Cell structure:

     The plasma membrane is a fragile, transparent, bag-like structure which surrounds the cell contents and
     separates them from the outside environment. This membrane has a core of two phospholipid layers (with
     the water soluble phosphate head facing to the outside of the cell and, in the bottom layer, into the cell),
     interspersed with cholesterol molecules to lend it rigidity and stability. Also throughout are proteins, which
     are responsible for most of the specialized functions of the membrane (channel proteins). Some of the
     proteins are enzymes; many are mounted on the outside to serve as receptors, or binding sites, for
     hormones or other chemical messengers. But most of the portions are involved in transport. They form
     pores through which water and small water soluble molecules or ions can move through. Others more
     actively move larger substances, or ions and such against the concentration gradient. Some of the proteins
     are glycoprotein: these proteins determine your blood type, act as receptor that certain bacteria, viruses
     or toxins can bind to, or work in cell to cell interactions. Microvilli are tiny finger-like projections of the
     plasma membrane to give more surface area for absorption. Membrane junctions vary in structure and
     role. In tight junctions, the plasma membranes are fused together, much like a zipper. These junctions
     connect cells into leak-proof sheets which stop substances from passing through the spaces between cells
     (blood vessels, intestines). Desmosomes are adhesion junctions that prevent mechanically stressed cells
     from tearing apart (skin). The membranes are bound together by proteins, the second with root-like
     structures. Gap junctions allow direct passage of chemical molecules, such as ions, from one cell to the
     next (heart, embryonic cells). These are mainly used for communication

     The cytoplasm is made up of the intracellular material that is outside the nucleus. It is the site of most
     cellular activity, and contains three parts: the cytosol, the organelles, and the inclusions. Cytosol it the
     semi-transparent fluid which suspends the other two. Dissolved into the cytosol (made up mostly of water),
     are various nutrients and other solutes. The organelles are the cellular machinery which do all of the cells
     work. Inclusions are not "work" units, but rather storage units for nutrients and such. They include fat
     droplets, glycogen granules, pigments, and water-containing vacuoles common in mucus

     Mitochondria are usually depicted as tiny, thread-like ot sausage shaped organelles. In the living cell,
     however, they are far more animated; squirming, lengthening, and changing shape almost constantly. The
     walls of the mitochondria consist of a double membrane, equal to two plasma membranes placed side by
     side. The outer wall is smooth, but the inner is riddled with shelf-like protrusions called cristae. Enzymes in
     the mitochondria carry out cellular respiration; that is, break down food using oxygen to produce adenosine
     triphosphate (atp). Thus, the mitochondria are often called the "powerhouses" of the cell. Cells such as liver
     or muscle cells use a great deal of energy, and so contain a large number of mitochondria. By comparison,
     some cells, such as an egg cell, have very few

     Ribosomes are small, round, dark organelles made up of proteins and ribosomal ribonucleic acid (rna). They
     are the sight of protein synthesis. They are both suspended in the cytoplasm, and attached to the rough
     endoplasmic reticulum

     The endoplasmic reticulum is a system of fluid-filled cisterns that twist their way about the cell. These
     conduits account for about half of the cell's membranes. They serve as a circulatory within the cell. The
     rough endoplasmic reticulum is made such by the presence to ribosomes, and move about the proteins the
     ribosomes make. It also manufactures the building materials of cellular membranes, lipids. The other type
     of endoplasmic reticulum is smooth. It plays no role in protein synthesis. It's function is cholesterol synthesis
     and breakdown, fat metabolism, and detoxification of drugs. These smooth endoplasmic reticulum are
     plentiful in liver cells, and cells that produce steroid-based hormones, such as the testes in males

     The Golgi apparatus appears as a stack of flattened membrane sacks, associated with numerous, small
     vesicles. Generally located in the vicinity of the nucleus, it serves as a traffic director of cellular proteins.
     Protein synthesis is completed here, and the proteins, hormones (protein and steroid -based alike), and
     enzymes are packages and sent out of the cells from the Golgi. As the substances are "tagged" for release,
     the sacks swell and pinch off to form secretory vesicles, which move to the cell's membrane and fuse with
     it, releasing the vesicle's contents. Some of the vesicles do not go to the plasma membrane. Some deliver
     their contents (enzymes) to digest a substance, thus becoming lysosomes. Lysosomes are especially
     abundant in white blood cells, which digest bacteria and other potentially harmful substances. The lysosomal
     membrane us usually quite stable, but when the cell is injured or deprived of oxygen, they self-digest,
     becoming "suicide sacks"

     Peroxisomes are membrane sacks which contain powerful oxidase enzymes. These enzymes aid in the
     digestion of fats and to detoxify a number of poisons, such as alcohol and formaldehyde. By removing and
     transferring hydrogen atoms from them to oxygen, the cell produces hydrogen peroxide. Although
     hydrogen peroxide is a normal byproduct of cellular functions, it can cause damage if too much
     accumulates. The peroxisome enzymes convert the hydrogen peroxide into water and oxygen gas. Though
     the perixisomes look like lysomes, their membranes appear to have formed directly from the budding of
     the rough endoplasmic reticulum

     The cytoskeleton is an elaborate network of (from largest to smallest) microtubules, intermediate
     filaments, and microfilaments. They support, hold, and change the cells shape and structure

     The two sets of paired centrioles are rod-shaped organelles that lie close to the nucleus. They are made up
     of microtubules, and migrate to apposing side of the nucleus during mitosis to pull apart the chromosomes,
     forming mitotic spindles

     The nucleus is the "control center" of the cell. It contains the deoxyribonucleic acid (dna), which codes for
     all life. More specifically, it contains the blueprints for proteins. Most often, the nucleus is elliptical or
     spherical, but conforms to the shape of the cell. In an elongated cell, for instance, the nucleus is also
     stretched. The nucleus has three distinct regions: The nuclear membrane is a double-membrane barrier.
     Between the two layers is a fluid filled space, or "moat". At various places, the two membranes join and
     fuse, forming the nuclear pores. Passage through this membrane is somewhat freer than other membranes
     through it large pores. The dna, however, cannot leave. Within the membrane, a jelly-like fluid called
     nucleoplasm suspends the nucleoli and the chromatin The nucleus contains one or more roundish bodies
     called nucleoli. These are the sites where ribosomes are born. When a cell is not dividing, the dna forms a
     lose network of bumpy threads called chromatin. When the cell divides, the chromatin forms denser,
     rod-like bodies called chromosomes