Photosynthesis in Higher Plants Test - 5

Photosynthetically active radiation. Photosynthetically active radiation, often abbreviated PAR, designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis.

Plastids are small bodies found free in most plant cells. These are found absent in fungi, certain bacteria, algae and multicellular animals. The plastids can be categorized in: Chromoplasts: Coloured (other than green) plastids. Chloroplasts: Green coloured plastids Leucoplasts: Colorless plastids. All these three are double membrane bound structures, which are mainly used for trapping radiation and storage purposes. New plastids arise from proplastids by a fission like process.

Chlorophyll a is the principal photosynthetic pigment while chlorophyll b is an accessory one.They have a similar structure consisted of a porphyrin-like ring (with a magnesium atom coordinated in the center) and a long hydrophobic hydrocarbon tail. Their structural difference is that chlorophyll b has an aldehyde group (-CHO) around the ring while chlorophyll a has a methyl group (-CH₃).

Park and Pon (1963) and Park and Biggins (1964) aimed at the description of the composition and organisation of the quantasomes. Assuming that the analysis of quantasomes has to precede new statements on them ("However, before an adequate molecular model of quantasome structure can be constructed, chemical analysis and localizations of substances within the quantasome must be performed" - Park and Pon 1963, p. 106), Park and Pon calculated the molecular mass of a 20 nm quantasome on the basis of manganese content at 960 kDa. Thereby 55 % of the mass were lipid compounds, whereas only 25 % were carotenoids and chlorophylls. However, Park and Biggins calculated a molecular mass of 2000 kDa, a length of 18 nm, a width of 15.5 nm and a thickness of 10 nm from the measurements of volume and density. This was twice as much as Park and Pon had calculated. Shibuya and Maruo (1965) found that all sulpholipids of the chloroplast were located in the quantasomes.

Chlorophyll is the green molecule in plant cells that carries out the bulk of energy fixation in the process of photosynthesis. Chlorophyll itself is actually not a single molecule but a family of related molecules, designated chlorophyll a, b, c, and d. Chlorophyll a is the molecule found in all plant cells. Chlorophyll d is found only in marine red algae, but chlorophylls b and c are common in freshwater. The molecular structure of chlorophylls a, b and d consists of a ring-like structure called a porphyrin and a long organic phytol "tail". In the center of the porphyrin ring is a magnesium molecule. Chlorophyll c lacks the phytol chain.

So, the correct answer is option B.

There is clear division of labour within the chloroplast. the membrane system is responsible for the light reaction ( trapping light energy and synthesis of ATP and NADPH) while dark reaction i.e., enzymatic reactions for the reduction of carbon dioxide into sugar using ATP and NADPH take place in the stroma.

Chlorophyll is a specialised light absorbing pigment which is found in the inner wall of granum. Each granum is a flat, sac-like structure in which light reaction of photosynthesis takes place.

Chloroplasts are photosynthesizing organelles, at which photosynthetic pigment chlorophyll captures, transforms and stores the energy from sunlight in the energy storage molecules NADPH and ATP while liberating oxygen from water in algal and plant cells. In a process referred to as the Calvin cycle, they use the ATP and NADPH to produce organic compounds from carbon dioxide.

Now, let us find the solution from the options-
- In the thylakoid membranes of the organelle, the chloroplast is a plastid containing chlorophyll (green pigment).
- Thylakoid can be described within the chloroplasts of a plant cell as a membrane-bound compartment.
-It is the area of photosynthesis light-dependent reactions.
- Thylakoid comprises a membrane of thylakoid containing thylakoid lumen.
-A load of thylakoid or lamella discs is known as grana.

Thus, the correct answer is option (A) Grana.

Note: At least 3 membrane systems are available for all chloroplasts: the outer chloroplast membrane, the thylakoid system, and the inner chloroplast membrane. There may be extra membranes encircling these three chloroplasts which are the product of secondary endosymbiosis.

Chloroplasts are the green plastids which take part in photosynthesis and temporary or  permanent storage of starch. These are discoid (disc-shaped) in higher plants with diameter of 4-6 μm and thickness of 2-4.μm.

A is the outer membrane, B-stroma, C-Grana, D-Starch granule.