Download Cell Walls and Boundaries Cells protect themselves by their cell

Cell Walls and Boundaries
Cells protect themselves by their cell membrane (and
cell wall in the case of plant cells). However, they will
need nutrients so somehow the cells must have a way
of allowing some things in while leaving some out.
 Think about your house, you want some people to
stay out while your friends and family are allowed
in.
 Two ways of doing this: passive transport and
active transport
Passive Transport = the movement of materials
across the cell membrane without using cellular
energy
 Every living cell exists in a liquid environment.
 One of the most important functions of the cell
membrane is to keep the cell’s internal
conditions relatively constant.
o Regulate the movement of molecules from
one side of the membrane to the other side.
 Types of passive transport: diffusion, facilitated
diffusion, osmosis and filtration.
Diffusion = process by which particles move from an
area of high concentration to an area of lower
concentration
 Is the driving force behind the movement of many
substances across the cell membrane.
 The cytoplasm of a cell is a solution of many
different substances dissolved in water.
o In any solution, solute particles tend to move
away from high concentration areas to lower
concentrated areas.
 Much like moving west from the east
coast to seek new land.
 Suppose a substance is present in unequal
concentrations on either side of a cell membrane
what will happen?
o Cell membranes consist of a phospholipid
bilayer with hydrophilic heads and
hydrophobic tails.
 If the substance can cross the cell membrane, its
particles will tend to move toward the area of less
concentration until it is even or at a state of
equilibrium
o At equilibrium, the particles will continue to
move across the membrane in both directions
= no net change
Facilitated Diffusion = molecules that cannot directly
diffuse across the membrane pass through special
protein channels (diffusion with help)
 Proteins act as carriers, or channels, making it
easy for certain molecules to cross the cell
membrane
 Facilitated diffusion does NOT require additional
use of cell’s energy.
 Used by bigger molecules or charged molecules
because the phospholipid bilayer only allows small
uncharged particles through without help.