Photosynthesis

Learning Objectives

Identify the starting material, product, and locations for both the light reactions and the carbon fixation reactions.
Understand how electron flow through the light reaction results in the production of ATP.
Compare and contrast photosynthesis and respiration.

Light reactions take place in the thylakoid membrane of the chloroplast. They start with H20 and NADPH, and produce e-, H+, O2, ATP, and NADP+. The Carbon Fixation Cycle takes place in the stroma of the chloroplast. It starts with CO2 and RuBP, and produces glucose.
The electron flow through the light reactions results in the production of ATP through the use of mechanical energy and magnetic pull. The electrons get carried down the electron transport chain, creating a more negative gradient inside that chain. This pulls the H+ (protons) through the thylakoid membrane, as their positve charge attraction to the negative electrons is stronger than the concentration gradient. The H+, once it escapes that magnetic pull, wants to go back down the concentration gradient, and does so by going through ATP synthase. ATP synthase then spins, connecting adenosine diphosphate and a phosphate group to create adeosine triphosphate, or ATP.
Both photosynthesis and respiration share an evolutionary origin (the biological soup dicussed earlier in the semester). Photosynthesis starts with the electron transport chain and then completes the Calvin Benson Cycle. The H+ gradient is higher on the inside of the thylakoid membrane. Cellular respiration starts with glycolysis, then goes to the citric acid cycle, then to the electron transport chain. The H+ gradient is higher on the outside of the mitochondrial inner membrane.

Chemical formula: 6CO2 + 12H20 + light = C6H12O6 + 6O2 + 6H2O
CO2 gets into the leaves via stomata; reaches chloroplasts in the mesophyll by diffusing through stomata
Stomata open and close; balancing act between gas exchange and evaporation of water

This is why drought conditions are bad; it reduces the ability for photosynthesis to happen
Less than 1% of H2O is used in photosynthesis
H2O is the source of electrons (e-) in photosynthesis; water is split, releasing O2 and e-. O2 is NOT from CO2. O2 is considered a byproduct.

Epidermal cells have no chlorphyll; mesophyll cells do
Chloroplasts have 2 outer membranes; thylakoid membrane arranged in grana (singular: granum)
Some of the reactions take place in different parts of the chloroplast (different membranes)
Strome is the liquid inside double membranes but outside thylakoid membrane; lumen are the spaces inside the grana made of thylakoid (inside the thylakoid membrane)
Two reactions take place: the light reactions and the Calvin Benson reactions
In light reactions, energy is captured by chlorophyll, electrons get excited and water is split, e- is given to electron carriers, and carried from the thylakoid membrane into the stroma

Several types of chlorophyll molecules:
- A magnesium end captures light
- A lipid tail anchors the molecule to the thylakoid membrane
About 250-400 pigment molecules are grouped in a light harvesting complex; these are called photosynthetic units
The arrangement of pigment molecules work together by transferring energy, eventually getting electrons excited enough to leave the chlorophyll; donates to the electron acceptor; molecules need to replenish their electrons; H2O gets donated e- to chlorophyll and gets split; becomes separate H+ and O2
It is a very difficult process in nature to split H2O, which is done easily by plants

RET: excess energy of an excited molecule, usually called the donor, is transferred to an acceptor molecule

2 types of photosynthetic units work together: Photosystem II (comes first) and Photosystem I
Similar to how each proton in the electron transport chain moved through reactions and a little bit of energy was lost each step
Light energy comes through, resonance energy transfer (excess energy of donor transferred to acceptor)
Chlorophyll molecules excited by light energy, donates that energy to Pheophytin molecules; chlorophyll loses/is missing electrons, burrows from water (creates H+ and O2)
H+ generated by oxidation of water helps to create a proton gradient used by ATP synthase to generate ATP; energized electrons transferred to plastiquinone are ultimately ysed to reduce NADP+ to NADPH or are used in non-cyclic electron flow
Plastocyanin is the chemical molecule that begins Photosystem I in tandem with more light energy; same process of excitation and donation until they are finally given to NADPH e- carrier
Important to recognize that beecause of the slow dissipation of energy there are 2 photosymthems, where we need an extra imput of light energy to drive the whole process
Light reactions can also be called "2-scheme"

Lumen = inside of the flattened thylakoid stacks
Strome = outside of the stacks
Reactions inside the membranes; chlorophyll that has lost e- graces from H2O splitting and creating proton gradient
NADPH carries e- into stroma; e- move horizontally, H+ move vertically to create gradient and produce ATP
- Powers ATP synthase
- Similar evolutionary origins (endosymbiosis of free-living prokaryotes); biochemical pathways might come from very similar origins

Light reactions take place in the thylakoid membranes to get energy from light; electrons come from high energy state in presense of water, produces H+ and O2; Calvin Benson can take place at night, does not need light (extended darkness = will run out of ATP needed for Calvin Benson cycle)

Common evolutionary origin, but different pathways and proteins
Respiration has straight line glycolysis, the citric acid cycle, and the electron transport chain. Photosynthesis is reversed: first the electron transport chain, then the Calvin Benson cycle
Respiration breaks down sugar and gets e- out, uses O2
Both use e- and H+ moving across a membrane, but H+ greater inside for photosynthesis and greater outside for respiration