Question

Essay questions:

1. Alternations of generations life cycle in plants.

2.mechanism of action of receptor kinases and gprotein couple receptors in plants.

3. Mechanisms of transport in plants.

4. Action of plant hormones

5. Processes of greening response

Answer 1

Answer 1: The alteration of generation life cycle occur to those plants and some algae that have distinct haploid (sexual) and diploid (asexual) phases. The multicellular haploid phase (having 'n' chromosome number, known as gametophytes) alters with multicellular deploid phase (having '2n' chromosome, known as Sporophyte).

The haploid gametophytes produces gametes having 'n' chromosomes. Gametes from different organisms or same organism of same species fuse together to form a deploid zygote having one set of chromosome from each parent. The deploid zygote develops into a matured deploid sporophyte which produces haploid spores through meisis. The spores contain half the number of chromosome as Sporophyte. The haploid spores again develop into haploid gametophyte and the cycle continues.

This cycle of alternate haploid and deploid phases is referred to as 'Alteration of Generation'.

Answer 2: The receptor kinase is a single pass transmembrane protein that specifically binds to the peptides and plays a vital role in cell growth, differentiation, metabolism and motility.

When a peptide ligand binds with the receptor kinase, it polymerises, specifically, dimerises with the near by receptor kinase. This lead to autophospholyration of the receptors, which are tyrosin kinases. As soon as the receptor autophosphorylates, it binds with the adapter protein and also binds to Guanine nucleotide exchange factor (GEF protein). The GEF protein activates Ras through GDP to GTP exchange. This further activates a cascade of protein kinases which ultimately activates the transcrption process. Ras inactivates when GTP is hydrolysed to GDP. The process stops on inactivation of Ras protein.

G-proteins are a group of a large family of transmembrane proteins. G-proteins are typically a seven pass transmembrane heteromeric protein comprised of alpha, beta and gamma subunits. Alpha unit has the ability to bind to Guanosine Triphosphate (GTP) and Guanosine Diphosphate (GDP) and entire G-protein is bound to near by receptor . When there is no receptor, G-protein alpha unit is bound to GDP. When it receives a signal, conformational change occurs, G-protein with alpha sub unit attaches to GTP. G-protein dissolves into two subunits, alpha and beta-gamma dimer, and no longer attached to the receptor complex. Both alpha sunbnit and beta-gamma dimer relay various messages through a number of secondary messengers to carry out various cell processes. cAMP, IP3 and DAG are examples of few secondary messengers that carry out distinct cell functions.

G-protein gets inactivated as soon as the alpha subunit hydrolyses the attached GTP into GDP. Inactive G-protein again form hetromer with alpha, beta, gamma and receptor complex to form G-protein Coupled Receptora (GPCR).

Answer 3: In plants, basically two types of transport phenomenon occur:

1.) Transportation of carbohydrate from leaves to the other parts. This transportation takes place through Phloem. The process of the tranportation of carbohydrate from leaves to other parts is known as translocation.Sugar is pumped through Phloem sieve tubes by active transport which is bidirectional. Since water is transported through Xylem in the neighbourhood through osmosis, it creates pressure for movement of sugar and water to next sieve tube and so on. At the required place, sugar is taken out of the sieve tubes and the water follows the same. The leftover water is returned into the xylem that creates continuous flow of the phloem.

2.) Transportation of water: Water moves froom roots to the leaves through Xylem by:

(i) capilary action i.e., upward movement of water through coehsion force of water to the walls of small tubes xylem.

(ii) Due to effect of transpiration: loss of water from the leaves due to transpiration draw the continuous flow of water from roots to leaves.

Answer 4: Plant hormones plays role of chemical messengers to carry out various functions at various stage of plant growth. the five major plant hormones and their functions are:

1) Auxin: This hormone is mainly involved in plant growth and cell expansion.

2) Gibbereline: It has simillar effects as auxin but has very distinct function. It helps in stem elongation and strengthening of stems between nodes.

3) Cytokinin: It delays the natural aging process of cell. It encourages cell division and hence helps in repair process.

4) Ethylene: It helps in ripening of fruits. It is the only gaseous molecule that is regarded as hormone in plants.

5) Abscisic acid: This hormone is produced when the plant is facing water stress.