Chapter 10 Photosynthesis

Objectives Describe the structure and function of a chloroplast. Describe methods of solar capture and ATP production. Describe how carbohydrates are synthesized. Compare and contrast the 3 modes of photosynthesis.

Key Terms ATP Synthase Chloroplast Chlorophyll Stroma Thylakoid light reactions Calvin cycle Antenna complex Ribulose biphosphate PGAL Photosystem Photorespiration CAM photosynthesis C3 Photosynthesis C4 Photosynthesis Chemiosmosis

Do you know this reaction? 12 + 6 H2O

Photosynthesis occurs in 2 stages.

Solar Energy Capture

Solar energy

Most available energy is not used 42% of solar energy reaches Earth’s surface 2% of the 42% utilized by plants, remainder becomes heat Only 0.1% - 1.6% is incorporated into plant material. Photosynthesis uses the portion of the electromagnetic spectrum known as visible light.

Photosynthetic Pigments

Electromagnetic Spectrum

Absorption Spectra

Photosynthetic Pigments Animation: Light and Pigments Chlorophyll a and b Absorbs violet, blue and red better than other colors. Green NOT absorbed! Animation: Light and Pigments

Photosynthetic Pigments Chlorophyll a - main photosynthetic pigment chlorophyll b - accessory pigments; broadens the spectrum used for photosynthesis carotenoids - accessory pigments that absorb excessive light that would damage chlorophyll

Leaf Structure

Chlorophyll (Terpene)

Structure & Function of Chloroplasts

Chloroplasts Internal membranes, thylakoids, are organized into grana. Thylakoid membranes house pigments for capturing light and the machinery to produce ATP. clustered together to form a photosystem acts as an antenna, gathering light energy harvested by multiple pigment molecules

Chloroplasts

Interaction of Light with Chloroplasts

Stage 1 Light-dependent Reactions

(Solar Energy Capture) The Light Reactions (Solar Energy Capture) In membranes of thylakoids Involves two light-gathering units Photosystem I Photosystem II Converts sunlight to chemical energy (ATP). O2 is a "waste product"

Photosynthesis Output Increases linearly at low light intensities but lessens at higher intensities. Reaches a saturation point

Electron Pathways Cyclic – electrons originate & return to PSI reaction center ATP is produced Noncyclic – electrons leave PSII and go to PSI H2O is oxidized yielding H+, e- and O2 NADP+ becomes NADPH

Photosystem Network of pigments that channels excitation energy gathered by any of the molecules to the reaction center. reaction center allows photon excitation to move away from chlorophylls and is the key conversion of light to chemical energy

Photosystem II complex (PSII) (P680 - Oxygen evolving apparatus)                                                                                                                 Membrane protein complex found in photosynthetic organisms (higher plants, green algae and cyanobacteria) Harnesses light energy to split H2O into O2, protons and electrons. Responsible for the production of atmospheric oxygen Also involved in the production of a substantial proportion of the global biomass.

Reaction Center Allows photon excitation to move away from chlorophylls and is the key conversion of light to chemical energy

Photosystem Function electron is joined with a proton to make hydrogen Bacteria use a single photosystem, Photosystem I (P700) electron is joined with a proton to make hydrogen electron is recycled to chlorophyll

Photosystem Function Plants use two photosystems photosystem I (P700) and II (P680) generate power to reduce NADP+ to NADPH with enough left over to make ATP two stage process: photosystem II – I. noncyclic photophosphorylation ejected electrons end up in NADPH

Cyclic Electron Pathway

Linear Electron Pathway

Chlorophylls a & b and accessory pigments (Chlorophyll a) Chlorophylls a & b and accessory pigments

ATP production tied to an electro- chemical gradient. Chemiosmosis ATP production tied to an electro- chemical gradient.

Stage 2 Light-independent Reactions

Carbohydrate Synthesis

Sometimes called light-independent reactions. Calvin Cycle Sometimes called light-independent reactions. Essential Question Does the Calvin cycle continue running in a plant kept in the dark?

Calvin Cycle Takes place in the stroma Makes sugar from CO2 (CO2 becomes CH2O) Energy supplied by ATP made by light reaction Electrons supplied by NADPH G3P (glyceraldehyde-3-phosphate = PGAL)

Light-Independent Reactions

P3G (glyceraldehyde-3-phosphate = PGAL) Product of Calvin cycle Important biochemical reactant

Modes of Photosynthesis C3 C4 CAM

C3 Photosynthesis Most plants are C3. (Kentucky Bluegrass) Stomata open during the day Photosynthesis throughout the leaf. CO2 enters Calvin Cycle directly - fixed using RuBP carbolylase More efficient than C4 and CAM under cool and moist conditions and under normal light (fewer enzymes and no specialized anatomy).

C4 Pathway Plants adapted to warmer environments deal with the loss of CO2 in two ways: C4 conducted in mesophyll cells, Calvin cycle in bundle sheath cells creates high local levels of CO2 to favor carboxylation reaction of rubisco isolates CO2 production spatially

C4 Photosynthesis CO2 undergoes “preliminary fixation into malate (a C4 molecule) before entering Calvin cycle. Malate stored in large vacuoles in mesophyll cells (CO2 fixation is partitioned by space) Hot, dry climates Photosynthesis takes place in inner cells Include several thousand species in at least 19 families. (crabgrass, corn, and many summer annuals)

C4 Photosynthesis (Cont’d) Adaptive Value: Photosynthesizes faster than C3 plants under high light intensity and high temperatures Better water use efficiency because PEP Carboxylase brings in CO2 faster and does not need to keep stomata open as much (less water lost by transpiration)

C4 Photosynthesis

Leaf Structure

Comparison of C3 and C4 Leaf Anatomy

CAM Photosynthesis (Crassulacean Acid Metabolism ) The CO2 converted to an acid and stored during night as an acid. Stomata open at night, usually closed during the day. (Reduces loss of water vapor) Daytime - acid broken down and the CO2 is released to RUBISCO for photosynthesis Include many succulents, also some orchids and bromeliads.

CAM Photosynthesis Adaptive Value Better Water Use Efficiency than C3 under arid conditions (transpiration rates are lower, no sunlight, lower temperatures, lower wind speeds, etc.)

CAM Photosynthesis Adaptive Value CAM-idle under extreme conditions. Stomata closed night and day. O2 from photosynthesis is used for respiration, CO2 from respiration used for photosynthesis. Cannot CAM-idle forever. Survival of dry spells and rapid recovery when water is available again. (No dormancy)

Carbon Fixation

Photorespiration O2 is incorporated into RuBP, which undergoes additional reactions that release CO2. decreased yields of photosynthesis

THAT’S ALL FOLKS