15 Photosynthesis

• Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and glucose

  
• The process begins with sunlight and water, where chlorophyll molecules in the plant's leaves absorb sunlight and split water molecules into hydrogen and oxygen
• The hydrogen atoms are used to create glucose, and the oxygen atoms are released as a by-product of the reaction
• The glucose produced is used by the plant as food to build new cells and produce energy, while the oxygen released is essential for life on Earth
• In photosynthesis, the oxidation of glucose during respiration yields ATP to fulfill an organism's energy needs
• Autotrophs, like plants, use photosynthesis to convert CO2 and water into glucose using light energy from the sun
• Photosynthesis takes place in the chloroplast in two stages: the light-dependent stage and the light-independent stage (Calvin cycle)
• The light-dependent stage converts light energy into chemical energy in the form of NADPH and ATP, which are then used in the light-independent stage to fix atmospheric carbon into sugar molecules
• Chloroplasts are the site of photosynthesis and contain features like the chloroplast envelope, stroma, thylakoids (grana), and intergranal lamella
• Photosynthetic pigments like chlorophyll a, chlorophyll b, and carotenoids absorb visible light for photosynthesis, with chlorophyll a being the primary pigment directly involved in the process
• The absorption spectrum of chlorophyll a does not exactly match the action spectrum
• Each pigment has its specific absorption spectrum, with different peaks and breadth, and they do not absorb green light strongly
• Carotenoids, besides their role in photosynthesis, have a photo-protective function by absorbing and dissipating excess light energy
• The electromagnetic spectrum from the sun provides light of varying wavelengths, with only visible light being absorbed by photosynthetic pigments in plants
• Earth's atmosphere allows only certain wavelengths of light through, with visible light being one of them
• The absorption spectrum and action spectrum of photosynthetic pigments can be measured using a spectrophotometer
• The action spectrum represents the efficiency of photosynthesis at different wavelengths of light
• The process begins with sunlight and water being absorbed by chlorophyll molecules in the plant's leaves
• Chlorophyll molecules use energy from sunlight to split water molecules into hydrogen and oxygen
• The glucose produced is used by the plant as food to build new cells and produce energy
• The oxygen released by photosynthesis is essential for life on Earth as it is the gas we breathe and is used by other animals and plants
• Chlorophyll a is not the only photosynthetic pigment in photosynthesis; accessory pigments like chlorophyll b and carotenoids broaden the spectrum of wavelengths over which photosynthesis can occur by channeling absorbed energy to chlorophyll a
• When a chlorophyll molecule absorbs a photon of light, one of the pigment molecule’s electrons is elevated from its ground state to its excited state
• The excited electron quickly falls back to ground state, releasing excess energy as heat and fluorescence
• The released energy can be passed to another chlorophyll molecule or captured by the primary electron acceptor molecule in the reaction center of a photosystem
• Chlorophyll molecules cluster together to form a functional unit called a photosystem, embedded in the thylakoid membrane of the chloroplast
• A photosystem consists of a reaction center surrounded by light-harvesting complexes made up of photosynthetic pigment molecules bound to proteins
• There are two types of photosystems: photosystem I (PS I) and photosystem II (PS II), which cooperate in the light-dependent reactions of photosynthesis
• In the non-cyclic light-dependent reaction, both PS II and PS I are involved, driving the synthesis of NADPH and ATP through electron flow and photolysis of water
• The main stages of the non-cyclic light-dependent reaction include photoactivation at PS II, photolysis of water, electron transport from PS II to PS I, photoactivation at PS I, and electron transport from PS I to NADP+
• During the photolysis of water, an enzyme splits a water molecule into two electrons, two hydrogen ions, and an oxygen atom, with the oxygen released as a by-product
• Electrons pass from the primary electron acceptor of PS II to PS I via an electron transport chain, where redox reactions occur as electrons are transferred down the chain, leading to the formation of ATP through non-cyclic photophosphorylation
• In the cyclic light-dependent reaction, only PS I is involved, leading to the production of ATP but not NADPH
• In photosynthesis, sunlight is absorbed by chlorophyll molecules in the plant's leaves, and water is transported from the roots to the leaves
• Chlorophyll molecules use sunlight energy to split water molecules into hydrogen and oxygen
• In the light-dependent reactions of photosynthesis, cyclic light-dependent reaction involves only photosystem I (PS I)
• In cyclic light-dependent reaction, the photo-excited electron from P700 is captured by PS I's primary electron acceptor and passed on to the first electron transport chain
• Energy lost during the electron's travel down the chain is coupled to the formation of ATP, producing only ATP in cyclic light-dependent reaction
• Cyclic light-dependent reaction does not produce NADPH or oxygen as there is no photolysis of water
• The light-dependent reactions, comprising cyclic and non-cyclic reactions, produce more ATP than NADPH to meet the higher ATP demand in the Calvin cycle