Introduction to Photosynthesis

1. Introduction to Photosynthesis • Photos = light • Synthi = to put together • Biochemical conversion of light energy into chemical energy by autotrophs Adapted from: G. von Bargen and J. Nafftzinger

2. Science as a Process • Greeks believed that plants derive their substance from soil. • Jan van Helmont showed that the materials that make up plants does not come from the soil.

3. Science as a Process • Willow tree gained 164 lbs, but the soil only lost a few ounces. • Plants must make their own food!

4. Science as a Process • Joseph Priestly determined that plants put something into the air that animals need. • Priestly is credited with the discovery of oxygen as a plant by-product!

5. Science as a Process • Mouse in jar alone, dies. • Mouse in jar with a plant, lives.

6. Science as a Process • Melvin Calvin figured out the steps of the photosynthetic process! • The Nobel Prize in Chemistry 1961 "for his research on the carbon dioxide assimilation in plants“ • Calvin and his team determined the exact reactants and products of photosynthesis by radioactively labeling atoms.

7. What is needed? • Raw materials: • Water, sunlight, carbon dioxide • Cellular components: • Chloroplasts • Biochemical components: • Chlorophyll (or some other pigment, but usually chlorophyll) • Electron acceptor (NADP) • ATP

8. light 6CO2 + 6H2O C6H12O6 + 6O2 chlorophyll Chemical equation

9. Light from the sun is composed of a range of wavelengths Sunlight is called white light, but it is actually made up of a wide range of wavelengths (colors), including red, green, and blue This is a form of electromagnetic radiationThe colors of the spectrum of visible white light are not absorbed equally by chloroplasts

10. Some substances called pigments can absorb light The main pigment used to absorb light in photosynthesis is chlorophyll The structure of chlorophyll allows it to absorb some colors or wavelengths of light better than others Red and blue light are absorbed more than green The green light that chlorophyll cannot absorb is reflected This makes chlorophyll and, therefore, chloroplasts and plant leaves look green Packets of light (photons) from visible light spectrum excite electrons on chlorophyll, causing them to jump an energy level

11. Spectra for Photosynthesis Note the wavelengths at which chlorophylls a and b are active Also note carotenoids’ activity Absorption Spectrum shows amount light absorbed by pigments at each wavelength Action Spectrum shows rate of photosynthesis at each wavelength All pigments combine absorb greater range, but green is absorbed less

12. Where does it happen? • In the leaves of plants • In chloroplasts of autotrophic protists • On membranes of autotrophic bacteria • Organisms that can perform photosynthesis are called autotrophs, whereas those that cannot are called heterotrophs

14. Leaf structure Epidermis Covered with cuticle Mesophyll cover wide area to maximize light absorption Palisade Upper layer Contain chloroplasts Spongy Note the air spaces Gas exchange occurs here Contain fewer chloroplasts

15. Stomates • Pores on underside of leaf • Guard cells on either side control opening and closing

16. Chloroplast structure Thylakoidflat, inner membranes have chlorophyll embedded, photosystems, and ETCs; large SA allows for maximum light absorption Site for light dependent reactions Small space inside thylakoids allow H+ to accumulate Granastacked structures of thylakoid forming compact, circular piles Stroma liquid, watery matrix is site for enzymes fixation and light independent reaction, also known as Calvin Cycle

17. Chloroplast Structure

18. Chlorophyll a & b: Main Photosynthetic Pigments Water insoluble Large heme group with Mg+2 in center Accessory pigments like carotenoids absorb light chlorophyll does not Aid in photosynthesis by passing energy to chlorophyll Absorbs two wavelengths of light: Blue: 420 nm Red: 660 nm

19. Chlorophyll a • Is only pigment that directly participates in the light- dependent reactions • Other pigments add energy to chlorophyll a or dissipate excessive light energy • Absorption of light elevates an electron to a higher energy orbital (increased potential energy)

20. Photosynthesis Basics • In plants, photosynthesis occurs in the chloroplasts • Thylakoid membrane • Grana • Stroma • Photosynthesis uses CO2, water, and light to produce glucose and O2.

21. Photosynthesis Basics • Where do the reactants come from? • CO2 • Air; enters through pores in the leaf’s surface called stomata • Water • Acquired by the roots and transported to leaves • Light Energy • From the Sun

22. Overview • Photosynthesis occurs in a two-pathway process. • Light Dependent Reactions • Light Independent Reactions

23. Overview • Light-dependent reactions • Chlorophyll and other molecules of the thylakoids capture sunlight energy • Sunlight energy is converted to the energy carrier molecules ATP and NADPH • Oxygen gas is released as a by-product

24. Overview • Light-independent reactions • Enzymes in the stroma synthesize glucose and other organic molecules from CO2 using the chemical energy stored in ATP and NADPH

25. How does it happen? Two major phases: Light-dependent energy-harvesting Light-independent carbon-fixing

27. Photosynthesis Rap