Class 11 Biology Chapter 8 Cell: The Unit of Life

Key Concept: Organelle Interactions, Molecular Basis, Real-world Application

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

The assertion states that chloroplasts and mitochondria cooperate to optimize energy production processes in plant cells through photosynthesis and respiration. This is true, as both organelles play distinct yet interconnected roles in cellular energy dynamics. Chloroplasts are responsible for capturing light energy and converting it into chemical energy stored in glucose molecules through photosynthesis. Mitochondria then utilize this glucose in aerobic respiration to generate ATP, which powers various cellular activities.

The reason further explains how chloroplasts capture light energy to synthesize glucose, providing a substrate for mitochondrial respiration. This explanation correctly aligns with the assertion, showing a clear link between the two organelles in their contribution to the cell’s energy management system. Thus, both the assertion and the reason are valid, and the reason accurately explains the assertion.

Key Concept: Comparative Analysis, Evolutionary Significance

a) Oil-rich seeds can survive longer periods due to efficient energy storage

[Solution Description] Glyoxysomes are essential in converting stored lipids to sugars through the glyoxylate cycle in oil-rich seeds. This provides energy necessary for seedling development before photosynthesis begins. In carbohydrate-rich seeds, the immediate availability of glucose means less reliance on lipid conversion. Oil-rich seeds have an advantage in surviving longer dormancy periods and unfavorable conditions by relying on energy-dense lipids.

Key Concept: Inter-Organelle Interaction, Real-World Application

d) By transferring shortened fatty acyl chains

[Solution Description] Peroxisomes play a critical role in shortening very long chain fatty acids through beta-oxidation. Once shortened, these fatty acids can then be transferred to mitochondria for complete oxidation. The coordination between peroxisomes and mitochondria ensures efficient lipid metabolism; peroxisomes handle the preliminary reduction in chain length, while mitochondria finish the oxidation process, producing additional ATP. Disruption in this interaction may lead to metabolic disorders due to inefficient oxidation of lipids.

Key Concept: Enzyme Interaction, Biochemical Pathways

c) Beta-oxidation of fatty acids

[Solution Description] The enzymes within microbodies such as lipases play crucial roles in the breakdown of fatty acids through beta-oxidation. If there is a deficiency in lipases, this process will be disrupted, leading to the accumulation of fatty acids and an overall decrease in energy production from these sources. This disruption would primarily affect lipid metabolism.

Key Concept: Chromosome Dynamics, Chromosome Anomalies

b) Failure in proper segregation during anaphase

[Solution Description] A kinetochore is crucial for the attachment of spindle fibers to the centromere during mitosis and meiosis. If a chromosome lacks a properly functioning kinetochore, it cannot attach to the spindle apparatus correctly. This improper attachment leads to failure in proper segregation during anaphase because the spindle fibers cannot pull the chromatids to opposite poles effectively. As a result, one of the daughter cells may receive an extra copy of the chromosome while the other receives none, leading to aneuploidy.

Key Concept: Chromosome Functionality, Nucleolar Activity

b) Enlarged nucleoli facilitate enhanced ribosomal RNA synthesis

[Solution Description] Rapidly dividing cancer cells often exhibit structural adaptations in their chromosomes and nucleoli. Acrocentric chromosomes, which have long arms allowing for quick replication, and enlarged nucleoli, responsible for increased ribosomal RNA synthesis, contribute to an enhanced capability for protein production. This supports rapid growth and division by ensuring that protein machinery required for these processes is abundant.

Key Concept: Complex Interactions, Structural-Functional Correlation

b) It enhances regulatory potential by permitting only selective passage through nuclear pores.

