Photosynthesis Light-Dependent Reactions & Calvin Cycle Section 8.2

Fig (a) Plants (c) Unicellular protist 10 µm 1.5 µm 40 µm (d) Cyanobacteria (e) Purple sulfur bacteria (b) Multicellular alga Photosynthesis occurs in plants, algae, some prokaryotes ENDERGONIC PROCESS-stores energy ANABOLIC PROCESS-Builds up molecules BioFlix: Photosynthesis BioFlix: Photosynthesis

Structures of Photosynthesis Leaves are the major locations of photosynthesis Chlorophyll - the green pigment within chloroplasts CO 2 enters and O 2 exits the leaf through microscopic pores called stomata

Fig. 10-3a 5 µm Mesophyll cell Stomata CO 2 O2O2 Chloroplast Mesophyll Vein Leaf cross section Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

Component of a Chloroplast Thylakoid – Saclike photosynthetic membranes –Light-dependent reactions occur here Granum – Stack of thylakoids Stroma – Region outside the thylakoid membrane –Reactions of the Calvin Cycle occur here

DRAW THIS CHLOROPLAST DIAGRAM IN YOUR NOTES

The Photosynthesis Equation 6 CO H 2 O + Light energy  C 6 H 12 O O 2

The Two Stages of Photosynthesis: A Preview The light reactions (in the thylakoids): –Split H 2 O –Release O 2 –Reduce NADP + to NADPH –Generate ATP from ADP The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH The Calvin cycle begins with carbon fixation, incorporating CO 2 into organic molecules (most importantly, glucose) Photosynthesis consists of the Light Reactions (the photo part) and Calvin Cycle (the synthesis part)

Light- Dependent reaction –Occurs in thylakoid –Uses H 2 O and light to produce ATP, NADPH, and O 2 –NADPH is an electron carrier Calvin cycle (Light-Independent Reaction) –Occurs in stroma –uses carbon dioxide, ATP, and NADPH to produce sugars (aka food, glucose, carbohydrates, etc.)

Light-Dependent Reactions Electron Transport Chain This stage of photosynthesis includes an electron transport chain (ETC). The molecules of the electron transport chain use high- energy electrons to push H+ ions from the stroma into the inner thylakoid space, producing ATP & NADPH in the process (Don’t worry about the details of the ETC). The ATP & NADPH are then used in the next stage, the light-independent reactions (Calvin Cycle).

Calvin Cycle

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i +

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 [CH 2 O] (sugar)

The light reactions convert solar (light) energy to the chemical energy of ATP and NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig Reflected light Absorbed light Light Chloroplast Transmitted light Granum Chloroplasts are solar-powered chemical factories –Their thylakoids transform light energy into the chemical energy of ATP & NADPH –ATP &NADPH are then used to power the Calvin Cycle (light- independent reactions)

The Nature of Sunlight Light is a form of electromagnetic energy The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see Wavelength is the distance between crests of waves Wavelength determines the type of electromagnetic energy

UV Fig Visible light Infrared Micro- waves Radio waves X-rays Gamma rays 10 3 m 1 m (10 9 nm) 10 6 nm 10 3 nm 1 nm 10 –3 nm 10 –5 nm nm Longer wavelength Lower energyHigher energy Shorter wavelength

Light and Pigments Pigments – light absorbing chemicals Chlorophyll – principle pigment in plants –Chlorophyll a –Chlorophyll b –Carotenoids –Xanthophyll

Why do leaves change colors? Chlorophyll a Chlorophyll b

Why Do Leaves Change Color?

Leaves change color in the Fall because chlorophyll begins to break down and disappear, exposing the other pigments – the carotenoids (orange, yellow, and reds) and xanthophylls (yellows).

Factors Affecting Photosynthesis Water supply Amount of sunlight Temperature

Types of Photosynthesis C3 Photosynthesis C4 Photosynthesis CAM Photosynthesis

C3 Photosynthesis : C3 plants. Called C3 because the CO2 is first incorporated into a 3- carbon compound. Stomata are open during the day. Photosynthesis takes place throughout the leaf. Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy).. Most plants are C3.

C4 Photosynthesis : C4 plants. Called C4 because the CO2 is first incorporated into a 4- carbon compound. Stomata are open during the day. Adaptive Value: Photosynthesizes faster than C3 plants under high light intensity and high temperatures Has better Water Use Efficiency because they do not need to keep stomata open as much (less water lost by transpiration) C4 plants include several thousand species in at least 19 plant families. Examples: corn, sugar cane, grasses

CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. Adaptive Value: –Better Water Use under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.). Examples: succulents such as cacti, pineapple, and also some orchids and bromeliads