Photosynthesis Using the sun to make useful forms of energy Sunlight plays a much larger role in our sustenance than we may expect: all the food we eat and all the fossil fuel we use is a product of photosynthesis, which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems

Photosynthesis 6 CO2 + 6H2O + light energy C6H12O6 + 6O2 Photosynthesis is the conversion of light energy into chemical energy by living organisms. How can sunlight be the source of energy for virtually every living thing? General Equation: chlorophyll 6 CO2 + 6H2O + light energy C6H12O6 + 6O2 Photosynthesis Song http://www.youtube.com/watch?v=PIvn7b4LbMc Why is chlorophyll above the arrow, instead of being part of the equation? Chlorophyll absorbs light energy and begins the process of photosynthesis.

Photosynthetic Organisms Plants, algae, some protists, and cyanobacteria all contain chlorophyll Chlorophyll Absorbs light energy and begins photosynthesis Chlorophyll a: blue-green Chlorophyll b: yellow-green Composed of porphyrin ring and long hydrocarbon tail All photosynthetic organism use chlorophyll a as the primary pigment Several types of chlorophyll are found in photosynthetic organisms Depends on wavelength of light they absorb – chlorophyll a absorbs best at blue and red and reflects at blue-green Chlorophyll B absorbs best at blue-green and orange, so it reflects a yellowish green Chlorophyll a is also used in commercial products as a dye – cleverly named natural green

Chlorophyll Chlorophyll a contains –CH3 at R Chlorophyll b contains –COH at R R Porphyrin contains magnesium ion surrounded by hydrocarbon ring with alternating single and double bonds. Delocalized electrons (meaning not associated with a single atom) absorb light energy and begin photosynthetic process Chlorophyll a contains –CH3 (methyl) at R Chlorophyll b contains –COH (aldehyde) at R Porphyrin ring Mg ion in centre, surrounded by hydrocarbon ring. This ring contains the electrons that absorb light energy Phytol Chain Hydrocarbon tail anchors the molecule to a membrane

Prokaryotic Autotrophs: Cyanobacteria Largest group of photosynthetic prokaryotes Unicellular, but grow in colonies Live in different environments: oceans, freshwater lakes, rivers, on rocks and soil Dense blooms produce toxins that pose environmental hazard First organisms to use sunlight in the production of organic compounds from water and CO2 Contain chlorophyll a to carry out photosynthesis and d : photosynthetic pigments called phycobilins Closely related to chloroplasts  endosymbiotic theory - Another example of photosynthetic organisms - These are the autotrophs formerly known as blue-green algae – kind of like the artist ‘prince’ Grow rapidly in nutrient rich water and know to create cyanobacterial blooms – which could be toxic Dense blooms usually occur in water that is rich in nitrates/phosphates usually caused by fertilizer run off from homes or farms Produced oxygen on a large scale, therefore paved the way for heterotrophs on Earth Endosymbiotic theory states that larger cells (probably ancestral to today’s eukaryotes) engulfed ancestors of cyanobacteria to create a mutually beneficial relationship – That is cyanobacteria were spared from the harsh environmental conditions and the host benefited from the food that these photosynthetic organelles produced QUESTION: What other organelle might be a product of endosymbiosis (discussed in cell resp)? – mitochondria Since they are prokaryotes – do not possess membrane bound organelles

Eukaryotic Autotrophs: Algae, Protists, Plants Contain chlorophyll within chloroplasts Chloroplasts give leaves, stems characteristic green colour Leaves are the primary photosynthetic organs of most plants To undergo photosynthesis, a plant cell must contain chlorophyll and obtain carbon dioxide, water and light energy from its environment - In order to photosynthesize, plant cells must contain chlorophyll, which is green pigment, therefore plants can only photosynthesize within the green structures- such as stems, and especially leaves

Leaves: The Photosynthetic Organs of Plants Waxy and water resistant, protection Allows light to pass to mesophyll Vascular bundle – transport water, minerals, carbohydrates Chloroplasts are abundant, location of most of photosynthesis Main function of leaves is photosynthesis Cross-section of a leaf Starting from the top QUESTION about cuticle: what would be the consequence of temperature rising past a given optimal temperature?  denaturing of enzymes The cuticle prevents against water loss and in addition protects the leaf from absorption of excessive light Chloroplasts are abundant in spongy and mesophyll layers – most photosynthesis occurs here. Guard cells are descriptively named as they guard or protect plants by opening or closing openings in the leaves in order to exchange gases and allow water vapour to escape, however if too much water is lost, the cells become flaccid and the stomata are closed QUESTION: Why would plants have evolved with stomata on the bottom of the leaves? – increase SA for absorption, prevent water loss from the elements like wind, beads of water can act like a magnifying glass and scorch the leaves. Vascular bundles are the same as veins – composed of xylem and phloem cells QUESTION what do each of those cells carry and where? Regulate exchange of CO2 and O2, allow water to escape by transpiration Create openings called stomata (sing. stoma)

Transpiration and Photosynthesis Transpiration is the evaporation of water from leaves creates a “transpiration pull” that helps to move water, minerals and other substances upward produces an evaporative cooling effect that prevents overheating Conditions that promote transpiration cause guard cells to reduce size of stomata Why would water need to move upward in a plant? Assists with transport of water, minerals and other substances from roots (where absorbed) to leaves (where used) in combination of protection from waxy cuticle, the process of transpiration allows for a cooling effect – literally letting off some steam WHAT are some conditions that would promote transpiration? – sunny, warm, dry, windy weather

Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells. http://wps.prenhall.com/esm_krogh_biology_3/0,8750,1135943-,00.html

Opening and Closing Stomata In addition to transpiration stomata allow CO2 to diffuse into air spaces within the leaf’s mesophyll layers Guard cells control the size of stomata Stomata would be opening, guard cells around

Open and Closed Stomata When K+ ions diffuse out of the guard cells, water also moves out by osmosis and the guard cells become flaccid and the stomata closes. Stomata are usually closed during the night. Terminal attachment Radial cellulose microfibrils When K+ ions diffuse into the guard cells, water also moves in by osmosis and the guard cells swell, opening the stomata. Stomata are usually open during the day. The size of the guard cell changes when water moves, by osmosis (low solute concn  high solute concn), into or out of the cell The direction of osmosis follows the diffusion of potassium ions across the guard cell membrane While potassium ions diffuse passively through membranes, their movement is coupled with the active transport of H+ions through membrane associated proton pumps therefore changes in size of stomata are dependent on the availability of ATP

Chloroplasts The Evolution of Organelles Photosynthesis factories of plants and algae Protein-rich semiliquid material in the interior of chloroplast Lamellae- unstacked thylakoids between grana Membrane bound, flattened sacs that stack to form granum (columns) Chlorophyll embedded in thylakoid membranes