CELL THE FUNDAMENTAL UNIT OF LIFE

 

CELL THE FUNDAMENTAL UNIT OF LIFE

What is a Cell?

• A cell is the smallest living unit of life.
• Cell was discovered by Robert Hooke (1665).

👉 Easy line: Cell = building block of life.

 

Cell Theory

The "Cell theory" was formulated by two biologists, M.J.Schleiden (1838), and T.Schwann (1839.

Cell theory states that

• Bodies of the living beings are made up of cells. The cell is the basic unit of life. 

• Activities of an organism are the sum total of activities of its cells. 

• Every new cell arises from pre-existing cells.
• Basic structure of cell

Cell Wall

• It is the outermost rigid, freely permeable layer found outside the plasma membrane in all the plant cells, bacteria, blue-green algae, some protists and all fungi. 

• It is absent in animal cells. 

• Cell walls protects the plant from the outside environment. It also provides a definite shape to the cell. 

• Cell wall of plant cells is formed of cellulose 

• It is formed of chitin in most of the fungi. 

Nucleus (Headquarter of the cell)

• Discovered by – Robert Brown (1831) "Nucleus is a double membrane bound dense protoplasmic body, which controls all cellular functions . Nucleus is considered as controller or director of the cell. 

• Structure : It is made up of following four components

• Nuclear membrane,Nucleoplasm ,nucleolus and chromatin threads.

Functions of nucleus 

• It controls all the metabolic activities of the cell. 

• It regulates cell cycle. 

• It contains genetic information and is concerned with the transmission of hereditary traits from one generation to another.

 

Endoplasmic Reticulum

• Endoplasmic reticulum is a complex network of membrane bound channels or sheets, tubules and vesicles. It does not work alone. 

• The ER works closely with the Golgi apparatus and ribosomes. It creates a network of membranes found through the whole cell. 

• Two types of Endoplasmic Reticulum ; 

• Smooth and Rough Smooth ER/Agranular ER : It has smooth membranes which do not bear ribosomes. It acts as a storage organelle. It is important in the synthesis and storage of steroids. It is mainly made up of vesicles and tubules. IT HELPS in detoxifying harmful drugs.

• Rough ER/Granular ER : It has a rough membrane because a number of ribosomes are attached to its outer surface. RER is engaged in protein synthesis. It is mainly made up of cisternae. Tubules are very few.

Golgi complex (Traffic police of cell)

• Discovered by Camillo Golgi (1898) in nerve cells of owls. Golgi carried out a revolutionary method of staining individual nerve and cell structures. 

• Position : It is located near the nucleus.

• It is involved in cell secretion and acts as storage, modification and condensation or packaging membrane. 

• It forms the lysosomes and secretory vesicles. 

• It is the site of formation of glycolipids and glycoproteins.  

• Synthesis of cell wall material (polysaccharide synthesis). 

• Cell plate formation on during plant cell division.

 

 

 

 

 

 

Lysosomes (Little enzyme packages) 

• Lysosomes are spherical bag-like structures covered by a single membrane. 

• Lysosomes hold enzymes that were created by the cell. 

• The purpose of the lysosome is to keep the cell clean by digesting any foreign materials as well as worn-out cell organelles. 

• They might be used to digest food or break down the cell when it dies. 

Mitochondria (Powerhouse of cell) 

• Mitochondria are double membrane bound organelles of eukaryotic cells.

• Mitochondria are semi autonomous organelles bound by an envelope of two unit membranes and filled with a fluid matrix. 

• The outer membrane is smooth and has porous proteins which form channels for the passage of molecules through it. 

• The inner membrane is semipermeable. 

• It usually produces numerous infolds called cristae. 

• The cristae greatly increases the inner surface area of the mitochondria to hold a variety of enzymes. 

• Cristae bears minute, regularly spaced tennis racket shaped particles known as F1 particles or oxysomes. 

• Oxysomes are concerned with ATP synthesis. Matrix contain various respiratory enzymes. 

Plastids

• Plastids are major organelles found in the cells of plants and algae. 

