Explain Structural Organisation of a Cell 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.

This page offers students with study source material that has been compiled by a team of subject-matter experts. Solving NCERT Solutions for Class 9 Science enables students to get the highest score on the CBSE Class 9 Science Term II test.

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.

Fill Up The Form & Our Team Will Provide You With All Help

Advantages of Studying from CBSE Class 9 Science Notes

  1. It helps in the easy understanding of ideas.
  2. These notes are quite useful for quickly revising the whole chapter.
  3. It includes all important subjects, ideas, and dates.
  4. By reading through these notes, students will have a better understanding of which subject they should focus on.

Explain Structural Organisation of a Cell

Structural Organisation of a Cell

Microscopic examination studies indicated that all cells have three fundamental characteristics, namely the plasma membrane, the nucleus, and the cytoplasm. Due to the existence of these characteristics, all internal processes and interactions between the cell and its environment are possible.

Plasma Membrane or Cell Membrane

This is the cell’s outermost living, thin, and sensitive layer. It serves as a barrier between the cell’s contents and its external environment.

See also  CBSE NCERT Class 9 Maths Chapter 15 Probability

The presence of lipids and proteins (as phospholipids) allows the plasma membrane to be flexible. It permits the cell to absorb food and other substances from the surrounding environment. This is referred to as endocytosis, e.g. Amoeba obtains food by this mechanism, which is facilitated by finger-like structures known as pseudopodia.

Functions of Plasma Membrane

  1. It assists in the cell’s shape maintenance.
  2. It works as a mechanical barrier, preventing the cell’s interior contents from spilling out.
  3. It protects against microbes and foreign substances.
  4. It supports the entry of certain selected elements into and out of the cell. As a result, the cell membrane is semipermeable, selectively permeable, partly permeable, and variably permeable.
  5. It is modified to fulfil many functions, for example, microvilli in the human body for absorption.
  6. Due to its semi permeability, the cell is able to maintain homeostasis.

Among the functions mentioned above, the most important is the transfer of


Transport Across the Membrane

Substances may be transported through a membrane with or without the expenditure of energy (active transport) (passive transport).

Transport Across the Membrane by Diffusion

    • Diffusion is the natural movement of material (solid, liquid, or gas) from a location of greater concentration to a region of lower concentration.
    • For example, CO2 (a waste product of the cell that must be expelled) accumulates at a greater quantity within the cell. The concentration of CO2 in the external environment of the cell is lower than the concentration within the cell.
    • CO2 diffuses out of the cell as a result of this concentration difference. Similarly, O2 enters the cell through diffusion when the amount or concentration of O2 lowers inside the cell.
    • In gases, diffusion is quicker than in liquids and solids. It is involved in gaseous exchange both inside and between cells, as well as between the cell and its external environment.
    • Along with gaseous exchange, diffusion aids an organism in getting nutrients from its surroundings.

Transport Across the Membrane by Osmosis

    • Osmosis is the process by which water molecules pass through a selectively permeable membrane along a concentration gradient. The quantity of material dissolved in water also affects the transport of water through the plasma membrane.
    • Thus, osmosis is also described as the movement of water molecules across a semipermeable membrane from a location of greater concentration to a region of lower concentration. Unicellular freshwater creatures and the majority of plant cells get water by osmosis.
    • Osmosis is also shown by the absorption of water by plant roots. Osmosis may be seen in a cell put in a solution with varying concentrations (such as hypotonic, isotonic and hypertonic).
See also  CBSE NCERT Class 9 English Chapter 10 Supplementary The Beggar

Isotonic Solution: The medium surrounding a cell has the same amount of water as the water contained inside the cell.

Water passes through the cell membrane in both directions, but the quantity arriving remains constant relative to the amount leaving. As a result, there is no general flow of water. As a consequence, no overall change in cell size is seen.

Hypotonic Solution: The medium or solution around the cell has a greater concentration of water than the solution inside the cell (or the outside solution is very diluted).

The cell absorbs water and swells as a result of endosmosis. This occurs because water molecules are allowed to move in both directions via the cell membrane. However, more water enters the cell than exits.

