Cytoplasm & Cytoplasmic Organelles

• Cytoplasm


  cyto = cellular material between the plasma membrane + the nucleus

  site of most cellular activities

  consists of 3 major elements - cytosol, organelles + inclusions

  
• Cytosol


  cytosol = viscous fluid in which other cytoplasmic elements suspended

  dissolved in the cytosol, which is largely water, r proteins, salts, sugars + a variety of other solutes
• Inclusions


  inclusions = chem substances that may be present depending on the cell type

  e.g. stored nutrients like glycogen granules in liver + muscle cells, lipid droplets in fat cells + melanin granules in certain skin + hair cells
• Cytoplasmic Organelles


  the organelles r the metabolic machinery of the cell

  each type of oragnelle carries out a specific function for the cell - synthesising proteins, generating ATP + etc
• Importance of Organelles' Membranes


  most organelles r bounded by a membrane similar in composition to the plasma membrane

  = enable the organelles to maintain an internal enviro diff from that of the surrounding cytosol

  this compartmentalisation = crucial to cell functioning
• Mitochondria


  mitochondria = thread-like membranous organelles

  provides the cell most of its ATP supply

  the density of mitochondria in a particular cell reflects that cell's energy requirements + mito generally cluster where the action is
• Mitochondria - Double Membrane


  a mitochondrion = enclosed by 2 membranes, each w the general structure of the plasma membrane

  the outer membrane is smooth + featureless

  inner membrane folds inward, forming shelflike cristae that protrude into the matrix, a gel-like substance w in the mitochondrion
• Mitochondrial Matrix


  teams of enzymes, some dissolvedd in the matrix + others forming part of the crista membreane, break down intermediate products of food fuels (glucose + others) to water + CO2 --- ATP generation
• Ribosomes


  Ribosomes r small granules composed of proteins + a variety of rRNAs

  each ribosome has 2 globular subunits that fit together

  sites of protein synthesis
• Free Ribosomes


  free ribosomes float freely in the cytosol

  they make soluble proteins that function in the cytosol, as well as those imported into mito + some other organelles
• Membrane-Bound Ribosomes


  membrane-bound organelles r attached to membranes, forming a complex = RER

  they synthesise proteins destined either for incoporation into cell membranes/lysosomes/for export from the cell
• Endoplasmic Reticulum


  the endoplasmic reticulum = an extensive system of interconnected tubes + parallel sacs = cisterns

  the cavities of cisterns r filled w fluid

  coiling + twisting thru the cytosol, the ER is continuos w the outer nuclear membrane

  2 distinct varieties = RER + SER
• Rough Endoplasmic Reticulum


  the external surface of the RER is studded w ribosomes

  proteins assembled on these ribosomes thread their way into the fluid-filled interior of the ER cisterns

  when complete, the newly made proteins r enclosed in vesicles for their journey to the Golgi apparatus where they undergo further processing
• Functions of the RER - Producing Secretory Proteins


  its ribosomes manufacture all proteins secreted from cells

  for this reason, the RER is particularly abundant + well developed in most secretory cells, antibody-producing immune cells + liver cells, which produce most blood proteins
• Functions of the RER - Membrane Factory


  manufactures the integral proteins + phospholipids that form part of all cellular membranes

  w in the cisterns, sugar groups r attached to those proteins that will eventually face the extracellular enviro

  the enzymes that calyse lipid synthesis have their active sites on the external face of the RER, where the needed substrates r available
• Smooth Endoplasmic Reticulum


  the SER is continuous w the RER + consists of tubules arranged in a looping network
• SER's Enzymatic Functions


  its enzymes catalyse reactions involved in:

  breaking down glycogen to form free glucose (in liver cells esp)

  store calcium ions in most cell types - skeletal + cardiac muscels cells have a special SER = sarcoplasmic reticulum that stores calcium + releases it as a trigger for contraction
• Golgi Apparatus


  Golgi apparatus consists of stacked + flattened membranous sacs, associated w swarms of tiny membranous vesicles

  traffic director for cellular proteins

  its major function is to modify, concentrate + package the proteins + lipids made at the RER + destined for export from the cell
• Lysosomes


  Lysosomes = spherical membraneous organelles containing activated hydrolytic enzymes

  lysosomal enzymes can digest almost all kinds of bio mols

  they work best in acidic conditions

  abundant in phagocytes, the cells that dispose of invading bacteria + cell debris