Fluidity of Cell Membranes: Importance and Mechanisms - Prof. P. Garretson, Study notes of Biology

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Chapter 5

Cell Membrane

Structure & Function

Biol 1001 Fall 2013

Fluid Mosaic Model  Cell membranes = patchwork (mosaic) of different proteins  (^) proteins within fluid of phospholipd bilayer  Constantly shift and flow  Membrane components remain constant  Distribution of proteins and phospholipids changes over time

Fluidity of Membrane  Based on orientation and bonds of phospholipids 

Hydrophilic heads

 Orient towards watery environments  Form hydrogen bonds with water molecules

 hydrophobic tails

 Orient towards center of membrane  Some with unsaturated fatty acids in tail

more fluid less fluid

 More unsaturated fatty acids  (^) More double bonds  (^) More kinks  (^) More fluid membrane  Fewer unsaturated fatty acids  (^) Fewer double bonds  (^) Fewer kinks  (^) Less fluid membraane Fig 5-

Why is Fluidity of Bilayer Important?  Cells must be able to change shape

 stiff cells break apart cell death

 Cell membranes must be in constant motion

 vacuoles bring materials into cell

 Membrane bound organelles need to:

 Move materials throughout cell (e.g. lysosomes)  merge with membranes of other organelles (e.g. Golgi)

Cell membranes  Surround cell and organelles(Eukaryotes)  Functions

 Phospholipid bilayer

 Isolate cell contents from environment 

Embedded proteins

 Transport essential molecules & ions  (^) Between cell and environment  (^) Between organelles and cytoplasm  Regulate biochemical reactions  create attachments in & between cells  allow communication between cells

Embedded Proteins  1000s possible  Functions based on type of protein  (^) attatchment  (^) communication  (^) Receptor proteins  (^) recognition proteins  (^) regulate biochemical reactions  enzymes  (^) transport  (^) channel proteins  (^) carrier proteins

Attachment Proteins  anchor cell membranes  (^) Anchors cell in place within tissues  Link cytoskeleton to extracellular matrix  Helps maintain cell shapes  Link cytoskeleton to plasma membrane  Helps cells adhere to and move along surfaces  Form connections between adjacent cells

Receptor Protein Example  Hormone: Adrenaline (epinephrine)  (^) “Fight or Flight Hormone”  Binds to specific receptor protein on muscle cells  Stimulate muscle cells to breakdown glycogen  (^) Produces energy for muscle contraction receptor hormone (cytoplasm) (extracellular) Hormone binds to receptor Hormone binding activates receptor, changing its shape Activated receptor stimulates response in cell 1 2 3 Fig 5-

Recognition Proteins  Glycoproteins that serve as “cell markers”  (^) Identify cell as being “self”, not foreign  (^) Prevent immune system from attacking your own cells  Examples  (^) MHC glycoprotein  (^) red blood cell sugar groups  (^) Identify blood cell type as A, B, AB or O  During transfusions - blood types must match or immune system will attack & destroy red blood cells  (^) organ tissue cells  During transplants – cells must match or organ will be rejected by body

Transport Proteins  Regulate movement of hydrophilic molecules in & out of cell  2 Types  channel proteins  Allow water and certain ions to cross membrane  carrier proteins  Molecule attaches to binding site of protein  Molecule transported across membrane & released

Substance Movement Across Membranes

Types of Gradients  temperature gradient  (^) Flow of energy from heigh temperature region to low temperature region  electrical gradient  (^) Drives the movement of ions  (^) Pressure gradient  (^) Movement of ions and molecules to reach a state of equilibrium  Concentration gradient  (^) Difference in concentration of a substance between two parts within a fluid or across a membrane

Concentration Gradient  Movement of ions and molecules  From high to low concentration  (^) Until equilibrium is met  Substance becomes evenly distributed  Ex: Dye in water  At first - dye is more concentrated in one area  Dye molecules disperse evenly throughout water  End result: uniform color of solution