Cell Membrane

• The cell membrane defines the boundary of the cell and serves as a barrier between the external and internal environments
  • All materials entering or exiting the cell must pass through the plasma membrane
  • Plasma membrane controls passage in two ways:
    • Physical process (e.g. diffusion)
    • Specific transport mechanisms

Diffusion

• Diffusion is the random movement of particles of a substance from a region of high concentration to one of lower concentration
  • Diffusion of gases is the fastest of
  • Diffusion in liquids can be slow, particularly in the absence of convective currents, movement of up to 1 m can take a year
  • Diffusions in solids extremely slow

Diffusion on the Biological Scale

• Biological organisms generally inhabit and maintain warm aqueous environments
  • Small distances: cell diameters of 10-50mm
    • Movement across a cell diamenter by a small molecule such as an amino acid would require only 0.5sec

Effects of Temperature and Pressure

• In nature diffusion are rarely a function of concentration alone
  • Useful to think in terms of free energy(DG)
    • High local concentration have large amounts of potential energy
    • Diffusion away from a region of high concentrations reduces free energy (-DG)
  • Opposing gradients of free energy will produce a net movement relative to the DG gradients

Membrane Permeability

• Suppose a molecule of H2O can pass through a membrane, while an ion or molecule of sugar can not pass through the same membrane
  • Differentially permeable (selectively permeable)
• Movement of H2O through a membrane is referred to as osmosis

Osmosis

Free energy of water molecules is always decreased if osmotically active substances (dissolved or suspended particles) are present in the water.

• Osmotic concentration of a fluid is the number of dissolved particles per unit volume
  • Bears a direct relationship to the fluids free energy
• Osmotic potential (pure H2O = 0) decreases with increasing osmotic concentrations (Y < 0)

Permeability of Biological Membranes

• Differences in the selective permeability between biological membranes is quite large
  • RBC's are 100x more permeable than are the membranes of Amoeba
    • Most cells are permeable to H2O, small sugars amino acids but impereable to polysaccharides, proteins and other large molecules
    • Hypertonic; Isotonic, Hypotonic

How the Cell Membrane Works

• Lipid Bilayer: Membranes held together by water
  • Phospholipid bilayer: self assembly suggest by Danielle and Davson (1930's)
    • Bilayer with aqueous environments on both sides
    • Referred to as a liposome
  • Fluid Mosaic Model (Singer and Nicholson, 1972)
    • Lipid bilayer and protein associations
      
      

Membrane Fluidity

• Membrane structure is not static, but rather fluid within the plane of the bilayer
  • Fluidity of a membrane is greatest when the lipid composition is highly unsaturated and it contains no cholesterol
    • Speed of lipid movement is 2mm/sec, a very rapid rate considering bacterial cells are 2mm in length
  • Cholesterol assists in maintaining properties by influencing lipid packing

Membrane Structure

• Protein associations with the fluid-mosiac membrane
  • Peripheral proteins associated with the surface of the lipid bilayer
  • Integral proteins are located wholly or partially within the lipid bilayer

Mechanisms for the Passage through the Membrane

• Membrane channels: the simplest mechanism of transport
  • Membrane channel movement follows diffusion gadients (Passive Processes)
    • Membrane pores are protein lined holes spanning the bilayer
      • Some degree of selectivity due the charge of the R-groups on the amino acids lining the pores
    • Facilitated Diffusion allows specific molecules to selective move down a concentration gradient mediate by a membrane protein channel

Cooperative Channels

• Movement of two molecules across a membrane
  • Symport: Movement of two molecules similtaneously across a membrane in the same direction mediate by a membrane protein.
  • Antiport:Movement of two molecules in opposite directions simultaneous by a membrane protein

Movement of Charged Particles across Membranes

• Electrostatic gradients across membrane result from an unequal distribution of charged particle.
  • Biological membranes have a net negative charge on the inside (about -70 mV)

This unequal charge distribution contributes to the movement of oppositely charge molecules toward the interior of the cell (Electrochemical Gradient)

