UNIT 1 – Eukaryotic Cell: Structure and Functions (Q&A) | MZO-001 MSCZOO | IGNOU

SAQ 1 

i. What is a Cell? What Are the Essential Characteristics of Cells?

A cell is the basic structural, functional, and biological unit of all living organisms. It is the smallest unit of life and is often referred to as the "building block of life." Cells carry out essential functions such as metabolism, energy production, and reproduction. Organisms can be unicellular (made of a single cell) or multicellular (composed of many cells).

Discovery of the Cell

Robert Hooke discovered the cell structure in 1665 by observing a thin slice of cork, specifically from the cork cambium of the cork oak tree (Quercus suber). The cork cambium is a layer of tissue found in the bark of plants that produces cork. Under his microscope, Hooke saw small, hexagon box-like compartments, which he named "cells" because they resembled the small rooms. These compartments were the cell walls of dead plant cells, a key observation that laid the foundation for the study of cell biology.

Discovery of the First Living Cell

The first living cell was discovered by Anton van Leeuwenhoek in 1674. Using his improved microscope, he observed single celled organisms, including bacteria and protozoa, in pond water. These microscopic organisms, which he called "animalcules," were the first living cells ever observed, marking a key moment in the development of microbiology and cell biology.

Types of Cells

Cells are classified based on various characteristics: 

1. Based on Nucleus:

1. Prokaryotic Cells: These cells lack a true nucleus. Instead, their genetic material (DNA) is located in a region called the nucleoid, but it is not enclosed by a membrane. Examples - Bacteria and Archaea.
2. Eukaryotic Cells: These cells have a true, membrane-bound nucleus where their genetic material is stored. Examples - Plants, animals, fungi, and protists.

2. Based on Cell Wall:

1. Plant Cells: Plant cells have a rigid cell wall made of cellulose outside their cell membrane, which provides structure and support.
2. Animal Cells: Animal cells lack a cell wall and are enclosed only by a flexible cell membrane.

3. Based on the Number of Cells:

1. Unicellular Organisms: These organisms are made up of only one cell. The single cell performs all life functions. Examples - Bacteria, Amoeba, and some algae.
2. Multicellular Organisms: These organisms are composed of many cells that work together, with different cells specializing in various functions. Examples - Humans, plants, fungi, and animals.

Important Cell Discoveries

• First Cell: Robert Hooke discovered the cell structure in 1665 by observing a thin slice of cork, specifically from the cork cambium of the cork oak tree (Quercus suber).

• First Living Cell: Discovered by Anton van Leeuwenhoek in 1674. He observed microorganisms (protists and bacteria) in pond water.

• Nucleus: Discovered by Robert Brown in 1831. He observed it in orchid cells, realizing its central role in cellular function.

• Sperm Cell: Discovered by Anton van Leeuwenhoek in 1677. He observed spermatozoa using his microscope and described their motion.

• Egg Cell (Ovum): The mammalian egg cell was first described by Karl Ernst von Baer in 1827.

Sizes of Cells

• Mycoplasma (smallest cell) : 0.1–0.3 µm
• Bacterial cells : 1–5 µm
• Red blood cells : 6–8 µm
• Plant cells : 10–100 µm
• Human egg cell or ovum cell (largest human cell) : ~100 µm
• Ostrich egg (largest cell) : ~120 mm (12 cm)
• Acetabularia (largest unicellular green algae) : Up to 10 cm
• Human nerve cells (longest human cell) : Up to 1 meter (1000 mm) in length
• Sclerenchyma fibres (longest cells in plant) : 55 cm long

Essential Characteristics of Cells

Cells are the basic units of life with several key features:

Structural Unit of Life:

Cells are the basic units of structure and organization in living organisms. They provide the structural basis for the body's tissues and organs.

Functional Unit of Life:

Cells carry out all necessary functions to sustain life, including metabolism, energy production, and reproduction.

Surrounded by a Plasma Membrane:

Each cell is enclosed by a plasma membrane that separates its internal environment from the external environment, regulating the movement of substances in and out of the cell.

Contain Genetic Material (DNA):

Cells house DNA, the genetic blueprint that directs all cellular activities and ensures the transmission of genetic information from one generation to the next. In eukaryotic cells, DNA is contained within a nucleus, while in prokaryotic cells, it is located in the nucleoid region.

Cytoplasm:

The cytoplasm is a gel-like substance within the cell membrane that contains the organelles and is the site for most cellular processes. It is composed of cytosol and the organelles suspended in it.

Organelles:

Eukaryotic cells contain membrane-bound organelles, each with specific functions, such as the nucleus (genetic control center), mitochondria (energy production), endoplasmic reticulum (protein and lipid synthesis), and Golgi apparatus (modification and packaging of proteins). Prokaryotic cells lack membrane-bound organelles but still perform essential functions.

Energy Production and Metabolism:

Cells perform metabolic processes that convert nutrients into energy. In eukaryotic cells, mitochondria are the primary sites of cellular respiration and energy (ATP) production.

Reproduction and Heredity:

Cells have the ability to reproduce by dividing. Eukaryotic cells undergo mitosis (for growth and repair) and meiosis (for sexual reproduction), ensuring genetic continuity.

Response to Stimuli:

Cells can respond to environmental stimuli, a characteristic essential for survival. This includes responses to chemical signals, physical interactions, and changes in their environment.

Growth and Development:

Cells grow and develop according to their genetic instructions, contributing to the overall growth and development of the organism.

Cellular Communication:

Cells communicate with each other through chemical signals and physical contacts, coordinating activities within tissues and organs.

ii) Explain the fluid mosaic model of the plasma membrane.

The fluid mosaic model is one of the most widely accepted and foundational concepts that explains the structural organization of the plasma membrane in living cells. It was first proposed by S.J. Singer and Garth L. Nicolson in 1972 and remains relevant today, though slightly refined with modern findings. Before we move into the components and structure, it is important to understand that this model helps explain how the membrane remains flexible, selectively permeable, and functionally active for processes like transport, cell signaling and communication.

