Chapter 5

• membrane:

  • the walls that divides the cell into separate compartments in order to create different conditions for different chemical processes
• Cell membrane function:

  • protective barrier for the cell (plasma membrane)
  • shape of the cell (plasma membrane)
  • cell motility (some cells)
  • cellular compartments (organelle membrane)
  • exchange surface for the material and signals in and out of the cell organelles
  • energy storage (membrane potential)
• Bilayer of phospholipid molecules:

  • composes the biological membranes

  
• Phospholipids move within and between the membrane layers

  • lateral movement - 10^7 times/second
  • flip flopping - rare once/month

  
• type of phospholipids influences membrane viscosity:

  • fluid: unsaturated hydrocarbon tails prevent packing enhancing membrane fluidity
  • viscous: saturated hydrocarbon tails pack together, increasing membrane viscosity

  
• Cholesterol:

  • in animal cells increases membrane viscosity making it difficult for the membrane proteins to do their jobs

  
• membrane viscosity:

  • a biologically active membrane must be kept in a fluid state

  factors that affects fluidity:

  • Transition temperature
  • cholesterol level
• Transition temperature:

  • temperature at which membrane turns into liquid state from a frozen state
  • transition temp can be affected by:
  • saturation level of fatty acids: the more unsaturated they are the lower the temperature
  • length of the fatty acid chain: the shorter the chain the lower the temp

  
• cholesterol level:

  • the higher the cholesterol the lower the fluidity

  
• evolution of differences in membrane lipid composition:

  • variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions
  • ability to change the lipid compositions in response to temperature changes has evolved in organisms that live with temperatures vary
• Glycosylation:

  • the process of covalently attaching a carbohydrate to a protein or lipid
  • glycolipid: carbohydrates attached to lipid
  • Glycoprotein: carbohydrate attached to protein
• glycosylation functional consequences:

  • carbohydrates attached to lipids and proteins as recognition signals for other cellular proteins
  • membrane glycoproteins and glycolipids often play a role in cell surface recognition
  • carbohydrates have a a protective effect. cell coat or glycocalyx - carbohydrate rich zone on the cell surface shielding cell from mechanical and physical damage
  • glycosylation may aid in protein folding, protect a protein from extracellular factors that could harm its structure
• plasma membrane permeability:

  • the plasma membrane is a selectively permeable barrier between the ell and its external environment. (some substances can move across and some cant)
  • the phospholipid bilayer is a barrier to the diffusion of hydrophilic substances
  • Hydrophobic interior makes formidable barrier to the movement of ions and hydrophilic molecules
• the plasms membrane structure ensures:

  • essential molecules (glucose and amino acids) enter a cell
  • metabolic intermediates remain in the cell
  • waste products exit
• high permeability: (easy access)

  • gasses
  • very small uncharged polar molecules

  ex.

  • CO2
  • N2
  • O2
  • ethanol
• Moderated permeability:(possible difficult access)

  • water H2O
  • urea H2NCONH2
• Low permeability:(aided access)

  • polar organic molecules

  ex.

  • glucose
• very low permeability:(special access if needed)

  • ions
  • Na+, K+, Mg2+, Ca2+, Cl-
  • charged polar molecules and macromolecules
  • animo acids
  • ATP
  • Proteins
  • polysaccharides
  • nucleic acids (DNA, RNA)

  
• Biological membrane contain proteins and sugars associated with lipids

  • integral (transmembrane) proteins
  • passing through membrane
  • peripheral proteins
  • attached to surface of membrane
  • anchored proteins
  • connected to membrane through lipid or sugar

  
• integral proteins of membrane:

  • although they have lateral mobility
  • 10-70% of them are anchored to either extracellular matric or cytoskeletal matrix

  
• Membrane proteins:

  • most of membrane functions are carried out by membrane proteins

  membrane proteins can associate with the lipid bilayer in different ways:

  • Integral membrane proteins - can be removed only with detergents
  • peripheral membrane proteins - can be released by more gentle extraction that leaves lipid bilayer intact

  
• Membrane proteins orient in membrane with respect to their amphipathic features as well as their unique function

  
• transmembrane proteins are first inserted into the ER membrane:

  • transmembrane segment:
  • if a sequence within the polypeptide contains a stretch of 20 amino acids that are mostly hydrophobic
  • ER signal sequence is cleaved
  • creates a membrane protein with a single transmembrane segment
  • some polypeptides may contain two or more transmembrane segments
  • an additional transmembrane segment is integrated into the membrane:
  • each time a polypeptide sequence contains a stretch pf 20 hydrophobic amino acids

  
• fluid mosaic model:

  • mosaic - proteins of membrane
  • phospholipids of membrane

  
• Types of membrane proteins by function:

  • transport
  • enzymatic activity
  • signal transduction
  • cell-cell recognition
  • intercellular joining
  • attachment to the cytoskeleton ad extracellular matrix (ECM)
• Transport proteins:

  • left:
  • a protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute
  • right:
  • other transport proteins shuttle a substance from one side to the other by changing shape
  • some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane

  
• enzymatic activity:

  • a protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution
  • in some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway

  
• signal transduction:

  • a membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone
  • the external messenger (signal molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein

  
• cell-cell recognition:

  • some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells
  • this type of cell-cell binding is usually short lived compared to intercellular joining

  
• intercellular joining:

  • membrane proteins of adjacent cells may hook together in various kinds of junctions, such as a gap junctions
  • this type of binding is more long lasting than cell-cell recognition
  • (difficult to tear them apart)

  
• attachment to the cytoskeleton and extracellular matrix (ECM)

  • microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins
  • a function that helps maintain cell shape and stabilizes the location of certain membrane proteins
  • proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes

  
• Synthesis of membrane components and their orientation in the membrane:

  
• cell membrane synthesis:

  • new phospholipids are manufactured by enzymes bound to the cytosolic surface of the smooth ER using free fatty acid
  • the enzymes deposit the newly made phospholipids exclusively in the cytosolic half of the bilayer

  
• diffusion:

  • diffusion is migration of molecules from their high concentration to the region of their low concentration
  • caused by random thermodynamic movement of molecules (brown movement)

  
• diffusion of one solute across artificial membrane:

  
• diffusion of two solutes across artificial membrane:

  
• diffusion rate is under the effect of:

  • distance
  • the shorter the distance the quicker the gradient is eliminated
  • molecule size
  • the smaller the molecule the faster the diffusion
  • temperature
  • the higher the temperature the faster the thermal motion of molecules and thus the diffusion
  • steepness of concentration gradient
  • the bigger the concentration difference the faster the diffusion rate
  • membrane surface area
  • the larger the surface area of membrane the faster the diffusion

  what is the gradient:

  • it is the gradual change of something

  
• Osmosis:

  • diffusion of water
  • water moves against the gravity force because of osmosis

  
• The process of osmosis: tonicity

  • isotonic
  • hypertonic
  • hypotonic
• hypotonic:

  • solution with a lower concentration of solute than the solution on the other side of the membrane
  • cells placed in a hypotonic solution will swell as the water enters, this which lead to cell lysis