Definition
A eukaryotic cell contains membrane-bound organelles such as a nucleus, mitochondria, and an endoplasmic reticulum. Organisms based on the eukaryotic cell include protozoa, fungi, plants, and animals. These organisms are grouped into the biological domain Eukaryota. Eukaryotic cells are larger and more complex than prokaryotic cells found in domains Archaea and Bacteria.
Overview
A eukaryotic cell is one of two different types of cells. Organisms that are based on the eukaryotic cell are called “eukaryotes” and include plants, animals, fungi, and protists. The only organisms that are not based on the eukaryotic cell are organisms based on a prokaryotic cell structure. Those organisms are found in the domains Archaea and Bacteria. There are several differences between a eukaryotic cell and a prokaryotic cell that can help you fully understand what makes a cell eukaryotic.
Eukaryotic Cell vs Prokaryotic Cell
The difference between a eukaryotic cell and a prokaryotic cell is simple: eukaryotic cells have membrane-bound organelles. Within a prokaryotic cell (such as a bacteria) the DNA simply floats around the cytoplasm. While prokaryotic cells do have one type of organelle (ribosomes), these organelles are not covered in a plasma membrane.
By contrast, eukaryotic cells are full of membrane-bound organelles that divide the cell into many different compartments. The nucleus houses the DNA. The endoplasmic reticulum creates many chambers to carry out specific biochemical reactions. The Golgi apparatus folds and packages various proteins and cellular products. Lysosomes store digestive enzymes to break down incoming food. Plus, eukaryotic cells contain mitochondria to create ATP molecules from glucose and chloroplasts to create glucose from sunlight (only in plants and algae).
Characteristics of a Eukaryotic Cell
Eukaryotic cells contain a variety of organelles, which perform various functions within the cell (described in detail, below). All of the organelles are stabilized and given physical support through the cytoskeleton, which is also involved in sending signals from one part of the cell to the other. In eukaryotic cells, the cytoskeleton is composed mainly of three types of filaments: microtubules, microfilaments, and intermediate filaments. The watery solution that surrounds all the organelles in the cell is called cytosol.
The figure below shows the structure of a eukaryotic cell. This is an animal cell. The nucleus and other organelles are shown. The cytosol is the blue substance surrounding all of the organelles. Together, the cytosol with all organelles besides the nucleus is known as the cytoplasm.
Eukaryotic Cell Cycle
The cell cycle is the life cycle of a cell. During this cycle, it grows and divides. Checkpoints exist between all stages so that proteins can determine whether the cell is ready to begin the next phase of the cycle.
Quiescence (G0)
Quiescence, also known as senescence or resting, is a phase in which the cell is not actively dividing. It is also known as Gap 0, or G0. This stage is considered the start of the cell cycle, although it is one that cells can reach and then stop dividing indefinitely, which ends the cell cycle. Cells in the liver, stomach, kidney, and nervous system are all examples of cells that can reach this stage and remain in it for long periods of time. It can also occur when a cell’s DNA is damaged. However, most cells do not go into the G0 stage at all and can divide indefinitely throughout the life of an organism.
Interphase
During interphase, the cell grows and takes in nutrients in preparation for division. Interphase takes up about 90 percent of the cell cycle. It consists of three parts: Gap 1, Synthesis, and Gap 2.
- Gap 1 (G1) is also known as a growth phase. The cell gets larger and increases its stock of proteins, along with organelles such as the energy-producing mitochondria.
- Synthesis (S) is the phase in which DNA replicates. During synthesis, the chromosomes replicate so that each chromosome is made up of two sister chromatids. At the end of this phase, there is double the amount of DNA in the cell.
- Gap 2 (G2) is another growth phase. The cell becomes even larger in order to prepare for mitotic division, and the remaining organelles are duplicated.
Mitosis (M)
Mitosis, or M phase, is when the cell begins to organize its duplicated DNA for separation into two daughter cells. The chromosomes separate so that one of each chromosome goes into each daughter cell. This results in the daughter cells having identical chromosomes to the parent cell. Mitosis itself is divided into prophase, metaphase, anaphase, and telophase. Each phase marks various points in the DNA separation process. Mitosis is then followed by a process called cytokinesis, during which the cell separates its nuclei and other organelles in preparation for division and then physically divides into two cells.
Examples of a Eukaryotic Cell
Plant Cells
Plant cells are unique among eukaryotic cells for several reasons. They have reinforced, relatively thick cell walls made of cellulose that help maintain structural support in the plant. Each plant cell has a large vacuole in the center that allows it to maintain turgor pressure. Turgor pressure results from the water in a central vacuole pushing outward on the cell walls. Plant cells also contain organelles called chloroplasts which contain the molecule chlorophyll. This important molecule is used in the process of photosynthesis, which is how plants make sugar by using the energy found in light.
Fungal Cells
Like plant cells, fungal cells also have a cell wall, but their cell wall is made of chitin (the same substance found in insect exoskeletons). Some fungi have septa, which are holes that allow organelles and cytoplasm to pass between them. This makes the boundaries between different cells less clear. Most fungi live underground or in decaying organic matter, where the mycelial network can contain millions of interconnected cells.
Animal Cells
Animal cells do not have cell walls. Instead, they have only a plasma membrane. The lack of a cell wall allows animal cells to form many different shapes. This allows for the processes of phagocytosis (“cell eating”) and pinocytosis (“cell drinking”) to occur. Animal cells differ from plant cells in that they do not have chloroplasts and have many smaller vacuoles instead of a large central vacuole.
Protozoa
Protozoa are eukaryotic organisms that consist of a single cell. They can move around, eat other small organisms, and digest food within vacuoles. Some protozoa have many cilia, which are small, moveable hairs that allow them to swim around. Others use large flagella – structures that look something like a large tail – to swim through the water. Some protists also have a thin layer called a pellicle, which provides support to the cell membrane.
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