Describe the levels of organization in multicellular organisms, from cells to tissues to organs to organ systems to organisms, and relate specialized cells to the functions they perform (TEKS Biology, Reporting Category 1; systems and system models; structure and function).

What this topic is asking

The Biology TEKS ask you to describe how a multicellular organism is built up from cells, and to connect specialized cells to the jobs they do. For STAAR Reporting Category 1 you need the levels of organization in order, the idea that cells become specialized through differentiation, and the reasoning that links a cell's structure to its function. This is a systems and system models and structure and function topic.

The levels of organization

• Cell. The basic unit of life. All living things are made of one or more cells.
• Tissue. A group of similar cells that work together on the same task (for example, muscle tissue made of muscle cells).
• Organ. Several different tissues combined to perform a function (for example, the heart, which contains muscle, nerve, and connective tissue).
• Organ system. A group of organs that work together (for example, the circulatory system: heart, blood vessels, and blood).
• Organism. All the organ systems together making up a complete living thing.

Some questions extend this ladder beyond the organism into ecology (population, community, ecosystem); see levels of ecological organization.

Cell specialization and differentiation

A complex body needs many kinds of cell, each suited to a job. Although nearly every cell in your body carries the same DNA, cells become different by switching on different genes, a process called differentiation. A muscle cell, a nerve cell, and a red blood cell all start from the same genetic instructions but end up with very different structures because they express different parts of the code.

This is why structure matches function across the body:

• A red blood cell loses its nucleus and fills with hemoglobin to carry the most oxygen, and its small flexible shape lets it pass through narrow capillaries.
• A nerve cell is long with many branches to carry signals over distance and connect to many cells.
• A root hair cell has a long extension to increase the surface area for absorbing water.

Why organization matters

Organizing cells into tissues, organs, and systems means a body can do many things at once and far better than a single cell could. This division of labor is a systems argument: the whole is more capable than the sum of its parts, and a problem at one level (a damaged tissue) can affect the level above it (the organ). STAAR often presents this idea as a hot-spot or drag-and-drop item asking you to place structures at the right level.

Try this

Q1. Define a tissue and give one example. [2]

• Cue. A tissue is a group of similar cells that work together on the same task; for example, muscle tissue.

Q2. Explain how a red blood cell's structure suits its function of carrying oxygen. [2]

• Cue. It has no nucleus and is packed with hemoglobin so it can hold the maximum oxygen, and its small flexible shape lets it pass through capillaries.