Photosynthesis

Chapter 10

• Plants – autotrophs (provide own food given certain circumstances)

• Need CO2, other inorganic (non-carbon based) materials obtained from environment.

• Autotrophs - producers of biosphere - provide food to rest of food chain.

Structure

• Green parts have chloroplasts - leaves where most photosynthesis takes place.

• Green because of chlorophyll (green pigment inside of chloroplasts)

• Chlorophyll absorbs light energy to drive making of food in chloroplasts.

http://micro.magnet.fsu.edu/cells/chloroplasts/images/chloroplastsfigure1.jpg

• Chloroplasts found mostly in mesophyll (tissue in interior of leaf)

• CO2 enters, O2 leaves through stomata (microscopic pores in leaf)

• Leaves have veins - transfer water from roots to leaves.

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• Within chloroplasts - dense fluid (stroma)

• Thylakoid basic unit of photosynthesis.

• Each thylakoid stacked on top of each other (called a grana)

• Stroma fills in between grana.

• Formula for photosynthesis: 6H2O + 6CO2 + Light energy ----------> C6H12O6+ 6O2

• Water, carbon dioxide, and light combine to make glucose (sugar) and oxygen (waste)

http://static.howstuffworks.com/gif/irrigation-photosynthesis.gif

• Photosynthesis divided into 2 stages.

• 1Light reactions (part controlled by light) and 2Calvin cycle (also called dark reactions)

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/psoverview.gif

Light reactions

• Solar energy (sun) converted to chemical energy.

• Solar energy is in the form of waves, (electrochemical waves)

• Distance between peak of 2 electrochemical waves - wavelength.

• Wavelengths vary in distance from gamma rays to radio waves.

http://www.tortuga.com/science/Life%20is%20the%20Transformation%20of%20Solar%20Energy.jpg

• Entire range of radiation - electromagnetic spectrum.

• Visible light provides us with color scheme.

• Light can be absorbed, reflected or transmitted when it meets matter.

• Pigments absorb light.

• All wavelengths absorbed - black.• Chlorophyll a - pigment found in

chloroplasts.• Works best for blue and red light,

least with green.• Accessory pigments work with

chlorophyll a to absorb light.

http://biology.clc.uc.edu/graphics/bio104/chlorophyll.jpg

Structure of chlorophyll

• One accessory pigments - chlorophyll b (yellow colors)

• Chlorophyll b will transfer energy to chlorophyll a when it absorbs sunlight.

• Carotenoids dissipate light that may be harmful to chlorophyll a (also found in human eye)

Light reactions

• Pigments absorb all wavelengths of visible light except green (why chloroplasts appear green; does not absorb this color, reflects it)

• Chlorophyll used by 2 systems in plant (photosystem I and photosystem II)

http://www.arborsci.com/CoolStuff/Chlorophyll.jpg

• Photons of light strike pigments - electrons excited, transported through photosystems.

• Reaches specific chlorophyll - molecule (reaction center) - light reactions begin.

• Photosystem I absorbs wavelength best at 700nm (dark red); photosystems II - 680nm (lighter red colors)

http://content.answers.com/main/content/wp/en/d/da/Photosystems.png

• When excited electrons reach reaction center, some electrons enter electron transport chain (ETC) - generate energy (either reduced NADPH or ATP).

• 2 processes make that happen - 1cyclic photophosphorylation; 2noncyclic photophosphorylation.

http://www.stanford.edu/group/hopes/treatmts/ebuffer/f_j13electtrans.jpg

Cyclic photophosphorylation

• Occurs in photosystem I - make ATP.• ATP used to make glucose during

dark reactions.• Electrons in cyclic process move

from reaction center through ETC, than back to reaction center.

• Does not make oxygen or NADPH.

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/cyclicphos.gif

Noncyclic photophosphorylation

• Starts in photosystem II.• Electrons passed to reaction

center.• Then passed through ETC.•Not returned to reaction center;

sent to photosystem I.

Photosystems II Photosystem I

http://www.und.ac.za/und/icd/citte/paper/net2/fig1a.gif

• They lose electrons (not recycled like in cyclic process) but get them from water.

• Produce oxygen as waste.• Electrons sent to photosystem I

used to make NADPH.

• As electrons make their way through ETCs, protons pumped out of stroma into thylakoid membranes.

• Creates proton gradient.• Protons flow back into stroma

and produce ATP.• NADPH and ATP used in Calvin

cycle (with CO2) to make sugars.

http://www.bioloj.ca/12U_Biology/img/12U_Biology/photosynthesis/Fig8_14b.jpg

The Calvin cycle

• CO2 fixed into carbohydrates using ATP and NADPH from light reactions as energy.

• 1st step - CO2 fixed into 6 C sugar with 2 phosphate groups (ribulose biphosphate (or RuBP) )

• Done through enzyme - rubisco.

http://www.marietta.edu/~spilatrs/biol103/photolab/interMeta.gif

• RuBP 6 C compound - then split into 2 molecules (3-phosphoglycerate) which are both 3 C compounds.

• Each are phosphorylated by ATP, then reduced by NADPH - forms substance called G3P (form of sugar).

• For every 3 molecules of CO2 - 6 molecules of G3P.

• At end - 6 molecules of G3P.• 1 used by plant cell, other 5

recycled to regenerate RuBP to start process again.

• To make 1 G3P for plant, 9 molecules of ATP used, 6 molecules of NADPH used.

Alternate forms of photosynthesis

• Photosynthesis - C3 plants.• 1st product made is 3 C compound

(3-phosphoglycerate).• Rice, wheat, and soybeans - C3

plants.• Produce less food on hot, dry days

(stomata closed)

• No CO2 - no Calvin cycle.

http://www.emeraldinsight.com/fig/0170950403003.png

• Instead of CO2 being used, rubisco adds O2 to Calvin cycle.

• No ATP generated no food produced.

• Called photorespiration.• Wasteful product - not known

why it still occurs in plants.

C4 plants

• C4 plants - alternate form of carbon fixation before Calvin cycle.

• Plants like sugarcane, corn, members of grass family.

• Have different anatomy.• 2 different types of photosynthetic

cells: bundle-sheath cells and mesophyll cells.

http://www.biology.lsu.edu/webfac/dlongstreth/biol12014f02/lectur37.jpg

• Bundle-sheath cells tightly packed and found around veins of leaf.

• Mesophyll cells found between bundle sheath and surface of leaf (loosely packed).

http://gemini.oscs.montana.edu/~mlavin/b434/graphic/Leafc4m.jpg

• In bundle sheath cells, CO2 produced as well as pyruvate.

• Pyruvate sent back to mesophyll cells; CO2 used in bundle sheath cells to go into Calvin cycle.

• Then fixes CO2 with rubisco, like in C3 plants.

• C4 plants fix CO2 twice.

• Plants live in hot, dry environments (like corn and crab grass) where stomata have to close often.

• C3 plants - causes photorespiration; C4 plants - still able to fix carbon.

• At cooler temperatures, C3 plants much more effective.

http://www.usep.edu.ph/smarrdec/Image%20Gallery%20Pics/corn%20laak.JPG

CAM plants

• CAM plants found in hot and dry environments where stomata are closed during the day.

• Plants open stomata at night, fix carbon during this time.

• Store products of carbon fixation for Calvin cycle which happens during the day.

• http://www.fw.vt.edu/dendro/forestbiology/photosynthesis.swf