(a) Define cell. How is a plant cell different from an animal cell ? Describe a typical cell structurally and functionally, with…

(a) Cell: plant versus animal, and structure and function of a typical cell

A cell is the basic structural and functional unit of life, an organised unit enclosed by a plasma membrane that can carry out metabolism, growth and reproduction.

Plant and animal cells differ in these ways. Plant cells have a rigid cellulose cell wall outside the membrane; animal cells have none. Plant cells have plastids, notably chloroplasts; animal cells lack them. A large central vacuole bounded by the tonoplast, up to about 90% of cell volume, maintains turgor, whereas animal cells have small, temporary vacuoles. Higher plant cells lack centrioles, and animal cells have them. Plant cells divide by cell-plate formation, animal cells by a cleavage furrow. Plant cells communicate through plasmodesmata and animal cells through gap junctions. Glyoxysomes occur only in plant cells.

A typical (eukaryotic) cell has: the plasma membrane, a fluid-mosaic lipid bilayer controlling transport and signalling; the nucleus, with a double envelope, chromatin and nucleolus, storing genetic information and directing synthesis; rough endoplasmic reticulum (ribosome-studded, protein synthesis) and smooth ER (lipid synthesis, detoxification); Golgi apparatus, which modifies, sorts and packages proteins; mitochondria, the sites of aerobic respiration and ATP synthesis, with their own DNA; ribosomes, the sites of protein synthesis; plastids (chloroplasts for photosynthesis); lysosomes or the vacuole for digestion and storage; peroxisomes for detoxification of peroxides; and the cytoskeleton, giving shape and enabling movement.

(b) Physiological and molecular basis of heterosis

Heterosis is the superiority of the F1 hybrid over its parents in vigour, yield or adaptability. Physiologically, it shows as more rapid early growth and larger leaf area, higher photosynthetic rate, a more extensive root system with better nutrient uptake, greater enzyme activity and metabolic efficiency, balanced hormone levels and greater tolerance of stress. These effects arise from the genetic mechanisms below.

Genetic basis: the dominance hypothesis, in which favourable dominant alleles from one parent mask unfavourable recessives from the other; the overdominance hypothesis, in which the heterozygote at a locus outperforms either homozygote; and epistasis, in which favourable interactions between loci contribute.

Molecular basis: complementation of deleterious alleles in many genes across the genome; single-gene overdominance, shown by the SFT gene that raises yield in heterozygous tomato; non-additive gene expression in hybrids, with parental alleles expressed differently and regulatory networks rebalanced; and epigenetic effects, including changes in small RNAs and DNA methylation that alter growth and stress-response genes. Heterosis is greatest between divergent parents and is lost in later generations as heterozygosity falls.

(c) Male sterility, self-incompatibility, and limitations of CMS

Male sterility is the failure of a plant to produce functional pollen. Types: genetic (nuclear) male sterility (GMS), controlled by a recessive nuclear gene; cytoplasmic male sterility (CMS), controlled by mitochondrial genes; cytoplasmic-genetic male sterility (CGMS), where sterile cytoplasm and recessive nuclear alleles together cause sterility and dominant restorer (Rf) genes restore fertility; environment-sensitive male sterility, as in thermo-sensitive and photoperiod-sensitive genic male sterility in two-line rice; chemically induced sterility with gametocides; and transgenic sterility, such as barnase-barstar.

Self-incompatibility (SI) is the failure of a fertile plant to set seed on self-pollination, governed by the S locus. It is homomorphic or heteromorphic. Homomorphic SI is gametophytic (the pollen's own haploid S allele decides, as in Solanaceae, Poaceae and Rosaceae) or sporophytic (the diploid parent's S genotype decides, as in Brassica). Heteromorphic SI is associated with floral morphs, as in distyly in Primula.

Limitations of CMS in hybrid seed production: genetic vulnerability, because a single cytoplasm used widely is exposed to epidemics, as with Texas (T) cytoplasm in maize and southern corn leaf blight (1970) and the downy mildew epidemic on pearl millet hybrid HB-3 on Tift 23A cytoplasm (India, 1971); a narrow cytoplasmic base, with most rice hybrids using WA cytoplasm; the need for effective restorer lines, which are unavailable in some crops, with incomplete restoration reducing seed set; negative effects of sterile cytoplasm or linked restorer genes on yield or quality; instability of sterility under temperature; and the cost of maintaining A, B and R lines in isolation.