :)- 

It provides extra support and strength to the cell

It provides shape to the cell

It allows material to enter or leave the cell through tiny holes