BIO 10 Lecture 8 THE VITAL FORCE: PHOTOSYNTHESIS

Photosynthesis = the process by which living organisms harvest the energy of the sun to reduce CO 2 and H 2 O to energy-rich carbohydrates Recall that reduction involves adding electrons to molecules and is endergonic (requires energy) (C 6 H 10 O 5 ) n reduction CO 2 + H 2 O + SUNLIGHT

Only certain organisms can photosynthesize: Plants Cyanobacteria Algae The entire ecosystem runs on the work done by these organisms Organisms like ourselves, who rely entirely on respiration for energy, can only do so because the photosynthesizing organisms make the energy-rich molecules for us We can then eat those molecules and break them down into useable energy "chunks" as ATP

The Nature of Light Light is very mysterious –Electromagnetic waves as well as quantum “packets” of energy called photons –Light waves always travel at the same speed through a vacuum Doesn’t travel with respect to you, the Earth, or anything else Doesn’t “wiggle” anything –Not at all like sound waves or water waves –Wavelength determines the amount of energy carried by each photon

–Light waves have different wavelengths in a spectrum from gamma rays (most energetic, shortest) to radio waves (least energetic, longest), only a portion of which are visible. –Photosynthetic organisms are green because the pigment chlorophyll absorbs only certain wavelengths of light (mostly red and blue) –The wavelengths it does not absorb are radiated back to our eyes and, collectively, these appear green to our eyes

The Leaf: Site of Photosynthesis in Plants Most photosynthesis occurs here Like the mitochondrion, the chloroplast has a double membrane Unlike the mitochondrion, the chloroplast also contains membrane- bound inner compartments called thylakoids

Where Photosynthesis Takes Place Photosynthesis begins in the thylakoid membrane - Contains the chlorophyll molecules on Photosystems Photosynthesis ends in the stroma (with starch) -The stroma surrounds the thylakoids and is enclosed by the double membrane of the chloroplast Stroma

Photosynthesis Occurs in Two Phases: Light Dependent Phase Step 1: Chlorophyll absorbs a photon at Photosystem II and one of its electrons is kicked off onto an electron carrier molecule Step 2: Now lacking an electron, chlorophyll grabs one from a water molecule in the stroma, splitting the water into H+ and O 2 Step 3: The O2 diffuses out of the cell and is expired by the plant Step 4: H+ begins to build up in the stroma

Light Dependent Phase Step 5: The electron carrier molecule now transfers the electron it received from chlorophyll down a transport chain, similar to that in the mitochondrion Step 6: At each step, the energy released by the transport of the electrons is used to pump H+ ions against their gradient into the stroma This process produces an H+ gradient, with much higher concentrations of H+ inside the stroma than outside Step 7: The H+ ions flow out of the stroma with their concentration gradient through an ATP synthase enzyme pore, driving the production of ATP from ADP + P -This ATP will be used in the Light Independent Reaction to drive the synthesis of starch

Light Dependent Phase Step 8: At the end of the electron transport chain, the electrons are dumped onto a second set of chlorophyll molecules at Photosystem I Step 9: Light hits these chlorophyll molecules and kicks the electrons off them onto electron carriers Step 10: The electrons are passed down a second electron transport chain and are finally dumped onto NADP to form NADPH, a molecule that will carry electrons and hydrogens to the Light Independent (starch-building phase) of photosynthesis

ATP to Calvin cycle

Movement of ATP & NADPH to the Light Independent Reaction

Phase 2: Light-independent Reactions –ATP and NADPH are not good permanent storage molecules, so the plants convert the energy into carbohydrates Electrons from carriers are added to CO 2 (“breathed in” by the plant) and H 2 O to reduce these molecules to starch Reactions occur in the stroma of the thylakoid Main cycle of reactions is called the Calvin Cycle

Summary of Photosynthesis

Chemical Equations Photosynthesis 6CO 2 + 6H 2 O + sunlight  C 6 H 12 O 6 + 6O 2 Respiration C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + ATP

Short Review of Lecture 8 Compare the light dependent and light independent reactions of photosynthesis. What goes into each reaction and what is produced by each? In what ways are the processes of photosynthesis and respiration similar? In what ways are they different? How can you account for the similarities between the two processes?