Anatomy Of Eukariotic Cells

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Centrioles

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Found only in animal cells, these paired organelles are found together near the nucleus, located at right angles to each other. Each centriole is made of nine bundles of micro-tubules (three per bundle) arranged in a ring. They have a role in building cilia and flagella, during which time they are referred to as basal bodies.Apparently they organize the micro-tubules in the mitotic spindles during mitosis and meiosis. These structures are self-replicating and make copies of themselves just before cell division begins. As the cell prepares to divide, the centrioles separate and move toward opposite poles of the cell. As they're moving apart, they radiate micro-tubules in a spindle-shaped formation that spans the cell from pole to pole. The spindle fibers act as guides for the alignment of the chromosomes as they separate.

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Cilia and Flagella

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Cilia and flagella are made up of micro-tubules, which are composed of linear polymers of globular proteins called tubulin. The core (axoneme) contains two central fibers that are surrounded by an outer ring of nine double fibers and covered by the cellular membrane.These motile appendages are constructed by basal bodies (kinetostomes), which also function as centrioles. The basal body is located at the base of each filament, anchoring it to the cell and controlling its movement. Cilia and flagella have the same structure. The only difference is that the flagella are longer.For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms. In multicellular organisms, cilia function to move fluid or materials past an immobile cell as well as moving a cell or group of cells. The respiratory tract in humans is lined with cilia that keep inhaled dust, smog, and potentially harmful microorganisms from entering the lungs. Flagella are found primarily on gametes.

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Endoplasmic Reticulum

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The endoplasmic reticulum (ER) is an network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. The ER is a continuous membrane with branching tubules and flattened sacs that extend throughout the cytoplasm. It is connected to the double-layered nuclear envelope, providing a connection between the nucleus and the cytoplasm. There are two kinds of ER, rough and smooth. Rough ER is covered with ribosomes, giving it a bumpy appearance when viewed through the microscope. This type of ER is involved mainly with the production of proteins that will be exported, or secreted, from the cell. The ribosomes assemble amino acids into units of proteins, which are transported into the rough ER for further processing. Once inside, the proteins are folded into the correct three-dimensional formation in order to become a useful. Chemicals, such as carbohydrates or sugars, are added, then the ER either transports the completed proteins to areas of the cell where they are needed, or they are sent to the Golgi apparatus for export. Smooth ER has a smoother appearance because it does not have ribosomes attached to it. This portion of the ER is involved with the production of lipids (fats), carbohydrate metabolism, and detoxification of drugs and poisons. Smooth ER is also involved with metabolizing calcium to mediate some cell activities. In muscle cells, smooth ER releases calcium to trigger muscle contractions. Cells specializing in lipid and carbohydrate metabolism (brain, muscle) or detoxification (liver) usually have more of this type of ER.

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Golgi Apparatus

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The Golgi apparatus (GA), also called Golgi body or Golgi complex, is a series of five to eight cup-shaped, membrane-covered sacs that look something like a stack of deflated balloons. The GA is the distribution and shipping department for the cell's chemical products. It modifies proteins and lipids (fats) that have been built in the endoplasmic reticulum and prepares them for export outside of the cell. The number of GAs in each cell varies according to its function, but animal cells generally contain between ten and twenty per cell. Proteins and lipids built in the smooth and rough endoplasmic reticulum bud off in tiny bubble-like vesicles that move through the cytoplasm until they reach the GA. The vesicles fuse with the GA membrane and release the molecules into the organelle. Once inside, the compounds are further processed by the GA, which adds molecules or chops tiny pieces off the ends. Once completed, the product is extruded from the GA in a vesicle and directed to its final destination inside or outside the cell. The exported products are secretions of proteins or glycoproteins that are part of the cell's function in the organism. Other products are returned to the endoplasmic reticulum or become lysosomes.

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Lysosomes

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The main function of these microbodies is digestion. Lysosomes break down cellular waste products and debris from outside the cell into simple compounds, which are transferred out into the cytoplasm as new cell-building materials. Like other microbodies, lysosomes are spherical organelles contained by a single layer membrane. This membrane protects the rest of the cell from the lysosomes' harsh digestive enzymes that would otherwise damage it. Lysosomes originate in the Golgi apparatus, but the digestive enzymes are manufactured in the rough endoplasmic reticulum. Lysosomes are found in all eukaryotic cells, but are most numerous in disease-fighting cells, such as white blood cells. Some human diseases are caused by lysosome enzyme disorders. Arthritis inflammation and pain are related to the escape of lysosome enzymes.

