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Choose correct statements about cell organelles:
A. Endomembrane system includes Golgi complex, ER and mitochondria
B. Rough ER bears ribosomes on its surface
C. Both mitochondria and plastids have circular DNA
D. A network of protein filaments called cytoskeleton is present in eukaryotic cells
Options
1
C and E only
2
A and B only
3
A, B and C only
4
B, C and D only
Correct Answer
Option 4: B, C and D only
Solution
1

A ❌ — Mitochondria are NOT part of endomembrane system. Endomembrane = ER, Golgi, lysosomes, vacuoles, plasma membrane.

B ✅ — Rough ER DOES have ribosomes on its cytosolic surface. Correct.

2

C ✅ — Mitochondria AND plastids both have circular DNA (prokaryote-like). Correct.

D ✅ — Cytoskeleton (protein filament network) IS present in eukaryotic cells. Correct.

Correct = B, C and D only
A is WRONG — mitochondria are NOT part of endomembrane system
Theory: Cell Biology
1. Endomembrane System — Components

The endomembrane system is a network of membranes inside eukaryotic cells that work together in membrane trafficking. Components: Nuclear envelope (double membrane with nuclear pores). Endoplasmic reticulum (ER): Rough ER (with ribosomes) — protein synthesis and modification. Smooth ER (no ribosomes) — lipid synthesis, detoxification, Ca2+ storage. Golgi apparatus (Golgi complex): processes and sorts proteins from ER. Trans face (mature face) → cis face (forming face). Lysosomes: membrane-bound, contain hydrolytic enzymes (acid hydrolases). Vacuoles: storage, turgor pressure (plants). Plasma membrane: outer boundary. Vesicles: transport between compartments. NOT in endomembrane system: mitochondria, chloroplasts, peroxisomes, ribosomes. These are functionally and evolutionarily distinct from the ER-Golgi network.

2. Rough ER vs Smooth ER

Rough ER (RER): studded with ribosomes on cytosolic surface. Connected to outer nuclear envelope. Functions: protein synthesis (membrane-bound ribosomes synthesise secretory proteins, lysosomal proteins, membrane proteins). Co-translational translocation: as protein is made, it enters ER lumen through translocon (Sec61 complex). N-glycosylation: sugar chains added to proteins in ER. Protein folding: ER chaperones (BiP, calnexin, calreticulin). Quality control: misfolded proteins retained or sent to proteasome. Smooth ER (SER): no ribosomes. Functions: lipid and steroid synthesis, phospholipid production, detoxification (liver — P450 enzymes), Ca2+ storage and release (muscle cells = sarcoplasmic reticulum).

3. Mitochondria and Plastids — Endosymbiont Origin

Both mitochondria and plastids (chloroplasts) share features with prokaryotes supporting endosymbiont theory: Circular DNA (no histones, no nuclear envelope — like prokaryotic nucleoid). 70S ribosomes (like prokaryotes, unlike cytoplasmic 80S). Binary fission (divide independently of nucleus). Double membrane (inner membrane = original prokaryote membrane, outer = derived from host phagocytic vesicle). Size similar to bacteria. Mitochondria: descended from alpha-proteobacteria. Contains mtDNA encoding 13 proteins (mostly ETS complexes), 22 tRNAs, 2 rRNAs. ~16.5 kb in humans. Chloroplasts: descended from cyanobacteria. Contains cpDNA (~120-160 kb). Encodes ~80 proteins. This common circular DNA is why statement C is correct — both organelles have circular DNA. Plastids include: chloroplasts (photosynthesis), leucoplasts (storage — amyloplasts for starch), chromoplasts (carotenoids — red/orange/yellow).

4. Cytoskeleton — Three Types of Filaments

The cytoskeleton is a dynamic network of protein filaments in eukaryotic cells providing structural support and enabling movement. Three types: Microfilaments (actin filaments): 7 nm diameter. Made of actin (most abundant cytoskeletal protein). Functions: cell shape, amoeboid movement, cytokinesis (cleavage furrow), microvilli (brush border). Dynamic — undergo rapid polymerisation/depolymerisation. Intermediate filaments: 8-12 nm. Variable protein composition (keratins in epithelial cells, vimentin in mesenchymal cells, neurofilaments in neurons, lamins in nuclear lamina). Stable — provide mechanical strength. Microtubules: 25 nm diameter. Made of alpha and beta tubulin dimers. Form hollow tubes. Functions: cell shape, cilia and flagella (9+2 arrangement), spindle fibres (mitosis), axonal transport (neurons), organelle positioning. Dynamic instability — grow and shrink. Drugs targeting cytoskeleton: colchicine (inhibits tubulin polymerisation → prevents mitosis), taxol (stabilises microtubules → prevents depolymerisation → kills cancer cells), cytochalasin (inhibits actin polymerisation).

