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Virtually all energy on earth comes from sunlight. Plants use energy from the sun to make the bonds which hold organic molecules together. When these bonds are broken the energy is ultimately transferred to ATP, which is then moved about cells and organisms to power their needs. I. Photosynthesis overview
II. Sunlight provides the energy A. Organisms depend upon photosynthesis 1. Photosynthesis uses sunlight as a source of energy to produce carbohydrates 2. Photosynthetic organisms (algae, plants and a few other organisms) serve as ultimate source of food for most life 3. Most food chains start with photosynthesizers 4. The early atmosphere of the earth lacked oxygen and it took about 3 billion years of photosynthesis to produce the current 21% oxygen atmosphere we now enjoy. This oxygen:
B. Solar Radiation 1. Solar radiation is described in terms of its energy content and its wavelength 2. Photons are discrete packets of radiant energy that travel in waves 3. The electromagnetic spectrum of solar radiation is based on wavelength a. Energy content of photons is inversely proportional to wavelength 4. Only 42% of solar radiation that hits earth's atmosphere reaches surface; most is visible light a. Higher energy wavelengths are screened out by ozone layer in upper atmosphere b. Lower energy wavelengths are screened out by water vapor and CO2 c. Consequently, both the organic molecules within organisms and processes, such as vision and photosynthesis, are adapted to radiation that is most prevalent in the environment 5. Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum a. Pigment = a substance that absorbs visible light b. Two major photosynthetic pigments are chlorophyll a and chlorophyll b c. Chlorophylls absorb violet, blue, and red wavelengths; they reflect green, this is why leaves appear green d. Carotenoids = yellow-orange pigments which absorb light in violet, blue, and green regions e. When pigments absorb light, electrons are boosted to a higher energy level and the energy is captured in a chemical bond III. Photosynthesis occurs in chloroplasts A. Chloroplasts have two parts: 1. A double membrane encloses a fluid-filled space called the stroma or ground substance 2. Thylakoids = flattened sacs organized into stacks called grana 3. Chlorophylls and other pigments involved in absorption of solar energy are embedded within thylakoid membranes; these pigments absorb solar energy C. Photosynthesis has two sets of reactions 1. Light-dependent reactions = the energy from the sun is captured in energy carrying molecules 2. Light-independent reactions = energy carrying molecules from the light-dependent reactions are used to make carbohydrates IV. Light-dependent reactions A. Occur in the thylakoid membranes and require participation of one or two light-gathering units called photosystems
F. The electrons are replaced by the splitting of water molecules G. The energy in the ATP and NADPH are used to power the light-independent reactions V. Light-independent reactions (also called carbon fixation or Calvin -Benson cycle) A. Takes place in the stroma of the chloroplast |