Basic Concept of the Fluid Mosaic Model

According to the fluid mosaic model, the plasma membrane is viewed as a dynamic, semi-fluid bilayer of lipids with proteins embedded in it, much like boats floating in a sea. The word "fluid" refers to the lateral movement of lipids and some proteins within the membrane, while "mosaic" refers to the patchwork arrangement of various proteins that float freely or are anchored in the lipid bilayer.

Components of the Plasma Membrane

Before going into functional details, let's understand its major components, which will help us grasp how they interact in this model:

Phospholipids

• These form the basic structural framework in the form of a bilayer, with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward. This arrangement provides a semi-permeable barrier to ions and polar molecules.

Proteins

• These are embedded in the lipid bilayer and can be of two types:
1. Integral (intrinsic) proteins: Span across the membrane and are involved in transport, signal reception and anchoring.
2. Peripheral (extrinsic) proteins: Attached loosely on the surface and often serve as enzymes or structural supporters.

Cholesterol

• Present in animal cell membranes, cholesterol fits between phospholipids and regulates membrane fluidity, preventing it from becoming too rigid or too permeable.

Carbohydrates

• Found attached to proteins (glycoproteins) or lipids (glycolipids), mainly on the outer surface. They play a role in cell recognition, signaling and adhesion.

Key Features and Functions Based on the Model

Now that the components are known, we can understand how their interactions define the nature and function of the membrane:

1. Fluid Nature:

• Lipids and some proteins move laterally within the bilayer, allowing the membrane to be flexible and capable of self-healing, fusion and dynamic remodeling.

2. Selective Permeability:

• The arrangement of hydrophobic and hydrophilic regions enables control over entry and exit of substances, maintaining homeostasis.

3. Functional Domains:

• Membrane proteins are not randomly scattered. Some are grouped into functional domains or complexes, enabling efficient signal transduction and molecular transport.

4. Asymmetry:

• The inner and outer leaflets of the bilayer are not identical in composition. This asymmetry is important for functions like signaling and membrane trafficking.

iii) Which organelles are involved in photosynthesis?

Photosynthesis is a highly specialised biological process in which light energy is converted into chemical energy by autotrophic organisms like plants, algae and some protists. To understand which organelles are involved in photosynthesis, it is important to first clarify that the core reactions take place within specific compartments of the cell. However, some other organelles also support or regulate this process indirectly. Therefore, before discussing each organelle, it is essential to know that the primary site of photosynthesis is the chloroplast, but other organelles such as the endoplasmic reticulum, Golgi apparatus, and even the peroxisomes and vacuoles have supplementary roles in supporting photosynthesis-related functions.

1. Chloroplasts

Chloroplasts are the main organelles directly involved in photosynthesis. They are double-membrane-bound structures found in the cytoplasm of plant and algal cells. These organelles originated from an ancient symbiotic cyanobacterium, a theory known as endosymbiosis, which is supported by the fact that chloroplasts contain their own DNA, ribosomes and can synthesise some of their own proteins.

The internal organisation of chloroplasts includes structures like grana and stroma, which play specific roles in photosynthesis. Inside the chloroplast, the internal membrane system is organised into stacked disc-like structures called grana, formed by thylakoids, which are the actual sites of light-dependent reactions. These thylakoids contain chlorophyll a, chlorophyll b and accessory pigments like carotenoids, which absorb solar energy. The stroma, the enzyme-rich fluid surrounding the grana, is the site of the Calvin cycle or light-independent reactions, where CO₂ is fixed into carbohydrates using ATP and NADPH. Chloroplasts also play a role in starch synthesis and fatty acid metabolism.

The structure of the chloroplast allows compartmentalisation, which is essential for separating light and dark reactions, ensuring high efficiency. The involvement of chloroplasts in photosynthesis was established by Andreas Schimper (1883) and further elaborated by Melvin Calvin during his work on the dark reactions in the 1950s, which led to the naming of the Calvin cycle.

2. Endoplasmic Reticulum (ER)

Although endoplasmic reticulum (ER) not directly performing photosynthesis, the rough ER helps in the synthesis and transport of proteins that are used in chloroplasts, including various enzymes required for photosynthetic pathways. The smooth ER plays a role in lipid synthesis that is necessary for membrane formation within the chloroplast.

3. Golgi Apparatus

The Golgi body modifies, packages, and transports proteins and lipids, many of which are necessary for chloroplast development and function. It is especially important for the transport of glycolipids and glycoproteins involved in chloroplast membrane composition.

4. Peroxisomes

During photorespiration, a process associated with photosynthesis in C₃ plants, peroxisomes work together with chloroplasts and mitochondria to metabolise 2-phosphoglycolate. This detoxification process helps recycle carbon atoms and protect the photosynthetic process.

5. Vacuole

The central vacuole of plant cells helps maintain turgor pressure, which is vital for keeping the leaf surface expanded to capture sunlight efficiently. It may also store ions and metabolites involved in photosynthesis.

iv) Why the mitochondrion is called the "powerhouse" of the cell?

The mitochondrion is commonly called the "powerhouse of the cell" because it is the main site for the production of adenosine triphosphate (ATP), which is the energy currency of cells. This label reflects the central role of mitochondria in cellular respiration, where they convert energy stored in food molecules into usable chemical energy. Before explaining how this happens, it is important to understand the background of this concept and how ATP generation is connected to the structure and function of mitochondria.

ATP is required for almost all cellular processes such as muscle contraction, active transport across membranes, cell division and biosynthesis. However, ATP cannot be stored in large amounts, so it must be constantly produced. Mitochondria carry out aerobic respiration, which is a highly efficient process that yields a large amount of ATP compared to anaerobic pathways. In fact, one molecule of glucose can yield up to 36 to 38 molecules of ATP in the presence of oxygen and most of this ATP is generated inside mitochondria.