5. Golgi Apparatus — Processing and Sorting

Golgi apparatus (discovered by Camillo Golgi 1898): stack of flattened membrane-enclosed cisternae. Functionally polarised: cis face (receiving face, nearest ER): receives vesicles from ER. Medial cisternae: modification. trans face (shipping face, facing plasma membrane): sorts proteins for different destinations. Functions: Post-translational modification: glycosylation (add/trim sugar chains), phosphorylation, sulfation, proteolytic cleavage. Protein sorting: routes proteins to: lysosomes (M6P tag), plasma membrane (secretory vesicles), secretion (constitutive and regulated). Lipid modification and synthesis. Membrane trafficking: Golgi is the central distribution hub of the secretory pathway. COPI vesicles: retrograde transport (Golgi → ER). COPII vesicles: anterograde transport (ER → Golgi). Clathrin-coated vesicles: trans-Golgi → endosomes/lysosomes/plasma membrane.

6. Lysosomes — Digestive Organelles

Lysosomes: membrane-bound organelles containing ~60 types of acid hydrolases (optimally active at pH 4.5-5.0). Membrane proteins: V-type H+-ATPase maintains acidic pH. LAMP proteins (lysosomal membrane proteins) protect membrane. Functions: Autophagy: digest worn-out organelles (mitophagy for old mitochondria). Phagocytosis: digest pathogens engulfed by macrophages/neutrophils. Heterophagy: digest extracellular material brought in by endocytosis. Autolysis: cell self-destruction (controlled by lysosome membrane integrity). Formation: from trans-Golgi network (TGN) → lysosomal proteins tagged with M6P → sorted to early endosomes → late endosomes → lysosomes. Lysosomal storage diseases: enzyme deficiencies → substrate accumulation. Tay-Sachs: hexosaminidase A deficiency. Gaucher: glucocerebrosidase deficiency. Pompe: alpha-glucosidase deficiency. These diseases cause progressive neurological and organ damage.

7. Vacuoles — Storage and Turgor

Vacuoles are membrane-bound spaces filled with water and dissolved substances. Plant cell central vacuole: occupies 80-90% of mature plant cell volume. Surrounded by tonoplast membrane. Functions: storage of water, ions, metabolites, waste products, pigments (anthocyanins in flower vacuoles). Turgor pressure: water moves into vacuole by osmosis → pushes protoplast against cell wall → turgidity → plant rigidity (wilting = loss of turgor). Digestion in some plants: vacuoles contain hydrolytic enzymes (similar to lysosomes). Animal cell vacuoles: smaller, multiple. Food vacuoles (phagosomes) in protozoans. Contractile vacuoles: expel excess water in freshwater protozoans (osmoregulation). Autophagic vacuoles: contain cytoplasmic debris for digestion.

8. Cell Fractionation — Studying Organelles

Cell fractionation is the technique used to isolate specific organelles for biochemical study. Method: Homogenisation: cells disrupted by blending, sonication, or pressure → lysate. Differential centrifugation: centrifuge at increasing speeds → heavier particles pellet first. 600g (10 min): nuclei, unbroken cells. 3000g (10 min): mitochondria, chloroplasts. 10,000g (20 min): lysosomes, peroxisomes. 100,000g (60-90 min): microsomes (ER fragments, ribosomes, small vesicles). Supernatant (remaining): cytoplasm + soluble proteins. Density gradient centrifugation: separate organelles by density on sucrose or Ficoll gradient. Provides purer fractions. Uses: identified which enzymes are in which organelle. Established that Krebs cycle enzymes are in mitochondrial matrix, ETS on inner membrane. Identified lysosomal enzymes. Confirmed ribosome structure (70S vs 80S). Location of photosystems in thylakoid membranes.

