Plant Physiology: Light Reactions - Free MCQ Practice Test with solutions,

Photosynthesis takes place in the green leaves of plants and other green parts of plants like stem etc. The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplasts, which contain the specialized light-absorbing green pigments, the chlorophylls. When chlorophyll absorbs light, it gets excited and emits electrons. These chlorophylls are found in photosynthetic units called Photosystem I and Photosystem II. Each unit has a specific reaction centre which contains pigment molecules. These molecules absorb light of different wavelengths and emit electrons. Due to the photon of light, electrons of chlorophyll get excited. These electrons are picked up by an electron acceptor which passes them to an electron transport system of cytochromes. The excitement of electrons of chlorophyll b photon of light is the first step of photosynthesis.

The energy of light captured by pigment molecules called chlorophylls, in chloroplasts is used to generate high- energy electrons with great reducing potential. These electrons are used to produce NADPH₂ as well as ATP in a series of reactions called the light reactions because they require light.

Photophosphorylation is the conversion of ADP to ATP using the energy of sunlight by activation of PSII. This involves the splitting of the water molecule in oxygen and hydrogen protons (H⁺), a process known as photolysis.

The reason of algae float to the surface during the day & sink at night is due to photosynthesis. In Day time, the algae is producing oxygen. When enough Oxygen is produced during the day , it gets trapped in bubbles and it can lift the clumps up to the surface.

Photophosphorylation is the process in which ATP is produced, which is called as energy currency of the cell. During this process, ADP is phosphorylated to give ATP in presence of light energy.
So, the correct answer is option B.

The chlorophyll pigments which are excited give up their electrons and to compensate for the loss of electrons, water is split to release four H⁺ ions and four electrons and O². The electrons that are lost from the PSII enter into an electron transfer chain or ETC.

Photosynthesis occurs in two stages- light reaction and dark reaction. During the light reaction reduced NADPH_2​ and ATP are synthesized. These products are used subsequently to reduce carbon dioxide into sugars. Thus, the purpose of ATP in photosynthesis is to transfer energy from light reaction to dark reaction.

Photosynthesis occurs in two stages, light reaction, and dark reaction. During the light reaction reduced NADPH2 and ATP are synthesized. These products are used subsequently to reduce carbon dioxide into sugars. The purpose of ATP in photosynthesis is to transfer energy from light reaction to dark reaction. The purpose of NADP is to transfer hydrogen from light reaction to dark reaction in the form of reduced NADPH_2.

Carbon dioxide fixation does not occur during the light reaction of photosynthesis. Instead, it takes place during the dark reaction (Calvin cycle). The light reaction involves processes such as light absorption, water splitting, and the formation of ATP and NADPH. Carbon dioxide fixation is not related to the light reaction.

Photosynthesis occurs in two stages- light reaction and dark reaction. During the light, reaction water is oxidized using energy from light and reduced NADPH₂ and ATP are synthesized. The oxidation of water is called photolysis. Oxygen is released as a by-product during the photolysis of water. The NADPH₂ and ATP are used subsequently to reduce carbon dioxide into sugars.

Photolysis of water occurs at grana i.e., lumen side of grana thylakoid membrane with the help of water splitting complex or OEC (oxygen evolving complex). This step is associated with PS- II of Z- scheme.
Three minerals Mn ion, Ca+  +,Cl− are associated with splitting of water.

The evolution of oxygen during the light-dependent steps in photosynthesis (Hill reaction) was proposed and proven by British biochemist Robin Hill. He demonstrated that isolated chloroplasts would make oxygen (O2) but not fix carbon dioxide (CO2).

Hill's reaction or light reaction takes place in thylakoids and the end products of this reaction are ATP and NADPH2. Blackman's reactions or dark reactions take place in the stroma of the chloroplast. In dark reactions, sugars are synthesized from carbon dioxide by using ATP and NADPH2, which are end products of Hill's reaction.

Hence, the correct answer is Option D.

Photophosphorylation is the conversion of ADP to ATP using the energy of sunlight by activation of PSII. This involves the splitting of the water molecule in oxygen and hydrogen protons (H⁺), a process known as photolysis.

Chlorophyll, any member of the most important class of pigments involved in photosynthesis, the process by which light energy is converted to chemical energy through the synthesis of organic compounds. Chlorophyll is found in virtually all photosynthetic organisms, including green plants, prokaryotic blue-green algae (cyanobacteria), and eukaryotic algae. It absorbs energy from light; this energy is then used to convert carbon dioxide to carbohydrates.

The NADPH2 is synthesized in non-cyclic photophosphorylation in the light phase of photosynthesis. This is used subsequently in the dark phase to reduce carbon dioxide into sugars. NADPH2 is also called as reducing power. The substance provides the hydrogen molecules to perform reduction of the other substance. 

Thus, the correct answer is option D.

Ionisation of water or splitting of water during photosynthesis is called as photolysis of water. It occurs during light phase and water is split into oxygen, protons and electrons. The protons are released into thylakoid lumen. From thylakoid lumen, during ATP synthesis, electrons pass on to the stroma through ATPase system. The electrons are passed on to a series of electron acceptors and ultimately donated to NADP, which also takes protons from chloroplast stroma to produce reduced NADPH2. Thus ultimately protons from water are passed on to NADP.

