CBSE Class 9 Science The Fundamental Unit of Life Chapter ( Handwritten Notes )

NEW LEARNET EDUCATION

CHAPTER 5

THE FUNDAMENTAL UNTI OF LIFE

NOTES OF FULL CHAPTER

Life, as we know it, took billions of years to evolve. From the very first precursor of life to the multitude of multicellular organisms that we see around us today :

All About Cells

It is the structural and functional unit of life.

→ Cell is termed as the structural unit of life as it provides structure to our body.

→ Cell is considered as the functional unit of life as all the functions of the body take place at cell level.

Discovery of cell:

→ Discovered by Robert Hooke in 1665.

→ Robert Brown in 1831 discovered the nucleus in the cell.

Cell Theory:

Cell theory states that:

→ All living organisms are composed of cells.

→ Cell is the fundamental unit of life.

→ All new cells come from pre-existing cells.

Cellular respiration

Cellular respiration is the process by which the food releases energy in the mitochondria. Cells absorb glucose from the food and burn it to produce energy.

Types of Organisms on the Basis of Number of Cells

There are two kinds of organisms on the basis of cells:

(i) Unicellular Organisms: The organisms that are made up of single cell and may constitute a whole organism, are named as unicellular organisms.

For example: Amoeba, Paramecium, bacteria, etc.

(ii) Multicellular Organisms: The organisms which are composed of a collection of cells that assume function in a coordinated manner, with different cells specialized to perform particular tasks in the body, are named as multicellular organisms.

For example: Plants, human beings, animals, etc.

Shape and Size of Cells

→ Cells vary in shape and size. They may be oval, spherical, rectangular, spindle shaped, or totally irregular like the nerve cell.

→ The size of cell also varies in different organisms. Most of the cells are microscopic in size like red blood cells (RBC) while some cells are fairly large like nerve cells.

The cells can be categorized in two types:

1. Prokaryotic Cell  

2. Eukaryotic Cell

1. Prokaryotic cell
Prokaryotic cells are cells in which true nucleus is absent. They are primitive and incomplete cells. Prokaryotes are always unicellular organisms. For example, archaebacteria, bacteria, blue green algae are all prokaryotes.

2. Eukaryotic Cell
Eukaryotic cells are the cells in which true nucleus is present. They are advanced and complete cells. Eukaryotes include all living organisms (both unicellular and multicellular organisms) except bactera and blue green algae.

Difference Between Prokaryotic and Eukaryotic Cells:

S. No.	Prokaryotic cell	Eukaryotic cell
1.	Size of cell is generally small (1-10 mm).	Size of cell is generally large (5-100 mm).
2.	Nucleus is absent.	Nucleus is present.
3.	It contains single chromosome.	It contains more than one chromosome.
4.	Nucleolus is absent.	Nucleolus is present.
5.	Memrane bound cell organelles are absent.	Memrane bound cell organelles such as mitochondria, plastids, endoplasmic reticulum, golgi apparatus, lysosomes, etc., are present.
6.	Cell division takes place by fission or budding.	Cell division takes place by mitotic or meiotic cell division.

Prokaryotic & Eukaryotic cells

Two types of cell; Prokaryotic and Eukaryotic cells. Prokaryotic cells are primitive and lack well defined nucleus. Eukaryotic cells are more advanced and have well defined nucleus.The nucleus has its own membrane called nuclear membrane. 

Cell membrane

• Cell membrane is the outer covering of a cell.
• It is made up of phospho-lipid bilayer membrane.
• It is selectively permeable in nature.
• The structure of a cell membrane is best described by the fluid mosaic model.

Diffusion

The movement of molecules from a region of their high concentration to a region of their lower concentration is known as diffusion.

Osmosis in selectively permeable membrane

Osmosis is the movement of water across a semi-permeable membrane. Osmosis is a selective process since the membrane does not allow all molecules to pass through it. Water is usually the only free flowing molecule across this membrane.

Isotonic, hypotonic solutions, hypertonic solutions

• Isotonic solutions are those which have the same solute and pH concentration as the surrounding body fluid or the cytoplasm.
• Hypotonic solutions contain lesser amount of solute concentration compared to the surrounding fluid and can force the cell to rupture due to excess input of water into the cell.
• Hypertonic solutions contain higher concentration of solute compared to the surrounding fluid and thus push water out of cell, shrinking it.

Cell walls in plants

Plant cells are different from animals cells due to the presence of a cell wall. The cell wall is made of cellulose and gives a rigid structure to the plant cell.

Cell Organelles

Endocytosis

Endocytosis is the invagination of cell membrane, followed by pinching off forming a membrane bound vesicle. This is commonly seen in Amoeba.

Nucleus in cells

Nucleus is the processing unit of the cell. It is a double membrane bound organelle which contains the genetic material for inheritance.

Chromosomes

During the growth phase of the cell, the chromatin condenses into a much thicker structure called chromosome.

Chromatin

Chromatin is a thread like structure which serves as the genetic material present inside the nucleus of the cell. It is made up of DNA and protein molecules. The DNA contains the hereditary information needed for the structure and function of the organism.