The process begins in the cytoplasm with glycolysis, which breaks down glucose into pyruvate. This pyruvate enters the mitochondrial matrix where it undergoes the Krebs cycle (also known as the citric acid cycle), releasing high-energy electrons. These electrons are then passed through the electron transport chain (ETC), which is located in the inner mitochondrial membrane. As electrons flow through the ETC, protons are pumped across the membrane to generate a proton gradient. Finally, this gradient powers ATP synthase, an enzyme that synthesizes ATP from ADP and inorganic phosphate. This entire process is called oxidative phosphorylation.

The reason mitochondria are so efficient is due to their unique double membrane structure and the presence of their own DNA and ribosomes, which allow them to produce some of their own proteins and enzymes needed for energy metabolism. This evolutionary origin is explained by the endosymbiotic theory, which suggests that mitochondria evolved from free-living aerobic bacteria.

Thus, mitochondria are called the "powerhouse" of the cell because they are the central site where biochemical energy is converted into a usable form, supporting all vital cellular activities.

v) Which organelle contains enzymes for cellular respiration?

The organelle that contains the enzymes for cellular respiration is the mitochondrion. This organelle plays a central role in energy metabolism by housing the enzymes and components necessary for the complete breakdown of glucose and other molecules to produce ATP (adenosine triphosphate), which is the primary energy currency of the cell. Before directly entering into the details of the enzymes, it is helpful to understand how the mitochondrion is structurally designed to perform this role efficiently.

Mitochondria are double-membrane-bound organelles found in almost all eukaryotic cells. The outer membrane serves as a protective layer, while the inner membrane is extensively folded into structures known as cristae, which increase the surface area for housing respiratory enzymes. The matrix, the innermost compartment, contains the enzymes for the Krebs cycle, while the inner membrane houses the electron transport chain (ETC) and ATP synthase, which are essential for the final steps of ATP production.

There are three main steps of aerobic respiration and specific enzymes are located in different mitochondrial regions:

1. Krebs Cycle (Citric Acid Cycle)
• Krebs cycle (also known as citric acid cycle) Takes place in the mitochondrial matrix and involves enzymes like citrate synthase, isocitrate dehydrogenase and succinate dehydrogenase. These enzymes help break down pyruvate into carbon dioxide while transferring high-energy electrons to NAD⁺ and FAD.
2. Electron Transport Chain (ETC)
• This step occurs on the inner mitochondrial membrane. Enzyme complexes I to IV (NADH dehydrogenase, cytochrome bc₁ complex, cytochrome oxidase, etc.) transfer electrons and pump protons across the membrane, creating a proton gradient.
3. ATP Synthesis
• The proton gradient created by the ETC powers the enzyme ATP synthase, which uses this potential energy to synthesize ATP from ADP and inorganic phosphate.

Thus, the mitochondrion contains all the essential enzymes for aerobic cellular respiration, allowing it to efficiently convert the chemical energy in food molecules into usable energy for cellular functions.

vi) Why mitochondria and chloroplast are called semi-autonomous?

The term semi-autonomous means that an organelle has some level of independence, but not complete independence, in performing its functions and maintaining itself. In eukaryotic cells, mitochondria and chloroplasts are called semi-autonomous organelles because they possess certain unique features that allow them to perform some of their own functions independently from the nucleus, but still depend on the nucleus for many essential components.

Introduction to the Concept of Semi-Autonomy

• Before we understand why mitochondria and chloroplasts are called semi-autonomous, it is important to know that most organelles in a eukaryotic cell depend completely on the nucleus for their proteins, enzymes and replication. However, mitochondria and chloroplasts are exceptions to this. They show several prokaryotic-like features which support the idea of their partial independence. These organelles are also central to energy-related functions: mitochondria in aerobic respiration and chloroplasts in photosynthesis.

Historical Insight

• This concept is also supported by the Endosymbiotic Theory proposed by Lynn Margulis in 1967, which suggests that mitochondria and chloroplasts originated from free-living bacteria that entered into a symbiotic relationship with ancestral eukaryotic cells. Over time, most of their genes were transferred to the nucleus, but they retained a few for essential internal processes.

Features That Make Them Semi-Autonomous

The four major features that make mitochondria and chloroplasts semi-autonomous:

1. Presence of Their Own DNA

Both mitochondria and chloroplasts contain circular, double-stranded DNA, similar to bacterial DNA. This DNA can replicate independently of the nuclear DNA. However, this DNA only codes for a small number of proteins.

2. Presence of Ribosomes

They contain 70S ribosomes, which are like the ribosomes found in prokaryotes, not the typical 80S ribosomes of the cytoplasm. These help in the synthesis of some of their own proteins.

3. Ability to Undergo Self-Replication

Mitochondria and chloroplasts divide by binary fission, just like bacteria. This is another evidence of their semi-independent behavior.

4. Partial Dependence on Nuclear Genes

Even though they make some of their own proteins, most of their proteins are encoded by nuclear DNA. These proteins are synthesized in the cytoplasm and imported into the organelles. Thus, they cannot function completely without the nucleus.

vii) Mention any two advantages of the extensive network of the endoplasmic reticulum.

To understand the advantages of the extensive network of the endoplasmic reticulum (ER), it is important to know that the ER is a large, interconnected membranous structure found throughout the cytoplasm of eukaryotic cells. It is divided into two types: rough ER (with ribosomes) and smooth ER (without ribosomes). The broad and interconnected nature of the ER offers several benefits. Out of these, two key advantages are explained below:

1. Efficient Intracellular Transport

The extended network of tubules and cisternae in the ER allows for smooth and efficient transport of proteins, lipids and other molecules within the cell. Since the ER extends throughout the cytoplasm and connects directly with the nuclear envelope and sometimes the plasma membrane, it forms a highway-like system that helps in the rapid movement of substances. This is particularly useful for proteins synthesized by the rough ER, as they are quickly delivered to the Golgi apparatus or other destinations.

