Many living things are made of special cells.
These cells have a tiny center. This center holds important plans for life. Plants and animals both have them. Even tiny things in water have them.
Can you find these cells? They are all around us!
Many living things have special cells.
These cells have a tiny center. This center holds the plans for life.
Plants and animals both have these cells.
Some cells are very small. Other cells are as big as a whale!
These cells can be all alone. Some cells live together in big groups. They are all around us!
Most living things on Earth are eukaryotes.
A eukaryote is a living thing with special cells. These cells have a nucleus. The nucleus is a tiny center that holds DNA. DNA is like a set of plans for life.
Eukaryotes are very different from bacteria. Eukaryote cells are much larger. They can be 10,000 times bigger than bacteria! Some are tiny single cells. Others are huge, like a blue whale. Plants, animals, and fungi are all eukaryotes.
These cells have many small parts called organelles. One part is the mitochondrion. It acts like a power plant. It makes power for the cell.
Plants also have chloroplasts. These parts use light to make food. This way of making food is called photosynthesis.
Eukaryotes can also make new life in two ways. They can use mitosis to make copies. They can also use sex. This is called meiosis. This helps make life more varied.
Eukaryotes are a huge group of living things. They include animals, plants, fungi, and seaweeds.
Some eukaryotes are just one tiny cell. Others are very large, like the blue whale. These whales can weigh up to 190 tonnes. Even though there are fewer eukaryotes than bacteria, they are much bigger. This means they make up most of the Earth's biomass. Plants alone make up over 81% of all life on Earth.
What makes a eukaryote special is its cell. Every eukaryotic cell has a nucleus. The nucleus is a center that holds DNA. DNA is kept in bundles called chromosomes. The nucleus is wrapped in a double layer called a nuclear envelope. This envelope has tiny holes called nuclear pores. These pores let materials move in and out of the center.
Inside the cell, there are many small parts called organelles. These parts work together in a system. One part is the endoplasmic reticulum. It helps move and change proteins. Another part is the Golgi apparatus. This part acts like a station to modify cell products. Many cells also have a cytoskeleton. This is a network of tiny structures that give the cell its shape. It also helps move things around inside the cell.
Scientists think these cells grew through a process called symbiogenesis. This is when different tiny living things live together. Long ago, an ancient cell swallowed a bacterium. That bacterium became the mitochondrion. Mitochondria are the powerhouses of the cell. They turn sugars or fats into energy called ATP. Later, some cells swallowed cyanobacteria. These became chloroplasts. Chloroplasts allow plants to make food from light.
Eukaryotes can make new life in two different ways. They can use mitosis to make exact copies of themselves. They can also use sexual reproduction. This involves meiosis, which is a special way to divide cells. In this process, two cells called gametes fuse together. This fusion creates a new zygote. This cycle helps keep life varied and interesting.
Eukaryotes are a massive and diverse group of organisms belonging to the domain Eukaryota. This group includes everything from microscopic single cells to massive animals and plants. They are distinguished from prokaryotes, such as Bacteria and Archaea, by a specific cellular structure. The defining feature of a eukaryote is a membrane-bound nucleus. This nucleus acts as a protected center that stores the cell's DNA. Within the nucleus, DNA is organized into linear bundles known as chromosomes.
The internal workings of a eukaryotic cell are highly organized through an endomembrane system. This system consists of various membrane-bound structures called organelles. The nucleus is wrapped in a double membrane called the nuclear envelope. This envelope contains nuclear pores that regulate the movement of materials. Nearby, the endoplasmic reticulum serves as a network for protein transport and maturation. It includes the rough endoplasmic reticulum, which is covered in ribosomes that synthesize proteins. These proteins then travel to the Golgi apparatus, a stack of flattened vesicles that modifies and prepares cell products for delivery.
Two specific types of organelles illustrate how complex these cells can be. Mitochondria are often called the "powerhouse of the cell" because they provide energy. They do this by oxidizing sugars or fats to produce a molecule called ATP. Mitochondria have two membranes, and their inner membrane is folded into structures called cristae. Another important organelle is the plastid, such as the chloroplast found in plants. Chloroplasts contain chlorophyll and use photosynthesis to produce organic compounds like glucose.
Scientists believe these complex organelles originated through a process called symbiogenesis. This theory suggests that different organisms lived together in a way that became permanent. It is thought that an anaerobic Promethearchaeota archaeon merged with an aerobic proteobacterium. This merger resulted in the first mitochondria. Later, a second episode of symbiogenesis occurred when a cyanobacterium was taken in. This event led to the creation of plants with chloroplasts.
Eukaryotic cells also rely on a cytoskeleton to maintain their shape and organization. The cytoskeleton is a dynamic network of protein filaments and tubules. It provides structural support and acts as a track for moving materials. Microfilaments made of actin allow cells to change shape or move. Motor proteins, such as dynein and kinesin, work along microtubules to transport cargo. Some eukaryotes also use long projections called flagella or shorter structures called cilia for movement. These structures are made of tubulin and are quite different from the flagella found in prokaryotes.
Eukaryotes exhibit incredible variety in size and biological organization. They can be unicellular, such as the tiny picozoans that are under 3 micrometers across. They can also be multicellular, like the blue whale, which can weigh up to 190 tonnes. While eukaryotes are a minority in terms of the total number of organisms, they dominate the Earth's biomass. The total global biomass of eukaryotes is 468 gigatons, compared to only 77 gigatons for prokaryotes. Within this total, plants alone account for more than 81% of Earth's biomass.
Reproduction in eukaryotes adds another layer of complexity. They can reproduce asexually through mitosis, which creates identical copies. They can also reproduce sexually through a process involving meiosis. During meiosis, cells produce haploid gametes, which contain only one copy of each chromosome. When two gametes fuse during fertilization, they form a diploid zygote with two sets of chromosomes. This sexual cycle allows life to alternate between haploid and diploid phases. This process is thought to be a primordial characteristic of the entire eukaryotic lineage.
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