Gated Channels

• Gated channels is another way to control the specific movement across membranes

Gated channels allow the transmission of information between cells into a other signals capable of communicating to the interior of the cell

• External signals (Hormones,etc)
• Receptors (Membrane channel proteins that respond to the external signal)

Mobile Carriers

• Mobile carriers could transport a molecule from one side of a membrane to the other
  • No specific evidence for this type of mechanism
  • Valinomycin acts in a manner in which we would expect a mobile carrier to function

Active Transport Mechanisms

• Active transport or "pumps" do not rely on free energy gradients
  • Use stored chemical energy to move substances against their concentration gradient
    • ATP generally provides the required energy
  • Useful in acquiring necessary cell building block and ridding the cellular interior of unwanted substances
    • Sodium potassium pump

• Reduction of the diffusing substances internal concentration
  • Complexation with another internal component or compound
  • Reduces the free concentration and thereby maintains the diffusion potential

Large Scale Movement: Endocytosis

• Endocytosis moves large substances or quantities of material across a membrane without the need for a specific channel.
• Endocytosis is a membrane mediated process
  • Phagocytosis (Cellular eating)
  • Pinocytosis (Cellular drinking)
  • Receptor mediated (Specific Absorption)

Receptor-mediated Endocytosis

• Absorption of materials at selective binding sites
  • Specific receptor molecules are clustered at absorption sites on the membrane
  • Once the receptors are "loaded", vesicles are formed and detached from the membrane
    • Absorption sites appear as coated pits
      • Clathrin protein mediate and assist in the internalization of the vesicles

Endocytosis of Cholesterol

• Cholesterol is transported in blood by low-density-lipoproteins (LDL)
  • Cholesterol is necessary to construct new membranes
    • When synthesis is occurring specific LDL receptor are deployed in the existing membrane
    • Receptors bind the LDL and are internalized as a vesicle which is referred to as an endosome
  • Endosomes fuse with lysosome that breakdown and recycle the materials endocytosed

Exocytosis

• Essential the reverse of endocytosis
  • Membrane mediated movement of materials from the interior of the cell to the exterior
  • Important in the secretion of hormones, extracellular surface materials and release of waste products
  • Exocytosis is directed process, in that the vesicles require specific binding proteins to fuse with a target membrane

Outside the Membrane

• Many organism have an exterior wall
  • Plants bacteria and fungi
    • Formation of cell walls in plants
      • Complex polysaccharide, principally cellulose
      • Primary cell wall (elastic as long as growth continues)
      • Middle lamella (principally pectin)
      • Secondary cell wall (interior to the primary cell wall and is composed primarily of lignin)
    • Fungi and bacteria differ in their cell wall components
      • Primary component is an amino acid sugar (glucosamine)

Cellular Coats and their Function

• Unlike plants, bacteria and fungi: Animal cell exteriors are not independent entities
  • The exterior coat is referred to as the glycocalyx
    • Glycocalyx is composed of glycolipids and glycoproteins
    • Glycocalyx provides a recognition site on the surface that allows cells to recognize similar cellular types

Limitation to Cell Size

• Larger size improves an individuals ability to harvest more of the available resources
  • Size can not simply be achieved increasing the size of a singular cell
    • Tripling the cell volume, only increases the cell surface by two fold
    • Since metabolic needs must be satisfied through membrane process, there is an upper limit to effective cellular size

Alternative Strategy of Multicellularity

• Simple aggregations of identical single cells clusters are present in nature
  • 32 cell disks of algae and amoeboid clusters of slime molds
• Complex aggregations of cells allows for the partitioning of labor (specialization's)
  • Certain cells have been shown to aggregate to confer particular functions
    • Propulsion, feeding and reproduction

Differing Mechanisms of Cellular Aggregation

• Intercellular matrices
  • Fibroblasts secrete collagen and elastin form a structural network
• Intercellular connections providing strength and adherence
  • Spot and belt desosomes
  • Tight and gap junctions
  • Plasmadesmata