Cell - Structure & Functions Summary Class 8 NCERT Summary

Cell

Discovery of the Cell

• Cells are the fundamental units that make up all living organisms.
• The cell was first identified by Robert Hooke in the year 1665 when he observed a slice of cork under a microscope.
• During his observations, Hooke coined the term "cell" to describe these tiny units of life.
  

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Cell Theory

• Schleiden and Schwann proposed the cell theory. 
• According to cell theory: Cells are the basic structural and functional units of life. 
• All living organisms are made up of one or more cells.
• New cells arise from pre-existing cells.

Types of Organisms on the basis of cell

1. Unicellular organisms

• Unicellular organisms are those that consist of a single cell, such as Amoeba and Paramecium.
• In these organisms, all vital functions like digestion, respiration, and excretion are performed by that one cell.

2. Multicellular organisms

• Multicellular organisms are made up of more than one cell, such as humans, cows, and roses.
• In these organisms, cells are specialized for different functions, with specific cells taking on particular roles.

Shape of the Cells

• Most of the cells have a definite shape.
• Some cells such as that in Amoeba have no definite shape.

• The human red blood cell (RBC) is spherical-shaped.

• The muscle cells in humans are spindle-shaped.
  
• The human nerve cells have elongated branched structure. 
• In plants and bacteria, the cell is enclosed in a protective covering called cell wall, which gives shape and rigidity to the cells.

Size of the Cells

• The smallest cells in bacteria range from 0.1 to 0.5 micrometres in size.
• The largest cells, like an ostrich's egg, can measure around 170 mm x 130 mm.
• While there is some link between cell size and the size of the organism, it is not a strict correlation.

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Cell Structure and Functions

• In multicellular organisms, each organ system is made up of multiple organs.
• Organs, in turn, are composed of tissues.
• Tissues consist of groups of similar cells that work together to perform a specific function.
• Organisms with only one cell are called unicellular organisms. Examples include Amoeba and Paramecium.
• In unicellular organisms, a single cell carries out all essential functions like digestion, respiration, and excretion.
• Organisms with more than one cell are known as multicellular organisms. Examples include Humans and Cows.
• In multicellular organisms, cells specialize in different tasks, showing a division of labor. Each type of cell is responsible for specific functions in the body.

Types of cell

There are two types of cells:

1. Prokaryotic cells: These cells lack a well-defined nuclear membrane. The genetic material is dispersed throughout the cytoplasm. Examples include bacteria and cyanobacteria.
2. Eukaryotic cells: These cells possess a nucleus enclosed by a distinct nuclear membrane. Examples include plant and animal cells.

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Components of the cell

Cell membrane

• It is the protective layer that surrounds the cell.
• Cell membrane selectively allows the entry of only some substances and prevents the movement of other materials. 
• Hence, it checks the transport of substances in and out of the cell. 

Cell wall

• In plants, an extra protective covering of a polysaccharide, cellulose is present. 
• It is called cell wall that protects plant cells from environmental variations. 

Cytoplasm

• It is a jelly-like substance present between cell membrane and nucleus. 
• It contains various cell organelles such as mitochondria, Golgi bodies, lysosomes etc.

Nucleus

• It is a dense spherical body located at the centre of the cell.
• It is surrounded by porous nuclear membrane.
• It contains spherical body called nucleolus.
• It also contains thread-like structures called chromosomes. 

Chromosomes

• These are the structures that carry genes and play an important role in inheritance. 
• Genes are the structural and functional unit of inheritance.
• The entire living substance in a cell is known as protoplast.

Vacuoles

• Vacuoles are fluid-filled membrane-bound structures in the cell.
• In plant cells, a single large vacuole is present.
• In animal cells, numerous small vacuoles are present.
• The membrane of the vacuole is called tonoplast.
• This membrane encloses a fluid called cell sap. 

Plastids

• They are present only in plant cells.
• Plastids that contain green colour pigment chlorophyll are known as chloroplasts.
• It is the chlorophyll that gives green colour to the leaves.
• Chloroplast traps solar energy and utilizes this energy to manufacture food for the plant. 

Plastids are of two types:
(i) Leucoplasts
(ii) Chromoplasts 

• Leucoplasts are colourless and are used to store food while chromoplasts are plastids containing pigments.
• Chloroplasts are a type of chromoplasts. 

Chloroplasts consist of two regions:
(i) grana (stacks of sac like membrane bound structures that contain pigment chlorophyll)
(ii) stroma (ground substance containing enzymes and starch grains)

Endoplasmic Reticulum (ER)
(i) Rough Endoplasmic Reticulum (RER): It is important for the synthesis and packaging of proteins.
(ii) Smooth Endoplasmic Reticulum (SER): It acts as storage organelle. It also helps in lipid (fat) synthesis. 

Golgi Apparatus

• It is made up of parallel arranged membrane-bound vesicles called cisternae. 
• It helps in storage, modification, and packaging of products in vesicles. 
• It helps in formation of glycoproteins and glycolipids.

Lysosomes

• It is a membrane-bound structure that holds variety of enzymes. 
• Rich in all types of hydrolytic enzymes, which are active at acidic pH. 
• It is involved in the digestion of carbohydrates, proteins, lipids, and nucleic acids.

Mitochondria

• It is a double membrane-bound structure. 
• The inner membrane of mitochondria is deeply folded to form cristae. 
• Cristae increase the surface area in the organelle.
• It is the site of cellular respiration and hence known as ‘power house of cell’. 
• They have their own circular DNA.
• They divide by fission.

Differences between plant and animal cells

Diagram of Plant Cells and Animal Cells