lecture 10 Photosynthesis: The Light Dependent Reactions

photosynthesis

a process by which plants utilize light energy from the sun, CO₂ from the atmosphere and H₂O from the Earth (soil) to produce glucose

why do plants do photosynthesis

Plants (and some prokaryotic organisms) are considered to be autotrophs, meaning they can synthesize their own food

Plants also undergo cellular respiration (and certain reactions of photosynthesis that do not require light energy) during periods of darkness

what other organisms are photosynthetic

many eukaryotic and prokaryotes (autotrophs)

• Cyanobacteria (primarily undergo cyclic photophosphorylation- more on this later!) 

• Various types of algae

photosynthesis overview

two phases are the light-dependent reactions which take place in the thylakoid membrane and the Calvin Cycle which take place in the stroma of the chloroplasts

light dependent reactions: in two different protein complexes (photosystems), photons (packets of light energy) and water are used to power the transport of electrons, so oxygen can be synthesized, producing ATP. The electron carrier NADPH is also produced

The NADPH and ATP from the light-dependent reactions come out of the thylakoid membrane and into the stroma (cytoplasm equivalent) of the chloroplast, where a series of enzymes catalyze the 5 reactions that make up the Calvin Cycle

The product is two three-carbon sugars (G3P molecules) are produced, and can be “fixed” into glucose to serve as long-term food storage for the plant

chloroplast

contains:

• An inner and outer membrane

• The stroma, or “cytosol” of the chloroplast

• Thylakoids, or thin membrane-bound structures that contain their own internal fluid, or lumen. Stacks of thylakoids are known as grana (plural of granum). 

what is the most abundant photopigment in chloroplasts and what are the different types

Chlorophyll

Chlorophyll a and Chlorophyll b

chlorophyll absorb light in the high-frequency range of the visible light spectrum (between 400 - 500 nm). Thus, keeping plants in a purple-to-blue light environment would not have a significantly detrimental effect on their ability to undergo photosynthesis. Additionally, most chlorophyll pigments will absorb some light energy in the 600 - 700 nm wavelength range (corresponding to yellow-orange-red light), though notably not as much light energy is absorbed in this range as in the higher energy region of the visible light spectrum

Chlorophyll reflects green light (in the roughly 500 - 600 nm wavelength range), and thus a plant that is only exposed to this wavelength of light would not be able to undergo photosynthesis (at least the light-dependent reactions). 

Comparing and Contrasting Cellular Respiration and Photosynthesis

Cellular Respiration and Photosynthesis are similar in that: 

• They both produce a form of energy (ATP in cellular respiration, glucose in photosynthesis).

• They both consist of a series of redox reactions 

Cellular Respiration and Photosynthesis are different in that photosynthesis is largely anabolic (reducing CO₂ to form glucose), while cellular respiration is largely catabolic (oxidizing glucose to generate ATP). 

Photosystem II

• the first Photosystem of the ETC of photosynthesis and takes place in the thylakoid membrane

• Oxidation of H₂O to O₂ occurs here, catalyzed by the energy harnessed from light stimulating the reaction center. 

• In exchange, the electron carrier plastoquinone (Pq) is reduced, transporting the electrons from this photosystem to the next photosystem in the chain. 

• Protons (H⁺) are also pumped from the stroma into the thylakoid lumen, to generate a proton gradient. 

Photosystem I

• the second Photosystem in the ETC of photosynthesis and takes place in the thylakoid membrane

• contains the reaction center P700. 

• More light energy (photons) enter and excite accessory pigments in this reaction center.

• Additionally, electrons from plastocyanin are passed along to chlorophyll a, which in turn passes these electrons to ferredoxin, the final electron carrier in the chain. 

• Protons (H⁺) are also pumped from the stroma into the thylakoid lumen at this photosystem. 

P680 reaction center

within this center of Photosystem II, a series of accessory pigments are the first to receive and “pass along” the light energy from incoming photons, ultimately reaching chlorophyll to then be transferred to (or reduce) plastoquinone

For every 2 photons, a single oxygen atom is produced from the splitting of water

P700 Reaction Center

the reaction center of photosystem I, are the first to “receive” and “pass along” the light energy (photons) from the sun. 

For every 2 photons, 2 electrons are transferred from: 

• Plastocyanin (Pc), to

• Chlorophyll a, to

• Ferredoxin (Fd), to

• NADP⁺ (to become NADPH)

cyclic phosphorylation

when uncouplers bind to Photosystem II, which causes no oxygen is produced from water, no NADPH is made from NADP⁺

but ATP is still produced

noncyclic (z-formation) photophosphorylation

involves both photosystems I and II, results in the production of oxygen (at photosystem II) from water, as well as the production of NADPH from NADP⁺, and ATP from ADP