CBSE Class 11 Biology Revision Notes Chapter 8

CBSE Class 11 Biology Revision Notes Chapter 8 – Cell The Unit of Life

Chapter 8 of Class 11 Biology introduces students to the basic unit of life, the Cell. There are numerous living and nonliving species on Earth, and cells are the fundamental components of all living beings. Cells, their microstructure, and the functions of each micro organelle are all covered in this chapter. To prepare for this chapter in-depth, students can access Revision Notes by Extramarks which summarise all the core concepts covered in this chapter for students’ reference.

NCERT Revision Notes for Class 11 Biology Cell: The unit of life

The cell is the smallest, most fundamental structural and functional unit of living beings, and it is also known as the “building block of life.” Cells are self-contained and capable of carrying out biological functions. Plants and animals alike are made up of one or more cells, all of which are derived from pre-existing cells. Robert Hooke was the first person to notice cells in a piece of cork.

Different Types of Cell

The human body contains hepatocytes in the liver, nephrons in the kidney, neurons in the brain, and other types of cells in other different organs. Tissues are made up of several cells. These tissues have distinct functions.

Cell Theory

The idea of the Cell was first proposed in 1839 by two scientists, Schleiden, a German botanist, and Schwann, a British biologist. The contemporary cell hypothesis comprises the following:

• Cells make up every living organism.
• Living organisms’ basic structural and functional unit is thought to be the cell.
• Rudolf Virchow described how all cells develop from pre-existing cells through division.
• All energy is exchanged within the cells.
• Inherited information is stored in cells and transmitted from cell to cell during cell division.
• The chemical composition of all cells is the same.

Structure of Eukaryotic Cells

Eukaryotic cells contain organised nuclei and organelles which are enveloped by membrane-bound organelles.

Membrane of Plasma

The plasma membrane is a dynamic, fluid structure that divides the cell’s interior from the outside environment and is found at the cell’s external boundary. It has selective permeability. Jonathan Singer and Garth Nicolson proposed the fluid mosaic concept of the plasma membrane in 1972. According to this idea, the membrane is a quasi-fluid structure in which proteins are embedded throughout the lipid bilayer, which gives the membrane fluidity and flexibility. Two layers of amphipathic molecules with polar heads and nonpolar tails make up the bilayer.

The primary forces that organise the lipid bilayer are hydrophobic interactions. The plasma membrane contains three types of lipids and two types of proteins. Phospholipids, glycolipids, and sterol are lipids, while peripheral enzymes and integral proteins are proteins. Proteins that are weakly bound to the bilayer and can be withdrawn are known as peripheral proteins. Integral proteins are proteins bound to the lipid bilayer and cannot be easily removed.

Cell Wall

A stiff non-living cell wall structure surrounds the plasma membrane. The cell wall is present in plant and fungal cells, and it gives the cell its shape. It also defends the cell from mechanical injury, infection, and undesired macromolecule invasion.

Cell walls play a crucial role in cell-to-cell communication and transport. The primary wall, middle lamella, and secondary wall are the three sections of the cell wall. The connections between the cytoplasm of surrounding cells and the middle lamella are known as plasmodesmata.

Ribosomes

Ribosomes are RNA and protein-containing cell organelles, hence the name ribonucleoprotein. Ribosomes collaborate to turn messenger RNA (mRNA) into proteins. In working ribosomes, small subunits read mRNA, while bigger subunits synthesise a polypeptide chain of amino acids. Eukaryotic cells have two types of ribosomes: cytosolic and organellar. The ribosome is the 70S in prokaryotes and the 80S in eukaryotes, where S stands for sedimentation coefficient. The sedimentation rate determines the particle size by dividing the velocity by centrifugal acceleration.

Endoplasmic Reticulum

It is present in eukaryotic cells and is a massive single membrane-bound intracellular compartment. The lumen is a membrane-enclosed sac created by an interconnected network of closed and flattened membrane-bound structures. Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) are two forms of ER that differ in the presence or absence of ribosomes (SER). When ribosomes are present in ER, it gives the structure a rough aspect, which is why it is called rough ER. Smooth ER is defined as the absence of ribosomes in the ER membrane.

Cotranslational translocation is the process of proteins generated by ribosomes located on the RER membrane entering the lumen. Proteins undergo five changes in the lumen before they arrive at their final destination. These changes include carbohydrate addition and processing, disulfide bond formation, correct folding, particular proteolytic cleavages, and multimeric protein assembly. The SER is involved in processes, including crucial lipid synthesis, steroid hormone production, glucose metabolism, detoxification, and calcium regulation.

Golgi Complex/Golgi Apparatus

It is a membrane-bound organelle that is a component of the endomembrane system. The Golgi complex is made of flattened membrane sacs known as cisternae and is found in the cytoplasm of eukaryotic cells. A Golgi stack usually has four to eight cisternae. The cis face and the trans face are the two faces of each Golgi stack. Both sides are referred to as the entry and exit faces, respectively. Protein packaging and secretion are two of the Golgi apparatus’ tasks.

