Plant Physiology: Photosynthesis and Respiration

Abscisic acid (ABA) acts as a stress hormone that triggers stomatal closure. It does this by inhibiting H+ pumps and promoting the rapid efflux of K+ ions from guard cells, which leads to water loss and turgor reduction.

The phrase 'Transpiration is a necessary evil' is a well-known botanical statement attributed to Curtis, who argued that while transpiration is essential for water transport, it also causes significant water loss.

CAM (Crassulacean Acid Metabolism) plants keep their stomata closed during the day to minimize water loss in arid environments and open them at night to take in CO2.

Excessive fertilizer in the soil creates a hypertonic environment relative to the plant roots. This causes water to move out of the root cells into the soil via exosmosis, leading to plasmolysis and plant death.

An essential element is defined by Arnon and Stout as one that is required for the plant to complete its life cycle, and its function cannot be replaced by another element.

Chlorophyll is a magnesium-porphyrin complex. Magnesium (Mg) is the central metal ion in the chlorophyll molecule, so it remains after the organic components are burnt.

Cyclic photophosphorylation involves only Photosystem I (PS I). Electrons cycle back to the reaction center, generating only ATP without the production of NADPH or oxygen.

Molybdenum is a critical component of the enzyme nitrogenase, which is required for biological nitrogen fixation in plants.

Chlorophyll is a lipid-soluble pigment. It does not dissolve in water but is soluble in organic solvents like acetone, alcohol, or ether.

Transpiration rate is inversely proportional to relative humidity (RH). High humidity reduces the vapor pressure gradient between the leaf interior and the atmosphere, slowing down transpiration.

Leghemoglobin is an oxygen-binding protein found in the root nodules of leguminous plants. It maintains low oxygen levels to protect the oxygen-sensitive nitrogenase enzyme.

Glucose is a monosaccharide that enters glycolysis directly, making it the most readily available and instantaneous source of energy for cellular respiration.

The chemical formula for chlorophyll b is C55H70O6N4Mg. It differs from chlorophyll a (C55H72O5N4Mg) by having an aldehyde group instead of a methyl group.

Chlorophyll a has a methyl (-CH3) group at the third carbon position, whereas chlorophyll b has an aldehyde (-CHO) group at that same position.

In CAM plants like Opuntia, organic acids (malic acid) are decarboxylated at night to release CO2 for the Calvin cycle. Because the CO2 is internally sourced and not consumed from the atmosphere, the net gas exchange results in an RQ of zero.

The action spectrum of photosynthesis shows that the highest rates of photosynthesis occur in the red and blue regions of the light spectrum, with red light being highly effective.

Cellular respiration is a catabolic process because it involves the breakdown of complex organic molecules like glucose into simpler molecules, releasing energy in the process.

Non-cyclic photophosphorylation (Z-scheme) involves both Photosystem II and Photosystem I, resulting in the production of both ATP and NADPH.

Photosynthesis rate is influenced by light intensity, quality, and temperature. Duration of light (photoperiod) affects flowering but is not a direct factor in the immediate rate of photosynthesis.

The Respiratory Quotient (RQ) for carbohydrates like glucose is 1, because the volume of CO2 released equals the volume of O2 consumed during aerobic respiration.

Fatty substances are highly reduced and require more oxygen for complete oxidation relative to the CO2 produced, resulting in an RQ value less than 1 (typically around 0.7).

In aerobic respiration, the complete oxidation of one molecule of glucose typically yields a theoretical maximum of 38 ATP molecules in prokaryotes (though often 36 in eukaryotes due to shuttle costs).

Anaerobic respiration is the process by which organisms produce energy in the absence of oxygen.

 The end products of respiration in plants are CO₂, H₂O and energy.

In C3 photosynthesis, the first stable product of carbon fixation is 3-phosphoglyceric acid (3-PGA). It is formed when carbon dioxide reacts with ribulose-1,5-bisphosphate.