Question

3) Describe the photosynthesis-relevant characteristics of light.

Answer 1

Q3) Photosynthesis is a process in which green plants and some other organisms produce food with the help of sunlight, carbon-dioxide and water, in this process light energy is converted into chemical energy.

The overall reaction of photosynthesis is

6CO2 + 6H2O (+ Light energy) = C6H12O6 + 6O2

Carbon-dioxide + water (+ light energy) = glucose + oxygen

Sunlight plays an important role in synthesizing food in these organisms. In green plants a photosynthetic pigment called chlorophyll is found, which helps in absorbing light as a result light is used to convert carbon-dioxide and water into carbohydrates, which is the source of energy for various metabolic processes in plant body.

Color of light is also important in photosynthesis, chlorophyll absorbs blue, violet and red rays of light. Rate of photosynthesis is more in presence of blue and red light and is less in presence of green light.

Based on the type of pigment present in a plant, it absorbs the light of a particular wavelength. Chlorophyll-a when present, it absorbs blue light having wave length 430(nm) and red light of wavelength 662(nm) and it reflects green light that’s why these plants appears green in color.

Chlorophyll-b found in green algae and plants they absorb light of 453 nm and 642nm wavelength.

Carotenoid pigments absorb green light of wavelength 460nm and 550nm, these plants appear red, orange or yellow as they are not able to absorb these lights. Light intensity also plays an important role in photosynthesis when intensity of light is more there is more photosynthesis, when the intensity of light is low the rate of photosynthesis is less.

In chemical reaction point of view photosynthesis is a light dependent oxidation-reduction process, (oxidation means removal of electron from a molecule and reduction means addition or gain of electron by a molecule), in plants energy of light is used for oxidation of water as a result oxygen gas (O2), hydrogen ions (H+) and electrons are produced. The removed electrons and hydrogen ions are transferred to carbon-dioxide which helps in the reduction process of it producing organic products.

In photosynthesis there are two phases of reaction they are 1) Light reaction or Hill reaction and 2) Dark reaction or Blackman’s reaction.

In light reaction which takes place in presence of light, here light is captured and in dark reaction which takes place in absence of light but is dependent on the end product of light reaction dark reaction is a process which is controlled by the enzymes.

During light reaction chlorophyll captures light and solar energy and converts it into chemical energy in the form of ATP molecules. This takes place because light energy helps in splitting the water molecules producing oxygen and hydrogen ions, hydrogen ions are used to make ATP. These ATP molecules help in providing energy at the time of dark reaction of photosynthesis and they are converted into ADP molecules. ADP molecules formed during dark reaction again converted into ATP molecules at the time of light reaction, thus both these reactions are interdependent. So light is necessary for photosynthesis to provide energy for the synthetic reactions. Synthesis of ATP molecules in presence of light is called photophosphorylation.