Solved Exercise With Answers And Reasons

SECTION 1: MULTIPLE CHOICE QUESTIONS

1. Which one of the following eukaryotic cell structures does not contain DNA? (a) Nucleus (b) Mitochondrion (c) Endoplasmic reticulum (d) Chloroplast

Answer: (c) Endoplasmic reticulum

Reason: The nucleus houses the majority of the cell's DNA (nuclear genome). Mitochondria and chloroplasts are semi-autonomous organelles that contain their own circular DNA (mtDNA and cpDNA, respectively), supporting the endosymbiotic theory. The endoplasmic reticulum (both rough and smooth) is a membrane network involved in protein and lipid synthesis but does not contain DNA. This distinction highlights the specialized roles of organelles: genetic storage vs. biosynthetic functions. Edge case: Some viruses may interact with ER, but native eukaryotic ER lacks DNA.

2. Which of the following is not an accurate description of a chromosome? (a) It is a coloured body localized in the nucleus (b) It is a protein and nucleic acid complex (c) It is the cellular structure that contains the genetic material (d) In eukaryotes, it is composed of many DNA molecules attached end to end

Answer: (d) In eukaryotes, it is composed of many DNA molecules attached end to end

Reason: Chromosomes are indeed visible as coloured bodies (hence "chromo" = colour) during cell division, consist of DNA wrapped around histone proteins (nucleoprotein complex), and serve as the vehicle for genetic material. However, in eukaryotes, each chromosome typically contains one very long linear DNA molecule complexed with proteins, not multiple DNA molecules joined end-to-end. Prokaryotes have a single circular chromosome. This nuance is important for understanding replication, condensation, and genetic stability—multiple independent molecules would complicate segregation during mitosis/meiosis.

3. A centriole is an organelle that is: (a) Present in the centre of a cell's cytoplasm (b) Composed of microtubules and important for organizing the spindle fibres (c) Surrounded by a membrane (d) Part of a chromosome

Answer: (b) Composed of microtubules and important for organizing the spindle fibres

Reason: Centrioles are cylindrical structures made of nine triplets of microtubules arranged in a 9+0 pattern. They form the core of the centrosome, which organizes the mitotic spindle during cell division (critical for chromosome separation). They are not membrane-bound, are located near the nucleus (not strictly "centre"), and are unrelated to chromosomes. In animal cells, they also form basal bodies for cilia/flagella. Plants generally lack centrioles but use other microtubule organizing centers. This reflects evolutionary differences in cytoskeletal organization.

4. The rough endoplasmic reticulum is: (a) An intracellular double-membrane system to which ribosomes are attached (b) An intracellular membrane that is studded with microtubular structures (c) A membranous structure found within mitochondria (d) Only found in prokaryotic cells

Answer: (a) An intracellular double-membrane system to which ribosomes are attached

Reason: The rough ER (RER) consists of interconnected flattened sacs (cisternae) with ribosomes on the cytosolic side, giving it a "rough" appearance. It is the primary site for synthesis of secretory and membrane proteins. Option (b) confuses it with microtubules; (c) describes cristae; (d) is incorrect as prokaryotes lack membrane-bound organelles. The RER connects to the nuclear envelope, enabling efficient co-translational translocation. In high-secretory cells (e.g., pancreatic cells), RER is abundant.

5. In the nucleus of eukaryotic cells, the genetic material is complexed with protein and organized into linear structures called: (a) Centrioles (b) Histones (c) Chromosomes (d) Plasmids

Answer: (c) Chromosomes

Reason: DNA wraps around histone octamers to form nucleosomes, which further condense into chromatin and, during division, into visible chromosomes. Histones are the proteins, not the structures. Centrioles are cytoskeletal; plasmids are small circular DNA in bacteria/yeast. Linear chromosomes ensure proper segregation via centromeres and telomeres. This organization allows regulation of gene expression through chromatin remodeling.

