PHOTOSYNTHESIS NAZNEEN JAHAN 1

THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH 2

Almost all plants are photosynthetic autotrophs, as are some bacteria and protists –Autotrophs generate their own organic matter through photosynthesis –Sunlight energy is transformed to energy stored in the form of chemical bonds (a) Mosses, ferns, and flowering plants (b) Euglena (c) Cyanobacteria THE BASICS OF PHOTOSYNTHESIS 3

Light Energy Harvested by Plants & Other Photosynthetic Autotrophs 6 CO H 2 O + light energy → C 6 H 12 O O 2 4

Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Life on earth made possible by sun. All Oxygen in the atmosphere was put there by photosynthesis All energy stored in coal and oil was supplied by photosynthesis. AN OVERVIEW OF PHOTOSYNTHESIS Carbon dioxide WaterGlucoseOxygen gas PHOTOSYNTHESIS 5

6 Question: Where does photosynthesis take place?

7 Stomata (stoma) Pores in a plant’s cuticle through which water vapor and gases (CO 2 & O 2 ) are exchanged between the plant and the atmosphere. Guard Cell Carbon Dioxide (CO 2 ) Oxygen (O 2 ) Found on the underside of leaves Stoma

8 Mesophyll Cell of Leaf Cell Wall Nucleus Chloroplast Central Vacuole Photosynthesis occurs in these cells! Photosynthesis occurs in these cells!

9 Chloroplast Organellephotosynthesis Organelle where photosynthesis takes place. Granum Thylakoid Stroma Outer Membrane Inner Membrane Thylakoid stacks are connected together Thylakoid stacks are connected together

10 Question: Why are plants green?

11 Chlorophyll Molecules Located in the thylakoid membranes Located in the thylakoid membranes Chlorophyll have Mg + in the center Chlorophyll have Mg + in the center Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420 nm and red- 660 nm are most important) Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420 nm and red- 660 nm are most important) Plants are green because the green wavelength is reflected, not absorbed Plants are green because the green wavelength is reflected, not absorbed.

Chloroplasts contain several pigments Chloroplast Pigments –Chlorophyll a –Chlorophyll b –Carotene –Xanthophyll 12

13 Wavelength of Light (nm) Short waveLong wave (more energy)(less energy)

Excited state ee Heat Light Photon Light (fluorescence) Chlorophyll molecule Ground state 2 (a) Absorption of a photon Excitation of chlorophyll in a chloroplast ee 14

15 Parts of Photosynthesis

16 Two Parts of Photosynthesis Two reactions make up photosynthesis: 1.Light Reaction or Light Dependent Reaction - Produces energy from solar power (photons) in the form of ATP and NADPH. SUN

17 Two Parts of Photosynthesis 2. Calvin Cycle or Light Independent Reaction Also called Carbon Fixation or C 3 Fixation Also called Carbon Fixation or C 3 Fixation Uses energy (ATP and NADPH) from light reaction to make sugar (glucose). Uses energy (ATP and NADPH) from light reaction to make sugar (glucose).

Photon Water-splitting photosystem NADPH-producing photosystem ATP mill Two types of photosystems cooperate in the light reactions 18

19 Light Reaction (Electron Flow) Occurs in the Thylakoid membranes Occurs in the Thylakoid membranes During the light reaction, there are two possible routes for electron flow: During the light reaction, there are two possible routes for electron flow: A.Cyclic Electron Flow B.Noncyclic Electron Flow

20 Cyclic Electron Flow Occurs in the thylakoid membrane. Occurs in the thylakoid membrane. Uses Photosystem I only Uses Photosystem I only P700 reaction center- chlorophyll a P700 reaction center- chlorophyll a Uses Electron Transport Chain (ETC) Uses Electron Transport Chain (ETC) Generates ATP only Generates ATP only ADP + ATP P

Cyclic Photophosphorylation Process for ATP generation associated with some Photosynthetic Bacteria Reaction Center => 700 nm 21

Noncyclic Photophosphorylation Photosystem II regains electrons by splitting water, leaving O 2 gas as a by-product 22

