Detailed Notes – Chapter 8 Cell: The Unit of Life

Chapter 8 Cell: The Unit of Life

1. Introduction to Life and Cells:

   • Living and non-living things coexist in our environment.
   • Central question: What distinguishes living organisms from inanimate objects?
   • Answer: The presence of cells, the basic unit of life, is the key characteristic that defines living organisms.

2. Composition of Organisms:

   • All organisms, regardless of their size or complexity, are composed of cells.
   • Cells are the fundamental structural and functional units of life.

3. Types of Organisms Based on Cell Composition:

   • Unicellular Organisms: Composed of a single cell.
   • Multicellular Organisms: Composed of many cells.

4. Unicellular Organisms:

   • Examples: Bacteria, certain types of algae, amoebas.
   • Functionality: Despite being single-celled, they exhibit all characteristics of life, such as metabolism, growth, response to stimuli, and reproduction, within that single cell.

5. Multicellular Organisms:

   • Examples: Humans, animals, plants.
   • Complexity: Composed of numerous cells organized into tissues, organs, and organ systems.
   • Specialization: Cells within multicellular organisms often have specialized functions, contributing to the overall function and survival of the organism.

8.1 WHAT IS A CELL?

1. Definition of a Cell:

   • A cell is defined as the fundamental structural and functional unit of all living organisms.
   • It is the smallest entity that exhibits the characteristics of life.

2. Capabilities of Unicellular Organisms:

   • Unicellular organisms are capable of independent existence.
   • They can perform all essential functions of life within a single cell.
   • This highlights the significance of cells as the basic unit of life.

3. Importance of Complete Cell Structure:

   • Only a complete structure of a cell ensures independent living for unicellular organisms.
   • Any structure less than a complete cell cannot sustain independent life.

4. Historical Contributions:

   • Anton Van Leeuwenhoek was the first to observe and describe live cells.
   • Robert Brown discovered the nucleus, a significant component of the cell.
   • The invention and improvement of microscopes, particularly the electron microscope, played a crucial role in revealing the structural details of cells.

8.2 CELL THEORY

1. Matthias Schleiden’s Observations (1838):

   • German botanist Matthias Schleiden examined plants and found that they were composed of different types of cells.
   • He observed that these cells formed the tissues of plants.

2. Theodore Schwann’s Findings (1839):

   • British zoologist Theodore Schwann studied animal cells and noted that they had a thin outer layer, now known as the plasma membrane.
   • Schwann also discovered that plant cells had a unique feature, the cell wall.
   • He proposed the hypothesis that both animals and plants are composed of cells and cell products.

3. Formulation of the Cell Theory:

   • Schleiden and Schwann collaborated to formulate the cell theory.
   • Initially, it did not address how new cells were formed.

4. Rudolf Virchow’s Contribution (1855):

   • Rudolf Virchow expanded the cell theory by proposing that cells divide and new cells are formed from pre-existing cells.
   • His statement, “Omnis cellula-e cellula,” meaning “every cell from a cell,” emphasized the idea that all cells come from pre-existing cells.

5. Modern Understanding of the Cell Theory:

   • The cell theory as understood today consists of two main principles:
     • All living organisms are composed of cells and products of cells.
     • All cells arise from pre-existing cells.

8.3 AN OVERVIEW OF CELL

1. Cell Types:

   • Plant cells, such as those found in onion peels, have a distinct cell wall as the outer boundary, followed by the cell membrane.
   • Human cheek cells have an outer membrane as their delimiting structure.

2. Nucleus:

   • Both plant and animal cells contain a dense, membrane-bound structure called the nucleus.
   • The nucleus houses chromosomes, which contain genetic material, DNA.

3. Cell Classification:

   • Cells with membrane-bound nuclei are termed eukaryotic.
   • Those lacking a membrane-bound nucleus are termed prokaryotic.

4. Cytoplasm:

   • A semi-fluid matrix called cytoplasm occupies the volume of both prokaryotic and eukaryotic cells.
   • It is the main site of cellular activities, where various chemical reactions occur to maintain cellular functions.

5. Organelles:

   • Eukaryotic cells contain membrane-bound organelles such as the endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles.
   • Prokaryotic cells lack membrane-bound organelles.

6. Ribosomes:

   • Non-membrane bound organelles called ribosomes are found in all cells, both eukaryotic and prokaryotic.
   • They are involved in protein synthesis.