Solution Hypertonic: The medium surrounding a cell contains less water than the cell itself (i.e. outside solution is very concentrated). Water passes through the cell membrane in both directions, but more water exits the cell than enters it this time. As a consequence, the protoplasm of the cell… shrinks (exosmosis).

Cell Wall

Outside the plasma membrane, it is a tough, non-living coating. It is present in the cells of plants and fungi. It is permeable. It is mostly composed of cellulose, a complex material that gives plants their structural strength.

Functions of Cell Wall

    • Plants, fungus, and bacteria cells have a cell wall that enables them to endure hypotonic circumstances without bursting.

In the hypotonic medium, cells prefer to absorb water by osmosis. The cell swells, exerting pressure on the cell wall. The wall applies equal pressure to both sides of the enlarged cell. Plant cells benefit from a thicker cell wall, which allows them to tolerate more changes in their surrounding media than animal cells do.

  • It has small pores known as pits. They allow for the crossing of cell walls by fine cytoplasmic strands (or cytoplasmic bridges) called plasmodesmata. Plant cells communicate with one another through these cytoplasmic channels.


It is a phenomenon that occurs when a live plant cell is maintained in a hypertonic solution and loses water by osmosis. It is the process by which protoplasm shrinks or contracts away from the cell wall.


It is often referred to as the cell’s brain. It is encased in a double-layered membrane known as the nuclear membrane. It has multiple pores referred to as nuclear pores. They transport substances from the nucleus to the cytoplasm.

See also  CBSE Class 9 Geography Chapter 2 Physical Features of India Notes

Chromosomes are contained in the nucleus. They are only apparent as rod-shaped structures when the cell is on the verge of dividing. It is surrounded by a liquid ground substance known as nucleoplasm. It includes substances from the nucleolus and chromatin.

The nucleolus is a roughly circular structure located inside the nucleolus. It is membrane-free. It is referred to as the ribosome factory.

Chromatin is a network of long, thread-like structures that is intertwined. During cell division, it condenses to produce chromosomes.

Functions of Nucleus

    • It aids in the transfer of inherited characteristics from parents to children.
    • It is in charge of the cell’s metabolic functions. When the protoplasm is gone, it dries up.
    • The nucleus is critical for cellular reproduction. A cell splits into two new cells throughout this procedure.
    • It controls cell growth and maturation by guiding the cell’s chemical activity.

Due to the lack of a nuclear membrane in certain species, such as bacteria, the nuclear area of the cell is poorly defined. In these species, the nuclear region is entirely composed of nucleic acid. The term “nucleoid” refers to such an

indeterminate nuclear area.

Prokaryotes are organisms whose cells lack a nuclear membrane

(pro = primitive, karyote = karyon = nucleus).

Additionally, prokaryotes lack cytoplasmic organelles. Thus, the majority of functions are conducted by poorly developed cytoplasmic regions. For example, chlorophyll is connected with membrane vesicles in photosynthetic prokaryotic bacteria. It lacks plastids, which are found in photosynthetic eukaryotes.

Eukaryotes are organisms with a well-defined nucleus enclosed in a nuclear membrane. 

Plant and animal cells are subcategories of eukaryotic cells. These are also very distinct from one another in a variety of ways.


Cytoplasm refers to the vast portion of each cell that is contained by the cell membrane. It is the volume of fluid contained inside the plasma membrane. It is composed of several specialized cell organelles, each of which serves a distinct role in the cell.

Functions of Cytoplasm

      • It facilitates material exchange between cell organelles.
      • It serves as a source for important substances like amino acids, carbohydrates, vitamins, and iron, among others.
      • It serves as the location of many metabolic processes, including glycolysis.


It is the cell’s life material. It is made up of two components: cytoplasm and nucleoplasm.

CBSE Class 9 Science Detailed Notes​

error: Content is protected !!
Scroll to Top

get all notes now!

Now you can get all notes as PDF or Printed Book. Get them now for fast revision and better marks in exams