PHOTOSYNTHESISPHOTOSYNTHESISThe Process That Feeds the Biosphere

•All organisms either directly or indirectly depend on photosynthesis •Autotrophs-Produce organic molecules from CO2 & other inorganic materials

•Heterotrophs-

*Plants, algae, & some protists are photoautotrophs, they use light to synthesize glucose

Depend on organic compounds produced by other organisms

“self-feeders” Consumers

All heterotrophs depend on autotrophs for food and the oxygen that’s a biproduct of

photosynthesis

Photosynthsis Converts Light Energy to Chemical Energy of Food

Which organelle is responsible for photosynthesis??

Chloroplasts

What is the chemical energy that autotrophs produce??Glucose

What are the raw materials that go into photosynthesis? What are the products? 6CO2 + 6H2O + Light Energy C6H12O6 + 6O2

Which plant organ is the primary site of photosynthesis?Leaves

Anatomy of the Leaf & Its Chloroplasts

How do we know what we know about photosynthesis??

Tracking Atoms:The OVERALL chemical reaction of photosynthesis is the reverse of cell respiration, but the process of photosynthesis is NOT the reverse of cell respiration:

What’s the equation for cell respiration?C6H12O6 + 6O2 6CO2 + 6H2O + ATP energy

What’s the equation for photosynthesis?6CO2 + 6H2O + light energy C6H12O6 + 6O2

Sunlight

Photosynthesis

Cellularrespiration

Heat

Relationship Between Cell Respiration & Photosynthesis

Photosynthesis Equation in its Simplest Form:

CO2 + H2O [CH2O] + O2

General formula for a carbohydrate

The Splitting of WaterThe Splitting of WaterBefore the 1930’s, it was widely believed that the O2 produced by plants is a result of the splitting of CO2 during photosynthesis

Thanks to C.B. van Niel & others, we now know that the O2 produced is a result of the splitting of a water molecule during photosynthesis:

Van Niel’s Experimental ResultsSulfur bacteria: CO2 + 2H2S [CH2O] + H2O + 2S

Plants: CO2 + 2H2O [CH2O] + H2O + O2

General: CO2 + 2H2X [CH2O] + H2O + 2X

Conclusion: photosynthetic organisms require a hydrogen source which is split during photosynthesis

Later Tests Using Oxygen-18 Isotope Markers Confirmed the Same….

Exp #1: When the oxygen in water is labeled…CO2 + 2H2O [CH2O] + H2O + O2

Exp #2: When the oxygen in carbon dioxide is labeled…CO2 + 2H2O [CH2O] + H2O + O2

Tracking Atoms Through Photosynthesis….

Photosynthesis as a Redox ProcessPhotosynthesis as a Redox ProcessCell Resp Photosynthesis

Employs redox reactions

Employs redox reactions

e- lose potential energy as they move down ETC

e- gain potential energyrequires energy from light

By-product = H2O By-product = O2

Gain or Lose?

Gain orLose?

? ?

Spinach leaf chromatographyMash a bit of spinach lead on the paper – the more pigment you get on the paper the better•Place the paper in a beaker with alcohol. •Be careful not to let the spinach line touch the alcohol•Watch carefully, without disturbing.

Two Stages of Photosynthesis: Light Reactions & Calvin Cycle

Light Reactions:•Light is absorbed by chlorophyll

•Energy from the light splits H2O•H from water reduces NADP+ (an e- acceptor) to NADPH•Chemiosmosis makes ATP from ADP. This process is known as photophosphorylation•Products of light reactions:Energy = ATP & NADPHBy-product = O2

Occurs in thylakoid

Two Stages of Photosynthesis: Light Reactions & Calvin Cycle

The Calvin Cycle:•CO2 from atmosphere enters chloroplast & is incorporated into organic molecules = carbon fixation

•NADPH from the light rxns reduces the fixed carbon (by donating H) to carbohydrate; ATP from light rxns is required for this step

•Product of Calvin Cycle:glucose

Occurs in stroma

The Light Reactions Convert Solar Energy to the Chemical Energy of

ATP & NADPHThe Nature of Sunlight (electromagnetic energy):•Light has properties of both waves & particles (photons)

•Visible light is the part of the EMS that drives photosynthesis

•The shorter the wavelength, the higher the energy

Photosynthetic Pigments are Light Receptors•Pigments, like chlorophyll either absorb, transmit, or reflect light

•The wavelength of light that is reflected is the color that we detect with our eyes

•Which wavelength (color) of Light is least effective for Photosynthesis??

Green

These accessory pigments allow chloroplasts to take advantage of a broader spectrum

Excitation of Chlorophyll by Light•Energy from a photon of light raises a pigment molecule’s e- from ground state to an unstable excited state •When the e- drops back down to its ground state, energy is emitted in the form of heat & light•Chloroplasts in plants don’t fluoresce because the excited e- is passed to an e- acceptor rather than dropping back down to ground state

A closer look at The Light Reactions

Light Reactions • Occurs in the

thylakoid membranes at regions called photosystems

• Reactants: light and water

• Products: oxygen, ATP, NADPH

A photosystem is a bundle of pigment molecules and proteins•Photosystems act as an antenna for light

Made of 2 chlorophyll “a” molecules & a primary e- acceptor

Special because they can use light to boost an e- to the next energy level…light energy has now been converted to chemical energy

• The light reactions are carried out by:– Photosystem II

• contains chlorophyll• Strips electrons from water

– An electron transport chain • makes ATP

– Photosystem I • contains chlorophyll• Gives electrons to NADPH

In between PS II and PS I is an ETC that works just like the one

in Cellular Respiration

• This is called: Non-Cyclic Flow of Electrons– Electrons move in a Z pattern from PS II to PS I, not a cycle– This process makes the ATP and NADPH for the Calvin Cycle

Photosystem I also carries out Cyclic Electron Flow

•Only ATP is produced, not NADPH

Cyclic e- Flow•This is because the Calvin Cycle needs more ATP than can be made by Non-Cyclic Electron Flow

Processing Time

• Label the diagram Include:

WaterLightPS IIETCPS IOxygen

H+e-ATPNADPHATP synthaseADP + Pi