The Light Dependent Reaction of Photosynthesis “Go Into the Light”

Overview of Photosynthesis

Photosystem I and photosystem II Photosystems Clusters of chlorophyll and other pigments found in the grana of the chloroplasts Photosystem I and photosystem II

Light Dependent Reaction Pigments in photosystem II absorb solar energy from the sun Electrons on pigment molecules (especially chlorophyll) increase their energy levels (become excited)

Light Dependent Reaction Excited electrons are taken from the pigments and carried from photosystem II to photosystem I by electron transport chain

Light Dependent Reaction Water is broken into 2 hydrogen atoms and 1 oxygen atom Electrons are taken from hydrogen atoms to form H+ ions

Light Dependent Reaction Electrons from hydrogen atoms replace electrons on the chlorophyll Oxygen atoms combine to form O2 molecule and are released as waste

Light Dependent Reaction As electrons pass thru the electron transport chain they release enough energy to form ATP molecules from ADP molecules

Light Dependent Reaction At the end of the electron transport chain hydrogen ions attached to NADP+ to make NADPH

Overview of Photosynthesis

Review Questions Where in the chloroplasts does the light reaction occur? What is a photosystem? How many photosystems are involved in the light reaction? Which photosystem absorbs energy from sunlight? What happens to the excited electrons from photosystem II? How does the chlorophyll replace its lost electrons? What happens to the hydrogen atoms from the water molecule? What happens to the oxygen atoms from the water molecule? Where does the energy come from to form ATP molecules? What happens to the hydrogen ions in the light reaction? What are the three products of the light reaction of photosynthesis?

The Light Independent Reaction of Photosynthesis “Welcome to the Dark Side”

Overview of Photosynthesis

The “Dark” Reaction Actually can occur in sunlight also Called dark since it does not require sunlight to occur Occurs in the stroma of the chloroplast

Material from the Light Reactions ATP from light reaction supplies the energy from the dark reaction Hydrogens from NADPH are released and supply energy for the dark reaction

6 carbon dioxide (CO2) molecules enter the cycle from the atmosphere The Calvin Cycle 6 carbon dioxide (CO2) molecules enter the cycle from the atmosphere Each carbon dioxide combines in with a 5-carbon sugar already present to make six 6-carbon sugars

New 3-carbon sugars are formed when the compounds are rearranged The Calvin Cycle The six 6-carbon sugars are rearranged into twelve 3-carbon sugars by using the energy from ATP and NADPH ATP becomes ADP and NADPH becomes NADP+ and are sent back to light reaction New 3-carbon sugars are formed when the compounds are rearranged

ATP becomes ADP and goes back to the light reaction The Calvin Cycle Two of the 3-carbon sugars are removed from the cycle and combine to form glucose The other ten 3-carbon sugars are rearranged into six 5-carbon sugars and the cycle begins again using energy from ATP ATP becomes ADP and goes back to the light reaction

Overview of Photosynthesis

Factors Affecting the Rate of Photosynthesis Amount of water: shortage of water can stop the process Temperature: photosynthesis occurs best between 0 and 35-degrees Celsius Light Intensity: more light will speed up process but there is a maximum speed no matter how much light is present

Review Questions What chemicals from the light reaction are used in the dark reaction? Where in the chloroplast does the dark reaction take place? Why is the name “dark” reaction not really accurate in describing this process? Where do the 5-Carbon sugars come from at the start of the Calvin Cycle? What three substances are necessary for the Calvin Cycle to begin? Where does the energy come from to keep rearranging the carbon containing molecules in the Calvin Cycle? How many three carbon compounds have to taken from the Calvin Cycle in order to make a glucose molecule? What products of the Calvin Cycle are sent back to the light reaction? Explain why shortage of water, temperature and light intensity each affect the rate of photosynthesis.