[Solution Description] The nuclear envelope consists of two membranes with a perinuclear space, where the outer membrane is continuous with the endoplasmic reticulum and bears ribosomes. This structural arrangement allows for compartmentalization of functions such as protein synthesis within the ER, and ribosomal assembly which occurs at the nuclear membrane. Furthermore, the presence of nuclear pores facilitates selective transport of RNA and proteins across the nuclear envelope. These pores provide regulated pathways that allow specific molecules to move in and out of the nucleus while maintaining a distinct internal environment. This combination of continuity with the ER and regulated pore transport contributes to efficient cellular function.

Key Concept: Real-World Application, Evolutionary Significance

c) It allows intricate regulatory control over gene expression by segregating transcription and translation processes.

[Solution Description] The nuclear envelope provides a distinct separation between the nucleoplasm and the cytoplasm. This compartmentalization allows for greater regulation and protection of genetic material, reducing the likelihood of damage and increasing efficiency in gene expression. By isolating transcription in the nucleus, it prevents simultaneous translation, thus allowing eukaryotic cells to perform more complex regulatory activities, ultimately fostering better adaptation and survival in diverse environments.

Key Concept: Real-world Application, Evolutionary Significance

b) Conversion from chloroplast to chromoplast, impacting photosynthetic efficiency

[Solution Description] Brown leaves may indicate insufficient chlorophyll production due to malfunctioning chloroplasts. Normally, chloroplasts provide an evolutionary advantage of photosynthesis through efficient light absorption by chlorophylls. A lack thereof could mean either defective chloroplasts or their conversion into chromoplasts, altering pigmentation.

Key Concept: Ribosomal Functionality, Biochemical Pathways

d) By synthesizing enzymes essential for the light reactions.

[Solution Description]

Chloroplasts contain 70S ribosomes which are responsible for synthesizing certain proteins needed for photosynthesis. These proteins include components of the thylakoid membrane where light-dependent reactions occur. By translating chloroplast genes into proteins, 70S ribosomes facilitate the biochemical pathways necessary for converting light energy into chemical energy through the synthesis of ATP and NADPH in photosynthesis.

Key Concept: Arrangement Analysis

b) Nine triplets

[Solution Description]

The centrioles are composed of nine peripheral fibrils made of tubulin protein, and each fibril is arranged in a triplet configuration. This means that around the hub at the center, there are nine triplets of microtubules forming the structure of a centriole.

Key Concept: Functional Differences

c) Form the spindle apparatus during cell division

[Solution Description] Microtubules are cylindrical structures composed of tubulin protein and perform several functions in the cell. They play a crucial role in cell division as they form the spindle apparatus which helps segregate chromosomes during mitosis. Additionally, microtubules facilitate intracellular transport by providing tracks for motor proteins like dynein and kinesin to carry cellular cargoes. They also contribute significantly to maintaining the structural integrity and shape of cells.

Key Concept: Ribosome Density

b) Size and density

[Solution Description] The Svedberg unit is a measure of sedimentation rate during centrifugation. It indirectly indicates the size and density of particles. A higher Svedberg value typically suggests greater density.

Key Concept: Functional Importance

c) To store oils and fats

[Solution Description] Leucoplasts function primarily as nutrient storage structures. Specifically, elaioplasts are responsible for storing oils and fats within plant cells. This function helps in storing energy-dense compounds that can be accessed by the plant when needed.

Key Concept: Pigment Function

b) They act as antioxidants.

[Solution Description] Carotenoid pigments are not involved in photosynthesis inside chromoplasts but instead play a crucial role as antioxidants. These pigments help protect plant cells by scavenging harmful free radicals.

Key Concept: Thylakoid Function

b) Absorb light and convert it into chemical energy

[Solution Description]

The thylakoids are membrane-bound structures within the chloroplasts where the light-dependent reactions of photosynthesis occur. These reactions involve capturing light energy to produce ATP and NADPH, which are then used in the Calvin cycle (dark reactions) taking place in the stroma.

Key Concept: Organelle Division

d) By binary fission, like bacterial replication.

