Download Biol 178 Lecture 17

Bio 178 Lecture 17
Photosynthesis
Copyright: J. Elson-Riggins
Reading
•
Chapters 9 & 10
Quiz Material
•
Questions on P 184 & 206
•
Chapter 9 & 10 Quizzes on Text Website
(www.mhhe.com/raven7)
Outline
• Respiration (Cntd.)
• Photosynthesis
 Summary
 Experiments
 Light
 Pigments
Regulation of Aerobic Respiration
Feedback inhibition regulates aerobic respiration
at committing points of glycolysis and the Krebs
cycle.
Glycolysis:
• Enzyme phosphofructokinase inhibited by high [ATP]
but activated by high [ADP] and low [citrate].
Krebs Cycle:
• Enzyme pyruvate decarboxylase inhibited by high
[NADH] and high [ATP].
• Enzyme citrate synthetase inhibited by high [ATP].
Regulation of Glucose Metabolism
Food Chains
• Autotrophs
“Self-feeders” - organisms able to synthesize organic
molecules (for food) from inorganic molecules.
• Heterotrophs
Organisms unable to synthsize organic molecules from
inorganic molecules - must feed on other organisms
(energy derived from autotrophs).
• Efficiency of Oxidative Metabolism
Heterotrophs using oxidative metabolism capture ~32% of
the energy consumed in autotrophs.
Food chain lengths are limited by the loss of 2/3 available
energy at each trophic level  Max 3 or 4 trophic levels.
Food Chain
Summary of Aerobic Respiration
Summary of Photosynthesis
• Energy Source
Sunlight. Only 1% of light energy reaching the earth is
captured by photosynthesis.
• Who Photosynthesizes?
Some bacteria, some protists, & plants.
• Photosynthetic Stages
(a) Energy capture.
(b) Use of captured energy to make ATP & NADPH.
(c) Use of ATP & NADPH to make glucose from
atmospheric CO2.
Summary of Photosynthesis (Cntd.)
• Is Light Required for the Entire Photosynthetic
Process?
Light Reactions - Processes (a) and (b) can only occur in
the presence of light.
Calvin Cycle - (c) can occur in the absence of light.
• Simplified Equation for Photosynthesis
6CO2 + 6H2O + Light  C6H12O6 + 6O2
• Sites of Photosynthesis in Plants
Light Reactions - Across the thylakoid membrane.
Calvin Cycle - Stroma.
Photosynthetic Machinery of Plants
Photosynthetic Machinery of Plants
Photosynthetic Machinery of Plants
Important Photosynthesis Experiments
• Soil, Water, and Light
Read about van Helmont, Priestly, & Ingenhousz.
• Light Independent Reactions - Blackman (1905)
Experiment - Tested the effects of light intensity, [CO2], &
temperature on photosynthesis.
Results - High light intensity  limited by temp & [CO2]
Low light intensity  limited by amount of light
Conclusions - Initial set of light reactions that are largely
temp & CO2 independent and a second set of light
independent reactions that are temp & CO2 dependent.
Blackman’s Results
• Light dependent reactions are independent of temperature, light independent
reactions are dependent on temperature.
• These results suggested that photosynthesis can be broken down into 2 stages.
http://www.biologie.uni-hamburg.de/b-online/e24/24.htm
Important Photosynthesis Experiments (Cntd.)
• Light and Reducing Power
1930s van Niel proposed that in plants H2O is the electron
donor and the reducing power generated by the splitting of
H2O is used to convert CO2 into organic matter (carbon
fixation).
Question 1 (1950s) – Overall Question – What is the
electron donor? (Part I - Where does the Oxygen come
from?)
Experiment - Give plants H218O.
Results - 18O ends up in O2, not C6H12O2:
6CO2 + 6H218O  C6H12O2 + 618O2
Conclusion - O2 comes from the splitting of H2O, not CO2.
Important Photosynthesis Experiments
Question 2 - What is the source of reducing power?
Experiment 1 (Hill, 1950s):
In the presence of light isolated chloroplasts can reduce a
dye and release O2.
Later shown that electrons transferred from H2O to
NADP+.
Experiment 2 (Arnon et al., 1950s):
In the presence of light but absence of CO2, isolated
chloroplasts accumulate ATP and NADPH. When CO2 is
added ATP and NADPH do not accumulate and the CO2 is
assimilated into organic molecules.
Light
• The Electromagnetic Spectrum
• Photons
Small packets of light energy.
How much light energy is contained in a photon?
Dependent on  (inversely proportional to ).
Absorption Spectra and Pigments
• Absorption Spectrum
(a) What happens when the energy of a photon is
absorbed by a molecule?
Electrons are boosted to a higher energy level.
(b) Which photons can be absorbed?
An electron can only be boosted to a higher energy level if
it absorbs exactly the right amount of energy - only
photons with the “correct”  will be absorbed by a specific
atom.
 Each molecule will have a characteristic absorption
spectrum.
Absorption Spectrum of Chlorophyll & Carotenoids
Absorption Spectra and Pigments (Cntd.)
• Pigments
Molecules that absorb specific wavelengths of light.
• Plant Pigments
(a) Chlorophyll
Chlorophyll a: Absorbs violet-blue & red light.
The principle photosynthetic pigment.
Directly converts light energy to chemical
energy.
Chlorophyll b: Absorption spectra shifted slightly towards
the green of that of a.
Accessory pigment.
Chlorophylls reflect green light.
Pigments (Cntd.)
(b) Carotenoids
Absorb mainly in the blue and green  and reflect orange
and yellow .
Accessory Pigments.
Structure of Chlorophyll
Chlorophyll
• Structure
Porphyrin head with a central magnesium atom attached
to a hydrocarbon tail.
• Action Spectrum (Englemann, 1882)
Experiment - Used a prism to direct different  onto
Spirogyra. Tested rate of O2 production with bacteria.
Results - Bacteria accumulated around the parts of the
alga illuminated by red & violet light.
Conclusion - Chlorophyll is responsible for photosynthesis.
Englemann’s Experiment - Action Spectrum
Carotenoids
• Structure
Carbon rings linked to chains with alternating single and
double bonds.
http://cas.bellarmine.edu/chem117a/im
ages/molecules/Lycopene.gif
• Function
Assist photosynthesis by absorbing  not efficiently
absorbed by chlorophylls.