Name _______________________
BIOLOGY
CHAPTER 6 NOTES PART 1
PHOTOSYNTHESIS
PHOTOSYNTHESIS- process by which light energy is transferred to chemical energy
Autotrophs- Most use photosynthesis
Heterotrophs- Must eat autotrophs or other hererotrophs
Ex: A caterpillar (heterotroph) eats grass (autotroph); a bird (heterotroph) eats the caterpillar
Photosynthesis is an example of a BIOCHEMICAL PATHWAY- a complex set of reactions in which
the product of one reaction is consumed (used) in the next reaction
-
- during this reaction, O2 is released
While only autotrophs perform photosynthesis, both autotrophs and heterotrophs perform a
process called CELLULAR RESPIRATION
- CO2 and water are given off as wastes
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LIGHT ABSORPTION AND CHLOROPLASTS
LIGHT REACTIONS- initial reactions in photosynthesis
- a cell may have 50 or more chloroplasts
CHLOROPLAST STRUCTURE
2 Membranes: outer and inner
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- GRANA- stacks of thylakoids
- STROMA- solution surrounding thylakoids
DRAW A CHLOROPLAST!!!
LIGHT
Light from the sun appears white, but is made up of a variety of colors
- the resulting colors ranging from red at one end to violet at the other is called the ____________
____________________________
- Light travels though space as waves of energy, similar to waves that travel through a body of
water
-waves are measured in WAVELENGTHS- the distance between crests in a wave
PIGMENT- a compound that absorbs light
- most pigments absorb some colors more strongly than others
-
- the light that is reflected or transmitted by the pigment no longer appears white
Ex: A shirt reflects or transmits the color yellow and absorbs all other colors
WHAT COLOR IS THE SHIRT?
INTRO. TO PHOTOSYNTHESIS
Photosynthesis can be divided into 3 sets of reactions:
1.
2. Light reactions (light-dependent reactions)
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3.
PIGMENTS IN CHLOROPLASTS
The most important pigments in chloroplasts are called CHLOROPHYLLS
- these 2 types absorb different colors of light
CHLOROPHYLL aCHLOROPHYLL b- absorbs mostly blue light
- neither absorbs much green light; green light is reflected or transmitted
ONLY chlorophyll a is directly involved in the light reactions
-
Other accessory pigments are:
THESE ARE OFTEN COLLECTIVELY CALLED CAROTENOIDS
- carotenoids absorb colors that chlorophyll a cannot, and enables plants to capture more of the
energy in light
-
LIGHT REACTIONS/LIGHT DEPENDENT REACTIONS & LIGHT ABSORPTION
BY CHLOROPHYLL
Chlorophyll and other carotenoids form what are called antenna complexes
-
- these reaction centers consist of a pair of chlorophyll a molecules
- Excited electrons are transferred to electron acceptors, leaving the reaction center in an oxidized
state
The antenna complex along with the chlorophyll a molecules in the reaction center make up a
________________________________
There are 2 types: photosystem I and photosystem II
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Light reactions begin when accessory pigment molecules in both photosystems absorb light
Through both photosystems, electrons are transferred through 2 electron transport chains to an
electron carrier called NADP+
-
- This NADPH is needed for the dark reactions to occur
Water contributes the electrons to keep the light reactions going and to continue making NADPH:
THIS IS WHY PLANTS NEED WATER!
Notice the O2 given off
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CHEMIOSMOSIS
As electrons pass down along one of the electron transport chains during the light reactions,
protons (H+) are pumped inside of the thylakoid
- this process is called CHEMIOSMOSIS
-
SUMMARY OF LIGHT REACTIONS
- Water keeps the light reactions going by replenishing electrons and giving off O 2 as a by- product
- ATP is made by chemiosmosis in the light reactions and is also needed in the dark reactions
DARK REACTIONS/LIGHT INDEPENDENT REACTIONS
1. The dark reactions do not need light to occur, however, they need the products of the light
reactions (ATP and NADPH)
2. The dark reactions occur in the STROMA
- What is the stroma again?
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3.
4. Carbon fixation is by the Calvin Cycle (dark reactions; also called C3 cycle)
- named after American scientist Melvin Calvin
STEPS OF CALVIN CYCLE
STEP 1
CO2 diffuses into the stroma and it is bonded to a 5-carbon compound called _______ to
form a 6-carbon compound.
STEP 2
The 6-carbon compounds immediately split to form 2 3-carbon molecules of a compound
called _________.
STEP 3
Next each PGA compound is converted to another 3-carbon molecule called glyceraldehyde
3-phosphate (G3P) in a 2-part process:
1.
2. The resulting compound then receives a proton (H+) from NADPH and releases a phosphate
group, producing G3P
NOTE:
STEP 4
One of the G3P molecules leaves the Calvin cycle and is used to make organic compounds
(carbohydrates) in which energy is stored for later use
STEP 5
The remaining G3P molecules are converted back to RuBP through the addition of phosphate
groups from ATP molecules
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**SEE CALVIN CYCLE DIAGRAM ON THE LAST PAGE OF THIS PACKET
OTHER CARBON-FIXING PATHWAYS
- The Calvin cycle is the most common method of carbon fixation
-
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- Most water loss in plants occurs through small pores called STOMATA, found on the underside of
leaves
- In hot, dry conditions, plants can partially close their stomata and reduce loss of water
- These conditions cause the Calvin Cycle to not operate properly
C4 PATHWAY
In the C4 pathway, CO2 is bound to an intermediate compound before it enters the Calvin Cycle
- Plants that use this pathway are called C4 plants
- The 4-carbon compound then undergoes a series of reactions until the CO 2 is finally transferred
to the RuBP
ADVANTAGES OF C4:
- Allows plants to fix carbon 4 times faster and thus grow faster in hot dry conditions
DISADVANTAGES OF C4:
- Requires more energy, but these plants grow where there is always abundant sunlight
Ex:
CAM PATHWAY
In the CAM pathway, CO2 is brought in at night, when conditions are cooler
- stomata are closed during the day to prevent water loss
- the CO2 taken in at night is stored in a compound called __________________________ (giving
it the name Crassulacean Acid Metabolism)
- the acid then releases the CO2 into the stroma of the leaves during the day where it then bonds
with RuBP and enters the Calvin Cycle.
ADVANTAGES OF CAM:
DISADVANTAGES OF CAM
Ex:
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SUMMARY OF PHOTOSYNTHESIS
Balanced equation for photosynthesis:
- the 6 CO2 are fixed during the dark reactions into the resulting carbohydrates
-
- the C6H12O6 is a product, which can be used directly by the plant or used to make more advanced
carbohydrates
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