Chapter 6 Where It Starts – Photosynthesis (Sections 6.1 - 6.4)

6.1 Green Energy Autotrophs make their own food using energy from the environment and inorganic carbon sources such as CO2 In photosynthesis, plants and other autotrophs capture light energy and use it to build sugars from water and CO2 Heterotrophs get energy and carbon from molecules that other organisms have already assembled

Key Terms photosynthesis Metabolic pathway by which most autotrophs capture light energy and use it to make sugars from CO2 and water autotroph Organism that makes its own food using carbon from inorganic molecules, and energy from the environment heterotroph Organism that obtains energy and carbon from organic compounds assembled by other organisms

6.2 Sunlight as an Energy Source Visible light drives photosynthesis, which begins when photons (discrete packets of light energy) are absorbed (from the sun) by photosynthetic pigment molecules (including chlorophyll) Pigments are molecules which reflect a specific wavelength of light. White light hits chlorophyll, but only green is reflected (into our eyes.) Photosynthesis converts light energy to chemical energy (stored in the bonds which hold sugar together).

Properties of Light Light energy travels in waves, and is organized as photons Visible light is a very small part of the spectrum (beyond ROYGBIV) of electromagnetic energy radiating from the sun We see light of particular wavelengths as different colors (they strike cells in the back of our eyes called “cones”) wavelength Distance between crests of two successive waves of light

Properties of Light range of most radiation reaching Earth’s surface range of heat escaping from Earth’s surface shortest wavelengths (highest energy) longest wavelengths (lowest energy) visible light gamma rays x-rays ultraviolet radiation near-infrared radiation infrared radiation microwaves radio waves 400 500 600 700 A Wavelengths of visible light (in nanometers) Figure 6.2 Properties of light. A Electromagnetic spectrum of radiant energy, which undulates across space as waves that we measure in nanometers. Visible light makes up a very small part of the spectrum. Raindrops or a prism can separate its different wavelengths, which we see as different colors. B Light is organized as packets of energy called photons. The shorter a photon’s wavelength, the greater its energy. B Higher energy Lower energy Fig. 6.2, p. 94

ANIMATION: Wavelengths of light 7

Pigments: The Rainbow Catchers Pigments are molecules that absorb light of particular wavelengths; photons that are not captured by a pigment are reflected as its characteristic color Chlorophyll a, the main photosynthetic pigment, absorbs violet and red light, and so it appears green Accessory pigments absorb additional wavelengths (those visible in leaves in fall after chlorophyll has degraded)

Key Terms chlorophyll a pigment An organic molecule that selectively absorbs light of certain wavelengths Main photosynthetic pigment in plants

Some Accessory Pigments Accessory pigments color familiar roots, fruits, and flowers and (some) in autumn leaves: Beta-carotene in carrots (orange) Zeaxanthin in corn (yellow) Lycopene in tomatoes (red) Anthocyanin in violets (blue)

Photosynthetic Pigments Figure 6.3 Examples of photosynthetic pigments. A Collectively, photosynthetic pigments are capable of absorbing almost all visible light wavelengths. B Structures of two photosynthetic pigments. The light-trapping ring structure of chlorophyll is almost identical to a heme group. Heme groups are part of hemoglobin, which is a red pigment

Pigment Function Absorbing a photon excites electrons in the pigment and boosts them to a higher energy level (next principle energy level or electron shell) Photosynthetic cells can capture energy emitted from an electrons as they return to a lower energy level When the energy is passed to a special pair of chlorophylls, the reactions of photosynthesis begin

Pigment Structure The light-trapping part of a pigment is an array of atoms in which single bonds alternate with double bonds Such arrays easily absorb photons, so pigment molecules function a bit like antennas

6.3 Exploring the Rainbow In 1882, botanist Theodor Engelmann identified the colors of light (red and violet) that drive photosynthesis in a photosynthetic alga (Chladophora) His results constituted an absorption spectrum, which is a graph that shows how efficiently the different wavelengths of light are absorbed by a substance

Engelmann’s Experiment Engelmann directed light through a prism so that bands of colors crossed a water droplet on a microscope slide

Engelmann’s Experiment bacteria alga 400 500 600 700 Figure 6.4 Discovery that photosynthesis is driven by particular wavelengths of light. Theodor Engelmann used the green alga Chladophora A in an early photosynthesis experiment B. His results constituted one of the first absorption spectra. C Absorption spectra of chlorophylls a and b, β-carotene, and two phycobilins reveal the efficiency with which these pigments absorb different wavelengths of visible light. Wavelength (nanometers) B Engelmann directed light through a prism so that bands of colors crossed a water droplet on a microscope slide. The water held a strand of Chladophora and oxygen requiring bacteria. The bacteria clustered around the algal cells that were releasing the most oxygen—the ones that were most actively engaged in photosynthesis. Those cells were under red and violet light. Fig. 6.4b, p. 96

Absorption Spectra

Wavelength (nanometers) phycoerythrobilin 100 chlorophyll b phycocyanobilin chlorophyll a β-carotene 80 60 Light absorption (%) 40 20 Figure 6.4 Discovery that photosynthesis is driven by particular wavelengths of light. Theodor Engelmann used the green alga Chladophora A in an early photosynthesis experiment B. His results constituted one of the first absorption spectra. C Absorption spectra of chlorophylls a and b, β-carotene, and two phycobilins reveal the efficiency with which these pigments absorb different wavelengths of visible light. Wavelength (nanometers) C Absorption spectra of a few photosynthetic pigments. Line color is the characteristic color of each pigment. Fig. 6.4c, p. 96

Key Concepts The Rainbow Catchers The flow of energy through the biosphere starts when chlorophylls and other photosynthetic pigments absorb the energy of visible light In plants, some bacteria, and many protists, that energy ultimately drives the synthesis of glucose and other carbohydrates

6.4 Overview of Photosynthesis Photosynthesis occurs in chloroplasts Plant chloroplasts have two outer membranes, and are filled with a semifluid matrix called stroma Stroma contains an inner, folded thylakoid membrane which forms stacks of disks (thylakoids) connected by channels

Key Terms chloroplast Organelle specialized for photosynthesis in plants and some protists stroma Semifluid matrix between the thylakoid membrane and the two outer membranes of a chloroplast thylakoid membrane A chloroplast’s highly folded inner membrane system; forms a continuous compartment in the stroma

The Chloroplast

The Chloroplast two outer membranes of chloroplast stroma part of thylakoid membrane system: Figure 6.5 The chloroplast: site of photosynthesis in the cells of typical leafy plants. The micrograph shows chloroplast-stuffed cells of a moss, Plagiomnium affine. thylakoid compartment, cutaway view Fig. 6.5b, p. 97

Photosynthesis An overall formula for photosynthesis: 6CO2 (carbon dioxide) + 6H2O (water) → light energy → C6H12O6 (glucose) + 6O2 (oxygen) Photosynthesis is a series of reactions that occur in two stages: light-dependent reactions and light-independent reactions

Key Terms light-dependent reactions First stage of photosynthesis (“photo-”) Occur at the thylakoid membrane Convert light energy to chemical energy (ATP, NADPH) light-independent reactions Second stage of photosynthesis (“-synthesis”) Occur in the stroma Use ATP and NADPH to assemble sugars from water and CO2

Two Stages of Photosynthesis

Key Concepts What Is Photosynthesis? Photosynthesis has two stages in the chloroplasts of plants and many types of protists In the first stage, sunlight energy is converted to chemical energy Molecules that form in the first stage of photosynthesis power the formation of sugars in the second stage