BIO 10 BIO 10 Lecture 8Lecture 8BIO 10 BIO 10 Lecture 8Lecture 8

THE VITAL FORCE: THE VITAL FORCE:

PHOTOSYNTHESISPHOTOSYNTHESIS

Photosynthesis = the process by which living organisms harvest the energy of the sun to reduce CO2 and H2O to energy-rich carbohydrates

Recall that reduction involves adding electrons to molecules and is endergonic (requires energy)

(C6H10O5)n

reductionCO2 + H2O +

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 PhaseStep 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 O2

Step 3: The O2 diffuses out of the cell and is expired by the plantStep 4: H+ begins to build up in the stroma

Light Dependent PhaseStep 5: The electron carrier molecule now transfers the electron it received from chlorophyll down a transport chain, similar to that in the mitochondrionStep 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 PhaseStep 8: At the end of the electron transport chain, the electrons are dumped onto a second set of chlorophyll molecules at Photosystem IStep 9: Light hits these chlorophyll molecules and kicks the electrons off them onto electron carriersStep 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 CO2

(“breathed in” by the plant) and H2O 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

• 6CO2 + 6H2O + sunlight C6H12O6 + 6O2

• Respiration• C6H12O6 + 6O2 6CO2 + 6H2O + 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?