Honors Biology

 What is Photosynthesis?
 6 CO
2
+ 6 H
2
O → C
6
H
12
O
6
+ 6 O
2

 light energy
Photosynthesis is the process of converting light energy into food energy (glucose, chemical energy stored in chemical bonds).
Photosynthesis carried out by plants and algae.

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Cross-section of a Leaf:
6 CO
2
+ 6 H
2
O → C
6
H
12
O
6
+ 6 O
2 light energy

 Cross-section of a Leaf:
 Palisade Mesophyll
 Spongy Mesophyll
 Guard cell
 Stomata
 Epidermis
 Cuticle
 Vein

What is Solar Energy and Visible Light?
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Visible Light is made of photons that travel at various wavelengths and frequencies.
Photons (packets of energy) are formed when an electron is energized and moves to a higher energy level, which is unstable. When the electron falls back to its original energy level, then a photon is released that travels at a particular wavelength.
The longer the wavelength, the lower the energy. The shorter the wavelength, the higher the energy.

What is Solar Energy and Visible Light?
 Visible light is only one form of energy in the
Electromagnetic Spectrum.
 Different colors of light have different wavelengths.
 Which color of light contains the most energy?
Least energy?

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Each plant cell contains 40-50 chloroplasts.
Chloroplasts contain pigments that either absorb or reflect different wavelengths of visible light.
Absorbed wavelengths are used for photosynthesis.
Each thylakoid membrane contains 250-400 chlorophyll pigments

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Reaction Center Pigment = Chlorophyll a
Antennae/Accessory Pigments = Chlorophyll b, carotenoids, phycobilins (red algae)
What is the absorption spectra for each pigment?
(spectrophotometer)
Chlorophyll a and b – red, orange, yellow, blue, and violet
(reflects green)
Carotenoids – violet, blue, and green (reflects red, orange, and yellow)
Phycobilins – violet, blue, green, yellow, and orange (reflects red)

Photosynthesis is Composed of 2 Reactions:
 What are the two main reactions of Photosynthesis? Reactants?
Products? Where located?
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
Light-Dependent reactions = Energy capturing reactions, captures solar energy and traps it as ATP and NADPH. Occurs in
Thylakoid membranes.
Light-Independent reactions = Synthesis reactions, uses the ATP and NADPH and captured CO
Stroma.
2 to make glucose. Occurs in the

What factors affect the rate of photosynthesis?
 Amount of Visible light and CO
2
:
 Amount of H
2
O, Minerals, Temperature:

1. Photosystem II absorbs light energy and transfers an excited electron to the Primary electron acceptor.

2. Primary Electron Acceptor molecule transfers high energy electrons to the Electron Transport System, which establishes a
H + ion gradient across the thylakoid membrane.

3. Energy from the Electron Transport Chain establishing a H + ion gradient is used for Chemiosmotic Phosphorylation production of ATP. (Photophosphorylation)

4. The electron from the Electron Transport
System is accepted by the reaction center chlorophyll of Photosystem I.
5. The Electron Acceptor molecule of Photosystem
I sends its electron to ultimately form NADPH.

Where does the light reaction take place?

At the thylakoid membrane
What are the inputs to the light reactions?
 Light energy, water
What are the products of the light reaction?
 ATP, NADPH, and O
2

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Synthesis reactions that occur in the stroma
CO
2

+ ATP + NADPH → Glucose
Carbon dioxide – comes into the leaf via stomata
 ATP and NADPH – from the Light reactions
No light required but typically occurs during the day

1. Carbon Dioxide
Fixation
2. Carbon Dioxide
Reduction
3. Regeneration of
RuBP

 What are the two reactions of photosynthesis?
 How are the light reactions connected to the
Calvin cycle?

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C
3
Plants
C
4
Plants
CAM Plants
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The pathways of photosynthesis discussed in this presentation are for C
3 plants.
Examples: wheat, rice, oats
 Optimum temperature = 15-25°C

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4
Mesophyll cells fix CO
2 during the day.
into a C
4 molecule (oxaloacetate)

Uses PEPCase enzyme instead of Rubisco to fix CO
2
C
4 molecule pumped to Bundle sheath cells where Calvin
Cycle occurs.
Examples: sugar cane, corn, bermuda grass, crabgrass
Net photosynthetic rate is greater than C
3 plants.
More heat tolerant, optimum temperature = 30-40°C

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Like C
3 plants, Mesophyll cells fix CO
2 in the Calvin Cycle, but it occurs at night. Stomata only open at night to bring in
CO
2 and reduce water loss.
Like C
4 plants, CO
2 is fixed into a C
4 molecule and then stored in large vacuoles in mesophyll cells to be used the following day when ATP and NADPH are available from the light reactions.
 Uses PEPCase enzyme instead of Rubisco to fix CO
2
Examples: succulents like cacti that live in deserts (CAM =
Crassulacean-acid metabolism)
Low photosynthetic rate, variable
Most heat tolerant, optimum temperature = 35°C

3
4
 What are the differences between C
3
, C
4
, and
CAM plants?

 http://www.emc.maricopa.edu/faculty/farabee/
BIOBK/BioBookPS.html