2. Large Surface Area for Metabolic Activities

The extensive and folded membranes of the ER provide a vast surface area, which is highly beneficial for carrying out various cellular activities. The rough ER offers ample space for ribosomes to carry out protein synthesis, while the smooth ER provides surfaces for lipid synthesis, detoxification of harmful substances (especially in liver cells), and storage of calcium ions (especially in muscle cells). This large area helps increase the overall efficiency of these functions within the cell.

viii) What is the function of peroxisomes in plant cells?

To understand the importance of peroxisomes in plant cells, it is necessary to know that these are small, membrane-bound organelles found in the cytoplasm. Although they are present in both animal and plant cells, in plant cells, they are particularly important due to their involvement in photorespiration and lipid metabolism. These organelles contain oxidative enzymes such as catalase and oxidase, which are responsible for removing harmful substances like hydrogen peroxide.

Role in Photorespiration

This is the most specific and unique function of peroxisomes in plant cells. During photosynthesis, under certain conditions like high oxygen and low carbon dioxide levels, a side process called photorespiration occurs. In this process, the chloroplasts produce glycolate, which is transported to peroxisomes. Inside the peroxisomes, glycolate is converted into glycine.

This reaction releases hydrogen peroxide (H₂O₂) as a by-product, which is highly toxic to the cell. The enzyme catalase in peroxisomes quickly converts H₂O₂ into water and oxygen, thus detoxifying the cell and preventing damage. Hence, peroxisomes are essential for maintaining the balance during photorespiration.

Role in Lipid Breakdown during Seed Germination

Another key function of peroxisomes in plant cells is the beta-oxidation of fatty acids, especially during seed germination. In the early stages of germination, seeds convert stored fats into sugars to provide energy for growth. This takes place in specialized peroxisomes called glyoxysomes, which are present in fat-storing tissues of plant seeds. Glyoxysomes help convert fatty acids into succinate, which can then enter the mitochondria and be used for energy production through cellular respiration.

Additional Role in Detoxification

Peroxisomes also help in detoxifying harmful by-products of metabolism. Besides hydrogen peroxide, they also help neutralize other reactive oxygen species (ROS), thereby maintaining the health of the cell.

ix) Explain the following terms: (a) chromatin network (b) chromosomes (c) Nucleosome (d) Solenoid Model

(a) Chromatin Network

In the nucleus of eukaryotic cells, chromatin refers to the complex of DNA and histone proteins. It is the molecular material that carries genetic information in a packed form. When observed under a microscope during the interphase of the cell cycle, this chromatin appears as a loose, thread-like network, which is called the chromatin network. Hence, chromatin is the substance made of DNA and proteins, while the chromatin network refers to its visible, mesh-like arrangement inside the nucleus when the cell is not dividing.

Structure and Components of the Chromatin Network

The chromatin network consists of multiple nucleosomes, where DNA is wrapped around histone protein cores. This arrangement allows the extremely long DNA strands to be compacted efficiently and organized in a way that maintains accessibility for transcription, replication and DNA repair. This network is finely distributed and can be seen as a delicate web inside the nucleus during interphase.

Types of Chromatin in the Network

There are two types found in the chromatin network:

1. Euchromatin: Lightly stained, loosely packed and transcriptionally active regions of DNA.
2. Heterochromatin: Densely stained, tightly packed and generally transcriptionally inactive areas.

This dual nature gives flexibility to the cell in managing gene expression depending on physiological conditions.

(b) Chromosomes

Chromosomes are thread-like structures made up of DNA and histone proteins. They are found inside the nucleus of eukaryotic cells and are responsible for the storage, expression and transfer of genetic information from one generation to the next. During cell division, chromatin condenses to form visible chromosomes, ensuring proper distribution of genetic material.

Structure of Chromosomes

Each chromosome consists of two identical sister chromatids joined at a point called the centromere. These chromatids carry the same genetic information. Chromosomes ensure the equal distribution of genetic material during cell division and maintain hereditary continuity.

Types of Chromosomes

There are two major bases for the classification of chromosomes:

1. Based on Centromere Position

This classification depends on where the centromere is located along the length of the chromosome.

1. Metacentric: Centromere is exactly in the middle, forming two equal arms.
2. Submetacentric: Centromere is slightly off-center, producing one short and one long arm.
3. Acrocentric: Centromere is close to one end, making one very short and one very long arm.
4. Telocentric: Centromere is at the end of the chromosome, appearing to have only one arm (not found in humans).

2. Based on Role in Sex Determination

This classification is based on whether the chromosome determines the sex of an individual or not.

1. Autosomes: These chromosomes control general body characteristics. Humans have 22 pairs of autosomes.
2. Sex Chromosomes (Allosomes): These determine the sex of the individual. In humans, females have XX and males have XY sex chromosomes.

(c) Nucleosome

To understand nucleosome properly, it is important to know that eukaryotic DNA is very long and cannot fit inside the nucleus unless it is packed in an organised way. This organised packaging starts with a basic unit known as the nucleosome. A nucleosome is the basic structural and functional unit of chromatin in eukaryotic cells. It plays a crucial role in DNA packaging by enabling long DNA strands to be compacted within the nucleus in a highly organised way. The term "nucleosome" was first introduced by Roger Kornberg in 1974, who also explained its "bead-like" appearance under the electron microscope. A nucleosome consists of DNA wrapped around histone proteins. This structure is essential for compacting DNA within the nucleus, ensuring its proper organisation for processes like transcription and replication.

Structure of Nucleosome

The nucleosome consists of a core particle and linker DNA. The core particle is an octamer of histone proteins, with two copies each of histones H2A, H2B, H3 and H4. Around this octamer, approximately 147 base pairs of DNA are wrapped in 1.65 left-handed superhelical turns. This DNA-histone complex is tightly coiled to form a structure that resembles "beads on a string." The core nucleosome is connected by linker DNA, which varies in length (approximately 20-80 base pairs) and connects adjacent nucleosomes. A fifth histone protein called H1 binds to the linker DNA between nucleosomes. It helps in stabilizing the structure and supports the formation of higher-order chromatin arrangements such as the 30 nm fiber. The H1 histone functions like a clamp and strengthens the DNA's firm attachment to the histone core.

