Chapter 8 Photosynthesis Photosynthesis is the major pathway by which energy and carbon are incorporated into carbohydrates In photosynthesis water is oxidized to form oxygen and carbon dioxide is reduced to form carbohydrates Photosynthesis consists of two sets of reactions 1 the Calvin cycle in which carbon dioxide is reduced to form carbohydrates and 2 light harvesting reactions in which ATP and NADPH are generated to drive the Calvin cycle The evolution of photosynthesis involved horizontal gene transfer in bacteria and endosymbiosis in eukaryotes In eukaryotes photosynthesis takes place in chloroplasts the Calvin cycle occurs in the stroma and the light harvesting reactions take place on the thylakoid membrane The Calvin cycle is a three step process that results in the incorporation of carbon dioxide into carbohydrates The three steps of the Calvin cycles are 1 carboxylation 2 reduction and 3 regeneration The first step is the addition of CO2 to the 5 carbon sugar RuBP o This step is catalyzed by the enzyme rubisco considered the most o The resulting 6 carbon compound immediately breaks down into two abundant protein on earth 3 carbon compounds the second step is phosphorylation of the 3 carbon compounds by ATP followed by reduction by NADPH to produce 3 carbon triose phosphate molecules that are exported from the chloroplast into the cytosol where they are used to build larger sugars the third step is the regeneration of RuBP from five 3 carbon molecules starch formation provides chloroplasts with an osmotically inactive way of storing carbohydrates the light harvesting reactions use sunlight to produce ATP and NADPH required by the Calvin cycle visible light is absorbed by chlorophyll o the absorbed light energy can be released as heat reemitted as light fluorescence or transferred to an adjacent chlorophyll molecules o special chlorophyll molecules in the reaction center transfer both energy and electrons thus initiating the photosynthetic electron transport chain The electron transport chain consists of multisubunit protein complexes and diffusible compounds o Water is the electron donor and NADP is the final electron acceptor Antenna chlorophylls transfer absorbed light energy to the reaction center Reaction centers are located within pigment protein complexes The linear transport of electrons from water to NADPH requires the energy photosystems input of two photosystems Photosystem II pulls electrons from water resulting in the production of oxygen and protons on the lumen side of the membrane o Photosystem I passes electrons to NADP producing NADPH for use in the Calvin cycle The buildup of protons drives ATP synthase to produce ATP on the stroma side of the membrane where it is used by the Calvin cycle Photosynthesis faces several challenges to its efficiency An imbalance between the light harvesting reactions and the Calvin cycle can lead to the formation of reactive oxygen species Protection from excess light energy includes antioxidant molecules that neutralize reactive oxygen species and xanthophyll pigments that dissipate excess light energy as heat Rubisco can act catalytically on oxygen as well as on carbon dioxide o When it acts as an oxygenase there is a loss of energy and reduced carbon from the Calvin cycle Rubisco has evolved to favor carbon dioxide over oxygen but the cost of this selectivity is reduced speed The synthesis of carbohydrates via the Calvin cycle results in significant energy losses largely due to photorespiration The maximum theoretical efficiency of photosynthesis is approximately 4 of total incident solar energy Chapter 21 Evolution Genetic variation is the result of differences in DNA sequences Visible differences among members of a species phenotypic variation are the result of differences at the DNA level genetic variation as well as the influence of the environment Mutation a change in DNA sequence is the ultimate source of genetic variation o This variation is reconfigured by recombination Mutations can be somatic in body tissues or germ line in gametes o Germ line mutations are the only ones that can be passed on to the next generation when a mutation occurs in a gene it creates a new allele information on allele frequencies is key to understanding patterns of genetic variation an allele frequency is the number of occurrences of a particular allele divided by the total number of occurrences of all alleles of that gene in a population in the past population geneticists relied on observable traits determined by a single gene to measure genetic variation but this method is not generally useful as most traits are influenced by many genes and by the environment allele frequencies can be measured by protein gel electrophoresis DNA sequencing is now the standard technique for measuring allele frequencies Evolution is a change in the frequency of alleles or genotypes over time Allele frequencies change in response to selection migration mutation and genetic drift o Primary mechanisms of evolution Hardy Weinberg equilibrium describes situations in which allele frequencies do not change o Provides a null hypothesis agains which to test whether or not evolution is occurring in a population Hardy Weinberg equilibrium makes 5 assumptions o No selection o No migration o No mutation o No sampling error due to small population size o Random mating Hardy Weinberg equilibrium allows allele frequencies and genotype frequencies to be calculated from each other Natural selection leads to adaptation which enhances the fit between an organism and its environment Natural selection is the differential reproductive success of genetic variants Under natural selection a harmful allele is eliminated over time and a beneficial one increases in frequency o Natural selection does not affect the frequency of neutral mutations Natural selection can maintain alleles at intermediate frequencies by balancing selection Changes in phenotype show that natural selection can be stabilizing directional or disruptive In artificial selection a form of directional selection a breeder governs the selection process Sexual selection involves the evolution of traits that increase an individual s access to members of the opposite sex Migration mutation and genetic drift are non adaptive mechanisms of evolution populations sequences Migration involves the movement of alleles between populations gene flow and tends to have a homogenizing effect Mutation is the ultimate source of