Imagine if you could absorb another creature and gain its abilities. What if you swallowed a small bird and suddenly could fly, or if you engulfed a cobra and could produce venom? In the history of life, especially during the evolution of complex cells known as eukaryotic cells, similar events actually happened. One organism would absorb another, and together they would become a new organism with combined powers.
Around 2 billion years ago, the only living things on Earth were prokaryotes. These are simple, single-celled organisms without complex structures inside them. Let’s look at three types of these early organisms. One was a large, simple cell that could absorb other materials by wrapping its membrane around them. Another was a bacterial cell that could turn sunlight into sugar through photosynthesis. The third type used oxygen to break down materials like sugar to release energy for life processes.
Sometimes, the large cells would absorb the photosynthetic bacteria. These bacteria would live inside the larger cell and reproduce, creating a partnership. If you saw this, you might think it was a single organism, with the green bacteria acting like a heart that pumps blood.
This process, where one organism lives inside another, is called endosymbiosis. But it didn’t stop there. If other bacteria joined in, these cells evolved into complex structures, developing parts called chloroplasts and mitochondria. These parts work together to capture sunlight, produce sugar, and break down that sugar using oxygen, which was starting to appear in Earth’s atmosphere.
This story is what scientists call the endosymbiotic theory, which explains how complex cells evolved. There is a lot of evidence supporting this theory, and here are three main pieces:
Chloroplasts and mitochondria multiply just like ancient bacteria. If they are destroyed in a cell, new ones can’t be made; they can only replicate themselves.
Chloroplasts and mitochondria have their own DNA and ribosomes. Their DNA is circular, like that of ancient bacteria, and shares many genes. The ribosomes in chloroplasts and mitochondria also resemble those of ancient bacteria, different from the rest of the eukaryotic cell.
Chloroplasts and mitochondria have two membranes: an inner and an outer one. The inner membrane has specific lipids and proteins not found in the outer membrane. The outer membrane came from the larger cell during endosymbiosis, while the inner one was retained from the absorbed organism. These membranes are similar to those of ancient bacteria.
Biologists use this theory to explain the origin of the diverse range of eukaryotic organisms. For example, green algae that grow on swimming pool walls were absorbed by a larger eukaryotic cell with flagella, leading to the formation of euglena. Euglena can perform photosynthesis, break down sugar using oxygen, and swim in pond water. As the theory predicts, the chloroplasts in euglena have three membranes, as they had two before being engulfed.
The endosymbiotic theory shows how organisms combined powerful abilities, making them better adapted to life on Earth. This led to an evolutionary leap, resulting in the microorganisms, plants, and animals we see today.
Imagine you are a large cell that has just absorbed a photosynthetic bacterium. Create a comic strip that illustrates this process of endosymbiosis. Show how the bacterium becomes a part of you and how you both benefit from this partnership. Use drawings and captions to tell the story of how complex cells evolved.
Using clay or other craft materials, build a 3D model of a eukaryotic cell. Include and label the mitochondria and chloroplasts. Explain to your classmates how these organelles are evidence of the endosymbiotic theory, highlighting their unique features such as their own DNA and double membranes.
Participate in a role-playing game where each student takes on the role of a different type of cell or organelle. Act out the process of endosymbiosis, with some students being the larger cells and others being the bacteria. Discuss how the relationships between these cells evolve over time and lead to the development of complex cells.
Engage in a classroom debate about the endosymbiotic theory. Divide into two groups: one supporting the theory with evidence and the other challenging it. Use the evidence provided in the article, such as reproduction, DNA, and membranes, to support your arguments. This will help you understand the scientific process and how theories are developed and tested.
Research a modern eukaryotic organism, such as euglena or green algae, that demonstrates the principles of endosymbiosis. Prepare a short presentation explaining how this organism supports the endosymbiotic theory. Highlight the features that show the integration of different cellular abilities and how this has contributed to the organism’s survival and adaptation.
Here’s a sanitized version of the provided YouTube transcript:
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What if you could absorb another organism and take on its abilities? Imagine if you swallowed a small bird and suddenly gained the ability to fly, or if you engulfed a cobra and were then able to produce venom. Throughout the history of life, particularly during the evolution of complex eukaryotic cells, such events occurred frequently. One organism absorbed another, and they united to become a new organism with combined abilities.
