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) CO 2 enters, O 2 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: 6H 2 O + 6CO 2 + Light energy > C 6 H 12 O 6 + 6O 2 Water, carbon dioxide, and light combine to make glucose (sugar) and oxygen (waste)

Photosynthesis divided into 2 stages. 1 Light reactions (part controlled by light) and 2 Calvin 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.

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 - 1 cyclic photophosphorylation; 2 noncyclic 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.

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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 IIPhotosystem I

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 CO 2 fixed into carbohydrates using ATP and NADPH from light reactions as energy. 1 st step - CO 2 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.

2 nd step - Each are phosphorylated by ATP, then reduced by NADPH - forms substance called G3P (form of sugar). 3 rd step- regenerate 5 C RuBP the CO2 acceptor.

For every 3 molecules of CO 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. 1 st product made is 3 C compound (3-phosphoglycerate). Rice, wheat, and soybeans - C3 plants. Produce less food on hot, dry days (stomata closed) No CO 2 - no Calvin cycle.

Instead of CO 2 being used, rubisco adds O 2 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, CO 2 produced as well as pyruvate. Pyruvate sent back to mesophyll cells; CO 2 used in bundle sheath cells to go into Calvin cycle. Then fixes CO 2 with rubisco, like in C3 plants. C4 plants fix CO 2 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.

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.

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