6. Which of the following statements does not apply to the nuclear envelope? (a) It is a double membrane (b) It is continuous with the endoplasmic reticulum (c) It has pores through which material enters and leaves (d) It has infoldings to form cristae

Answer: (d) It has infoldings to form cristae

Reason: The nuclear envelope is a double membrane with nuclear pores (nuclear pore complexes) for transport of mRNA, proteins, etc., and is continuous with the RER. Cristae are infoldings of the inner mitochondrial membrane that increase surface area for ATP synthesis. This distinction prevents confusion between nuclear transport and mitochondrial respiration.

7. Lysosomes are formed by budding from which cellular organelle? (a) Smooth endoplasmic reticulum (b) Golgi apparatus (c) Rough endoplasmic reticulum (d) Nucleus

Answer: (b) Golgi apparatus

Reason: Lysosomes bud from the trans-Golgi network as primary lysosomes containing hydrolytic enzymes (acid hydrolases). They fuse with endosomes/phagosomes for digestion. This pathway ensures proper enzyme targeting via mannose-6-phosphate tags. RER synthesizes the enzymes, but maturation occurs in the Golgi. Defects (e.g., I-cell disease) highlight this trafficking importance.

8. All peroxisomes carry out this function: (a) Break down fats and amino acids into smaller molecules that can be used for energy production by mitochondria (b) Digest macromolecules using the hydrolytic enzymes they contain (c) Synthesize membrane components such as fatty acids and phospholipids (d) Control the flow of ions into and out of the cell

Answer: (a) Break down fats and amino acids into smaller molecules that can be used for energy production by mitochondria

Reason: Peroxisomes perform β-oxidation of very long-chain fatty acids and amino acid metabolism, producing H₂O₂ (detoxified by catalase). Lysosomes handle hydrolytic digestion (b); SER does lipid synthesis (c); ion control is membrane proteins (d). Peroxisomes are single-membrane organelles crucial in liver/kidney cells. In plants, glyoxysomes convert fats to sugars.

9. How would the absence of peroxisomes in a cell affect its metabolism, and what would be the likely symptoms? (a) The cell would be unable to carry out oxidative phosphorylation, leading to reduced ATP production. (b) The cell would accumulate hydrogen peroxide, leading to oxidative stress and potential cellular damage. (c) The cell would have impaired protein synthesis, leading to muscle weakness. (d) The cell would fail to produce lipids, causing membrane instability

Answer: (b) The cell would accumulate hydrogen peroxide, leading to oxidative stress and potential cellular damage.

Reason: Peroxisomes neutralize reactive oxygen species (ROS) like H₂O₂ via catalase. Absence leads to oxidative damage to DNA, proteins, and lipids. This relates to disorders like Zellweger syndrome (peroxisome biogenesis disorders) causing neurological issues and early death. Oxidative phosphorylation occurs in mitochondria (a); protein synthesis in ribosomes/ER (c); lipid synthesis in SER (d). Implications include inflammation and accelerated aging.

10. Which of the following does not apply to chloroplasts? (a) They contain chlorophyll and the enzymes required for photosynthesis. (b) They contain an internal membrane system consisting of thylakoids. (c) They synthesize ATP. (d) They are bounded by two membranes, the inner of which is folded into the cristae.

Answer: (d) They are bounded by two membranes, the inner of which is folded into the cristae.

Reason: Chloroplasts have a double membrane envelope, chlorophyll in thylakoids (stacked into grana), and produce ATP via photophosphorylation. Cristae are specific to mitochondria. This differentiates photosynthetic (chloroplast) from respiratory (mitochondrial) organelles. Chloroplasts also have their own DNA and ribosomes.

11. What is the correct sequence of membrane compartments through which a secretory protein moves from synthesis to release from the cell? (a) SER → Golgi apparatus → RER → Cell membrane (b) Cell membrane → Golgi apparatus → RER → SER (c) RER → Golgi → Cell membrane → SER (d) RER → SER → Golgi apparatus → Cell membrane

Answer: (d) RER → SER → Golgi apparatus → Cell membrane (Note: While transport vesicles move directly from RER to cis-Golgi, the option reflects the interconnected ER system where proteins may transit through regions of smooth ER before Golgi processing.)

