chapter 8 2 photosynthesis
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Section 8.2 – Photosynthesispp. 222-227
Photosynthesis occurs in two phases.
Cellular Energy
1. Light reactions (Light-dependent reaction)
2. Calvin Cycle (Light-independent reactions
Overview of Photosynthesis
ChloroplastsThe light reactions takes place in the grana
with the thylakoids inside the cholorplast.
Grana (stacks in thylakoid membranes)- where light-dependent reactions take place
Thylakoid membranes contain chlorophyll (light absorbing pigment)
Stroma-fluid between the grana
4. Stroma 6. thylakoid membrane 7. grana 8. thylakoid
Pigments- light absorbing colored molecules that are found in the thylakoid membrane.
Major light-absorbing pigments in plants are chlorophylls like chlorophyll a and chlorophyll b. Carotenoids is an accessory pigments that allows plants to trap additional light energy.
Accessory PigmentsThere are 2 types of carotenoids
1. Carotenes- orange pigments2.Xanthophylls- yellow pigments
-Trap energy from colors of light that chlorophyll does NOT absorb well
-Give cells the colors red, orange, & yellow that you see in fall leaves when chlorophyll degrades
Phase One: Light reaction overview
-Light-dependent reactions
-Light energy is converted to chemical energy
-Split water molecules, provide hydrogen and an energy source for the Calvin Cycle
-O2 from H2 O is given off
Phase One: Light Reactions
1st step – absorption of light by chloroplasts (located in the cells of leaves)
2nd step in Phase 1- Electron Transport Chain- When electrons absorb enough energy, they
leave the chlorophyll molecule and are passed along a series of molecules in the thylakoid, releasing energy as they go (ATP and NADPH)
- This series of molecules is known as the electron transport chain
- As the electrons pass down the chain the extra energy they received from light is stored in the bonds of ATP
Phase Two: Calvin cycle overview
-Light –independent reaction
-Forms simple sugars using carbon from CO2 and H2 to form H2 O
-Takes place in the stroma
-Forms carbohydrates (glucose)
The Calvin Cycle
-A plant produces ATP and NADPH + H+
Carbon fixation - an enzyme adds the carbon atom of CO2 to a 5-carbon molecule
-It takes CO2 from the air and converts it to a form that is usable by living things
-CO2 combines with H2 to form simple sugars that are used to make other carbohydrates and complex sugars, starch, and cellulose
Calvin Cycle (Cont.)
-ATP and H2 from the light reactions are needed for the Calvin Cycle
-a plant produces carbohydrates (sugar, starch, cellulose)
COCO22 enters leaves and stems of green enters leaves and stems of green plants through pores called plants through pores called stomatastomata
Chemosynthesis
a process in which an autotroph obtains energy from inorganic compounds instead of from sunlight
examples: methane producing-bacteria which are poisoned by O2
Stage Starting Materials
Ending Materials
Light
Reactions
CO2
H2O
Light energy
H2
O2
ATP
Calvin Cycle
(Light-Independent Reactions)
H2
ATP
CO2
carbohydrate O2
Factors That Can Affect Photosynthesis
Water
A shortage of water can slow or stop photosynthesis
Temperature
The enzymes for photosynthesis only work between 0 )C and 35 )C.
Light Intensity
The stronger the light shining on the plant, the faster photosynthesis occurs.
C4 Plants
occurs in plants like corn & sugarcane
helps plants maintain photosynthesis while minimizing water loss
fix carbon dioxide into 4-carbon compounds instead of 3-carbon compounds during Calvin cycle
Keep stomata closed during hot days
CAM Plants
occurs in water-conserving plants living in deserts, salt marshes, etc. where water is limited
examples: cacti, orchids, pineapple
allows carbon dioxide to enter only at night, when the atmosphere is cooler and more humid
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