*Cell:The smallest structural and functional unit of an organism, typically microscopic and consisting of cytoplasm and a nucleus enclosed in a membrane. Microscopic organisms typically consist of a single cell, which is either eukaryotic or prokaryotic.

*The difference between plant and animal cells have several differences and similarities. For example, animal cells do not have a cell wall or chloroplasts but plant cells do. Animal cells are round and irregular in shape while plant cells have fixed, rectangular shapes.

Plant and animal cells are both eukaryotic cells, so they have several features in common, such as the presence of a cell membrane, and cell organelles, like the nucleus, mitochondria and endoplasmic reticulum.

Muscle: A tissue composed of cells or fibers, the contraction of which produces movement in the body.

Blood: The fluid that circulates in the principal vascular system of human beings and other vertebrates, in humans consisting of plasma in which the red blood cells, white blood cells, and platelets are suspended.

Nerve: One or more bundles of fibers forming part of a system that convey simpulses of sensation, motion, etc., between the brain or spinal cord and other parts of the body.

In unicellular (single-celled) organisms, the single cell performs all life functions. It functions independently. However, multicellular (many celled) organisms have various levels of organization within them. Individual cells may perform specific functions and also work together for the good of the entire organism. The cells become dependent on one another.

The digestive tract is dependent upon the respiratory system, because your digestive tract functions by using muscular contractions to break up food and move it along the tract. Smooth muscle in the stomach churns food into a liquid, and contractions of the intestine move food through the system. These muscles depend upon oxygen in order to function -- without oxygen, your digestive tract would stop working.