Photosynthesis

Getting Energy• Autotrophs- make their own energy (usually

from the sun) Ex. plants

• Heterotrophs- get energy from other organisms

Ex. animals, funguses

Photosynthesis • Photosynthesis- using energy from the sun to create

glucose. Plants use glucose to make ATP and Sugars• Reactants- Carbon Dioxide, Sunlight, Water• Products – Oxygen Glucose

• Sun + 6 CO2+ 6 H2O C6H12O6 + 6 O2

Reactants:

Fig. 10-4

6 CO2

Products:

12 H2O

6 O26 H2OC6H12O6

Chloroplasts• Found in the mesophyll (interior tissue) of leafs• Thylakoids -(located inside chloroplasts) help capture

sunlight for plants. They are arranged in stacks called grana.

• Stroma - region of fluid filled space outside the thylakoids

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Chlorophyll• Chlorophyll – the pigment in chloroplasts.• Pigments are substances that absorb visible light

(different pigments absorb different light)• Chlorophyll transmitts green light, absorbs all others.*• There are other pigments such as Chlorophyll B and

carotenoids present in plants.

• Spectrophotometer - measures a pigment’s ability to absorb various wavelengths of light

• sends light through pigments and measures amount of light transmitted

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Spectrophotometer

• absorption spectrum - is a graph plotting a pigment’s light absorption versus wavelength

• The absorption spectrum of chlorophyll suggests that violet-blue and red light work best for photosynthesis

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Absorption Spectrum

Action Spectrum• action spectrum – tells you which wavelength of light

actually drives photosynthesis the best.

• Engelmann made an action spectrum using algae and bacteria. (the more 02 the algae released, the more the bacteria grew)

What’s light got to do with it?• Chloroplasts use light photons to help take

electrons from water.

• The electrons are then added to CO2 to make glucose.

• (A Redox Reaction)

• Sun + 6 CO2+ 6 H2O C6H12O6 + 6 O2

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

e-e-

Reduction/Oxidation (Redox) Reaction• Reduction – to gain electrons• Oxidation – to loose electrons.

• Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

• OIL RIG or LEO GER

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Light Reaction (the “photo” part)• Light reaction requires sunlight

– In thylakoid– Happens in the light– Water is oxidized– Produces NADPH & ATP

Photosystem• The thylakoid membranes contain photosystems • photosystem – pigment molecules attached to proteins

(light harvesting complex) that funnels light energy into a reaction center where electrons are transferred.

• Light comes in electrons come out.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Photosystem II (PII)• Light first enters photosystem II (discovered 2nd)• The light excites chlorophyll molecules, who excited

other chlorophyll molecules by passing on photons.

Splitting Water• Meanwhile, water is split into Hydrogen and Oxygen

by enzymes. (so don’t forget to water your plants!)• The Oxygen is released as waste (good for us)• The Hydrogen is kept

P680• P680 (likes 680nm light) – a special chlorophyll

molecule in photosystem II that takes electrons from the hydrogen that came from the split water. (it really likes electrons)

• The water has been oxidized

Primary Electron Acceptor • Remember the photons being passed by the

chlorophyll? • They get sent to P680.

P680 gets so exited that it

loses the electrons it received

from water.• Primary Electron Acceptor – a

molecule in the reaction center that

receives electrons from P680.

Electron Transport Chain• Linear Flow - The primary electron acceptor sends the

electrons down the electron transport chain to Photosystem I (discovered 1st).

• On the way down, movement of the electrons is used to make ATP.

ATP Production • As electrons move, they cause the Hydrogens

(protons) from water to move out of the membrane into the thylakoid space.

• As hydrogens build up in the thylakoid space they are forced back through the membrane through ATP synthase into the stroma.

ATP Synthase • ATP synthase – enzyme that makes ATP as

hydrogens pass through.

Photosystem I (PI)• Meanwhile the electrons from the ETC have reached

photosystem I. (similar to photosystem II)• P700 in photosystem I is going to receive electrons

coming from the ETC. (the electrons are not coming from water this time.)

Photosystem I cont’d• Light excites chlorophyll molecules in PI who excite

other chlorophyll molecules by passing on photons.• The photons get sent to P700 who gets so excited that

it loses the electrons it got from the ETC.

Cyclic Flow• The primary electron acceptor receives the electrons

from P700 .• The primary acceptor can then send the electrons

back to the top of the ETC so they call come down again and make more ATP. This is cyclic flow.

NADPH• The primary acceptor in PI can also send the electrons

to an electron carrier called NADP+.• Once NADP+ gets electrons to carry (gets reduced) it

becomes NADPH.

Moving On• The NADPH will take electrons to the stroma to

begin the second cycle. ATP will also go to the stroma to help.*

Calvin Cycle (the “synthesis” part)• Calvin cycle ~ AKA Light-Independent reaction

– In stroma– Happens in both light & dark– CO2 is Reduced– Uses ATP and NADPH from Light Reaction– Produces Glucose

• Carbon fixation- Incorporating Carbon dioxide

• A 5 carbon sugar named RuBP awaits CO2

• An enzyme named rubisco adds one CO2 to RuBP to make a 6 carbon sugar. (this happens three times)

• The 6 carbon sugar then splits into two 3 carbon molecules

phase 1 Carbon Fixation

phase 2 Reduction• Reduction – adding electrons • ATP is used to add a

phosphate to the 3

carbon molecules

(makes the molecule

unstable)• NADPH then reduces

the 3 carbon molecule,

replacing the newly added

phosphate with electrons.• 3 carbon molecule

is now called G3P.

phase 3 Regeneration• Regeneration – replacing RuBP• 1 Molecule of G3P is sent out to make glucose

(2 G3P’s make 1 glucose)• 5 Molecules of G3P are

used to make RuBP

(remember this all happens

3 times)*

Fig. 10-UN2

Regeneration ofCO2 acceptor

1 G3P (3C)

Reduction

Carbon fixation

3 CO2

CalvinCycle

6 3C

5 3C

3 5C

Fig. 10-UN4

C3 Plants• C3 Plants – (most plants)initial fixation of CO2, via

rubisco, forms a three-carbon compound

• On hot, dry days, plants close stomata, which conserves H2O but also limits intake of CO2

• So some plants use strategies besides C3.

C4 Plants• C4 plants –counteract hot dry days by fixing CO2 into

four-carbon compounds in mesophyll cells.• It requires the enzyme PEP carboxylase because it

can fix CO2 even when there isn’t much of it.

C4 Cont’d• The four-carbon compounds are send to bundle-

sheath cells, where they release CO2 that is then used in the Calvin cycle.

• Increases CO2 for the calvin cycle when stomata are closed to save water.

CAM Plants• Cam plants use CAM to fix CO2 into 4 carbon

molecules.• To do this, CAM plants open their stomata at night

• Stomata close during the day, and CO2 is released from the 4 carbon molecules and used in the Calvin cycle

You should now be able to:

1. Describe the structure of a chloroplast

2. Describe the relationship between an action spectrum and an absorption spectrum

3. Trace the movement of electrons in linear electron flow

4. Trace the movement of electrons in cyclic electron flow

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. Describe the role of ATP and NADPH in the Calvin cycle

6. Describe two important photosynthetic adaptations that cope with hot dry conditions.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings