Chap 10 — Photosynthesis

Why are several different pigments used in photosystems? What is the main pigment used?

•Different pigments absorb light better at different wavelengths. the different pigments allow plants to obtain as much energy as possible by using multiple wavelengths of light •Chlorophyll is the main pigment used

What are the parts of a photosystem, and how do they function together to harvest light? Identify the first redox reaction of photosynthesis.

•Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent reactions . In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll; that energy is converted into stored chemical energy in the form of NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate). Protein complexes and pigment molecules work together to produce both NADPH and ATP •Photosystem I is referred to by the wavelength at which its reaction center best absorbs light, or P700; photosystem II is also known by this characteristic, or P680 •[H.] ions reduce the carbon in CO2

According to the absorption spectrum of chlorophyll, which wavelengths/colors of visible light does it absorb? Which wavelengths are reflected and transmitted?

•Chlorophyll a absorbs light in the blue-violet region, chlorophyll b absorbs red-blue light, and both a and b reflect green light (which is why chlorophyll appears green)

What two products are made by the light reactions for use in the Calvin Cycle?

• ATP and NADPH

Define the terms autotroph and heterotroph. Which are producers and which are consumers? How do they fit in the carbon cycle? Identify to which group each of the following organisms (from the lab course) belong: onion, Elodea, potatoes, yeast, cyanobacteria, Lactobacillus, Paramecium, Amoeba, Euglena, rotifers, and humans. In terms of large classes of organisms, which have at least some members that perform photosynthesis?

• Autotroph: Self-feeders; they sustain themselves without eating anything derived from other living beings • Heterotroph: get energy from compounds produced by other organisms

Explain what carbon fixation is. Why is it important for both animal and plant life?

• Carbon fixation is the conversion of carbon dioxide into organic compounds during photosynthesis. It mostly refers to the processes found in autotrophs, usually driven by photosynthesis, whereby carbon dioxide is changed into sugars • It is part of the process for food conversion

Name the three membranes and the three compartments found in a chloroplast. In which membrane are the enzymes of the light reactions embedded? In which compartment are the enzymes of the Calvin Cycle found? Indicate where in photosynthesis — the light reactions or the Calvin Cycle — each reactant is consumed and each product is produced.

• Chloroplasts are found in the cells of mesophyll, the interior tissue of the cell. The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast). Thylakoids may be stacked in columns called grana. Chloroplasts contain stroma, a dense interior fluid. • Light reactions occur in the thylakoids. • Calvin cycle occurs in the stroma. • ATP and NADPH

What are the three phases of the Calvin Cycle, and what does each accomplish?

• In phase 1 (Carbon Fixation), CO2 is incorporated into a five-carbon sugar named ribulose bisphosphate (RuBP). The enzyme which catalyzes this first step is RuBP carboxylase or rubisco. It is the most abundant protein in chloroplasts and probably the most abundant protein on Earth. The product of the reaction is a six-carbon intermediate which immediately splits in half to form two molecules of 3-phosphoglycerate. • In phase 2 ( Reduction), ATP and NADPH2 from the light reactions are used to convert 3-phosphoglycerate to glyceraldehyde 3-phosphate, the three-carbon carbohydrate precursor to glucose and other sugars. • In phase 3 (Regeneration), more ATP is used to convert some of the of the pool of glyceraldehyde 3-phosphate back to RuBP, the acceptor for CO2, thereby completing the cycle.

Which coenzyme carries electrons in photosynthesis? How is it similar to and different from NAD+? At what point in photosynthesis is the coenzyme reduced? Where is it oxidized?

• NADP+ • NADP+ is simply NAD+ with a third phosphate group attached • NADP+ - is reduced to NADPH in the light stage and the NADPH is oxidized to NADP+ in the Calvin Cycle.

Write the summary equation for the process of photosynthesis. Which reactant becomes oxidized and which becomes reduced. Explain how each reactant is imported into a leaf, and how each product is exported from the leaf.

• Photosynthesis: 6CO2 + 6H2O + energy --> C6H12O6 + 6O2

What is photorespiration, and what conditions lead to it? What are some of the ways different plant groups deal with such conditions?

• The process by which in the presence of light a plant consumes oxygen and releases carbon dioxide (instead of fixing carbon dioxide) during photosynthesis, resulting in a decrease in photosynthetic output since no ATP is produced and carbon (as well as nitrogen in the form of ammonia) is lost inevitably •When plants become water stressed, they close their stomata to prevent further water loss by transpiration. Water stress is most likely under hot, dry conditions. •Common adaptations are called C4 metabolism and CAM metabolism

What is the original source of electrons for the manufacture of sugar? What kind of energy excites these electrons?

• Water is split, providing a source of electrons and protons. The light reactions generate ATP and increase the potential energy of electrons by moving them from water to NADPH. The Calvin cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. The Calvin Cycle forms sugar from CO2. O2 is created when electrons are stripped away. Electrons are excited by ATP.

Describe the steps of the light reactions. Summarize the path energy takes from entering the light reactions as a photon to moving onward as energized electrons.

•During the light reactions, there are two possible routes for electron flow: cyclic and linear 1. A photon hits a pigment and its energy is passed among pigment molecules until it excites P680 2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+) 3. H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680 -P680+ is the strongest known biological oxidizing agent -O2 is released as a by-product of this reaction 4. Each electron "falls" down an electron transport chain from the primary electron acceptor of PS II toPS I 5. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane -Diffusion of H+ (protons) across the membrane drives ATP synthesis 6. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor 7. Each electron "falls"down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd). 8. The electrons then reduce NADP+ to NADPH, which is then available for the Calvin cycle - The energy changes of electrons during linear flow through the light reactions can be shown in a mechanical analogy

How many CO2 molecules are consumed in one cycle of the Calvin Cycle? How many CO2 must enter the cycle to produce one molecule of the G3P? What kind of compound is G3P? How many cycles would be required for a leaf cell to make one molecule of glucose from G3P?

•Each turn of the Calvin Cycle uses 1 CO2. • 3 CO2 (3 cycles) are needed for 1 G3P (glyceraldehyde 3-phosphate) • G3P is not a glucose, but a three-carbon sugar • For the net synthesis of one molecule of G3P, the cycle must take place three times, fixing three molecules of CO2