Functions of Nucleosome

The nucleosome performs multiple essential functions:

• It helps in condensing and packaging the DNA into a compact form so that it fits inside the nucleus.

• It acts as a protective unit for DNA, preventing it from physical damage and enzymatic degradation.

• It plays a regulatory role in gene expression, as the wrapping of DNA influences which genes are active or silent.

• It also helps in chromatin remodeling, DNA replication and repair processes by making specific regions accessible or inaccessible as needed.

(d) Solenoid Model

The solenoid model is a well-known hypothesis that explains the secondary level of DNA packaging in eukaryotic cells. After the primary level of compaction, where DNA is wrapped around histone proteins to form nucleosomes, further folding is required to fit the large eukaryotic genome inside the nucleus. In 1976, scientists Finch and Klug proposed the solenoid model to describe how nucleosomes organize into a higher-order helical structure known as the 30 nm chromatin fiber. This model plays a crucial role in making chromatin more compact, yet still accessible for essential processes like replication and transcription.

Structure

In the solenoid model, nucleosomes are arranged in a spiral or helical fashion forming a hollow tube-like fiber. About six nucleosomes are present per turn of the solenoid. The linker DNA, which connects each nucleosome, bends in such a way that it helps the nucleosomes to pack closely. Histone H1 is very important in this model as it binds to the linker DNA and stabilizes the folding. The overall diameter of this fiber is around 30 nanometres and this folding leads to a much more condensed form of chromatin compared to the "beads on a string" structure.

Functions

The solenoid model is important for several reasons:

• Efficient DNA Packaging: It helps in compacting the long DNA strands so that they can fit inside the limited space of the nucleus.

• Regulation of Gene Expression: The coiling influences the accessibility of DNA to transcription factors, playing a role in gene regulation.

• Support during Cell Division: The compact structure is essential for the proper segregation of chromosomes during mitosis and meiosis.

• Protection of Genetic Material: The coiled arrangement provides mechanical protection to the DNA from damage.

x) What is the function of the nucleolus in the cell?

The nucleolus is a dense, spherical structure found inside the nucleus of eukaryotic cells. It is not surrounded by a membrane, yet it is one of the most prominent sub-nuclear structures visible under a microscope. The nucleolus plays a very important role in gene expression and cellular metabolism. Understanding its functions is essential because of its direct connection to protein synthesis, cell growth and cell cycle regulation.

Function of the Nucleolus

The primary function of the nucleolus is ribosome biogenesis, which involves the synthesis and assembly of ribosomal RNA (rRNA) and ribosomal proteins to form ribosomes. These ribosomes are essential for protein synthesis in the cell. Besides this primary role, the nucleolus also performs several additional functions that contribute to cellular processes like stress response, cell cycle regulation, and RNA modification.

Primary Function:

1. Ribosome Biogenesis:

The primary and most well-known function of the nucleolus is the synthesis of ribosomes. This process is called ribosome biogenesis. Ribosomes are the molecular machines responsible for protein synthesis, and without them, cells cannot produce proteins necessary for their structure and function.

Within the nucleolus, the ribosomal RNA (rRNA) genes located on nucleolar organizer regions (NORs) of specific chromosomes are actively transcribed by RNA polymerase I. This transcription produces a large precursor molecule of rRNA (45S pre-rRNA in humans), which is then processed and cleaved into smaller functional rRNA units (18S, 5.8S and 28S rRNAs). These rRNA molecules are then assembled together with ribosomal proteins, which are imported from the cytoplasm into the nucleus, to form immature ribosomal subunits. These subunits are later transported to the cytoplasm where they combine to form functional ribosomes.

Additional Functions:

Besides ribosome biogenesis, the nucleolus performs some additional roles as well:

1. Regulation of Cell Cycle and Stress Response:

The nucleolus plays an important role in sensing cellular stress and DNA damage. It interacts with tumor suppressor proteins such as p53. Under stress conditions, p53 is activated partly through nucleolar mechanisms, leading to cell cycle arrest or apoptosis. Thus, the nucleolus indirectly contributes to genome stability and cancer prevention.

2. Assembly of Signal Recognition Particles (SRPs):

The nucleolus is involved in the formation of SRPs, which are essential complexes that help in the proper targeting of proteins to the endoplasmic reticulum during translation. This is important in the secretory pathway of eukaryotic cells.

3. Modification of Small Nuclear RNAs (snRNAs):

The nucleolus also hosts small nucleolar RNAs (snoRNAs) and related proteins that modify snRNAs and rRNAs. These modifications include methylation and pseudouridylation, which are necessary for proper rRNA folding and function.

4. Storage and Maturation of Regulatory Molecules:

It serves as a temporary storage site for several nuclear proteins, such as those involved in DNA repair, transcription regulation and cell cycle control. Some proteins mature in the nucleolus before being transported to their final destination in the nucleus or cytoplasm.

5. Involvement in Aging and Senescence:

Recent studies suggest that changes in nucleolar size and activity are markers of cellular aging and senescence. An enlarged or fragmented nucleolus is often associated with aged or stressed cells.

6. Pathological Role in Diseases:

Nucleolar hypertrophy (enlargement) is frequently observed in cancer cells and is used as a diagnostic marker. Disruptions in nucleolar structure or function are also linked to neurodegenerative diseases like Alzheimer's and Parkinson's disease.

SAQ 2

i) How are sclerenchyma and collenchyma different with respect to structure and function?

Sclerenchyma and collenchyma are two important types of simple permanent tissues in plants. Both are specialized for support, but they differ in their structure, composition and functional roles. These differences help the plant maintain its shape, stand upright and survive in various environmental conditions. Let us understand their differences based on structure and function.

