CBSE Class 11 Biology Notes Chapter 8 Cell the Unit of Life

CBSE Class 11 Biology Notes Chapter 8: In Chapter 8 of CBSE Class 11 Biology, you'll learn about cells, which are like tiny building blocks of life. These notes help you understand what cells are and how they work.

CBSE Class 11 Biology Notes Chapter 8 Cell the Unit of Life Overview

CBSE Class 11 Biology Notes Chapter 8 Cell the Unit of Life PDF

CBSE Class 11 Biology Notes Chapter 8 Cell the Unit of Life

Discovery of Cell

Cell Theory

Overview of Cell

Prokaryotic Cell

Eukaryotic Cell

Difference Between Prokaryotic Cell and Eukaryotic Cell

Characteristic	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent	Present
Nuclear Membrane	Absent	Present
DNA Organization	Singular circular DNA	Multiple linear chromosomes
Organelles	Few or none	Many, including mitochondria, Golgi apparatus, etc.
Ribosomes	Smaller	Larger
Cell Size	Generally smaller (1-10 μm)	Generally larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Cell Structure and Function

The structure of a cell is intricately linked to its function, as each component plays a specific role in maintaining the cell's integrity and enabling its various functions. Here's an overview of some key cellular structures and their functions:

Cell Membrane (Plasma Membrane): The cell membrane acts as a barrier, separating the internal environment of the cell from the external environment. It regulates the passage of substances in and out of the cell, ensuring that essential molecules are taken in while waste products are expelled.

Cell Wall (in plant cells): The cell wall provides structural support and protection for plant cells. It helps maintain cell shape and prevents the cell from bursting when internal pressure increases. Additionally, the cell wall allows for the exchange of water, nutrients, and gases between adjacent cells.

Cytoplasm: The cytoplasm is the gel-like substance that fills the interior of the cell. It houses various organelles and cellular structures, providing a medium for chemical reactions to occur. The cytoplasm also serves as a site for the movement of organelles and other cellular components.

Nucleus: The nucleus houses the cell's genetic material, DNA, and controls the cell's activities by regulating gene expression. It is surrounded by a double membrane called the nuclear envelope, which separates the nucleus from the cytoplasm. Within the nucleus, the DNA is organized into structures called chromosomes.

Organelles: Organelles are specialized structures within the cell that perform specific functions. Examples include:

• Mitochondria: Responsible for generating energy in the form of ATP through cellular respiration.
• Endoplasmic Reticulum (ER): Involved in protein synthesis and lipid metabolism. Rough ER has ribosomes attached to its surface, while smooth ER does not.
• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport within or outside the cell.
• Lysosomes: Contain enzymes that break down waste materials and cellular debris.
• Chloroplasts (in plant cells): Site of photosynthesis, where light energy is converted into chemical energy in the form of glucose.

Fluid Mosaic Model

Functions of Cell Membrane

Physical barrier: It separates the internal environment of the cell from the external environment, providing protection to cellular components and allowing different activities to occur independently inside and outside the cell.

Structural support: The membrane supports the cell's structure by protecting the cytoskeleton, a network of protein fibers that maintains the cell's shape and integrity.

Selective permeability: The membrane regulates the passage of molecules in and out of the cell, allowing essential substances like water, oxygen, and carbon dioxide to pass through while controlling the movement of ions and other polar molecules.

Endocytosis and exocytosis: The membrane facilitates the processes of endocytosis, where the cell engulfs large molecules or particles from the extracellular fluid, and exocytosis, where it releases substances into the extracellular space.

Cell signaling: Specialized proteins and carbohydrates on the membrane's surface play a crucial role in cell signaling and communication, allowing cells to respond to external signals and interact with one another.

What is Cell Wall?

Middle lamella: Contains polysaccharides called pectins, facilitating cell adhesion between neighboring cells.

Primary cell wall: Found in developing plant cells, consisting of hemicellulose fibers and pectin polysaccharides embedded in a matrix of cellulose microfibrils.

Secondary cell wall: Present in some plant cells, reinforced with lignin in addition to cellulose and hemicellulose, providing strength and water-conducting capabilities to vascular tissues.

Support: Providing structural support and mechanical strength to cells, regulating cell growth, and facilitating cell division.

Control cell growth: Signaling the cell cycle to initiate cell division and expansion.

Control diffusion: Regulating the passage of molecules into and out of the cell.

Communication: Facilitating communication between plant cells through channels called plasmodesmata.

Defense: Acting as a barrier against pathogens and preventing water loss.

Storage: Storing carbohydrates for energy usage during plant growth and development, particularly in seeds.

Endomembrane System

Endoplasmic Reticulum (ER): The endoplasmic reticulum is a network of tubular structures dispersed throughout the cytoplasm. It exists in two forms: rough ER (RER), which has ribosomes on its surface, and smooth ER (SER), which lacks ribosomes. RER is involved in protein synthesis and secretion, while SER plays a role in lipid synthesis and detoxification.

Golgi Apparatus: The Golgi apparatus is responsible for processing and packaging proteins and lipids, particularly those intended for export from the cell. It consists of flattened sacs called cisternae, which are stacked parallel to each other. The Golgi apparatus synthesizes glycoproteins and glycolipids and sorts and packages molecules into vesicles for transport.