[Solution Description] Mitochondria divide by binary fission, which is a simple process where the organelle replicates its DNA and then splits into two separate mitochondria. This method ensures each new cell gets an appropriate number of mitochondria during cell division.

Key Concept: Structural Function

d) By increasing surface area for ATP production

[Solution Description] The cristae are foldings of the inner membrane of mitochondria, significantly increasing the surface area available for hosting the protein complexes required for the electron transport chain. This increase in surface area enhances the capacity for ATP production during aerobic respiration.

Key Concept: Enzyme Location

c) Matrix

[Solution Description] The Krebs cycle, also known as the citric acid cycle, takes place in the matrix of the mitochondria. This is because the matrix contains the necessary enzymes that facilitate this process.

Key Concept: Membrane Dynamics

b) Transport of ions into vacuoles

[Solution Description] The tonoplast is a membrane surrounding the vacuole in plant cells. It plays a crucial role in transporting ions and other solutes into and out of the vacuole, helping maintain cell turgor pressure and regulating cytoplasmic ion concentration.

Key Concept: Comparative Function

c) Cell adhesion and communication

[Solution Description] The middle lamella is primarily composed of calcium pectate and acts as a glue to hold neighboring plant cells together. While plasmodesmata function as channels that facilitate intercellular communication. A common function they share is supporting cell-to-cell interaction and communication within plant tissues.

Key Concept: Chloroplast Function

b) Thylakoid

[Solution Description] The thylakoids, which are membrane-bound structures within the chloroplast, are where light-dependent reactions occur. These reactions capture and convert sunlight into chemical energy in the form of ATP and NADPH.

Key Concept: Role of ATP in Transport

d) Active transport

[Solution Description] The process that requires ATP to transport ions against their concentration gradient is active transport. Active transport mechanisms, such as the Na\+/K\+ pump, utilize energy from ATP hydrolysis to move sodium and potassium ions across the cell membrane against their respective concentration gradients.

Key Concept: Lipid Arrangement

b) Tails face each other, polar heads face outward

[Solution Description] The phospholipids in the cell membrane are arranged with their polar heads facing outward towards the aqueous environment and their hydrophobic tails facing inward, away from water. This orientation creates a bilayer that forms the structural basis of the membrane.

Key Concept: Chromatin Naming

d) Walther Flemming

[Solution Description] Walther Flemming, a German biologist, is credited with naming chromatin. In 1879, he used aniline dyes to stain and observe thread-like structures during cell division, which he termed “chromatin.”

Key Concept: Structure of the Nucleus

c) Perinuclear space

[Solution Description] The space between the two layers of the nuclear envelope is known as the perinuclear space. This space plays a role in regulating the exchange of materials between the nucleus and cytoplasm.

Key Concept: Function Identification

b) Spindle apparatus formation

[Solution Description] Centrioles form the spindle fibers which are essential for chromosome movement during cell division in animal cells. They aid in organizing the microtubules that separate chromosomes during mitosis.

Key Concept: Basic Structure

d) 9+2

[Solution Description] The axoneme of eukaryotic cilia and flagella follows a 9+2 arrangement of microtubules, consisting of nine peripheral doublets and two central singlets.

Key Concept: Membrane Composition

b) Phospholipids

[Solution Description] The primary lipid component of the cell membrane is the phospholipid. These molecules arrange themselves into a bilayer, with hydrophilic heads facing outward and hydrophobic tails facing inward, forming the basic structure of the cell membrane as described by the fluid mosaic model.

Key Concept: Basic Structure

a) Axoneme

[Solution Description] The core structure of cilia and flagella is known as the axoneme. It consists of microtubules arranged in a specific pattern.

Key Concept: Passive Transport

c) Osmosis

[Solution Description] Passive transport involves the movement of substances across the cell membrane without the use of cellular energy. Osmosis is a type of passive transport where water moves across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.

Key Concept: Basic Structure

b) Microtubule

[Solution Description] The cytoskeleton consists of filamentous structures like microtubules, microfilaments, and intermediate filaments.

