Plastids
By Derek

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  • Plastids are cells that store specific things.
  • They are large cytoplasmic organelles found in the cells of most plants, but they are not found not in animal cells..
  • Plastids form small colorless bodies called proplastids.
  • Once formed certain kinds of plastids can be converted into other types.
  • For ex. chlorophyll can be synthesized with a leucoplast and light.
  • There are three plastid categories- Leucoplasts (white or colorless plastids that store starch granules) , Chromoplasts (colored plastids that store pigment molecules) and Chloroplasts which are essential in the photosynthetic process.
Chromoplasts - color centers

  • There are two main types of chromoplasts -- cartenoids which store yellow and orange pigment and chlorophyll which stores green pigment.
  • Chloroplasts contain chlorophyll which contains green pigment and some carotenoids which hold yellow or orange pigment.
  • Chlorophyll traps radiant sun energy then manufactures complex organic molecules(particularly glucose) from simple raw organic materials.

Leucoplasts - energy storage

  • Leucoplasts are organelles where starch, oil and protein are stored.
  • The ones filled with starch are common in seeds and storage roots and stems (i.e. carrots and potatoes).
  • Starch is deposited as a grain or a group of grains

Chloroplasts

  • Chloroplasts serve as the site of photosynthesis.
  • Leaves have about 500,000 chloroplasts per square millimeter of the surface. They are found primarily in cells of the mesophyll, green tissue of the leaf's interior. Each mesophyll cell contains 30-40 chloroplasts.
  • They contain chlorophyll, the green pigment that absorbs energy from sunlight for use in photosynthesis, which means putting together with light. Photosynthesis is the process by which plants make food. In green plants, sunlight captured by chlorophyll enables carbon dioxide from the air to unite with water and minerals from the soil and create food. This process also releases oxygen into the air, and people and animals must have this oxygen to breathe. Energy from the sun splits water molecules into hydrogen and oxygen. The hydrogen joins with carbon from the carbon dioxide to produce sugar. The sugar--together with nitrogen, sulfur, and phosphorus from the soil--helps a plant make the fat, protein, starch, vitamins, and other materials that it needs to survive.
  • Chlorophyll gives plants their color.
  • The chloroplasts of most plants are shaped like disks or lenses. Under a microscope, they can be suspended in the part of the cell called the cytoplasm. These lens shapes measure about 2-4 um to 4-7 um. These organelles are divided into three fundtional compartments by a system of membranes.
    a. Intermembrane Space
    i. The chloroplast is bound by a double membrane which partitions its contents from the cytosol.
    b. Thylakoid Space
    i. Thylakoids form another membranous system within the chloroplast.
    ii. Chlorophyll is found in the thylakoid membranes.
    iii. Thylakoids function in the steps of photosynthesis that initially convert light energy to chemical energy.
    iv. the thylakoid membrane segregates the interior of the chloroplast into two compartments: Thylakoid space = Space inside the thylakoid. Grana = stacks of thylakoids in a chloroplast.
    c. Stroma
    i. Viscous fluid outside the thylakoids.
    ii. Those steps that use chemical energy to convert carbon dioxide to sugar occur in the Srtoma.
  • Chloroplasts are one of several types of specialized plant-cell structures called plastids.
  • There are several forms of chlorophyll. The most common forms in plants are chlorophyll a and chlorophyll b. They absorb most of the long wavelengths (red rays) and the short wavelengths (blue-violet rays) of visible light. They absorb the middle wavelengths (green rays) least effectively.

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