7. Additional Organelles in Eukaryotic Cells:

   • Chloroplasts (in plants) and mitochondria contain ribosomes and are involved in energy production.
   • Animal cells may have a non-membrane-bound organelle called a centrosome, which aids in cell division.

8. Cell Size, Shape, and Activities:

   • Cells vary greatly in size, shape, and activities.
   • Examples of cell size range from Mycoplasmas, the smallest cells at 0.3 µm, to ostrich egg cells, the largest isolated single cell.
   • Human red blood cells are about 7.0 µm in diameter, while nerve cells are among the longest.
   • Cell shapes vary from disc-like to irregular, often corresponding to their specific functions.

8.4 PROKARYOTIC CELLS

1. Representative Organisms:

   • Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).

2. Size and Reproduction:

   • Prokaryotic cells are generally smaller and multiply more rapidly than eukaryotic cells.

3. Shape Variation:

   • Prokaryotic cells may vary greatly in shape and size, with four basic shapes observed: bacillus (rod-like), coccus (spherical), vibrio (comma-shaped), and spirillum (spiral).

4. Basic Organization:

   • Prokaryotic cells have a fundamental organizational similarity despite their diverse shapes and functions.
   • They possess a cell wall surrounding the cell membrane, except in mycoplasma.
   • The cytoplasm fills the interior of the cell, which lacks a well-defined nucleus.
   • The genetic material is typically naked, not enclosed by a nuclear membrane.
   • In addition to the genomic DNA (single chromosome/circular DNA), many bacteria contain small circular DNA molecules called plasmids, which confer unique phenotypic characteristics, such as antibiotic resistance.

5. Organelles and Inclusions:

   • Prokaryotic cells lack membrane-bound organelles found in eukaryotic cells, except for ribosomes.
   • However, they have unique structures called inclusions.
   • Prokaryotes also possess mesosomes, which are specialized differentiated forms of the cell membrane characterized by infoldings.

8.4.1 Cell Envelope and its Modifications

1. Composition of the Cell Envelope:

   • The cell envelope of most prokaryotic cells, especially bacterial cells, is chemically complex and consists of three tightly bound layers.
   • These layers include the outermost glycocalyx, followed by the cell wall, and then the plasma membrane.
   • Although each layer performs distinct functions, they act together as a single protective unit.

2. Gram Staining Classification:

   • Bacteria can be classified into two groups based on differences in their cell envelopes and their response to the Gram staining procedure.
   • Gram-positive bacteria retain the stain, while Gram-negative bacteria do not.

3. Glycocalyx:

   • The glycocalyx varies in composition and thickness among different bacteria.
   • It can be a loose sheath called the slime layer or a thick and tough capsule.

4. Cell Wall:

   • The cell wall determines the shape of the cell and provides structural support to prevent the bacterium from bursting or collapsing.

5. Plasma Membrane:

   • The plasma membrane is selectively permeable and interacts with the external environment.
   • Structurally, it is similar to the plasma membrane of eukaryotic cells.

6. Mesosomes:

   • Mesosomes are special membranous structures formed by extensions of the plasma membrane into the cell.
   • They assist in cell wall formation, DNA replication and distribution, respiration, secretion processes, and increasing the surface area of the plasma membrane.

7. Chromatophores:

   • In some prokaryotes like cyanobacteria, chromatophores are membranous extensions into the cytoplasm that contain pigments.

8. Flagella, Pili, and Fimbriae:

   • Motile bacteria have thin filamentous extensions from their cell wall called flagella.
   • Flagella are composed of three parts: filament, hook, and basal body.
   • Pili and fimbriae are surface structures of bacteria that do not play a role in motility.
   • Pili are elongated tubular structures made of a special protein, while fimbriae are small bristle-like fibers.
   • Fimbriae help bacteria attach to surfaces such as rocks in streams and host tissues.

8.4.2 Ribosomes and Inclusion Bodies

1. Ribosomes:

   • In prokaryotes, ribosomes are associated with the plasma membrane of the cell.
   • They are approximately 15 nm by 20 nm in size and are composed of two subunits: 50S and 30S units, which together form 70S prokaryotic ribosomes.
   • Ribosomes serve as the site of protein synthesis within the cell.
   • Multiple ribosomes may attach to a single mRNA molecule, forming a chain called polyribosomes or polysome.
   • The ribosomes of a polysome work together to translate the mRNA into proteins.

