From Cells to Organism Help (page 2)

Introduction to from Cells to Organisms

Concerning, the lowest levels of biological organization, remember that the chemical level includes subatomic particles, atoms, and molecules. And just beyond the macromolecules lie the organelles. Finally, recall that the cell level begins “The Life-line,” because it is the lowest living level of biological organization. It is now appropriate for us to ask, “Just what is it that we mean, by living level?” For an answer, just go back to Chapter 1, and review the section on the characteristics of living things. These are the characteristics that first appear in the Pyramid at the cell level, and then continue upward through the entire organism.

Tissues

Right above the cell is the tissue level. A tissue is a collection of similar cells, plus the intercellular ( in -ter- SELL -yew-lar) material located “between” them. There are four basic or primary types of tissue (Figure 2.3). These are epithelial ( eh -pih- THEE -lee-al) tissue, muscle tissue, connective tissue , and nervous tissue . By basic or primary, it is meant that all of the specific types of tissues found in living things (especially humans and animals) are modifications or specializations of these four types.

Epithelial denotes something present “upon” ( epi -) the “nipples” ( theli ), such as the nipples in humans and related animals (like a bear, monkey, or your family cat or dog). Epithelial tissue is a covering and lining tissue. It covers body surfaces in general, and lines cavities within the body interior. Epithelial tissue, for example, forms the outermost layer of the human skin. As evident from Figure 2.3, A, epithelial tissue is almost entirely cellular in nature, with little or no intercellular material between its cells. Connective tissue (Figure 2.3, B), in great contrast, includes a lot of intercellular material between its cells. Frequently, this intercellular material contains long, slender rods – connective tissue fibers . Such fibers help connective tissue do its main job, which is to directly or indirectly connect body parts together. The fearsome teeth in the jaw of a shark, for instance, are firmly anchored into their sockets by connective tissue fibers. Nevertheless, these teeth sometimes break off, and then re-grow. Muscle tissue (Figure 2.3, C) consists of long, slender, muscle fibers . These muscle fibers are actually cells that contract or shorten, thereby creating body movements. Nervous tissue (Figure 2.3, D) is the major tissue for communication and control within the body’s internal environment. (The internal environment is everything deep in from the surface of the skin.) The nervous tissue largely does its communicating by means of neurons ( NUR-ahns ), the nerve cells. Neurons (nerve cells) within the nervous tissue inform the brain when the body has been damaged, usually resulting in the sensation of pain.

Fig. 2.3 The basic or primary types of body tissues.