| Talk about the characteristics of a plant cell. | Plant cells have some specific characteristics that differ from the animal cells. At the ultra-structural level, the cell is surrounded by a cell wall; the cytoplasm encloses a large vacuole and plastids. Cellulose is the main component of the cell wall. Starch is the main carbohydrate reserve. Chlorophylls, are the photosynthetic pigments. They are light energy receptors necessary for photosynthesis and give the green color to the plastids (chloroplasts) in which they are found. However, these criteria do not have an absolute value and does not apply to all green plants. |
| What is photosynthesis and what are the photosynthetic organelles? | During photosynthesis, plants, algae and photosynthetic bacteria capture light energy to synthesize complex organic molecules (sugars) from simple inorganic elements (water and carbon dioxide), while releasing the oxygen. Photosynthesis is carried out in the chloroplasts of plant cells or in specialized regions of the cell membrane (thylakoids) of prokaryotes. |
| Talk about the carried out processes of photosynthesis. | Chlorophyll molecules absorb light energy to synthesize energy-rich molecules ATP and NADPH used for plant metabolism (the light dependent phase of photosynthesis). The chemical energy contained in these molecules allows, through a series of reactions, the carbon sequestration from atmospheric carbon dioxide to form carbohydrates (the dark phase or Calvin cycle). The overall reaction of photosynthesis is: 6 CO2 + 12 H2O → C6H12O6 + 6 H2O + 6 O2. |
| Talk about the roles of plant organs in photosynthesis. | In eukaryotic cells, photosynthesis takes place in the chloroplasts of chlorophyll parenchyma of leaves (mesophyll). Carbon dioxide enters the leaf and oxygen exits through the stomata. The water and minerals collected from the roots flow through the stem by a conductive tissue (xylem) and reach the leaves by the ribs. The sugars produced by photosynthesis, leave the leaves by the ribs and are carried by the conductive tissue of the organic sap (phloem), to non-chlorophyllous organs. The reactions of the light dependent phase of photosynthesis take place in the thylakoid membranes found inside the chloroplast. The reactions of the dark phase take place in the stroma of the chloroplast. |
| Talk about the chloroplast structure. | A chloroplast is wrapped by a double membrane. Inside, its cavity is filled with a dense liquid (stroma) and it is crossed by membranous sacs called thylakoids. Stacks of thylakoids form grana (singular granum), their membrane define a compartment, the thylakoid space. The thylakoids are interconnected by stromal thylakoid (or intergranal thylakoids) |
| What are photosynthetic pigments and where are they localized? | Chloroplasts contain pigments capable of absorbing photons (particles of light energy). The main photosynthetic pigments are chlorophyll "a" and the accessory pigments are chlorophyll "b" and carotenoids. These pigments absorb certain wavelengths (red, blue) more effectively than others. Chlorophylls are integrated into the thylakoid membrane and are linked by membranous proteins. |
| Talk about the excitation of chlorophyll. | When the chlorophyll molecules (or other pigmented molecules) absorb a photon of light, an electron is excited and passes from one layer to another, more distant from the atom nucleus. The excited electron therefore has more energy. This excited state is unstable and the molecule returns to its initial state. In the thylakoid membranes of chloroplasts, there are electron acceptors that receive the excited electrons. |
| What is the photosystem? | A photosystem comprises a collecting antenna and a reaction center. In the thylakoid membranes, photosynthetic pigments form a complex antenna or antenna. The antennas pigment molecules absorb light and transmit the energy from a molecule to another until the reaction center is reached. This center includes a pair of specialized molecules of chlorophyll "a". These are the only molecules that can transfer electrons to an electron acceptor. |
| Talk about the types of photosystems and their activity | There are two photosystems: photosynthesis photosystems I and II, numbered in the order of their discovery. They differ in the chlorophyll "a" molecule of the reaction center. Photosystem I “has" the P700 reaction center ("P" stands for pigment and "700" optimal absorption peak in nanometers), which means it does not absorb wavelengths longer than 700 nm and photosystem II has the center P680.
The chlorophyll "a" molecules of the reaction centers are identical except that they are associated with different proteins. Both photosystems are connected by a chain of electron transport. They operate simultaneously and continuously. |
| Talk in general about photochemical reactions | During the photochemical reactions, the electrons excited by light are transmitted through a chain of electron transport. There are 2 types of photochemical reactions: cyclic phosphorylation and acyclic phosphorylation.
