NCERT Class 11 Biology Chapter 8 Notes Cell The Unit Of Life- Download PDF Notes

NCERT Class 11 Biology Chapter 8 Notes Cell – The Unit of Life explores the cell as the fundamental structural and functional unit of life. These NCERT notes provide a concise and well-structured summary of key concepts, including the definition of living and non-living organisms, characteristics of cells, types of cells, differences between prokaryotic and eukaryotic cells, cell theory, and cell organelles.

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This Story also Contains

1. NCERT Class 11 Biology Chapter 8 Notes
2. Most Important Questions Class 11 Biology Chapter 8 Cell: The Unit Of Life
3. Approach to Solve Questions of Class 11 Biology Chapter 8
4. Chapter-Wise NCERT Class 11 Notes Biology

These NCERT Class 11 Biology Notes simplify complex topics, making them easier to understand and ideal for quick revision before CBSE Board exams and competitive exams like NEET. With NCERT Solutions and downloadable PDF notes, students can efficiently reinforce their understanding and save time during exam preparation.

Also, students can refer to:

• NCERT Solution for Class 11 Biology Chapter 8 Cell: The Unit of Life
• NCERT Exemplar Class 11 Biology Chapter 8 Solutions Cell: The Unit of Life

NCERT Class 11 Biology Chapter 8 Notes

If we observe, we find that there are both living and non-living things around us. But what gives an organism its life? It is the cell, which is the building block of life, that exists in all living organisms.

All living organisms are composed of cells. Bacteria and other organisms are unicellular, consisting of a single cell that carries out all the functions of life. Multicellular organisms, such as human beings, consist of numerous cells acting in unison.

The first cell of life was viewed and reported by Anton Van Leeuwenhoek, and subsequently, the nucleus was found by Robert Brown. Microscope, and then electron microscope, made scientists familiar with the intricacies of the cell's structure.

Cell Theory

Matthias Schleiden, a German botanist, researched plants in 1838 and discovered that all plants consist of various forms of cells, which collectively constitute plant tissues.

At about the same period, British zoologist Theodore Schwann (1839) examined animal cells and found that they have a thin outer covering, termed the plasma membrane. He also described that plant cells possess a cell wall that is not found in animal cells.

Schleiden and Schwann collectively developed the cell theory.

But their theory didn't account for the way in which new cells are created. In 1855, Rudolf Virchow expanded on the theory by declaring that new cells come from existing cells.

Modern Cell Theory Holds That:

• All living things consist of cells and their byproducts.
• New cells are produced only from existing cells via cell division.

Prokaryotic Cells

Prokaryotic cells are tiny compared to eukaryotic cells and reproduce rapidly. They are also available in various shapes, which are as follows:

• Bacillus (rod-shaped)
• Coccus (spherical)
• Vibrio (comma-shaped)
• Spirillum (spiral-shaped)

Even though prokaryotic cells differ from each other in terms of shape and functionality, they follow a common basic structure. All prokaryotes except mycoplasma have a cell wall outside the cell membrane.

The cytoplasm is a jelly-like liquid within the cell. Prokaryotes do not have a distinct nucleus since their genetic material is not covered with a nuclear membrane. Rather, their DNA occurs freely in the cytoplasm.

Plasmids and Genetic Material

• Plasmids occur in many bacteria as small circular DNA fragments beyond their primary DNA.
• Plasmids provide bacteria with unique characteristics, including antibiotic resistance.
• Researchers employ plasmids in genetic engineering to introduce foreign DNA into bacteria.
• In contrast to eukaryotic cells, prokaryotic cells do not have membrane-bound organelles other than ribosomes.

Cell Structure and Envelope

Prokaryotic cells possess a complex protective layer known as the cell envelope, which is composed of:

• Glycocalyx (outer layer) – May be a slime layer (loose covering) or a capsule (tough outer coating).
• Cell Wall (middle layer) – Gives strength and support.
• Plasma Membrane (inner layer) – A phospholipid bilayer that protects the cell and facilitates communication.
• Bacteria are differentiated as Gram-positive or Gram-negative by their cell wall structure, by the method known as Gram staining, invented by Hans Christian Gram.

Movement and Surface Structures

Flagella and cilia facilitate movement, propelled by a basal body.

Pili and fimbriae are short,t hair-like structures facilitating surface attachment for bacteria.

