Photosynthesis Capturing sunlight to produce organic compounds

Overview Overall equation:  CO 2 + H 2 O  C 6 H 12 O 6 + O 2 2 Main Stages:  I. Light Reactions: occurs in the membrane and interior of thylakoids.  II. Calvin Cycle (light-independent reactions): occurs in stroma  As in respiration, the products of earlier reactions and pathways are used in later reactions and pathways.

Light Reactions “Big idea”:  energy from sunlight is used to excite e- of an electron transport chain (ETC).  ETC is used to generate ATP and NADPH.  These molecules “carry” energy to the next stage.

The second stage: Carbon fixation and the Calvin Cycle  “Big idea”:  energy from ATP and NADPH is used to build a simple organic compound that is a precursor of larger compounds, such as carbs, fats, proteins.  Carbon for organic compounds is acquired through carbon fixation (CO 2 from atmosphere).

Leaf Cross-Section

Chloroplasts

Photosynthetic Cell (3-D)

Inside a chloroplast Chloroplast Thylakoids: membrane- bound areas in the shape of flattened discs. Stroma: Fluid that surrounds the thylakoids.

Thylakoid sunlight H2OH2OO2O2 CO 2 Carbohydrates

Light Absorption: How It Works Requires the use of pigments (chlorophylls a and b and carotenoids)  Pigments are clustered together in the thylakoid membrane in groups of a few hundred  A cluster of pigments = “photosystem” Pigments absorb LIGHT energy and convert it to CHEMICAL energy

Light Absorption: How It Works Chlorophylls absorb violet, blue, and red but reflect green

Light Absorption: How It Works Carotenoids absorb blue and some green but reflect yellow, orange, and brown Between the cholorphylls and the carotenoids, the majority of the visible spectrum is absorbed by the plant

Light Reactions: a closer look Water is split to provide e- to ETC. H+ ions and oxygen are produced.

Light is absorbed by pigments in P.S. II. Each pigment absorbs a specific range of wavelengths. When light is absorbed by chlorophyll a, electrons become “excited” (gain energy). The “excited” e- move to a primary e- acceptor and are then passed down the ETC.

Light is absorbed by pigments in P.S. II. Each pigment absorbs a specific range of wavelengths. When light is absorbed by chlorophyll a, electrons become “excited” (gain energy). The “excited” e- move to a primary e- acceptor and are then passed down the ETC.

Light reactions (cont’d) As e- move down the ETC they lose energy. The energy of the e- is used to pump H+ into the thylakoid.

When e- enter P.S. I, light is absorbed and e- become “excited” again. Electrons move down another ETC and are taken by NADP+ to make NADPH. The e- in NADPH are still at a relatively high energy level.  i.e.: the energy of sunlight is now stored as chemical PE in an organic molecule.

The final step Chemiosmosis: movement of H+ (protons) through ATP synthase transfers energy which is used to produce ATP.  The high concentration of H+ was built by e- moving down ETC = potential energy.  The energy of the e- moving down the ETC came from the sun.  The energy of sunlight is now stored as chemical PE in an organic molecule.

Chemiosmosis The high concentration of H+ was built by e- moving down ETC = potential energy. The energy of the e- moving down the ETC came from the sun. The energy of sunlight is now stored as chemical PE in an organic molecule.

Thylakoid (stack = granum) sunlight Stroma Overview of Photosynthesis

Photosynthesis The “Dark” Reactions aka: Light-independent reactions

The Photosynthesis Equation CO 2 + H 2 O → C 6 H 12 O 6 + O 2 Which reactant was used in the light reactions (stages 1 and 2)?  H 2 O (split to provide electrons for photosystem II) Which product was produced in the light reactions?  O 2 (a byproduct of splitting water molecules) CO 2 and C 6 H 12 O 6 were not involved in the light reactions- must be involved the third stage

Stage 2: Calvin Cycle During this stage, carbon is “fixed”. What does “carbon fixation” refer to?  Changing inorganic carbon (like CO 2 ) to organic carbon (molecules with C bonded to other C) Summary of the Calvin Cycle:  CO 2 molecules combine with an organic compound. Energy from NADPH and ATP is used to make PGAL. PGAL is then used to build organic compounds (like glucose).

Stage 2: The Details 1. A CO 2 molecule combines with RuBP (5-C) to create a 6-C molecule C molecule immediately splits into 2 3-C molecules (PGA) 3. Energy from ATP and NADPH is used to change PGA into PGAL 4. Some PGAL is used to regenerate RuBP to keep Calvin cycle going; the rest is used to make organic compounds (like glucose)

Stage 2: The Details 6-C compound splits 2 PGA 2 PGAL

Stage 2: The Starting Materials Carbon fixation requires ATP, NADPH, RuBP, and CO 2  ATP and NADPH: made during the light reactions  RuBP: regenerated at the end of each cycle  CO 2 : Some is created by the plant during cell respiration and the rest is taken in from the atmosphere through openings in the plant leaves called stomata.

Leaf Cross-Section

Guard Cells and Stomata CO 2 enters plant leaves through openings called stomata. Guard cells on either side of the stomata empty or fill with water to open and close the stomata.

Guard Cells and Stomata

The Problem with Stomata When stomata open to allow CO 2 in, it also allows H 2 O to escape. So taking in CO 2 comes at the expense of losing water.  Not an issue for C 3 plants (most plants are this type) because they exist in temperate climates where water loss isn’t such a problem  Major problem for plants that exist in climates that are hot and/or dry  The solution: alternative carbon fixation

Alternative Carbon Fixation The C 4 Pathway  C 4 plants partially close their stomata during the hottest part of the day (reduces the water loss, but also the amount of CO 2 coming in)  Contain enzymes that “fix” CO 2 into 4-C compounds when CO 2 level is low  Breakdown 4-C compounds later on to release CO 2 (which can then be used in Calvin cycle)  C 4 plants include corn, sugar cane, and crabgrass

Alternative Carbon Fixation The CAM Pathway  Plants living in the hottest and driest climates  Open stomata at night and close during day (opposite of other plants)  Take in CO 2 at night and “fix” into various organic compounds  CO 2 released from these compounds during the day and used in Calvin cycle  CAM plants include cacti and pineapple