Key Concept: Structure Function Match

c) Cristae

[Solution Description] The cristae are folds within the inner membrane of the mitochondria that increase the surface area available for biochemical reactions involved in aerobic respiration.

Key Concept: Mitochondrial Structure

b) Increase surface area for chemical reactions

[Solution Description] Cristae are folds of the inner mitochondrial membrane that increase the surface area available for chemical reactions, particularly those involved in aerobic respiration such as the electron transport chain and ATP synthesis.

Key Concept: Subunit Sizes

b) 60S and 40S

[Solution Description] Eukaryotic ribosomes have a total sedimentation coefficient of 80S, and they are made up of two subunits: a large subunit with a sedimentation coefficient of 60S and a small subunit with a sedimentation coefficient of 40S.

Key Concept: Ribosome Type

b) 70S

[Solution Description] Chloroplasts contain 70S ribosomes, which are smaller than the 80S ribosomes found in the cytoplasm. These ribosomes are involved in protein synthesis within the chloroplast.

Key Concept: Function Identification

c) Amoeba

[Solution Description] The contractile vacuole is an organelle found predominantly in certain single-cell organisms like Amoeba, where it plays a crucial role in maintaining osmotic balance and expelling excess water from the cell.

Key Concept: Enzyme Types

a) Lipases, Proteases, Carbohydrases

[Solution Description] Lysosomes contain a range of hydrolytic enzymes known as hydrolases, which include lipases, proteases, and carbohydrases. These enzymes help in the digestion of biological macromolecules such as proteins, lipids, carbohydrates, and nucleic acids within the lysosome.

Key Concept: Vesicle Movement

b) Cis face

[Solution Description] Vesicles from the endoplasmic reticulum (ER) fuse with the convex cis face of the Golgi apparatus and then move towards the trans face.

Key Concept: Basic Structure

c) Luminal and extra-luminal

[Solution Description] The endoplasmic reticulum divides the intracellular space into two distinct compartments known as luminal and extra-luminal. The lumen is the internal space inside the ER, whereas the cytoplasm constitutes the extra-luminal space.

Key Concept: Composition Identification

d) Chitin

[Solution Description] In algae, the cell wall is primarily composed of cellulose, galactans, mannans, and minerals such as calcium carbonate. Chitin is not a component of the cell wall in algae; it is found in fungi.

Key Concept: Basic Concept

c) Lipid bilayer

[Solution Description] The cell membrane’s fluid nature is primarily due to the lipid bilayer. Lipids can move laterally within the layer, providing fluidity which is crucial for membrane functionality.

Key Concept: Passive Transport

d) Osmosis

[Solution Description] Osmosis is the movement of water molecules through a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration, without requiring energy input.

Key Concept: Organelle Identification

c) Nucleus

[Solution Description] The nucleus is considered the control center of the cell because it contains genetic material (DNA), which regulates all cellular functions.

Key Concept: Mechanism Analysis, Complex Process Understanding

b) Multiple ribosomes bind to a single mRNA, increasing the translation rate and efficiency.

[Solution Description] Polysomes allow multiple ribosomes to translate a single mRNA strand simultaneously, leading to more efficient protein production. As each ribosome translates the mRNA, it contributes to the overall rate of protein synthesis, thus enhancing the capacity and speed at which proteins are produced from a single mRNA.

Key Concept: Ribosome Evolution, Ribosome Assembly

c) Prokaryotic ribosome’s simplicity allows swift assembly and disassembly.

[Solution Description] The smaller prokaryotic ribosomes are more efficient for translation under rapid growth conditions due to their reduced complexity, which allows quicker assembly and disassembly during protein synthesis. This efficiency is beneficial in rapidly changing environments where adaptability is crucial.

Key Concept: Advanced Structural Analysis, Evolutionary Significance

b) By facilitating energy-efficient movement using proton motive force.