Internal Structures

• Mesosomes – Folding of the cell membrane that helps in cell wall construction and replication of DNA.
• Chromatophores – Present in photosynthetic bacteria such as cyanobacteria, they contain pigments for photosynthesis.
• Ribosomes – Small organelles in the cytoplasm that assist in protein synthesis. They are categorised according to their rate of sedimentation in Svedberg (S) units.

Eukaryotic Cells

Eukaryotic cells are larger and more complex than prokaryotic cells. They have a well-defined nucleus enclosed by a nuclear membrane and contain membrane-bound organelles that perform specialised functions. Eukaryotic cells can be unicellular (like amoebas) or multicellular (like plants and animals).

Cell Structure

Eukaryotic cells have three main parts:

1. Plasma Membrane – A phospholipid bilayer that protects the cell and controls what enters and exits.

2. Cytoplasm – A fluid-filled space containing organelles that carry out life functions.

3. Nucleus – The control centre of the cell, containing DNA and responsible for growth and reproduction.

Cell Organelles and Their Functions

• Nucleus – Contains chromosomes (DNA) and directs cell activities.

• Endoplasmic Reticulum (ER) – Helps in protein and lipid synthesis.

  • Rough ER has ribosomes and makes proteins.

  • Smooth ER makes lipids and detoxifies chemicals.

• Golgi Apparatus – Packages and transports proteins and lipids.

• Mitochondria – The powerhouse of the cell, producing energy (ATP) through cellular respiration.

• Lysosomes – Contain digestive enzymes to break down waste.

• Plastids (only in plants) – Help in photosynthesis (chloroplasts) and store food (leucoplasts).

• Vacuoles – Store water, nutrients, and waste; larger in plant cells.

Cell Wall and Cytoskeleton

• Cell Wall (only in plant cells) – Provides structure and support.

• Cytoskeleton – A network of protein filaments that maintains cell shape and helps in movement.

Differences Between Plant and Animal Cells

• Plant Cells have a cell wall, large vacuole, and chloroplasts for photosynthesis.

• Animal Cells have centrioles and lysosomes, which help in cell division and digestion.

Eukaryotic cells allow complex life forms to exist by efficiently carrying out essential biological functions through their specialised organelles.

Also Read

• NCERT Books for Class 11
• NCERT Syllabus for Class 11

Plant Cell VS. Animal Cell

Cell Organelles

Cell organelles are specialised structures contained within a cell that have specific functions to maintain the cell alive and in its optimum working condition. They are membrane-bound in eukaryotic cells, while prokaryotic cells have most of them missing, except for ribosomes. Here is a detailed description of each organelle and what it does.

Nucleus – The Control Centre

• The nucleus is the largest organelle in eukaryotic cells.

• It contains genetic material (DNA), which controls cell activities.

• The nuclear membrane encloses the nucleus and has pores for the exchange of materials.

• Inside the nucleus, the nucleolus is responsible for making ribosomes.

Endoplasmic Reticulum (ER) – The Transport Network

• A network of membranes that transports materials within the cell.

• Types:

  • Rough ER (RER): Studded with ribosomes, helps in protein synthesis.

  • Smooth ER (SER): Lacks ribosomes, produces lipids, and detoxifies harmful substances.

Ribosomes – The Protein Factories

• Tiny structures are responsible for protein synthesis.

• Found freely floating in the cytoplasm or attached to the rough ER.

• Present in both prokaryotic and eukaryotic cells.

Golgi Apparatus – The Packaging and Dispatch Centre

• A stack of flattened sacs that modifies, sorts, and packages proteins and lipids.

• Sends these materials to their destinations within or outside the cell.

• Produces lysosomes in animal cells.

Mitochondria – The Powerhouse of the Cell

• Mitochondria produce energy (ATP) through cellular respiration.

• They have a double membrane, with the inner membrane forming folds called cristae to increase surface area for energy production.

• They contain their own DNA and ribosomes, allowing them to reproduce independently.

Lysosomes – The Digestive System of the Cell

• Small vesicles filled with enzymes that break down waste materials, old cell parts, and foreign substances.

• Help in cell defence by destroying harmful invaders.

• Known as suicidal bags, as they can cause cell self-destruction if necessary.

Vacuoles – Storage Units

• Fluid-filled sacs are used for storing water, nutrients, and waste.

• Large and central in plant cells to provide support.

• Smaller in animal cells and helps in waste disposal.