Structural Differences:

Sclerenchyma cells are dead at maturity and have extremely thick cell walls due to the uniform deposition of lignin, a complex organic polymer. These cells lack protoplasm and usually have very narrow lumens. They are rigid and occur as either fibres (long and narrow) or sclereids (short and irregular). Because of their hardness, they often make plant parts like seed coats and nut shells hard and tough.

On the other hand, collenchyma cells are living and have unevenly thickened cell walls. These thickenings are mainly due to the deposition of cellulose, hemicellulose and pectin, especially at the corners of the cells. Collenchyma cells retain the protoplasm and have more flexibility. They usually occur in the cortex of stems and petioles, often below the epidermis.

Functional Differences:

The main function of sclerenchyma is to provide mechanical strength and rigidity to mature plant parts. As they are dead, they do not help in metabolic activities. They form the main support tissue in hard, non-growing regions such as vascular bundles, seed coats and nutshells.

Collenchyma, in contrast, offers flexible mechanical support to growing parts of the plant. Since the cells are living, they can elongate and adapt to the growing needs of the plant. They are mostly found in young stems, petioles and leaf midribs, where they help in bending and swaying without breaking.

ii) What are the characteristics of sclerenchyma cells?

Sclerenchyma is a type of simple permanent tissue found in higher plants, mainly responsible for providing mechanical strength and support. These cells are specially adapted to offer rigidity to plant parts that are no longer elongating or growing. The word "sclerenchyma" is derived from the Greek word scleros, meaning "hard", which reflects the tough nature of these cells.

The following are the key characteristics of sclerenchyma cells:

1. Dead at Maturity:

Sclerenchyma cells are non-living when they reach maturity. They lose their protoplasm, making them metabolically inactive. This feature is important because their primary role is to provide support, not participate in physiological processes.

2. Highly Thickened Cell Walls:

One of the most important features of sclerenchyma is the presence of very thick secondary cell walls. These thick walls are uniformly thickened and are rich in lignin, a complex and hard organic substance that adds rigidity and impermeability to the cell wall.

3. Narrow Lumen:

Due to the heavy deposition of lignin on the inner surface of the cell wall, the lumen (central cavity) of the sclerenchyma cell becomes very narrow or almost absent. This makes the cells extremely rigid and strong.

4. Two Main Types – Fibres and Sclereids:

There are two types of sclerenchyma cells based on shape and location:

1. Fibres: These are long, narrow and tapered cells usually arranged in bundles. They are commonly found in stems, bark and vascular tissues.
2. Sclereids: These are short, irregular-shaped cells. They provide stiffness and hardness to plant parts like seed coats, nutshells and the gritty texture in pear fruits.

5. No Intercellular Spaces:

Sclerenchyma cells are usually tightly packed, with little or no intercellular space, which contributes to the compactness and strength of the tissue.

6. Provides Mechanical Support:

The main function of sclerenchyma cells is to provide structural support to those parts of the plant that have stopped growing. They are especially abundant in mature regions of the plant, such as stems, roots and vascular bundles.

iii) Which tissue occurs in the outermost cell layer of plant organs?

In all primary plant organs such as roots, stems, leaves, flowers and fruits, the outermost protective covering is formed by the epidermal tissue system. This system is a collective term used to describe the outer cell layers that protect the internal tissues of the plant. The epidermis is the main and most prominent component of this system. In simpler terms, we can say that epidermal tissue is the entire tissue system, while the epidermis is the actual single outer layer of cells that we directly observe in plant organs.

The epidermal tissue system not only includes the epidermis, but also contains various specialized structures like stomata (with guard cells), trichomes (hair-like projections) and root hairs, each of which plays a functional role in plant physiology. It forms the first line of defence of the plant body.

Structure of Epidermis

The epidermis generally consists of a single layer of parenchymatous cells, which are flat, compactly arranged and do not have intercellular spaces. In aerial parts of the plant, these cells are often covered with a waxy cuticle, which helps in preventing excessive water loss. In roots, the cuticle is generally absent, and some epidermal cells develop into long, tubular root hairs that enhance water and mineral absorption.

Additional structures like stomata help in gaseous exchange, trichomes provide protection against herbivores and excessive sunlight, and guard cells regulate the opening and closing of stomata.

Functions of Epidermal Tissue

• Protection: It safeguards the inner tissues from mechanical injury, desiccation, pathogens and harmful radiation.

• Water regulation: The cuticle and stomata help reduce water loss and regulate transpiration.

• Gas exchange: Stomata allow controlled exchange of oxygen and carbon dioxide.

• Absorption: In roots, root hairs increase surface area and help absorb water and minerals from the soil.

• Temperature regulation and defense: Trichomes reflect sunlight, insulate the plant surface and protect against herbivory.

iv) What type of cells are fibroblasts?

Fibroblasts are specialized connective tissue cells that play a crucial role in maintaining the structural framework of tissues by producing extracellular matrix (ECM) components, such as collagen, elastin and glycosaminoglycans. They are a type of mesodermal-derived cell, meaning they originate from the mesoderm layer during embryonic development.

Structure of Fibroblasts

Fibroblasts are spindle-shaped (elongated) cells with a large oval or elongated nucleus. The cytoplasm is basophilic, meaning it stains easily with basic dyes due to the presence of ribosomes and rough endoplasmic reticulum, which are involved in protein synthesis. The presence of numerous cytoplasmic extensions allows fibroblasts to spread throughout the connective tissue, making them highly adaptable to the surrounding environment.

In their active state, fibroblasts are metabolically active, and they exhibit rich rough endoplasmic reticulum and Golgi apparatus which are involved in the production and secretion of proteins such as collagen, elastin, fibronectin and proteoglycans. These proteins are essential for the formation of the ECM, which provides structural support to tissues and organs.

Types of Fibroblasts

There are two primary types of fibroblasts:

1. Active fibroblasts:
• These cells are metabolically active and have a prominent nucleus with an abundant rough endoplasmic reticulum. These cells are involved in the production of extracellular matrix (ECM) components like collagen, elastin, and glycosaminoglycans. They are larger, with an abundant cytoplasm and well-developed organelles for protein synthesis.
2. Quiescent fibroblasts:
• Also known as fibrocytes, these cells are in a resting state and are less active in protein synthesis compared to their active counterparts. They have a smaller cytoplasm and a more condensed nucleus. Their primary function is to remain in a resting state until they are activated during injury or stress.

