Exercises
Which of the following is not correct?
(a) Robert Brown discovered the cell.
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells are formed from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.
Answer
(a) Robert Brown discovered the cell — this is not correct. The cell was discovered by Robert Hooke in 1665. Robert Brown discovered the nucleus of the cell in 1831. Schleiden and Schwann correctly formulated the cell theory. Virchow correctly proposed "Omnis cellula-e cellula" (cells arise from pre-existing cells). Unicellular organisms do carry out all life activities within a single cell.
Q2. New cells generate from
(a) bacterial fermentation
(b) regeneration of old cells
(c) pre-existing cells
(d) abiotic materials
Ans. (c) pre-existing cells. This is the principle of cell theory as proposed by Rudolf Virchow: "Omnis cellula-e cellula," meaning every cell arises from a pre-existing cell through the process of cell division.
Q3. Match the following
Column I — Column II
(a) Cristae — (i) Flat membranous sacs in stroma
(b) Cisternae — (ii) Infoldings in mitochondria
(c) Thylakoids — (iii) Disc-shaped sacs in Golgi apparatus
Ans. (a) Cristae → (ii) Infoldings in mitochondria
(b) Cisternae → (iii) Disc-shaped sacs in Golgi apparatus
(c) Thylakoids → (i) Flat membranous sacs in stroma
Q4. Which of the following is correct
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well defined cell wall.
(c) In prokaryotes, there are no membrane bound organelles.
(d) Cells are formed de novo from abiotic materials.
Ans. (c) In prokaryotes, there are no membrane bound organelles — this is correct. Prokaryotic cells lack membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi bodies, etc. Option (a) is wrong because prokaryotes lack a well-defined nucleus. Option (b) is wrong because animal cells lack a cell wall. Option (d) is wrong because cells arise only from pre-existing cells.
What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
Answer
A mesosome is a characteristic membrane structure formed by extensions of the plasma membrane into the cell in prokaryotes. These extensions may appear as vesicles, tubules, or lamellae.
Functions of mesosome
1. It helps in cell wall formation during cell division.
2. It plays a role in DNA replication and distribution to daughter cells during cell division.
3. It increases the surface area of the plasma membrane, thereby increasing the efficiency of respiration (it carries respiratory enzymes — analogous to mitochondria in eukaryotes).
4. It is also involved in secretion processes.
How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
Answer
Neutral solutes move across the plasma membrane by the process of simple diffusion — they move along their concentration gradient (from higher to lower concentration) without requiring energy. The rate of diffusion depends on the size of the molecule and the concentration gradient.
Polar molecules cannot move across the lipid bilayer by simple diffusion because the hydrophobic core of the membrane creates a barrier for charged or polar substances.
Polar molecules are transported by two mechanisms:
1. Facilitated diffusion: Polar molecules move down their concentration gradient with the help of specific transport proteins (carrier proteins or channel proteins) embedded in the membrane. This does not require energy (ATP).
2. Active transport: When molecules need to move against their concentration gradient, energy (ATP) is required. Specific carrier proteins (pumps) use ATP to move substances across the membrane (e.g., the Na⁺-K⁺ pump).
Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
Answer
The two double membrane-bound organelles are:
1. Mitochondria
2. Chloroplasts
Characteristics and Functions of Mitochondria
• Mitochondria are sausage-shaped or cylindrical organelles, 0.2–1.0 µm in diameter and 1–4.1 µm in length.
• They have a double membrane: outer membrane (smooth) and inner membrane (folded into cristae).
• The inner compartment is called the matrix, which contains circular DNA, ribosomes (70S), and enzymes of the Krebs cycle.
• The cristae increase the surface area for enzyme activity related to respiration.
• Function: Site of aerobic cellular respiration; produce ATP (energy currency of the cell). Called the "powerhouse of the cell."
Characteristics and Functions of Chloroplasts
• Found in green plant cells and algae.
• Disc-shaped or lens-shaped organelles, 4–6 µm in length and 1–3 µm in thickness.
• Outer and inner membranes enclose the stroma (matrix).
