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 (

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 CO 2.

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 6CO 2 + 6H 2 O + Light  C 6 H 12 O 6 + 6O 2 Sites of Photosynthesis in Plants Light Reactions - Across the thylakoid membrane. Calvin Cycle - Stroma.

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, [CO 2 ], & temperature on photosynthesis. Results - High light intensity  limited by temp & [CO 2 ] Low light intensity  limited by amount of light Conclusions - Initial set of light reactions that are largely temp & CO 2 independent and a second set of light independent reactions that are temp & CO 2 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.

Important Photosynthesis Experiments (Cntd.) Light and Reducing Power 1930s van Niel proposed that in plants H 2 O is the electron donor and the reducing power generated by the splitting of H 2 O is used to convert CO 2 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 H 2 18 O. Results - 18 O ends up in O 2, not C 6 H 12 O 2 : 6CO 2 + 6H 2 18 O  C 6 H 12 O O 2 Conclusion - O 2 comes from the splitting of H 2 O, not CO 2.

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 O 2. Later shown that electrons transferred from H 2 O to NADP +. Experiment 2 (Arnon et al., 1950s): In the presence of light but absence of CO 2, isolated chloroplasts accumulate ATP and NADPH. When CO 2 is added ATP and NADPH do not accumulate and the CO 2 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 O 2 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. ages/molecules/Lycopene.gif Function Assist photosynthesis by absorbing not efficiently absorbed by chlorophylls.