Class 8 Cell – Structure and Functions Notes

Cell – Structure and Functions Notes

Cell:-

The cell is the basic structural and functional unit of our life.

• Cell was discovered by Robert Hooke in 1665.
• Different cells have different shapes like round, spherical, elongated, spindle (long and pointed at both ends) depending on their specific functions.
• Some cells have changing shape like amoeba and white blood cell (WBC).
• The shape and size of the cell is related to its function. For example, nerve cells, both in the elephant and rat, are long and branched because they perform the same function, that of transferring messages.
• Smallest cell in the world:- Mycoplasma (PPLO-Pleuro pneumonia like organims)
• Largest cell in the world:- Ostrich egg
• Smallest cell in the human body:- Sperm cell
• Largest cell in the human body:- Female Ovum (Egg)

Unicellular organisms:-

The single celled organisms are called unicellular organisms. 
Ex:- Amoeba, Paramoecium, Bacteria, Chlamydomonas etc.

Multicellular organisms:-

The organisms that are composed of more than one cells are called multicellular organisms. 
Ex:- Some fungi, plants, animals etc.

Tissue:-

A group of cells performing a particular function is called tissue.

Levels of organisation:-

Cells make up tissues, tissues make up organs, and organs make up organ systems.

Plasma/Cell membrane:-

The outer covering of the cell is known as cell membrane.

• Cell membrane is made up of proteins & lipids.
• Cell membrane controls the entry & exit of the materials in & out of the cell. So, cell membrane is called selectively permeable membrane.

Cell Wall:-

The another rigid outer covering in addition to the plasma membrane found in plant cell is called cell wall.

Cytoplasm:-

The fluid content present between the plasma membrane and nucleus is called cytoplasm.

• The cell organelles like mitochondria, golgi bodies, ribosomes etc are present in the cytoplasm.

Nucleus:-

• It is generally spherical and located in the centre of the cell.
• The outer covering of the nucleus is called nuclear membrane.
• Nucleus contains nucleolus and thread-like structures called chromosomes.
• Chromosomes carry genes which help in transfer of characters from the parents to the offspring.
• The chromosomes can be seen only when the cell divides.

Protoplasm:-

The entire content of a living cell present inside the cell membrane is known as protoplasm.

Differences between prokaryotic and eukaryotic cell:-

S. No.	Prokaryotic Cell	Eukaryotic Cell
1.	Nuclear membrane is absent.	Nuclear membrane is present.
2.	Its size is generally smaller.	Its size is generally larger.
3.	Single chromosome is present.	More than one chromosomes are present.
4.	Membrane bound cell organelles are absent.	Membrane bound cell organelles are present.
5.	Ex:- bacteria, cyanobacteria (blue green algae) etc.	Ex:- Fungi, plants, animals etc.

Some cell organelles:-

Vacuoles:-

• Vacuoles are small sized in animal cells while large sized in plant cells.

Plastids:-

• Plastids are present in only plant cell.
• There are two types of plastids:-
  • Chromoplasts (coloured plastids)
  • Leucoplasts (white or colourless plastids)
• Chromoplasts containing the pigment chlorophyll (green pigment) are known as chloroplasts.
• Plastids have their own DNA & ribosomes.

Mitochondria:-

• Mitochondria are known as powerhouses of the cell.
• Mitochondria release energy in the form of ATP (Adenosine triphopshate).
• They have their own DNA & ribosomes. 

Differences between plant cell and animal cell :-

S.No	Plant cell	Animal cell
(i)	Plastids are present.	Plastids are absent.
(ii)	Cell wall is present.	Cell wall is absent.
(iii)	Single and large vacuole is present.	More than one & small vacuoles are present.