 |
Cellular Control Center: Nucleus: The nucleus is the "brain" of the cell. It contains all the DNA for the cell. The nucleus is covered in a nuclear envelope. This envelope is covered with holes that allow mRNA to pass in and out of the nucleus. Most nuclei also contain a small, dense region called the nucleolus. The nucleolus is where the assembly of ribosomes begins. Organelles That Store, Clean Up, and Support: Vacuoles and Vesicles: Vacuoles store materials like water, salts, proteins, and carbohydrates. In plant cells, there is a single, large, central vacuoles filled with liquid. The pressure from this central vacuole helps increase the rigidity of the cell and makes it possible for the plants to stand up. Vesciles are found in nearly all eukaryotic cells and help store and move materials between cell organelles, as well as to and from the cell surface. Lysosomes: Lysosomes break down lipids, carbohydrates, and proteins into small molecules that can be used by the rest of the cell. They are also involved in breaking down organelles that are past their usefulness. They are essentially the "cleanup crew" of the cell. They remove the "junk" that might otherwise accumulate and clutter up the cell. A good number of of serious human diseases can be traced to lysosomes that fail to function properly. Biologists once thought they were found only in animal cells, but they now realize they are in a few, specialized, plant cells. The Cytoskeleton: The cytoskeleton helps the cell maintain its shape and is also involved in its movement. The cytoskeleton is essentially the rebar that holds the cell up. The cytoskeleton is usually made up of microfilaments and microtubules. The microfilaments are threadlike structures made up of a protein called actin. They form extensive networks in some cells and produce the framework that supports the cell. Microtubules are hollow structures made up of proteins called tubulins. Usually, they play critical roles in maintaining cell shape. They are also important in cell division in making a structure called the "mitoic spindle" which helps to seperate the chromosomes. Organelles That Build Proteins: Ribosomes: Ribosomes are the areas where proteins are assembled. Each ribosome is like a small machine in a factory, turning out proteins on orders that come from its DNA "boss". Cells that are especially active in protein synthesis often contain large numbers of ribosomes. Endoplasmic Reticulum: The endoplasmic reticulum is where lipid components of the cell are assembled, along with proteins and other materials that are explorted from the cell. The portion of the ER involved in protein synthesis is called the rough ER. It is called this because it has ribosomes on the surface. Newley made proteins leave these ribosomes and are inserted into the rough ER where they may be chemically modified. Golgi Apparatus: The golgi apparatus modifies, sorts, and packages proteins and other materials from the endoplasmic reticulum for storage in the cell or release outside the cell. When proteins produced in the rougher ER are done they move into the golgi apparatus. The golgi apparatus then acts as a "customization shop" by modifying cells for their end tasks. Organelles That Capture and Release Energy: Chloroplasts: Chloroplasts are the biological equivalents of photovolic arrays. They capture energy from sunlight and convert it into food that contains chemical energy in a process called photosynthesis. Two membranes surround chloroplasts. Inside the organelle are large stacks of other membranes, which contain the green pigment chlorophyll. Mitochondria: Mitochondria convert the chemical energy stored in food into compounds that are more convenient for the cell to use. They have two membranes just like chloroplasts which enclose the organelle. In humans, all or nearly al of our mitochondria come from the cytoplast of the egg cell. Mitochondria also contain their own genetic information in the form of small DNA molecules. Cellular Boundaries: Cell Walls: Many organisms have cell walls in addition to cell membranes. The main purpose of these walls is to support, protect, and shape the cell. Cell walls are found in most plant cells, but not in animal cells. Cell Membranes: All cells have cell membranes. These membranes are almost always made of of a double-layered sheet called a lipid bilayer. This bilayer is formed by the fact that the lipid molecules that make it up have a hydrophilic head that likes water and two hydrophobic tails that dislike water. The cell membrane regulates what enters and leaves the cell and also protects and supports the cell. |
|
