(a) Describe along with illustrated diagrams the structure and function of mitochondria and endoplasmic reticulum. 8+7=15 (b)(i)…

The eukaryotic cell is a highly compartmentalized unit where organelles like mitochondria and the endoplasmic reticulum (ER) drive energy metabolism and biosynthesis.

Part (a): Mitochondria and ER Mitochondria are double-membrane-bound organelles. The outer membrane is permeable, while the inner membrane is folded into cristae, increasing surface area for the electron transport chain (ETC). The matrix contains mitochondrial DNA (mtDNA), 70S ribosomes, and enzymes for the Krebs cycle. [Diagram: Longitudinal section showing outer membrane, inner membrane with cristae, matrix, and mtDNA loops.] Functionally, mitochondria are the site of aerobic respiration, generating ATP via oxidative phosphorylation. In thermogenic plants like Symplocarpus, the alternative oxidase (AOX) pathway dissipates energy as heat, facilitating pollination. They also regulate apoptosis and maintain calcium homeostasis.

Endoplasmic Reticulum (ER) is a continuous membrane network. Rough ER (RER) is studded with ribosomes, while Smooth ER (SER) lacks them. [Diagram: Schematic of RER with ribosomes and SER tubules connected to the nuclear envelope.] RER is the primary site for synthesizing secretory and membrane proteins, initiating N-linked glycosylation. SER is involved in lipid and steroid synthesis, detoxification of xenobiotics, and calcium storage. In plants, calreticulin acts as the primary calcium-binding protein in the ER lumen, buffering cytosolic Ca²⁺ signals.

Part (b)(i): Cytoplasmic Male Sterility (CMS) CMS is a maternally inherited trait where pollen is non-functional due to mutations in the mitochondrial genome, while female fertility remains normal. This is caused by the expression of chimeric open reading frames (ORFs) that produce toxic peptides. Example: In maize, the Texas male sterile (T) cytoplasm contains the urf13 gene. The toxicity of the URF13 protein is conditional, requiring interaction with the Bm-T toxin (or methomyl) to disrupt inner membrane integrity. Restoration of fertility occurs when nuclear Restorer of fertility (Rf) genes suppress mitochondrial gene expression. This system is crucial for hybrid seed production in crops like maize and sorghum, where sterile female lines (A-lines) are crossed with restorer lines (B-lines) to produce F1 hybrids.

Part (b)(ii): Molecular Basis of Sex Determination In dioecious plants, sex determination is often chromosomal. In Carica papaya, the XY system involves a large Y chromosome containing the MSY (Male Sterility Y) locus. The MSY gene encodes a MADS-box transcription factor that represses floral organ identity genes, leading to male flower development. In Silene latifolia, the Y chromosome is highly degenerate, and sex determination involves dosage compensation of X-linked genes. Epigenetic modifications, such as DNA methylation, also play a role in stabilizing sex-specific gene expression. Environmental factors can occasionally influence sex expression, but the genetic framework remains the primary determinant.

Part (c): Protein Synthesis and Structure Protein Synthesis involves two main stages: transcription and translation.

1. Transcription: RNA Polymerase II binds to the promoter region of a gene. The pre-mRNA undergoes 5' 7-methylguanosine capping, spliceosomal intron removal, and 3' polyadenylation to form mature mRNA.
2. Translation: The mRNA binds to the 40S ribosomal subunit. Initiation factors position the start codon (AUG) with the initiator tRNA (Met-tRNA). The 60S subunit joins to form the 80S ribosome.

• Elongation: Aminoacyl-tRNAs enter the A-site. Peptide bond formation is catalyzed by the peptidyl transferase center of the 25S rRNA in the large subunit. The ribosome translocates, moving the peptidyl-tRNA to the P-site.
• Termination: Upon reaching a stop codon, release factors trigger the release of the polypeptide chain. Post-translational modifications (e.g., phosphorylation, glycosylation) occur in the ER and Golgi apparatus.

Protein Structure and Function: Proteins have four levels of structure. The primary structure is the linear amino acid sequence. Secondary structures (α-helices, β-sheets) are stabilized by hydrogen bonds. Tertiary structure is the 3D folding driven by R-group interactions. Quaternary structure involves multiple polypeptide subunits. Functions are diverse: enzymes catalyze metabolic reactions; structural proteins (e.g., cellulose synthases) provide support; transport proteins (e.g., aquaporins) move substances; and signaling proteins (e.g., receptors) transmit information.

In conclusion, the intricate coordination between mitochondrial energy production, ER-based protein processing, and genetic regulation of traits like CMS and sex determination underscores the integrative complexity of plant cell biology.