Board – CBSE

Std- 9

Topic- Fundamental Unit of Life

Revision Notes

Introduction

• “Cell is the structural and functional unit of life. It is the basic unit of life”.
• It was discovered by Robert Hook in 1831 in a cork slice.
• Leeuwenhoek (1674) discovered the free-living cells in pond water.

The cell theory

• Cell is the basic unit of life and was presented by two biologists, Schleiden and Schwann.
• The cell theory by Virchow suggests that all cells arise from pre-existing cells.

Types of organisms

• Unicellular Organism: These organisms are single-celled which perform all the functions. Examples: Amoeba, paramecium, bacteria.
• Multicellular Organism: Many cells are grouped together to perform a different function in the body and also form various body parts. Examples: fungi, plants, animals.

Types of cells

There are two types of cells:

(i) Prokaryotes

(ii) Eukaryotes

Difference between Animal Cell and Plant Cell

Diffusion

• The random movement of a substance from a region of high concentration to the region of low concentration is called diffusion.
• Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. The cell also obtains nutrition from the environment.

Osmosis

• The movement of water molecules through the selectively permeable membrane along the concentration gradient is called osmosis.
• Plant cells tend to obtain water through osmosis.

Behave of cell in Hypotonic or Hypertonic or Isotonic solution

Plasma membrane or Cell membrane

• The outermost covering of the cell separates the contents of the cell from the external environment.
• It regulates the entry and exit of substances through the cell (selectively permeable membrane).
• It is made up of lipid and protein.

Cell Wall

• Cell wall is another rigid outer covering in addition to the plasma membrane found in a plant cell.
• The plant cell wall is mainly composed of cellulose.

The function of the Cell wall

• Cell wall permits plants, fungi, and bacterial cells to withstand hypotonic external media without bursting.
• Prevent cells from extreme conditions.

Plasmolysis

• When a living plant cell loses water through osmosis, it causes shrinkage or contraction of the cell away from the cell wall. This phenomenon is known as plasmolysis.

Nucleus

Composition of Nucleus

• The nucleus has a double-layered covering called a nuclear membrane.
• The nuclear membrane has pores that allow the transfer of material.
• The nucleus contains chromatin fiber, (Chromosomes are visible in a dividing cell).
• Chromosomes are composed of DNA and protein.

Functions of chromosomes

• Chromosomes contain hereditary information.
• DNA molecules contain the information necessary for cellular activity.
• Functional segments of DNA are called genes.

Functions of Nucleus

• The nucleus plays a central role in cellular reproduction.
• It directs the chemical activities of the cell.

Nucleoid

• In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane.
• Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

Cytoplasm

• The cytoplasm is the fluid inside the plasma membrane.
• It also contains many specialized cell organelles.

Function of Cytoplasm

• It helps in the exchange of material between cell organelles.
• It stores vital chemicals such as amino acid, glucose, vitamins, and iron, etc.
• It is the site of certain metabolic pathways such as glycolysis.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is a large network of membrane bound sheets.
• It looks like long tubules or round or oblong bags (vesicles).

Types of Endoplasmic Reticulum

(i) Rough endoplasmic reticulum (RER)

(ii) Smooth endoplasmic reticulum (SER)

Functions of Endoplasmic Reticulum

• RER has ribosomes attached to its surface. Ribosomes are the main site of protein synthesis.
• The SER helps in the synthesis of lipids.
• Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis.
• Some other proteins and lipids function as enzymes and hormones.
• One function of the ER is to serve as a channel for the transport of materials.
• In the liver cells, SER plays a crucial role in detoxifying many poisons and drugs.

Golgi Apparatus

• The Golgi apparatus consists of membrane-bound vesicles arranged parallel to each other in stacks called cisterns.
• These membranes often have connections with the membranes of ER.

The function of Golgi Body

• The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.
• Its functions include the storage, modification, and packaging of products in vesicles. The Golgi apparatus is also involved in the formation of lysosomes.

Lysosomes

• They keep the cell clean by digesting any foreign material.
• Lysosomes have a membrane-bounded structure filled with digestive enzymes.

Functions of Lysosomes

• Lysosomes break foreign materials entering the cell.
• When a cell gets damaged, lysosomes may burst and the enzymes digest their own cells.
• Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.

Mitochondria

• Mitochondria are known as the powerhouses of the cell

Structure of mitochondria

• Double membrane structure,
• The outer membrane is very porous.
• The inner membrane is deeply folded to increase surface area

Functions of mitochondria

• Released energy by respiration in the form of ATP (Adenosine triphosphate) molecules.
• Mitochondria have their own DNA and ribosomes. Therefore, mitochondria can make some of their own proteins.

Plastids

• Plastids are present only in plant cells.
• There are three types of plastids:
• Chromoplasts (colored plastids).
• Leucoplasts (white or colorless plastids).
• Chloroplasts (contains chlorophyll).

Structure of Plastids

• The internal structure consists of numerous membrane layers embedded in the stroma.
• Plastids also have their own DNA and ribosomes like mitochondria and are similar to their structure.

Function of Plastids

• Chloroplasts are important for photosynthesis in plants.
• Leucoplasts are primarily organelles in which materials such as starch, oils, and protein granules are stored.

Vacuoles

• Vacuoles are storage sacs for solid or liquid contents.
• They are small-sized in animal cells while plant cells have very large vacuoles.

Function of vacuoles

• In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell
• Many important substances are stored in vacuoles like amino acids, sugars, and proteins.
• In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed.

Video Summaries

1)Introduction

2)Basic Structure of Cell

3)Types of Cell

4)Cell Structure: Cell wall and Cell Membrane

5) Cell Organelles: Part-1

6)Cell Organelles: Part-2