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PHOTOSYNTHESIS INTRODUCTION All photosynthetic organisms use a green coloured pigment called chlorophyll which absorbs light energy The two most common photosynthetic pigments are chlorophyll a and chlorophyll b Both pigments have a porphyrin ring attached to a long hydrocarbon tail The porphyrin ring contains a magnesium atom at the centre surrounded by a hydrocarbon ring with alternating double bonds. Delocalized electrons in the alternating double-single bonds absorbs light energy and start the photosynthetic process Chlorophyll molecule Figure 2 Chlorophyll molecules contain a porphyrin ring and long hydrocarbon tail. The hydrophobic tail anchors the molecule into a membrane. The porphyrin ring contains electrons that absorb light energy and begin the process of photosynthesis. Chlorophyll a has a methyl group (_CH3) at position _R in the porphyrin ring, whereas chlorophyll b contains an aldehyde group(_CHO) in that position Types of Photosynthetic Organisms 1) Prokaryotic Autotrophs: Cyanobacteria (Bluegreen algae) Evolved 2.5 to 3.4 BYA, very important because excess O2 put itno atmosphere and formed protective ozone layer Uses chlorophyll a and d found in folds of the cell membrane No cell walls, nucleus, or organelles 2) Eukaryotic Autotrophs: True Algae, Photosynthetic Protists, and Plants Have chlorophyll a, b, and / or c along with accessory pigments in membrane bound chloroplasts SUMMARY of PHOTOSYNTHETIC ORGANISMS Photosynthesis is carried out by plants, algae, photosynthetic protists, and cyanobacteria. • Electrons in the porphyrin ring of chlorophyll absorb light energy and begin the process of photosynthesis. • The endosymbiotic theory proposes that an ancestor of cyanobacteria was engulfed by an ancestor of today’s eukaryotic cell and gave rise to plant cells. • Algal and plant cells contain chloroplasts. They are most abundant in the mesophyll and guard cells of plant leaves. • The following is an overall equation for photosynthesis: chlorophyll 6CO2(g) _ 6H2O(l) _ light energy [CH2O](aq) _ 6O2(g) • Stomata are open in the daytime and closed at night. Light-activated proton pumps in guard cell membranes cause potassium ions to move from neighbouring epidermal cells into guard cells. As a result, water moves by osmosis into guard cells, causing them to swell. Increasing turgor pressure within guard cells causes their membranes to buckle and stomata to open. As the concentration of sucrose in guard cells decreases in the evening, water moves out of the cells and stomata close. • Chloroplasts have an outer membrane and an inner membrane. The interior space contains a semiliquid material called stroma with a system of membranebound sacs called thylakoids, some of which are stacked on top of one another to form grana. Thylakoid membranes contain chlorophyll molecules and electron transport chain. transport chains.