Chapter 7 Photosynthesis: Using light energy to produce organic molecules

I. Study of Photosynthesis u Before 1600: Ancient Greeks proposed that plants obtained all nourishment from soil. u Mid 1600s: Jan Baptista van Helmont performed experiments to determine where plants obtain nourishment from. F Planted small willow tree in a pot with soil. F Added water only, for five years. F Plant gained 75 kilograms. F Soil had lost 60 grams.

Add water only for five years Plant: 75 kg gained Soil: 0.06 kg lost Helmont’s Experiment

I. Study of Photosynthesis Helmont’s conclusions: F Plants do not obtain all nutrients from soil (correct) F They obtain most nutrients from water (wrong).

I. Study of Photosynthesis u 1770: Joseph Priestley consistently observed that a candle in a closed jar would not burn out if a mint plant was added to the jar. Priestley’s conclusion: Plants “restore” the air depleted by a candle. u Late 1770s: Jan Ingenhousz showed that plants require light to “restore” the air in a closed jar. u 1800s: Photosynthesis reaction elucidated. u 1950s: Discovered that oxygen produced by plants comes from water.

Photosynthesis Requires Water, Carbon Dioxide, and Light

II. Net Reaction of Photosynthesis 6CO 2 + 6H 2 O + LIGHT ENERGY ---> C 6 H 12 O 6 + 6O 2 REDOX REACTION u Carried out by photosynthetic autotrophs (plants, algae, and some bacteria). Also called producers. u Produce 200 billion tons of organic material from CO 2 every year. u Almost all living organisms obtain energy directly or indirectly from photosynthesis. F Exceptions: A few bacteria metabolize sulfur or iron and some animals that eat them, don’t depend on photosynthesis.

Photosynthesis is a Redox Reaction

III. Chloroplasts are site of photosynthesis in eucaryotes u All green parts of a plant carry out photosynthesis. u Most chloroplasts are found in leaves, specifically in mesophyll, green tissue in interior of leaves. u Green color is due to chlorophyll, a light absorbing pigment. u In bacteria, photosynthesis occurs on extensions of the cell membrane. u Stomata: Pores in leaf for exchange of CO 2 and O 2

Chloroplasts are the Site of Photosynthesis

Areas of Chloroplast Important for Photosynthesis Thylakoids: Membrane “discs” arranged in stacks (grana) which contain chlorophyll and other important molecules. Site where solar energy is trapped and converted into chemical energy (light reactions). Thylakoid Membrane: Site of ATP synthesis. Stroma: Thick fluid outside thylakoid membranes, surrounded by interior membrane. Site of sugar synthesis (dark reactions).

Chloroplasts Have Three Sets of Membranes

IV. Plants produce oxygen by “splitting” water n Water is used as a source of H and electrons to reduce CO 2 6CO 2 + 6H 2 O + ENERGY ---> C 6 H 12 O 6 + 6O 2 Where does the free oxygen come from? CO 2 or H 2 O Label the CO 2 or H 2 O with radioactive O 18 u CO 2 + 2H 2 O > CH 2 O + H 2 O + O 2 **** Free oxygen comes from the splitting of water, not CO 2.

Oxygen Generated by Photosynthesis Comes from Water

IV. Light reactions trap energy and electrons required to make sugar from CO 2 n Light reactions: Require light. Convert light energy to chemical energy of ATP and reducing power of NADPH u Occur in the thylakoid membranes of chloroplast u Water is split with energy from sun into free O 2, H and electrons. u Reduce NADP + to NADPH: High energy electrons and H obtained from splitting of H 2 O u Photophosphorylation: Light energy is used to produce ATP from ADP + P i u ATP synthesis is driven by chemiosmosis Input: ADP, NADP+, water, and light. Output: ATP, NADPH, and O 2.

Light Dependent Reactions: Light Energy Trapped by Chlorophyll is Used to Split Water, Make NADPH & ATP

V. Light Independent (Dark) reactions (Calvin Cycle) make sugar from CO 2 n Calvin Cycle: Uses ATP and NADPH produced by light reactions to reduce CO 2 to glyceraldehyde-3-phosphate u Occurs in the stroma of chloroplast u Don’t need light directly. u Carbon fixation: Process of gradually reducing CO 2 gathered from atmosphere to organic molecules u NADPH provides H and electrons to reduce CO 2 and ATP provides energy. Input: CO 2, ATP, and NADPH. Output: Sugars, ADP, and NADP+.

Light Independent Reactions: Sugar Synthesis

NOTE: u Light reactions: Transform light energy into usable form of chemical energy (ATP and NADPH). Water is split to obtain H. u Light independent reactions (Calvin cycle): Use chemical energy (ATP and NADPH) to drive the endergonic reactions of sugar synthesis.

Light and Dark Reactions of Photosynthesis

V. Properties of light energy: A portion of the electromagnetic spectrum n Visible light: A small portion of the electromagnetic energy spectrum which cells in our retina can detect ( wavelength: nm). u Wavelength: Distance between two “crests” of light wave u Photon: Discrete “particles” of light energy u Energy: The amount of energy is INVERSELY proportional to the wavelength of light

Smaller wavelengthLonger wavelength Gamma X-rays UV visible infrared microwaves radio rays light light light waves Visible light spectrum Wavelength in nanometers: VIOLET BLUE GREENYELLOW ORANGE RED Higher EnergyLower Energy

White Light is a Spectrum of Different Lights Isaac Newton ( ) separated white light by passing it through a prism.

Chlorophyll Absorbs Portions of Visible Light Spectrum Green light is reflected by chlorophyll

VI. Pigments allow plants to absorb various wavelengths of light n Pigments: Molecules that absorb light energy u Black object: All wavelengths are absorbed u White object: All wavelengths are reflected u Green object: All wavelengths BUT green are absorbed n Absorption spectrum: Shows wavelengths absorbed by a certain pigment

n Plants use different pigments to capture light energy, each has its own unique absorption spectrum u Chlorophyll a: Primary light absorption pigment u Chlorophyll b: Accessory light absorption pigment u Carotenoids: Accessory light absorption pigments

Structure of a Chlorophyll Molecule

IX. How is ATP produced in photosynthesis? n Answer: By chemiosmosis through the development of a proton gradient in the chloroplast. n NOTE: The synthesis of ATP in both oxidative phosphorylation and photosynthesis is linked to the production of a proton gradient. u Electron transport chain pumps H + to create gradient u ATP synthase: Uses energy as H + flows “downhill” to drive the synthesis of ATP from ADP and P i

ATP Production Requires a Proton Gradient

Photosynthesis Helps Counteract the Greenhouse Effect u u The earth’s atmosphere contains about 0.03% of carbon dioxide. u u Carbon dioxide traps solar energy in the atmosphere, making the earth about 10 o C warmer than it would otherwise be. u u Since the mid 1800s, the atmospheric levels of carbon dioxide have risen steadily due to the burning of fuels and forests. u u The “Greenhouse Effect” refers to the global warming that is caused by increased atmospheric carbon dioxide levels. u u Global warming may cause polar ice caps to melt, which in turn could cause massive coastal flooding and other problems. u u Plants use up about half of carbon dioxide generated by humans and other organisms.

Greenhouse Effect: Heat is Trapped by Carbon Dioxide