A small mass of protoplasm bounded externally by a semipermeable membrane, usually including one or more nuclei and various nonliving products, capable alone or interacting with other cells of performing all the fundamental functions of life, and forming the smallest structural unit of living matter capable of functioning independently.

General cell information:

Cell Membrane - Is responsible for the controlled entry and exit of molecules. "Gate Keeper"
Centriole - Grows into a spindle which is responsible for mitosis, for separating replicated chromosomes.
Cytoplasm - Controls cell metabolism including signal pathways, intercellular receptor. "Cell Jello"
Cilia - Projections from the cell, designed to move the cell or substances around the cell. "Tiny Hairs"
Cytoskeleton - Helps maintain the cell shape, motility and internal movement. "Framework"
ER (Rough) - Important to synthesis of proteins with help of ribosomes.
ER (Smooth) - Important in synthesis of lipid and membrane proteins.
Golgi Bodies - Important for packaging macromolecules for transport around the cell. "Packagers"
Lysosome - Intercellular digestion that also kills bacteria. "Suicide Sacs"
Mitochondria - Provides the energy a cell needs. "Powerhouse"
Nucleus - Is where DNA, which responsible for providing the cell with its unique characteristics, is located. "Brain"
Chromosomes - Is a structure of DNA that carries the genes and functions of the cell.
Nuclear Membrane - A structure that binds the nucleus within the cell.
Nucleolus - Produces ribosomes, moves the rough ER and is critical in protein synthesis.
Peroxisome - Responsible for protecting the cell from its own production of toxic peroxide.
Phagocytic Vesicle - The mouth of the cell that stores food.
Pinocytic Vesicle - Stores water for the cell.
Vacuole - Plays a role in intracellular digestion and the release of cellular waste products. "Storage"
Ribosome - Serves as the site of assembly for polypeptides encoded by RNA. "Protein Factory"

DETAILED ANIMAL CELL INFORMATION

Mitochondria

These organelles are the power houses of the cell and contain the molecular machinery for the conversion of energy. This conversion changes glucose into adenosine tri-phosphate (ATP), the energy of every cell. Mitochondria are mostly protein, but some lipid, DNA and RNA are present. These generally cylindrical organelles have an outer membrane surrounding an inner membrane that folds a facility for the transport of enzymes.

Ribosomes

Ribosomes are small organelles composed of ribosomal RNA and 80 some different proteins. RNA is synthesized in the nucleolus and the ribosomal subunits are constucted from RNA. Once assembled, the subunits pass through the nuclear pores to the cytoplasm where they take part in protein sysnthesis. Some ribosomes are free in the cytoplasm. Other ribosomes are attached to the endoplasmic reticulum (ER) where the protein is formed within the interior to the ER. These proteins are used for storage or incorporation into membranes.

Endoplasmic Reticulum

Within the cytoplasm of cells is a 3-dimensional maze of connecting and branching channels made by a continuous membrane. This is called the endoplasmic reticulum (ER). ER can be classified as rough ER, when ribosomes are attached to the cytosolic side of the membrane, or smooth ER when no ribosomes are present. Rough ER is prominent in cells that are making proteins for export such as digestive enzymes, hormones, structural proteins or antibodies. The main function of rough ER is the separation of proteins destined for export from the cell or for intracellular use. Phospholipid synthesis also occurs in the rough ER.

Smooth ER lacks attached ribosomes and often appears more tubular than rough ER. Smooth ER allows the cell a certain ability to perform a variety of specialized functions. It is necessary for steroid sysnthesis, metabolism and detoxification of substances in the liver to take place in the smooth ER.

Golgi complex

The Golgi is a curved membrane stack that concentrates and packages proteins for export or storage. It also adds directions for the destination of the protein package. Proteins made within the rough ER but are transported off in vesicles to the Golgi. There the vesicles fuse with the membrane and the components are further modified, concentrated and packaged. Therefore, the Golgi shows a polarity in that one side accepts incoming vesicles (convex) and the final product vesicles bud off the opposite side (concave]. In fact, scientists' studies have shown that the enzymes present within the Golgi are at different levels of the membrane stack.

Lysosomes

Lysosomes are membrane bound vesicles containing more than 40 enzymes that can digest most biological macromolecules. These organelles are the sites of intracellular digestion that are more numerous in cells performing phagocytosis. The limiting membrane keeps the digestive enzymes separate from the cytoplasm. The most common lysosomal enzymes are acid phosphatase, ribonuclease, and deoxyribonuclease. The enzymes function mostly at pH 5 and are inactive at the pH of the cytosol (7.2). This taken with the limiting membrane protects the cell from digesting itself. Lysosomal enzymes are synthesized on the rough ER, trasferred to the Golgi for modification and packaging. The cellular machinery attaches a directional signal to the enzymes that allows the ER and Golgi to sort these proteins and separate them to forming lysosomes.

Primary lysosomes are small concentrated sacs of enzymes that are not digesting anything. Primary lysosomes fuse with a phagocytic vacuole to become secondary lysosomes where digestion begins. As the substances are digested the nutrients diffuse through the lysosomal membrane to the cytosol. Residual bodies are formed when indigestable things remain in the vacuoles. In cells with a long life span such as cardiac muscle cells, residual bodies are more numerous.

