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What are ribosomes? Ribosomes are responsible for assembling the Proteins of the Cell. Ribosomal Subunits are synthesized by the Nucleolus. Depending on the Protein Production level of a Particular Cell, Ribosomes may Number in the millions.

Ribosome attached to Endoplasmic reticulum, Image courtesy of The Virtual Cell.

What are their distinguishing characteristics? 

Ribosomes are typically composed of Two subunits: a Large Subunit and a Small Subunit. These Two Units Join together when the Ribosome Attaches to Messenger rna to Produce a Protein in the Cytoplasm (cyto-).

Small and Large Ribosome Subunits, Image courtesy of The Virtual Cell. There are Two Places that Ribosomes usually Exist in the cell: suspended in the Cytosol and Bound to the Endoplasmic reticulum. These Ribosomes are called Free Ribosomes and Bound Ribosomes respectively. In both Cases, the Ribosomes usually Form Aggregates called polysomes (poly-) (Also known as polyribosomes).

Liver Cell Ribosomes and polyribosomes, Copyright Dennis Kunkel. Free Ribosomes usually Make Proteins that will Function in the Cytosol While Bound Ribosomes usually Make Proteins that are exported or Included in the Cells Membranes. Interestingly Enough, Free Ribosomes and Bound Ribosomes are interchangeable and the Cell can Change their Numbers according to Metabolic Needs.

Ribosomes - Protein CONSTRUCTION TEAMS Cells Need to Make Proteins. Those Proteins Might be used as Enzymes or as Support for other Cell Functions. When you Need to Make Proteins, you Look for Ribosomes. Ribosomes are the Protein builders or the Protein synthesizers of the Cell. They are Like construction guys who Take one Amino acid at a Time and Build Long Protein Chains.

Ribosomes could be in Many Places around the Cell. You Might Find them Floating in the Cytoplasm. Those Floating Ribosomes Make Proteins that will be used Inside of the Cell. Other Ribosomes are found on the Endoplasmic reticulum. Endoplasmic reticulum with Ribosomes attached is called Rough. It Looks bumpy Under a Microscope. Those attached Ribosomes Make Proteins that will be used Inside the Cell and Proteins made for Export (Outside the Cell).

Two Pieces A Ribosome is not Just one Piece. There are Two Pieces or Subunits. Scientists named them 60-S and 30-S. When the Cell Needs to Make Protein, Mrna is created in the Nucleus. The Mrna is then sent Into the Cell to the Ribosomes. When it is Time to Make the Protein, the Two Subunits come together and combine with the Mrna. The Two Pieces Lock onto the Mrna and Start the Protein synthesis.

Mixing AND Matching Amino acids

The Process of making Proteins is quite Simple. We Just explained that Mrna is made in the Nucleus and sent Into the Cell. The Mrna combines with the Ribosome Subunits. Another Nucleic acid Lives in the Cell - Trna, which Stands for Transfer rna, and it is bonded to Amino acids. With the Mrna offering Instructions, the Ribosome connects to a Trna and Pulls Off the Amino acids. Slowly the Ribosome Makes a Long Amino acid Chain that Becomes a Protein.

· One or more per Cell · Spherical Shape · Denser than surrounding Cytoplasm

	Chromosomes - Usually in the Form of chromatin- Contains Genetic information- Composed of Dna- Thicken for Cellular division- Set Number per Species (i.e. 23 Pairs for human)
	Nuclear membrane - Surrounds Nucleus- Composed of Two Layers- Numerous Openings for Nuclear Traffic
	Nucleolus - Spherical Shape- Visible when Cell is not dividing- Contains Rna for Protein manufacture

	·	Collective Term for Cytosol and Organelles contained Within ·	Colloidal Suspension ·	Cytosol mainly composed of Water with Free-Floating Molecules ·	Viscosity constantly Changes

Centrioles - Paired Cylindrical Organelles Near Nucleus- Composed of Nine Tubes, each with Three tubules- Involved in Cellular division- Lie at Right Angles to each other Chloroplasts - A plastid usually found in Plant Cells- Contain Green Chlorophyll where Photosynthesis Takes Place Cytoskeleton - Composed of microtubules- Supports Cell and Provides Shape- Aids Movement of Materials in and out of Cells Endoplasmic reticulum - Tubular Network fused to Nuclear membrane- Goes Through Cytoplasm onto Cell membrane- Stores, Separates, and Serves as Cells Transport system- Smooth type: Lacks Ribosomes- Rough Type (pictured): Ribosomes embedded in Surface Golgi apparatus - Protein 'packaging Plant- A Membrane Structure found Near Nucleus- Composed of numerous Layers forming a Sac Lysosome - Digestive 'plant' for Proteins, Lipids, and Carbohydrates- Transports undigested Material to Cell membrane for removal- Vary in Shape depending on Process Being carried out- Cell Breaks Down if Lysosome explodes Mitochondria - Second largest Organelle with unique Genetic Structure- Double-layered Outer membrane with Inner Folds called cristae- Energy-producing Chemical reactions Take Place on cristae- Controls level of Water and other Materials in Cell- Recycles and Decomposes Proteins, Fats, and Carbohydrates, and Forms Urea Ribosomes - Each Cell contains thousands- Miniature 'protein factories'- Composes 25% of Cells mass- Stationary type: embedded in Rough Endoplasmic reticulum- Mobile type: injects Proteins Directly Into Cytoplasm Vacuoles - Membrane-Bound Sacs for Storage, Digestion, and Waste removal- Contains Water solution- Contractile Vacuoles for Water removal (in Unicellular organisms)

