#AP Biology: Endosymbiotic Theory - Your Night-Before Guide

Hey there, future AP Bio superstar! Let's break down the endosymbiotic theory, a key concept that ties together evolution, cell structure, and a bit of cellular history. This guide is designed to be your quick, easy-to-use resource for test day. Let's get started!

#The Basics: Prokaryotic vs. Eukaryotic Cells

#Prokaryotic Cells

• Simple Structure: Think of these as the OG cells. They're small and lack membrane-bound organelles, like a nucleus.
• Circular DNA: Their DNA is a single, circular loop floating in the cytoplasm. 🧬
• Ancient Lineage: These guys have been around for about 4 billion years, making them the earliest forms of life on Earth.

#Eukaryotic Cells

• Complex Structure: These are the more advanced cells with membrane-bound organelles (nucleus, mitochondria, etc.).
• Linear DNA: Their DNA is organized into multiple linear chromosomes within the nucleus.
• Relatively Recent: Eukaryotes appeared about 1.8 billion years ago, evolving from prokaryotic ancestors.

#Endosymbiotic Theory: A Cellular Merger

#What is it?

The endosymbiotic theory explains how eukaryotic cells acquired their membrane-bound organelles like mitochondria and chloroplasts. It proposes that:

1. An early eukaryotic ancestor engulfed a prokaryotic cell (likely an aerobic bacterium).
2. Instead of being digested, the prokaryote became an endosymbiont (a cell living inside another cell).
3. Over time, this symbiotic relationship became permanent, with the endosymbiont evolving into an organelle.

#The Selective Advantage

• Energy Boost: The engulfed prokaryotes were capable of producing extra energy (ATP) through cellular respiration or photosynthesis, giving the host cell a major advantage.
• Survival and Reproduction: Cells with these endosymbionts were more successful at surviving and reproducing, leading to the evolution of modern eukaryotic cells.

#Evidence for Endosymbiosis

#Key Discoveries

1. Organelle Division: In 1883, Andreas Schimper noticed that plastids divide similarly to bacteria.
2. Organelle DNA: Mitochondria and plastids have their own DNA, which is circular and more similar to prokaryotic DNA than eukaryotic DNA.
3. Double Membrane: Mitochondria and chloroplasts have a double membrane. The inner membrane is from the original prokaryote, and the outer membrane is from the host cell's engulfing process.

#Visualizing the Process

• Engulfment: A larger cell engulfs a smaller prokaryotic cell.
• Endosymbiosis: The smaller cell lives inside the larger one.
• Evolution: Over time, the smaller cell evolves into an organelle.

• Mitochondria: Evolved from aerobic bacteria.
• Chloroplasts: Evolved from photosynthetic bacteria (like cyanobacteria).

#Final Exam Focus

#High-Priority Topics

• Prokaryotic vs. Eukaryotic Cells: Know the key differences in structure and DNA organization.
• Endosymbiotic Theory: Understand the steps, evidence, and significance of this theory.
• Mitochondria and Chloroplasts: Focus on their double membrane, DNA, and role in energy production.

#Common Question Types

• Multiple Choice: Expect questions about the evidence for endosymbiosis, the differences between prokaryotes and eukaryotes, and the function of mitochondria and chloroplasts.
• Free Response: Be prepared to explain the endosymbiotic theory, analyze data supporting it, or compare and contrast prokaryotic and eukaryotic cells.

#Last-Minute Tips

• Time Management: Quickly scan questions and prioritize those you're confident about.
• Common Pitfalls: Don't confuse the endosymbiotic theory with the origin of the nucleus. Focus on mitochondria and chloroplasts.
• Strategies: Use diagrams and mnemonics to remember key concepts. Practice explaining the theory to yourself or a friend.

#

Practice Question

Practice Questions

#Multiple Choice

1. Which of the following pieces of evidence supports the endosymbiotic theory? a) The presence of a nucleus in eukaryotic cells b) The presence of linear DNA in mitochondria c) The double membrane surrounding mitochondria and chloroplasts d) The lack of ribosomes in prokaryotic cells

2. According to the endosymbiotic theory, which of the following is the most likely ancestor of mitochondria? a) An archaea b) A photosynthetic bacterium c) An aerobic bacterium d) A virus

#Free Response Question

Question:

Explain the endosymbiotic theory and provide three pieces of evidence that support it. In your explanation, be sure to describe the origin of mitochondria and chloroplasts within eukaryotic cells. How does the theory explain the double membrane structure of these organelles?

Scoring Breakdown:

• (1 point): Explanation of the endosymbiotic theory (engulfment of a prokaryote by a eukaryote).
• (1 point): Identification of the origin of mitochondria (aerobic bacteria).
• (1 point): Identification of the origin of chloroplasts (photosynthetic bacteria).
• (1 point): Evidence 1: Organelle division similar to bacteria.
• (1 point): Evidence 2: Presence of circular DNA in organelles.
• (1 point): Evidence 3: Double membrane structure of organelles.
• (1 point): Explanation of the double membrane (inner membrane from the prokaryote, outer from the engulfing cell).