unit 1

• purpose of internal membranes

  Eukaryotic cells have a relatively small surface area to volume ratio as a result of their size. The plasma membrane of eukaryotic cells is therefore too small an area to carry out all the vital functions carried out by membranes. Eukaryotic cells have a system of internal membranes, which increases the total area of membrane and provides a larger surface area for vital functions to take place.
• Endoplasmic Reticulum (ER)


  forms a network of membrane tubules continuous with the nuclear membrane. Rough ER (RER) has ribosomes on its cytosolic face while smooth ER (SER) lacks ribosomes.
• the golgi apparatus

  series of flattened membrane discs. The discs are connected allowing molecules to move within the Golgi apparatus. The Golgi apparatus is adjacent to the endoplasmic reticulum.
• Lysomes

  membrane-bound organelles containing a variety of hydrolases that digest proteins, lipids, nucleic acids and carbohydrates. The interior of the lysosomes are acidic allowing optimal function of the enzymes it contains.
• vesicles


  transport materials between membrane compartments. They consist of an aqueous solution enclosed by a lipid bilayer.
• synthesis of lipids

  Lipids are synthesised in the smooth endoplasmic reticulum (SER) and inserted into its membrane
• where does the synthesis of proteins occur

  begins in cytosolic ribosomes. The synthesis of cytosolic proteins is completed there, and these proteins remain in the cytosol.
• transmembrane (integral) proteins


  carry a signal sequence, which halts translation and directs the ribosome synthesising the protein to dock with the ER, forming RER.

  
  

  Translation continues after docking, and the protein is inserted into the membrane of the ER.
• signal sequence


  short stretch of amino acids at one end of the polypeptide that determines the eventual location of a protein in a cell.
• what happens once proteins are in the ER

  they are transported by vesicles that bud off from the ER and fuse with the Golgi apparatus
• proteins moving through the golgi apparatus

  As proteins move through the Golgi apparatus they undergo post-translational modification

  Molecules move through the Golgi discs in vesicles that bud off from one disc and fuse to the next one in the stack
• post-translational modification

  refers to covalent modifications which are made to proteins after translation

  
  

  The addition of carbohydrate groups is the major post-translational modification; enzymes catalyse the addition of various sugars in multiple steps to form the carbohydrates.
• vesicles leaving the golgi apparatus

  take proteins to the plasma membrane and lysosomes. Vesicles move along microtubules to other membranes and fuse with them within the cell

  
  

  then packaged into secretory vesicles. These vesicles move to and fuse with the plasma membrane, releasing the proteins out of the cell.
• Where are secreted proteins translated?

  translated in ribosomes on the RER and enter its lumen
• proteolytic cleavage


  Many secreted proteins are synthesised as inactive precursors and require proteolytic cleavage to produce active proteins. Proteolytic cleavage is another type of post-translational modification
• amino acids

  building blocks of proteins; sequence determines protein structure
• Monomer


  a molecule that may bind chemically to other molecules to form a polymer; proteins are polymers of amino acid monomers.
• peptide bonds

  link amino acids to for a polypeptide

  
  

  covalent and very strong, they are difficult to break.
• amino acid grouping

  can be grouped according to their properties. All amino acids have a central carbon with four groups attached (an amine (NH2), a carboxylic acid (COOH), a hydrogen and a variable R group).
• amino acid classes

  according to their R groups:

  
  

  basic (positively charged);

  acidic (negatively charged);

  polar;

  hydrophobic.
• Modulator

  Modulators regulate the activity of the enzyme when they bind to the allosteric site. Upon binding a modulator, the conformation of an allosteric enzyme will change and this alters the affinity of the active site for the substrate. Modulators may be positive or negative.
• primary structure of protein

  The order in which the amino acids are synthesised into the polypeptide.
• What name is given to the covalent bonds which link sulfur atoms

  disulphide bridges
• secondary structure of protein

  Hydrogen bonding along the backbone of the protein strand results in regions of secondary structure .

  The secondary structure of a protein is stabilised by hydrogen bonds between atoms of the same chain

  
  

  alpha helices

  turns

  beta plated sheets
• tertiary structure of protein

  The polypeptide folds into a tertiary structure. Folding at this level is stabilised by many different interactions between the R groups of the amino acids.

  brought about by charge effects, such as interactions of the R groups in hydrophobic regions
• hydrogen bonding and disulphide bridges

  A hydrogen bond is an electrostatic attractive interaction which occurs between a hydrogen atom and an electronegative atom, such as oxygen or nitrogen. A disulfide bond (also known as a disulfide bridge) is a covalent bond between two thiol (SH) groups.
• quaternary structure of a protein

  results when a protein consists of multiple polypeptide subunits

  describes the spatial arrangement of the subunits
• prosthetic group


  a non-protein unit tightly bound to a protein and necessary for its function, e.g. haem in haemoglobin
• temp influence on R groups

  will affect the structure (and function) of a protein. Increasing temperature disrupts the interactions that hold the protein in shape; the protein begins to unfold, eventually becoming denatured.
• pH influence on R groups

  The charges on acidic and basic R groups are affected by pH. As pH increases or decreases from the optimum, the normal ionic interactions between charged groups are lost, which gradually changes the conformation of the protein until it becomes denatured.
• ligand binding

  a substance that can bind to a protein. R groups not involved in protein folding can allow binding to ligands. Binding sites will have complementary shape and chemistry to the ligand. As a ligand binds to a protein-binding site the conformation of the protein changes. This change in conformation causes a functional change in the protein.
• Co-operativity

  binding of a substrate to one subunit imcreases the affinity of another subunits active site for the subsequent substrate molecule

  
  

  This is of biological importance because the activity of allosteric enzymes can vary greatly with small changes in substrate concentration.
• protein kinase

  catalyse the transfer of a phosphate group to other proteins
• protein phsophatases

  dephosphorolate
• what occurs when adding a phosphate group

  adds negative charges in the side chain of amino acids. This can change a protein's structure by altering interactions with nearby amino acids;
• Glucose symport

  Transporter protein that simultaneously transports glucose and sodium across the membrane. Sodium facilitated the transport of glucose.
• Membrane proteins involved in transfixing signals from outside a cell to its interior

  G protein or enzyme (linked receptor)
• Channel


  Protein that allows passive transport of molecules across a membrane.
• Ligand gated

  Channel protein that is opened by the bonding of a signal molecule to allow diffusion of a specific ion.
• Transporter

  Transmembrane protein that changes conformation to move molecules across the membrane. Transport can be facilitated or active.