• Within stroma are membranous stacks called grana (singular: granum), composed of thylakoids. Grana are connected by stroma lamellae.
• Thylakoid membranes contain chlorophyll and other pigments.
• Chloroplasts also contain circular DNA and 70S ribosomes.
• Function: Site of photosynthesis — light reactions occur in thylakoids; dark reactions (Calvin cycle) occur in stroma. Called the "kitchen of the cell."
Diagram description — Mitochondria (longitudinal section)
The diagram shows an oval structure with outer membrane (smooth), inner membrane (folded into finger-like projections = cristae), matrix (inner space containing ribosomes, circular DNA, and granules), and inter-membrane space between outer and inner membrane.
Diagram description — Chloroplast (cross-section)
The diagram shows outer membrane, inner membrane, stroma (matrix), grana (stacks of thylakoids), thylakoid membrane containing photosynthetic pigments, stroma lamellae (intergranal lamellae) connecting grana, and stroma containing DNA, ribosomes, and starch granules.
What are the characteristics of prokaryotic cells?
Answer
Characteristics of prokaryotic cells:
1. Small size: Generally 1–10 µm in size.
2. No true nucleus: Genetic material (circular DNA) is present in the nucleoid region — not enclosed by a nuclear membrane.
3. No membrane-bound organelles: Lack mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.
4. Cell wall: Present in most prokaryotes; made of peptidoglycan (murein) in bacteria.
5. Ribosomes: 70S type (50S + 30S subunits), smaller than eukaryotic ribosomes.
6. Plasma membrane: Present; may form mesosomes (infoldings that aid in respiration and cell division).
7. Capsule: Many bacteria have a glycoprotein or polysaccharide capsule outside the cell wall for protection.
8. Flagella: Simpler structure than eukaryotic flagella, composed of flagellin protein.
9. Plasmids: Small, extra-chromosomal circular DNA may be present.
10. No histone proteins: DNA is not associated with histones (though some basic proteins are present).
Multicellular organisms have division of labour. Explain.
Answer
In multicellular organisms, billions of cells are present. It is not possible for every cell to perform all the functions of the body. Therefore, different types of cells become specialized to perform specific functions. This specialization of cells and differentiation of tasks among different cells/tissues/organs is called division of labour.
For example:
• Muscle cells are specialized for contraction and movement.
• Nerve cells (neurons) are specialized for transmission of electrical signals.
• Gland cells secrete enzymes or hormones.
• RBCs (erythrocytes) are specialized for oxygen transport (contain hemoglobin).
• Epithelial cells provide protection and lining.
This division of labour leads to greater efficiency, and each cell type is perfectly suited to its specific role. Cells are organized into tissues, tissues into organs, and organs into organ systems, with each level having a distinct function. This is one of the key advantages of multicellularity.
Cell is the basic unit of life. Discuss in brief.
Answer
The cell is considered the basic structural and functional unit of life because:
1. All living organisms are made up of cells — either a single cell (unicellular) or many cells (multicellular).
2. All metabolic activities — respiration, digestion, growth, reproduction — occur within cells.
3. Cells contain the genetic material (DNA) necessary for heredity and control of cellular activities.
4. New cells arise only from pre-existing cells (cell theory).
5. Even in multicellular organisms, each cell independently carries out essential metabolic processes.
The cell theory (Schleiden, Schwann, and Virchow) states:
• All living organisms are composed of cells.
• The cell is the basic unit of life.
• All cells arise from pre-existing cells.
A cell has all the properties of life: metabolism, growth, reproduction, and response to stimuli. Therefore, cell is rightly called the basic unit of life.
What are nuclear pores? State their function.
Answer
Nuclear pores are small, circular openings or perforations present on the nuclear envelope (the double membrane surrounding the nucleus). They are formed by the fusion of the inner and outer nuclear membranes at specific points. Each nuclear pore is about 100 nm in diameter and is associated with a complex protein structure called the nuclear pore complex.
