Biology 102Biology 102

Photosynthesis 1Photosynthesis 1

Lecture OutlineLecture Outline

1.1. Back to the dinosaurs…Back to the dinosaurs…

2.2. What is photosynthesis (and “who” does What is photosynthesis (and “who” does it)?it)?

3.3. The light-dependent reactionsThe light-dependent reactions

1. Back to the dinosaurs…1. Back to the dinosaurs…

2. What is photosynthesis?2. What is photosynthesis?

Photo: “light”Photo: “light”Light energy is stored in the energy of chemical Light energy is stored in the energy of chemical

bonds.bonds. Synthesis: “build”Synthesis: “build”

Organic molecules are built from inorganic Organic molecules are built from inorganic moleculesmolecules

Carbon-carbon bonds are formed!Carbon-carbon bonds are formed!

The formulas for photosynthesis and The formulas for photosynthesis and cellular respirationcellular respirationWrite it down!Write it down!

The relationship between photosynthesis The relationship between photosynthesis and cellular respirationand cellular respiration

Organisms that photosynthesizeOrganisms that photosynthesize

Eukaryotes that Eukaryotes that photosynthesizephotosynthesizePlants (all!)Plants (all!)Some protistsSome protists

Marine phytoplanktonMarine phytoplankton Freshwater and marine Freshwater and marine

algaealgae

Prokaryotes that Prokaryotes that photosynthesizephotosynthesizeExample: Example:

CyanobacteriaCyanobacteria

Leaves (plants only) and chloroplasts Leaves (plants only) and chloroplasts (eukaryotes only)(eukaryotes only)

Chloroplasts within mesophyll cellsChloroplasts within mesophyll cells Epidermis is thin and transparent (why?)Epidermis is thin and transparent (why?)

Photosynthesis involves two sets of Photosynthesis involves two sets of reactionsreactions

The light-dependent reactions (“photo”) The light-dependent reactions (“photo”) Light energy is captured from the sun and turned into Light energy is captured from the sun and turned into

chemical energychemical energyThe energy is temporarily stored within energy carrier The energy is temporarily stored within energy carrier

moleculesmolecules

The light-The light-inindependent reactions (“synthesis”)dependent reactions (“synthesis”)Temporarily-stored energy is used to build the glucose Temporarily-stored energy is used to build the glucose

molecule from carbon dioxide and watermolecule from carbon dioxide and waterEnergy is stored in the carbon-carbon bonds!Energy is stored in the carbon-carbon bonds!

3. The light-dependent reactions3. The light-dependent reactions

What is accomplished?What is accomplished?Light energy is turned into chemical energyLight energy is turned into chemical energy

Overview of what happensOverview of what happensLight is captured by pigments in the Light is captured by pigments in the

chloroplastchloroplast Electrons of the pigments “absorb” the energyElectrons of the pigments “absorb” the energy

Electrons are transferred between compoundsElectrons are transferred between compoundsATP and NADPH are both formedATP and NADPH are both formed

What is light?What is light? Light is one type of energy within the Light is one type of energy within the

electromagnetic spectrumelectromagnetic spectrum Different wavelengths of light have different Different wavelengths of light have different

amounts of energyamounts of energy

What wavelengths of light do What wavelengths of light do different plant pigments absorb?different plant pigments absorb?

Note that the color Note that the color you see is the color you see is the color that is that is notnot absorbed, but absorbed, but rather reflected!rather reflected!

How is the energy of light actually How is the energy of light actually captured?captured?

Summary of light-dependent reactionsSummary of light-dependent reactions

1.1. Light energy is “absorbed” Light energy is “absorbed” by pigments of the light-by pigments of the light-harvesting complex and harvesting complex and passed to a special passed to a special chlorophyll molecule, the chlorophyll molecule, the “reaction center”“reaction center”

2.2. An electron in the reaction An electron in the reaction center chlorophyll is excited center chlorophyll is excited and “jumps” from the and “jumps” from the chlorophyll molecule to…chlorophyll molecule to…

3.3. An electron acceptor at a An electron acceptor at a higher energy levelhigher energy level

4.4. The electron is passed between molecules The electron is passed between molecules of the electron transport systemof the electron transport system

The energy gained (from moving the electron The energy gained (from moving the electron to lower energy compounds) is used to to lower energy compounds) is used to synthesize ATPsynthesize ATP

5.5. The electron ends up in The electron ends up in another chlorophyll another chlorophyll molecule (Photosystem molecule (Photosystem I)I)

6.6. An electron in the An electron in the reaction center reaction center chlorophyll is excited chlorophyll is excited and “jumps” from the and “jumps” from the chlorophyll molecule chlorophyll molecule to…to…

7.7. An electron acceptor at An electron acceptor at a higher energy level a higher energy level

8.8. The electrons are transferred along a The electrons are transferred along a second electron transport chain (along second electron transport chain (along with an Hwith an H++ ion…) to a molecule of NADPH ion…) to a molecule of NADPH

Where did the mysterious H+ come Where did the mysterious H+ come from? What about the missing electron?from? What about the missing electron?

9.9. It’s all about the water!It’s all about the water!

Review of light-dependent reactions Review of light-dependent reactions

End result of light-dependent reactionsEnd result of light-dependent reactions

Energy from sun was captured and Energy from sun was captured and converted to chemical energy converted to chemical energy ATP and NADPHATP and NADPH

Water was split to provide e- and HWater was split to provide e- and H++ and and generated oxygen generated oxygen as a bi-productas a bi-product!!