Cell Specialization

Essential Question

How does a cell’s structure match the job it does?

Your body has many kinds of cells, and they do not look alike. In this lesson, you’ll learn how cells become specialized, how a cell’s structure fits its job, and how cells are organized into tissues, organs, and organ systems.

Same DNA, Different Genes

• Most body cells carry the same DNA
• Each cell type uses only some of its genes
• Different genes make different proteins
• Specialization is called differentiation

A multicellular organism starts as one cell, which divides many times. Most of the cells that result carry the same DNA, but they do not use all of it. A cell uses some genes and leaves others unused, and the proteins it makes decide how it is built and what it does. This process, called differentiation, produces specialized cells. A muscle cell makes large amounts of the muscle protein myosin, and a developing red blood cell makes hemoglobin. (Mature red blood cells in humans lose their nucleus, so they carry no DNA at all.)

Structure Fits Function

• Red blood cells carry oxygen
• Muscle cells contract
• Root hair cells absorb water
• Intestinal cells absorb food

Each specialized cell has a shape and set of parts that help it do its job. A mature red blood cell has no nucleus, which leaves more room for hemoglobin, the protein that carries oxygen. A muscle cell is long and full of protein fibers that contract, and it has many mitochondria. A root hair cell has a long projection that increases the surface through which it absorbs water. The intestinal lining cell has tiny folds called microvilli that increase the surface through which it absorbs food.

Organelles Match the Job

• Energy-hungry cells have more mitochondria
• Protein-secreting cells have more rough ER and Golgi
• Leaf cells that photosynthesize have chloroplasts
• Storage cells may have large vacuoles

Cells also differ in the number of organelles they contain. A cell that uses a lot of energy, such as a heart muscle cell, has many mitochondria. A cell that makes and releases large amounts of protein has extensive rough ER and Golgi apparatus. A leaf cell that carries out photosynthesis has many chloroplasts, and a storage cell may have a large vacuole. The numbers in the graph are approximate sample values used to show the pattern. Real counts vary by cell and by organism.

Levels of Organization

• Similar cells form a tissue
• Tissues form an organ
• Organs form an organ system
• Organ systems form an organism

Specialized cells do not work alone. Similar cells working together form a tissue, and tissues that work together form an organ. Several organs that work together form an organ system, and organ systems working together make up the whole organism. For example, heart muscle cells make up heart muscle tissue, which is part of the heart, which is part of the circulatory system. Each level depends on the one below it, so a problem with specialized cells can affect the whole organism.

↑ Back to top

---

Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Same DNA, Different Proteins

A muscle cell and a developing red blood cell from the same person have the same DNA. Use the model to explain why the muscle cell makes myosin and the red blood cell makes hemoglobin.

Show the solution

1. Same DNA: both cells carry the myosin gene and the hemoglobin gene.
2. Different genes used: the muscle cell uses the myosin gene, and the red blood cell uses the hemoglobin gene.
3. Result: the proteins a cell makes give it its structure and function, so the two cells differ.
4. Answer: Each cell uses a different part of the same DNA, so it makes different proteins.

Example 2: Which Cell Needs the Most Energy?

The graph shows the approximate number of mitochondria in four kinds of cells (sample values). Write a claim, evidence, and reasoning about which cell uses the most energy.

Show the solution

1. Claim: the heart muscle cell uses the most energy.
2. Evidence: it has about 5,000 mitochondria, compared with about 1,000 in the liver cell, 300 in the skin cell, and 0 in the mature red blood cell.
3. Reasoning: mitochondria make ATP, so more mitochondria means more ATP. A heart muscle cell contracts constantly, so it needs a lot of ATP.
4. Answer: The heart muscle cell, with about 5,000 mitochondria.

Example 3: Order the Levels

Use the model to put these in order from smallest to largest: circulatory system, heart muscle cell, human, heart, heart muscle tissue.

Show the solution

1. Cell: the heart muscle cell is the smallest unit.
2. Tissue: many similar heart muscle cells form heart muscle tissue.
3. Organ: the heart is made of several tissues.
4. Organ system: the heart, blood vessels, and blood make up the circulatory system.
5. Organism: the circulatory system is one system of the human.
6. Answer: Heart muscle cell, heart muscle tissue, heart, circulatory system, human.

↑ Back to top

---

Practice

Match cell structures to their jobs, fill in the levels of organization, read a graph of organelle numbers, and explain how cells with the same DNA can be different. Try it on your own first, then check your work with the answer key.

• Cell Specialization Practice Worksheet (PDF)
• Cell Specialization Practice Answer Key (PDF)

↑ Back to top

---

Lesson Slides

• 6.4 Cell Specialization Slides (PDF)

↑ Back to top

---

Big Ideas

• Differentiation produces specialized cells that use different genes
• A cell’s structure fits its function
• Organelle numbers match the job a cell does
• Cells form tissues, organs, organ systems, and organisms

Back to the essential question: cells in a body carry the same DNA but use different genes, so they make different proteins and take on different structures. Each structure, from a red blood cell’s missing nucleus to a muscle cell’s many mitochondria, fits the job of the cell. Specialized cells work together in tissues, organs, and organ systems. Next, you’ll look at how the cell membrane controls what enters and leaves.

↑ Back to top

---

↑ Back to top