• Plastids are the site of manufacture and storage of important chemical compounds used by the cell. 

• Plastids often contain pigments used in photosynthesis and the types of pigments present can change or determine the cell's color. 

• Plastids are responsible for photosynthesis, storage of products like starch.Chloroplasts are the sites of photosynthesis. 

• They contain enzymes and coenzymes necessary for the process of photosynthesis. 

Plastids are of three types- 

• Chromoplasts - For pigment synthesis and storage. Chromoplasts are red, yellow and orange in color and are found in petals of flowers and in fruits. Their color is due to two pigments, carotene and xanthophyll. 

• Leucoplasts - Leucoplasts are colorless or white plastid. They occur in plant cells not exposed to light, such as roots and seeds. they are also involved in the formation of oil and proteins. 

• Chloroplasts -  Chloroplasts are probably the most important among the plastids since they are directly involved in photosynthesis. They are usually situated near the surface of the cell and occur in those parts that receive sufficient light. e.g. the palisade cells of leaves. The green color of chloroplasts is caused by the green pigment chlorophyll.

 How do substances like CO ₂ and water move in and out of the cell? Discuss.

Solution: CO ₂ moves by diffusion. The cellular waste accumulates in high concentrations in the cell, whereas the concentration of CO ₂ in the external surroundings is comparatively lower. This difference in the concentration level inside and outside of the cell causes the CO ₂ to diffuse from a region of higher (within the cell) to a lower concentration. H ₂ O diffuses by osmosis through the cell membrane. It moves from a region of higher concentration to a lower concentrated region through a selectively permeable membrane until equilibrium is reached.

4. Why is the plasma membrane called a selectively permeable membrane?

Solution: The plasma membrane is called as a selectively permeable membrane as it permits the movement of only certain molecules in and out of the cells. Not all molecules are free to diffuse.

o  Why are lysosomes known as suicide bags?

Solution: When there is damage to the cell and when revival is not possible, lysosomes may burst, and the enzymes digest their own cell. Consequently, lysosomes are known as suicide bags.

o  Where are proteins synthesised inside the cell?

Solution: Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also referred to as protein factories. Ribosomes are particles that are found attached to the rough endoplasmic reticulum.

 Why are lysosomes known as suicide bags?

Solution: When there is damage to the cell and when revival is not possible, lysosomes may burst, and the enzymes digest their own cell. Consequently, lysosomes are known as suicide bags.

9. Where are proteins synthesised inside the cell?

Solution: Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also referred to as protein factories. Ribosomes are particles that are found attached to the rough endoplasmic reticulum.

1. Make a comparison and write down ways in which plant cells are different from animal cells.

Solution: The following table depicts the differences between plant cells and animal cells.

Characteristic	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Shape of Cell	With distinct edges, the shape is either rectangular or square-shaped.	Round and irregular shape
Nucleus	Present. It lies on one side of the cell	Present. It lies in the centre of the cell
Lysosomes	Rarely present	Always present
Plastids	Present	Absent
Structure of Vacuoles	Single or a few large vacuoles that are centrally located	Presence of numerous and small vacuoles

 

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	Generally small (1-10 μm)	Generally large (5-100 μm)
Nuclear Region	Not well defined; nucleoid instead of nucleus	Well-defined; surrounded by a nuclear membrane
Chromosomes	Single chromosome	More than one chromosome
Membrane-bound Organelles	Absent	Present

 

o  What would happen if the plasma membrane ruptures or breaks down?

Solution: If the plasma membrane ruptures or breaks down, molecules of some substances will freely move in and out of the cell. Normally, the plasma membrane acts as a barrier that controls the exchange of materials through processes like osmosis and diffusion

o  What would happen to the life of a cell if there was no Golgi apparatus?

Without the Golgi apparatus, cells would be unable to properly package and transport materials essential for their functioning. This would impair processes such as secretion, cell signaling, and the maintenance of cellular structure.

 

NUTRITION IN AMOEBA-------

If ribosomes are removed from the cell no protein formation takes place.