Plastids – The Colour and Food Factory (Only in Plant Cells)

• Chloroplasts – Contain chlorophyll and perform photosynthesis.

• Chromoplasts – Provide pigments to flowers and fruits.

• Leucoplasts – Store starch, proteins, and fats.

Cytoskeleton – The Support System

• A network of protein filaments that gives the cell its shape and structure.

• Helps in cell movement and internal transport.

Peroxisomes – The Detox Centres

• Contains enzymes that break down fatty acids and harmful substances like hydrogen peroxide.

• Help in the detoxification of the cell.

Cilia and Flagella – The Movement Organelles

• Cilia – Small, hair-like structures that help in movement and sensing the environment.

• Flagella – Long, whip-like structures that help in cell movement, seen in sperm cells and bacteria.

Centrosome and Centrioles – The Cell Division Helpers (Only in Animal Cells)

• The centrosome contains centrioles, which help in cell division by forming the spindle fibers during mitosis.

Subject-Wise NCERT Exemplar Solutions

• NCERT Exemplar Class 11 Solutions
• NCERT Exemplar Class 11 Maths
• NCERT Exemplar Class 11 Physics
• NCERT Exemplar Class 11 Chemistry
• NCERT Exemplar Class 11 Biology

Most Important Questions Class 11 Biology Chapter 8 Cell: The Unit Of Life

This chapter explains the structure, function, and significance of cells as the basic units of life, covering organelles, cell types, and cellular processes.

Q1. What is the importance of a vacuole in a plant cell?
A. Stores proteins
B. Maintains turgor pressure and stores cell sap
C. Site of photosynthesis
D. Controls cell division

Answer:
The vacuole is a membrane-bound space in plant cells that stores sap, water, and waste materials. It maintains turgor pressure against the cell wall, helping the cell keep its shape and fluid balance.
Hence, the correct option is B. Maintains turgor pressure and stores cell sap.

Q2. What are satellite chromosomes?
A. Chromosomes without centromeres
B. Chromosomes with secondary constrictions and small fragments called satellites
C. Chromosomes found only in bacteria
D. Chromosomes that carry mitochondrial DNA

Answer:
Satellite chromosomes have an additional or secondary constriction at the distal end, forming small outgrowths called satellites. Examples include chromosomes 13, 14, 15, 16, 21, and 22.
Hence, the correct option is B. Chromosomes with secondary constrictions and small fragments called satellites.

Q3. Which cell organelle is found only in animal cells?
A. Chloroplast
B. Cell wall
C. Centrosome
D. Vacuole

Answer:
Centrosomes are present exclusively in animal cells and play a key role in cell division.
Hence, the correct option is C. Centrosome.

Q4. What are the two main types of cells based on the presence or absence of a nucleus?
A. Prokaryotic and Eukaryotic
B. Plant and Animal
C. Bacterial and Viral
D. Single-celled and Multicelled

Answer:
Cells are classified as prokaryotic (without a membrane-bound nucleus) and eukaryotic (with a membrane-bound nucleus).
Hence, the correct option is A. Prokaryotic and Eukaryotic.

Q5. Why are mitochondria called the "powerhouses" of the cell?
A. They store genetic material
B. They synthesise proteins
C. They produce energy in the form of ATP through aerobic respiration
D. They digest waste materials

Answer:
Mitochondria generate cellular energy by producing ATP through aerobic respiration, supplying power to the cell.
Hence, the correct option is C. They produce energy in the form of ATP through aerobic respiration.

Subject Wise NCERT Solutions

• NCERT Solutions for Class 11 Mathematics
• NCERT Solutions for Class 11 Chemistry
• NCERT Solutions for Class 11 Physics

• NCERT Solutions for Class 11 Biology

Approach to Solve Questions of Class 11 Biology Chapter 8

To answer Cell: The Unit of Life questions well, adopt this easy-to-follow approach:

1. Understand the general definition of a cell, its discovery, and why it is referred to as the structural and functional unit of life.
2. Learn the distinction between prokaryotic and eukaryotic cells, their structures, and examples.
3. Study the structure and functions of all major cell organelles, such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, chloroplasts, and vacuoles.
4. Recall the cell membrane and cell wall structure and function, as well as their protective, supportive, and substance transport roles.
5. Practice drawing neat, labelled diagrams of cell types, organelles, and special structures such as cilia and flagella (with their 9+2 arrangement)

Chapter-Wise NCERT Class 11 Notes Biology