Cell Structure and Function : Biology : class 11

 

Part A: Cell Theory, Cell Organelles, and Biomolecules

 

Multiple Choice Questions (MCQs) - 1 Mark Each

 

Find your Answers Below

Which of the following statements is NOT a part of the modern cell theory?

a) All living organisms are composed of cells and products of cells.

b) All cells arise from pre-existing cells.

c) Cells contain hereditary material which is passed on to the daughter cells.

d) All cells have a cell wall.

 

The fluid mosaic model for the structure of plasma membrane was proposed by:

a) Schleiden and Schwann

b) Robert Brown

c) Singer and Nicolson

d) Rudolf Virchow

 

Which of the following cell organelles is responsible for the synthesis of lipids and steroids?

a) Rough Endoplasmic Reticulum (RER)

b) Smooth Endoplasmic Reticulum (SER)

c) Golgi apparatus

d) Ribosomes

 

The "powerhouse" of the cell is:

a) Ribosome

b) Lysosome

c) Mitochondrion

d) Chloroplast

 

Which of the following is present in a prokaryotic cell?

a) Nucleus

b) Mitochondria

c) Ribosomes

d) Endoplasmic Reticulum

 

The main function of the nucleolus is:

a) DNA replication

b) RNA synthesis

c) Ribosome synthesis

d) Protein synthesis

 

A plant cell wall is primarily made up of:

a) Peptidoglycan

b) Chitin

c) Cellulose

d) Glycogen

 

Which organelle is involved in packaging materials to be delivered either to intracellular targets or secreted outside the cell?

a) Endoplasmic Reticulum

b) Mitochondria

c) Golgi apparatus

d) Lysosomes

 

Lysosomes are also known as "suicidal bags" because they contain:

a) Oxidative enzymes

b) Hydrolytic enzymes

c) Catalase

d) Peroxidase

 

The 9+2 arrangement of microtubules is characteristic of:

a) Centriole

b) Cilia and Flagella

c) Basal body

d) Microfilaments

 

Which of the following is a structural polysaccharide?

a) Starch

b) Glycogen

c) Cellulose

d) Glucose

 

The primary structure of a protein refers to its:

a) Alpha-helical or beta-pleated sheet conformation

b) Three-dimensional folding due to interactions between R-groups

c) Linear sequence of amino acids

d) Arrangement of multiple polypeptide subunits

 

Which type of bond links nucleotides together to form a nucleic acid strand?

a) Glycosidic bond

b) Peptide bond

c) Ester bond

d) Phosphodiester bond

 

The building blocks of proteins are:

a) Monosaccharides

b) Fatty acids

c) Amino acids

d) Nucleotides

 

Enzymes are primarily:

a) Carbohydrates

b) Lipids

c) Proteins

d) Nucleic acids

 

The site on an enzyme where the substrate binds is called the:

a) Allosteric site

b) Regulatory site

c) Active site

d) Inhibitory site

 

Which of the following describes competitive inhibition of an enzyme?

a) The inhibitor binds to a site other than the active site.

b) The inhibitor binds reversibly to the active site, competing with the substrate.

c) The inhibitor permanently alters the enzyme's active site.

d) The inhibitor increases the enzyme's activity.

 

The process of DNA replication occurs during which phase of the cell cycle?

a) G1 phase

b) S phase

c) G2 phase

d) M phase

 

Crossing over takes place during:

a) Prophase of Mitosis

b) Anaphase I of Meiosis

c) Prophase I of Meiosis

d) Metaphase II of Meiosis

 

The number of chromosomes in a human somatic cell is 46. After mitosis, the daughter cells will have:

a) 23 chromosomes

b) 46 chromosomes

c) 92 chromosomes

d) Variable number of chromosomes

 

MCQ Answer Key:

 

d) All cells have a cell wall.

 

c) Singer and Nicolson

 

b) Smooth Endoplasmic Reticulum (SER)

 

c) Mitochondrion

 

c) Ribosomes

 

c) Ribosome synthesis

 

c) Cellulose

 

c) Golgi apparatus

 

b) Hydrolytic enzymes

 

b) Cilia and Flagella

 

c) Cellulose

 

c) Linear sequence of amino acids

 

d) Phosphodiester bond

 

c) Amino acids

 

c) Proteins

 

c) Active site

 

b) The inhibitor binds reversibly to the active site, competing with the substrate.

