ch 3 gaining admission: the cell membrane Flashcards
the cell membrane is like the
- gatekeeper
- guardian
- border guard
despite being only 6 to 10 nanometers thick and visible only through an electron microscope, the cell membrane keeps the cell’s
cytoplasm in place and lets only select materials enter and depart the cell as needed
this semipermeability or selective permeability is a result of a
double layer (bilayer) of phospholipid molecules interspersed with protein molecules
the outer surface of each layer is made up of tighly packed
hydrophilic (or water-loving) polar heads
inside between the two layers you find
hydrophobic (water-fearing) nonpolar tails consistinf of fatty acid chains
since the phospholipids give the membrane a rather
- fluid structure
- cholesterol molecules are embedded to provide stability and make it less permeable to water-soluble substances
both cytoplasm and the matrix, the environment outside of the cell, are primarily
water
the polar heads electrostatically attract
polar water molecules
the membrane’s interior made up of
non-polar fatty acid molecules (the phospholipid tails) creating a dry middle layer
Lipid-soluble molecules can pass through this layer but water soluble molecules such as
- amino acids
- sugars
- many proteins
cannot
these molecules must enter the cell through little tunnels created by channel proteins
in addition to forming protein channels, other proteins do not span the membrane but stick out from the
outer surface, sometimes with a carbohydrate attached
protein channels serve numerous purposes for example
- creating receptors for hormones
- binding site for other cells
- aiding in recognition of self versus non-self
the cell membrane is designed to hold the cell
together and to isolate it as a distinct functional unit
although the cell membrane can spontaneously
- repair minor tears
- severe damage to the membrane will cause the cell to disintegrate
the membrane is picky about which molecules it
lets in or out
the cell membrane allows movement across its barrier by
- diffusion
- osmosis
- active transport
diffusion
atoms, ions, and molecules are in constant
motion, taking a straight-line path until they inevitably crash into another particle and bounce off with a new trajectory
as a result the particles will naturally flow to areas of lower concentration (more collisions means more movement)
diffusion
diffusion
the movement of molecules from high to low concentration
diffusion
As long as the molecules can move through any barricade that may be in their way (the cell membrane for ex) the body does not need to invest any
energy in their movement
diffusion
The kinetic energy of the particles is the driving force moving them down their
concentration gradient
diffusion
even once equilibrium is reached diffusion
continues
there just is not a net change (one randomly moves out so anotehr moves in)
equilibrium: molecules are equally distributed throughout
diffusion
