Class Starter
•Write down the primary role that sunlight plays in living systems, and then define
photosynthesis.
9.2 Photosynthesis
Harvesting lights energy
Photosynthesis
 is the process that provides energy for almost all life.

Chloroplasts are the organelles that convert light energy into chemical energy.
o stroma – fluid within the inner membrane contains the thylakoid
membrane.
o thylakoids - flat, disc-like sacs that are arranged in stacks contain
molecules that absorb light energy for photosynthesis.
Light
•a form of electromagnetic radiation
–energy that can travel through empty space in the form of waves.
•Sunlight contains all of the wavelengths of visible light which we see as different colors.
•pigment
–is a substance that absorbs certain wavelengths (colors) of light and reflects all
of the others.
•In plants, light energy is harvested by pigments that are located in the thylakoid
membrane of chloroplasts.
Chlorophyll
•a green pigment in chloroplasts
•absorbs light energy to start photosynthesis
•absorbs mostly blue and red light and reflects green and yellow light, which makes
plants appear green.
•Plants also have pigments called carotenoids which help plants absorb additional light
energy.
•When light hits a thylakoid, energy is absorbed by many pigment molecules and
eventually transferred to electron carriers.
Checkpoint
•Draw a picture of a chloroplast and label the stroma and thylakoid.
–Where is light absorbed?
•What is the role of pigments in photosynthesis?
–What is the name and color of the main pigment?
Two Electron Transport Chains
•Electrons from the electron carrier are used to produce new molecules, including ATP,
that temporarily store chemical energy.
•During photosynthesis, one electron transport chain provides energy to make ATP,
while the other provides energy to make NADPH.
•Both chains use energy from electrons excited by light.
Two Electron Transport Chains, cont.
Producing ATP
•Step 1: Electrons excited by light leave the chlorophyll molecules.
•An enzyme splits water molecules to replace these electrons.
• Oxygen gas is formed and released into the atmosphere
Two Electron Transport Chains, cont.
Producing ATP
•Step 2: Excited electrons transfer some of their energy to pump H+ ions into the
thylakoid.
•This process creates a concentration gradient across the thylakoid membrane.
Checkpoint
•What are the electrons from the electron carriers are used to make?
•What is the role of the first and second electron transport chains?
•Once the electrons have been excited by light and left, what is split to replace the
electrons?
–What other important element is produced?
Two Electron Transport Chains, cont.
Producing ATP
•Step 3: The energy from diffusion of H+ ions through the channel portion of ATP
synthase is used to catalyze a reaction in which a phosphate group is added to a molecule
of ADP, producing ATP.
Two Electron Transport Chains, cont.
Producing NADPH
•Step 4: Light excites electrons in another chlorophyll molecule.
•The electrons are passed on to the second chain and replaced by the de-energized
electrons from the first chain.
Two Electron Transport Chains, cont.
Producing NADPH
•Step 5: Excited electrons combine with H+ ions and NADP+ to form NADPH.
•NADPH is an electron carrier that provides high-energy electrons needed to store
energy in organic molecules.
Checkpoint
•In order to produce ATP, hydrogen passes through __________, by joining _____ and
_____.
•After light excites the electrons for a second time, what is formed as a result?
–Where do the electrons come from that replace the ones that left?
Producing Sugar
•The first two stages of photosynthesis depend directly on light because light energy is
used to make ATP and NADPH.
•In the final stage of photosynthesis, ATP and NADPH are used to produce energystoring sugar molecules from the carbon in carbon dioxide.
•The use of carbon dioxide to make organic compounds is called carbon dioxide
fixation, or carbon fixation.
Producing Sugar, continued
Light-independent reactions
•reactions that fix carbon dioxide
•also called dark reactions.
•Calvin cycle
–The most common method of carbon fixation.
–Atmospheric carbon dioxide is combined with other carbon compounds to produce
organic compounds.
–ATP and NADPH supply some of the energy required in these reactions.
Factors that Affect Photosynthesis
3 environmental factors that affect photosynthesis
•light intensity
•carbon dioxide concentration
•temperature
•Although different plants are adapted to different levels of light, the photosynthesis rate
increases with increases in light intensity until all of the pigments in a chloroplast are
being used.
•Photosynthesis is most efficient in a certain range of temperatures.
Checkpoint
•What did the first part of photosynthesis produce?
•The energy from the previous answer is used to make _______ from _______.
–This process is called?
–Name of the process where this is completed?
•What are three factors that effect photosynthesis?
Summary
•In plants, light energy is harvested by pigments located in the thylakoid membrane of
chloroplasts.
•During photosynthesis, one electron transport chain provides energy used to make ATP,
while the other provides energy to make NADPH.
Summary
•In the final stage of photosynthesis, chemical energy is stored by being used to produce
sugar molecules from the carbon in the gas carbon dioxide.
•Light intensity, carbon dioxide concentration, and temperature are three environmental
factors that affect photosynthesis.