Frequently Asked Questions
1. Why is mitochondria NOT part of the endomembrane system? ⌄
The endomembrane system consists of organelles that exchange material via vesicle transport and share membrane composition and origin. Mitochondria are excluded because: (1) They have a separate evolutionary origin — from endosymbiotic alpha-proteobacteria. (2) They do not exchange membrane via vesicles with ER or Golgi. (3) Their inner membrane has a completely different lipid composition (rich in cardiolipin — a phospholipid unique to bacteria and mitochondria). (4) They have their own DNA and protein synthesis machinery. Similarly, chloroplasts and peroxisomes are NOT part of the endomembrane system. This is a very common NEET trap question.
2. What is co-translational import into the ER? ⌄
Co-translational import: protein synthesis and ER entry occur simultaneously. Process: Ribosome begins translation of mRNA encoding a secretory/membrane protein. Signal sequence (first ~20 amino acids) emerges from ribosome. SRP (Signal Recognition Particle) binds to signal sequence. SRP-ribosome complex docks at SRP receptor on ER membrane. Ribosome binds translocon (Sec61 complex) — a protein channel in ER membrane. Translation resumes — polypeptide is threaded directly into ER lumen as it is made. Signal sequence is cleaved by signal peptidase. Protein folds in ER lumen with help of chaperones. Contrast: post-translational import into mitochondria and chloroplasts — proteins are fully synthesised in cytoplasm first, then imported through protein import complexes (TOM/TIM complex for mitochondria, TOC/TIC for chloroplasts).
3. What type of DNA do mitochondria have? ⌄
Mitochondrial DNA (mtDNA): circular, double-stranded. In humans: 16,569 base pairs. Located in the mitochondrial matrix. Multiple copies per mitochondrion (2-10 copies). Encodes: 13 proteins (all components of oxidative phosphorylation complexes I, III, IV, V), 22 tRNAs, 2 rRNAs. Most mitochondrial proteins (~1000+) are encoded by nuclear genes and imported. Inheritance: strictly maternal (sperm mitochondria are destroyed after fertilisation). No recombination → changes only by mutation → used as molecular clock for tracing maternal lineage. Mitochondrial diseases: mutations in mtDNA → MELAS syndrome, Leigh syndrome, MERRF — affect tissues with high energy demand (brain, muscle, heart). Heteroplasmy: mixture of mutant and normal mtDNA in cells.
4. What are the three types of cytoskeletal filaments? ⌄
Three cytoskeletal filament types: Microfilaments (7 nm): made of actin. Dynamic (polymerise/depolymerise rapidly). Cell shape (cortical actin), amoeboid movement, microvilli, cleavage furrow in cell division, muscle contraction (thin filaments). Drug: cytochalasin inhibits actin polymerisation. Intermediate filaments (8-12 nm): many protein types depending on cell type (keratins, vimentin, desmin, neurofilaments, lamins). Stable, not dynamic. Provide tensile strength, nuclear lamina. Mutations in keratins cause skin blistering diseases (epidermolysis bullosa). Microtubules (25 nm): made of tubulin. Dynamic instability. Spindle fibres (mitosis/meiosis), cilia/flagella axoneme (9+2 arrangement), axonal transport, organelle positioning. Drugs: colchicine inhibits polymerisation (gout treatment, anti-mitotic), taxol stabilises microtubules (anticancer), vincristine/vinblastine inhibit polymerisation (cancer chemotherapy).
5. What is the role of the Golgi apparatus? ⌄
Golgi apparatus (Golgi complex) is the processing and sorting centre of the secretory pathway. Receives proteins from ER (via COPII vesicles). Processing: modification of N-linked glycans (add/trim sugars), addition of O-linked glycans, phosphorylation, sulfation, proteolytic processing of proproteins. Sorting: trans-Golgi network (TGN) sorts proteins into different vesicles based on sorting signals. Destinations: lysosomes (M6P-tagged proteins), plasma membrane (secretory/membrane proteins), regulated secretory granules (hormone-secreting cells). Constitutive secretion: continuous. Regulated secretion: triggered by signal (e.g., insulin secretion on glucose stimulation). Discovery: Camillo Golgi, 1898, using his own silver staining method. For a long time the Golgi was thought to be an artifact of staining — electron microscopy confirmed its reality in the 1950s.
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