Ligth Dependent and Calvin Cycle

• RuBP (ribulose bisphosphate) is regenerated at the end of the cycle, allowing it to be used again.
• CO2 enters the chloroplast through diffusion from the stroma into the thylakoid space.
• The Calvin cycle is also known as the light-independent reactions or dark reaction.
• Photosynthesis occurs in two stages - light dependent reactions and light independent reactions.
• Light dependent reactions occur in chloroplast thylakoid membranes and involve splitting water molecules using energy from sunlight.
• In photosynthesis, light is used to make organic compounds, especially sugars, in plants.
• The process of photosynthesis is extremely important for virtually all life on earth because these organic compounds are what we, and our food are all made of.
• When light strikes a leaf, the light energy is used to drive a series of chemical reactions that ultimately make sugars and other organic molecules.
• Overall, carbon dioxide and water are taken in by the plant and used to make glucose and oxygen gas.
• Photosynthesis can be divided into two sets of reactions: the light-dependent reactions and the Calvin Cycle.
• The light-dependent reactions depend on light and are the first set of reactions in photosynthesis.
• Leaves are made of plant cells, and inside these cells are special organelles called chloroplasts that do photosynthesis.
• Each chloroplast has many disks called thylakoids with pigments (like chlorophyll) that absorb light.
• Thylakoids can be thought of as solar panels in the chloroplast, as they absorb the light energy from the sun.
• The thylakoid membrane is where pigments such as chlorophyll are found and is where light gets absorbed.
• Photosystem II, when it absorbs light, causes electrons in chlorophyll to gain energy (or get excited) from the light.
• These excited electrons leave chlorophyll and move to an electron transport chain.
• Since chlorophyll lost electrons, those electrons need to be replaced.
• To replace them, water is split and its electrons go to chlorophyll.
• NADPH carries electrons and hydrogens to the next set of reactions in photosynthesis, the Calvin Cycle.
• NADPH is an electron carrier and is another key product of the light-dependent reactions.
• When the electrons reach the end of the first electron transport chain, they go to photosystem I where light excites them once again.
• The electrons travel down a second, shorter electron transport chain where they are accepted by a molecule called NADP+.
• A special enzyme called ATP synthase allows these hydrogen ions to passively diffuse from high to low concentration.
• Plants make oxygen when they do photosynthesis.
• ATP is a key product of the light-dependent reactions.
• When water is split, electrons, hydrogen ions, and oxygen are produced.
• The flow of hydrogen ions through ATP synthase causes ATP synthase to spin and produce ATP, similar to how water flowing through a turbine produces power at a hydroelectric dam.
• When NADP+ accepts the electrons, it also accepts hydrogen and becomes NADPH.
• Both ATP and NADPH are critical products of the light-dependent reactions that are needed to make sugar in the Calvin cycle, which we will examine in our next video.
• This creates a high concentration of hydrogen ions inside the thylakoid.
• If you’d like to try the light dependent reactions yourself, check out the link for the Photosynthesis Interactive at BioMan Biology.
• The electron transport chain transports electrons, using the energy in the electrons to pump hydrogen ions (or protons) across the thylakoid membrane, into the thylakoid.
• The Calvin cycle, also known as the light independent reactions or dark reactions, is the second set of reactions in photosynthesis that occurs after the light dependent reactions.
• Two key products from the light reactions, ATP and NADPH, are needed to make the Calvin cycle work.
• The Calvin cycle takes place in the fluids surrounding the thylakoids, known as the stroma.
• ATP provides the energy and NADPH provides the electrons and hydrogens that are needed to reduce carbon dioxide to build sugars in the Calvin cycle.
• In the first phase of the Calvin cycle, carbon fixation, an enzyme called rubisco takes CO2 from the air and adds it to a five-carbon compound called RuBP, a process that makes a six-carbon compound that immediately splits in half to make two molecules of three-phosphoglycerate.
• For every three molecules of CO2 brought into the Calvin cycle, there's a net gain of one G3P.
• In the reduction phase of the Calvin cycle, ATP from the light reactions provides energy to make an intermediate compound, which can then be reduced or gain electrons and hydrogen.