8.3

What happens during the light-dependent reactions?

-The light-dependent reactions use energy from sunlight to produce oxygen and convert ADP and NADP+ into the energy carriers ATP and NADPH

photosystem

-Thylakoids contain clusters of chlorophyll and proteins known as photosystems

-surrounded by accessory pigments

-essential to the light-dependent reactions

-absorb sunlight and generate high-energy electrons that are then passed to a series of electron carriers embedded in the thylakoid membrane

-Light absorption by the photosystems is just the beginning of light-dependent reactions

electron transport chain

-The light-dependent reactions begin when pigments in photosystem II absorb light (called photosystem II simply because it was discovered after photosystem I)

-Light energy is absorbed by electrons in the pigments found within photosystem II, increasing the electrons’ energy level

-These high-energy electrons (e–) are passed to the electron transport chain

-series of electron carrier proteins that shuttle high-energy electrons during ATP-generating reactions

-As light continues to shine, more and more high-energy electrons are passed to the electron transport chain

-the thylakoid membrane contains a system that provides new electrons to chlorophyll to replace the ones it has lost (these new electrons come from water molecules)

-Enzymes on the inner surface of the thylakoid break up each water molecule into 2 electrons, 2 H+ ions, and 1 oxygen atom

-The 2 electrons replace the high-energy electrons that have been lost to the electron transport chain

-As plants remove electrons from water, oxygen is left behind and is released into the air

-The hydrogen ions left behind when water is broken apart are released inside the thylakoid

-Energy from the electrons is used by the proteins in the chain to pump H+ ions from the stroma into the thylakoid space

-At the end of the electron transport chain, the electrons themselves pass to a second photosystem called photosystem I

ATP synthase

-H+ ions cannot cross the membrane directly, so the thylakoid membrane contains ATP synthase (a protein) that spans the membrane and allows H+ ions to pass through it

-Powered by the gradient, H+ ions pass through ATP synthase and force it to rotate

-As it rotates, ATP synthase binds ADP and a phosphate group together to produce ATP (chemiosmosis)

-this process enables light-dependent electron transport to produce not only NADPH (at the end of the electron transport chain), but ATP as well

Calvin cycle (light-independent reactions)

-The ATP and NADPH formed by the light-dependent reactions contain an abundance of chemical energy, but they are not stable enough to store that energy for more than a few minutes

-During the light-independent reactions (referred to as the Calvin cycle), plants use the energy that ATP and NADPH contain to build stable high-energy carbohydrate compounds that can be stored for a long time

-6 carbon dioxide molecules from the atmosphere are combined with 6 5-carbon molecules in the first step of the cycle —> produces 12 3-carbon compounds

-Energy from ATP and high-energy electrons from NADPH are used to convert the 3-carbon molecules to higher-energy forms: 2 of these 3-carbon molecules are removed from the cycle to produce sugars, lipids, amino acids, and other compounds, the remaining 3-carbon molecules are converted back into 5-carbon forms that are used to start the cycle again

What happens during the light-independent reactions?

-During the light-independent reactions, ATP and NADPH from the light-dependent reactions are used to produce high-energy sugars

What factors affect photosynthesis?

-Among the most important factors that affect photosynthesis are temperature, light intensity, and the availability of water