Test: What is Photosynthesis? - NEET MCQ

Photosynthesis is an enzyme regulated anabolic process which involves the synthesis of organic compounds, inside the chlorophyll containing cells, from CO₂ and H₂O with the help of sunlight as a source of energy. Oxygen evolved during photosynthesis comes from water. The equation for photosynthesis can be summarised as follows.

Light is the visible part of electomagnetic radiations. Visible light consists of radiations having a wavelength between 390-760 nm (or 399-430nm), blue of indigo (430-470nm), blue green or blue (470-500nm), green (500-580nm), yellow (580-600nm), orange (600-650nm), orange red (650-600nm) and red (660-760nm).

• Carboxylation is the fixation of CO₂ into a stable organic intermediate.
• Carboxylation is the most crucial step of the Calvin cycle where CO₂ is utilised for the carboxylation of RuBP.
• This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.
• Since this enzyme also has an oxygenation activity, it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.

• Chromatiaceae is a family of purple sulphur bacteria, e.g. Chromatium.
• These are short, Gram-negative rods, ~1 µm in diameter and 3-4 µm long.
• They are able to use sulphur and sulphide as the sole photosynthetic electron donor and sulphur can be oxidised to sulphate.
• These bacteria use an inorganic sulphur compound, such as hydrogen sulphide, as an electron donor.

Both statements (A) and (B) are correct about photosynthesis reaction.
A. In this process, solar energy is converted into chemical energy.
B. In photosynthesis, CO₂ and H₂O are used to give glucose and oxygen in the presence of sunlight with the help of green pigment chlorophyll.

Ruben and Kamen (1941), while working on chlorella (unicellular green alga ) found that oxygen liberated during photosynthesis comes from water. 

(i) When normal H₂​O radioactive CO₂​(i.e.CO₂¹⁸​) were used, normal O₂​ is evolved. 

(ii) When normal CO₂​ radioactive H₂​O(i.e.H₂​O¹⁸) were used, radioactive O₂​ (i.e.O₂^18​) is evolved. 

nCO_2​+nH_2​O¹⁸→(CH₂​O)n​+O₂¹⁸​↑ (radioactive oxygen)+H₂​O 

So the correct option is 'D'.

Light reactions or the 'photochemical' phase include light absorption, water splitting, and oxygen release. The splitting of water is associated with the PS II - water is split into H⁺, [O] and electrons. This creates oxygen, one of the net products of photosynthesis and the formation of high-energy chemical intermediates, ATP and NADPH₂. The light reactions capture energy from sunlight, which they change to chemical energy that is stored in molecules of NADPH₂ and ATP.

It takes 12 more ATPs than C₃ cycle to replenish this PEP because additional 2 ATPs are needed for every CO₂ molecule fixed.

Van Niel (1897-1885), on the basic of his studies with puple and green sulphur bacteria, demonstrated that phoosynthesis is a light dependent reaction in which hydrogen from a oxidisable compound reduces CO₂ to form sugar.

In green sulphur bacteria, when H₂S instead of H₂O was used as hydrogen donor, no O₂ was evolved.

6CO₂+12H₂S→C6H₁₂O₆+6H₂O+125

From the above observation and conclusions, van Niel proposed that oxygenic photosyntheis of all organisms is actually an oxidation-reduction reaction involving action of hydrogen of water over carbon dioxide to form organic ompounds. He also propounded that oxygen is evolved from water.

Chlorophyll a is the primary photosynthetic pigment, which is bluish green in the pure state. It has an empirical formula C_55​H_72​O₅​N₄​ Mgand molecular weight of 893. It absorbs more red wavelength of light than violet blue . It is soluble in a number of organic solvents such as petroleum ether. 

The graphical curve showing the amount of energy of different wavelengths of light absorbed by a pgment is called absorption spectrum,. It is studies with the help of spectrophotometer. The absorption spectra of chorophylls a and b show that they absorb maximum light in the blue-violet and red wavelengths. The pgments are often known after the wvelength which is absorbed to the maximum e.g., chl a₆₇₃ Chl a₆₈₃(P₆₈₀), Chl a₇₀₃(P₇₀₀).

Chlorophyll a is found in almost all photosynthetic plants except bacteria. It is called primary photosynthetic pigment because it performs primary reactions of photosynthesis which involve conversion of light energy into chemical energy. Other photosynthetic pigments like chlorophyll b, xanthophylls and carotenes are called accessory pigments. They absorb light energy of different wavelengths, broaden the spectrum of light absorption and hand over the energy to chlorophyll a through electron spin resonance and also protect chlorophyll a from photo-oxidation.

The most abundant plant protein in the world is RuBisCo, which is an enzyme that catalyses the first step in carbon fixation. RuBisCo is found in plants, algae, cyanobacteria and certain other bacteria. It takes part in photosynthesis and converts inorganic CO₂ into organic forms containing C-C bonds and H atoms.

Chlorophyll present in chloroplasts traps solar energy.