Summary BSC 1010 Structure and function of plasma membranes Review

Components and Structure
 Functions
o Defining the outer border of all cells and organelles
o Managing what enters and exits the cell
 Selectively permeable
 Allows some materials to freely enter or leave but other materials
cannot move freely
o Receiving external signals and initiating cellular responses
o Adhering to neighboring cells
 Fluid Mosaic Model
o A mosaic of components that give the membrane a fluid character
o Plasma membrane has three principal components
 Lipids, proteins, carbohydrates
o Cholesterol is a lipid comprised of four fused carbon rings
o Proportions may vary, but typically:
 Protein: 50%
 Lipids: 40%
 Carbs: 10%
o Carbohydrates present only on membrane exterior surface and are attached to proteins,
forming glycoproteins, or attached to lipids, forming glycolipids
o Asymmetric; inner surface differs from the outer surface
 Interior surface proteins anchor fibers of the cytoskeleton to the membrane
 Exterior surface proteins bind to the extracellular matrix
 Glycoproteins bind to substances the cell needs to import
 Phospholipids
o Main fabric is composed of an amphiphilic (having both hydrophilic and hydrophobic
parts) lipid molecule called phospholipid
 A phosphate group (polar)
 A glycerol molecule
 2 fatty acid chains
 Each fatty acid can be either saturated or unsaturated
 Carbons are saturated with hydrogen - all single C-C bonds
 Unsaturated when at least one double C=C bond occurs
o Phospholipid Bilayer
 Phospholipids arrange themselves in a bilayer
 Polar heads face outward
 Hydrophobic tails face inward
 Proteins
o Second major component of membranes
o Function as transporters, receptors, enzymes, or in binding and adhesion
o Integral membrane proteins- integrated completely into the bilayer
 One or more regions that are hydrophobic are others that are hydrophilic
 Hydrophobic regions can be α helical or beta sheets
 Locations and number of regions determine how they arrange within the bilayer
 Beta-blocker pills block the effects of the hormone epinephrine, aka adrenaline
o Peripheral membrane proteins- occur only on the surfaces
 Attached to either integral proteins or to phospholipids

,  May serve as enzymes
 Part of the cell's recognition sites, components of signaling pathways
o Rhodopsin is a light-sensitive receptor protein involved in visual phototransduction
 Mutations causing blindness disease
o Membrane Proteins
 Carbohydrates
o Oligosaccharide carbohydrates
o Located on exterior surface of the plasma membrane, bound to either proteins or to
lipids
 Membrane Fluidity
o Plasma membrane needs to be flexible but not so fluid that it cannot maintain its
structure
o Fluidity is affected by:
 Phospholipid type- phospholipids with saturated fatty acids can pack together
more closely than those with unsaturated FA
 Temperature- cold temperatures compress molecules making membranes more
rigid
 Cholesterol- located within the fatty acid layer, acts as a fluidity buffer; keeping
membranes fluid when cold and from not getÝng too fluid when hot
Passive Transport
 Plasma membrane is selectively permeable
o Cytosol solutions to differ from extracellular fluids
o Transport across a membrane can be either
 Passive transport- requiring no energy
 Active transport- requiring energy (ATP)
 Passive Transport
o Diffusion- when a substance from an area of high concentration moves down its
concentration gradient
 In membranes this occurs through the lipid bilayer
 Factors that affect diffusion rates
 Concentration gradients- greater difference, faster diffusion
 Mass of the molecules- smaller molecules diffuse more quickly
 Temperature- molecules move faster when temperatures are higher
 Solvent density- dehydration increases density of cytoplasm, reduces
diffusion rates
 Solubility- more nonpolar (lipid-soluble) materials diffuse faster
 Surface area- increased surface area speeds up diffusion rates
 Distance travelled- the greater the distance, the slower the rates;
important factor affecting upper limit of cell size
 Pressure- in some cells, blood pressure forces solutions through
membranes, speeding up diffusion rates
o Net movement ceases once equilibrium is achieved
o Only small, nonpolar molecules
o Facilitated passive transport moves substances down their concentration gradients
 Channel proteins
 Top, bottom, and inner core are composed of hydrophilic amino acids
 Some are always open while others are "gated"