What Are Cell Organelles? CBSE NCERT Notes for Class 9 Science Chapter 4 Structure Of The Atom

CBSE NCERT Notes for Class 9 Science Chapter 4 Structure Of The Atom​

Chapter 4 Structure of NCERT Solutions for Class 9 Science At skillyogi, the Atom gives students with answers to all of the questions in the NCERT Class 9 textbook, which is aligned with the CBSE board.

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CBSE Class 9 Science notes will assist students in studying the topic thoroughly and clearly.

These CBSE Class 9 Science notes were written by subject experts who made the study material very basic, both in terms of language and format.

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What are Cell Organelles?

Cell Organelles

To sustain their sophisticated structure and function, large and complex cells need a high level of chemical activity. These cells use membrane-bound structures to maintain these activities distinct from one another. These structures, dubbed cell organelles, serve specialised duties inside the cell. This is the primary distinguishing feature between eukaryotic and prokaryotic cells.

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Endoplasmic Reticulum (ER)

It is a vast network of tubes and sheets that are membrane-bound. It spreads into the cytoplasm from the outer nuclear membrane. It seems to be lengthy tubules encased in round and oblong bags (vesicles).

The ER membrane resembles the plasma membrane in structure.

It is classified into three types: cisternae, vesicles, and tubules. ER may be classified into two categories according on the nature of its membrane:

  • Rough Endoplasmic Reticulum (RER) : It includes ribosome particles, which contribute to its rough surface. The ribosomes are where proteins are synthesised. The RER is mostly composed of cisternae.
  • Smooth Endoplasmic Reticulum : It contributes to the synthesis of fat molecules, or lipids. It is composed mostly of vesicles and tubules. Due to the lack of ribosomes, its surface is smooth. In various cells, ER takes on a variety of forms. It is always composed of a network of vesicles and tubules.
  • Functions of Endoplasmic Reticulum
  1. SER’s production of fat and lipid molecules contributes to the formation of the cell membrane. This is referred to as membrane biogenesis.
  2. All functioning cells have ribosomes, which serve as sites for protein synthesis. The endoplasmic reticulum transports the proteins generated here throughout the cell.
  3. SER is required for the detoxification of poisons and drugs in vertebrate liver cells (group of animals).
  4. ER synthesises additional proteins and lipids that act as enzymes and hormones.
  5. It produces a network structure that facilitates the transfer of materials, particularly proteins. It is involved in transport between different areas of the cytoplasm or between the cytoplasm and the nucleus.
  6. It supports the cells mechanically.
  7. It serves as a scaffolding for the cytoplasm. It serves as a surface for a number of the cell’s metabolic operations.

Golgi Apparatus

It is composed of a network of membrane-bound, fluid-filled vesicles, large spherical vacuoles, and flattened, smooth cisternae. These are parallel to one another. Each one of these stacks is referred to as a cistern. The Golgi apparatus (or dictyosomes) develops from the smooth ER membrane. As such, it is another component of a complex cellular membrane system.

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The substance synthesised close to the Endoplasmic Reticulum (ER) is packed and distributed throughout the cell through the Golgi apparatus.

Functions of Golgi Apparatus

  1. In vesicles, the Golgi apparatus stores, changes, and packs products.
  2. It is involved in the lysosome formation process.
  3. In certain cases, it converts simple sugars to complex sugars.
  4. It has a role in the synthesis of the cell wall and plasma membrane.


These are the cell’s waste disposal systems. Lysosomes are membrane-enclosed sacs containing digesting enzymes. The rough endoplasmic reticulum synthesises these enzymes. Lysosomes are sometimes referred to as a cell’s suicide bags. When cellular metabolism is disrupted or the cell is destroyed, lysosomes may burst, allowing the enzymes to digest their own cell. They are not present in red blood cells.

Functions of Lysosomes

  1. They contribute to the cell’s cleanliness by digesting. foreign substance that penetrates the cell, as well as worn-out organelles. As a result, they are referred to as scavengers and cellular housekeepers.
  2. They eliminate foreign material by slicing it up with their powerful digestive enzymes. These enzymes are capable of degrading all organic compounds.
  3. During starvation, the lysosomes decompose stored food and provide energy to the cell through autophagy.


Milker discovered mitochondria for the first time in 1880. It is a cell organelle bordered by two membranes. The outer membrane is quite porous. The inner membrane is deeply folded into projections resembling fingers known as cristae. It generates a large surface area for chemical processes that generate ATP.

Intermembrane space is the space between the outer and inner membranes. Mitochondrion is a self-replicating organelle (semi autonomous). In animal cells, it is the biggest organelle.

Functions of Mitochondria

  1. It creates energy for the cell’s many functions. It is referred to as the cell’s powerhouse. Mitochondria are the organelles responsible for cellular respiration. They provide the cell with the energy it requires in the form of ATP (Adenosine Triphosphate). This ATP is referred to as the cell’s energy currency.
  2. When a cell needs energy, the ATP molecule degrades. It generates energy for the body’s metabolic processes.
  3. Mitochondria are unusual organelles since they contain their own DNA and ribosomes. As a result, they are capable of producing some of their own proteins.
  4. They serve as intermediates for the synthesis of a variety of chemicals, including fatty acids, steroids, and amino acids.
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These are present only in plant cells. Plastids’ internal organisation is composed of many membrane layers embedded in a substance called stroma. Externally, plastids resemble mitochondria. They are constructed in two layers. They are self-contained in terms of DNA and ribosomes.

Types of Plastids

  1. Chloroplasts : These are the chlorophyll-containing plastids (a green pigment). They provide a green colour to the plant. In addition to chlorophyll, chloroplasts contain a variety of yellow or orange pigments. It is a semiautonomous cellular structure. Chloroplasts are sometimes referred to as the cell’s kitchen. Function These are necessary components of photosynthesis in plants.
  2. Chromoplasts : These are plastids that are coloured (except green).

Chromoplasts are responsible for the coloration of flowers and fruits. They have a high concentration of carotenoid pigments and lipids.

  1. Leucoplasts : These are the plastids that are white or colourless. They are capable of transforming into different forms of plastids. Function Leucoplasts are structures that store carbohydrates (amyloplasts), oils (elaioplasts), and protein granules (aleuroplasts).


These are the containers that contain solid or liquid contents. Animal cells have small vacuoles, while plants have big vacuoles. Some may take up between 50% and 90% of the whole cell capacity. A membrane termed the tonoplast surrounds the vacuole.

Functions of Vacuoles

  1. Vacuoles contain cell sap and give cells in plants their turgidity and rigidity.
  2. Numerous chemicals are held in vacuoles, including amino acids, carbohydrates, organic acids, and proteins.
  3. Consumed food items are kept in food vacuoles in Amoeba.
  4. In some unicellular organisms, vacuoles also play an important role in expelling excess water and some wastes from the cell. 
  5. Vacuoles also play a significant part in the expulsion of excess water and certain wastes from some unicellular organisms.

CBSE Class 9 Science Detailed Notes​

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