Rough endoplasmic reticulum: Rough endoplasmic reticulum appears"pebbled" by electron microscopy due to the presence of numerousribosomes on its surface. Proteins synthesized on these ribosomescollect in the endoplasmic reticulum for transport throughout thecell.

Smooth endoplasmic reticulum: Throughout the eukaryotic cell, especially thoseresponsible for the production of hormones and other secretory products, is a vastnetwork of membrane-bound vesicles and tubules called the endoplasmicreticulum, or ER for short. The ER is a continuation of the outer nuclear membraneand its varied functions suggest the complexity of the eukaryotic cell. The smoothendoplasmic reticulum is so named because it appears smooth by electronmicroscopy. Smooth ER plays different functions depending on the specific cell typeincluding lipid and steroid hormone synthesis, breakdown of lipid-soluble toxins inliver cells, and control of calcium release in muscle cell contraction.

Vacuole: A vacuole is a membrane-bound sac that playsroles in intracellular digestion and the release of cellularwaste products. In animal cells, vacuoles are generallysmall.

Mitochondria: Mitochondria provide the energy a cell needs to move, divide, produce secretory products,contract in short, they are the power centers of the cell. They are about the size of bacteria but mayhave different shapes depending on the cell type. Mitochondria are membrane-bound organelles, andlike the nucleus have a double membrane. The outer membrane is fairly smooth. But the inner membraneis highly convoluted, forming folds (cristae) as seen in the cross-section, above. The cristae greatlyincrease the inner membrane's surface area. It is on these cristae that food (sugar) is combined withoxygen to produce ATP the primary energy source for the cell.

Cell membrane: Every cell is enclosed in a membrane, a double layer of phospholipids (lipid bilayer). Theexposed heads of the bilayer are "hydrophilic" (water loving), meaning that they are compatible with waterboth within the cytosol and outside of the cell. However, the hidden tails of the phosopholipids are"hydrophobic" (water fearing), so the cell membrane acts as a protective barrier to the uncontrolled flow ofwater. The membrane is made more complex by the presence of numerous proteins that are crucial to cellactivity. These proteins include receptors for odors, tastes and hormones, as well as pores responsible forthe controlled entry and exit of ions like sodium (Na+) potassium (K+), calcium (Ca++) and chloride (Cl-).

Peroxisomes: Peroxisomes are membrane-bound packets of oxidativeenzymes. In animal cells, peroxisomes protect the cell from its ownproduction of toxic hydrogen peroxide. As an example, white blood cellsproduce hydrogen peroxide to kill bacteria. The oxidative enzymes inperoxisomes break down the hydrogen peroxide into water and oxygen.

Lysosomes: Lysosomes contain hydrolytic enzymesnecessary for intracellular digestion. They are common inanimal cells, but rare in plant cells. Hydrolytic enzymes ofplant cells are more often found in the vacuole.

Golgi Apparatus: The Golgi apparatus is a membrane-bound structure with a singlemembrane. It is actually a stack of membrane-bound vesicles that are important inpackaging macromolecules for transport elsewhere in the cell. The stack of larger vesiclesis surrounded by numerous smaller vesicles containing those packaged macromolecules.The enzymatic or hormonal contents of lysosomes, peroxisomes and secretory vesicles arepackaged in membrane-bound vesicles at the periphery of the Golgi apparatus.

Cytoskeleton: As its name implies, the cytoskeleton helps to maintaincell shape. But the primary importance of the cytoskeleton is in cellmotility. The internal movement of cell organelles, as well as celllocomotion and muscle fiber contraction could not take place withoutthe cytoskeleton.

Centrosomes: The centrosome, or MICROTUBULE ORGANIZING CENTER (MTOC), is an area in the cellwhere microtubules are produced. Plant and animal cell centrosomes play similar roles in cell division,and both include collections of microtubules, but the plant cell centrosome is simpler and does nothave centrioles. During animal cell division, the centrioles replicate (make new copies) and thecentrosome divides. The result is two centrosomes, each with its own pair of centrioles. The twocentrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules growinto a "spindle" which is responsible for separating replicated chromosomes into the two daughtercells.

Cytosol: The cytosol is the "soup" within which all the other cellorganelles reside and where most of the cellular metabolism occurs.Though mostly water, the cytosol is full of proteins that control cellmetabolism including signal transduction pathways, glycolysis,intracellular receptors, and transcription factors.

Nucleus: The nucleus is the most obvious organelle in any eukaryoticcell. It is enclosed in a nuclear membrane and communicates with thesurrounding cytosol via numerous nuclear pores. Within the nucleusis the DNA responsible for providing the cell with its uniquecharacteristics. The DNA is similar in every cell of the body, butdepending on the specific cell type, some genes may be turned on oroff that's why a liver cell is different from a muscle cell, and amuscle cell is different from a fat cell. When a cell is dividing, thenuclear chromatin (DNA and surrounding protein) condenses intochromosomes that are easily seen by microscopy.