Chapter 6

PhotosynthesisPhotosynthesis

Capturing Light Energy• All energy needed by living things comes

directly or indirectly from the Sun

• Energy moves through organisms in a living system

• Autotrophs vs. Heterotrophs

• Energy is released as bonds are broken.

• Some as heat the rest is stored, temporarily, as ATP.

• Energy is stored when bonds are made.

ATP• Energy storing compound

• Nucleotide structure

• As phosphate is removed, energy is released from ATP.

• ATP--> ADP + P---> AMP + P

• As phosphate is added, energy is stored in ATP.

• AMP + P ---> ADP + P ---> ATP

Photosynthesis Intro.• Example of a biochemical pathway

• 6CO2 + 6H20 + Sunlight---> C6H12O6 + 6O2 + Energy

• Reverse reaction is Cellular Respiration (chapter 7)

• Use of sunlight (radiant energy) to make chemical energy (ATP)

• Involves Chloroplasts located in the leaves of plants

• 2 types of reactions take: Light and Dark reactions

Chloroplasts• Light reactions begin with the absorption of

light

• Contain a double membrane like mitochondria

• Thylakoids are individual sacs stacked to form a grana and are surrounded by the stroma

• Thylakoids contain pigments that absorb the different wavelengths of light

• Chlorophyll a and b

• Accessory pigments (carotenoids, xanthophylls)

Physics of Light• White light contains different wavelengths of color

• Visible spectrum (light) is what is seen by the eye

• ROY G BIV

• red region has long wavelengths

• blue region has short wavelengths

Light Reaction• AKA: Light Dependent Reaction

• Photosystems II and I are involved

• Light excites molecules in Photosystem II causing them to move to an electron acceptor which then passes them through an ETC to replace ‘lost’ electrons in photosystem I

• Light also excites molecules in Photosystem I causing them to move to an electron acceptor which then passes them through an ETC to form NADPH

• Hydrolysis of water allows for electrons to replace ‘lost’ electrons in photosystem II. Hydrogen atoms stay inside the thylakoid while Oxygen diffuses out of the chloroplast.

• 2H20---> 4H+ + 4e’ + O2

Chemiosmosis

• Concentration of protons across the thylakoid causes ATP to form

• ATP synthase (protein located in the thylakoid membrane) harnesses energy from concentration gradient causing ADP to add another P to make ATP

• ATP synthase also acts like a carrier protein and an enzyme

http://youtu.be/Oi2_n2wbB9o

Recap of Light Rx

• Light Dependent Reactions absorb light energy in structures called thylakoids, which contain pigments.

• Electrons in photosystems II and I are excited and move via the ETC

• Water is split to replace ‘lost’ electrons for PS2

• NADPH and ATP are made

Calvin Cycle• AKA: Light Independent Reaction or Dark

Reaction

• Products made during the Light Reaction start the Dark Reaction (biochemical pathway)

• Dark Reaction takes place in the stroma

• Carbon Dioxide is used to make organic compounds (carbon fixation) by enzyme controlled reactions

• CO2 joins a 5-carbon sugar called RUBP making a 6-carbon sugar

• 6-carbon sugar splits into 2 3-carbon sugars PGA

• ATP and NADPH from the light reaction are used to convert PGA into PGAL

• PGAL is converted back into RUBP using more ATP and into organic molecules

http://youtu.be/o1I33Dgcc_M

Recap of Dark Rx

• ATP and NADPH from Light Reaction are used in the Dark Reaction

• RUBP combines with a CO2 to make 2 PGA molecules

• ATP then NADPH rearrange PGA into PGAL

• Some PGAL is used to make organic compounds; the rest to remake RUBP so cycle can continue

Alternative Pathway

• C3 plants use Calvin cycle to fix Carbon Dioxide

• C4 plants covert CO2 into a 4-carbon compound before going to calvin cycle (corn, sugar cane, crabgrass)

• CAM plants close stomata during day and open them at night fixing CO2 into organic compounds to use during the day in the calvin cycle (pineapple, cactuses)

Rate of Photosynthesis

• Rate at which a plant carries out photosynthesis is affected by the environment

• Light intensity

• Carbon Dioxide levels

• Temperature