2. Inclusion Bodies:

   • Reserve materials in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies.
   • Unlike organelles, inclusion bodies are not bound by any membrane system and are freely present in the cytoplasm.
   • Examples of inclusion bodies include phosphate granules, cyanophycean granules, and glycogen granules.
   • Gas vacuoles are also considered inclusion bodies and are found in certain photosynthetic bacteria such as blue-green and purple-green bacteria.

8.5 EUKARYOTIC CELLS

1. Classification of Eukaryotes:

   • Eukaryotes encompass a diverse range of organisms, including protists, plants, animals, and fungi.

2. Compartmentalization of Cytoplasm:

   • Eukaryotic cells exhibit extensive compartmentalization of cytoplasm due to the presence of membrane-bound organelles.

3. Nucleus:

   • Eukaryotic cells possess an organized nucleus surrounded by a nuclear envelope, which separates the genetic material from the cytoplasm.

4. Genetic Material:

   • The genetic material in eukaryotic cells is organized into chromosomes, consisting of DNA and associated proteins.

5. Structural Complexity:

   • Eukaryotic cells have a variety of complex locomotory and cytoskeletal structures, which are involved in cellular movement and support.

6. Differences Between Plant and Animal Cells:

   • Plant cells and animal cells exhibit differences in structure and composition.
   • Plant cells have cell walls, plastids (such as chloroplasts), and a large central vacuole, which are absent in animal cells.
   • Animal cells possess centrioles, which are typically absent in plant cells.

8.5.1 Cell Membrane

1. Composition of the Cell Membrane:

   • The cell membrane is mainly composed of lipids and proteins.
   • The major lipids are phospholipids arranged in a bilayer, with polar heads facing outward and hydrophobic tails facing inward.
   • Cholesterol is also present in the membrane, contributing to its stability.

2. Protein and Carbohydrate Components:

   • Cell membranes also contain proteins and carbohydrates.
   • The ratio of protein to lipid varies among different cell types.
   • Membrane proteins can be classified as integral (partially or totally buried in the membrane) or peripheral (lying on the surface of the membrane).

3. Fluid Mosaic Model:

   • Proposed by Singer and Nicolson in 1972, the fluid mosaic model describes the membrane as a quasi-fluid structure with proteins embedded within the lipid bilayer.
   • The fluidity of the membrane allows lateral movement of proteins, which is important for various cellular functions.

4. Functions of the Cell Membrane:

   • The cell membrane is involved in cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, and other essential processes.
   • One of its crucial functions is the selective transport of molecules across it.

5. Selective Permeability:

   • The membrane is selectively permeable to molecules present on either side of it.
   • Passive transport allows molecules to move across the membrane without requiring energy, such as simple diffusion and osmosis.
   • For polar molecules, carrier proteins facilitate their transport across the nonpolar lipid bilayer.
   • Active transport, such as the Na+/K+ pump, requires energy (ATP) to transport ions or molecules against their concentration gradient.

8.5.2 Cell Wall

1. Composition of the Cell Wall:

   • The cell wall is a non-living rigid structure that forms an outer covering for the plasma membrane of fungi and plants.
   • It is composed of various substances depending on the organism. In algae, it is made of cellulose, galactans, mannans, and minerals like calcium carbonate. In plants, it consists of cellulose, hemicellulose, pectins, and proteins.

2. Functions of the Cell Wall:

   • Provides structural support and shape to the cell.
   • Protects the cell from mechanical damage and infection.
   • Facilitates cell-to-cell interactions.
   • Acts as a barrier to undesirable macromolecules.

3. Primary and Secondary Cell Wall:

   • In young plant cells, the primary wall is capable of growth. As the cell matures, the primary wall diminishes, and a secondary wall is formed on the inner side of the cell, towards the plasma membrane.

4. Middle Lamella:

   • The middle lamella is a layer mainly composed of calcium pectate.
   • It acts as a cementing material, holding or gluing neighboring cells together.

5. Plasmodesmata:

   • Plasmodesmata are channels that traverse the cell wall and middle lamellae.
   • They connect the cytoplasm of neighboring cells, allowing for communication and transport of substances between adjacent cells.

