Cell - Structure & Functions Summary Class 8 NCERT Summary

Cell

Discovery of the Cell

• Cells are the basic structural units and the building blocks of all living organisms.
• Cell was discovered by Robert Hooke in 1665 after observing a piece of cork under a magnifying device.
• Robert Hooke coined the term “cell”.
  

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

• Unicellular Organisms
• Multicellular Organisms

1. Unicellular organisms

• Number of Cells Organisms made of only a single cell are called unicellular organisms.
  For example: Amoeba and Paramecium
• All the basic functions such as digestion, respiration, excretion, etc. in these organisms performed by a single cell.

2. Multicellular organisms

• Organisms made up of more than one cells are called multicellular organisms. For example: Humans, cow, rose, etc.
• In these organisms, the cells show division of labour as particular set of cells are involved in performing a specific body function. 

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 cell is 0.1 to 0.5 micrometre in bacteria.
• The largest cell is of size 170 mm x 130 mm, which is the egg of an ostrich.
• Size of a cell has no relation with the size of an organism. 

Cell Structure and Functions

• In multicellular organisms, each organ system is made up of several organs.
• Organs are further made up of tissues.
• Tissues are groups of similar cells performing a specific function.
• Number of cells Organisms made up of only a single cell are called unicellular organisms.
  For example: Amoeba and Paramecium
• Single cell in unicellular organisms performs all the basic functions such as digestion, respiration, and excretion.
• Organisms made up of more than one cells are called multicellular organisms.
  For example: Humans, cow, etc.
• In multicellular organisms, the cells show division of labour as a particular set of cells are involved in performing a specific body function. 

Types of cell

There are two types of cells.

• Prokaryotic cells
• Eukaryotic cells

1. Prokaryotic cells
Cells which do not have a well defined nuclear membrane and the nuclear material lies freely in the cytoplasm of the cell. For example: bacteria, blue green algae.

2. Eukaryotic cells
Cells having nucleus with well defined nuclear membrane. For example: plant and animal cells

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