AP Biology Text Chapter 10 Photosynthesis AP Biology Text Chapter 10

Photosynthesis Photosynthesis nourishes almost all living organisms either directly or indirectly. Two main processes: Light-dependent Light-independent

Green Plants Leaves are the major site of photosynthesis. Chloroplasts containing the pigment chlorophyll are found mainly in mesophyll cells forming the tissues in the interior of the leaf. O2 exits and CO2 enters the leaf through microscopic pores called stomata. Stomata Mesophyll Cell Chloroplast

Chloroplast structure Each chloroplast has two membranes around a central aqueous space, the stroma. (Light- independent reaction) In the stroma is a system of membraneous sacs, the thylakoids. The thylakoids may be stacked in columns called grana. (Light-dependent) Chlorophyll is located within the thylakoids.

Photosynthetic Equation 6CO2 + 12 H2O + light energy → C6H12O6 + O2 + 6H2O Water appears on both sides of the equation because 12 molecules of water are consumed and 6 new water molecules are formed.

Two stages of photosynthesis The light reactions (light-dependent): convert solar energy to chemical energy. The Calvin cycle (light-independent): uses energy from the light reactions to incorporate CO2 from the atmosphere into sugar. Begins with the incorporation of CO2 into organic molecules, in a process known as carbon fixation.

Pigments absorb light energy Photosynthetic Pigments absorb light energy and use it to provide energy to carry out photosynthesis Plants have 2 groups of pigments Chlorophylls (a and b) Green: absorb red, blue and violet Carotenoids Yellow, Orange and Red: absorb blue, green and violet Absorbing different colors allows for more light to be absorbed

Organization of pigments In the thylakoid membrane, chlorophyll is organized along with proteins and small organic molecules into photosystems. A photosystem is composed of a reaction center surrounded by a light-harvesting complex. Two photosystems PS I also P700 PS II also P680 At the reaction center is a primary electron acceptor, which accepts an excited electron from the reaction center chlorophyll a. This transfer is the first step in the light reactions

Light is absorbed by PS and electrons flow through transport chains

Noncyclic electron flow IMPORTANT: ATP and NADPH are formed This is the predominant route for flow of electrons and results in the production of ATP and NADPH. Overview: Light  P680 (oxygen released)  ATP produced  P700  NADPH produced (goes to Calvin Cycle)

Cyclic electron flow IMPORTANT: the sole purpose of cyclic photophosphorylation is to produce ATP. No NADPH is produced and no oxygen is released. Calvin cycle uses a lot of ATP CP is done when chloroplast is running low on ATP

Calvin Cycle Main part of light-independent reaction Doesn’t need light, but only occurs in light Uses products from light reaction (ATP and NADPH) Cyclic process: produces 3-carbon sugar PGAL or G3P Carbon Fixation: Carbon enters leaf as CO2, becomes fixed as PGAL Net synthesis of one G3P molecule Nine ATP and 6 NADPH molecules were used. G3P is the starting material for metabolic pathways that synthesize other organic compounds, including glucose. ADP & NADP are passed to the light reactions and used to regenerate ATP and NADPH.

Alternative mechanisms for Carbon Fixation These include plant adaptations that prevent dehydration may also reduce availability of CO2 On hot, dry days, most plants (C3 plants) close their stomata to conserve water. When stomata close, CO2 levels drop as CO2 is consumed in the Calvin Cycle At the same time, O2 levels rise as the light reaction converts light to chemical energy

Alternative mechanisms for Carbon Fixation - C4 Plants C4 plants fix CO2 into a four-carbon compound Several thousand plants use this including sugar cane and corn Modification for dry environments Minimize water loss and maximize sugar production

CAM plants These plants open stomata during the night and close them during the day. Temperatures are typically lower at night & humidity is high Includes cacti, pineapples At night, CAM plants fix CO2 into a variety of organic acids in mesophyll cells. During the day, the light reactions supply ATP and NADPH to Calvin cycle and CO2 is released from organic acids.