Reason: Secretory proteins are synthesized on RER ribosomes, folded in the ER lumen, then packaged into vesicles that go to the Golgi for modification (glycosylation, sorting), and finally to secretory vesicles that fuse with the plasma membrane (exocytosis). The ER is a continuous system (RER transitions to SER in some areas). This endomembrane system ensures quality control and targeted delivery. Disruptions cause diseases likecystic fibrosis (trafficking defects).

12. How does the process of facilitated diffusion differ from active transport? (a) Facilitated diffusion requires energy, active transport does not (b) Facilitated diffusion does not require energy, active transport does (c) Both processes require energy (d) Both processes do not require energy

Answer: (b) Facilitated diffusion does not require energy, active transport does

Reason: Facilitated diffusion uses carrier/channel proteins to move substances down their concentration gradient (passive, no ATP). Active transport (e.g., Na⁺/K⁺ pump) moves substances against the gradient using ATP energy. Both are mediated by proteins (unlike simple diffusion). This is fundamental to maintaining electrochemical gradients, cell volume, and nerve signaling. Examples: Glucose via GLUT transporters (facilitated) vs. sodium-glucose symporter (secondary active).

SECTION 2: SHORT QUESTIONS

1. Compare the resolution and magnification of light microscope and electron microscope?

Light Microscope (LM):

Magnification: Typically up to 1,500× (practical limit around 1,000–2,000× with oil immersion).
Resolution: About 0.2 μm (200 nm) due to the wavelength of visible light (400–700 nm). Limited by diffraction.

Electron Microscope (EM):

Magnification: Up to 100,000× or more (TEM and SEM).
Resolution: About 0.1–0.5 nm (much higher because electrons have much shorter wavelengths than visible light).

Key Comparison and Implications: Light microscopes are suitable for viewing live cells and larger structures (e.g., whole cells, nuclei, some organelles) in colour with simpler preparation. Electron microscopes provide ultra-structural details (e.g., cristae, ribosomes, viral particles) but require dead, vacuum-prepared, heavy-metal-stained specimens. Nuances: Resolution is the ability to distinguish two close points; EM revolutionized cytology by confirming organelle details. Edge case: Confocal laser scanning microscopes improve LM resolution somewhat, but EM remains superior for nanoscale. In educational contexts like FSc, LM is used for basic observation while EM supports advanced research.

2. State the cell theory. How we can validate it? What are the exceptions to cell theory?

Cell Theory (Classical): All living organisms are composed of one or more cells; the cell is the basic unit of structure and function; all cells arise from pre-existing cells (via cell division).

Modern Extensions: Cells contain hereditary information (DNA) passed during division; metabolic reactions occur within cells.

Validation Methods:

Microscopic observation (Hooke, Leeuwenhoek, Schleiden, Schwann, Virchow).
Experimental evidence: Cell fractionation, tissue culture, and observing mitosis/meiosis.
Modern tools: Electron microscopy, DNA staining, and single-cell sequencing confirm uniformity and continuity.

Exceptions/Exceptions to Strict Cell Theory:

Viruses and prions: Acellular infectious agents; they replicate only inside host cells (not truly "living" by some definitions).
Striated muscle fibres and giant algae (e.g., Acetabularia): Multinucleate or coenocytic (multiple nuclei in shared cytoplasm).
Aseptate fungal hyphae: Coenocytic structure.
Mature mammalian red blood cells (erythrocytes): Lack nucleus and organelles but derive from nucleated precursors. These highlight that while cells are fundamental, some specialized structures deviate for functional advantages (e.g., increased efficiency in transport or size).

3. The table below compares the process of diffusion, facilitated diffusion and active transport. Fill in the blank cells, using the words “YES” or “NO”.

Process Description	Simple Diffusion	Facilitated Diffusion	Active Transport
Is ATP required?	NO	NO	YES
Are carrier proteins involved?	NO	YES	YES
Is direction of transport always from higher to lower concentration?	YES	YES	NO

Explanation and Nuances:

Simple Diffusion: Passive movement of small nonpolar molecules (O₂, CO₂, steroids) directly through the lipid bilayer down the concentration gradient.
Facilitated Diffusion: Passive but uses channel or carrier proteins for polar/charged molecules (e.g., glucose via GLUT).
Active Transport: Energy-dependent (primary uses ATP; secondary uses ion gradients) to move against gradient (e.g., Na⁺/K⁺ pump). Implications: These mechanisms maintain homeostasis; active transport is crucial for nerve impulses and nutrient uptake in low-concentration environments. Edge case: Some transporters can work in both modes depending on conditions.

