Friday, December 26, 2008


Movement is an important characteristic of living things. Animals are able to move from one place to another. But plants are restricted to some types of movements induced by water currents, turbidity, osmosis, etc. Animals are able to move because of the presence of contractile muscle fibers. Prokaryotes and other single-celled eukaryotic organisms move by hair-like structures called flagella and cilia or parts of pro top lasts (pseudopodia) as in the case of amoeba.

Amoeboid Movement

This type of movement is observed in some single-celled eukaryotes such as amoebas, slime molds, and eukaryotic cells such as lymphocytes of humans. Amoebas move by means of pseudopods (a foot-like extension at the front of a cell) attaching and detaching from the substratum. Locomotion may be correlated with the forward flow of fluid cytoplasm (endoplasm) into advancing pseudopodia through a surrounding gel-like ectoplasmic tube. The ectoplasm forms at the pseudopodia tip in a region called the fountain zone. As the amoeba advances, the ectoplasmic tube "liquefies" at the posterior end to form endoplasm. As the cell moves, new pseudopodia are formed in the direction of movement while the earlier ones are withdrawn. Amoeboid movement has a striking similarity with cytoplasmic movement known as cyclosis. The cytoplasmic streaming movement is a common characteristic of all plant and animal cells. This has an important role in intracellular transport. The cytoplasmic streaming movement and amoeboid movement by pseudopodia may be dependent on the contractile activity of protein fibers such as actin and myosin present in the cytoplasm along with the cytoskeleton.

Movement by Cilia and Flagella

Bacteria move using a rotating flagellum. Non-animal eukaryotes achieve motion in many ways, including moving by cilia and flagella, growing roots and hyphae, or dispersing gametes, spores, and seeds with the help ofwifd, water, and animals.

Cilia and flagella are the main locomotory organs (organelle) of prokaryotes such as bacteria and other single-celled organisms. Both flagella and cilia have identical internal structures. Microtubules form the internal structures of both cilia and flagella. They are also involved in other cellular movements such as movement of chromosomes and streaming movement of protoplasts, etc. The combined stroking movement of cilia propels certain unicellular organisms, such as paramecia, through their environment and moves fluid and particles over the surface of ciliated cells of higher eukaryotes. Flagella provide the propulsion for motile cells ranging from bacteria to sperm of higher eukaryotes. The main difference between cilia and flagella is in their stroking pattern and also in their length and number per cell. Flagella beat with a symmetrical undulation that is propagated as a wave along entire length of flagella. Flagella are very long and less in number. Flagellated cells usually carry one or few flagella. Ciliated cells usually have thousands of cilia all over the cell surfaces and are very short. A cilium beats symmetrically· with a fast stroke in one direction followed by slower recovery stroke. Cilia and flagella are also present in many multicellular organisms. There are some types of tissues in higher organisms where cilia are present. For example, in mammals the ciliated epithelium helps in the transport of certain materials on the internal surfaces as in the case of the movement of mucus in the respiratory tract. The sperm of most animals move with help of single flagella.

Muscle and Movement

Even though all organisms can move directly or indirectly, the animals have an especially impressive range of movement. The presence of muscles contributes a special ability for animals in movement. The contraction of muscle tissue powers animal movement. Muscle contains thin filaments of the proteins actin and myosin; myosin filaments lie between the actin filaments within a sarcomere, the basic contractile unit of muscle. A series of sacromeres attached end-to-end makes up a single fiber called myofibrils. A number of myofibrils bundled together form a muscle fibril. Myofibrils are bundled to form a muscle fiber, which is further bundled to constitute a functional body muscle (e.g., the human biceps). Since muscles only produce force when contracting, they must work as opposing pairs to give the full range of possible motion.

Sarcomeres appear as bands under a microscope. Each sarcomere is bounded on either side by dark lines, the Z discs. The cells contain the contractile proteins, actin and myosiJ\, which are arranged in a very specific manner. Z discs form the anchor points for the actin filaments. From this point actin filaments extend toward the center of the sarcomere. Myosin filaments are present in between the actin filaments toward the middle.

Muscles contract as the actin and myosin filaments slide past one qnother. The filaments do not decrease in length during the process of contraction and relaxation. ATP binds to the myosin head at a specific binding site and releases myosin from the actin filament. Myosin hydrolyzes the ATP into ADP and Pi. (Note that myosin is an ATPase.) Some of the energy is used to change the position of the myosin from a "bent" low-energy state, to an "open" high-energy state. Energized myosin binds to a site on the actin filament. ADP and Pi are released from the myosin head, and this causes the head to "spring back" to its original bent configuration moving the actin filament toward the sarcomere center, pulling the Z discs inward causing contraction of the muscle. The muscles are anchored to the skeleton for exerting the force caused by the contraction. Following contraction, the muscle fibers return to their original position.

Muscle strength is increased somewhat by stretching existing muscle cells through exercise, but mostly by increasing the number of muscle cells. In contrast, muscle speed is mainly influenced by muscle length and the percentage of fast or slow muscle fibers.

Muscles and skeleton work together to control the strength and speed of animal movement. The physical model of a lever system explains how vertebrate skeleton and muscles interact to achieve adaptive movement.

Animals move by swimming, running, or flying, with all three requiring the production of thrust. Environmental drag, caused by friction with the surrounding molecules and the pressure of those molecules sticking to the body, resists thrust, and thereby restricts animal's motion. Natural selection has favored organisms that minimize drag.

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