Functions of nuclear pores
1. They allow selective transport of materials between the nucleus and the cytoplasm.
2. mRNA molecules synthesized in the nucleus are transported to the cytoplasm through nuclear pores for translation.
3. Ribosomal subunits synthesized in the nucleolus are exported to the cytoplasm through nuclear pores.
4. Proteins required in the nucleus (e.g., histones, transcription factors) are imported from the cytoplasm through nuclear pores.
5. ATP and other molecules also pass through nuclear pores.
Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
Answer
Both lysosomes and vacuoles are membrane-bound organelles and part of the endomembrane system, but they differ significantly in function:
Lysosomes
• Lysosomes are small vesicles containing powerful hydrolytic enzymes (acid hydrolases) that function optimally at acidic pH.
• They are involved in intracellular digestion — they digest worn-out organelles (autophagy), bacteria/foreign particles (phagocytosis), and cellular debris.
• They help in programmed cell death (autolysis) — they are called "suicide bags" of the cell.
• They recycle cellular components and are important in defense against pathogens.
Vacuoles
• Vacuoles are membrane-bound sacs that store various substances.
• In plant cells, the central vacuole (tonoplast-bound) is large and maintains cell turgidity (osmoregulation), stores water, pigments (anthocyanins), metabolic wastes, and nutrients.
• In animal cells, vacuoles are small and temporary; food vacuoles digest food, contractile vacuoles regulate water balance in protists.
• Vacuoles provide structural support in plants.
Thus, lysosomes function mainly in digestion and recycling, while vacuoles mainly in storage, osmoregulation, and structural support.
Describe the structure of the following with the help of labelled diagrams.
(i) Nucleus
(ii) Centrosome
Answer
(i) Nucleus:
The nucleus is the most prominent organelle in eukaryotic cells, usually spherical, 5–25 µm in diameter.
Structure
• Nuclear envelope: Double membrane (outer and inner) with nuclear pores. The outer membrane is continuous with rough endoplasmic reticulum and bears ribosomes.
• Nuclear pores: Circular perforations in the nuclear envelope that allow transport of materials.
• Nucleoplasm (nuclear matrix): Semifluid material filling the nucleus, contains nucleotides, enzymes, and ions.
• Chromatin: Network of DNA and protein (histones) threads. During cell division, chromatin condenses into chromosomes.
• Nucleolus: One or more spherical bodies within the nucleus; site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
Diagram description — Nucleus
Outer nuclear membrane → Nuclear pore → Inner nuclear membrane → Nucleoplasm → Chromatin network → Nucleolus (dense, darkly stained body) → Rough ER continuous with outer nuclear membrane.
(ii) Centrosome:
The centrosome is an organelle found in animal cells and some lower plants; it acts as the main microtubule-organizing center (MTOC).
Structure
• Centrosome consists of two cylindrical structures called centrioles, arranged perpendicular to each other.
• Each centriole has a cartwheel arrangement of nine triplets of microtubules (9 + 0 arrangement — no central microtubules), each triplet made of three microtubules (A, B, C).
• Centrioles are surrounded by a cloud of amorphous, electron-dense material called pericentriolar material (PCM) or centrosphere, from which spindle microtubules nucleate.
• During cell division, centrosomes form the spindle apparatus (aster).
• Centrioles also give rise to basal bodies of cilia and flagella.
Diagram description — Centrosome
Two barrel-shaped centrioles at right angles to each other, each showing nine sets of triplet microtubules in cross-section, surrounded by pericentriolar material, with astral rays (microtubules) radiating outward.
What is a centromere? How does the position of centromere form the basis of classification of chromosomes? Support your answer with a diagram showing the position of centromere on different types of chromosomes.
Answer
Centromere is a specialized region on a chromosome that holds the two chromatids together. It is the attachment site for spindle fibers (kinetochore) during cell division and plays a crucial role in the movement of chromosomes toward the poles during mitosis and meiosis.
Classification of chromosomes based on centromere position
1. Metacentric: Centromere is at the middle/center of the chromosome, forming two equal arms. Chromosome is V-shaped.
2. Sub-metacentric: Centromere is slightly off-center, making one arm slightly shorter than the other. Chromosome is L-shaped or J-shaped.
