BIOLOGY 3 Marks Question

Chlorophyll-a molecules act as antenna molecules. They get excited by absorbing light and emit electrons during cyclic and non-cyclic photophosphorylations. They form the reaction centers for both photosystems I and II. Chlorophyll-b and other photosynthetic pigments such as carotenoids and xanthophylls act as accessory pigments. Their role is to absorb energy and transfer it to chlorophyll-a. Carotenoids and xanthophylls also protect the chlorophyll molecule from photo-oxidation. Therefore, chlorophyll-a is essential for photosynthesis. If any plant were to lack chlorophyll-a and contain a high concentration of chlorophyll-b, then this plant would not undergo photosynthesis.

The shady side of the plant/potted plants kept in the shade will have more concentration of chlorophyll and hence a darker green colour of leaves. This is because of the following two reasons:

1. The chloroplasts of the mesophyll cells are irregularly arranged, and their orientation is in vertical files along the walls in strong light.
2. Photo oxidation of chloroplasts occurs in bright light and its non-oxidation occurs in shaded areas.

1. NADPH is oxidised at stage E.
2. A-RuBP carboxylase oxygenase (RuBisCO), B-Reduction, C-Regeneration
3. ATP is converted to ADP at stages D and F.

Light reactions or the 'Photochemical' phase include following steps:

• Light absorption
• Water splitting
• Oxygen release
• The formation of high-energy chemical intermediates, ATP and NADPH.

Several complexes are involved in the process. The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II). These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC are made up of hundreds of pigment molecules bound to proteins. Each photosystem has all the pigments (except one molecule of chlorophylla) forming a light harvesting system also called antennae. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680.

1. Photorespiration is the light dependent process of oxygenation of ribulose biphosphate (RuBP) and release of carbon dioxide by the photosynthetic organs of plant.
2. Photorespiration does not produce energy or reducing power, rather it consumes energy. Further, it undoes the work of photosynthesis. Therefore, photorespiration is a highly wasteful process. It is not essential for survival.
3. The end product of this process is phosphogly cerate.
4. The site of photorespiration is chloroplast whereas; peroxisome and mitochondria are required for completing the process.

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is a redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and also the electrons needed to convert carbon dioxide into carbohydrate, which is a reduction reaction. In general outline, photosynthesis is the opposite of cellular respiration, where glucose and other compounds are oxidized to produce carbon dioxide, water, and release chemical energy.

Light is rarely a limiting factor for photosynthesis because light saturation occurs at 10% of the full sunlight. So, except for plants in shade or dense forest; light is rarely a limiting factor. Photosynthetic organisms in oceans are found at various depths and the amount of light available to them is enough for carrying out photosynthesis. Moreover, these organisms show great variations in photosynthetic pigments. These pigments help these organisms to carry out photosynthesis even in low light conditions.

The first option is wrong because growing only those plants from which we get food would disturb the ecosystem. The second option is the correct one, because a plant may not be directly useful for us, but it is useful for us in indirect way. Let us take example of grass. We do not use grass as food and hence the grass may look useless for us. But the grass is the main fodder for many herbivores. Some of those herbivores are domesticated for milk and meat. Moreover, any living being is important for maintaining the balance of the ecosystem.

The cyclic photophosphorylation is shown in the above figure.

1. The given figure represents structure of chloroplast. They are photosynthetic organelles in plants. This structure is present in plant cell.
2. Yes, these can be passed on to the progeny. Chloroplasts are semi-autonomous cell organelles as they possess DNA. During cytokinesis chloroplast as well as other cell organelles are distributed to the daughter cells.
3. In the given figure, part marked as (1) is stroma of chloroplast where dark reaction of photosynthesis takes place. Marked part (2) is extra nuclear DNA or chloroplast DNA. Chloroplast DNA is responsible for replication of chloroplast.

1. Plants are also called lungs of the nature because, they perform the exact function as lungs do in mammals. They produce oxygen, which is necessary for all forms of life. So, in essence since our lungs keep us alive in the same way the trees keep our lungs alive.

Thus, they are considered to be the part of our lung existence.

2. Chlorophyll-b and other accessory pigments are necessary as these are light absorbing compounds found in photosynthetic organisms. These pigments enable a wider range of wavelength of incoming light to be utilised for photosynthesis.
3. Chlorophyll-a structure is such that it gets damaged by bright/high light intensities. It is protected by, carotenoid pigments.

Although all cells in the green part of a plant have chloroplasts, most of the energy is captured in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, can contain between 450000 and 800000 chloroplasts for every square millimeter of leaf (nearly 60-70 chloroplasts/ cell). The surface of the leaf is uniformly coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to reduce heating. The transparent epidermal layer allows light to pass through to the palisade mesophyll cells, where most of the photosynthesis takes place. The green stems are also capable of performing photosynthesis.

Group A will show more photosynthesis. Chlorophyll absorbs maximum light in the blue region of the spectrum, i.e., 400-450nm and hence, photosynthetic rate will also be high. Group B will show negligible amount of photosynthesis or no photosynthesis. Chlorophyll does not absorb any light in the green region, i.e., 500-550nm, but reflects green so, plants appear green.

1. The splitting of water occurs on the inner side of the thylakoid membrane; hence protons remain in the lumen of thylakoids.
2. The primary electron acceptor, located on the outer side of the membrane, transfers its electrons to a hydrogen acceptor; so with the electron transport, the proton also moves to the lumen of thylakoids.
3. The NADP reductase enzyme, located on the outerside of the membrane uses protons from the stroma for the reduction of NADP, thus decreasing the concentration of protons in the stroma.

Temperature does not influence light reactions of photosynthesis but affects the enzyme controlled dark reactions. The optimum temperature for photosynthesis is 18 to 35°C. When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every 10°C rise in temperature. Above the optimum temperature, the rate of photosynthesis shows an initial increase for short duration but later declines.

Plant which is kept in dark with proper ventilation cannot carry out photosynthesis. In this case, the plant will survive till its reserve food material is not exhausted completely, afterwards, it will die. Such a plant can survive in only one condition, i.e., when provided with artificial light source like a lightened bulb or tube light along with water. So it can carry out photosynthesis and grow.