Cytoplasm

Cytoplasm is the fluid found inside the cell. It gives the structure to the cell and houses different organelles of the cell.

Organelles

Organelles are structures present in the cytoplasm of the cell that help in several functions of the cell.

Endoplasmic Reticulum

Endoplasmic reticulum is a membrane like cell organelle that plays an integral role in the interpretation of the genetic information present in the nucleus.

Rough ER

Rough ER are the ones that have ribosomes on it. The ribosome is made up of nucleic acids and proteins. They are the site of protein synthesis. The Rough ER is also involved in the modification and folding of protein.

Smooth ER

Smooth ER do not have ribosomes and thus are not involved in protein synthesis. They are however, involved in the lipid metabolism and detoxifying poisonous molecules.

Golgi Apparatus

Golgi Apparatus is also called the post office of the cell. They package and transport the proteins across the cytoplasm.

Lysosomes

They are referred to as suicide bags of the cell as they contain potent enzymes that can digest a cell. Lysosome also help in defense by attacking a foreign object.

Mitochondria

Mitochondria are also called power plant of the cell. They generate ATP via the electron transport chain. They also have a DNA called mtDNA, which makes them semi-autonomous organelle.

Plastids

There are various types of plastids in different cells based on the pigment they contain. The chloroplast is the plastid where the photosynthesis occurs. Some of the other plastids are leucoplast and chromoplast.

Vacuoles

Vacuoles are large vesicles that hold water or air in them and give structural rigidity to the cell. Vacuoles are common in plant cells. In animals the vacuoles are either very small or absent.

Comparison between plant & animal cells

Plants cells are different from animals cells structurally. Plant cells have cell wall and chloroplast which are missing in animals cells. Plants cells also have large vacuoles which are either very small or missing in animal cells. The nucleus is present at the center of the cell in animal cells and at the periphery in plant cells.

The cell is the fundamental unit of life. All the life activities are carried out by cells. The organisms are classified based on the number of cells present in them. Unicellular organisms are single-celled, while multicellular organisms have a large number of cells.

Unicellular organisms are believed to be one of the earliest forms of life on earth. Eventually, more complex multicellular organisms evolved from these unicellular life forms over the aeons. Multicellular organisms have specialized cells with complicated cell organelles, which unicellular organisms typically lack.

In an ecosystem, plants have the role of producers while animals have taken the role of consumers. Hence, their daily activities and functions vary, so do their cell structure. Cell structure and organelles vary in plants and animals, and they are primarily classified based on their function. The difference in their cell composition is the reason behind the difference between plants and animals, their structure and functions.

Each cell organelle has a particular function to perform. Some of the cell organelles are present in both plant cell and the animal cell, while others are unique to just one. Most of the earth’s higher organisms are eukaryotes, including all plant and animals. Hence, these cells share some similarities typically associated with eukaryotes.

For example, all eukaryotic cells consist of a nucleus, plasma membrane, cytoplasm,  peroxisomes, mitochondria, ribosomes and other cell organelles.

As stated above, both plant and animal cells share a few common cell organelles, as both are eukaryotes. The function of all these organelles is said to be very much similar. However, the major differences between the plant and animal cells, which significantly reflect the difference in the functions of each cell.

Difference Between Animal Cell and Plant Cell:

S. No.	Animal cell	Plant cell
1.	Animal cells are generally small in size.	Plant cells are larger than animal cells.
2.	Cell wall is absent.	Plasma membrane of plant cell is surrounded by a rigid cell wall of cellulose.
3	Plastids are absent except in case of protozoan Euglena.	Plastids are present.
4.	Here vacuoles are many, small and temporary.	They have a permanent and large central sap vacuole.
5.	They have centrosome and centrioles.	They lack centrosome and centrioles.

Structure of Plant cell and Animal cell:

The major differences between the plant cell and animal cell are mentioned below:

Plant Cell	Animal Cell
Cell Shape
Square or rectangular in shape	Irregular or round in shape
Cell Wall
Present	Absent
Plasma/Cell Membrane
Present	Present
Endoplasmic Reticulum
Present	Present
Nucleus
Present and lies on one side of the cell	Present and lies in the centre of the cell
Lysosomes
Present but are very rare	Present
Centrosomes
Absent	Present
Golgi Apparatus
Present	Present
Cytoplasm
Present	Present
Ribosomes
Present	Present
Plastids
Present	Absent
Vacuoles
Few large or a single, centrally positioned vacuole	Usually small and numerous
Cilia
Absent	Present in most of the animal cells
Mitochondria
Present but fewer in number	Present and are numerous
Mode of Nutrition
Primarily autotrophic	Heterotrophic

Conclusion

Both plant and animal cells comprise membrane-bound organelles, such as endoplasmic reticulum, mitochondria, the nucleus, Golgi apparatus, peroxisomes, lysosomes. They also have similar membranes, such as cytoskeletal elements and cytosol. The plant cell can also be larger than the animal cell. The normal range of the animal cell varies from about 10 – 30 micrometres and that of plant cell range between 10 – 100 micrometres.