CBSE NCERT Notes for Class 9 Science Chapter 5 The Fundamental Unit Of Life

CBSE NCERT Notes for Class 9 Science Chapter 5 The Fundamental Unit Of Life

We do know that matter is composed of particles organized in a certain manner. While the particles in gases are well separated and may move freely, the particles in solids are closely packed, with little space for movement.

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.

Discovery of Cell

Under the primitive microscope, Robert Hooke (1665) studied a thin slice of cork. Cork, he discovered, consists of little box-like structures resembling honeycombs.

He dubbed these boxes cells. Cork is derived from the bark of a tree. The cell is derived from the Latin term for a little chamber’. Cells have the following fundamental characteristics:

  1. They are capable of independence.
  2. They include information about a person’s ancestors.
  3. They are capable of carrying out all life-sustaining processes on their own.
  4. They are chemically related and have similar metabolic functions.

Cellular Composition In Different Organisms

They are classified into two types based on the number of cells present in various organisms:

  1. Multicellular organisms (having many cells).
  2. Unicellular organisms (having single cell)

Shape Of Cells

Certain cells, such as most plant and animal cells, have a stable form, whereas others, such as WBCs and amoeba, constantly change their shape. Fixed shaped cells may be elliptical (e.g. fat cell), spherical (e.g. ovum), spindle-shaped (e.g. smooth muscle cell), knobbed thread-shaped (e.g. sperm), discoidal (e.g. RBC), or elongated (e.g. nerve cell).

Size Of Cells

The size of the cell varies widely, from the smallest Mycoplasma cell (0.1-0.5 gm) to the extremely huge Ostrich egg cells (18 cm). The longest cells in the human body are the nerve cells, which may measure up to 90 cm in length.


Each living cell is capable of performing many essential processes that define living organisms.

  1. Multicellular organisms such as humans carry out these activities via the division of labour. Different sections of the human body are specialised to perform certain activities, such as the heart, which is designed to pump blood, the stomach, which is designed to digest food, and the kidney, which is designed to filter urine.
  2. Cells are defined by their shape and size in relation to the function they perform.
  3. Within a single cell, division of labour is also visible. Each cell has unique components referred to as cell organelles. These provide the organism with the ability to live and execute special activities.
  4. Organelles and protoplasm combine to form the fundamental unit of life defined as the cell. Each kind of cellular organelle serves a unique function. For instance, getting nutrients, breathing, removing waste, or producing offspring. Mitochondria is the organelle that generates energy for the 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.


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.

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 chemicals.

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).

  • 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.

  • Hypertonic Solution: 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).


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.

Plasmolysis : 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.

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 specialised 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.

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

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.


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.


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.

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.


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

  • 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.

  • 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.

  • 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. Vacuoles also play a vital role in the elimination of excess water and some wastes from some unicellular organisms.
  5. Vacuoles also play a significant part in the expulsion of excess water and certain wastes from some unicellular organisms.

NCERT questions & answers from The Fundamental Unit Of Life

Who discovered the cell, and how ?

Answer: Robert Hooke (1665). He thinly cut a piece of cork and inspected it with his homemade microscope. Cork has a variety of minute box-like structures named cells, as identified by the scientist (cellulae, later reduced to cells). His work was published in the book Micrographia.

Why is a cell called a structural and functional unit of life ?


Structural Unit: A live organism is composed of at least one cell. As a result, the cell is a fundamental structural unit of life. Unit of Function.

All of an organism’s vital processes are contained within its cells. Cells can also become specialised to execute certain jobs, such as muscular contraction or nerve impulse transmission. As a result, cells are essential components of life.

How do substances like CO2 and water move into and out of the cell ? Discuss. (CCE 2011)

Answer: CO2 moves into and out of cells by diffusion while water does it through osmosis.

Diffusion. It is the movement of particles from high-concentration regions to low-concentration regions of various substances.’

(i) Internally, a respiring cell produces more CO2. As a result, the internal concentration of the compound increases. CO2 escapes the cell into the external medium due to the lower CO2 concentration in the external medium.

(ii)CO2 is consumed during photosynthesis in photosynthetic cells. Its intracellular concentration is less than that of the surrounding medium.

As a result, CO2 diffuses from the cell’s exterior to its interior.

Osmosis: It is the process by which water is transferred from a zone of higher concentration (clean water or dilute solution) to a zone of lower concentration (strong solution) separated by a semipermeable membrane.

Semipermeable plasma membrane Cell sap is an excellent solvent. External water enters the cell via endosmosis as a result of this proclivity until the cell’s wall pressure overcomes it.

If the external medium contains a particularly strong solution, water will escape from the cell into it. The scientific term for this process is exosmosis.

Why is plasma membrane called selectively permeable membrane ? (CCE 2012)

Answer: The membrane of the cell acts as a semipermeable water barrier. It enables the entry of gases via diffusion. Ions, sugars, and amino acids are actively transported across the plasma membrane. Other materials, on the other hand, are impervious to the plasma membrane. As a result, it is selectively permeable.

Why are lysosomes known as suicide bags ? (CCE 2011, 2012, 2013)

Answer: Lysosomes contain digestive enzymes that are capable of digesting all forms of organic matter. When their covering membrane ruptures, as it does in response to cell damage, the digestive enzymes flow over the contents of the cell and digest them. Lysosomes are referred to as suicide bags because they are organelles that, when ruptured, can kill the cells that contain them.

What would happen if the plasma membrane ruptures or breaks down ?

Answer: Cytoplasm and cell organelles will leak, lysosomes will burst, and cellular contents will be digested.

What would happen to the life of a cell if there was no Golgi apparatus ? (CCE 2011)


There would be no intracellular digestion and cleansing, no molecular complexing, no excretion, and no synthesis of a new plasma membrane without the lysosome.

Which organelle is known as power house of the cell ? Why ? (CCE 2011, 2013)


Mitochondria are referred to as the cell’s powerhouse because they synthesise the majority of the ATP (adenosine triphosphate) molecules required for chemical synthesis, mechanical function, and other cellular processes.

Where do the lipids and proteins constituting the cell membrane get synthesised ?

Answer: Proteins are synthesised over ribosomes of RER while lipids are synthesised over SER.

How does Amoeba obtain food ?


The plasma membrane of an amoeba is malleable. Amoeba contributes to food particle engulfment. A phagosome engulfs and transports the ingested food particle into the Amoeba’s body.

Phagosomes and lysosomes form digestion or food vacuoles when they interact. The food vacuole is where digestion occurs. Consumption of the digested meal results in its absorption into the cytoplasm. The process by which undigested material is ejected from the cell is called exocytosis.

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