(Light dependent phase) |
| Talk about the cyclic chain first step (Light absorption and its effects) | Photosystem I can operate independently of photosystem II. The molecules of the P700 reaction center absorb light, the excited electron moves to a primary acceptor molecule that transfers its additional electron to a secondary acceptor and so on along the electron transport chain. Finally, the electrons return to the reaction center of photosystem I. This transfer of electrons is coupled to a proton pump from the outside to the inside of the thylakoid membrane creating a proton motive force capable of producing ATP from ADP and Pi. This ATP synthesis in chloroplast due to light is called phosphorylation. |
| How does photosystem II attribute to the photosynthesis? | These reactions involve the two photosystems (I and II). There is production of O2, ATP and NADP+ is reduced to NADH and H+. The NADP+ is the final electron acceptor and the electron donor is a water molecule. The O2 is released into the atmosphere. Photosystem II absorbs light captured by molecules of the antenna and pass to the molecules of the P680 reaction center which loses an electron. This excited electron exits from P680 and passes into a chain of electron acceptors. The electron-deficient molecule of P680 (oxidant) is capable of sensing and capturing the electrons from the electrons donor H2O. Photolysis is the lysis, in the presence of light, of the molecule of H2O into ½O2 + 2H + +2ē. |
| How do photosystems I and II cooperate for photosynthesis? | The pairs of electrons pass from photosystem II to the photosystem I by an electron transport chain. This passage generates a proton gradient or proton motive forces on both sides of the thylakoid membrane which activate the ATP synthesis. Photosystem I is excited by a light photon. The excited electron is then transmitted in a short chain of electron transport to NADP+. This can reduce the NADP+ into NADPH. The electrons removed from the P700 molecule are replaced by those coming from photosystem II. |
| What is the ending point of transferred electrons? | In conclusion, the photons picked up by the two photosystems allow the photolysis of water molecules. Electrons will move continuously in one direction only, from water to NADP+ and a proton gradient that allows the formation of molecules rich in ATP energy and NADPH. These molecules are used in the Calvin cycle. The O2 released from chloroplasts cell fate and eventually leaves through the stomata. |
| Talk in general about the Calvin cycle. | The Calvin takes place in the chloroplast stroma. This is the last step of photosynthesis where ATP and NADPH, generated during the photochemical reactions, are used to fix and to reduce carbon dioxide in the synthesis of simple sugars. The cycle takes place in three steps
Dark phase. |
| Talk about CO2 fixation step. | The Rubisco ( ribulose - bisphosphate - carboxylase ), an enzyme representing more than 40 % of soluble protein in most leaves, plays a key role in the so-called C3 plants (most green plants such as trees, potato and beet) . This enzyme fixes each of the 3 CO2 molecules (3x1C) to 1 molecule of ribulose 1,5-diphosphate (RUBP 3x5C), a 5-carbon sugar.
6 molecules of a 3 carbons product, the phosphoglyceric acid (PGA 6x3C), are formed after passing through an unstable intermediate.
The first product of the cycle contains 3 carbon atoms, which is why the Calvin cycle is also known as the C3 pathway. |
| Talk about the Reduction step. | 6 PGA molecules are reduced by 6 NADPH and 6 ATP to produce 6 molecules of glycerate 3-phosphate (G3P). |
| Talk about the refreshing of RUBP step. | One of the 6 G3P molecules leaves the cycle for the synthesis of sugars and other cellular materials. The other 5 G3P molecules remain in the cycle to regenerate the RUBP (initial reagent of the cycle). The regeneration of RUDP requires 3 ATP molecules. The RUBP is ready to receive CO2 and the cycle starts again. A carbon enters the cycle at each time. The cycle occurs three times, producing one molecule of glycerate 3 - phosphate (G3P). Each time the RUBP is regenerated. Most of the fixed carbon is converted into starch and sucrose. |
| How do plants differ in types of photosynthesis? | Plants have various mechanisms carbon fixation step in photosynthesis. The photosynthesis type of plant is determined by the number of carbon atoms of the organic molecule formed first when the CO2 is fixed. |
| Talk about the mechanism of C3 plants. | The first step of the Calvin cycle consists of fixing a molecule of CO2 (carboxylation) on the ribulose 1,5-diphosphate by the Rubisco , to give a compound of 3 carbon atoms , PGA. A large majority of plants, including trees work under this mechanism. The CO2 fixed by Rubisco comes initially from the diffusion of atmospheric CO2 through stomata, then in a dissolved form, through the leaf cells to the chloroplast stroma. |
| Talk about the mechanism of C4 plants. | Stomatal closure in a warm and dry atmosphere causes a lack of CO2 in the chlorenchyma. This problem is solved by the so-called C4 as corn or sugar cane plants. These plants having this mechanism have another enzyme fixing CO2, phosphoenolpyruvate carboxylase (PEP carboxylase) located in the leaf mesophyll. Atmospheric CO2 binds first to phosphoenolpyruvate (PEP) to produce oxaloacetate, a 4-carbon molecule in the mesophyll cells. The oxaloacetate is converted to malate (or aspartate, depending on the species), which passes from the mesophyll cells to the bundle sheath cells. Malate is then decarboxylated and CO2 enters into the Calvin cycle reacts with RUBP, where the Rubisco is present, to form the PGA. The C4 pathway occurs in the mesophyll, but the Calvin cycle occurs in the bundle sheath. |
| Talk about the mechanism of CAM plants. | Crassulacean Acid Metabolism.
They differ from C4 plants by the fact that carbon sequestration is not separated in space (mesophyll / bundle sheath), but over time (day / night).
CO2 fixation on phosphoenolpyruvate (PEP) and the production of oxaloacetate take place at night when the stomata are open. The oxaloacetate is converted to malate which quickly is stored in the vacuole overnight as malic acid. During the day, when the stomata are closed, malic acid is included in the vacuole overnight as malic acid. During the day, when the stomata are closed, malic acid is included in the vacuole and the CO2 is transferred to RUBP of Calvin cycle. In Crassulaceae, the stomata close during the day, which reduces water loss through transpiration. |