Initially chlorophyll captures the electron while ionization of water because trapping of the light energy is the key to photosynthesis. The first event is the absorption of light by a photoreceptor molecule and it is found in chloroplasts of green plants i.e. chlorophyll a.

Cyclic photophosphorylation produces ATP. Phosphorylation refers to the use of light energy to ultimately provide the energy to convert ADP to ATP, thus replenishing the universal energy currency in living cells. This process uses the only photosystem I and the chlorophyll P ₇₀₀.

The Calvin cycle has three stages.
1)carbon fixation
2)reduction
3)regeneration.
At the end of each Calvin cycle net gain is 9ATP and 6 NADPH2.In each Calvin cycle, 3 CO₂ molecules are fixed with RuBP to form 2 molecules of 3PGA in the first step. Later two molecules of 3PGA through a series of reactions regenerate 2RuBP in the third stage. So, for the fixation of one CO₂ carbon dioxide molecule through the Calvin cycle requires 3ATP and 2NADPH2.
So, the correct answer is 'option B'.

During non-cyclic electron flow, the electron hole in P-680 (reaction center of pigment system- II) is filled by electrons obtained by photolysis of water. The photolysis is an oxidative process and means splitting of water in the presence of light to form oxygen, protons and electrons. As a result, there is the evolution of oxygen and the electron move through Mn-protein bound to PS- II.

The most important method of ATP synthesis is phosphorylation of ADP using inorganic phosphate and energy difference across membranes. The energy difference is provided by the electrochemical gradient of protons. When the energy of light is used to derive the electrochemical gradient of protons across the thylakoid membranes the process of ATP synthesis is called as photophosphorylation. When the energy obtained by oxidation of reduced coenzymes is used to establish electrochemical gradient of protons across the inner mitochondrial membrane the process of ATP synthesis is called as oxidative phosphorylation. During both photo and oxidative phosphorylation electrons are exchanged between iron containing cytochrome proteins.

During non-cyclic electron flow, the electron ejected on behalf of PS I by the reaction centre P_7​0​0​ moves uphill and reduces an iron-sulphur protein. From here, it moves downhill to ferredoxin (Fd) and then to NADP⁺, which is reduced to NADPH₂​. The enzyme which actually forms reduced NADPH_2​ is ferredoxin NADPH oxidoreductase.

so, the correct answer is option B.

Both respiration and photosynthesis both require Cytochromes.
Both respiration and photosynthesis involve electron transfer across a series of electron carriers embedded in the inner mitochondrial membrane and the thylakoid membranes respectively. Cytochromes are responsible for ATP production through electron transport and are iron-containing proteins. 

During photosynthesis, water (H₂O) is oxidized, which means it loses electrons. Carbon dioxide (CO₂) is reduced, which means it gains electrons. 

During noncyclic electron flow the electron hole in P-680 is filled by electrons obtained by photolysis of water. As a result, there is an evolution of oxygen and the electron move through Mn-protein bound to PS II. In this transport Mn⁺⁺ is oxidised to Mn⁺⁺⁺ and then reduced to Mn⁺⁺ in a cyclic manner. The manganese thus transfers electrons from water to photosystem II and thus plays an important role in electron transport in the process of photosynthesis.

• The photosynthetic pigments are organized into two groups of light-harvesting systems.
• They are mainly composed of many pigments bonded to proteins.
• These light-harvesting systems are present within PS I and PS II.

Photosynthesis is the process in which autotrophs such as green plants use carbon dioxide and water in presence of sunlight to synthesize nutrients such carbohydrates in the form of glucose and release oxygen. In this carbon dioxide is reduced to glucose and water is oxidized to release oxygen.

Option A is correct.

The electron released from P₆₈₀ is first accepted by the primary electron acceptor, pheophytin, which is a chlorophyll a molecule lacking the central magnesium ion.

From pheophytin the electron is transferred to plastoquinone (PQ). PQ accepts two electrons and two protons to form PQH₂.

Electrons then pass through the cytochrome b₆f complex and plastocyanin to reach P-700 in photosystem I; from there they are transferred to ferredoxin and ultimately to NADP⁺ via ferredoxin-NADP+ reductase.

• Four electrons and four protons are produced at the end of the photolysis of water.
• Two molecules of water get converted into 4H⁺ and 4e^– along with free oxygen.
• The ions used for this splitting is Mn²⁺, Ca²⁺, and Cl^–.

• Calvin cycle is an example of light independent reaction in which carbohydrate is formed by the reaction of carbon dioxide and water by the utilisation of assimilatory power (ATP, NADPH) formed during light-dependent phase of photosynthesis. The site of Calvin cycle is the stroma of chloroplast.
• Thylakoid is a membranous-bound compartment present in the chloropplast. Photosystem I and photosystem II are present on the thylakoid membrane to carry out light-dependent reactions of photosynthesis. These two systems are involved in the absorption of different wavelengths of light for the production of ATP and NADPH.
• Chlorophyll pigments are embedded in the thylakoid membrane. They are responsible for absorbing light of different wavelengths to carry out light-dependent photosynthetic reactions.
• Water splitting reaction occurs in the lumen of thylakoid during light-dependent phase of photosynthesis. Photolysis of water is the first step of photosynthesis. Splitting of water provides electrons for the photosynthetic electron chains as well as protons for the establishment of proton gradients.