Non-Membrane-Bound Cell Organelles (Ribosomes, Centrioles, Cytoskeleton)
Ribosomes
- Non-membrane-bound; present in all living cells.
- Location & destiny of proteins
- Free in cytoplasm ➜ proteins used inside same cell.
- Bound to rough ER ➜ proteins for secretion, membranes, or lysosomes.
- Also found in mitochondria & chloroplasts.
- Structure & Composition
- Made of rRNA + ribosomal proteins.
- Prokaryotes ≈ 60\% rRNA / 40\% protein; eukaryotes ≈ 50\% : 50\%.
- Subunits (Svedberg units)
- Eukaryote: small 40S + large 60S ➜ 80S ribosome.
- Prokaryote: small 30S + large 50S ➜ 70S ribosome.
- Eukaryotic ribosome assembled in nucleolus, exported via nuclear pores.
- Function: Protein synthesis (translation)
- Reads mRNA codons.
- Matches each codon with complementary tRNA carrying specific amino acid.
- Forms peptide bonds, elongating polypeptide until stop codon.
- Newly made protein released for folding/modification (cytosol in prokaryotes; Golgi pathway in eukaryotes).
Centrioles
- Cylindrical organelles of 9 triplets of microtubules.
- Usually occur as a perpendicular pair inside the centrosome (near nucleus).
- Key Roles
- Organize mitotic/meiosis spindle; ensure accurate chromosome segregation.
- Form basal bodies that nucleate cilia & flagella.
- Absence ➜ spindle errors, mitotic delay, defective cilia/flagella.
Cytoskeleton
- Dynamic network of protein filaments in eukaryotic cytoplasm.
- Components & Functions
- Microtubules
- Tubulin polymers; maintain shape, vesicle/organelle transport, form mitotic spindle.
- Microfilaments (actin)
- Cell shape, cortical tension, motility, contractile ring in cytokinesis.
- Intermediate filaments
- Tensile strength, structural stability.
- Cell Division Structures
- Mitotic spindle: microtubules nucleated from duplicated centrosomes (late G_{2} phase).
- Contractile ring: overlapping actin-myosin ring; constricts to split cytoplasm during cytokinesis.
- Other Roles: intracellular transport, organelle positioning, cell locomotion, mechanical support.