Bio 178 Lecture 17Photosynthesis

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.

• HeterotrophsOrganisms unable to synthsize organic molecules from inorganic molecules - must feed on other organisms (energy derived from autotrophs).

• Efficiency of Oxidative MetabolismHeterotrophs 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 Photosynthesis6CO2 + 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 lightConclusions - 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

http://www.biologie.uni-hamburg.de/b-online/e24/24.htm

• 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.

Important Photosynthesis Experiments (Cntd.)• Light and Reducing Power1930s 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 H2

18O.

Results - 18O ends up in O2, not C6H12O2:

6CO2 + 6H218O C6H12O2 + 618O2

Conclusion - O2 comes from the splitting of H2O, not CO2.

Important Photosynthesis ExperimentsQuestion 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.)• PigmentsMolecules 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• StructureCarbon rings linked to chains with alternating single and double bonds.

http://cas.bellarmine.edu/chem117a/images/molecules/Lycopene.gif

• Function

Assist photosynthesis by absorbing not efficiently absorbed by chlorophylls.