 

b) S phase

 

c) Prophase I of Meiosis

 

b) 46 chromosomes

 

Short Questions (2-3 Marks)

 

Question: State the two main tenets of the Cell Theory.

Answer:

The two main tenets of the Cell Theory are:

 

All living organisms are composed of cells and products of cells.

 

All cells arise from pre-existing cells. (This part was added by Rudolf Virchow, "Omnis cellula e cellula").

 

Question: List two key differences between a prokaryotic and a eukaryotic cell.

Answer:

 

Nucleus: Prokaryotic cells lack a true nucleus (genetic material is in a nucleoid region), while eukaryotic cells possess a well-defined nucleus enclosed by a nuclear membrane.

 

Membrane-bound organelles: Prokaryotic cells lack membrane-bound organelles (e.g., mitochondria, ER, Golgi), whereas eukaryotic cells have various membrane-bound organelles. (Other differences: size, cell wall composition, ribosomes type, presence of histone proteins).

 

Question: What is the Endomembrane System? Name the organelles that constitute it.

Answer: The Endomembrane System is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins. The organelles that constitute it are the Endoplasmic Reticulum (ER), Golgi apparatus, Lysosomes, and Vacuoles.

 

Question: Describe the primary function of ribosomes. Where are they found in a cell?

Answer: Ribosomes are the sites of protein synthesis (translation) in both prokaryotic and eukaryotic cells. In eukaryotic cells, they can be found freely in the cytoplasm or attached to the outer surface of the Rough Endoplasmic Reticulum (RER). In prokaryotic cells, they are exclusively found in the cytoplasm.

 

Question: Explain the concept of fluid mosaic model of the plasma membrane.

Answer: The fluid mosaic model, proposed by Singer and Nicolson (1972), states that the plasma membrane is a quasi-fluid structure where proteins are embedded within or associated with a bilayer of phospholipids. The fluid nature allows for lateral movement of proteins and lipids, giving it a 'mosaic' appearance. This fluidity is crucial for various functions like cell growth, formation of intercellular junctions, secretion, endocytosis, and cell division.

 

Question: Differentiate between rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Answer:

 

Rough Endoplasmic Reticulum (RER): Has ribosomes attached to its surface, giving it a rough appearance. It is primarily involved in the synthesis and folding of proteins destined for secretion or insertion into membranes.

 

Smooth Endoplasmic Reticulum (SER): Lacks ribosomes on its surface, hence appears smooth. It is involved in lipid synthesis, steroid hormone synthesis, detoxification of drugs and poisons, and calcium ion storage.

 

Question: What are microbodies? Give an example and its function.

Answer: Microbodies are small, membrane-bound organelles that contain various enzymes. They are specialized for specific metabolic reactions. An example is Peroxisomes, which contain enzymes involved in the breakdown of fatty acids and amino acids, producing hydrogen peroxide as a byproduct. They also contain catalase, which breaks down harmful hydrogen peroxide into water and oxygen.

 

Question: List the major components of the cytoskeleton and their general roles.

Answer: The major components of the cytoskeleton are:

 

Microtubules: Hollow cylinders involved in cell shape, intracellular transport, and forming cilia, flagella, and spindle fibers.

 

Microfilaments (Actin filaments): Solid rods involved in cell shape, muscle contraction, amoeboid movement, and cytokinesis.

 

Intermediate Filaments: Rope-like structures providing mechanical strength and supporting cell shape; important in maintaining cell integrity.

 

Question: What is the significance of vacuoles in plant cells?

Answer: In plant cells, vacuoles are large, membrane-bound sacs. Their significance includes:

 

Storage: Storing water, sap, excretory products, and other materials not useful to the cell.

 

Turgidity: Maintaining turgor pressure against the cell wall, providing rigidity and support to the plant.

 

Waste disposal: Sequestration of waste products.

 

Hydrolysis: Containing hydrolytic enzymes, similar to lysosomes in animal cells.

 

Question: What are nucleic acids? Name the two main types.

Answer: Nucleic acids are complex macromolecules that carry genetic information and play a crucial role in gene expression. They are polymers of nucleotides. The two main types are Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA).

 

Question: Briefly describe the primary structure of a protein.

Answer: The primary structure of a protein refers to the unique linear sequence of amino acids linked together by peptide bonds. This sequence is determined by the genetic code in the DNA and dictates all higher levels of protein structure, ultimately determining the protein's function.