8.5.3 Endomembrane System

1. Definition and Components:

   • The endomembrane system refers to a network of membranous organelles within eukaryotic cells.
   • It includes the following components:
     • Endoplasmic reticulum (ER)
     • Golgi complex (Golgi apparatus)
     • Lysosomes
     • Vacuoles

2. Coordination of Functions:

   • Although each membranous organelle within the endomembrane system has distinct structures and functions, they work together to coordinate various cellular processes.
   • These processes may include protein synthesis, modification, packaging, transport, and degradation.

3. Exclusion of Other Organelles:

   • Mitochondria, chloroplasts, and peroxisomes are not considered part of the endomembrane system.
   • This is because their functions are not coordinated with the components of the endomembrane system.
   • Mitochondria are involved in energy production (cellular respiration), chloroplasts are involved in photosynthesis (found only in plant cells), and peroxisomes are involved in various metabolic processes, including the breakdown of fatty acids and detoxification of harmful substances.

 8.5.3.1 The Endoplasmic Reticulum (ER)

1. Structure of ER:

   • Electron microscopic studies reveal a network of tiny tubular structures scattered in the cytoplasm known as the endoplasmic reticulum (ER).
   • The ER divides the intracellular space into two distinct compartments: the luminal compartment (inside the ER) and the extra-luminal compartment (cytoplasm).

2. Types of ER:

   • ER can be classified into two main types based on its appearance and function:
     • Rough Endoplasmic Reticulum (RER): It has ribosomes attached to its outer surface, giving it a rough appearance. RER is involved in protein synthesis and secretion.
     • Smooth Endoplasmic Reticulum (SER): It appears smooth since it lacks ribosomes. SER is involved in lipid synthesis and various other metabolic processes.

3. Functions of RER:

   • RER is frequently observed in cells actively involved in protein synthesis and secretion.
   • It is extensive and continuous with the outer membrane of the nucleus.
   • RER plays a crucial role in the synthesis of proteins, particularly those destined for secretion or insertion into the cell membrane.

4. Functions of SER:

   • SER is the major site for the synthesis of lipids.
   • In animal cells, SER is involved in the synthesis of lipid-like steroidal hormones.

8.5.3.2 Golgi apparatus

1. Discovery and Structure:

   • Camillo Golgi first observed densely stained reticular structures near the nucleus in 1898, which were later named Golgi bodies after him.
   • The Golgi apparatus consists of many flat, disc-shaped sacs or cisternae with diameters ranging from 0.5µm to 1.0µm.
   • These cisternae are stacked parallel to each other and are concentrically arranged near the nucleus.
   • The Golgi complex has distinct convex cis or forming face and concave trans or maturing face.

2. Function:

   • The Golgi apparatus primarily functions in packaging materials to be delivered either to intracellular targets or secreted outside the cell.
   • Materials to be packaged, in the form of vesicles from the endoplasmic reticulum (ER), fuse with the cis face of the Golgi apparatus and move towards the maturing face.
   • This close association with the ER explains why the Golgi apparatus is often found near the endoplasmic reticulum.
   • Proteins synthesized by ribosomes on the endoplasmic reticulum are modified in the Golgi cisternae before being released from its trans face.
   • The Golgi apparatus is an important site for the formation of glycoproteins and glycolipids.

8.5.3.3 Lysosomes

1. Formation and Structure:

   • Lysosomes are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus.
   • They are typically spherical organelles enclosed by a single membrane.

2. Enzyme Content:

   • Lysosomes contain a wide range of hydrolytic enzymes, collectively known as hydrolases.
   • These enzymes include lipases (for lipid digestion), proteases (for protein digestion), carbohydrases (for carbohydrate digestion), and nucleases (for nucleic acid digestion).

3. Optimal pH:

   • Lysosomal enzymes are optimally active at an acidic pH.
   • The acidic environment within lysosomes is maintained by proton pumps in the lysosomal membrane that actively transport protons (H⁺ ions) into the lysosome.

4. Function:

   • The primary function of lysosomes is intracellular digestion or autophagy.
   • They break down various biomolecules, including carbohydrates, proteins, lipids, and nucleic acids, into their constituent components.
   • Lysosomes play a crucial role in cellular waste disposal, recycling of cellular components, and regulation of cellular homeostasis.
   • They are also involved in the degradation of foreign substances engulfed by the cell through processes such as endocytosis and phagocytosis.

