Where It Starts - Photosynthesis What Is Photosynthesis? The Rainbow Catchers Making ATP and NADPH Making Sugars Alternate Pathways
What is Photosynthesis? Energy flow through ecosystems begins when photosynthesizers intercept sunlight Photosynthesis is the process by which plants, some protists and some prokaryotes convert light energy from the sun into chemical energy
Autotrophs Meaning self-feeders Ø Make food using energy from environment and carbon from inorganic molecules Ø Ultimate source of organic molecules for other living organisms Ø Producers of the biosphere
Heterotrophs Consumers Obtain carbon from organic compounds assembled by other organisms
Solar Energy Sunlight is a type of energy called electromagnetic energy Light energy travels in rhythmic waves and is organized in packets of energy known as photons
Wavelength: the distance between the crests of waves/ less than one nm to more than one km Photons travelling at the same wavelength carry the same amount of energy
Electromagnetic spectrum: entire range of wavelengths/ range of all types of electromagnetic radiation
Capturing a Rainbow Ø Photosynthesizers use pigments to capture light of specific wavelengths
Why do cells use more than one photosynthetic pigment? Ø Chlorophyll a: most common photosynthetic pigment in plants and protists Absorbs violet, red, and orange light Ø Accessory pigments harvest additional light wavelengths
Engelmann s Experiment Photosynthesis is driven best by particular wavelengths of visible light/ blue and red light
Where does photosynthesis take place in plants?
Two outer membranes Stroma: thick fluid in the inner compartment of the chloroplast Chloroplasts Thylakoid membrane: much folded inner membrane Thylakoids: interconnected disks
The Overall Equation of Photosynthesis Anabolic Pathway
In the Chloroplast
Light-Dependent Reactions
The Thylakoid Membrane
Light harvesting complex Circular arrays of chlorophylls, accessory pigments and protein molecules Capture light energy and bounce it toward photosystems
Photosystem Cluster of pigments and protein that initiate the light reactions by donating electrons to the electron transport chain Pair of special Chlorophyll a molecules Primary electron acceptor
The Thylakoid Membrane photosystem II/680 nm photosystem I/700 nm
The Noncyclic Pathway
The Cyclic Pathway Ø Electrons ejected from photosystem I enter an electron transfer chain, and then return to photosystem I/ not accepted by NADP +
Ø Under intense light conditions, photosystem II may become non-functional Ø Similarities with non-cyclic pathway: Ø Hydrogen ions are moved into the thylakoid compartment, driving ATP formation Ø Differences from non-cyclic pathway: Ø NADPH and oxygen gas are not produced
The Light-Independent Reactions Calvin Benson cycle Ø Build sugars in the stroma of chloroplasts Ø Not powered by light energy Ø Driving force is ATP and NADPH that formed in the light-dependent reactions Ø Uses carbon atoms from CO 2 to make sugars
The Calvin-Benson Cycle Ø Carbon fixation: carbon from an inorganic source gets incorporated into an organic molecule
The Calvin-Benson Cycle Ø To synthesize one glucose molecule, the Calvin cycle uses 6 CO 2, 18 ATP and 12 NADPH
Adaptations to Climate Stomata are tiny gateways for gases Open stomata: Ø Allow CO 2 to diffuse from the air into photosynthetic tissues Ø Allow O 2 to diffuse out of these tissues into the air Closed stomata: Ø Conserve water on hot, dry days Ø Limit the availability of CO 2 for the light-independent reactions; sugar synthesis slows
Photorespiration: Rubisco attaches oxygen instead of carbon dioxide to ribulose bisphosphate
C3 Plants The initial product is a 3-C molecule (3PGA): C 3 plants Ø Use only the Calvin Benson cycle to fix carbon Ø When CO 2 concentration declines, rubisco uses oxygen as a substrate in photorespiration (Rubisco attaches oxygen instead of carbon dioxide to ribulose bisphosphate) Ø Sugar production declines C 4 and CAM Plants
Ø Close stomata on dry days, but their sugar production does not decline Ø Minimize photorespiration by fixing carbon twice in two cell types Ø Mesophyll cells Ø Bundle-sheath cells C4 Plants Ø Examples: corn and bamboo
CAM Plants Ø C4 plant that conserves water by fixing carbon twice, at different times of day Ø Conserve water even in desert regions with extremely high daytime temperatures Ø Open stomata at night when lower temperatures minimize evaporative water loss Ø Examples: pineapple, some succulents and cacti
Comparing C 4 and CAM Plants
Significance of Photosynthesis Photosynthesis is the process that is responsible for the presence of oxygen in our atmosphere It is estimated that photosynthesis makes about 160 billion metric tons of carbohydrates per year No other process is more important than photosynthesis to the welfare of life on Earth