Test: Photosynthesis in Higher Plants - Assertion and Reasoning - NEET MCQ

Chloroplasts function as the site of photosynthesis in eukaryotic photoautotrophs. Inside the leaves, the chloroplasts occur mostly in the mesophyll cells along their walls for easy diffusion of gases and receiving optimum quantity of incident light. Within the chloroplast there is the membranous system consisting of grana, the stroma lamellae, and the fluid stroma. The membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH.

Anthocyanin pigments only give colouration since the epidermal cells mainly have potential colouring pigments. It is responsible for blue, red, pink and purple colours, observed in different parts of plants such as petals, stamens and fruits etc. Anthocyanins are also important for attracting insects for pollination and seed dispersal. Hence, anthocyanin pigments are not accessory photosynthetic pigments

Chlorophyll a is found in all photosynthetic plants except bacteria. Hence, it is called as universal photosynthetic pigment. It is also called primary photosynthetic pigment because it performs primary reaction of photosynthesis which involves conversion of light into chemical energy. Other photosynthetic pigments like chlorophyll b, carotenes and xanthophylls are called accessory pigments.

In bacteria, photosynthesis utilizes light of wavelength more than 700 nm and their reaction centre is B-890.

Although the efficiency of photosynthesis is uniform over most of the spectrum, it declines significantly in the red, i.e., at wavelength of 680 nm and above. This phenomenon is called red drop. However, it was shown by Emerson that if light at 680 nm is supplemented with light of a shorter wavelength (< 600 nm), the quantum efficiency of photosynthesis in the red can be restored to normal.

Six molecules of CO₂ enter Calvin cycle to produce one hexose molecule whereas 18 ATP, 12 NADPH⁺ and H⁺ molecules are used up. The light reaction of photosynthesis results in ATP and NADPH₂ formation.

Cyclic pathway of photosynthesis appeared first in some eubacterial species. It is supposed to be the first evidence of production of ATP in the presence of light. During non-cyclic photophosphorylation photolysis of water takes place. Under the influence of light energy and the catalytic action of chlorophyll, water a substance of low energy value, is split up into oxygen and hydrogen. Oxygen is used in the chloroplast. Non-cyclic photophosphorylation is the only natural process which adds molecular oxygen to the atmosphere

In case of cyclic photophosphorylation, the electron, while passing between ferredoxin and plastoquinone and/or over the cytochrome complex the electron loses sufficient energy to form ATP from ADP and inorganic phosphate.

Each molecule of ribulose-1, 5-biphosphate fixes one molecule of carbon dioxide with the addition of water, thereby resulting in the formation of two molecules of 3-phosphoglyceric acid (3-PGA). The fixation and reduction of one molecule of CO₂ requires three molecules of ATP and two of NADPH, coming from the photochemical reactions.

The grana stacks of membranes are enriched in PS II and LHC (Light harvesting centre), while there is little ATP synthetase. On the other hand, a fraction of stroma thylakoids is rich in PS I and ATPase and poor in PS II and LHC.

In case of cyclic photophosphorylation, the electron, while passing between ferredoxin and plastoquinone and/or over the cytochrome complex the electron loses sufficient energy to form ATP from ADP and inorganic phosphate.

The synthesis of ATP via electron flow through the ETS, with oxygen as the terminal electron acceptor, is known as oxidative phosphorylation and takes place in mitochondria. In contrast to the oxidative phosphorylation of mitochondria, O2 is not used in photophosphorylation of chloroplasts and NADP+ is the last electron acceptor.

Less than 1% of the total water absorbed is utilized in photosynthesis. The rest is lost in transpiration. Even a slight increase in transpiration reduces the leaf hydration that cuts down photosynthesis by causing stomatal closure and hence decreased CO2 absorption, loss of leaf turgidity, reduced absorption of solar radiations and decrease in enzymatic activity.

The overall equation of photosythesis is CO₂ + 2H₂O + n(hv) → (CH₂O)₆ + H₂O + O₂ The standard free-energy change for the synthesis of hexose from CO² and H₂O is ΔG° = + 686 kcal. As six molecules of CO₂ are involved in forming one molecule of hexose, the energy input per CO₂ molecule will be 114 kcal.

The energy balance of photosynthesis:

represents a storage of 686,000 calories per mole. This amount of energy is provided by a total of 12 NADPH and 18 ATP molecules, which represent 750,000 calories. The efficiency reached by the PCR cycle is thus as high as 90% (686/750 x 100 = 90%).

Thylakoid membranes possess photosynthetic pigments and coupling factors. Coupling factors are involved in ATP synthesis. Photosynthetic pigments include chlorophyll a, chlorophyll b, carotenes and xanthophylls. They occur in specific groups called photosystems (previously quantasomes). There are two photosystems. The grana lamellae have both PS I and PS II and the stroma lamellae lack PS II as well as NADP reductase enzyme.

Electron transport in photosynthesis produces a proton gradient. The gradient develops inside the thylakoid lumen in chloroplasts. Proton pump is energised by electron flow. It creates a proton gradient or high concentration of H⁺ in the lumen. The proton gradient is broken down due to movement of protons through transmembrane channels, CF₀ of ATPase. The breakdown of the gradient provides enough energy to cause a conformational change in the F₁ particle of the ATPase, which makes the enzyme synthesise several molecules of energy-packed ATP.

The phenomenon of breaking up of water into hydrogen and oxygen in the illuminated chloroplast is called photolysis or photocatalytic splitting of water. Light energy, an oxygen evolving complex and an electron carrier are required. It is attached to the inner surface of the thylakoid membrane. The complex has four Mn ions. Light energised changes in Mn (Mn²⁺, Mn3⁺, Mn⁴⁺) removes electrons from OH^– component of water forming oxygen. The electrons released during photolysis of water are picked up by P₆₈₀ photocentre of photosystem II.

Water splitting complex is associated with PS-II, which itself is physically located on the inner side of the membrane of the thylakoid.

Cyclic pathway of photosynthesis first appeared in some eubacterial species for ATP synthesis. Non-cyclic photophosphorylation is the only natural process, which adds oxygen to the atmosphere by photolysis of water. Non-cyclic photophosphorylation first appeared or originated in cyanobacteria.