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Chapter 10 Photosynthesis
Photosynthesis
Process by which plants use light energy to make food (not energy).
Ps General Equation
6 CO2 + 6 H2O ---> C6H12O6 + 6 O2
Requires:
Chlorophyll
Light
Early Question(Just listen)
Does the Oxygen in sugar come from the CO2 or from the H2O ?
Model:
CO2 + 2 H2O ----> CH2O + O2
Proof Used 18O as a tracer.
CO2 + 2 H2O ----> CH2O + O2
CO2 + 2 H2O ----> CH2O + O2
Both experiments confirm that water is split.
O2 is a waste product of Ps that altered life on earth.
Ps
Has two chemical reactions:
1. Light Reaction
2. Dark Reaction
Names are from “light” as a requirement, not where or when they occur.
Light
A form of electromagnetic radiation.
Visible light has the right energy for use in Ps.
Too Hot Too Cold
Just Right
Action Spectrum
Not all colors are useable to the same degree for Ps.
Red and Blue light - absorbed and used in Ps.
Green light - reflected or transmitted.
Photosynthesis Pigments
1. Chlorophylls
2. Accessory Pigments
Chlorophylls Has CHON and Mg. Several types possible. Molecule has a lipophilic tail-
can go through membranes Contains Mg in a reaction
center.
Accessory Pigments
Absorb light energy and transfer it to chlorophyll.
Ex: Carotene (orange). Xanthophyll (yellow)
Fall Leaf Colors Chlorophyll breaks down. N and Mg salvaged and
moved into the stem for next year.
Accessory pigments remain behind, giving the various fall leaf colors.
Chloroplast Structure(Annotate your diagram)
Double outer membrane. Inner membrane folded and
stacked into grana. (Each disk is a thylakoid)
Stroma - liquid that surrounds the thylakoid membranes.
Photosystems
Collection of pigments that serve as a light trap.
Made of chlorophyll and the accessory pigments.
Two types known: PSI, PSII
Cyclic Photophosphorylation
Uses PSI only. Produces ATP. Requires light.
Linear or Noncyclic Photophosphorylation
Uses PSI and PSII. Splits water, releasing H+,
a pair of e-, and O2. Produces ATP and NADPH.
(e- carrier similar to NADH)
Light Reaction
Same thing as Noncyclic Photophosphorylation.
Location - grana of the chloroplast.
Function - to split water and produce ATP and NADPH.
Light Reaction
Light Water ADP NADP+
O2
ATP NADPH
Requirements Products
Chemiosmosis Model Light energy is used to pump
H+ across a membrane. When the H+ diffuses back,
ATP is generated. The chloroplast produces ATP
in the same manner as the mitochondria in Rs.
Chemiosmosis
H+ are pumped into the thylakoid space.
ATP and NADPH are made when the H+ diffuse into the stroma.
Comment
There can be a 3 pH unit difference between the thylakoid space and the stroma.
Dark Reactions How plants actually make food
(carbohydrates). Don't require light directly to
run. Also known as the
Calvin cycle or C3 Ps.
Dark Reactions
Function - to use ATP and NADPH (from light rxn) to build food from CO2
Location - stroma of the chloroplast.
Rubisco
Ribulose BisPhosphate Carboxylase.
Enzyme that adds CO2 to an acceptor molecule.
Most important enzyme on earth.
C3 Ps
6 CO2
18 ATP 12 NADPH
C6H12O6
18 ADP 12 NADP+
Requirements Products
END of PART 1!!
Photorespiration
When Rubisco accepts O2
instead of CO2 as the substrate.
Generates no ATP. Decreases Ps output by as
much as 50%.
Photorespiration
Happens when conditions are too dry that stomata close
May reflect a time when O2 was less plentiful and CO2 was more common.
Stomata
Holes in the plant leaf that allow for gas exchange
Close them to prevent water loss
Alternate Ps Methods
1. C4 Ps
2. CAM Ps
C4 Ps Uses a different enzyme to
initially capture CO2
Still uses C3 Ps to make sugar, but only does so in the bundle sheath cells.
PEP Carboxylase
Enzyme used for CO2 capture in C4 Ps.
Can use CO2 down to 0 ppm. Prevents photorespiration.
C4 Ps
Found in 19 plant families. Characteristic of hot regions
with intense sunlight, but plenty of water
Examples - sugarcane, Bermuda grass, crab grass
C3 Ps vs C4 Ps
Photorespiration
Shade to full sun
High water use
Cool temperatures
Slow to moderate growth rates
Cool season crops
No Photorespiration
Full sun only
Moderate water use
Warm temperatures
Very fast growth rates
Warm season crops
CAM Ps Crassulacean Acid
Metabolism Found in plants from arid
conditions where water stress is a problem.
Examples - cacti, succulents, pineapples, many orchids.
CAM Ps Open stomata at night to take
in CO2.
The CO2 is stored as a C4 acid. During the day, the acid is
broken down and CO2 is fixed into sugar.
CAM plants
Avoid H2O stress by keeping stomates closed during the day.
Generally have slow growth.
Ps to Rs ratios
Predict what will happen Ps > Rs Ps = Rs Ps < Rs
Ps:Rs Ratios Reflect a plant’s balance in
making food and using food.1. Ps > Rs, energy available
for growth and reproduction.2. Ps = Rs, no growth, but don’t
die either.3. Ps < Rs, death by starvation
Comments - Ps:Rs
Rs happens 24 hours a day. Ps only in light. If Ps < Rs, best solution is to
increase the amount of light.
Factors That Affect Ps
1. Light - quantity and quality.
2. Temperature - too hot or too cold.
3. CO2 - often limits C3 plants.
4. Minerals - especially N P K and Mg.
Importances of Ps
1. Food - either directly or indirectly comes from plants.
2. Oxygen in the air.
3. CO2 balance.
4. Plant products.
5. Life on Earth.
Summary
Know the main Ps equation. Know Light Reaction. Know Dark Reaction. Alternate Ps forms. Ps:Rs ratios.