Photosynthesis in Higher Plants

Phytochrome is an amorphous photoreceptor chromoprotein. It exists in two interconvertible forms, i.e. P_r and P_fr.

P_r (inactive) $\underset{\mathrm{Far} \mathrm{red}}{\overset{\mathrm{Red} \mathrm{light}}{\rightleftharpoons }}$ P_fr (active)

Photosynthetic bacteria are of two types- Oxygenic and Anoxygenic. They do not have chloroplast, and photosynthesis takes place within the cell. In cyanobacteria, thylakoids lie naked in the cytoplasm and attached to them are minute structures called phycobilisomes which contain the pigments. Bacteria have only one photosystem i.e. type I reaction centre (PS I- 700) in green sulphur bacteria and type II (PS II- 680) in non sulphur bacteria.

Photorespiration is a metabolic pathway that occurs in plants in the presence of light, in which ribulose bisphosphate carboxylase/oxygenase the (RuBisCO) enzyme involved in carbon dioxide fixation with ribulose bisphosphate, accepts oxygen, in place of carbon dioxide, resulting in the formation of a twocarbon compound, glycolate. Most of the fixed carbon represented by the glycolate can be salvaged by a series of reactions-the glycolate pathway involving the peroxisomes and mitochondria, and returned to the chloroplasts. However, some of the carbon is lost as carbon dioxide. Unlike respiration there is no production of ATP.

Collenchyma is a specialized type of parenchyma which functions as a supporting tissue. The cell walls are irregularly thickened due to the deposition of cellulose and pectin. Hence, it is a living tissue.

The opening and closing of stomata is a function of turgor changes in guard cells.

In light,  malic acid is produced in guard cells which dissociates into hydrogen and malate ions resulting in intake of  K⁺ ions through H⁺ - K⁺ ion exchange. There is increase in concentration of K⁺ and malate ions in guard cells, endosmosis of water into guard cells occurs resulting in increased turgor pressure which results in stomatal opening. These are concerned with gaseous exchange during photosynthesis and respiration and water loss through transpiration.

Photophosphorylation refers to the use of light energy from photosynthesis to provide the energy to convert ADP to ATP.  It was discovered by D. Arnon and coworkers.

Stomata are meant for gaseous exchange but are also the main source of transpiration. In majority of plants stomata open in the presence of light, i.e. in daytime and close in darkness (night).

In the process of photorespiration, uptake of oxygen and evolution of carbon- dioxide are light dependent. RuBP carboxylase acts as RuBP oxygenase. This process occurs in chloroplast and required the help of peroxisomes and mitochondria.

Short Day Plants or SDPs are the plants that require a long period of darkness. These plants can flower in complete darkness if supplied with exogenous nutrients.

Blackman formulated the principle of limiting factors. He studied the effect of CO₂ concentration, light intensity and temperature on the rate of photosynthesis.