4. Categorize the organelles as (i) single membrane bounded, (ii) double membrane bounded and (iii) lacking any membrane.

(i) Single membrane bounded:

Endoplasmic Reticulum (RER & SER)
Golgi apparatus
Lysosomes
Peroxisomes / Glyoxysomes
Vacuoles (especially large central vacuole in plants)
Plasma membrane (though technically the cell boundary)

(ii) Double membrane bounded:

Nucleus (nuclear envelope)
Mitochondria
Chloroplasts (in plant cells)

(iii) Lacking any membrane:

Ribosomes
Centrioles (in animal cells)
Cytoskeletal elements (microtubules, microfilaments, intermediate filaments)
Nucleolus (within nucleus)
Proteasomes (in some classifications)

Context: This categorization reflects the endomembrane system (single/double) vs. non-membrane structures. Double membranes support endosymbiotic origins for mitochondria/chloroplasts.

5. State two functions of the proteins in the plasma membrane.

Transport: Channel proteins (facilitated diffusion), carrier proteins (active/passive transport), and pumps maintain ion gradients and nutrient uptake.
Cell recognition and signaling: Glycoproteins act as receptors for hormones/signals, antigens for immune recognition, and adhesion molecules for cell-cell junctions.

Additional Nuances: Proteins also provide structural support (e.g., spectrin) and enzymatic activity. Fluid mosaic model emphasizes their dynamic lateral movement. Implications: Defects cause diseases like cystic fibrosis (CFTR channel).

6. State two features that mitochondria have in common with prokaryotes.

Own circular DNA and ribosomes: Mitochondrial DNA (mtDNA) is circular like bacterial chromosomes; they have 70S ribosomes (prokaryotic type).
Binary fission: Mitochondria divide independently by fission, similar to bacteria, and possess a double membrane consistent with endosymbiosis.

Implications: Strong evidence for endosymbiotic theory—ancient aerobic bacteria engulfed by ancestral eukaryotic cells.

7. List three ways in which prokaryotic cells differ from eukaryotic cells.

Nucleus: Prokaryotes lack a true membrane-bound nucleus (nucleoid region); eukaryotes have a defined nucleus.
Organelles: Prokaryotes lack membrane-bound organelles (e.g., no mitochondria, ER, Golgi); eukaryotes have them.
Size and complexity: Prokaryotes are generally smaller (1–10 μm) with simpler organization; eukaryotes are larger (10–100 μm) with compartmentalization.

Other Differences: Prokaryotic DNA is circular and lacks histones; cell wall contains peptidoglycan (bacteria); reproduction mainly binary fission. These enable rapid adaptation in prokaryotes vs. specialization in eukaryotes.

8. List the structures and molecules, which can cross the nuclear envelope.

Small molecules/ions: Via passive diffusion through nuclear pores.
mRNA, tRNA, ribosomal subunits: Exported to cytoplasm for protein synthesis.
Proteins (e.g., histones, transcription factors, DNA polymerases): Imported via nuclear localization signals through nuclear pore complexes (NPCs).
Ribosomal proteins and other macromolecules: Bidirectional transport.

Mechanism: Nuclear pore complexes (large protein assemblies) allow selective, energy-dependent transport for larger molecules (>40–60 kDa). This regulates gene expression tightly.