Functions of Fibroblasts

• Collagen synthesis: Fibroblasts produce collagen, an essential structural protein, which forms the extracellular matrix and provides tensile strength to connective tissues.

• Tissue repair and wound healing: Fibroblasts are pivotal in wound healing. They migrate to the site of injury, proliferate, and synthesize extracellular matrix components necessary for tissue repair.

• Elastin production: In addition to collagen, fibroblasts also produce elastin, a protein that provides elasticity to tissues such as the skin, lungs and blood vessels.

• Secretion of growth factors: Fibroblasts secrete various growth factors, including transforming growth factor-beta (TGF-β), which are essential for the regulation of tissue homeostasis and repair processes.

• Regulation of extracellular matrix composition: Fibroblasts play a key role in maintaining the balance between the synthesis and degradation of extracellular matrix components, thus ensuring tissue integrity.

• Role in inflammation: During tissue injury, fibroblasts participate in inflammatory responses by secreting cytokines and other signaling molecules that help in modulating immune responses.

TERMINAL QUESTIONS

1. What are the different functions of plasma membrane?

The plasma membrane, also called the cell membrane, is a selectively permeable biological barrier that surrounds the cytoplasm of all living cells. It plays a crucial role in maintaining cellular integrity and homeostasis by regulating the movement of substances in and out of the cell. It is mainly composed of a phospholipid bilayer with embedded proteins, cholesterol and carbohydrates, which together allow the membrane to carry out several specialized functions.

Main Functions of Plasma Membrane

1. Selective Permeability:

One of the most important functions of the plasma membrane is to regulate the entry and exit of substances. It allows only certain molecules (like oxygen, water and glucose) to enter, while waste materials and unwanted substances are removed. This selective permeability is due to the lipid bilayer and specific transport proteins.

2. Transport of Materials:

The membrane facilitates both passive transport (like diffusion and osmosis) and active transport (through protein pumps and channels) to move substances such as ions, nutrients and waste materials across the membrane. Active transport uses energy in the form of ATP to move substances against the concentration gradient.

3. Cell Communication and Signal Transduction:

The plasma membrane contains receptor proteins that receive signals (such as hormones and neurotransmitters) from other cells or the external environment. These signals are then transmitted inside the cell, leading to appropriate cellular responses. This is called signal transduction.

4. Cell Recognition:

Glycoproteins and glycolipids present on the external surface of the plasma membrane help in recognizing and interacting with other cells. These components act as markers or identity tags, which are important in immune response and tissue organization.

5. Structural Support and Shape Maintenance:

The plasma membrane helps maintain the structural integrity and shape of the cell. It also anchors the cytoskeleton (internal protein framework), which provides mechanical support and helps in cell movement.

6. Cell Adhesion:

The plasma membrane helps neighboring cells attach to each other and to the extracellular matrix. This cell adhesion is important in forming tissues and organs, and in processes like wound healing and embryonic development.

7. Endocytosis and Exocytosis:

The plasma membrane is actively involved in endocytosis (engulfing of external particles or fluids into the cell) and exocytosis (expelling substances like enzymes and hormones from the cell). These processes help in nutrient intake, immune responses and secretion of cellular products.

8. Barrier Function:

The membrane acts as a physical barrier that protects the internal cellular environment from external harm, including toxins, pathogens and mechanical injury.

2. Elucidate the basic structure of nucleosome.

In eukaryotic cells, the DNA is extremely long and must be efficiently packed to fit within the confines of the nucleus. This packaging is achieved without tangling or damaging the genetic material with the help of the nucleosome, which is the basic unit responsible for this organization. It is not only responsible for compacting the DNA but also plays an essential role in regulating access to genetic information during processes such as transcription, replication and DNA repair. The nucleosome acts as the first level of chromatin organization and serves as a structural and functional framework for further compaction.

Core Structure of the Nucleosome

The nucleosome consists of two main components: the core particle and the linker DNA. The core particle is built around a histone octamer, which is made up of two molecules each of histones H2A, H2B, H3 and H4. These histones are small, positively charged proteins that interact with the negatively charged DNA. Around this histone octamer, about 147 base pairs of DNA are wound in 1.65 left-handed superhelical turns, forming a tight and stable DNA-protein complex.

This tightly wound DNA around the core histones gives rise to a characteristic appearance resembling "beads on a string" when observed under an electron microscope. Each bead is a nucleosome and the string connecting them is known as linker DNA, which spans about 20 to 80 base pairs between adjacent nucleosomes.

Role of Linker Histone H1 and Higher-Order Structure

The linker histone H1 binds to the region where the DNA enters and exits the core particle. Its presence helps to stabilize the nucleosome and assists in further compaction. With the help of H1, nucleosomes fold into more condensed structures like the 30 nm fiber, allowing the DNA to be packaged into even more compact forms such as chromatin loops and chromosomes.

This organized structure is dynamic and can be modified chemically (for example, by acetylation or methylation of histone tails), which influences how tightly DNA is packed and whether certain genes are accessible for expression.

3. Write a brief account on the origin of mitochondria and chloroplast.

Mitochondria and chloroplasts are essential organelles in eukaryotic cells. Mitochondria are involved in aerobic respiration and ATP production, while chloroplasts are the site of photosynthesis in plant and algal cells. The widely accepted explanation for their origin is the Endosymbiotic Theory.

The earliest idea related to the symbiotic origin of plastids was proposed by Konstantin Mereschkowski in 1905, who suggested that chloroplasts evolved from autotrophic cyanobacteria like ancestors through a process of symbiogenesis. Later in the 1920s, Ivan Wallin, an American biologist, extended this idea to mitochondria, proposing that they originated from aerobic bacteria. However, these early ideas received little acceptance at that time.

