Unit 4 Notes Photosynthesis

Converts light energy into food energy 6CO H 2 O light, enzymes, chlorophyll C 6 H 12 O 6 + 6O 2 + 6H 2 O

Photosynthesis  Two types of autotrophic organisms  Photosynthetic—make food from sun’s energy  Chemosynthetic—make food from other inorganic molecules  2 Steps to Photosynthesis  Light dependent reactions—need light and chlorophyll to make ATP and NADPH  Light independent reactions—take ATP and NADPH + CO 2 to make glucose (or other sugars)

Plant Structure Leaf Structure

Plant Structure  Leaf Parts  Cuticle—waxy—provides protection from insects, environmental damage, etc., and keeps water from getting out  Epidermis—defense/barrier—reduces water loss  Palisade Mesophyll—structure (lots of chloroplasts)  Spongy Mesophyll—air flow (lots of chloroplasts)  Xylem—vascular tissue that moves water  Phloem—vascular tissue that moves sugars  Stomata—openings on bottom of leaf—lets CO 2 in and O 2 and H 2 O out

Plant Structure  Leaf Parts (continued)  Guard Cells—cells around the opening of the stomata—open when water flows in by osmosis  Open  Triggers = Light / Decrease CO 2 / Internal Clock  K + enters cell  water potential decreases  osmosis in  cells become turgid  stomata opens  Close  Triggers = No Light / Increased Temperature / Lose H 2 O / Increase CO 2  K + leaves  osmosis out  cells become flaccid  stomata close

Plant Structure

 Chloroplasts  Site of Photosynthesis  The membranes (thylakoids) within the chloroplast give it the ability to build up concentration gradients for “whooshing”— chemiosmosis

Light Dependent Reactions Light energy (photons) is absorbed by pigments in the thylakoid membranes PigmentAbsorbsReflects Chlorophyll aRed/BlueGreen Chlorophyll bRed/BlueGreen CarotenoidsBlue/GreenOrange/Yellow

Light Dependent Reactions  Photosystems—Light Harvesting Units  Photosystem I—has chlorophyll p700 (longer wavelength)  Photosystem II—has chlorophyll p680 (shorter wavelength)  Pigments absorb light energy and transfer the energy to other pigments until it is absorbed by a photosystem  The photosystem gathers enough energy to raise 1e - to a higher energy level (can be used for ETS!)

Light Dependent Reactions  Cyclic Photophosphorylation—Electron transfer with PS I  Electrons from PSI go through an ETS and create ATP--electrons then goes back to PSI  Electrons are cycling!  Not enough energy for most plants

Light Dependent Reactions  Non-cyclic Photophosphorylation—Electron transfer with PS I & PS II  PS II gathers light energy which raises electron to ETS—ATP is made by chemiosmosis  Electron goes to PS I and then is raised to a 2 nd ETS—NADP is reduced to NADPH  Electrons in PS II are replaced by photolysis of H 2 O

Light Dependent Reactions

Light Independent Reactions  Reactions occur on the stromal surface of the thylakoid  Need:  Enzymes  Ribulose Biphosphate (RuBP)—a 5-Carbon sugar  CO 2 (From Air)  NADPH  ATP Plant Makes From Non-Cyclic Photophosphorylation

Light Independent Reactions— Calvin Cycle  Carbon Dioxide Fixation:  6CO 2 + 6RuBP  6-6C molecules (unstable); so they split into 12-3C molecules called PGA (phosphoglyceric acid)  Reduction  Each PGA is phosphorylated by ATP and reduced by NADPH into 12 molecules of PGAL (phosphoglyceraldehyde)  Regeneration of RuBP  10 of 12 PGAL remake RuBP—need ATP to do it!  2 of 12 PGAL make 6C sugar (glucose)

Light Independent Reactions— Calvin Cycle

Cellular Respiration & Photosynthesis CategoryCellular RespirationPhotosynthesis PurposeConvert stored chemical energy (glucose) to usable energy (ATP Convert solar energy to stored chemical energy (glucose) WhoALL CELLSPhotosynthetic Autotrophs WhereCytoplasm & Mitochondria Chloroplast Steps1.Glycolysis 2.Intermediate Step 3.Kreb’s Cycle 4.ETS 1.Light Dependent 2.Light Independent

Cellular Respiration & Photosynthesis Sun Photosynthesis Glucose + Oxygen Gas Cellular Respiration Carbon Dioxide ATPWork

Photosynthesis in Dry Environments  C 3 Plants—Plants that fix CO 2 with Rubisco & make 3-C PGA  In hot / dry environment, must close stoma to conserve water  Closing of stoma reduces access to CO 2  Rubisco fixes O 2 instead of CO 2 —leading to photorespiration—consumes oxygen gas and makes carbon dioxide without making ATP (actually uses ATP)

Photosynthesis in Dry Environments  C 4 Plants—Plants have an alternative mode of fixing CO 2 that is more efficient  Leaf structure is slightly different  Bundle sheath cells packed around veins  Mesophyll cells loosely packed around  Calvin Cycle limited to Bundle Sheath Cells  Process  PEP carboxylase fixes CO 2 making a 4C product  4C product is sent to bundle sheath cells and releases CO 2 for Calvin Cycle  Process requires ATP but is more efficient than CR

Photosynthesis in Dry Environments

 CAM Plants—crassulacean acid metabolism  Plants open stomata at night & close during the day—helps conserve water  CO 2 enters stomata at night & is incorporated into organic acids stored in central vacuoles  CO 2 is released from acids in the morning to enter Calvin Cycle

Photosynthesis in Dry Environments