9. Distinguish each of the following pairs.

a. Exocytosis and endocytosis

Exocytosis: Fusion of secretory vesicles with plasma membrane to release contents outside the cell (e.g., hormones, neurotransmitters). Expels material.
Endocytosis: Intake of materials by invagination of plasma membrane forming vesicles (e.g., nutrients, signals). Brings material in. Both are vesicle-mediated and energy-requiring but opposite in direction.

b. Phagocytosis and pinocytosis

Phagocytosis ("cell eating"): Engulfment of large solid particles (bacteria, debris) by pseudopodia; common in immune cells (macrophages). Forms phagosome.
Pinocytosis ("cell drinking"): Uptake of fluids and dissolved solutes via small vesicles; non-specific and continuous in most cells. Both are forms of endocytosis but differ in scale and specificity.

c. Peroxisome and glyoxysomes

Peroxisomes: Single-membrane organelles in most eukaryotic cells; contain catalase for H₂O₂ breakdown and β-oxidation of fatty acids.
Glyoxysomes: Specialized peroxisomes in plant seeds/germinating seedlings; contain enzymes for glyoxylate cycle to convert fats into sugars (gluconeogenesis). Both are oxidative but glyoxysomes are plant-specific for lipid mobilization.

10. What are the main functions of lysosomes?

Intracellular digestion: Break down macromolecules (proteins, lipids, carbohydrates, nucleic acids) using hydrolytic enzymes in acidic pH.
Autophagy: Digestion of worn-out organelles and cellular debris.
Phagocytosis support: Fuse with phagosomes in immune cells to destroy pathogens.
Recycling: Release breakdown products for reuse (e.g., amino acids).

Implications: Lysosomal storage diseases (e.g., Tay-Sachs) occur due to enzyme deficiencies, leading to substrate accumulation. They act as the cell's "suicide bags" if ruptured.

11. Describe the role of the Golgi body in forming lysosomes.

The Golgi apparatus modifies, sorts, and packages proteins from the ER. Lysosomal enzymes are tagged with mannose-6-phosphate in the cis-Golgi, sorted in the trans-Golgi network, and bud off as primary lysosomes (transport vesicles). These mature into functional lysosomes upon fusion with endosomes. This ensures accurate delivery and prevents premature enzyme activity. Disruption affects cellular waste management.

12. What are histones? Where are these found in eukaryotic cells?

Histones are small, basic (positively charged) proteins rich in lysine and arginine. They act as spools around which DNA wraps to form nucleosomes—the fundamental unit of chromatin.

Location: Found in the nucleus of eukaryotic cells, complexed with DNA to form chromosomes/chromatin. Five main types: H1 (linker), and core histones (H2A, H2B, H3, H4).

Role: DNA packaging (compaction), gene regulation via modifications (epigenetics—acetylation, methylation), and protection from damage. Absent in prokaryotes.

13. What do you mean by “stem cell”? What are the main usages of stem cells?

Stem cells are undifferentiated cells capable of self-renewal (dividing to produce more stem cells) and differentiation into specialized cell types.

Types: Totipotent (zygote), pluripotent (embryonic), multipotent (adult/somatic, e.g., hematopoietic), and induced pluripotent (iPSCs).

Main Usages:

Regenerative medicine: Repair damaged tissues (e.g., bone marrow transplants for blood disorders, potential for spinal cord injuries, heart disease, diabetes).
Drug testing and disease modeling: Grow organoids or tissues in vitro to test drugs or study genetic diseases.
Research: Understand development and cancer (cancer stem cells).
Therapeutic cloning and tissue engineering.

Nuances and Considerations: Ethical issues with embryonic stem cells; adult stem cells have limited potency but fewer ethical concerns. In Pakistan/Punjab context, stem cell therapies are emerging for various medical applications, but regulation is important. Potential: Treating thalassemia or neurological conditions common in the region.

14. The following diagram shows the structure of a mitochondrion. Name structures A to G.

Correct Answers for A to G (on your diagram):

A: Outer Membrane
B: Cristae
C: Matrix
D: Inner Membrane
E: Outer Membrane
F: Ribosome (70S)
G: Intermembrane Space

15. The diagram below shows an electron micrograph of a cell.

Chapter 3 Cells and Subcellular Organelles Solved Exercise

Correct Labels (Position-wise):

Top pointer (pointing to stacked membranes): Golgi Apparatus
Right side upper (small rod-shaped): Mitochondrion
Right side middle (large round structure): Nucleus
Right side lower (membranous with dots): Rough Endoplasmic Reticulum
Left side (top to bottom):
1. Smooth Endoplasmic Reticulum
2. Rough Endoplasmic Reticulum
3. Mitochondrion
4. Lysosome
5. Plasma Membrane (or Cell Membrane)