3.1 – Photosynthetic Organisms PHOTOSYNTHESIS - Summary Chemical Equation

1. 3.1 – Photosynthetic Organisms PHOTOSYNTHESIS - Summary Chemical Equation CHLOROPHYLL (Fig. 2, P. 139) chloropyll 6CO2 + 6H2O + Light Energy  C6H12O6 + 6H2O + 6O2 • all photosynthetic organisms use one of the several forms of the pigment chlorophyll to absorb light and begin the process of photosynthesis • the chlorophyll molecule consists of a light-absorbingporphyrin ring and a hydrophobic, membrane-anchoringhydrocarbon tail • the porphyrin ring has a central Mg atom surrounded by a hydrocarbon ring with alternating single and double bonds in which delocalized electrons absorb light energy • the functional (R) group in the porphyrin ring distinguishes forms of chlorophyll from each other (i.e. a and b) and determines what wavelength of light can be absorbed Prokaryotic Autotrophs: Cyanobacteriacyanobacteria (blue-green algae) “bloom” in nutrient-enriched waters, form symbiotic relationships with fungi (lichens)

2. Eukaryotic Autotrophs: Photosynthetic Protists, Algae, Plants - eukaryotes have membrane-bound organelles including DNA in a porous nuclear membrane • much larger than prokaryotes with several chromosomes compared with one ring-shaped chromosome plus plasmid •  protists are all eukaryotic but polyphyletic (arise from many diverse origins) • arose 1.5 BYA as specialized cells became incorporated into prokaryotes and became organelles • ALGAE (obligate autotrophs – classified by photosynthetic pigment within chloroplasts) • Diatoms (Chrysophyta) - xanthophyll masks chlorophyll a,c •  10 000 species; unicellular golden algae (“plant in a glass box”) •  Earth’s most important producers (phytoplankton in all oceans) • Dinoflagellates (Pyrrophyta) - chlorophyll a,c – 1100 species •  unicellular, cause of “red tide”; Earth’s second most important producer • Green Algae (Chlorophyta) - chlorophyll a,b – 7000 species •  precursors to land plants as cellulosic cell walls, similar chloroplasts, biflagellate gametes resemble those in primitive plants • Brown Algae (Phaeophyta) - fucoxanthin masks chlorophyll a,b – 1500 species; vegetatively complex (holdfast, stipe, blade) •  commonly known as “kelps”, can grow up to 60 m long along west coast of NA • Red Algae (Rhodopyta) - carotenes mask chlorophyll – 4000 species •  most are marine, often calcified; complex filamentous forms •  true plants have multicellular embryos surrounded by protective tissue as compared to an unprotected single zygote in algae; true plants have vascular tissue and more complex, specialized organs (roots, stems, leaves)

3. Leaves • - are the major sites of photosynthesis in plants • - usually have a blade in which vascular bundles and support tissue branch into veins which - are parallel in monocots and netlike in dicots • - can be simple (continuous, undivided blade and margin) such as maple, oak, and poplar, or compound (blade is divided into two or more leaflets) such as locust • Leaf Structure • - one-cell layer of epidermis coated with waxy cuticle covers the upper and lower surfaces of the leaf blade • - most of the interior cells are mesophyll which contain many chloroplasts (the site of photosynthesis) • - one or two layers of tightly-packed palisade mesophyll under the upper epidermis gives way to loosely-packed spongy mesophyll through which the veins run, carrying water and nutrients to and products of photosynthesis away from the leaves • large air spaces allow for efficient gas (CO2 , O2) and water vapour exchange between the leaf and the external environment via stomata (pores) on the lower epidermis •  stomata control transpiration (loss of water from the leaf) by opening and closing of guard cells as they absorb or lose water •  water lost through transpiration is replaced by water pulled up through xylem from the roots

4. Opening and Closing Stomata • the direction of osmosis follows the diffusion of K+ ions in and out of guard cells surrounding stomata • diffusion of K+ ions is coupled with the active transport of H+ ions through membrane-associated proton pumps, requiring energy from ATP • thicker inner walls of guard cells cause them to buckle outward as they become turgid, increasing the size of the stoma • when K+ ions move out, water follows by osmosis, the guard cells become flaccid and the stoma closes • generally, stomata are open during the day and closed at night • light activates blue-light receptors in guard cells, stimulating proton pumps as protons move out, the resulting electrochemical gradient causes the diffusion of K+ ions in, followed by osmosis • decrease in CO2 around guard cells as photosynthesis begins in surrounding mesophyll also results in osmosis into guard cells • decrease of sucrose concentration in guard cells results in diffusion of K+ ions out of cells followed by osmosis and closing of stomata • CHLOROPLASTS (Fig. 13, P. 144) • photosynthetic organelles in eukaryotic autotrophs with their own DNA • outer and inner membranes enclose a semi-liquid stroma within which 30-50 membrane-bound thylakoids are stacked into about 60 grana joined by lamellae (unstacked thylakoids) • photosynthesis occurs in the stroma and within the thylakoid membrane which contains chlorophyll and electron transport chains • Section 3.1 Questions – P. 145, #2, 3, 5, 6, 7