FUNDAMENTAL UNIT OF LIFE -CELL ORGANELLS

 

 Diffusion 

• The spontaneous movement of a substance from a region of high concentration to the region of low concentration is called diffusion. 

• Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. Cell also obtains nutrition from the environment. 

Osmosis 

• The movement of water molecules through selectively permeable membrane along the concentration gradient is called osmosis. 

• Plant cell tend to obtain water through osmosis.

Plasmolysis 

• When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis. 

Nucleus 

• It is called the brain of the cell as it controls all the activities of cell. 

→ Composition of Nucleus 

• The nucleus has a double layered covering called nuclear membrane. 

• The nuclear membrane has pores which allow the transfer of material from inside the                     nucleus to the cytoplasm. 

• The nucleus contains chromosomes, which are visible as rod-shaped structures only when               the cell is about to divide.

→ Functions of chromosomes  

 • Chromosomes contain information for inheritance of features from parents to next                         generation in the form of DNA (Deoxyribo Nucleic Acid) molecules .Chromosomes are                    composed of DNA and protein.  

 • DNA molecules contain the information necessary for constructing and organising cells. 

 • Functional segments of DNA are called genes. 

 • In non-dividing cell, this DNA is present as part of chromatin material. 

 • Chromatin material is visible as entangled mass of thread like structures. Whenever the                  cell is about to divide, the chromatin material gets organised into chromosomes and                        perform cell division.

→ Functions of Nucleus  

 • The nucleus plays a central role in cellular reproduction. It is the process by which a                        single divides and forms two new cells.  

Nucleoid 

• In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. 

• Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

ncert biology chapter-THE FUNDAMENTAL UNIT OF LIFE

 

what is cell ?        

Introduction:- 

Cells are both the structural and functional units of life. They are the basis for all living things. Robert Hooke discovered them in 1665, using a primitive microscope. Leeuwenhoek (1674), using a significantly improved microscope, discovered them in pond water. The nucleus in the cell was discovered by Robert Brown in 1831. Purkinje defined protoplasm as the fluid substance that makes up the cell in 1839.

THE CELL THEORY

Schleiden and Schwann proposed the theory that plants and animals are composed of cells, and the cell is the basic unit of life. Virchow further elaborated this theory by stating that all cells acquire their characteristics from pre-existing cells.

 → Types of organisms 

Organisms are typically divided into two types based on the number of cells they contain: 

(i) unicellular organisms and

(ii) multicellular organisms.

 (i) Unicellular Organisms: These organisms are single celled and can perform all the functions.. Example: Amoeba, paramecium, bacteria.

(ii) Multicellular Organism: A multicellular organism (MCO) is made up of various cells that together perform different functions in the body and also form a variety of body parts. Examples are fungi, plants, and animals.

→ Types of cells based on nucleus structure.  

• There are two types of cells on the basis of nucleus structure : 

(i) Prokaryotic cells, and

(ii) Eukaryotic cells

Prokaryotic Cell

Difference between Prokaryotes cell and Eukaryotes cell

Difference between Animal cell and Plant cell

Plasma membrane or Cell membrane 

A cell's plasma membrane or cll membrane is the most outer envelope of the cell, which separates its interior from its surroundings. The plasma membrane allows some materials to enter and exit the cell due to which it is also known as selectively permiable membrane.

• As it restricts some other materials from passing through. This membrane is called the selectively permeable membrane.  It is made up of lipids and proteins.

→ Properties of Plasma membrane 

It is flexible (composed of organic molecules called lipids and proteins).Cellular flexibility allows it to absorb food and other substances from the external environment. It is called endocytosis. Through this process, amoeba acquire food.

 → Functions of Plasma membrane 

It permits the movement of some materials within and outside the cell.  It prevents movement of other materials not needed by the cell by acting as a selectively permeable membrane.

Cell Wall

 • The cell wall is another rigid outer covering found on plant cells, in addition to the plasma membrane.The cell wall is located outside the plasma membrane.

• The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance which provides structural strength to plants. 

→ Function of Cell Wall 

Because of the cell walls, plants, fungi, and bacteria are capable of surviving in very thin (hypotonic) media without bursting. In such a medium, cells tend to take up water through osmosis.Pressure builds up against the cell wall as the cell swells. The wall exerts a similar amount of pressure on the swelling. Due to the cell wall, cells can withstand greater changes in the surrounding medium than animal cells.