Fundamental unit of life class 9 Notes

Fundamental unit of life  Class 9 notes  Chapter 5

 Cells :

• Cells was discovered by Robert Hooke in 1665.
• Cell is a Latin word for ‘a little room’.
• Every organism are made of  cells.
• On the basis of  number of cells present in the body organisms can be classified into unicellular (single celled organisms) and multicellular organisms(composed of many cells).

Structure of cell :

 

Plasma membrane :

• The Plasma membrane (cell membrane) is the outermost covering of a cell.
• It separates the contents of the cell from its external environment.
• It is made up of phospholipid bilayer membrane.
• It is selectively permeable membrane.
•  Gases such as carbon dioxide or oxygen can also move in or out from a high concentration to a low concentration in a cell across the cell membrane by the process of diffusion.
• Cell membrane also allows water to move in and out of the cell toward a higher solute concentration  by the process of osmosis .

Isotonic, hypotonic and hypertonic solutions :

• Isotonic solutions are those which have the same solute  concentration as the cell, there will be no net movement of water across the cell membrane.
• Hypotonic solutions contain lesser amount of solute concentration(or higher water concentration) compared to the cell, the cell will gain water by osmosis .The overall result is that water enters the cell. The cell is likely to swell up.
• Hypertonic solutions contain a higher concentration of solute(or lower concentration of water) compared to the cell, the cell will lose water by osmosis. The overall result is that  more water leaves the cell than enters it. Therefore the cell will shrink.

Cell wall :

• Cell Wall is found in plant cells.
• The cell wall lies outside the plasma membrane.
• Cell wall is a rigid covering made up of cellulose which is a complex substance and  provides structural support to the plants.
• Plant cells are different from animal cells due to the presence of a cell wall.
• Cell walls allow the cells of plants, fungi and bacteria to withstand much greater changes in the surrounding medium than animal cells. 

Nucleus :

• The nucleus is small, round and   double layered covering  structure found in cell.
• The double layered membrane called nuclear membrane ,nuclear membrane has pores that  allow the transfer of material from  nucleus to its outside(cytoplasm).
• The nucleus contains some thread like structures called chromatin.
• Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.
• Chromosomes contain information for the inheritance of characters from parents to the next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.
• DNA molecules contain the information for constructing and organising cells, Genes are the Functional segments of DNA .
• In some organisms like bacteria, the nuclear region  may be poorly defined due to the absence of a nuclear membrane. Such organisms, whose cells lack a nuclear membrane, are called prokaryotes (Pro = primitive or primary , karyote  = nucleus).
• Organism like plants and animals ,the nuclear region well defined and  surrounded by a  nuclear membrane. Such organisms are called eukaryotes.

Nucleus

Prokaryotes	Eukaryotes
Generally small in size	Generally large in size
Nuclear region poorly defined	Nuclear region well defined
Nuclear Membrane is absent.	Nuclear Membrane is present.
Membrane-bound cell organelles are absent.	Membrane-bound cell organelles are present.
Chromosome single	More than one chromosome
Bacteria	Animal cell         Plant Cell

Cytoplasm :

• The cytoplasm is the fluid content  found inside the plasma membrane. 
• Cytoplasm also contains many specialised cell organelles.
•  These organelles performs a specific function for the cell.

• Cell organelles :

• Some important examples of cell organelles are as following.
• Endoplasmic Reticulum
• Golgi Apparatus
• Lysosomes
• Mitochondria
• Plastids

Endoplasmic Reticulum :

• Endoplasmic reticulum (ER) is a large network of membranous tubes and sheets.
• Endoplasmic reticulum (ER)  transport  materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus, due to which it is also called transport system of the cell.
• Endoplasmic reticulum (ER) provide surface for some biochemical activities of the cell.
• Endoplasmic reticulum (ER) can be classified into following  two types
• Rough endoplasmic reticulum (RER) 
• Smooth endoplasmic reticulum (SER) 
  

Rough endoplasmic reticulum (RER) :

• RER looks rough under the microscope because ribosomes attached to its surface.
• Ribosomes are the sites of protein synthesis. 

Smooth endoplasmic reticulum (SER) :

• Due to the absence of ribosomes, SER looks smooth under the microscope.
• SER involved in the manufacture of fat molecules, or lipids that are important for proper functioning of cell .
• SER plays a key role in the detoxification of many poisons and drugs.

Golgi apparatus :

• The Golgi apparatus was first described by Camillo Golgi .
• Golgi apparatus consists membrane bound vesicles (flattened sacs).
• Disk-shaped, stacked,  flattened sacs that make up the Golgi apparatus are called cisternae.
• Its main function is the packaging and transports the materials synthesised in Endoplasmic Reticulum to inside and outside the cell.

Lysosomes :

• Lysosomes are membrane-bound organelles.
• Lysosomes contain strong digestive enzymes capable of breaking down all organic material.
• Lysosomes help to keep the cell clean by digesting any foreign material like bacteria  or food, as well as old organelles or worn-out cell organelles.
• Lysosomes are also known as the ‘suicide bags’ of a cell as they contain strong digestive enzymes that can digest their own cell.

Mitochondria :

• Mitochondria is a membrane-bound organelle present in the cytoplasm of all eukaryotic cells
• Mitochondria is also known as the powerhouses of the cell.
• Mitochondria is surrounded by two membranes.
• The outer membrane is porous.  
• The inner membrane is deeply folded that form a layered structure called cristae. 
• Cristae helps in increasing the surface area  for ATP generating chemical reactions.
• ATP is known as the energy currency of the cell. 
• Mitochondria have their own DNA and ribosomes therefore it is  capable to produce some of their own proteins.

Plastids :

• Plastids are only present in plant cells.
• Plastids have their own DNA and ribosomes. 
• Plastids can be classified into two types-
• Chromoplasts (coloured plastids)
• Leucoplasts (white or colourless plastids)
• Chromoplasts containing the  green pigment chlorophyll are known as chloroplasts.
• Chloroplasts are important for photosynthesis in plants.
• Chloroplasts have many membranous layers embedded in surface called stroma.
• Leucoplast stores starch, oils and protein granules.

Vacuoles :

• Vacuoles are large vesicles that store solid or liquid contents.
• Vacuoles are large size in plant cells. 
• In animals, the vacuoles are either very small or absent.
• In plant cell , vacuoles provide turgidity and rigidity to the cell.