	 	Cell wall - Most commonly found in Plant Cells- Controls turgity- Extracellular Structure surrounding Plasma membrane- Primary Cell wall: extremely elastic- Secondary Cell wall: Forms around Primary cell wall after Growth is complete

Plasma membrane - Outer membrane of Cell that Controls Cellular Traffic- Contains Proteins (Left, gray) that Span Through the Membrane and allow Passage of Materials- Proteins are surrounded by a Phospholipid bi-layer.

All living Cells contain Ribosomes, tiny Organelles composed of approximately 60 percent Ribosomal rna (Rrna) and 40 percent Protein. However, though they are generally described as Organelles, it is important to Note that Ribosomes are not Bound by a Membrane and are much smaller than other Organelles. Some Cell Types may Hold a few million Ribosomes, But several thousand is more typical. The Organelles Require the Use of an Electron microscope to be visually detected.

Ribosomes are mainly found Bound to the Endoplasmic reticulum and the Nuclear envelope, as Well as Freely Scattered throughout the Cytoplasm, depending Upon whether the Cell is Plant, Animal, or Bacteria. The Organelles Serve as the Protein Production machinery for the Cell and are consequently most Abundant in Cells that are Active in Protein synthesis, such as Pancreas and Brain Cells. Some of the Proteins synthesized by Ribosomes are for the Cells Own Internal Use, especially those that are produced by Free Ribosomes. Many of the Proteins produced by Bound Ribosomes, However, are transported Outside of the Cell. In Eukaryotes, the Rrna in Ribosomes is organized Into four Strands, and in Prokaryotes, Three Strands. Eukaryote Ribosomes are produced and assembled in the Nucleolus. Ribosomal Proteins Enter the Nucleolus and combine with the four Rrna Strands to create the Two ribosomal Subunits (one Small and one Large) that will Make Up the completed Ribosome (See Figure 1). The Ribosome Units Leave the Nucleus Through the Nuclear Pores and unite Once in the Cytoplasm for the purpose of Protein synthesis. When Protein Production is not Being carried out, the Two Subunits of a Ribosome are separated. In 2000, the complete Three-dimensional Structure of the Large and Small Subunits of a Ribosome was established. Evidence based on this Structure suggests, as had Long been assumed, that it is the Rrna that Provides the Ribosome with its Basic Formation and functionality, not Proteins. Apparently the Proteins in a Ribosome Help Fill in Structural Gaps and enhance Protein synthesis, although the Process can Take Place in their Absence, albeit at a much slower Rate. The Units of a Ribosome are often described by their Svedberg (s) Values, which are based Upon their Rate of Sedimentation in a centrifuge. The Ribosomes in a Eukaryotic Cell generally have a Svedberg Value of 80S and are comprised of 40s and 60s Subunits. Prokaryotic cells, on the other Hand, contain 70S Ribosomes, each of which consists of a 30s and a 50s Subunit. As demonstrated by these Values, Svedberg Units are not Additive, so the Values of the Two Subunits of a Ribosome do not add Up to the Svedberg Value of the Entire Organelle. This is because the Rate of Sedimentation of a Molecule depends Upon its Size and Shape, rather than simply its Molecular weight. Protein synthesis Requires the Assistance of Two other kinds of Rna Molecules in Addition to Rrna. Messenger rna (Mrna) Provides the Template of Instructions from the Cellular Dna for building a Specific Protein. Transfer rna (Trna) Brings the Protein building Blocks, Amino acids, to the Ribosome. There are Three Adjacent Trna Binding sites on a ribosome: the aminoacyl Binding Site for a Trna Molecule attached to the Next Amino acid in the Protein (as illustrated in Figure 1), the peptidyl Binding Site for the central Trna Molecule containing the Growing Peptide Chain, and an Exit Binding Site to Discharge used Trna Molecules from the Ribosome. Once the Protein Backbone Amino acids are polymerized, the Ribosome Releases the Protein and it is transported to the Cytoplasm in Prokaryotes or to the Golgi apparatus in Eukaryotes. There, the Proteins are completed and released Inside or Outside the Cell. Ribosomes are very Efficient Organelles. A Single Ribosome in a Eukaryotic Cell can add 2 Amino acids to a Protein Chain Every Second. In Prokaryotes, Ribosomes can Work Even faster, adding about 20 Amino acids to a Polypeptide Every Second. In Addition to the most familiar Cellular locations of Ribosomes, the Organelles can Also be found Inside Mitochondria and the Chloroplasts of Plants. These Ribosomes notably Differ in Size and makeup than other Ribosomes found in Eukaryotic cells, and are more Akin to those Present in Bacteria and Blue-green algae Cells. The similarity of Mitochondrial and Chloroplast Ribosomes to Prokaryotic Ribosomes is generally considered Strong supportive evidence that Mitochondria and Chloroplasts evolved from Ancestral Prokaryotes.