PhotosynthesisPhotosynthesisCapturing the Energy in LightCapturing the Energy in Light

6CO2 + 6H2O -> C6H12O6 + 6O2

I. Energy for Life ProcessesI. Energy for Life Processes

A.A. Plants are autotrophs (make their Plants are autotrophs (make their own food)own food)

B.B. Photosynthetic organisms: plants, Photosynthetic organisms: plants, protists, algae, some bacteria.protists, algae, some bacteria.

C.C. Photosynthesis is a complex series Photosynthesis is a complex series of chemical reactions known as a of chemical reactions known as a biochemical pathway.biochemical pathway.

II. Light Absorption in II. Light Absorption in ChloroplastChloroplast The first reactions in photosynthesis The first reactions in photosynthesis

are called light reactionsare called light reactions A chloroplast contains thylakoids, A chloroplast contains thylakoids,

grana or granum (stacks ofgrana or granum (stacks ofthylakoids), and stroma (solution).thylakoids), and stroma (solution).

A.A. Light and PigmentsLight and Pigments1.1. Light travels through space as waves of energyLight travels through space as waves of energy

2.2. White light from the sun is composed of an array White light from the sun is composed of an array of colors called the visible spectrum (ROY G. BIV)of colors called the visible spectrum (ROY G. BIV)

3.3. Different colors have different wavelengths (nm)Different colors have different wavelengths (nm)

4.4. Pigments absorb certain colors of light and reflect Pigments absorb certain colors of light and reflect or transmit the colors of others or transmit the colors of others

B. B. Chloroplast PigmentsChloroplast Pigments

1. Chlorophylls: the type of pigment present in 1. Chlorophylls: the type of pigment present in the thylakoids (chlorophyll a and chlorophyll b).the thylakoids (chlorophyll a and chlorophyll b).

2. Only chlorophyll a is directly involved in the 2. Only chlorophyll a is directly involved in the light reactions of photosynthesis\light reactions of photosynthesis\

3. Chlorophyll b is called an accessory pigment (it 3. Chlorophyll b is called an accessory pigment (it assist chlorophyll a in capturing light energy).assist chlorophyll a in capturing light energy).

4. Carotenoids also function as accessory 4. Carotenoids also function as accessory pigmentspigments

5. In leaves (photosynthetic parts) 5. In leaves (photosynthetic parts) chlorophylls are more abundant and chlorophylls are more abundant and they hide the colors of the other they hide the colors of the other pigmentspigments

6. Fruits and flowers: colors of other 6. Fruits and flowers: colors of other pigments might showpigments might show

III. Electron TransportIII. Electron Transport Chlorophylls and carotenoids are Chlorophylls and carotenoids are

grouped in clusters referred to as a grouped in clusters referred to as a photosystem (photosystem I and photosystem (photosystem I and photosystem II)photosystem II)

Steps involved in the light reaction Steps involved in the light reaction (inside the thylakoid membrane)(inside the thylakoid membrane)

1.1. Light excites electrons in chlorophyll a Light excites electrons in chlorophyll a molecules of photosystem II.molecules of photosystem II.

2.2. These electrons move to the primary These electrons move to the primary electron acceptorelectron acceptor

3.3. The electrons are transferred along a The electrons are transferred along a series of molecules called the electron series of molecules called the electron transport chain (ETC)transport chain (ETC)

4. Light excites electrons in chlorophyll a 4. Light excites electrons in chlorophyll a (photosystem I) as the electrons move to (photosystem I) as the electrons move to the primary electron acceptor, they are the primary electron acceptor, they are replaced by electrons from photosystem II.replaced by electrons from photosystem II.

5. The electrons from5. The electrons fromphotosystem I arephotosystem I aretransferred alongtransferred alonga second ETC. Ata second ETC. Atthe end of the chainthe end of the chainthey combine withthey combine withNADPNADP++ and a proton and a proton(H(H++) to make NADPH.) to make NADPH.

A.A. Restoring Photosystem IIRestoring Photosystem II1.1. The replacement electrons (Step 4) are The replacement electrons (Step 4) are

provided by the splitting of water provided by the splitting of water molecules.molecules.

2.2. 2H2H22OO 4H 4H++ + 4e + 4e-- + O + O2 2

3.3. Protons (HProtons (H++) produced are left inside ) produced are left inside the thylakoid, oxygen diffuses out of the thylakoid, oxygen diffuses out of the chloroplast and can leave the the chloroplast and can leave the plant. For this reason, oxygen is called plant. For this reason, oxygen is called a byproduct of the light reaction.a byproduct of the light reaction.

IV. ChemiosmosisIV. Chemiosmosis A. This is a process in chloroplasts and A. This is a process in chloroplasts and

mitochondria where protons move mitochondria where protons move across a membrane (from high to low across a membrane (from high to low concentrations) which also concentrations) which also synthesizes ATP.synthesizes ATP.

B. There is a build up of protons inside B. There is a build up of protons inside the thylakoid (from the splitting of the thylakoid (from the splitting of water)water)

C. ADP + PC. ADP + P ATP (by the enzyme ATP (by the enzyme ATP ATP synthase)synthase)

D. The energy that drives the above D. The energy that drives the above reaction is from the movement of the reaction is from the movement of the protons inside the thylakoid to the protons inside the thylakoid to the stroma.stroma.

Describe the structure and function of the Describe the structure and function of the thylakoids of a chloroplast. thylakoids of a chloroplast.

What role do accessory pigments (hog What role do accessory pigments (hog breath fresheners) play in photosynthesis. breath fresheners) play in photosynthesis.

What happens to the electrons that are What happens to the electrons that are lost by photosystem II ? What happens to lost by photosystem II ? What happens to the electrons that are lost by photosystem the electrons that are lost by photosystem I. I.

Name the substances that are produced Name the substances that are produced when water molecules are broken down when water molecules are broken down during the light reaction. during the light reaction.

How is ATP made in the light reaction? How is ATP made in the light reaction? Explain how the light reactions would be Explain how the light reactions would be

affected if there were no concentration affected if there were no concentration gradient of protons across the thylakoid gradient of protons across the thylakoid membrane.membrane.