Life on Earth is SOLAR powered Photosynthesis (Ps) nourishes almost all living organisms Autotrophs - mainly Ps organisms (photoautotrophs) that make their own food (using sun E, CO 2, and H 2 O) Also called producers of the biosphere Exs = green plants and Ps protist groups (fig 10.2) Heterotrophs - get E from organic compounds produced by other organisms Also called consumers of the biosphere Exs = fungi, animals, & many protist groups Photosynthesis converts light E to chemical E of food Introduction to Photosynthesis ( )

The Process That Feeds the Biosphere Photosynthesis – Is the process that converts solar (light) energy into chemical energy Plants and other autotrophs – Are the producers of the biosphere

Plants are photoautotrophs They use the energy of sunlight to make organic molecules from water and carbon dioxide Figure 10.1

Photosynthesis Occurs in plants, algae, certain other protists, and some prokaryotes These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). (a) Plants (b) Multicellular algae (c) Unicellular protist 10  m 40  m (d) Cyanobacteria 1.5  m (e) Purple sulfur bacteria Figure 10.2

Heterotrophs – Obtain their organic material from other organisms – Are the consumers of the biosphere – Includes fungi, animals, many protist groups and many bacteria

Primarily found in leaves (mesophyll = main part of a leaf) Stomata = regulated holes in leaves where gas exchange occurs (what gases does a plant need to exchange for Ps?) Organelles enclosed by a double-membrane system (endosymbiosis) Stroma = internal fluid-filled cavity Thylakoids = system of interconnected membrane sacs (separates the stroma from the thylakoid space) Grana = stacks of thylakoids Chlorophyll = green pigment that absorbs light E = molecular bridge between sunlight and Ps activity Molecules are embedded in the thylakoid membrane system Chloroplasts – Sites of Ps within the cell

Chloroplasts: The Sites of Photosynthesis in Plants The leaves of plants – Are the major sites of photosynthesis Vein Leaf cross section Figure 10.3 Mesophyll CO 2 O2O2 Stomata

Chloroplasts – Are the organelles in which photosynthesis occurs – Contain thylakoids and grana – Stroma is the fluid in the internal cavity – Chlorophyll is imbedded in the thylakoid membranes Chloroplast Mesophyll 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tracking Atoms Through Photosynthesis: Photosynthesis is summarized as 6 CO H 2 O + Light energy  C 6 H 12 O O H 2 O OR CO 2 + H 2 O  [CH 2 O] + O 2 Overall Ps equation has been known since the 1800s The equation for Ps (fig 10.4) = reverse of respiration But carbohydrates are not made by simply reversing what happens in respiration BOTH processes occur in plant cells!

The Splitting of Water Chloroplasts split water into – Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules 6 CO 2 12 H 2 O Reactants: Products: C 6 H 12 O 6 6H2O6H2O 6O26O2 Figure 10.4

Photosynthesis as a Redox Process Photosynthesis is a redox process – Water is oxidized, carbon dioxide is reduced

Two stages of Ps (fig 10.5): 1. Light rxns: depend on light  make ATP & NADPH and give off O 2 NADPH = very similar in structure to NADH (just add a phosphate group to NADH) = the e- carrier Photophosphorylation = how ATP is generated (using chemiosmosis again) 2. Calvin cycle: use ATP and NADPH to fix C from the atmosphere into organic compounds Carbon fixation = initial incorporation of C into organic compounds Two Stages of Photosynthesis

The Two Stages of Photosynthesis Photosynthesis consists of two processes – The light reactions – The Calvin cycle

The Light Reactions The light reactions – Occur in the grana – Split water, release oxygen, produce ATP, and form NADPH

The Calvin Cycle The Calvin cycle – Occurs in the stroma – Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

An overview of photosynthesis H2OH2O CO 2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH 2 O] (sugar) NADPH NADP  ADP + P O2O2 Figure 10.5 ATP

Light Reactions The light reactions convert solar energy to the chemical energy of ATP and NADPH

Light Light = electromagnetic energy, which travels in waves Wavelength = distance between crests/troughs of waves (nm - km) – Smaller wavelengths = stronger light waves Electromagnetic spectrum (fig 10.6) = entire range of light – Visible light ( nm) important to biological systems Different pigments absorb different wavelengths and reflect others (what we see that makes them colored) – What wavelength of light do plants reflect?

The Nature of Sunlight Light – Is a form of electromagnetic energy, which travels in waves Wavelength – Is the distance between the crests of waves – Determines the type of electromagnetic energy

The electromagnetic spectrum – Is the entire range of electromagnetic energy, or radiation Gamma rays X-raysUVInfrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy Figure 10.6

The visible light spectrum – Includes the colors of light we can see – Includes the wavelengths that drive photosynthesis

Photosynthetic Pigments Photosynthetic pigments absorb specific wavelenths of light Absorption spectrum = a pigment’s light absorption vs. wavelength Spectrophotometer = instrument that measures absorbance of specific wavelengths (fig 10.8) Beam of light sent through solution  fraction of light transmitted at each wavelength measured

Photosynthetic Pigments: Light Receptors Photosynthetic Pigments – Are substances that absorb specific wavelengths within the visible light spectrum

Pigments – Reflect some light, which include the colors we see Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7

The spectrophotometer – Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength Transmitted light is NOT absorbed by that particular pigment

An absorption spectrum – Is a graph plotting light absorption versus wavelength Figure 10.8 White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue light

Photosynthetic Pigments Chlorophyll a (fig 10.10) absorption spectrum (fig 10.9a) Chlorophyll b = accessory pigment similar to chl. a When chlorophyll pigment absorbs light  energy boosts an e- to an orbital of higher energy level (pigment is in its excited state) If chlorophyll is isolated from chloroplast (fig 10.11)  fluoresces (emits light) in red-orange end of spectrum (E given off as heat) Carotenoids = other accessory pigments (hydrocarbons) reflecting various shades of orange/yellow/red (fig 10.9a) Most important function = photoprotection (absorb & dissipate excess light E)

Pigment Absorption Spectra The absorption spectra of chloroplast pigments – Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

Absorption spectra of three pigments in chloroplasts Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Absorption of light by chloroplast pigments Chlorophyll a (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Wavelength of light (nm) Chlorophyll b Carotenoids Figure 10.9

The action spectrum for photosynthesis Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis Rate of photosynthesis (measured by O 2 release) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. (b)

The action spectrum for photosynthesis Was first demonstrated by Theodor W. Engelmann Aerobic bacteria Filament of alga Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O 2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. (c) Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. CONCLUSION

Types of Chlorophyll Chlorophyll a – Is the main photosynthetic pigment Chlorophyll b – Is an accessory pigment C CH CH 2 C C C C C CN N C H3CH3C C C C C C C C C N C C C C N Mg H H3CH3C H C CH 2 CH 3 H C H H CH 2 H CH 3 C O O O O O CHO in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Figure 10.10

Other Pigments Other accessory pigments – Absorb different wavelengths of light and pass the energy to chlorophyll a

Excitation of Chlorophyll by Light When a pigment absorbs light – It goes from a ground state to an excited state, which is unstable Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground state Photon e–e– Figure A

Chlorophyll absorbs energy If an isolated solution of chlorophyll is illuminated – It will fluoresce, giving off light and heat – The excited electron drops back to the ground-state orbital. Figure B

Tomorrow, we will start with the different types of photosynthetic pigments, and which wavelengths of light each absorbs. We will also discuss the light reaction portion of photosynthesis. The light reaction produces ATP and NADPH which go to power the fixation and reduction of carbon dioxide into sugar by the Calvin Cycle.