Around 2 billion years ago, the only living organisms on Earth were prokaryotes, which are single-celled organisms lacking membrane-bound organelles. Let’s examine three types of these organisms. One was a large, simple cell that could absorb other materials by wrapping its membrane around them. Another was a bacterial cell that converted solar energy into sugar molecules through photosynthesis. The third used oxygen to break down materials like sugar and release energy for life processes.
The large cells would occasionally absorb the photosynthetic bacteria. These bacteria would then live inside the larger cell and reproduce as they normally would, creating a linked existence. If you encountered this arrangement, you might perceive it as a single organism, with the green bacteria functioning similarly to how your heart pumps blood.
This process of one organism living inside another is known as endosymbiosis. However, the endosymbiosis didn’t stop there. If other bacteria moved in as well, the cells of this species began to evolve into highly complex structures, developing components known as chloroplasts and mitochondria. These structures work together to harness sunlight, produce sugar, and break down that sugar using the oxygen that began to appear in the Earth’s atmosphere.
The absorption of other organisms was one way species adapted to changing environmental conditions. This narrative illustrates what biologists refer to as the endosymbiotic theory, which is the leading explanation for the evolution of complex cells. There is substantial evidence supporting this theory, and we can highlight three main pieces.
First, chloroplasts and mitochondria multiply in the same manner as ancient bacteria. If these structures are destroyed in a cell, new ones cannot be formed; they can only replicate themselves.
Second, chloroplasts and mitochondria contain their own DNA and ribosomes. Their DNA has a circular structure similar to that of ancient bacteria and shares many genes. The ribosomes of chloroplasts and mitochondria also resemble those of ancient bacteria, differing from the ribosomes found in the rest of the eukaryotic cell.
Lastly, consider the membranes involved in the engulfing process. Chloroplasts and mitochondria have two membranes surrounding them: an inner and an outer membrane. The inner membrane contains specific lipids and proteins not found in the outer membrane. This is significant because the outer membrane originated from the larger cell. During the endosymbiosis process, these structures were enveloped in that membrane while retaining their own as the inner one. The same lipids and proteins are present in the membranes of ancient bacteria.
Biologists utilize this theory to explain the origin of the diverse range of eukaryotic organisms. For instance, green algae that grow on swimming pool walls were absorbed by a larger eukaryotic cell with flagella, leading to the formation of what we now call euglena. Euglena can perform photosynthesis, break down sugar using oxygen, and swim in pond water. As predicted by the theory, the chloroplasts in these euglena have three membranes, as they had two before being engulfed.
The endosymbiotic theory allowed organisms to combine powerful abilities, making them better adapted to life on Earth. This resulted in species capable of much more than when they were separate, leading to an evolutionary leap that produced the microorganisms, plants, and animals we observe on the planet today.
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This version maintains the core ideas while removing any potentially sensitive or inappropriate content.
Cells – The basic structural and functional units of all living organisms, often called the building blocks of life. – All living things are made up of cells, which carry out essential functions to sustain life.
Evolution – The process by which different kinds of living organisms develop and diversify from earlier forms during the history of the Earth. – The theory of evolution explains how species adapt to their environments over time.
Bacteria – Microscopic single-celled organisms that can be found in diverse environments, some of which can cause disease. – Bacteria play a crucial role in ecosystems by breaking down organic matter and recycling nutrients.
Chloroplasts – Organelles found in plant cells and some algae that conduct photosynthesis, converting light energy into chemical energy. – Chloroplasts contain chlorophyll, which gives plants their green color and helps capture sunlight.
Mitochondria – Organelles known as the powerhouses of the cell, responsible for producing energy through cellular respiration. – Mitochondria convert glucose into ATP, providing energy for cellular activities.
Endosymbiosis – A symbiotic relationship in which one organism lives inside the cells of another organism, often leading to mutual benefits. – The theory of endosymbiosis suggests that mitochondria and chloroplasts originated from ancient symbiotic bacteria.
Organisms – Individual living entities that can carry out life processes independently. – Organisms can be as simple as a single-celled bacterium or as complex as a human being.
Photosynthesis – The process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. – Photosynthesis is essential for life on Earth as it provides oxygen and organic compounds used by most organisms.
Energy – The capacity to do work, which in biological systems is often stored in molecules like ATP. – Plants capture energy from sunlight and store it in the form of glucose during photosynthesis.
DNA – Deoxyribonucleic acid, the molecule that carries the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms. – DNA contains the information necessary to build and maintain an organism’s cells and pass genetic traits to offspring.