 

Question: What is the 'active site' of an enzyme? Why is it important?

Answer: The active site is a specific three-dimensional region on an enzyme molecule where the substrate binds. It is crucial because it is precisely shaped to fit the substrate (lock and key model) or undergoes slight conformational changes upon binding (induced fit model), facilitating the catalytic conversion of the substrate into products. The specificity and efficiency of an enzyme's action depend on its active site.

 

Question: Distinguish between saturated and unsaturated fatty acids.

Answer:

 

Saturated Fatty Acids: Contain only single bonds between carbon atoms in their hydrocarbon chain. They are 'saturated' with hydrogen atoms. They are typically solid at room temperature (e.g., animal fats).

 

Unsaturated Fatty Acids: Contain one or more double bonds between carbon atoms in their hydrocarbon chain. This causes kinks in the chain. They are typically liquid at room temperature (e.g., plant oils).

 

Question: What is the 'Cell Cycle'? Name its two main phases.

Answer: The cell cycle is the series of events that take place in a cell leading to its division and duplication of its DNA (DNA replication) to produce two daughter cells. Its two main phases are:

 

Interphase: A period of growth and DNA replication (G1, S, G2 phases).

 

M Phase (Mitosis or Meiosis): The actual cell division phase.

 

Question: State one key significance of mitosis and one key significance of meiosis.

Answer:

 

Significance of Mitosis: Essential for growth, tissue repair, replacement of dead cells, and asexual reproduction. It ensures that daughter cells receive an identical set of chromosomes as the parent cell.

 

Significance of Meiosis: Essential for sexual reproduction, as it produces haploid gametes (sperm and egg) from diploid cells. It also introduces genetic variation through crossing over and independent assortment of chromosomes, which is crucial for evolution.

 

Long Questions (5-6 Marks)

 

Question: Describe the structure and function of the Mitochondrion. Why is it called the "powerhouse of the cell"?

Answer:

Structure of Mitochondrion:

Mitochondria are double-membrane bound organelles, typically oval or cylindrical in shape.

 

Outer Membrane: Smooth and permeable to small molecules, forming the outer boundary. It contains porins.

 

Inner Membrane: Highly convoluted and folded inwards to form shelf-like structures called cristae. The cristae significantly increase the surface area for enzymatic reactions. The inner membrane is less permeable and contains specific transport proteins.

 

Outer Compartment (Intermembrane Space): The space between the outer and inner membranes.

 

Inner Compartment (Matrix): The fluid-filled space enclosed by the inner membrane. It contains a circular DNA molecule, 70S ribosomes, various enzymes (e.g., for Krebs cycle), and inorganic ions.

 

Function of Mitochondrion:

Mitochondria are the primary sites of aerobic respiration. They are responsible for:

 

ATP Synthesis: The main function is the production of adenosine triphosphate (ATP), the energy currency of the cell. This occurs through the electron transport chain and oxidative phosphorylation, which take place on the inner mitochondrial membrane.

 

Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix, producing ATP, NADH, and FADH2.

 

Fatty Acid Oxidation: Breakdown of fatty acids to generate acetyl-CoA.

 

Why it is called the "Powerhouse of the Cell":

Mitochondria are termed the "powerhouse of the cell" because they are the main sites where cellular respiration occurs, leading to the efficient generation of large amounts of ATP. ATP provides the necessary energy for almost all metabolic activities and cellular functions, much like a power plant generates electricity for a city. Without mitochondria, cells would be unable to produce sufficient energy to sustain life.

 

Question: Compare and contrast the structure of a plant cell and an animal cell.

Answer:

Similarities (Shared Features):

Both plant and animal cells are eukaryotic cells and share several fundamental structures:

 

Cell Membrane (Plasma Membrane): Both have a selectively permeable cell membrane that regulates the passage of substances.

 

Cytoplasm: Both contain cytoplasm, the jelly-like substance filling the cell, where organelles are suspended.

 

Nucleus: Both possess a well-defined nucleus containing the genetic material (DNA).

 

Mitochondria: Both have mitochondria for cellular respiration and ATP production.

 

Endoplasmic Reticulum (RER & SER): Both have ER for protein and lipid synthesis.

 

Golgi Apparatus: Both have Golgi for packaging and modifying proteins and lipids.

 

Ribosomes: Both contain ribosomes for protein synthesis.

