Lesson 12

12

Getting More Complex: Eukaryotes

What would happen if one living thing swallowed another and, instead of destroying it, kept it as a partner?

Eukaryotic cells contain a nucleus and other structures with specialized jobs. This lesson introduces their organization and the evidence that mitochondria began as free-living bacteria, helping explain a major change in the history of life.

Click here to hear the lesson video

Key Ideas

• Over time, some cells became more complex and developed special structures (organelles) inside.
• These are called eukaryotes, and they have a nucleus that holds the cell's DNA instructions.
• The endosymbiosis theory explains how eukaryotes formed: one cell absorbed another and they became partners.
• Mitochondria, the 'powerhouses of the cell,' were once free-living bacteria that became part of larger cells.
• Plants, animals, fungi, and YOU are all made of eukaryotic cells!

Spines

• DK's Science as You've Never Seen it Before: pg. 142-143
• Mammoth Science: Cells pg. 38
• Visual Timelines: Life on Earth pg. 16
• Alternative
  • Britannica Students Eukaryotes (https://kids.britannica.com/students/article/eukaryote/322731)
  • Berkley Endosymbiosis (https://evolution.berkeley.edu/it-takes-teamwork-how-endosymbiosis-changed-life-on-earth/evidence-for-endosymbiosis/)

Vocabulary

• Eukaryote: An organism whose cells have a membrane-bound nucleus and specialized organelles; includes all plants, animals, and fungi.
• Nucleus: The control center of a eukaryotic cell; contains the cell's DNA and directs cell activity.
• Organelle: A specialized structure inside a eukaryotic cell that performs a specific function.
• Mitochondria: Organelles that produce energy for the cell; the 'powerhouse of the cell.' Once free-living bacteria.
• Endosymbiosis: The theory that some organelles were once free-living bacteria absorbed into larger cells, forming a partnership.
• DNA: The molecule that carries genetic instructions for building, operating, and reproducing all living things.

Discussion Questions

1. What makes a eukaryotic cell different from a simpler cell?
   Sample Answer: Eukaryotic cells have a nucleus, a special compartment that holds the cell's instructions, plus other specialized structures called organelles.
2. What does the nucleus do?
   Sample Answer: It acts like a command center, storing the cell's DNA and controlling how the cell grows and works.
3. Can you name a living thing made of eukaryotic cells?
   Sample Answer: Plants, animals, fungi, and humans are all made of eukaryotic cells.
4. What is endosymbiosis?
   Sample Answer: It's the idea that mitochondria (and chloroplasts) were once free-living bacteria that got absorbed by larger cells. They formed a partnership that benefited both and led to more complex cells.

Digging Deeper

1. Why is the development of the nucleus such a major step in evolution?
   Sample Answer: A nucleus protected DNA, allowing more complex genetic instructions. Without it, cells couldn't specialize or form multicellular organisms.