Cell Specialization and Levels of Biological Organization

Cell specialization is the process by which cells become adapted to perform particular jobs, and the levels of biological organization are the steps from a cell to a tissue, organ, organ system and whole organism. In brief: a muscle cell contracts, a nerve cell carries signals and a red blood cell carries oxygen. Many similar cells form a tissue, several tissues form an organ, and organs working together form an organ system, as the heart, blood vessels and blood form the circulatory system.

Specialization is what allows multicellular organisms to be large and complex. This guide explains how cells specialize, gives examples of specialized animal and plant cells, describes each level of organization, compares the levels in a table, works through an example from cell to organism and corrects common mistakes.

What is cell specialization?

Almost all cells in a multicellular organism contain the same DNA, since they come from one fertilized egg by cell division. Nevertheless, they look and work differently. During development, different sets of genes are switched on in different cells, and this process is called differentiation. The cell makes particular proteins and gains a particular structure.

Specialization has a trade-off. A specialized cell does one job very well but depends on other cells for what it cannot do. A nerve cell cannot make its own oxygen supply, for example, and relies on the blood to deliver it.

Stem cells are the exception. They are unspecialized cells that can divide and turn into specialized cells, and they are important in growth and repair.

What are examples of specialized animal cells?

• Red blood cells: Biconcave disc shape, no nucleus in mammals and full of hemoglobin, so they carry as much oxygen as possible.
• Nerve cells (neurons): Long fibers and many branches that carry electrical signals over long distances.
• Muscle cells: Long cells with protein filaments that slide past each other and shorten the cell. They contain many mitochondria.
• Sperm cells: A tail for swimming, many mitochondria for energy and enzymes in the head to penetrate the egg.
• Egg cells: Large, with a food store and a protective layer that changes after fertilization.
• White blood cells: Flexible cells that can change shape to engulf bacteria or produce antibodies.
• Ciliated epithelial cells: Cells lining the airways with tiny hair-like cilia that sweep mucus and dust away from the lungs.

What are examples of specialized plant cells?

• Palisade mesophyll cells: Tall cells in leaves packed with chloroplasts to absorb light.
• Root hair cells: A long extension that increases the surface area for absorbing water and minerals.
• Xylem vessels: Dead, hollow cells joined end to end, with walls strengthened by lignin, forming tubes that carry water.
• Phloem sieve tube cells: Living cells with end plates containing pores, which carry sugars. Companion cells next to them supply energy.
• Guard cells: Pairs of cells that change shape to open and close stomata.

What are the levels of biological organization?

Life can be described at several levels, each built from the one before. In an organism, the main levels are as follows.

1. Cell: The basic unit of life. Example: a muscle cell.
2. Tissue: A group of similar cells that perform a shared function. Example: cardiac muscle tissue.
3. Organ: A structure made of two or more tissues that performs a particular function. Example: the heart, which includes muscle, connective, nervous and epithelial tissue.
4. Organ system: A group of organs that work together. Example: the circulatory system.
5. Organism: A complete living individual in which all systems cooperate.

Beyond the organism, ecology continues with populations, communities and ecosystems. Plants are organized in a similar way. Their tissues form organs such as roots, stems and leaves, and these form the root system and the shoot system.

At each level, new properties appear that the lower level does not have. A single heart muscle cell can contract, but only the whole heart can pump blood.

Levels of organization compared

Worked example: from cell to organism in the stomach

Use the stomach to practice naming the levels. The stomach is an organ in the digestive system of a mammal.

1. Cells: Mucus-producing cells, acid-producing cells, smooth muscle cells and nerve cells are present, each with different structure.
2. Tissues: Epithelial tissue lines the inside and secretes mucus and acid. Smooth muscle tissue in the wall churns the food. Connective tissue holds the layers together, and nervous tissue controls the muscle and secretions.
3. Organ: The stomach combines these tissues to break food down mechanically and chemically.
4. Organ system: The stomach is part of the digestive system, together with the esophagus, intestines, liver and pancreas.
5. Organism: The digestive system works with the circulatory system, which carries nutrients, and the nervous system, which controls it, in the whole animal.

If an exam asks to identify a level, count the types of tissue. One type of cell points to a tissue, two or more tissues point to an organ, and several cooperating organs point to a system.

How did scientists learn about tissues and organs?

The French anatomist Xavier Bichat proposed around 1800 that organs are made of distinct tissues, and he described 21 types, although he worked without a microscope. Later, microscopic studies by Schleiden and Schwann tied tissues to cells. The modern understanding of differentiation comes from developmental biology and genetics in the twentieth century, and stem cell research of the last decades has added much detail.

Why does this knowledge matter?

Medicine is organized around levels. Doctors diagnose problems in cells, such as sickle cell disease, in tissues, such as inflammation, and in organs, such as heart failure. Tissue engineering aims to grow replacement skin, cartilage and other tissues from a patient’s own cells. Organ transplantation replaces a whole failed organ. Understanding which cell type is affected helps scientists to design targeted treatments.

Which mistakes are common?

Mistake 1: Saying that specialized cells have different DNA

They have the same DNA but use different genes. A few exceptions exist, such as mature red blood cells in mammals, which lack a nucleus.

Mistake 2: Mixing up tissue and organ

A tissue is made of one cell type, while an organ contains several tissues.

Mistake 3: Calling a single organ a system

The stomach is an organ. A system is a group of organs, such as the digestive system.

Mistake 4: Assuming that plants have no organs

Roots, stems, leaves and flowers are plant organs, made of several tissues.