[Solution Description] The bacterial flagellum is composed of a simple structure with a filament, hook, and basal body, which allows for efficient energy use through rotational movement facilitated by a motor protein at the base. Unlike the more complex eukaryotic system that requires ATP breakdown, the bacterial flagella utilize proton motive force, providing an evolutionary advantage in terms of energy efficiency and adaptability to different environments.

Key Concept: Structural Interconnection, Integration with Other Structures

c) They stabilize the peripheral doublets and regulate bending.

[Solution Description] In cilia and flagella, the axoneme has a 9+2 arrangement of microtubules: nine pairs of peripheral doublets and two central microtubules. The radial spokes are protein complexes that extend from each of the nine outer microtubule doublets towards the central pair of microtubules. These spokes play a crucial role by maintaining structural integrity and regulating the dynein motors’ activity, facilitating the sliding of microtubules against one another, which is vital for generating movement. They connect the central sheath to the peripheral doublets, effectively transmitting signals necessary for coordinated motion.

Key Concept: Comparative Analysis, Evolutionary Perspective

d) It provides mechanical protection against osmotic pressure.

[Solution Description]

The cell wall of Gram-positive bacteria is primarily composed of a thick peptidoglycan layer that provides structural strength and rigidity. This layer protects against osmotic pressure changes and external stresses. The peptidoglycan’s dense and complex structure can resist environmental hazards, making these bacteria generally more resilient than those with thinner walls. Moreover, the absence of an outer membrane makes them impermeable to certain harmful substances like antibiotics which target outer membranes.

Key Concept: Real-world Application, Structural Complexity

c) Modified glycocalyx increases evasion of phagocytosis and affects classification.

[Solution Description] The glycocalyx, when present as a capsule, is a significant virulence factor in many bacterial species because it provides protection against phagocytosis by host immune cells. This feature enhances the bacterium’s ability to survive within hosts, contributing to pathogenicity. Additionally, variations in the glycocalyx composition and thickness can impact the bacterium’s classification since it influences interactions with various stains and responses to external environmental factors. Therefore, an altered glycocalyx can improve avoidance of immune response while also affecting classification properties by altering the staining pattern.

Key Concept: Mesosome and Chromatophore Comparison, Cell Envelope Adaptations

b) Increase in mesosome structures and thicker glycocalyx layer.

[Solution Description] Organisms in high-salinity environments develop adaptations in their cell envelopes to prevent desiccation and maintain osmotic balance. Additionally, enhanced respiration might involve structural modifications like an increase in mesosome density which can improve respiratory efficiency by providing more surface area for enzymatic activities. Glycocalyx changes or capsule formation might also occur to provide protection against saline conditions by reducing permeability and helping retain moisture.

Key Concept: Role in Metabolism

d) By acting as an energy reserve through glycogen storage

[Solution Description] Glycogen granules serve as energy reserves within prokaryotic cells. During times when external energy sources are scarce, these granules can be broken down into glucose molecules, providing a readily accessible form of energy to fuel cellular processes. This ability to store and later mobilize energy supports metabolic functions and enhances cell survival during periods of nutrient deprivation.

Key Concept: Polysome Formation

c) A polysome forms, enhancing translational efficiency.

[Solution Description] When multiple ribosomes attach to a single mRNA molecule, they form what is known as a polysome or polyribosome. This structure allows for simultaneous translation of the mRNA by several ribosomes, increasing the efficiency of protein synthesis as each ribosome translates the same mRNA sequence into a protein.

Key Concept: Structural Differences

c) Gram-positive bacteria have a thick peptidoglycan layer and stain purple.

[Solution Description] Gram-positive bacteria have a thick peptidoglycan layer and stain purple, while Gram-negative bacteria have a thin peptidoglycan layer, an additional outer membrane, and stain pink.