Lysosomes

It’s a single membrane-enclosed organelle that contains hydrolytic enzymes that degrade various chemicals. Hydrolytic enzymes include nucleases, proteases, lipases, glycosidases, phosphatases, phospholipases, and sulphatases. For maximum function, the enzyme requires an acidic environment inside the lysosomes with a pH of around 5.0. The lysosomal membrane still contains a proton pump. This proton pump moves the proton from inside the membrane using ATP. In macrophage phagocytosis, lysosomes digest both internal and external things, and they can break down virus particles or bacteria.

Vacuoles

Vacuoles are fluid-filled particles usually located in the cell’s cytoplasmic matrix. The vacuole is surrounded by a membrane called a tonoplast. The pH of the lumen is acidic, just like the pH of lysosomes. Plant cells have massive vacuoles, which hold water, dissolved inorganic ions, carbohydrates, enzymes, and other substances. Because it performs osmoregulation and pumps excess water out of the cell, it differs from another type of vacuole known as the contractile vacuole. In Amoeba, the vacuole is an example.

Mitochondria

It’s a location for aerobic respiration found in all eukaryotic cells. Because they manufacture ATP, the cell’s energy currency, they’re known as the powerhouse. They are cell organelles with circular DNA molecules and ribosomes bound by a double membrane. Intermembrane space refers to the gap between the outer and inner membranes. Because it is convoluted to produce cristae, the membrane structure is complicated. Cristae aid in the expansion of the mitochondrial surface area.

Cardiolipin, a phospholipid found abundantly in the inner membrane, renders it impervious to solutes. ATP synthase or F0-F1 ATPase enzyme complexes are located in the inner membrane and play a vital role in ATP molecules.

Plastids

They are double-membrane cell organelles found in algae and plant cells. They have double-stranded DNA and ribosomes in their structure, just like mitochondria. Leucoplast, chromoplast, and chloroplast are the three types of chloroplasts. Plastids contain a pigment involved in photosynthesis and are also involved in the production and storage of food.

The Chloroplast surrounds the stroma, a fluid-filled structure that contains a granum, a stack of sacs. The thylakoids are flattened membrane sacs that are arranged. Each granum is connected to the next by stroma lamellae, which are flat membranous tubules.

Chromoplasts are the plastids that are in charge of pigment synthesis and storage. They give the fruit, flowers, and ageing leaves distinct colours such as yellow, orange, or red.

On the other hand, the leucoplast is a colourless plastid found in plants’ unexposed parts. It is separated into three components that store starch, lipids in fats, and proteins. Amyloplast stores starch, Elaioplast stores lipids in fats, and proteinoplast stores proteins.

Nucleus

All eukaryotic cells, except RBCs, have a nucleus, which is a double membrane structure (red blood cells). The core carries DNA in the form of chromosomes, which are packed with histone proteins and regulate the activity and growth of the cell. A gene is a piece of DNA that includes the instructions for producing a particular protein.

The two types of chromosomes seen are euchromatin and heterochromatin, with euchromatin being a less compact structure that can be transcribed into mRNA. On the other hand, heterochromatin is a dense structure that cannot be translated into mRNA.

Large molecules cannot enter the cell through the nuclear membrane, a two-layered barrier. The endoplasmic reticulum is found in the outer layer. Nuclear pores in the membrane control the passage of solutes into and out of the nucleus. A perinuclear gap separates these two layers. The nucleolus is a solid, spherical structure inside the nucleus of eukaryotic cells. It aids in the assembly of ribosomes during protein synthesis. It vanishes and reappears during cell division.

Peroxisomes

Peroxisomes are tiny membrane-bound cell organelles found in almost all eukaryotic cells. They contain oxidative enzymes, which aid in a variety of metabolic processes. The breakdown of fatty acids is the primary function of peroxisomes. Peroxisomes can be made from the cell’s smooth endoplasmic reticulum. Matrix soluble proteins and membrane proteins are found in peroxisomes.

Cytoskeleton

It consists of a cytoplasmic network of proteinaceous filaments that gives mechanical support to the cells. The cytoskeleton keeps the cell in shape and aids in cell movement. It also arranges the organelles and makes it easier for solutes to travel around the cell.

Cilia and Flagella

They are hairy protrusions that aid in the movement of creatures. Cilia are structures that aid in adhesion, whereas flagella are comparatively whip-like appendages that help in the motion. They have a plasma membrane covering and a core of several microtubules parallel to the long axis. This core is referred to as an axoneme, and it has a 9+2 configuration. The number of radially organised microtubule doublet pairs is 9, and the centre pair of microtubules is 2.

Centrosomes and Centrioles

A pair of cylindrical structures known as centrioles make up the centrosome. Microtubule organisation is aided by centrioles parallel in the cytoplasm. It is composed of nine symmetrically aligned tubulin protein peripheral triplet fibrils. Spokes connect the central proteinaceous hub to the peripheral fibrils. Centrioles create a web of threads that run throughout the cell during cell division. Flagella are made up of them.

