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Lecture 12

BIOL 112 Lecture 12: Lecture 12

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Biology (Sci)
BIOL 112
Frieder Schoeck

Lecture 12 The overall reaction of photosynthesis is the reverse of respiration, therefore must require the same input of energy that sugar oxidation releases. This energy comes from the sun. In both cases (respiration and photosynthesis), we pump protons out of the cell. In the case of respiration, the energy comes from NADH. In the case of photosynthesis, the energy comes from the light. In photosynthesis we use NADP instead of NAD. NAD and NADP can be interconverted. NAD is used to accelerate the oxidation of sugars, therefore the [NAD] is kept high. NADPH is used for reductions to make sugars, therefore the [NADPH] is kept high. All photosynthetic reactions occur in chloroplasts. The membranes inside the chloroplast are called thylakoid membranes. Enzymes sit in these membranes and capture light energy. These light reactions produce ATP and NADPH in the thylakoid membranes. the Calvin cycle uses up that ATP and NADPH to convert gaseous CO2 into solid sugar in the stroma. The Calvin cycle is the opposite of the citric acid cycle. Visible light are the only wavelengths (from 400 to 700 nm) where chemicals can absorb energy, which can be converted to chemical energy; absorption results in colors (the color is the left-over light that was not absorbed). Absorption of light energy is used in vision and in photosynthesis. To be precise, energy absorption is an electron transition to a higher energy level (excited electron), which can facilitate chemical reactions. Vision occurs when the light wavelength is from 400 to 700 nm and photosynthesis occurs when the light is on the same narrow range of wavelength. This is because, it is exactly the right energy to excite an electron and lift it to a higher energy orbital. Higher energy lower/shorter wavelength electromagnetic radiation (X-rays) have too much power so they damage the electron by ionization (electrons are kicked out completely). Lower energy higher/longer wavelength electromagnetic radiation (microwaves) do not have enough power to lift the electron, so it just makes it wiggle (vibrational energy (heat) only). Alternating/conjugated double bonds found in chlorophyll or beta carotene result in delocalized electrons particularly suitable to be excited by certain wavelengths of visible light. The absorption spectrum shows the wavelengths at which a pure solution of chlorophyll or any other pigment absorbs light. Each peak corresponds to the excited state of an electron. Chlorophyll a, chlorophyll b, and carotenoids capture most of the light energy. An electron gets excited at the peak of the curves. To excite chlorophyll the ideal wavelength is between 400 and 500 nm. They absorb the blue, so what we see is the green. That Is why leaves are green. The action spectrum shows the wavelengths at which plants produce the most oxygen (most photosynthesis is occurring). Oxygen is a waste product of photosynthesis, therefore a measure of the amount of photosynthesis. Oxygen production can be measured directly after shining light of different wavelengths on plants, or it can be measured indirectly with the help of oxygen-seeking bacteria (they will preferentially move to wavelengths where there is the most oxygen). The strong overlap of the absorption spectrum and the action spectrum was the first evidence implying chlorophyll in photosynthesis. the solar spectrum is the electromagnetic radiation that arrives on earth from the sun. Luckily for us, the solar spectrum largely corresponds to the visible wavelengths. Earth has exactly the right distance from the sun to allow the development of life; planets closer to the sun have more damaging short wavelength radiation and no liquid water; planets further away from the sun have more long wavelength radiation and also no liquid water. Plant leaves turn yellow and red, because chlorophyll is digested and shuttled back into the tree stem during the winter; carotenoids are left behind resulting in the change from green to yellow to red in some plant leaves. What happens with an excited electron? An excited electron has potential energy. When an electron is excited it has 3 options. 1. The most common case: it might fall back to the ground state. this releases energy as light and heat. This is not useful, because the energy is lost (dispersed as heat/light); 2 and 3 occur during photosynthesis. 2. Resonance energy transfer: physical process. When two molecules are close together and one of the chlorophyll electrons falls back to the ground state. That releases energy that will be immediately used to excite the neighboring chlorophyll molecule (only the energy is transferred, not the electron). 3. Transfer of electrons: chemical process. There is the transfer of an electron, a redox reaction. There are about 100 chlorophylls plus carotenoids in the light harvesting complex. They are all connected to a
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