A major revival and refinement of the endosymbiotic theory came with Lynn Margulis in 1967, who in her seminal paper "On the Origin of Mitosing Cells" provided detailed evidence that both mitochondria and chloroplasts originated from free-living prokaryotes. She proposed that an ancestral eukaryotic cell engulfed an aerobic bacterium (which became mitochondrion) and later a photosynthetic cyanobacterium (which became chloroplast) in two separate events. This model gained strong support later through molecular and genetic studies.

Endosymbiotic Theory of Origin

According to this theory, mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by ancestral eukaryotic cells. These prokaryotes entered into a symbiotic relationship with the host cell, eventually becoming permanent internal components of the eukaryotic cell.

Mitochondria are believed to have originated from aerobic alpha-proteobacteria, while chloroplasts are thought to have evolved from photosynthetic cyanobacteria. Over time, most of their genes were transferred to the host nuclear genome, but they retained a small, independent genome for essential functions.

Structural and Genetic Evidence Supporting Endosymbiosis

The following structural and genetic features support the endosymbiotic origin of these organelles:

• Double Membrane Structure: Both mitochondria and chloroplasts possess a double membrane, similar to that of gram-negative bacteria, which suggests engulfment by endocytosis.

• Circular DNA: These organelles contain their own circular DNA, not associated with histones, much like bacterial DNA.

• 70S Ribosomes: They contain 70S-type ribosomes (prokaryotic type), unlike the 80S ribosomes found in the eukaryotic cytoplasm.

• Binary Fission: Both organelles replicate independently through binary fission, which is a typical bacterial method of reproduction.

• Gene Sequence Similarities: Molecular studies have shown that mitochondrial genes are closely related to alpha-proteobacteria, and chloroplast genes resemble cyanobacterial genes.

4. Discuss the different types of lysosomes.

Lysosomes are small membrane-bound organelles found in most animal cells. They are absent in most plant cells, although plant cells do have similar structures like vacuoles that perform some of the same functions. Lysosomes are an important part of the eukaryotic endomembrane system. These organelles were first discovered by the Belgian scientist Christian de Duve in 1955. Lysosomes are known as the digestive system of the cell, because they contain hydrolytic enzymes that help in the breakdown of biological substances like proteins, lipids, carbohydrates and nucleic acids.

Lysosomes are also called the "suicidal bags" of the cell. This is because under certain conditions like cellular damage, stress, or ageing, lysosomes may rupture and release their enzymes into the cytoplasm. These enzymes digest the cell's own components, leading to autolysis or self-destruction.

Types of Lysosomes

Functionally, lysosomes can be divided into three main types, each with distinct structural and functional features. These are Primary Lysosomes, Secondary Lysosomes (with two subtypes) and Residual Bodies.

1. Primary Lysosomes (Protolysosomes)

Primary lysosomes are newly formed vesicles that bud off from the trans-Golgi network. They are spherical in shape and enclosed by a single membrane. Inside, they contain hydrolytic enzymes like proteases, nucleases, lipases and glycosidases, but these enzymes are present in an inactive or latent form because they have not yet come in contact with any substrate.

Their main role is storage and transport of digestive enzymes to sites where they will be needed later. Once they encounter a vesicle containing foreign or damaged material (like phagosomes or autophagosomes), they fuse with it and form secondary lysosomes where active digestion takes place. Until this fusion, primary lysosomes remain inactive but ready to act.

2. Secondary Lysosomes (Digestive Lysosomes)

Secondary lysosomes are formed when primary lysosomes fuse with other vesicles containing material to be degraded. The enzymes inside become active due to the acidic internal pH (around 4.5–5.0), which is maintained by proton pumps on the membrane. These are the actual site of digestion inside the cell. Depending on the origin of the material to be digested, secondary lysosomes are further divided into two kinds:

A. Heterophagic Vacuoles or Heterosomes or Phagolysosomes:

• These are formed when primary lysosomes fuse with phagosomes or endosomes containing extracellular material such as bacteria, dead cells, or dust particles. These materials are often taken into the cell by phagocytosis or endocytosis. The resulting heterophagic vacuoles digest and neutralize foreign bodies, making them very important for defense against pathogens, especially in macrophages and neutrophils.

B. Autophagic Vacuoles (Autophagosomes + Lysosome):

• These are formed when primary lysosomes fuse with autophagosomes, which are vesicles containing the cell's own old or damaged organelles, like mitochondria, peroxisomes  or portions of the endoplasmic reticulum. This process is called autophagy, and it plays a critical role in cellular renewal and homeostasis. In starvation conditions, autophagy also helps by breaking down cellular components for energy.

3. Residual Bodies

After digestion is complete in secondary lysosomes, the indigestible material that remains behind is enclosed in vesicles called residual bodies. These contain substances like oxidized lipids, heavy metals and other inert materials. In some cells, these residual bodies are expelled out of the cell by exocytosis. However, in long-living cells such as neurons, cardiac cells, or skeletal muscle cells, they tend to accumulate inside the cytoplasm and form granules known as lipofuscin, which are also called aging pigments.

These residual bodies are indicators of cellular age and wear-and-tear and are especially prominent in aging tissues.

Read More:

• UNIT 1 – Eukaryotic Cell: Structure and Functions (Q&A) | MZO-001 MSCZOO | IGNOU

• UNIT 2 – Actin Filaments (Q&A) | MZO-001 MSCZOO | IGNOU
  

• UNIT 3 – Microtubules (Q&A) | MZO-001 MSCZOO | IGNOU

• UNIT 4 – Intermediate Filaments (Q&A) | MZO-001 MSCZOO | IGNOU

• UNIT 5 – Biology of Membrane and Transport of lons (Q&A) | MZO-001 MSCZOO | IGNOU

• UNIT 6 – Transepithelial Transport (Q&A) | MZO-001 MSCZOO | IGNOU

• UNIT 7 – Bulk Transport (Q&A) | MZO-001 MSCZOO | IGNOU