Photosynthesis is the synthesis of glucose from carbon dioxide and water in the presence of sunlight with oxygen as a by-product. It is the most important biochemical pathway and nearly all life depend on it. A simple general equation for photosynthesis is as follows:
6CO2 + 12H2O + light energy ______ C6H12O6 + 6O2 + 6H2O
Raw materials
• Water
• Carbon dioxide
Water and carbon dioxide are the raw materials of photosynthesis. Plants have mechanisms for intake and transport of these raw materials
Water present in soil, is absorbed by roots and root hairs through osmosis. This water is eventually transported to leaves through xylem vessels.
Air that enters through tiny pores stomata are found on the surface of leaves in most plants, and they let carbon dioxide diffuse into the mesophyll layer and oxygen diffuse out.
Through stomata co2 reaches into the air spaces present around mesophyll cells. This air carries CO2 which gets absorbed in the thin layer of water surrounding mesophyll cells. From here the carbon dioxide diffuses into mesophyll cells.
Mechanism of photosynthesis
Photosynthesis occurs in two phases
• Light reaction
• Dark reaction
photosynthesis occurs in the leaves and the specialized cell organelles responsible for this process is called the chloroplast..
Light Reaction of Photosynthesis
• Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts.
• The Grana, membrane-bound sacs like structures present inside the thylakoid functions by gathering light and is called photosystems.
• These photosystems have large complexes of pigment and proteins molecules present within the plant cells, which play the primary role during the process of light reactions of photosynthesis.
• There are two types of photosystems: photosystem I and photosystem II.
Under the light-dependent re When light strikes chlorophyll, it energizes electrons within that molecule. These electrons jump up to higher energy levels, the High-energy electrons are excited. excited electrons leave chlorophyll to participate in further reactions, leaving the chlorophyll and split water molecules into hydrogen ions, electrons and oxygen atom. this process is known as photolysis means splitting by light
actions, the light energy is converted to ATP and NADPH, which are used in the second phase of photosynthesis.
Excited electrons that have absorbed light energy are unstable. First, they pass down an ETC, which captures their energy and uses it to pump hydrogen ions by active transport into the thylakoids. These concentrated ions store potential energy by forming a chemiosmotic or electrochemical gradient - a higher concentration of both positive charge and hydrogen inside the thylakoid than outside As they flow, the ion channel enzyme ATP synthase uses their energy to chemically bond a phosphate group to ADP, making ATP.
Light re-energizes the electrons, and they travel down a second electron transport chain (ETC), eventually bonding hydrogen ions to NADP+ to form a more stable energy storage molecule, NADPH.
• During the light reactions, ATP and NADPH are generated by two electron-transport chains, water is used and oxygen is produced.
Dark Reaction of Photosynthesis
The light reaction produced ATP which is its energy source, and NADPH for reducing ability move toward the chloroplast stroma, the region between the thylakoid membrane and the organelle’s inner membrane just after completing the light reaction of photosynthesis.
The three basic phases of the Calvin cycle are
. It starts when carbon in the form of carbon dioxide enters through minute pores in the leaves called stomata, where they diffuse into the stroma of the chloroplast.
Carbon dioxide combine with a five-carbon molecule ribulose 1,5bisphosphate to form an unstable six carbon intermediate that breaks down to form two molecules of 3 phosphoglycerate. This step of the Calvin cycle is catalyzed by the enzyme are known as rubisco. Using the energy from ATP, the three-carbon compound, 1,3 bisphosphoglycerate molecule are produced. 1,3 bisphosphoglycerate are converted into a 3 carbon sugar glyceraldehyde 3 phosphate with the help of enzyme glyceraldehyde 3 phosphate dehydrogenase, in which NADPH from light reaction acts as the electron donor.
Some Glyceraldehyde 3 phosphate is utilized in making glucose, while others are recycled to continue the cycle by combining with a carbon acceptor that turns into Ribulose 1,5bisphosphate. The energy required to regenerate Glyceraldehyde 3 phosphate is derived from ATP.
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