Key Concept: Functional Integration, Detailed Process

c) By synthesizing and processing proteins critical for cell function

[Solution Description] Ribosomes associated with the rough endoplasmic reticulum (ER) play a key role in synthesizing proteins destined for secretion or for use in organelles like lysosomes. These proteins are synthesized into the lumen of the rough ER, where they undergo proper folding and post-translational modifications, such as glycosylation. Once properly folded and modified, these proteins are transported to the Golgi apparatus for further processing and eventual export from the cell or integration into cellular membranes. This process ensures that cells maintain their structure and respond effectively to external signals, thereby contributing to cellular homeostasis by ensuring essential proteins are available when needed.

Key Concept: Cellular Compartmentalization, Endomembrane System

c) To prevent potential damage to the cell by confining powerful hydrolytic enzymes to a specific organelle.

[Solution Description] Lysosomal enzymes are hydrolytic and capable of breaking down biomolecules. If these enzymes were free in the cytoplasm, they could destructively digest essential cellular components, leading to cell damage or death. By being enclosed within lysosomes, these enzymes remain inactive until they reach their target substrates inside the lysosome, thus preventing undesirable reactions in the cell.

Key Concept: Eukaryotic Cell Diversity, Basic Structure of Plant Cells

b) Impaired water transport and support

[Solution Description] Secondary cell walls in plants provide structural support and are crucial for functions like water transportation and mechanical strength. They are rich in lignin, which makes them rigid and resistant to decay. Without secondary walls, plants would have weaker stems and reduced ability to transport water via xylem, potentially leading to wilting and decreased growth. Additionally, their resistance to pathogens and environmental stressors would be compromised. Thus, the most significant consequences involve impaired support and vascular function.

Key Concept: Cell Shape and Function

c) Columnar

[Solution Description] Columnar cells are elongated and have microvilli on their surface, increasing surface area for efficient absorption, commonly found in the lining of the intestine.

Key Concept: Scientist Identification

d) Theodor Schwann

[Solution Description] The scientist who identified the plasma membrane in animal cells was Theodor Schwann. In 1839, he proposed that both animal and plant bodies are composed of cells, marking a significant contribution to cell theory.

Key Concept: Cell Theory Statements

c) All cells arise from pre-existing cells.

[Solution Description] Rudolf Virchow expanded on the cell theory by stating “Omnis cellula-e cellula,” which means all cells arise from pre-existing cells. This statement emphasizes the continuity of life through cellular reproduction.

Key Concept: Historical Figures

c) Robert Brown

[Solution Description] Robert Brown, a Scottish botanist, is credited with discovering the nucleus in plant cells in 1831 during his studies of orchid cells.

Key Concept: Cell Structure Comparison

c) Nucleus

[Solution Description] Prokaryotic cells lack membrane-bound organelles, which are characteristic of eukaryotic cells. One key example is the nucleus. Eukaryotic cells have a true nucleus enclosed by a nuclear envelope, whereas prokaryotes have genetic material that is not enclosed in a membrane. Mitochondria, which are involved in energy production, are also absent in prokaryotes and found only in eukaryotes.

Key Concept: Historical Figures

d) Robert Brown

[Solution Description] Robert Brown was the scientist who discovered the nucleus in plant cells in 1831. His discovery was fundamental to understanding the internal structure of cells.

Key Concept: Basic Definition

c) Cell

[Solution Description]

The cell is recognized as the fundamental structural and functional unit of all living organisms. It is the smallest unit capable of performing all vital activities that characterize life.

Key Concept: Basic Definition

d) Cell

[Solution Description] The cell is defined as the basic structural, functional, and biological unit of all living organisms. It is often called the “building block of life”.

Key Concept: True Characteristics

c) Mitochondria have their own DNA and help in energy production.

[Solution Description] Mitochondria are known as the powerhouse of the cell because they produce energy through oxidative phosphorylation. They have their own DNA, which is separate from the nuclear DNA, allowing them to carry out some replication and synthesis independently.