Photosynthesis: Where it all begins! AP Biology

Us versus Them Autotrophs make their own food (self- nourishing) Photoautotrophs use sunlight as the energy source Heterotrophs must feed on autotrophs, one another, or waste

Photosynthesis Is the main pathway by which carbon and energy enter the web of life. Where do we find carbon in living things?

An overview 12 H 2 O + 6 CO 2 6O 2 + C 6 H 12 O 6 + 6H 2 O Sunlight Two divisions: Light dependent reactions, which yields ATP and H+ Light independent reactions, which uses the products of the light dependent reactions to make glucose

An Overview Takes place in the chloroplasts Two outer membranes surrounding a mostly fluid interior called the stroma Another folded membrane is stacked in the stroma. The stacks of thylakoid discs are grana (granum) Double membrane Thylakoids Stroma

Light Dependent Reactions Sunlight splits water molecules Oxygen diffuses away Its electrons flow through electron transfer chains This forms ATP Coenzyme NADP picks up the electrons and hydrogen

Light Dependent Reactions

One Needs the Other

Light Independent Reactions Occur in the stroma Does not require light ATP gives up energy Coenzyme NADPH gives up electrons and hydrogen CO2 is dismantled for its C and O atoms Glucose is made

Not Really Glucose Glucose is quickly changed to sucrose, cellulose, or starch

Properties of Light Light travels in waves The distance between two crests is a wavelength The shorter the wavelength, the higher the energy All wavelengths combined appear as white

Photons The energy of light has a particle-like quality Energy, when absorbed, can be measured as packets called photons Each photon has a fixed amount of energy

Pigments Absorb wavelengths of light Most absorb only certain wavelengths Reflect back or transmit the others Chlorophyll looks green because it does NOT absorb green wavelengths

Accessory Pigments Carotenoids Phycobilin –Phycoerythrin –Phycocyanin Anthocyanins

Light Dependent Reactions A Closer Look Produces ATP and NADPH

Photosystems In the thylakoid membrane, pigments are organized in clusters called photosystems Photons of light are absorbed by pigment, and the pigment’s electrons get “excited.”

Excited Electrons When electrons of an atom absorb energy, they move to a higher energy level Energy entering a pigment destabilizes the arrangement of electrons. (They jump around)

Excited Electrons The excited electrons quickly return to a lower energy level, stabilizes, and some energy is released in the form of light or heat. (fluorescence.) In photosynthesis, this releasing energy gets passed on to another pigment in a random “walk” Some is lost as heat. The remaining energy matches to a wavelength that the photosystem’s reaction center can trap. The reaction center passes the energy in the form of excited electrons to an electron transport chain

The Photosystems P700 is photosystem I, and can be cyclic. P700 can cycle alone, or can receive electrons from P680 as part of a non-cyclic pathway. When it cycles alone, light energy excites electrons, boosting them to a higher energy level, and sending them through an electron transport chain. The end product of the electron transport chain is ATP.

Photosystems P680 is photosystem II and is non-cyclic. It receives energy from light, boosting electrons to a higher energy level and sending them through an electron transport chain to P700. The electrons it gives up from the pigments are replaced by the splitting of water (constant supply) p114

Photosystems Both electron transport chains use the energy from the “falling” electrons to pump H ions to the inside of the thylakoid membrane, resulting in a concentration gradient When H ions move through ATP synthase, the energy is used to attach ADP to P, making ATP, needed for the light indep.

How ATP is made Hydrogen ions from the splitting of water accumulate in the thylakoid membrane. Electron transport chains build up even more hydrogen ions in the thylakoid. Ions are pumped from the stroma into the thylakoid. Sets up a concentration gradient When they flow into stroma, they are used to form ATP from ADP and P (ATP synthase enzyme required)

This method of making ATP Is called the chemiosmotic model for ATP production Will also happen in the mitochrondria In which place do you think more ATP will be made?

Light Independent Reactions A Closer Look: takes the ATP and NADPH from light dependent and makes glucose

A Cycle Called the Calvin- Benson cycle. ATP drives the reactions NADPH delivers hydrogen and electrons CO2 provides the carbon

Photophosphorylation Is a specific type of Chemiosmosis Chemiosmotic theory refers to the method of building up H+ concentration gradient to make ATP.

Cyclic vs Non-cyclic Cyclic electron flow involves the P700 photosystem only Electrons are boosted in energy, passed thru an electron transport chain, producing ATP, and returned to the P700 photosystem

Cyclic vs Non-cyclic Noncyclic: electrons in P680 are boosted in energy, go thru the electron transport chain, producing ATP. The electrons still carry some energy, they move on to P700 and go through a second ETC, producing ATP AND NADPH Electrons do not return to the P680. NADP is the final electron acceptor.

Non-cyclic

C3, CF4, and CAM Plants Stomata (openings for gas and water exchange) close in dry weather to conserve water But this means that CO 2 in and O 2 out is also stopped

C3, CF4, and CAM Plants Halts the light independent reactions, but the light dependent reactions continue O 2 builds up and triggers an alternate pathway called photorespiration Inefficient backdoor way to make small amounts of glucose This is what happens to most plants (C3.) Diagram on page 117

C4 plants have a better answer O 2 also builds up here when stomata close, but an additional step keeps the CO 2 concentration higher than the O 2 concentration, so the inefficient photorespiration is not triggered. Involves two different cells Diagram page 117

CAM plants Like C4 plants, CAM plants use a C4 cycle and the Calvin-Benson cycle. But the cycles occur in the same cell, but one during day and the other at night.