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