Detailed Notes -3 – chapter 5 – class 9

Introduction to Cell Organelles:

Every cell has a membrane around it to separate its contents from the external environment.

1. • Large and complex cells, especially those in multicellular organisms, require numerous chemical activities to support their structure and function.
   • To compartmentalize these activities and keep them separate, cells use membrane-bound structures called organelles.

2. Distinction Between Eukaryotic and Prokaryotic Cells:

   • Eukaryotic cells, which include cells from multicellular organisms, have membrane-bound organelles.
   • Prokaryotic cells, such as bacteria, lack membrane-bound organelles and have a simpler internal structure.

3. Visibility of Organelles:

   • Some organelles are visible only with an electron microscope due to their small size and complex structure.

4. Examples of Cell Organelles:

   • The passage lists several important organelles: endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and plastids.
   • These organelles are crucial because they perform essential functions within the cell.

5. Functions of Organelles:

   • Each organelle has specific functions that contribute to the overall functioning of the cell.
   • For example, the endoplasmic reticulum is involved in protein synthesis and lipid metabolism, while mitochondria are the powerhouse of the cell, producing energy through cellular respiration.

5.2.5 (i) ENDOPLASMIC RETICULUM (ER)

Structure of Endoplasmic Reticulum (ER):

1. • The endoplasmic reticulum (ER) is described as a large network of membrane-bound tubes and sheets.
   • It can appear as long tubules or round/oblong bags known as vesicles.
   • The structure of the ER membrane is similar to that of the plasma membrane.

2. Types of ER:

   • There are two types of ER: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

3. Rough Endoplasmic Reticulum (RER):

   • RER appears rough under a microscope due to the presence of ribosomes attached to its surface.
   • Ribosomes are the sites of protein synthesis, and RER is involved in the manufacture of proteins.
   • The proteins synthesized on the RER are then transported to various parts of the cell as needed.

4. Smooth Endoplasmic Reticulum (SER):

   • SER does not have ribosomes attached to its surface.
   • It is involved in the synthesis of fat molecules or lipids, which are important for cell function.
   • Some of these lipids and proteins contribute to the formation of the cell membrane through a process known as membrane biogenesis.

5. Functions of Proteins and Lipids Produced by ER:

   • Proteins and lipids synthesized by the ER serve various functions within the cell.
   • Some contribute to building the cell membrane, while others function as enzymes and hormones.

6. Variability of ER:

   • While the appearance of ER may vary greatly in different types of cells, it always forms a network system.

5.2.5 (ii) GOLGI APPARATUS

1. Structure of Golgi Apparatus:

   • The Golgi apparatus consists of a system of membrane-bound vesicles, which are flattened sacs.
   • These vesicles are arranged approximately parallel to each other in stacks called cisterns.
   • The membranes of the Golgi apparatus often have connections with the membranes of the endoplasmic reticulum (ER), forming a complex cellular membrane system.

2. Discovery and Naming:

   • The Golgi apparatus was first described by Camillo Golgi, an Italian physician and biologist.

3. Functions of Golgi Apparatus:

   • The Golgi apparatus functions in the packaging and dispatching of materials synthesized near the ER to various targets inside and outside the cell.
   • It is involved in the storage, modification, and packaging of products in vesicles.
   • The Golgi apparatus may also play a role in the synthesis of complex sugars from simple sugars.
   • Additionally, the Golgi apparatus is involved in the formation of lysosomes, which are organelles containing enzymes responsible for cellular digestion.

4. Connection with Endoplasmic Reticulum (ER):

   • The Golgi apparatus has connections with the membranes of the endoplasmic reticulum, indicating coordination between these two organelles in cellular processes.

5.2.5 (iii) LYSOSOMES

1. Structure of Lysosomes:

   • Lysosomes are membrane-bound sacs filled with digestive enzymes.
   • These enzymes are produced by the rough endoplasmic reticulum (RER).

2. Function of Lysosomes:

   • Lysosomes serve as the waste disposal system of the cell.
   • They help maintain cell cleanliness by digesting foreign materials and worn-out cell organelles.
   • Foreign materials entering the cell, such as bacteria or food particles, as well as old organelles, are broken down by lysosomes.
   • Lysosomes contain powerful digestive enzymes capable of breaking down all organic material.