3. Acrocentric: Centromere is close to one end, making one arm very long and the other very short. Chromosome is J-shaped.
4. Telocentric: Centromere is at the terminal end (tip) of the chromosome, so only one arm is visible. Chromosome is rod-shaped (I-shaped).
Diagram description
The diagram shows four chromosomes:
— Metacentric: centromere in the middle, two equal arms (V-shape)
— Sub-metacentric: centromere slightly off-center, slightly unequal arms (L-shape)
— Acrocentric: centromere near the end, very short and long arms (J-shape)
— Telocentric: centromere at the very tip, one arm only (rod/I-shape)
Each chromosome is shown as having two chromatids joined at the centromere, with the short arm labeled as "p" and the long arm as "q."
Important Extra Questions
Differentiate between prokaryotic and eukaryotic cells.
Answer
Feature | Prokaryotic Cell | Eukaryotic Cell |
Nuclear membrane | Absent | Present |
Size | 1–10 µm (small) | 10–100 µm (large) |
Membrane-bound organelles | Absent | Present |
Ribosomes | 70S | 80S (cytoplasmic) |
Cell division | Binary fission | Mitosis/Meiosis |
Examples | Bacteria, Archaea | Plants, Animals, Fungi |
What is the endomembrane system? Name its components.
Answer
The endomembrane system refers to a group of interconnected membrane-bound organelles in eukaryotic cells that work together to synthesize, modify, package, and transport proteins and lipids. The key feature is that membranes and vesicles from one organelle can bud off and fuse with another.
Components
1. Endoplasmic reticulum (ER) — smooth and rough
2. Golgi apparatus (Golgi complex)
3. Lysosomes
4. Vacuoles
5. Plasma membrane (functionally connected)
Note: Mitochondria, chloroplasts, and peroxisomes are not part of the endomembrane system because they have independent origins and do not exchange vesicles with the others.
Describe the structure and functions of the Golgi apparatus.
Answer
The Golgi apparatus (discovered by Camillo Golgi) consists of stacks of flattened, membrane-bound cisternae (sacs). The stack has a cis face (receiving face, near ER) and a trans face (secreting face, toward plasma membrane).
Functions
1. Packaging and processing: Proteins and lipids synthesized in the ER are received, modified (glycosylation, phosphorylation), and packaged in the Golgi.
2. Sorting and dispatching: Golgi sorts molecules and sends them to their correct destinations (secretory vesicles → plasma membrane, lysosomes, or outside the cell).
3. Synthesis of cell wall polysaccharides in plants.
4. Formation of acrosome in sperm cells.
What is the significance of the 9+2 arrangement in cilia and flagella?
Answer
Cilia and flagella of eukaryotes have a characteristic internal structure called the axoneme, which shows a 9+2 arrangement of microtubules — nine doublets of microtubules arranged in a circle, plus two central singlet microtubules.
Significance
• The nine peripheral doublets are connected by dynein arms (ATPase) which generate sliding force between adjacent doublets, converting chemical energy (ATP) into mechanical movement.
• The central pair plus radial spokes coordinate the bending motion.
• This arrangement allows cilia to beat in a whip-like motion (for locomotion in unicellular organisms, or movement of fluids over epithelial surfaces) and flagella to propel cells (e.g., sperm motility).
What is the difference between smooth ER and rough ER? State the functions of each.
Answer
Rough ER (RER):
• Has ribosomes attached to the outer surface.
• Appears rough/granular under electron microscope.
• Functions: synthesis of proteins destined for secretion, membrane proteins, and lysosomal proteins; modifies proteins by glycosylation.
Smooth ER (SER)
• Lacks ribosomes on its surface.
• Appears smooth under electron microscope.
• Functions: synthesis of lipids (steroids, phospholipids), metabolism of carbohydrates, detoxification of drugs and poisons (in liver cells), storage of calcium ions (in muscle cells — as sarcoplasmic reticulum).