organelles

organelles

specialized intracellular structures; e.g. mitochondria, Golgi apparatus, centrioles, granular and agranular endoplasmic reticulum, vacuoles, microsomes, lysosomes, plasma membrane and fibrils (Table 1)

Table 1: Cell organelles

Structure	Characteristic
Cell membrane	Bimolecular lipid and protein membrane that surrounds the cell • Bi-layer of non-polar lipid molecules (phosphatidylcholine, phospharidylethanolamine, cholesterol) that acts as a barrier to water and hydrophilic solutes • Proteins: Structural proteins Protein pumps that actively transport ions such as sodium out of the cell Carrier proteins that carry molecules such as glucose into the cell Ion channel proteins, such as sodium ion channels in nerve fibres to which molecules of local anaesthetic attach to prevent the passage of the pain-inducing nerve impulse Receptor proteins, such as insulin receptors which facilitate the passage of glucose into the cell Immunoglobulin proteins, which form part of the immune response mechanism Enzyme proteins, such as alkaline phosphatase
Cell cytoplasm	The gel-like matter contained within the membrane envelop: • Contains and supports specialized organelles, including the cell nucleus, endoplasmic reticulum, the Golgi apparatus, mitochondria and lysozomes • Stores substances (glycogen and lipids) • Synthesizes and metabolizes essential substances (amino acids, fatty acids, monosaccharides) • Synthesizes and translates protein • Contains microtubules which help maintain the form of the cell and form intercommunication channels between organelles
Cell cytoskeleton	A complex network of structural elements (microtubules, intermediate filaments and microfilaments) which determine the shape of the cell, its ability to move and its response to external stimuli
Cell nucleus	Present in all eukaryotic cells capable of mitosis Contains the cell genome (DNA) and the means of replication and transcription of RNA Separated from the cytoplasm by the nuclear envelop (which disappears during mitosis and meiosis) Contains the nucleolus, where ribosomes are synthesized
Intercellular connections	There are two types of intercellular connections: tight junctions and gap junctions
Cell adhesion molecules (CAMs)	Structures (receptor integrins, immunoglobulin adhesion molecules, Ca+-dependent cadherins and carbohydrate-binding selectins) within the cell membrane that bond to laminins within the extracellular matrix to attach adjacent cells to one another and to the basal membrane