Selasa, 13 November 2012

PHOTOSYNTHETIC



Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms that use light as a source of energy carry out photosynthesis, since photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon. In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. However, there are some types of bacteria that carry out anoxygenic photosynthesis, which consumes carbon dioxide but does not release oxygen.
Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is a redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons needed to convert carbon dioxide into a carbohydrate, which is a reduction reaction. In general outline, photosynthesis is the opposite of cellular respiration, where glucose and other compounds are oxidized to produce carbon dioxide, water, and release chemical energy. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.
The general equation for photosynthesis is therefore:
2n CO2 + 2n DH2 + photons2(CH2O)n + 2n DO
Carbon dioxide + electron donor + light energy → carbohydrate + oxidized electron donor
In oxygenic photosynthesis water is the electron donor and, since its hydrolysis releases oxygen, the equation for this process is:
2n CO2 + 4n H2O + photons2(CH2O)n + 2n O2 + 2n H2O
carbon dioxide + water + light energy → carbohydrate + oxygen + water
Often 2n water molecules are cancelled on both sides, yielding:
2n CO2 + 2n H2O + photons2(CH2O)n + 2n O2
carbon dioxide + water + light energy → carbohydrate + oxygen
Other processes substitute other compounds (such as arsenite) for water in the electron-supply role; the microbes use sunlight to oxidize arsenite to arsenate:[12] The equation for this reaction is:
CO2 + (AsO33–) + photons → (AsO43–) + CO[13]
carbon dioxide + arsenite + light energy → arsenate + carbon monoxide (used to build other compounds in subsequent reactions)
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.
Most organisms that utilize photosynthesis to produce oxygen use visible light to do so, although at least three use infrared radiation.



Copyright : http://en.wikipedia.org/wiki/Photosynthesis

RIBOSOME

Ribosomes are the cellular organelles that carry out protein synthesis, through a process called translation. They are found in both prokaryotes and eukaryotes, these molecular machines are responsible for accurately translating the linear genetic code, via the messenger RNA, into a linear sequence of amino acids to produce a protein. All cells contain ribosomes because growth requires the continued synthesis of new proteins. Ribosomes can exist in great numbers, ranging from thousands in a bacterial cell to hundreds of thousands in some human cells and hundreds of millions in a frog ovum. Ribosomes are also found in mitochondria and chloroplasts.
Structure
The ribosome is a large ribonucleoprotein (RNA-protein) complex, roughly 20 to 30 nanometers in diameter. It is formed from two unequally sized subunits, referred to as the small subunit and the large subunit. The two subunits of the ribosome must join together to become active in protein synthesis. However, they have distinguishable functions. The small subunit is involved in decoding the genetic information, while the large subunit has the catalytic activity responsible for peptide bond formation (that is, the joining of new amino acids to the growing protein chain).
In prokaryotes, the small subunit contains one RNA molecule and about twenty different proteins, while the large subunit contains two different RNAs and about thirty different proteins. Eukaryotic ribosomes are even more complex: the small subunit contains one RNA and over thirty proteins, while the large subunit is formed from three RNAs and about fifty proteins. Mitochondrial and chloroplast ribosomes are similar to prokaryotic ribosomes.
In spite of its complex composition, the architecture of the ribosome is very precise. Even more remarkable, ribosomes from all organisms, ranging from bacteria to humans, are very similar in their form and function. Recent breakthroughs in studies of ribosome structure, using techniques such as scanning, cryo-electron microscopy, and X-ray crystallography, have provided scientists with highly refined structures of this complex organelle. One particularly exciting conclusion from studies of the large subunit is that it is ribosomal RNA (rRNA), and not protein, that provides the catalytic activity for peptide bond formation. That is, it forms the chemical linkage between the amino acids of the growing protein molecule.
Synthesis
The synthesis of ribosomes is itself a very complex process, requiring the coordinated output from dozens of genes encoding ribosomal proteins and rRNAs. Ribosomes are assembled from their many component parts in an orderly pathway. In eukaryotes, rRNA synthesis and most of the assemblysteps occur in a structure within the nucleus called the nucleolus. Eukaryotic ribosome synthesis is especially complicated, because the ribosomal proteins themselves are made by ribosomes in the cytoplasm (that is, outside of the nucleus), so they then must be imported into the nucleolus for assembly onto the nucleolus-derived rRNA. Once assembled, the nearly complete ribosomal subunits are then exported out of the nucleus and back into the cytoplasm for the final steps of assembly.
Ribosomes from a liver cell are represented by the darkened ares of the magnified image. These organelles contain RNA and use it for protein synthesis.Ribosomes from a liver cell are represented by the darkened ares of the magnified image. These organelles contain RNA and use it for protein synthesis.
The exact details of the in vivo ribosome assembly pathway (the process of ribosome assembly within the living cell) are still under investigation. Assembly in eukaryotic cells involves not only the components of the mature particles, but also dozens of auxiliary factors that promote the efficient and accurate construction of the ribosome during its assembly. However, bacterial ribosomes can be constructed in vitro using purified ribosomal proteins and rRNAs. These ribosomes appear to function normally in in vitro translation reactions.
Ribosome Function
Translation of messenger RNA (mRNA) by ribosomes occurs in the cytoplasm. In bacterial cells, ribosomes are scattered throughout the cytoplasm. In eukaryotic cells, they can be found both as free ribosomes and as bound ribosomes, their location depending on the function of the cell. Free ribosomes are found in the cytosol, which is the fluid portion of the cytoplasm, and are responsible for manufacturing proteins that will function as soluble proteins within the cytoplasm or form structural elements, including the cytoskeleton, that are found within the cytosol.
Bound ribosomes are attached to the outside of a membranous network called the endoplasmic reticulum to form what is termed the "rough" endoplasmic reticulum. Proteins made by bound ribosomes are intended to beincorporated into membranes, or packaged for storage, or exported outside of the cell. Ribosomes exist either as a single ribosome (that is, one ribosome translating an mRNA) or as polysomes (two or more ribosomes sequentially translating the same mRNA in order to make multiple copies of the same protein).
Ribosomes have the critical role of mediating the transfer of genetic information from DNA to protein. Ribosomes translate this code using an intermediary, the messenger RNA, which is a copy of the DNA that can be interpreted by ribosomes. To begin translation, the small subunit first identifies, with the help of other protein factors, the precise point in the RNA sequence where it should begin linking amino acids, the building blocks of protein. The small subunit, once bound to the mRNA, is then joined by the large subunit and translation begins. The amino acid chain continues to grow until the ribosome reaches a signal that instructs it to stop.
Many of the antibiotics used in humans and other animals to treat bacterial infections specifically inhibit ribosome activity in the disease-causing bacteria, without affecting ribosome function in the host-animal's cells. These antibiotics work by binding to a protein or RNA target in the bacterial ribosome and inhibiting translation. In recent years, the misuse of antibiotics has resulted in the natural selection of bacteria that are resistant to many of these antibiotics, either because they have mutations in the antibiotic's target in the ribosome or because they have acquired a mechanism for excluding or inactivating the antibiotic.