Lysosomes also participate in the turnover of cellular organelles. Cytoplasmic components become enclosed in a membrane that fuses with a primary lysosome. In bones, the lysosomal enzymes are released from osteoclasts to digest surrounding bone during the process of remodeling. Lysosomal enzymes are also involved in the process of inflammation.

Nuclear Membrane

The nuclear membrane is vital in all cells. It surrounds the nucleus which is the brain of the cell. The nuclear membrane is composed of two layers and has numerous openings for nuclear traffic.

Chromosomes

Genes are the units of inheritance from parent to child. At a molecular level they hold the information necessary to direct the synthesis of cellular components. This is accomplished via the genetic code. Genes are units of deoxyribonucleic acid (DNA) contained within the chromosomes. DNA structure is that of a long, double-stranded helix. Each strand of the helix is made of repeating nucleotide bases of only four types: adenine, cytosine, guanine and thymine. It is the sequence of these nucleotides over the length of each individual chromosome added together that make up the human genome. The nucleotides are composed of a phosphate, a certain sugar, and one of the four bases.

Cytoplasm

Cytoplasm is the gel-like material inside the cell membrane and outside the nucleus. Cytoplasm contains a large amount of water and many chemicals and structures that carry out the life processes in the cell. These structures that the cytoplasm contains are called organelles. The cytoplasm is the substance contained within the plasma membrane excluding, in eukaryotes, the nucleus. The cytosol makes up about 50% of the cell volume. It contains numerous cytoskeletal elements, organelles, vesicles and metabolic enzymes. The cytoplasm coordinates the movement of intracellular organelles, and provides a framework for the organization of enzyme pathways. All of the necessities of protein synthesis are found within the cytosol and it contains numerous enzymes that build large molecules and break down small molecules.

Cell Membrane

Cell Membrane is the thin and nearly invisible structure that surrounds the cytoplasm of the cell. It also connects the the endoplasmic reticulum, and the nuclear membrane. Each eukaryotic cell has as its boundary to the outside a cell membrane. The cell or plasma membrane is a lipid bilayer containing proteins, cholesterol, and oligosaccharides that functions as a selective barrier for entry and exit of substances. The plasma membrane, by limiting the transport of some things and facilitating the movement of others, helps to maintain the internal environment of the cell, which is different from the fluid outside the cell. Proteins are a very important part of the cell membrane and can be classified into two groups based on physical distribution. Integral proteins are within the cell membrane and may pass multiple times through the membrane. Peripheral proteins are loosely associated with membrane surfaces. On the external surface of the cell membrane are important parts of receptor molecules. Receptors are necessary to cellular signalling, adhesion, and recognition. The cell membrane is not static. It is changed by the addition of new membrane vesicles from the Golgi while removal takes place in the form of vesicles being formed and then fused with lysozomes for processing. This membrane trafficking is important in the cell function.

Peroxisomes

These are small oragnelles containing oxidative enzymes. Peroxisomes contain amino oxidases, hydroxyacid oxidase and catalase. Catalase protects the cell from hydrogen peroxide damage. Enzymes involved in lipid metabolism are also found in peroxisomes. Peroxisomal enzymes are synthesized on the free cytosolic ribosomes with a signal sequence that directs them to peroxisomes. As enzymes are added the peroxisome grows and then splits into two smaller peroxisomes.

Cytoskeleton

Within the cell is a complex network of filaments, anchor proteins, and protein motors that form a support and transportation scaffolding. This network provides shape to the cell and participates in cellular movement.

Cilia and Flagella

Cilia and flagella have at their core a motile, highly organized, microtubular structure. Sperm cells have a flagellum. Ciliated cells, such as those lining the respiratory tree, normally have numerous cilia Both types of cellular processes contain the same core organization arrangement of microtubules within a cell membrane covering. Cilia and flagella are motile structures. Movement is accomplished by the sliding of adjacent doublets over one another by an energy requiring process. The dynein arms have an ATP base activity and an affinity for tubulin. These arms are thought to bind along the surface of the adjacent doublet.

Centrioles

These cylindric organelles participate in cell division as microtubule organizing centers. A single pair of centrioles oriented at a 90 degree angle to one another is found near the Golgi complex in non-dividing cells. Before cell division, each centriole replicates itself. During mitosis a pair of centrioles moves to opposite poles of the cell to become organizing centers for the mitotic spindle.

Nucleus

The nucleus is the major organelle of all cells, in which the chromosomes are separated from the cytoplasm by the nuclear membrane. The nucleus is a spherical, sometimes elongated structure that contains the genetic material, the DNA, of the cell. The highly organized chromatin within is enveloped by a nuclear membrane that contains numerous pores for the transport of large macromolecules and ribosomal subunits to the cytoplasm.

Nucleolus

This sperical structure within the nucleus is composed of DNA, RNA and protein. The DNA present is that which codes for RNA, known as nucleolar organizers. Proteins synthesized in the cytoplasm enter through the nuclear pores and become associated with the newly made RNA in the nucleolus. Afterwards the ribosomal subunits migrate to the cytoplasm. The nucleolus disappears during cell division but reappears in the final stage of mitosis.

Vacuole

The vacuole is a membrane - bound sacs for storage, digestion, and waste removal. The vacuole contains a large water solution. The vacuole is extremely large in plant cells but rather small in animal cells.

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