1. What is Chlorophyll? Name two types and their molecular compostition.
Chlorophyll absorbs light energy and begins the process of photosynthesis. The two most common types are clorophyll a (blue-green) and chlorophyll b (yellow-green). Both molecules are composed of a porphyrin ring attached to a long hydro-carbon tail. The porphyrin ring contains a Magnesium atom at it's center surrounded by a hydrocarbon ring with alternating single and double bonds. Delocalized electrons in the alternating single-double bonds in the ring absorb light energy and begin the photosynthetic process. The only difference between chlorophyll a and b is that a contains a methyl group (-CH3) at R and b contains an aldehyde group (-COH) at R. This difference affects the type of light energy that the molecules can absorb. The hydrocarbon tail (a.k.a. Phytol tail or phytol chain) anchors the chlorophyll molecule in a membrane by associating with the hydrophobic regions of the phospholipid bilayer. All photosynthetic organisms primarily use chlorophyll a.
2. How do guard cells control the size of stomata?
Transpiration occurs threw the stomata which are controled by the guard cells. Guard cells control the size of a stoma by changing their shape in response to changes in environmental conditions. The stoma is open when the guard cells are turged (swollen) and closed when guard cells are flaccid (limp). The guard cells change their size when water moves by osmosis into or out of the cell. The direction of osmosis follows the diffusion of potassium ions, K+, across the guard cell's plasma membrane. The movement of K+ is coupled with the transport of H+ ions through membrane associated proton pumps. This means that the size of the stomata are dependent on the availability of ATP, a product of photosynthesis.
3. What dimensions does a typical plant cell chloroplast have?
Approximately 3um to 8um in length and 2um to 3um in diameter.
4. Describe the internal structure of a chloroplast.
Outer membrane followed by intermembrane space, then inner membrane. The space inside is called stroma. Within the stroma, there are grana which are stacks of thylakoid disks connected to one another by lamellae which are stretched out thylakoid disks. The thylakoids are made up of a membrane, thylakoid membrane, which is embedded with chlorophyll molecules to collect light-energy. The inside of the thylakoid disk is a water filled space called the thylakoid lumen.
5. What is the endosymbiotic theory?
The endosymbiotic theory proposes that an ancestor of cyanobacteria was engulfed by and ancestor of today's eukaryotic cells, and association that was mutually beneficial. The cyanocacterium was protected from a harsh external environment, while the eukaryotic host obtained food molecules produced by the engulfed photosynthetic bacterium. It is believed that this association eventually gave rise to plant cells.
6. What is the overall equation for photosynthesis?
Full equation:
6CO2 + 12H2O + light energy ----> C6H12O6 + 6H2O + 6O2
7. What is Cyclic Electron Flow
In some cases, excited electrons take a cyclic pathway that uses photosystem I only. A photon ejects an electron form chlorophyll P700 of photosystem I. The electron is passed to ferredoxing, Fd, and then goes throught the Q cycle, the b6-f complex (cytochrome chain), an dvack to chlorophyll P700. This cyclic pathway generates a proton gradient for chemiosmotic ATP synthesis, but does not release electrons to generate NADPH. Without NADPH, the reactions of carbon fixation cannot occur because electrons in the hydrongen atoms of NADPH are needed to reduce carbon dioxide.
8. What is the Calvin Cycle?
The reactions that convert carbon dioxide into carbohydrate molecules occur in the stroma of chloroplasts. The process occurs by a cyclic series of reactions.The details of the cycle were determined in the early 1960s by Melvin Calvin. It can be divided into 3 phases: carbon fixation, reduction reactions, and ribulose 1,5bisphosphate (RuBP) regeneration.
9. Name the main differences between photosynthesis and cellular restpiration.
-The Calvin cycle includes reactions taht are similar to reactions in cellular respiration but that occur in reverse.
-The proteins, quinones and cytochromes of the electron transport chains in photosynthetic membranes and in respiratory membranes are similar in structure and, in some cases, are exactly the same.
-Both processes use chemiosmosis to transorm energy from one form to another. In mitochondria, H+ ions are pumped from the matrix into the intermembrane space, with ATP synthesis occurring in the matrix. In chloroplasts, H+ ions are pumped from the stroma inot the thylakoid lumen, with ATP synthesis occurring in the stroma.
10. What is a photon and a photosystem?
A photon is light energy and a photosystem is a cluster of accessory pigments, chlorophyll molecules, proteins and electron aceptor embedded in the thylakoid membrane to collect light energy and transform it to potential energy in an electron which begins the process of photosynthesis.
11. What are alternative mechanisms of carbon fixation?
There are three. C3 plants, C4 plants and Cam plants. In C3 plants, photorespiration predominates, in C4 plants, the Calvin cycle predominates, there is no photorespiration and the carbon fixation is in mesophyll cells then again in the bundle-sheath cells and for CAM plants, the stomata open at night to minimize water loss and the Calvin cycle also predominates and there is no photorespiration.
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