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.
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.
* • 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)
Photosynthesis divided into 2 stages. • 1Light reactions (part controlled by light) and 2Calvin cycle (also called dark 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.
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)
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)
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.
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.
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.
The Calvin cycle • CO2 fixed into carbohydrates using ATP and NADPH from light reactions as energy. • 1st step - CO2 fixed into 5 C sugar with 2 phosphate groups (ribulose biphosphate (or RuBP) ) • Done through enzyme - rubisco.
So, RuBP 5 C compound adds one CO2 to make a 6 C • Then split into 2 molecules (3-phosphoglycerate) which are both 3 C compounds.
2nd step - Each are phosphorylated by ATP, then reduced by NADPH - forms substance called G3P (form of sugar). • 3rd step- regenerate 5 C RuBP the CO2 acceptor.
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.
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.
Bundle-sheath cells tightly packed and found around veins of leaf. • Mesophyll cells found between bundle sheath and surface of leaf (loosely packed).
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.