Classification is the organization of things into groups. One way scientists classifyglossary term (opens in a new window) prokaryotes is by the substances from which they are built. The two domains of prokaryotes, archaea and bacteriaglossary term (opens in a new window), have substantially different chemistries. These differences are numerous, but the main ones are that their cellglossary term (opens in a new window) walls are formed from different substances, and their cell membranes have a different structureglossary term (opens in a new window). In fact, archaea resemble eukaryotes more than they do bacteria. Archaea can also live in different places than bacteria. Some are called extremophiles because they like extreme environments. For example, some species live in boiling springs, while others thrive in toxic chemicals. Some organisms in the domain archaea live deep underwater by hydrothermal vents. Others can survive under icepacks, in the world’s saltiest conditions, or even in the radiationglossary term (opens in a new window) of space! Of course, not all archaea live in extreme habitats; many are found in regular soil and water or the guts of animals; most live in the ocean.

The domain bacteria is probably the one you are more familiar with. Bacteria all have cell walls, but some bacteria have protective outer membranes, and others don’t. Bacteria with the protective membraneglossary term (opens in a new window) are called “gram negative” because they will not take up a certain dye. “Gram positive” bacteria will soak up the dye into their cell walls. Gram stains helps physicians determine what antibioticglossary term (opens in a new window) should be used to fight the bacteria. For example, penicillin works very well against gram-positive bacteria, but poorly against gram negative. Tetracycline works well against gram negative, but not against gram positive.

Bacteria are often identified based on their shapes and the ways they are arranged. The main shapes of prokaryotes are bacillus (rod shaped), coccus (round), and spirillum (spiral). Other shapes include coccobacillus (oval), spirochetes (tighter spirals), and vibrio (curved rods). When a prokaryoteglossary term (opens in a new window) is arranged as a single cell, it is referred to as its shape (for example, a bacillus bacterium). If there is a pair of cells, the prefix diplo- is added, as in diplococcus.

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synovial joints are characterized by the presence of a joint cavity. the walls of this space are formed by the articular capsule, a fibrous connective tissue structure that is attached to each bone just outside the area of the bone’s articulating surface. the bones of the joint articulate with each other within the joint cavity.

friction between the bones at a synovial joint is prevented by the presence of the articular cartilage, a thin layer of hyaline cartilage that covers the entire articulating surface of each bone. however, unlike at a cartilaginous joint, the articular cartilages of each bone are not continuous with each other. instead, the articular cartilage acts like a teflon® coating over the bone surface, allowing the articulating bones to move smoothly against each other without damaging the underlying bone tissue. lining the inner surface of the articular capsule is a thin synovial membrane. the cells of this membrane secrete synovial fluid (synovia = “a thick fluid”), a thick, slimy fluid that provides lubrication to further reduce friction between the bones of the joint. this fluid also provides nourishment to the articular cartilage, which does not contain blood vessels. the ability of the bones to move smoothly against each other within the joint cavity, and the freedom of joint movement this provides, means that each synovial joint is functionally classified as a diarthrosis.

outside of their articulating surfaces, the bones are connected together by ligaments, which are strong bands of fibrous connective tissue. these strengthen and support the joint by anchoring the bones together and preventing their separation. ligaments allow for normal movements at a joint, but limit the range of these motions, thus preventing excessive or abnormal joint movements. ligaments are classified based on their relationship to the fibrous articular capsule. an extrinsic ligament is located outside of the articular capsule, an intrinsic ligament is fused to or incorporated into the wall of the articular capsule, and an intracapsular ligament is located inside of the articular capsule.

at many synovial joints, additional support is provided by the muscles and their tendons that act across the joint. a tendon is the dense connective tissue structure that attaches a muscle to bone. as forces acting on a joint increase, the body will automatically increase the overall strength of contraction of the muscles crossing that joint, thus allowing the muscle and its tendon to serve as a “dynamic ligament” to resist forces and support the joint. this type of indirect support by muscles is very important at the shoulder joint, for example, where the ligaments are relatively weak.

additional structures associated with synovial joints

a few synovial joints of the body have a fibrocartilage structure located between the articulating bones. this is called an articular disc, which is generally small and oval-shaped, or a meniscus, which is larger and c-shaped. these structures can serve several functions, depending on the specific joint. in some places, an articular disc may act to strongly unite the bones of the joint to each other. examples of this include the articular discs found at the sternoclavicular joint or between the distal ends of the radius and ulna bones. at other synovial joints, the disc can provide shock absorption and cushioning between the bones, which is the function of each meniscus within the knee joint. finally, an articular disc can serve to smooth the movements between the articulating bones, as seen at the temporomandibular joint. some synovial joints also have a fat pad, which can serve as a cushion between the bones.

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