 

Lysosomes: Present in animal cells and sometimes small, transient forms in plant cells.

 

Peroxisomes: Present in both.

 

Differences (Contrasting Features):

 

Feature	Plant Cell	Animal Cell
Cell Wall	Present, rigid, made of cellulose.	Absent.
Chloroplasts	Present, contain chlorophyll, perform photosynthesis.	Absent.
Vacuoles	Typically one large, central vacuole (tonoplast-bound), occupying up to 90% of cell volume.	Usually many small, temporary vacuoles, or none.
Centrioles	Absent (except in some lower plant forms).	Present, involved in cell division.
Cell Shape	Usually fixed, rigid, and rectangular/square due to cell wall.	Irregular or rounded, more flexible.
Storage Material	Starch.	Glycogen.
Plasmodesmata	Present, channels for cell-to-cell communication.	Absent (instead, have gap junctions).

 

Question: Discuss the different levels of protein structure (primary, secondary, tertiary, and quaternary).

Answer:

Proteins are complex macromolecules whose biological function is intimately tied to their intricate three-dimensional structure. There are four hierarchical levels of protein structure:

 

Primary Structure:

 

This is the simplest level, defined as the linear sequence of amino acids joined together by peptide bonds.

 

Each protein has a unique primary sequence, determined by the genetic code (DNA).

 

This sequence is fundamental because it dictates all higher levels of protein structure and ultimately its specific function. A single change in an amino acid can drastically alter protein function (e.g., sickle cell anemia).

 

Secondary Structure:

 

Refers to the local folding patterns of the polypeptide chain, stabilized by hydrogen bonds between the carbonyl oxygen and the amino hydrogen of the polypeptide backbone.

 

The two most common secondary structures are:

 

Alpha-helix (α-helix): A coiled structure resembling a spring, formed by hydrogen bonds between amino acids four residues apart. (e.g., keratin, myosin).

 

Beta-pleated sheet (β-pleated sheet): A flat, zig-zagging structure formed by hydrogen bonds between adjacent polypeptide strands (can be parallel or anti-parallel). (e.g., silk fibroin).

 

Tertiary Structure:

 

This is the overall three-dimensional shape of a single polypeptide chain, resulting from further folding and coiling of the secondary structures.

 

It is stabilized by various types of interactions between the R-groups (side chains) of amino acids, including:

 

Hydrogen bonds

 

Ionic bonds

 

Hydrophobic interactions (nonpolar groups cluster internally)

 

Disulfide bridges (covalent bonds between cysteine residues)

 

The tertiary structure is crucial for the protein's biological activity, as it forms the active sites of enzymes and binding sites for other molecules.

 

Quaternary Structure:

 

This level is present only in proteins composed of two or more polypeptide subunits (each having its own primary, secondary, and tertiary structure).

 

It describes the arrangement and interaction of these multiple polypeptide chains (subunits) to form a functional complex.

 

The interactions are similar to those in tertiary structure (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).

 

Example: Hemoglobin, which consists of four polypeptide chains (two alpha and two beta subunits).

 

These hierarchical levels of structure ensure that proteins fold into precise, stable conformations necessary for their diverse biological roles.

 

Question: Describe the process of Mitosis, highlighting its main stages and significance.

Answer:

Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. It is divided into four main stages, following interphase.

 

Stages of Mitosis:

 

Prophase:

 

Chromatin Condensation: Chromatin fibers condense and coil to become shorter, thicker, and visible as distinct chromosomes. Each chromosome has duplicated during S phase and consists of two sister chromatids joined at the centromere.

 

Centriole Movement: In animal cells, the duplicated centrioles begin to move to opposite poles of the cell, radiating out microtubules to form the astral rays.

 

Spindle Formation: The mitotic spindle (microtubules) begins to form between the separating centrosomes/centrioles.

 

Nuclear Envelope & Nucleolus Disappearance: The nuclear envelope starts to disintegrate, and the nucleolus disappears.

 

Metaphase:

 

Chromosome Alignment: The condensed chromosomes (each with two sister chromatids) migrate and align themselves along the equatorial plate (metaphase plate) of the cell, equidistant from the two poles.

 

Spindle Fiber Attachment: Spindle fibers (kinetochore microtubules) from opposite poles attach to the kinetochores (protein structures) present at the centromere of each chromosome.

 

Anaphase:

 

Centromere Splitting: The centromere of each chromosome divides, separating the two sister chromatids.