23 Noncyclic Electron Flow ADP +  ATP ADP +  ATP NADP + + H  NADPH NADP + + H  NADPH Oxygen comes from the splitting of H 2 O, not CO 2 Oxygen comes from the splitting of H 2 O, not CO 2 H 2 O  1/2 O 2 + 2H + H 2 O  1/2 O 2 + 2H + P

The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H + through that membrane – The flow of H + back through the membrane is harnessed by ATP synthase to make ATP – In the stroma, the H + ions combine with NADP + to form NADPH Chemiosmosis powers ATP synthesis in the light reactions 24

The production of ATP by chemiosmosis in photosynthesis Thylakoid compartment (high H + ) Thylakoid membrane Stroma (low H + ) Light Antenna molecules Light ELECTRON TRANSPORT CHAIN PHOTOSYSTEM IIPHOTOSYSTEM IATP SYNTHASE 25

A Photosynthesis Road Map Chloroplast Light Stack of thylakoids ADP + P NADP  Stroma Light reactions Calvin cycle Sugar used for  Cellular respiration  Cellulose  Starch  Other organic compounds 26

27 Calvin Cycle (C 3 fixation) 6CO 2 6C-C-C-C-C-C 6C-C-C 6C-C-C-C-C 12PGA RuBP 12G 3 P (unstable) 6NADPH 6ATP C-C-C-C-C-C Glucose (6C) (36C) (30C) (6C) 6C-C-C C3C3 glucose

28 Calvin Cycle Remember: C 3 = Calvin Cycle C3C3 Glucose

29 Photorespiration Occurs on hot, dry, bright days Occurs on hot, dry, bright days Stomates close Stomates close Fixation of O 2 instead of CO 2 Fixation of O 2 instead of CO 2 Produces 2-C molecules instead of 3-C sugar molecules Produces 2-C molecules instead of 3-C sugar molecules Produces no sugar molecules or no ATP Produces no sugar molecules or no ATP

30 Photorespiration Because of photorespiration, plants have special adaptations to limit the effect of photorespiration: 1.C 4 plants 2.CAM plants

31 C 4 Plants Hot, moist environments Hot, moist environments 15% of plants (grasses, corn, sugarcane) 15% of plants (grasses, corn, sugarcane) Photosynthesis occurs in 2 places Photosynthesis occurs in 2 places Light reaction - mesophyll cells Light reaction - mesophyll cells Calvin cycle - bundle sheath cells Calvin cycle - bundle sheath cells

32 C 4 Plants Mesophyll Cell CO 2 C-C-C PEP C-C-C-C Malate-4C sugar ATP Bundle Sheath Cell C-C-C Pyruvic Acid C-C-C-C CO 2 C3C3 Malate Transported glucose Vascular Tissue

C4 Pathway CO 2 is fixed into a 4- carbon intermediate Has an extra enzyme– PEP Carboxylase that initially traps CO 2 instead of Rubisco– makes a 4 carbon intermediate 33

34 CAM Plants Hot, dry environments Hot, dry environments 5% of plants (cactus and ice plants) 5% of plants (cactus and ice plants) Stomates closed during day Stomates closed during day Stomates open during the night Stomates open during the night Light reaction - occurs during the day Light reaction - occurs during the day Calvin Cycle - occurs when CO 2 is present Calvin Cycle - occurs when CO 2 is present

35 CAM Plants Night (Stomates Open)Day (Stomates Closed) Vacuole C-C-C-C Malate C-C-C-C Malate C-C-C-C CO 2 C3C3 C-C-C Pyruvic acid ATP C-C-C PEP glucose

C3 plants and C4 plants  The rate of photosynthesis in C4 plants is much faster ( 6 times) than that in C3 plants.  C4 plants thrive at much higher temperature than do the C3 plants.  C3 plants use RuBP carboxylase to fix CO2 and the first organic molecule created is phosphoglycerat(this thing has 3 C atoms=> the name C3).  C4 plants use PEP carboxylase to fix CO2 and the first organic molecule is oxalacetic acid(4 C atoms) 36

Summary of C4 Photosynthesis C4 Pathway – Separates by space (different locations) CAM Pathway – Separates reactions by time (night versus day) 37