Search results
Results from the WOW.Com Content Network
Many types of plant cells contain a large central vacuole, a water-filled volume enclosed by a membrane known as the tonoplast [3] that maintains the cell's turgor, controls movement of molecules between the cytosol and sap, stores useful material such as phosphorus and nitrogen [4] and digests waste proteins and organelles.
Chloroplasts (from the Greek chloros for green, and plastes for "the one who forms" [31]) are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules while freeing oxygen from water in plant and algal cells.
The oxidation of water is catalyzed in photosystem II by a redox-active structure that contains four manganese ions and a calcium ion; this oxygen-evolving complex binds two water molecules and contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction (Kok's S-state diagrams).
The reaction begins with the excitation of a pair of chlorophyll molecules similar to those in the bacterial reaction center. Due to the presence of chlorophyll a, as opposed to bacteriochlorophyll, Photosystem II absorbs light at a shorter wavelength. The pair of chlorophyll molecules at the reaction center are often referred to as P680. [1]
In land plants, the plastids that contain chlorophyll can perform photosynthesis, thereby creating internal chemical energy from external sunlight energy while capturing carbon from Earth's atmosphere and furnishing the atmosphere with life-giving oxygen. These are the chlorophyll-plastids—and they are named chloroplasts; (see top graphic).
Most plants store chemicals in the vacuole that react with chemicals in the cytosol. If the cell is broken, for example by a herbivore , then the two chemicals can react forming toxic chemicals. In garlic, alliin and the enzyme alliinase are normally separated but form allicin if the vacuole is broken.
[11] [12] Their photopigments can absorb the red- and blue-spectrum frequencies of sunlight (thus reflecting a greenish color) to split water molecules into hydrogen ions and oxygen. The hydrogen ions are used to react with carbon dioxide to produce complex organic compounds such as carbohydrates (a process known as carbon fixation ), and the ...
When either of the two chlorophyll a molecules at the reaction center absorb energy, an electron is excited and transferred to an electron-acceptor molecule. Photosystem I contains a pair of chlorophyll a molecules, designated P700 , at its reaction center that maximally absorbs 700 nm light.