Prokaryotic and Eukaryotic Cells

Essential Question

How are bacterial cells different from plant and animal cells?

A bacterium and a skin cell are both alive, but they are built very differently. In this lesson, you’ll compare the two main kinds of cells, prokaryotic and eukaryotic, see how their sizes compare, and use a magnification formula to find the actual size of a cell from a picture.

Two Kinds of Cells

• Prokaryotic cells have no nucleus
• Eukaryotic cells have DNA inside a nucleus
• Eukaryotic cells have membrane-bound organelles
• Both have a membrane, cytoplasm, DNA, and ribosomes

Biologists sort cells into two groups. Prokaryotic cells have no nucleus. Their DNA floats in the cytoplasm in a region called the nucleoid, and they usually have no membrane-bound organelles. Bacteria are prokaryotes. Eukaryotic cells keep their DNA inside a membrane-bound nucleus and have other organelles, which are compartments with their own membranes. Animals, plants, fungi, and protists are made of eukaryotic cells. Both kinds of cells have a cell membrane, cytoplasm, DNA, and ribosomes.

Inside a Prokaryotic Cell

• Cell wall gives shape and protection
• DNA is a loop in the nucleoid
• Small DNA rings called plasmids
• Many bacteria have a flagellum to swim

A typical bacterium is a single cell surrounded by a cell wall that gives it shape and protects it. Some also have a sticky outer capsule. Inside the cell membrane, the main DNA is a loop in the nucleoid, and many bacteria also carry small rings of DNA called plasmids. Ribosomes fill the cytoplasm, building proteins. Many bacteria have a flagellum, a whip-like tail that spins to move the cell.

Inside Eukaryotic Cells

• Nucleus holds the DNA
• Mitochondria are in animal and plant cells
• Plant cells also have a cell wall
• Plant cells have chloroplasts and a large vacuole

Animal and plant cells are both eukaryotic, so both have a nucleus and mitochondria. Plant cells also have features that animal cells typically lack. A rigid cell wall outside the membrane supports the plant, chloroplasts carry out photosynthesis, and a large central vacuole stores water and other materials. Notice that prokaryotic cells can have a cell wall too, but it is made of different materials from a plant cell wall. You will study each organelle in the next lesson.

How Big Are Cells?

• Most cells are too small to see without a microscope
• Bacteria are usually about 1 to 10 µm
• Many eukaryotic cells are 10 to 100 µm
• 1 mm = 1,000 µm

Cells are measured in micrometers (µm). One micrometer is one thousandth of a millimeter. The graph uses a log scale, because the sizes range from 0.1 µm to 120 µm, and gives approximate values. Bacteria are usually smaller than eukaryotic cells, but there are exceptions. The magnification formula connects a picture to the real cell: actual size = image size ÷ magnification. A cell that measures 10 mm in a photograph taken at ×5,000 is really 10 mm ÷ 5,000 = 0.002 mm, or 2 µm.

Where Eukaryotic Cells Came From

• The endosymbiotic theory explains mitochondria
• A large cell took in a bacterium
• The bacterium stayed and became a mitochondrion
• DNA, ribosomes, and membranes support the theory

The endosymbiotic theory proposes that mitochondria began as free-living bacteria that were taken into a larger cell and stayed there, to the benefit of both. Scientists support this theory with evidence. Mitochondria have their own circular DNA, ribosomes that resemble those of bacteria, and a double membrane, and they divide on their own inside the cell. The same theory explains chloroplasts, which also have these features.

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Worked Examples

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

Example 1: Finding Actual Size

A micrograph taken at ×5,000 shows a bacterium that measures 10 mm long. Find its actual length in micrometers. A typical animal cell is about 30 µm across. How many of these bacteria, placed end to end, would fit across the animal cell?

Quantity	Value
Image length	10 mm
Magnification	×5,000
Animal cell width	30 µm

Show the solution

1. Formula: actual size = image size ÷ magnification
2. Actual length: 10 mm ÷ 5,000 = 0.002 mm
3. Convert: 0.002 mm × 1,000 µm/mm = 2 µm
4. Fit across the cell: 30 µm ÷ 2 µm = 15 bacteria
5. Answer: The bacterium is 2 µm long, and 15 would fit across a 30 µm animal cell.

Example 2: Classify the Cell

A student studies three unknown cells. Use the table to classify each cell as prokaryotic, plant, or animal, and give the evidence.

Cell	Nucleus	Cell wall	Chloroplasts	Size
X	No	Yes	No	2 µm
Y	Yes	Yes	Yes	40 µm
Z	Yes	No	No	20 µm

Show the solution

1. Cell X: it has no nucleus, so it is prokaryotic. Its size, 2 µm, fits too.
2. Cell Y: it has a nucleus, a cell wall, and chloroplasts, so it is a plant cell.
3. Cell Z: it has a nucleus but no cell wall and no chloroplasts, so it is an animal cell.
4. Answer: X is prokaryotic, Y is a plant cell, and Z is an animal cell.

Example 3: Are Eukaryotic Cells Bigger?

A student claims, “Eukaryotic cells are larger than prokaryotic cells.” Use the graph to decide whether it supports the claim. Write a claim, evidence, and reasoning.

Show the solution

1. Claim: the data support the claim as a general pattern.
2. Evidence: the bacterium is about 2 µm, while the yeast cell is 5 µm, the red blood cell is 7.5 µm, and the skin cell is 30 µm. The egg cell is 120 µm, which is 60 times the bacterium’s length.
3. Reasoning: eukaryotic cells contain a nucleus and other organelles, so they usually need more room. This is a pattern with exceptions: a few bacteria are large, and the graph shows only a few cell types.
4. Answer: The graph supports it as a general pattern. Eukaryotic cells are usually larger, but there are exceptions.

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Practice

Label a bacterium, classify cells from their features, calculate actual size from a micrograph, and use size data to evaluate a claim. Try it on your own first, then check your work with the answer key.

• Prokaryotic and Eukaryotic Cells Practice Worksheet (PDF)
• Prokaryotic and Eukaryotic Cells Practice Answer Key (PDF)

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Lesson Slides

• 6.2 Prokaryotic and Eukaryotic Cells Slides (PDF)

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Big Ideas

• Prokaryotic cells have no nucleus; eukaryotic cells do
• Eukaryotic cells have membrane-bound organelles
• Plant cells also have a cell wall, chloroplasts, and a large vacuole
• Actual size = image size ÷ magnification
• Mitochondria probably began as bacteria taken into a larger cell

Back to the essential question: prokaryotic cells have no nucleus and usually no membrane-bound organelles, while eukaryotic cells keep their DNA in a nucleus and divide their work among organelles. Plant cells add a cell wall, chloroplasts, and a large vacuole. Prokaryotic cells are usually much smaller. Next, you’ll take a closer look at the organelles of eukaryotic cells.

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