3. Role in Cellular Metabolism:

   • During disturbances in cellular metabolism or when the cell is damaged, lysosomes may burst.
   • The enzymes released from lysosomes can then digest their own cell, aiding in the removal of damaged components.
   • Due to this ability to self-destruct, lysosomes are also referred to as the “suicide bags” of the cell.

5.2.5 (iv) MITOCHONDRIA

1. Function of Mitochondria:

   • Mitochondria are referred to as the powerhouses of the cell.
   • They release energy required for various chemical activities in the form of ATP (Adenosine triphosphate) molecules.

2. Structure of Mitochondria:

   • Mitochondria have two membrane coverings.
   • The outer membrane is porous, while the inner membrane is deeply folded.
   • These folds increase the surface area available for ATP-generating chemical reactions.

3. Role of ATP:

   • ATP is the energy currency of the cell.
   • The energy stored in ATP is used by the body for various purposes, including making new chemical compounds and performing mechanical work.

4. Unique Characteristics of Mitochondria:

   • Mitochondria have their own DNA and ribosomes.
   • This means that mitochondria are capable of producing some of their own proteins.

5.2.5 (V) PLASTIDS

1. Types of Plastids:

   • Plastids are organelles found only in plant cells.
   • There are two main types of plastids: chromoplasts and leucoplasts.
   • Chromoplasts are colored plastids, while leucoplasts are white or colorless plastids.

2. Functions of Chromoplasts and Chloroplasts:

   • Chromoplasts containing the pigment chlorophyll are known as chloroplasts.
   • Chloroplasts are essential for photosynthesis in plants, where they capture light energy to produce glucose.
   • Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.

3. Functions of Leucoplasts:

   • Leucoplasts primarily store materials such as starch, oils, and protein granules.

4. Internal Structure of Chloroplasts:

   • The internal organization of chloroplasts includes numerous membrane layers embedded in a material called the stroma.

5. Similarities with Mitochondria:

   • Plastids, like mitochondria, have their own DNA and ribosomes.

5.2.5 (vi) VACUOLES

1. Function of Vacuoles:
   • Vacuoles are storage sacs for solid or liquid contents within the cell.
   • In animal cells, vacuoles are typically small-sized, while plant cells have very large vacuoles.
   • The central vacuole in some plant cells can occupy a significant portion of the cell volume.
   • In plant cells, vacuoles are filled with cell sap, providing turgidity and rigidity to the cell.
2. Contents of Vacuoles:
   • Vacuoles store various substances important for the life of the cell, including amino acids, sugars, organic acids, and proteins.
   • In single-celled organisms like Amoeba, food vacuoles contain consumed food items.
   • Specialized vacuoles in some unicellular organisms help expel excess water and waste from the cell.
3. Importance of Cell Structure and Organization:
   • The organization of membranes and organelles within the cell determines its structure and ability to function.
   • This organization allows cells to perform essential functions such as respiration, obtaining nutrition, clearing waste material, and forming new proteins.
4. Significance of the Cell:
   • The cell is described as the fundamental structural unit of living organisms.
   • It is also referred to as the basic functional unit of life, highlighting its central role in biological processes.

Cell Division

1. Purpose of Cell Division:

   • New cells are formed in organisms for various reasons, including growth, replacing old or damaged cells, and producing gametes for reproduction.

2. Types of Cell Division:

   • There are two main types of cell division: mitosis and meiosis.

3. Mitosis:

   • Mitosis is the process by which most cells divide for growth and tissue repair.
   • During mitosis, a mother cell divides to form two identical daughter cells, each with the same number of chromosomes as the mother cell.

4. Meiosis:

   • Meiosis is a special type of cell division that occurs in cells of reproductive organs or tissues.
   • It involves two consecutive divisions and produces four new cells, each with half the number of chromosomes as the mother cell.
   • The cells produced by meiosis are gametes, which are required for sexual reproduction.

5. Reduction in Chromosome Number:

   • The chromosome number is halved in daughter cells produced by meiosis.
   • This reduction is necessary for sexual reproduction because when two gametes fuse during fertilization, they restore the original chromosome number in the resulting zygote.