Where It Starts – Photosynthesis Chapter 6 Where It Starts – Photosynthesis

How Did Energy-Releasing Pathways

6.1 How Do Photosynthesizers Absorb Light? Most autotrophs photosynthesize Light is organized in packets of energy called photons which move in waves Visible light: Small range of the light spectrum Insert label and caption for any figures here.

Properties of Light (cont’d.) visible light gamma rays x-rays ultraviolet radiation near-infrared radiation infrared radiation microwaves radio waves shortest wavelengths (highest energy) longest wavelengths (lowest energy) Figure 6.1 – Properties of light. 400 nm 500 nm 600 nm 700 nm A B

6.2 Why Do Cells Use More Than One Photosynthetic Pigment? In 1882, Theodor Engelmann tested the hypothesis that the color of light affects the rate of photosynthesis Used motile, oxygen-requiring bacteria to identify where photosynthesis was taking place Directed a spectrum of light across individual strands of green algae

Why Do Cells Use More Than One Photosynthetic Pigment? (cont’d.) In Engelmann’s experiment, oxygen- requiring bacteria gathered where blue and red light fell across the algal cells Conclusion: Blue and red light are best light for driving photosynthesis in these algal cells

Why Do Cells Use More Than One Photosynthetic Pigment? (cont’d.) phycoerythrobilin phycocyanobilin chlorophyll b chlorophyll a bacteria β-carotene algae Figure 6.3 - {Animated} Discovery that photosynthesis is driven best by particular wavelengths of visible light. 400nm 500nm 600nm 700nm 400nm 500nm 600nm 700nm Wavelength B C

Capturing a Rainbow Photosynthesis use pigments to capture light of specific wavelengths Chlorophyll α: most common photosynthetic pigment Accessory pigments harvest additional light wavelengths

Capturing a Rainbow (cont’d.) Figure 6.2 - Examples of photosynthetic pigments. Left, photosynthetic pigments can collectively absorb almost all visible light wavelengths. Right, the light-catching part of a pigment (shown in color) is the region in which single bonds alternate with double bonds. These and many other pigments (including heme, Section 5.5) are derived from evolutionary remodeling of the same compound. Animals convert dietary beta-carotene into a similar pigment (retinal) that is the basis of vision.

What Happens During Photosynthesis? (cont’d.) Photosynthesis is often summarized as: CO2 + water sugars + O2 light energy

6.3 What Happens During Photosynthesis? Photosynthesis converts light energy into the energy of chemical bonds Bond energy can then power the reactions of life.

What Happens During Photosynthesis? (cont’d.) In eukaryotes, photosynthesis takes place in chloroplasts Light-dependent reactions on thylakoid membrane Converts light energy to ATP and NADPH Light-independent reactions in the stroma ATP and NADPH drive synthesis of sugars from water and CO2

What Happens During Photosynthesis? (cont’d.) Light-dependent and light-independent reactions

6.4 How Do The Light-Dependent Reactions Work? Thylakoid membranes contain millions of photosystems Photosystem contain hundreds of chlorophylls and other molecules Water is the electron donor; electrons are energized by light absorbed by chlorophyll

How Do The Light-Dependent Reactions Work? (cont’d.) Figure 6.5 - A view of some components of the thylakoid membrane as seen from the stroma. Molecules of electron transfer chains and ATP synthases are also present, but not shown for clarity. photosystem light-harvesting complex

The Noncyclic Pathway (cont’d.) Light energy energizes electrons in photosystem II E.T.C. creates ion gradient; ATP synthases phosphorylate ADP; ATP is formed in the stroma Light energy energizes electrons in photosystem I Electrons move through a second electron transfer chain; NADPH forms

The Noncyclic Pathway (cont’d.) 1 light energy 5 light energy 8 electron transfer chain electron transfer chain 4 photosystem II 3 photosystem I 6 ATP synthase thylakoid compartment 2 7 H2O Figure 6.7 - {Animated} Light-dependent reactions, noncyclic pathway. ATP and oxygen gas are produced in this pathway. Electrons that travel through two different electron transfer chains end up in NADPH. stroma O2

6.5 How Do The Light-Independent Reactions Work? The Calvin–Benson cycle: Build sugars in the stroma of chloroplasts Driving force is ATP and NADPH that formed in the light-dependent reactions Uses carbon atoms from CO2 to make sugars (Carbon fixation)

Adaptations to Climate Stomata are tiny gateways for gases Open stomata: CO2 diffuse into photosynthetic tissues; O2 to diffuse out Closed stomata to conserve water on dry days Limit the availability of CO2 for the light- independent reactions; sugar synthesis slows

Adaptations to Climate (cont’d.) C3 plants use only the Calvin–Benson cycle to fix carbon When CO2 concentration declines, uses oxygen as a substrate in photorespiration When stomata are closed during the day, C3 plants lose carbon instead of fixing it

Adaptations to Climate (cont’d.) glycolate RuBP Calvin– Benson Cycle PGA Figure 6.9A Anatomical and biochemical specializations minimize photorespiration in C4 plants. sugar

Adaptations to Climate (cont’d.) C4 plants close stomata on dry days, but their sugar production does not decline Minimize photorespiration by storing carbon in its cells Examples: corn, switchgrass, and bamboo

Adaptations to Climate (cont’d.) CAM plants are like C4 plants that conserve water by fixing carbon Open stomata at night when low temperatures minimize water loss Examples: Pineapples, Jade Plants

Adaptations to Climate (cont’d.) Figure 6.10 - Crabgrass “weeds” overgrowing a lawn. Crabgrasses, which are C4 plants, thrive in hot, dry summers, when they easily outcompete Kentucky bluegrass and other fine-leaved C3 grasses commonly planted in residential lawns.

Adaptations to Climate (cont’d.) mesophyll cell bundle-sheath cell Figure 6.9 Anatomical and biochemical specializations minimize photorespiration in C4 plants. Micrographs show leaf cross sections.