Plants and Photosynthesis |
Can you name these seven cell parts? |
Chloroplasts are the site of photosynthesis. They circle inside the cell wall in movement called cyclosis. This movement can be seen in live Elodea or Anacharis leaves. When the water vacuole is full, the chloroplasts are pressed along the cell wall. When it is not, the cells drift more toward the center of the cell. |
Within this leaf, all of the individual cells can still be seen as small rectangular areas on both sides of the leaf. Plant cells have a rigid cell wall, large water vacuoles or vacuole, and chlorplasts. During cell division, they have a cleavage plate which separates the daughter cells after mitosis. |
Plants come in many varieties and each is important. Plants are responsible for recycling carbon dioxide into oxygen in our atmosphere. They are the producers, the autotrophs capable of converting the energy from sunlight to chemical energy to be used by the organisms that consume them. Plants grow in a variety of environments, they are aquatic, marine, grow in sand and soil, and many are adapted to growing in the most adverse conditions. Green plants contain chlorophyll which traps the energy of sunlight. Green plants store their food as starches or sugars. Food can be stored in the leaves, the roots, or the stems of the plant. |
Plant cells have rigid external walls made of cellulose and lignin. Plant cells need rigid walls because of internal turgor pressure caused by water pressure within the large water vacuole. Walls allow stems to stand erect and leaves to be displayed in the sun. |
The Process of Photosynthesis: |
Photosynthesis takes place in the chloroplasts within the leaf. Flattened membranous sacs called thylakoids are the site within the cell. On the thylakoids are functional groups of pigments which are referred to as photosystems. There are about 300 pigment molecules in each and these molecules are directly involved in photosynthesis. There is a reaction center in each system which is responsible for trapping light energy in the form of photons. Photosystem I is named P700 because it contains chlorophull a which absorbs light in the 700 nm wavelength. Photosystem II is named P680 because that is the wavelengh that chlorophyll a aborbs at in it. The photochemical reactions and the electron transport activities take place on the thylakoids. The Calvin Cycle or Carbon Dioxide Fixation Cycle take place in the stroma or reservoir of the thylakoid. |
What happens in photosystem I? CYCLIC: Light strikes the pigment molecules. An electron from P700 has its energy level raised and it is released. It attaches itself temporarily to an acceptor molecule. From here the electron gets passed from one acceptor molecule to another. Each step is catalyzed by an enzyme and the electron reduces its energy level with each acceptor. The energy released from this process is used to add a third phosphate to ADP making it ATP. In the reservoir, the excess hydrogen ions from the reaction are stored. These ATP will be used to synthesize carbohydrates in the Calving Cycle. |
NONCYCLIC: |
Excited electrons may also take a different path once they leave the chlorophyll a molecule. From the first acceptor, the electron may travel to ferridoxin, an iron containing acceptor. From here the electron goes to NADP (nicotinamide adenine dinucleotide phosphate) Since two electrons are following this pathway simultaneously, they are both accepted by NADP to become NAFPH by reduction.The energy stored in these hydrogen bonds will be used during the "dark reaction" or Calvin cycle when the chloroplast will manufacture the glucose molecule and release oxygen as a by-product of this reaction. |
PHOTOSYSTEM II: |
200 chlorophyll a molecules are in the reaction center for this process. (in the blue-greens and in bryophytes, the pigment here is chlorophyll b, in brown algae it is c, and in red algae it is d) In this system, the excited electron from P680 is passed to an acceptor designated as "Q". The electron then gets passed down a chain of acceptors, with each pass losing some of its energy. The P 680 pulls some electrons from water molecules leaving behind free hydrogen and oxygen. The hydrogens become associated with NADP and the molecular oxygen is released as a by-product. |
THE CALVIN CYCLE |
The Calvin Cycle takes place in the stroma of the thylakoid. NADP and ATP from Photosystems I and II are no used to attach carbon dioxide to a preexisting organic molecule. Enxymes are in the stroma that provide for this. The sugar, ribulose biphosphate (RuBp) and the carbon dioxide join to form an unstable compound. This compound will split into two three carbon molecules called PGA or phosphoglyceric acid. These two molecules are then reduced and form PGAL. Two of these molecules are then joined to form a six carbon sugar molecule called glucose. Some of the PGAL is used to replenish the RuBp which will serve as the starting point for the next Calvin Cycle. To form these bonds, the third phosphate in ATP is released and replaced with hydrogen. The energy is used to make the sugar. The needed hydrogen comes from the NADPH molecule formed in the light phases. |
Comparing respiration and photo- synthesis. Notice how carbon dio- ide and oxygen are used or produced in the two different cycles. |
carbon dioxide + 2 water + light = oxygen + sugar + water When light and chlorophyll are present. |
Photosynthesis site. |
Plant cell Lab |
Chromatography lab |
