Cells, Tissues, and Organs Study Guide

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Updated on Sep 21, 2011


One of the hallmark characteristics of living things is that they perform chemical reactions. These reactions are collectively known as metabolism. Cells, the basic units of life, can perform many of these metabolic reactions. In a multicelled organism, the cells group together to form tissues that perform the same functions. Tissues group together to form organs, and finally, several organs exist together in a system. In this lesson, we will see how and why this hierarchy is established.

The Basic Unit of Life

All organisms from the smallest single-celled protists to huge whales and giant redwood trees are based on tiny microscopic cells. The types and number of cells may vary, but the cell is the basic unit of life.

The cell is the minimum amount of organized living matter complex enough to carry out the functions of life as outlined in Lessons 1 and 2. In the most basic sense, a cell is made of a gelatinous living substance we call protoplasm, which contains many small structures, all surrounded by a membrane.

Cell Structure

The cell membrane separates the living cell from the rest of the environment. However, this membrane is not just a static solid wall. It must allow food molecules and oxygen to enter and wastes to exit. Thus, the cell membrane is semipermeable because it allows some things to pass through, but not others. It must also communicate and associate with the membranes of other cells.

Inside the cell membrane is a substance called protoplasm in which many tiny structures called organelles (because they act like small versions of organs) are suspended. Some of the more important organelles and their functions are listed in the following table.

Cells Tissues and Organs

Cellular Differentiation

Simple, one-celled organisms use a single cell to carry out all the necessary biochemical and structural functions. However, in multicelled animals, except very primitive ones (such as sponges), different types of cells are specialized to perform a particular function. Thus, a division of labor takes place; some cells do one task while others do another. This kind of specialization leads to groups of cells equally specialized for a specific function. These grouped cells are known as tissues.

Tissue Types

Tissues in plants and animals are categorized by their functions as shown in the following tables.

Cells Tissues and Organs

Organs and Organ Systems

As living organisms go through their life cycle from birth to death, they gain mass or grow, and their bodies change or develop. Single-celled organisms grow and develop very rapidly, whereas more complex, multicelled organisms take much longer. Organisms go through some very large changes as they age. Humans start out as a single fertilized cell and then grow and develop over several years to the adult stage. This growth and development is a process that separates life from nonlife. It also makes it necessary to group tissues together into larger units called organs. The organs perform more complicated functions than do the single tissues that make them up.

For example, the heart is made of cardiac muscle (a type of contractile tissue) with its valves made of conjunctive tissue. This allows the heart to have a specific shape and to beat in a complicated way that delivers blood to the lungs for oxygenation and then throughout the body. Simple cardiac tissue not grouped into a heart organ would not be able to do this.

As another example, look at tree roots as organs. They have a protective epidermis (a "skin") made of protective tissue that covers them. They also are composed of meristematic tissue that allows them to grow. Also part of the root organ is conductive tissue because roots are meant to absorb and transport water and utrients to the rest of the plant. Other examples of organs found in plants are flowers and leaves. In animals, however, many organs can be found, such as the brain, lungs, stomach, liver, eyes, ears, and so on.

Generally, in complex organisms like plants and animals, many organs are then grouped together into systems. A good example of an organ system is the digestive system in animals that consists of many organs (which in turn consist of many combinations of tissues). The digestive system contains the mouth, esophagus, stomach, small and large intestines, liver, pancreas, and gall bladder, to name some of the most well-known organs.

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