8.5.3.4 Vacuoles 

• Structure and Composition:

  • Vacuoles are membrane-bound organelles found in the cytoplasm of cells.
  • They contain water, sap, excretory products, and other materials that are not immediately useful for the cell.
  • The membrane surrounding the vacuole is called the tonoplast.

• Size and Volume:

  • In plant cells, vacuoles can occupy a significant portion of the cell volume, sometimes up to 90%.
  • Their large size and volume contribute to the rigidity and turgor pressure of plant cells.

• Functions:

  • Plant Vacuoles:
    • In plant cells, vacuoles play essential roles in maintaining cell structure, storage, and turgor pressure.
    • They store water, ions, sugars, pigments, and other substances.
    • The tonoplast facilitates the transport of ions and other materials into the vacuole, maintaining their concentration higher than in the cytoplasm.
  • Contractile Vacuoles in Amoeba:
    • In single-celled organisms like Amoeba, contractile vacuoles are important for osmoregulation and excretion.
    • They help regulate the water content of the cell by expelling excess water to maintain proper osmotic balance.
  • Food Vacuoles:
    • In many cells, particularly in protists, food vacuoles are formed by engulfing food particles through processes like phagocytosis.
    • These food vacuoles then fuse with lysosomes for digestion and nutrient absorption.

8.5.4 Mitochondria

1. Structure:

   • Mitochondria are double membrane-bound organelles, typically shaped like sausages or cylinders.
   • They are not easily visible under the microscope unless specifically stained.
   • Each mitochondrion consists of an outer membrane and an inner membrane, which divides its lumen into two aqueous compartments: the outer compartment and the inner compartment.
   • The inner compartment contains a dense homogeneous substance called the matrix.
   • The inner membrane forms infoldings called cristae, which increase the surface area for chemical reactions.
   • The two membranes have specific enzymes associated with mitochondrial function.

2. Size and Variability:

   • Mitochondria exhibit considerable variability in shape and size.
   • They typically have a diameter ranging from 0.2 to 1.0 µm and a length ranging from 1.0 to 4.1 µm.

3. Function:

   • Mitochondria are the sites of aerobic respiration, the process by which cells generate energy in the form of ATP (adenosine triphosphate).
   • They are often referred to as the “powerhouses” of the cell due to their role in ATP production.
   • The matrix of mitochondria contains DNA, RNA, ribosomes (70S), and components required for protein synthesis.
   • Mitochondria produce ATP through a series of metabolic pathways, including the citric acid cycle and oxidative phosphorylation.

4. Reproduction:

   • Mitochondria reproduce by a process called fission, where they divide to form new mitochondria.

8.5.5 Plastids

1. Presence and Observation:

   • Plastids are found in all plant cells and in some protists like euglenoids.
   • They are easily observed under the microscope due to their large size.

2. Types of Plastids:

   • Plastids can be classified into three main types based on the type of pigments they contain:
     • Chloroplasts: Contain chlorophyll and carotenoid pigments responsible for photosynthesis.
     • Chromoplasts: Contain fat-soluble carotenoid pigments like carotene and xanthophylls, giving plants yellow, orange, or red colors.
     • Leucoplasts: Colorless plastids that store various nutrients, such as amyloplasts (starch), elaioplasts (oils and fats), and aleuroplasts (proteins).

3. Structure of Chloroplasts:

   • Chloroplasts are lens-shaped, oval, spherical, discoid, or ribbon-like organelles with variable sizes (5-10µm in length and 2-4µm in width).
   • They are double membrane-bound organelles, with the inner membrane being relatively less permeable.
   • The space enclosed by the inner membrane is called the stroma.
   • Within the stroma, there are organized flattened membranous sacs called thylakoids, arranged in stacks called grana. The thylakoids contain chlorophyll pigments and are the site of light-dependent reactions of photosynthesis.
   • Stroma lamellae connect thylakoids of different grana.
   • The stroma contains enzymes required for carbohydrate and protein synthesis, as well as small, double-stranded circular DNA molecules and ribosomes.

4. Function of Chloroplasts:

   • Chloroplasts are the sites of photosynthesis, where light energy is captured and converted into chemical energy in the form of carbohydrates (such as glucose).
   • Thylakoids contain chlorophyll pigments that absorb light energy for photosynthesis.
   • The stroma contains enzymes necessary for the synthesis of carbohydrates and proteins.