2.3 Plasma Membrane, 2.4 Membrane Permeability

Plasma membrane

separates internal cell environment from external environment

compromised primarily of phospholipids

phospholipids are ampiphatic (have both hydrophilic and hydrophobic regions),forms a bilayer

polar hydrophilic heads (phosphate regions) are oriented TOWARD aqueous environment

nonpolar hydrophobic tails(fatty acid regions) are facing inwards, AWAY from aqueous environments

hydrophobic interior causes the membrane to be selectively permeable

structure is described by the fluid mosaic model

Fluid mosaic model

a model to describe the structure of the cell membrane

fluid: membrane is held together by weak hydrophobic interactions and can therefore move and shift

temperature affects fluidity

unsatured hydrocarbon tails help maintain fluidity at low temps(kinked tails prevent tight packing of phospholipids

cholesterol helps maintain fluidity at high and low temps

at high temps, cholesterol reduces movement so the phospholipids don’t spread too far

at low temps, cholesterol reduces tight packing of phospholipids

mosaic: comprised of many macromolecules

Membrane proteins

Integral proteins and peripheral proteins

Integral proteins

proteins that are embedded into the lipid bilayer

can be hydrophilic, hydrophobic, or both (determined by R groups/side chains)

hydrophilic= polar or ionic side chains, hydrophilic regions make up the interior of the channel or pore

hydrophilic regions are exposed to the cytosol

hydrophobic= nonpolar side chains

hydrophobic regions make up the protein surface and interact with fatty acids

Peripheral proteins

proteins that are not embedded into the lipid bilayer

loosely bonded to the surface

Membrane Carbohydrates

important for cell to cell recognition

glycolipids are carbohydrates bonded to lipids

glycoproteins are carbohydrates bonded to proteins,most abundant

Selective permeability

the ability of membranes to regulate the substances that enter and exit

this is due to the hydrophobic interior(non polar hydrophobic tails)

small,non polar, hydrophobic molecules have easy passage across the membrane ex. hydrocarbons,CO2,O2,N2(gases)

hydrophilic polar molecules, large molecules, and ions have difficult passage or protein assisted passage across the membrane due to the non polar hydrocarbon tails

small polar, uncharged molecules (like water and ammonia) can pass through in small amounts

Cell wall

some organisms also have cell walls that cover their plasma membrane like bacteria,archaea,fungi and plants

function is to provide structural boundary (defines and supports the cell’s shape), provide a permeability barrier for some substances, and protection from osmosis lysis(cell bursting when excess water goes into cell)

in plants, the cell wall is composed of cellulose, it’s thicker than plasma membranes, contains plasmodesmata(hole like structures in the cell wall filled with cytosol that connect with adjacent cells)