Structure of a Prokaryotic Cell

Single-celled organisms with no nucleus or membrane-bound organelles are known as prokaryotic cells.

Prokaryotes include bacteria, mycoplasma, cyanobacteria, and PPLOs, to name a few. Prokaryotes are substantially smaller than eukaryotes and divide much more quickly. The asexual division has historically been accomplished through binary fission and conjugation. They come in shapes and sizes, but they all have the same basic structure. Prokaryotes, except mycoplasma, have a cell wall that surrounds the cell. Plasmids are circular DNA extensions found in bacteria. They lack all cell organelles except ribosomes, responsible for protein synthesis.

They have one or more flagella in motile bacterial cells. A flagellum is a long whip-like structure of three main parts: filament, hook, and the basal body. A pilus can aid bacteria in conjugation, while Pili and Fimbriae aid in the substrate or host adhesion.

Cell, The Unit of Life, Notes Biology Chapter 8- Download

CBSE Class 11 Biology Chapter 8 notes are available to download from the official website of Extramarks. The revision notes contain a thorough explanation of the concepts along with several diagrams and illustrations.

Cell: Definition

According to Chapter 8, Biology Class 11, the cell is the fundamental unit of life. Unicellular animals can survive and conduct the core activities of life. Independent living is ensured by material than the entire structure. As a result, the cell is the fundamental structural and functional unit of all living organisms.

What is Cell Theory?

Matthias Schleiden, a German botanist, examined plants and discovered that they all contain different types of cells. These cells make up the tissues of plants.

Theodore Schwann, a modern British naturalist, studied numerous types of animal cells. Every cell had a thin outer shell, which he discovered. Today, this layer is referred to as the ‘plasma membrane.’ Schleiden and Schwann invented the term “cell theory.” The primary two points of cell theory, according to Extramarks Chapter 8 Class 11 Biology notes, are as follows:

• Each living entity is made up of cells and cell products.
• Every cell is made up of pre-existing cells.

Cell: A to the Point Analysis

• The cell membrane, which is found in all animal and plant cells, is made up of proteins.
• If you read Extramarks Biology Class 11 Chapter 8 notes carefully, you will see that the nucleus is a dense membrane-bound structure inside the cell.
• The genetic substance DNA’s basic unit is the nucleus.
• The cytoplasm is a semi-fluid matrix that makes up the cell’s interior arena.
• In addition to the nucleus, organelles are membrane-bound components found within the cell.
• Endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, and other organelles can be found inside the cell. In prokaryotic cells, these membrane-bound organelles are absent.
• The ribosome is an example of a non-membranous organelle. Both prokaryotic and eukaryotic cells contain this protein.
• Cells are classified according to their size, shape, and activity. Mycoplasmas are the smallest cells on the planet. A single isolated cell is what an ostrich egg is called.
• Human nerve cells are the cells in multicellular animals.
• There are also disc-like, polygonal, columnar, cuboid, and other forms.

What are Prokaryotic Cells?

The absence of membrane-bound nuclei distinguishes a prokaryotic cell. Bacteria, blue-green algae, Mycoplasmas, and other prokaryotic cells are examples.

Ribosomes and Inclusion Bodies

You will have a good understanding of the ribosome if you follow Extramarks Class 11 Biology Chapter 8 ‘cell, the unit of life’ notes. Both prokaryotic and eukaryotic cells have ribosomes. In prokaryotes, ribosomes are attached to the cell’s plasma membrane. Ribosomes are around 15 mm by 20 mm in size and are made up of two subunits, the 50S and 30S units. 70S bacterial ribosomes are made up of these two subunits. Phosphate, cyanophycean, and glycogen granules are examples of inclusion bodies.

What are Eukaryotic Cells?

The presence of membrane-bound nuclei characterises a eukaryotic cell.

Cell Membrane

• According to class 11 Biology Chapter 8 notes, the basic structure of a cell membrane is made of lipids and proteins.
• Phospholipids are the most crucial lipids.
• Later research into the cell membrane found that it is made of proteins and carbohydrates.

Cell Wall

• The cell wall is a stiff non-living structure that serves as the exterior coating for the plasma membrane of fungi and plants.
• The cell wall’s job is to provide the cell shape while protecting it from mechanical harm and infection.

Endomembrane System

The cell’s endomembrane system is made up of the following components:

• Endoplasmic Reticulum (ER): The cytoplasm is littered with tubular structures.

• Apparatus of Golgi: Near the nucleus, they are the heavily stained reticular structures.

• Mitochondria: It is the cell’s driving force. Mitochondria produce the cellular energy known as ATP.
• Plastids: Plant cells have parasitic plasmids. Chloroplasts, chromoplasts, and leucoplasts are the three types of chloroplasts.