Plant Growth and Hormones

Chlorophyll molecules are light trapping green pigments in plants. They are made up of a cyclic tetrapyrrole head and a long phytol tail attached to the head. One atom of magnesium is directly coordinated to the four pyrrole rings in the cyclic tetrapyrrole head. Thus, magnesium is an important structural component of chlorophyll molecules. The iron is an essential micronutrient for plants. It is required for synthesis of chlorophyll and has several other functions also. Plants also require an exposure to light in order to synthesize chlorophyll.  Apparently because in the absence of light chlorophylls are not required at all; trapping of light being there chief function. In absence of light plants will be thin and pale. Such plants are called etiolated plants.

Short day plants require the long duration of the dark period. If the dark period is interrupted with a brief exposure to red light (660 nm), the flowering will not take place. The inhibitory effect of red light on flowering during the critical dark period in short day plants can be overcome by a subsequent exposure to far-red rays. This is due to the fact that when exposed to far-red light, P$ _fr$ is converted to P$ _r$.

1. Extrinsic factors are those factors like light, water, temperature, availability of gases is the abiotic or environmental factors that control the growth of a plant. These are not controlled by the plant.
   
2. Intrinsic factors like growth regulators are released by the plant and are in direct control under the plant. They regulate process like cell differentiation, cell division etc.
   

• Plant growth regulators function as chemical messengers for intercellular communication. There are currently five recognized groups of plant hormones: auxins, gibberellins, cytokinins, abscisic acid ($ABA$) and ethylene.
• Indole-3-acetic acid ($IAA$, $3-IAA$) is the most common, naturally occurring plant growth regulator of the auxin class. 
  
• $2IP$ (Isopentenyl adenine) is a naturally occurring cytokinin that regulates cell division, development, and nutrient processing in plants.
• Zinc ($Zn$) is not a plant growth regulator. It is one of the eight essential micronutrients. It is needed by plants in small amounts, but yet crucial to plant development.
• So, the correct answer is '$IAA, 2IP, Zn$'.

Enzymes are biocatalyst. These are proteinaceous substances that are capable of catalysing chemical reactions of biological systems without themselves undergoing any change. The term enzymes were first used by Kuhne in 1877.

Morphogenesis is a process that controls the organized spatial distribution of cells during embryonic development. It can also take place in a matured organism in cell culture or in tumor cell masses. It is concerned with the shape of tissues. Plant growth regulators such as gibberellins, cytokinins, abscisic acid and ethylene control growth and development in plants. Gibberellins and cytokinins are the two instrumental hormones that play a key role in morphogenesis. Whereas auxins stimulate the plant cell to grow long.

Ethylene helps fruits ripen by making them softer, through the breakdown components of the cell walls, and sweeter, through the conversion of starches to sugars. Unlike the other plant hormones, ethylene is actually a gas and is distributed through the air, not through the plant body. One of the coolest things about ethylene is that it is released in a positive feedback loop, a little bit of ethylene causes more to be released, which causes even more to be released, and so on. A benefit of this fact is that you can take an unripe fruit (a pear, plum, or peach, for example) and put it in a paper bag with riper fruit (bananas work well for this) and ethylene will accumulate, making the unripe fruit soft and sweet. Indoleacetic acid, gibberellic acid and kinetin does not triggers the ripening process. 

Auxins have a cardinal role in the coordination of many growths and behavioral processes in the plant's life cycle and are essential for plant body development. So the correct option is ' Auxin'.

Ethylene gas is used for artificial ripening of fruits. Ethylene is also an important natural plant hormone, used to induce ripening process under controlled conditions of temperature. Ethylene treatment breaks down the green chlorophyll pigment in the exterior part of the peel and allows the yellow or orange carotenoid pigments to be expressed.

Phosphorylation of glucose is increased by insulin. Glucokinase is an enzyme which is responsible for phosphorylation of glucose to glucose-6-phosphate with the help of insulin, secreated by beta cells. Beta cells release insulin in response to rising levels of glucose. Insulin enables many types of cells to import and use glucose, and signals the liver to synthesize glycogen. While alpha cells produes more of glucagon. Glucagon serves as a signal to the liver to break down glycogen and release glucose into the blood. Glucokinase in beta cells serves as a glucose sensor, amplifying insulin secretion as blood glucose rises.

Insulin is not a phytohormones, while cytokinins, ethylene and auxins are examples of phytohormones. Phytohormones are produced within a plant and are essential for the plant growth. Insulin is a peptide hormone, produced within human beings, by beta cells in the pancreas. It regulates the metabolism of carbohydrates and fats.

Insulin is secreted by Islets of Langerhans of the pancreas and serves to lower the blood glucose level by stimulating the uptake of glucose by cells, specifically muscle cells, liver cells and adipose tissues cells.