explain schematically the mechanism of photosynthesis exhibited by plants like opuntia, aloevera in presence of light.
Answers
Answer:
Photosynthesis in Plants
There are numerous species of plants all over the world. Some have adapted to desert conditions while other have adapted to cold climates. There are also plants that could only survive in cool, moist areas with adequate sunlight. These differences in climatic conditions and ecosystems have resulted in different types of photosynthesis in plants. The three types of photosynthesis are C3, C4 and CAM photosynthesis.
Plants perform photosynthesis because it generates the food and energy they need for growth and cellular respiration. It is important to note that not all plants photosynthesize. Some are parasites and simply attach themselves to other plants and feed from them.
For plants to perform photosynthesis they require light energy from the sun, water and carbon dioxide. Water is absorbed from the soil into the cells of roots. The water passes from the root system to the xylem vessels in the stem until it reaches the leaves. Carbon dioxide is absorbed from the atmosphere through pores in the leaves called stomata. The leaves also contain chloroplasts which hold chlorophyll. The sun’s energy is captured by the chlorophyll.
Leaves are essential for the well-being of plants. Most of the reactions involved in the process of photosynthesis take place in the leaves.
The typical plant leaf includes the following
Upper and lower epidermis – the upper epidermis is the outer layer of the cells that controls the amount of water that is lost through transpiration.
Stomata – these are pores (holes) in the leaves that are responsible for the exchange of gases between the plant leaves and the atmosphere. Carbon dioxide is absorbed from the atmosphere and oxygen is released.
Mesophyll – these are photosynthetic (parenchyma) cells that are located between the upper and lower epidermis. These cells contain the chloroplasts.
Vascular bundle – these are tissues that form part of the transport system of the plant. Vascular bundles consist of xylem and phloem vessels which transport water, dissolved minerals and food to and from the leaves.
Process of Photosynthesis (step-by-step)
The light-dependent reactions and the Calvin Cycle are the two main stages of photosynthesis in plants.
Light-dependent Reactions
The first stage of photosynthesis is the light dependent reactions. These reactions take place on the thylakoid membrane inside the chloroplast. During this stage light energy is converted to ATP (chemical energy) and NADPH (reducing power).
Light is absorbed by two Photosystems called Photosystem I (PSI) and Photosystem II (PSII). These protein complexes contain light harvesting chlorophyll molecules and accessory pigments called antenna complexes. The photosystems are also equipped with reactions centers (RC). These are complexes of proteins and pigments which are responsible for energy conversion. The chlorophyll molecules of PSI absorb light with a peak wavelength of 700nm and are called P700 molecules. The chlorophyll molecules of PSII absorb light with a peak wavelength of 68Onm and are called P68O molecules.
The light dependent reactions begin in PSII.
A photon of light is absorbed by a P680 chlorophyll molecule in the light harvesting complex of PSII.
The energy that is generated from the light is passed from one P680 chlorophyll molecule to another until it reaches the reaction center (RC) of PSII.
At the RC is a pair of P680 chlorophyll molecules. An electron in the chlorophyll molecules becomes excited as a result of a higher level of energy. The excited electron becomes unstable and is released. Another electron is released following the capture of another photon of light by the light harvesting complex and the transfer of energy to the reaction center.
The electrons are transported in a chain of protein complexes and mobile carriers called an electron transport chain (ETC). Plastoquinone is the mobile carrier that transports the electrons from the reaction center of PSII to the Cytochrome b6f Complex as shown in the diagram above.
The electrons lost from PSII are replaced by splitting water with light in a process called Photolysis. Water is used as the electron donor in oxygenic photosynthesis and is split into electrons (e-), hydrogen ions (H+, protons) and oxygen (O2). The hydrogen ions and oxygen are released into the thylakoid lumen. Oxygen is later released into the atmosphere as a by-product of photosynthesis.
While the electrons pass through the ETC via Plastoquinone, hydrogen ions (protons) from the stroma are also tranferred and released into the thylakoid lumen. This results in a higher concentration of hydrogen ions (proton gradient) in the lumen.
As a result of the proton gradient in the lumen, hydrogen ions are transferred to ATP synthase and provide the energy needed for combining ADP and Pi to produce ATP.
Cytochrome b6f transfers the electrons to Plastocyanin which then transports them to Photosystem I.