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Micro-filaments

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Micro-filaments are solid rods made of globular proteins called actin and are common to all eukaryotic cells. Long chains of the molecules are intertwined in a helix to form individual microfilaments. These filaments are primarily structural in function and are an important component of the cytoskeleton, along with micro-tubules. In association with myosin, micro-filaments help to generate the forces used in cellular contraction and basic cell movements. They enable a dividing cell to pinch off into two cells and are involved in amoeboid movements of certain types of cells. They also enable the contractions of muscle cells.

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Microtubules

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These straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells (prokaryotes don't have them) and perform a number of functions. Microtubules form part of the cytoskeleton that gives structure and shape to a cell, serve as conveyor belts moving other organelles through the cytoplasm, are the major components of cilia and flagella, and participate in the formation of spindle fibers during cell division (mitosis). Microtubules can function individually or join with other proteins to create larger structures (e.g. cilia). These filaments are composed of linear polymers of tubulin, which are globular proteins, and can increase or decease in length by adding or removing tubulin proteins.

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Mitochondria

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Mitochondria (singular, mitochondrion) are oblong shaped organelles that are found in the cytoplasm of every eukaryotic cell. They occur in varying numbers, depending on the cell and its function. These organelles are the power generators of the cell, converting oxygen and nutrients into ATP (adenosine triphosphate). ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities. This process is called aerobic respiration and is the reason animals breathe oxygen. The mitochondrion is different from other organelles because it has its own DNA and reproduces independently of the cell in which it is found; an apparent case of endosymbiosis. Scientists hypothesize that millions of years ago small, free-living prokaryotes were engulfed, but not consumed, by larger prokaryotes; perhaps because they were able to resist the digestive enzymes of the engulfing organism. The two organisms developed a symbiotic relationship over time, the larger organism providing the smaller with ample nutrients and the smaller organism providing ATP molecules to the larger one. Eventually, the larger organism developed into the eukaryotic cell, the smaller organism into the mitochondrion. Nonetheless, there are a number of prokaryotic traits that mitochondria continue to exhibit. Their DNA is circular, as it is in the prokaryotes, and their ribosomes and reproductive methods (binary fission) are more like those of the prokaryotes.
Mitochondrial DNA can be used study different aspects of inheritance.

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Nucleus

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The nucleus is a highly specialized organelle that serves as the information and administrative center of the cell. This organelle has two major functions. It stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include intermediary metabolism, growth, protein synthesis, and reproduction (cell division). Only the cells of advanced organisms, known as eukaryotes, have a nucleus. Simpler one-celled organisms (prokaryotes), like the bacteria and cyanobacteria, don't have a nucleus. The spherical nucleus occupies about 10 percent of a cell's volume, making it the cell's most prominent feature. Most of the nuclear material consists of chromatin, the unstructured form of the cell's DNA that will organize to form chromosomes during mitosis or cell division. Also inside the nucleus is the nucleolus, an organelle that synthesizes protein-producing macromolecular assemblies called ribosomes. A double-layered membrane, the nuclear envelope, separates contents of the nucleus from the cellular cytoplasm. The envelope is riddled with holes called nuclear pores that allow specific types and sizes of molecules to pass back and forth between the nucleus and the cytoplasm. It is also attached to a network of tubules, called the endoplasmic reticulum.
Chromatin/Chromosomes - Packed inside the nucleus of every human cell is nearly 6 feet of DNA, which is divided into 46 individual molecules, one for each chromosome and each about 1.5 inches long. Nucleolus - The nucleolus is a membrane-less organelle within the nucleus that manufactures ribosomes, the cell's protein-producing structures.
Nuclear Envelope - The nuclear envelope is a double-layered membrane that encloses the contents of the nucleus during most of the cell's lifecycle. The space between the layers is called the perinuclear space and appears to connect with the rough endoplasmic reticulum. The nuclear envelope is perforated with holes called nuclear pores. These pores regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others. Building blocks for building DNA and RNA are allowed into the nucleus as well as molecules that provide the energy for constructing genetic material.

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Peroxisomes

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Peroxisomes function to rid the cell of toxic substances, in particular, hydrogen peroxide -- a common byproduct of cellular metabolism. These organelles contain enzymes that convert the hydrogen peroxide to water, rendering the potentially toxic substance safe for release back into the cell. Some types of peroxisomes, such as those in liver cells, detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to molecules of oxygen.Peroxisomes are similar in appearance to lysosomes.

