Biology Homework Chapter 7: Photosynthesis - The Light Cycles
Textbook assignment: Chapter 7:Photosynthesis: Using Light to Make Food, sections 1-9.
- 7.1 Autotrophs are producers: they make their own food using energy from the environment. Autotrophes not limited to photosynthetic organisms. Some organisms can also synthesize their food from available inorganic chemicals. Nor are all photosynthetic organisms plants: cyanobacteria and chloroplast-containing protists can also harvest light energy.
- 7.2 Most photosynthesis occurs in the green pigment chlorophyll, found in the thylakoids membranes within stacks of grana in chloroplast organelles. Chloroplasts are found in mesophyll tissues of all green plants, and in some non-green parts of various eukaryotes, but not in prokaryotes. The arrangement of thylakoids as flat pancake-like structures stacked on top of each other increases membrane area, similar to the internal cristae of mitochondria.
- 7.3 The overall reaction equation for photosynthesis is the reverse of the the overall reaction equation for cellular respiration. Water is both a reactant and a product. The reactant water is split, giving up oxygen to the atmosphere. We can trace the movement of individual atoms from reactant molecules to product molecules by using radioactive isotopes.
- 7.4 Photosynthesis is an example of a oxidation-reduction reaction, in which electrons move from one element to another. Compare the reaction diagrams with the redox reaction diagrams for cellular respiration. The cellular respiration reaction released energy; running it in reverse as photosynthesis requires energy input.
- 7.5 Photosynthesis has two stages: the first harvests energy from light and stores it in ATP and NADPH. as we shall see later, the Calvin cycle collects the synergy to complete synthesis of sugars and starches, and releases in NADP+ and ADP. These can then be used again by the light cycle reactions. In a way, they function as "taxis" for the high energy electrons, carrying the electrons to locations for dark cycle reactions, and returning empty to the location of light cycle reactions to pick up new electron "passengers".
Just as glycolysis and the Krebs cycle take place in different parts of an animal cell, light and dark reactions take place in different parts of the plant cell. In the Calvin cycle, carbon is "fixed" or combined with other elements, which prevents it from reacting further.
- 7.6 Electrons can only change from one energy state to another by absorbing or emitting light carrying energy equal to the energy difference between the two states. The wavelength of a light wave is a function of this amount of energy — so the color of the light tells us what energy was emitted, and only particular colors can cause the electrons in a given atom or molecule (such as the pigments chlorophyll and beta-carotene) to change orbits. If this doesn't make sense yet, review chapter 2 and ASK QUESTIONS!
- 7.7 Because reactions involve electron jumps, they can be driven by light absorption of the right wavelength of light. Chemicals which absorb and emit light easily are called pigments; they are responsible for most of the colors we see. The part of the molecule that can absorb this (usually a single metal atom) is the reaction center. Plants are green because they absorb light in the blue and red spectrums but reflect green and yellow light.
- 7.8 Plants use two photosystems to harvest energy at slightly different wavelengths. In photosystem I (so-called because it was discovered and described first by Calvin's team), light energy plus electron energy is used to accelerate and deliver high-energy electrons to an NADP+ carrier molecule, which becomes NADPH. In photosystem II, light energy alone drives the production of water molecules plus high-energy electrons. The electrons are directed back through the membrane, where the energy harvested as they pass through the membrane is used to create ATP to drive photosystem I. The products of the two systems are water and ATP (photosystem II), and NADPH (photosystem I).
- 7.9 Electrons in the photosynthetic chemiosmosis reaction follow a similar path to those in the cellular respiration reaction. Energy harvested as the electrons complete the electron transport chain between photosystem II and photosystem I is used to push hydrogen ions from a low concentration to a higher concentration area within the thylakoid (an example of active transport!). The hydrogen ions "fall" down the concentration gradient through a protein complex called ATP synthase. This protein gate harvests the energy of the falling hydrogen ion, and stores it in the phosphate bond it creates between ADP and a phosphate ion, making ATP.
Read the following weblecture before chat: Photosynthesis
Take notes on any questions you have, and be prepared to discuss the lecture in chat.
Perform the study activity below:
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- No quiz yet: the Chapter Quiz opens when we finish the chapter.
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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