Photosynthesis occurs in two phases. Cellular Energy 1. Light reactions (Light-dependent reaction) 2. Calvin Cycle (Light-independent reactions Overview of Photosynthesis
Chloroplasts The 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 Pigments There are 2 types of carotenoids 1. Carotenes- orange pigments 2.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 -O 2 from H 2 O is given off
Phase One: Light Reactions 1 st step – absorption of light by chloroplasts (located in the cells of leaves)
2 nd 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 CO 2 and H 2 to form H 2 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 CO 2 to a 5-carbon molecule -It takes CO 2 from the air and converts it to a form that is usable by living things -CO 2 combines with H 2 to form simple sugars that are used to make other carbohydrates and complex sugars, starch, and cellulose
Calvin Cycle (Cont.) -ATP and H 2 from the light reactions are needed for the Calvin Cycle -a plant produces carbohydrates (sugar, starch, cellulose)
CO 2 enters leaves and stems of green plants through pores called stomata
A shortage of water can slow or stop photosynthesis
The enzymes for photosynthesis only work between 0 ̊C and 35 ̊C.
The stronger the light shining on the plant, the faster photosynthesis occurs.
C 4 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