Membrane proteins and glycolipids mediate cell recognition events, which are seen in certain contexts such as immune responses.
The plasma membrane also mediates the flow of information to the cell in the form of messenger molecules such as hormones, a process known as signal transduction. Here, the binding of the signalling molecule to specific receptor proteins triggers off a series of enzymatic reactions in the plasma membrane, leading to an appropriate response by the cell.
The entire part of the cell inside the plasma membrane (excluding the nucleus in eukaryotes) constitutes the cytoplasm. The bulk of vital metabolic processes in the cell occurs within the cytoplasm. The cytoplasm of prokaryotes consists of a watery medium called the cytosol and suspended insoluble particles. In eukaryotes, the cytoplasm consists of the cytosol and various organelles. The cytosol contains dissolved ions, small molecules, and soluble macromolecules such as enzymes. The insoluble particles in the cytoplasm include the ribosomes, which form the protein-synthesizing machinery.
Ribosomes are "protein synthesizing machines" that associate with messenger RNA (mRNA) and catalyze the synthesis of proteins. Ribosomes can be considered to be the smallest cellular organelles. Each ribosome is composed of "several different ribosomal RNA (rRNA) molecules and more than 50 proteins, organized into a large subunit and a small subunit".
Ribosomes are present in three places in almost all eukaryotic cells: free in the cytoplasm, attached to the surface of the endoplasmic reticulum, and contained in the endoplasmic reticulum. In plant cells, ribosomes are also present in chloroplasts. When not active in the synthesis of proteins, the ribosomes exist as separate subunits.
The process of synthesizing a polypeptide using the information contained in an mRNA is called translation. A ribosome can take part in the translation of any mRNA to make different polypeptide products. Each individual amino acid is associated with a RNA molecule called the transfer RNA (tRNA). While the information required to synthesize the polypeptide is present in the mRNA and the constituent amino acids are borne by the tRNA, the ribosome is "simply the molecular machinery that accomplishes the task. Its structure enables it to hold the mRNA and the tRNAs in the right positions, thus allowing the growing polypeptide to be assembled efficiently.
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