Eukaryotic Cell Structure


Eukaryotic cells are usually larger and more complex than prokaryotic cells. They typically contain many internal structures and membranes, and often have very specialized roles. One of their major features is that their genetic material is contained within a membrane-bound nucleus. A very common way that is used to think about the structure and function of a eukaryotic cell is as a factory.
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Eukaryotic Cell - Electron Micrograph Image

Each organelle ("little organ") has a very specialized role within the eukaryotic cell, such as workers or machines have specific roles within a factory. The eukaryotic cell is typically split into two sections - the nucleus (the control-center, or the main office of a factory) and the cytoplasm that contains all of the organelles, which work together to run the cell, and therefore, run life. A cell must also have a boundary, which is the plasma cell membrane. Below is a break-down of the organelles typically found within a eukaryotic cell.



Nucleus

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Stained Nucleus - Electron Micrograph
The nucelus is the control-center of the cell. The nucleus contains almost all of a cell's DNA, which contains the codes and instructions for making proteins and other molecules. A double membrane, called the nuclear envelope, surrounds the nucleus. This membrane has thousands of pores, which allow for material to leave and enter the nucleus. Within the nucleus is the genetic material, DNA, which is bound to proteins in chromatin form. Usually this is spread throughout the nucleus, but will condense into structures called chromatids when the cell needs to divide.

Many nuceli also contain a small, dense structure called a nucleolus, which is where ribosomes start to get made.

Cytoplasm

The cytoplasm is the portion of the cell outside of the nucleus. Other organelles are located within the cytoplasm, as well as nutrients that the cell needs to use to run the different jobs within the cell.

Ribosomes

One of the most important jobs that happens within the cell is the making of proteins, which get assembled on ribosomes. Ribosomes are made up of particles of RNA and protein found throughout the cytoplasm. Ribosomes make proteins, a process called translation, using the instructions from the DNA within the nucleus.
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Close-Up of a Ribosome

Endoplasmic Reticulum

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Endoplasmic Reticulum
The endoplasmic reticulum, or ER, is an internal membrane system. This is where the lipid components of the cell membrane are assembled, as well as proteins and any materials that get exported from the cell.

The part of the ER where proteins get synthesized is the "rough ER," so called because ribosomes are located on its surface. Newly-made proteins leave the ribosomes and enter the rough ER, where they may be chemically modified. The proteins that are exported from the cell get synthesized here, while others are made on free ribosomes.

The other portion of the ER, known as the "smooth ER" due to its lack of ribosomes, may contain enzymes that have specialized roles, such as making membrane lipids or detoxifying drugs. Therefore, liver cells usually have large amounts of smooth ER.

Golgi Apparatus

When proteins get made in the rough ER, they then move next into the Golgi Apparatus.
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ER and Golgi Apparatus
This was discovered by the Italian scientist Camillo Golgi. The Golgi's role is to modify, sort out, and "package" proteins, as well as other materials from the ER, to either be stored in the cell, or be secreted outside of the cell. It makes any necessary final changes to proteins before they are sent off to their final destination.

Lysosomes

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Lysosomes
The cell needs some way to get rid of any unnecessary material or waste. This is the role of a lysosome, a small organelle that is filled with enzymes. These break down lipids, carbohydrates, and proteins into small molecules which can then be used by the cell if needed. The other major role that these small organelles play is the breakdown of organelles that are past their prime, and need to be replaced by new ones.


Vacuoles

Vacuoles play the storage role for the cell, storing things such as water, salts, proteins, and carbohydrates. In plant cells, there is often a central vacuole that is filled with liquid, with provides the pressure needed to support leaves and flowers. In single-celled organisms, a vacuole may have the ability the contract, which pumps out excess water.

Mitochondria

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Mitochondrion

Mitochondria have a very important role in the cell - they convert the energy in food into compounds that can then be used by the cell. Mitochondria have a unique structure that consists of a highly-folded double membrane. The citric acid cycle (Kreb's cycle) occurs in the mitochondria, as does the electron transport chain, which are both two very important steps of cellular respiration. It is believed that mitochondria may have evolved from a unicellular prokaryote that took on an endosymbiotic role in an eukaryote.
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Mitochondrial Structure


Mitochondria actually have their own genome, which supports the theory that they may have been a prokaryote. Interestingly, all mitochondrial DNA gets inherited from the mother, through the cytoplasm in the ovum. It is in the form of small DNA molecules.


Cytoskeleton


The cell needs a supporting structure, as well as a way of transporting molecules from one organelle or section of the cell to another. The cytoskeleton is made up of protein filaments, which help the cell to maintain its shape, as well as allow for movement. Two principal filaments, microfilaments and microtubules, make up the cytoskeleton. The microfilaments are made up of actin, and provide a flexible, yet sturdy structure. They can also promote movement by assembling and disassembling, such as in the movement of an amoeba - this is where the term pseudopod comes from, or "false foot."


Microtubules are made up of tubulin proteins, which help keep a cell's shape. In cell division, they form the mitotic spindle, which separate chromosomes, and in animal cells, they also make centrioles.

Plant Cells

Plant cells can vary from animal cells, and have some of their own unique organelles, such as a chloroplast, water vacuole, and cell wall.

Animated look at the inner workings of the cell and membrane structure (Musical and Narrated Versions):


Types of Eukaryotes


Some eukaryotes are actually single-celled organisms, such as a protists. Others form large, multi-cellular organisms. Animals, plants, and fungi are also eukaryotes.