 

Chromatid Movement: The separated sister chromatids (now considered individual chromosomes) are pulled towards opposite poles of the cell by the shortening of the kinetochore microtubules. Each pole receives an identical set of chromosomes.

 

Elongation: The cell also elongates due to the lengthening of non-kinetochore microtubules.

 

Telophase:

 

Chromosome Decondensation: The chromosomes arrive at their respective poles, decondense, and lose their individuality.

 

Nuclear Envelope Reformation: A new nuclear envelope reforms around each set of chromosomes at the poles.

 

Nucleolus Reappearance: The nucleolus reappears in each daughter nucleus.

 

Spindle Disappearance: The spindle fibers disappear.

 

Cytokinesis Begins: Cytokinesis (division of the cytoplasm) usually begins during late anaphase or telophase.

 

Significance of Mitosis:

 

Growth: Essential for the growth of multicellular organisms by increasing the number of cells.

 

Repair and Regeneration: Replaces damaged or dead cells, and repairs tissues (e.g., wound healing, regeneration of lost body parts).

 

Asexual Reproduction: The basis of asexual reproduction in many organisms (e.g., budding in yeast, vegetative propagation in plants).

 

Maintenance of Chromosome Number: Ensures that the daughter cells receive an identical and exact copy of the parent cell's genetic material, maintaining the chromosome number across generations of somatic cells.

 

Question: Explain the various types of enzymes based on the reactions they catalyze and describe the 'lock and key' model of enzyme action.

Answer:

Types of Enzymes (based on reactions they catalyze - Enzyme Commission (EC) Classification):

Enzymes are typically classified into six major classes based on the type of reactions they catalyze:

 

Oxidoreductases: Catalyze oxidation-reduction reactions (transfer of electrons or hydrogen atoms).

 

Example: Dehydrogenases, oxidases, reductases. (e.g., Alcohol dehydrogenase).

 

Transferases: Catalyze the transfer of a functional group (e.g., methyl, amino, phosphate) from one molecule to another.

 

Example: Transaminases, kinases. (e.g., Hexokinase transfers a phosphate group to glucose).

 

Hydrolases: Catalyze the hydrolysis (breaking of bonds by the addition of water) of various bonds (ester, ether, peptide, glycosidic, C-N, C-C, C-halide bonds).

 

Example: Digestive enzymes like proteases (pepsin, trypsin), lipases, amylases. (e.g., Sucrase hydrolyzes sucrose into glucose and fructose).

 

Lyases: Catalyze the removal of groups from substrates by mechanisms other than hydrolysis, leaving double bonds, or vice versa.

 

Example: Aldolases, fumarase. (e.g., Carbonic anhydrase breaks down carbonic acid).

 

Isomerases: Catalyze the interconversion of isomers (structural, geometrical, or positional isomers).

 

Example: Phosphohexose isomerase, mutases. (e.g., Glucose-6-phosphate isomerase converts glucose-6-phosphate to fructose-6-phosphate).

 

Ligases: Catalyze the joining together of two molecules, usually accompanied by the hydrolysis of ATP or a similar energy-rich compound.

 

Example: DNA ligase, synthetases. (e.g., Acetyl-CoA synthetase joins acetate and Coenzyme A).

 

Lock and Key Model of Enzyme Action:

The 'Lock and Key' model of enzyme action was proposed by Emil Fischer in 1894. This model describes the high specificity of enzyme-substrate binding.

 

Analogy: The enzyme is compared to a 'lock', and the substrate is compared to a 'key'.

 

Specific Shape: Just as a specific key can only open a particular lock, a particular enzyme has an active site with a unique, rigid, three-dimensional shape.

 

Perfect Fit: Only a substrate molecule with a complementary shape (fitting perfectly into the active site) can bind to the enzyme.

 

Enzyme-Substrate Complex: When the substrate fits into the active site, an unstable enzyme-substrate (ES) complex is formed.

 

Catalysis: Within the active site, the enzyme facilitates the chemical reaction, converting the substrate(s) into product(s).

 

Product Release: Once the reaction is complete, the product(s) detach from the active site, leaving the enzyme free to bind with another substrate molecule and repeat the catalytic cycle.

 

This model effectively explains enzyme specificity but has been refined by the 'Induced Fit' model, which suggests a slight conformational change in the active site upon substrate binding for an even tighter fit.