Chapter 10
Photosynthesis
Photosynthesis
• Process by which plants use light energy to
make food.
• A reduction process that makes complex
organic molecules from simple molecules.
Ps General Equation
6 CO2 + 6 H2O ---> C6H12O6 + 6 O2
Requires:
Chlorophyll
Light
Early Question
• 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: a redox process
• Hydrogens are added to Carbons.
• Water is a source for the Hydrogens.
• Complex covalent bonds are made.
Ps
• Has two chemical reactions:
1. Light Reaction
2. Calvin Cycle
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.
Comment
• In oceans, red light is lost or filtered out early
because it has lower energy.
• Only blue light which has higher energy can
reach the lower depths.
Result
• Many deep ocean fish are bright red in color.
• Why?
• They can’t be seen because there is no red
light to reflect their color.
Plants without chlorophyll
Photosynthesis Pigments
1. Chlorophylls
2. Accessory Pigments
Chlorophylls
• Has CHON and Mg.
• Several types possible.
• Molecule has a lipophilic tail that allows it
to dissolve into 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
• Double outer membrane.
• Inner membrane folded and stacked into
grana.
• 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
Photophsphorylation
• 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
Light Reaction
• Same thing as Noncyclic Photophsphorylation.
• Location - grana of the chloroplast.
• Function - to split water and produce ATP and
NADPH.
Light Reaction
Requirements
• Light
• Water
• ADP + Pi
• NADP+
Products
• O2
• ATP
• NADPH
Chemiosmosis Model
• The chloroplast produces ATP in the same
manner as the mitochondria in Rs.
• Light energy is used to pump H+ across a
membrane.
• When the H+ diffuses back, ATP is
generated.
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.
Calvin Cycle
• How plants actually makes food
(carbohydrates).
• Don't require light directly to run.
• Also known as C3 Ps.
Calvin Cycle
• Function - to use ATP and NADPH 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.
• Most abundant enzyme on earth.
Dark Reactions/Calvin Cycle
Requirements
• 6 CO2
• 18 ATP
• 12 NADPH
Products
• C6H12O6
• 18 ADP + 18 Pi
• 12 NADP+
Photorespiration
• When Rubisco accepts O2 instead of CO2 as
the substrate.
• Generates no ATP.
• Decreases Ps output by as much as 50%.
Photorespiration
• May reflect a time when O2 was less plentiful
and CO2 was more common.
Alternate Ps Methods
1. C4 Ps
2. CAM Ps
C4 Ps
• Uses a different enzyme to initially capture
CO2
• Separates CO2 capture from carbon fixation
into sugar.
• 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.
• Examples - sugarcane, Bermuda grass, crab
grass
C3 Ps
•
•
•
•
•
vs.
Photorespiration
Shade to full sun
High water use
Cool temperatures
Slow to moderate
growth rates
• Cool season crops
C4 Ps
•
•
•
•
•
•
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
• Tissues decrease in pH over night, rise in pH
during day.
• Avoid H2O stress by keeping stomates closed
during the day.
• Generally have slow growth.
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 survives.
3. Ps < Rs, death by starvation
Comments - Ps:Rs
• Rs happens 24 hours a day.
• Ps only in light.
• Plants overwinter on stored food made when
Ps > Rs.
• If Ps < Rs, best solution is to increase the
amount of light.
Factors That Affect Ps
1.
2.
3.
4.
Light - quantity and quality.
Temperature - too hot or too cold.
CO2 - often limits C3 plants.
Minerals - especially NPK 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 Calvin Cycle.
Alternate Ps forms.
Ps:Rs ratios.
Practice FRQ Question (long version)
• Explain how the molecular reactions of
photosynthesis transform light energy
into chemical bond energy. Include in
your discussion the relationship between
chloroplast structure and light reactions
and Calvin cycle.
U-Tube Links
• Light Reaction:
• http://www.youtube.com/watch?v=hj_WK
gnL6MI