Lysosomes: Lysosomes are membrane-bound vesicles containing hydrolytic enzymes produced by the Golgi apparatus. They function as the cell's "garbage disposal," breaking down unwanted molecules, such as lipids, proteins, carbohydrates, and nucleic acids, into smaller components that can be recycled or disposed of.

Vacuoles: Vacuoles are membrane-bound organelles found in the cytoplasm of cells. They contain various substances, including water, sap, and excretory materials. In plant cells, the central vacuole can occupy up to 90% of the cell's volume and is involved in storing water, maintaining turgor pressure, and storing ions, metabolites, and waste products. In other organisms like amoebas, vacuoles play roles in excretion, osmoregulation, and digestion.

Here are the functions of each component of the endomembrane system:

Endoplasmic Reticulum (ER):

Rough ER (RER):

• Synthesizes proteins destined for secretion or insertion into membranes.
• Provides a large surface area for protein synthesis due to the presence of ribosomes.

Smooth ER (SER):

• Synthesizes lipids, including phospholipids and steroids.
• Detoxifies harmful substances by enzymatic reactions.
• Stores and regulates calcium ions, which are important for signaling and muscle contraction.

Golgi Apparatus:

• Receives proteins and lipids from the ER and modifies them by adding carbohydrates (glycosylation), phosphates, or sulfates.
• Sorts and packages modified molecules into vesicles for transport to their final destinations, such as other organelles, the cell membrane, or secretion outside the cell.

Lysosomes:

• Contain hydrolytic enzymes capable of breaking down macromolecules, such as proteins, lipids, carbohydrates, and nucleic acids, into smaller components.
• Digest worn-out organelles, foreign particles, and engulfed pathogens through a process called autophagy or phagocytosis.
• Participate in programmed cell death (apoptosis) by degrading cellular components.

Vacuoles:

• Store water, ions, nutrients, and waste products, helping maintain cell turgor pressure and osmotic balance.
• Aid in the digestion of macromolecules through the hydrolytic enzymes present in certain types of vacuoles, such as lytic vacuoles in plant cells.

Vesicles:

• Transport molecules between organelles within the cell or to and from the cell membrane.
• Mediate the exchange of proteins, lipids, and other substances required for cellular processes, including secretion, endocytosis, and intracellular trafficking.

Mitochondria

Aerobic Respiration: Mitochondria are the primary sites for aerobic respiration, the process by which cells generate energy in the form of ATP (adenosine triphosphate) from oxygen and nutrients.

ATP Production: Through a series of biochemical reactions occurring in the inner mitochondrial membrane, mitochondria produce ATP, which serves as the primary energy currency of the cell.

Genetic Material: Mitochondria contain their own genetic material in the form of a single circular DNA molecule, as well as RNA molecules, ribosomes, and other components necessary for protein synthesis. This unique feature suggests that mitochondria have their own semi-autonomous genetic machinery.

Apoptosis: Mitochondria play a crucial role in programmed cell death, or apoptosis, by releasing specific proteins and enzymes that initiate and regulate this process. This mechanism helps maintain cellular homeostasis and eliminate damaged or unwanted cells.

Detoxification: In liver cells, mitochondria contribute to the detoxification of ammonia, a toxic byproduct of metabolism, by converting it into less harmful substances.

Plastids

Plastids are double-membrane bound organelles found in plant cells and euglenoids. They possess their own DNA and ribosomes. Plastids play crucial roles in various cellular processes and can be classified into three main types based on the pigments they contain:

Chloroplasts : These are double membrane-bound organelles similar to mitochondria. Chloroplasts contain chlorophyll and carotenoids, which capture light energy required for photosynthesis. They consist of a stroma surrounded by thylakoids, organized into grana. Chloroplasts are responsible for photosynthesis in plant cells.

Chromoplasts : Chromoplasts contain pigments such as carotene and xanthophylls, giving plant parts yellow, orange, or red colors. They are involved in the synthesis and storage of pigments, contributing to the coloration of fruits, flowers, and other plant structures.

Leucoplasts : Leucoplasts are colorless plastids that serve various storage functions. They can be further categorized into different types based on their contents:

• Amyloplasts : Store carbohydrates, particularly starch, in plant cells, such as in potatoes.
• Elaioplasts : Store oils and fats.
• Aleuroplasts : Store proteins, particularly in seeds.

Ribosomes

Cytoskeleton

Cilia and Flagella

Centrosome and Centrioles

Nucleus

Structure Of Nucleus

Nuclear Membrane : Also known as the nuclear envelope, it is a double-layered membrane that surrounds the nucleus, separating its contents from the cytoplasm. The outer membrane is continuous with the endoplasmic reticulum, while nuclear pores perforate the membrane, allowing the selective exchange of materials between the nucleus and cytoplasm.

Nucleoplasm : The nucleoplasm is the gel-like substance present inside the nucleus, similar to the cytoplasm in composition. It contains water, ions, enzymes, and various dissolved molecules, providing a medium for cellular activities within the nucleus.

Chromosomes : Chromosomes are thread-like structures composed of DNA molecules complexed with proteins called histones. They carry genetic information in the form of genes and are responsible for determining an organism's hereditary traits. During cell division, chromosomes condense to form distinct structures visible under a microscope.

Nucleolus : The nucleolus is a prominent structure found within the nucleus, typically one or more per nucleus. It is involved in the synthesis and assembly of ribosomal RNA (rRNA) and ribosomal subunits, essential for protein synthesis. The nucleolus appears as a dense region devoid of a membrane, containing DNA, RNA, and proteins.