Bibliography
Frank, Joachim. "How the Ribosome Works." American Scientist 86 (1998): 428-439
Garrett, Robert A., et al, eds. The Ribosome: Structure, Function, Antibiotics, and Cellular Interactions. Washington, DC: ASM Press, 2000
Karp, Gerald. Cell and Molecular Biology: Concepts and Experiments, 3rd ed. New York: John Wiley & Sons, 2002.



Copyright : http://www.bookrags.com/research/ribosome-gen-04/

LEAF


            A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants.
            Typically a leaf is a thin, flattened organ borne above ground and specialized for photosynthesis, but many types of leaves are adapted in ways almost unrecognisable in those terms: some are not flat (for example many succulent leaves and conifers), some are not above ground (such as bulb scales), and some are without major photosynthetic function (consider for example cataphylls, spines, and cotyledons).
            The function of leaves:
1.      Place of  photosynthesis
2.      Evaporation
3.      Storage of food reserves
4.      Vegetative propagation tool
      
      Structure of leaves:





            Parts of leaves :
            Complete leaf has the following parts:
1.      a petiole (leaf stalk)
2.      a lamina (leaf blade), and
3.      stipules (small structures located to either side of the base of the petiole)

            Complete leaf can be encountered in some crops, such as: Musa paradisiaca L. ,Areca catechu L.  , Bambusa sp. The leaves do not have the above three sections called incomplete leaf.

Example of complete leave :








Example of incomplete leave :


Leaf characteristics:

    • Simple and compound leaf structure:
      • Simple leaves have a single blade.
      • Compound leaves have more than one blade on a single petiole. The multiple blades of a compound leaf are called leaflets.
        • Palmately compound leaves have leaflets arranged like the fingers of a hand.
        • Pinnately compound leaves have leaflets arranged on either side of an axis, resembling a feather.
        • Trifoliolate leaves have leaflets arranged in threes, like clover.
        • Compound leaves are sometimes twice divided. These leaves are called twice-compound.
    • Leaf attachment:
      • Petiolate - The blade is attached to the stem by a petiole.
      • Sessile - The blade is attached directly to the stem without a petiole.
    • Leaf arrangement:
      • Opposite - Two leaves grow opposite each other at each node.
      • Alternate - One leaf grows at each node. The leaves alternate sides along the stem.
      • Whorled - Several leaves grow around a single node.
    • Leaf shapes:
      • Linear - Narrow from base to tip.
      • Elliptic - Oval-shaped.
      • Ovate - Wide at the base and narrow at the tip.
      • Cordate - Heart-shaped.
    • Leaf margins:
      • Entire - The edge of the leaf is smooth.
      • Serrate - The edge of the leaf is finely toothed.
      • Lobed - The edge of the leaf is deeply indented.
    • Leaf venation: The system of principal veins in the leaf blade.
      • Parallel - Major veins arise at the base, remain more or less parallel, and converge at the tip of the leaf.
      • Net-veined or Reticulate:
        • Pinnate - Major veins diverge from one large mid-vein, with smaller network connections between.
        • Palmate - Several large veins arise from the base of the leaf like the fingers of a hand.
    • Leaf surfaces: The presence or absence of hairs, the kinds of hairs, and the presence of other surface features, such as glands, combine to give many leaf characteristics. There are over 25 terms used to describe leaf surfaces. This amount of detail is beyond the scope of our class. 


Reference :
1. Haupt, Arthur Wing, Plant morphology. Publisher: McGraw-Hill 1953. Downloable from http://www.archive.org/details/plantmorphology00haup
2. http://en.wikipedia.org/wiki/Leaf
5. http://www.istockphoto.com/stock-photo-7861234-young-mango-leaf.php
6. http://staff.tuhsd.k12.az.us/gfoster/standard/botleaf.htm