LECTURES 8 & 9 PHOTOSYNTHESIS I & II

Understand how plants use solar energy to fix atmospheric CO2 into carbohydrates

First 4 videos

Describe in detail: The light-reactions of photosynthesis, the function of chlorophyll and carotenoids, the 'Z' scheme, how ATP and NADPH are made, how oxygen is evolved, the dark-reactions of photosynthesis, how the 'Calvin' cycle was discovered, the basic reactions in C3, C4, and CAM metabolism, the central role of RuBisCO

Light reactions: Light energy from the sun is captured by chlorophyll, exciting electrons in photosystem II and starting an electron transport chain through electron carriers. These electrons are used to join together NADP with a proton to form NADPH. The electron transport chain also causes protons to be pumped into the thylakoid space, building up a proton gradient between the thylakoid space and stroma. The proton gradient is used for chemiosmosis, which involves protons moving through ATP synthase to provide the energy to form ATP from ADP.

Function of chlorophyll and carotenoids: Chlorophyll a: key light-capturing pigment that participates directly in the light reactions, chlorophyll b: accessory pigment. Violet-blue and red light are absorbed by chlorophyll, green light reflected. Chlorophyll consists of a porphyrin ring (light absorbing "head" with magnesium atom at center) and a hydrocarbon tail (interacts with hydrophobic regions of proteins in thylakoid membrane). Carotenoids: group of yellow and orange hydrocarbon accessory pigments. Carotenoids broaden the spectrum of colours that can drive photosynthesis and carry out photoprotection - absorbing and dissipating excess light energy that could damage chlorophyll or interact with oxygen, posing a danger to the cell.

Z-scheme: The z-scheme shows the path of electron flow and reduction potentials of the components in photosynthesis. The chlorophyll a molecules in PSII are P680 (absorb light of wavelength 680 nm) and the chlorophyll a molecules in PSI are P700. Light energy entering PSII excites electrons in antenna pigments, causing energy to be transferred between antenna pigments until reaching the P680 chlorophyll and exciting electrons in the chlorophylls. These electrons reach the primary acceptor and move through electron carrier molecules (plastoquinone, cytochrome complex, plastocyanin) resulting in a drop in reduction potential until they reach PSI. Light energy excites electrons in the P700 chlorophyll molecules of PSI, and the electrons from the electron transport chain replace these electrons. The excited electrons are moved to the primary acceptor of PSI. From there they move through a second electron transport chain through ferredoxin (Fd). NADP+ reductase catalyses the transfer of electrons from Fd to NADP+ to create NADPH.

How oxygen is evolved: Oxygen is evolved when water-splitting enzymes split water into protons and oxygen. Water provides electrons for the electron transport chain and protons for the proton gradient. Oxygen is a waste product that diffuses out of the plant.

Dark reactions: The dark reactions involve the Calvin cycle, which takes place in the stroma of chloroplasts. The Calvin cycle consists of 3 steps: carbon fixation/carboxylation, reduction, and regeneration. Carbon fixation involves CO2 bonding with RuBP (ribulose bisphosphate) to form 2 molecules of 3-phosphoglycerate (3PGA), catalysed by RuBisCO. 3PGA is then reduced by 2NADPH molecules, using 2 molecules of ATP. The result is triose phosphate. For every 6 molecules of triose phosphate formed, 5 will be used to regenerate RuBP , while one will be used to produce glucose phosphate. Regeneration uses 1 ATP molecule.

'Calvin' cycle discovery: Discovered by Melvin Calvin. Calvin added radioactive (14C) bicarbonate to illuminated algae to be incorporated during photosynthesis and then plunged the algae into alcohol to stop the reaction. He then examined the results using paper chromatography. When radioactive carbon decayed it could be seen on X-ray film, which allowed Calvin to determine the pathway of photosynthetic CO2 fixation.

C3, C4, and CAM metabolism:

C3 plants: Initial carbon fixation occurs via rubisco; the first organic product of carbon fixation is 3-phosphoglycerate (3 carbon compound). During hot, dry days, stomata are closed and there is a lack of CO2. Rubisco then fixes O2 instead, which leaves the chloroplast and gets converted to CO2. This process is called photorespiration and requires ATP in order to be carried out, and thus decreases photosynthetic output (sugar is not produced). Photorespiration does, however, help protect the plant against the harmful effects of the products of the light reactions which build up when CO2 intake is low. Examples of C3 plants: wheat, soybeans, rice.

C4 plants: C4 plants are plants that have evolved to minimise photorespiration. They preface the Calvin cycle with an alternate mode of carbon fixation that forms a 4-carbon compound as its product. C4 plants have two types of photosynthetic cells: bundle-sheath cells and mesophyll cells. When CO2 first enters the cells, PEP carboxylase adds the CO2 to PEP in the mesophyll cells, forming oxaloacetate (4 carbon compound). The 4-carbon compounds are then transported to bundle-sheath cells through plasmodesmata in the form of malate, where they release CO2 which is assimilated into the Calvin cycle. PEP carboxylase has a high affinity for CO2 and a low affinity for O2, meaning that carbon fixation is more efficient in hot, dry weather. Examples of C4 plants: corn, sugarcane.

CAM plants: CAM (Crassulacean Acid Metabolism) plants open their stomata at night and close them during the day (reverse of what other plants commonly do). losing stomata during the day helps plants conserve water, but they also take in less CO2. Organic acids made during the night are stored in mesophyll cells. They release CO2 during the day when the light reactions supply ATP and NADPH for the Calvin cycle. Common in succulent plants, cacti, pineapples.

RuBisCO: RuBisCO (Ribulose 1,5-Bisphosphate Carboxylase-Oxygenase) is the enzyme involved in binding CO2 and RuBP. The reaction is metabolically irreversible. RuBisCO is one of the most abundant enzymes occurring in nature, and makes up about 50% of the soluble protein present in plants.

	Created by Karolina K about 5 years ago

Biology Note on Photosynthesis, created by Karolina K on 18/04/2019.