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Plasma Membrane

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All living cells, prokaryotic and eukaryotic, have a plasma membrane that encloses their contents. The membrane has two functions. First, it is a boundary holding the cell constituents together and keeping other substances out. Second, it is permeable, allowing nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated. The membrane is made of a two molecule thick layer (bilayer) of phospholipids, an oily substance found in all cells. This layer is embedded with many diverse proteins and has carbohydrates attached to its outer surface. The lipids in the membrane can exist either in a gel-like, nearly solid, state or in a liquid-like state, which gives the lipid molecules more mobility. In living cells, the membrane seems to be in a transition between the two states, depending on physical conditions and what lipids and proteins are present in the membrane layer.

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Ribosomes

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All living cells contain ribosomes, tiny organelles composed of approximately 60 percent RNA and 40 percent protein. In eukaryotes, ribosomes are made of four strands of RNA. In prokaryotes, they consist of three strands of RNA. Ribosomes are scattered throughout the cytoplasm and are the protein production sites for the cell. Some of the proteins are synthesized for the cell's own use, particularly in single-celled organisms. In multicellular organisms, many of the proteins produced by a specialized cell, e.g. antibodies, will be transported and used elsewhere in the organism. Eukaryote ribosomes are produced and assembled in the nucleolus. Three of the four strands are produced there, but one is produced outside the nucleolus and transported inside to complete the ribosome assembly. Ribosomal proteins enter the nucleolus and combine with the four strands to create the two subunits that will make up the completed ribosome. The ribosome units leave the nucleus through the nuclear pores and unite once in the cytoplasm. Some ribosomes will remain free-floating in the cytoplasm, creating proteins for the cell's use. Others will attach to the endoplasmic reticulum and produce the proteins that will be "exported" from the cell.
Protein synthesis requires the assistance of two other RNA molecules. Messenger RNA (mRNA) provides instructions from the cellular DNA for building a specific protein. Transfer RNA (tRNA) brings the protein building blocks, amino acids, to the ribosome. Once the protein backbone amino acids are polymerized, the ribosome releases the protein and it is transported to the Golgi apparatus. There, the proteins are completed and released inside or outside the cell.

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Cell Wall(Plant Only)

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One of the most important distinguishing features of plant cells is the presence of a cell wall, which serves a variety of functions.
The cell wall protects the cellular contents; gives rigidity to the plant structure; provides a porous medium for the circulation and distribution of water, minerals, and other small nutrient molecules; and contains specialized molecules that regulate growth and protect the plant from disease. A structure of great tensile strength, the cell wall is formed from fibrils of cellulose molecules, embedded in a water-saturated matrix of polysaccharides and structural glycoproteins. Many plant cells have both a primary cell wall, while the cell is growing, and a secondary cell wall, which is produced inside the primary wall after the cell has stopped growing. Conduits called plasmodesmata penetrate both the primary and secondary cell walls, providing pathways for transporting cytoplasmic molecules from one cell to another. The primary chemical component of cell walls is cellulose, which is made up of several thousand glucose molecules linked end to end. Other chemicals that make up cell walls are lignins, which add rigidity, and waxes, such as cutin and suberin, which reduce water loss from cells. Only a few proteins are found in cell walls, but they serve an important role in regulating the size of the cell. Enzymes initiate reactions that form the structural networks of molecules, help protect plants against fungal invasions by breaking fragments off of the cell walls of the fungi, and degrade the cell walls to induce fruit to ripen and leaves to fall in autumn.

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Vacuole (Plants Only)

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Vacuoles are membranous sacs consisting mostly of water containing various dissolved sugars, salts, proteins, and other nutrients. Each plant cell has a large, single vacuole that typically takes up most of the room in the cell.Some contain pigments that give certain flowers their colors. The central vacuole also contains plant wastes that taste bitter to certain insects and animals, thus discouraging them from consuming the plant.The plant vacuole stores compounds, helps in plant growth, and plays an important structural role for the plant. Under optimal conditions, the vacuoles are filled with water to the point that they exert a turgor pressure against the cell wall. This helps maintain the structural integrity of the plant, along with the support from the cell wall. When plant cells can't obtain the water they need, pressure in the vacuole is reduced and the plant wilts.