Transcript The Cell (aka Plasma) Membrane intro mini

The Cell (aka Plasma) Membrane
intro mini-movie
Functions of the Cell Membrane
 Found in all cell types
 Is more flexible than a cell wall
 Controls what enters and exits the
cell
 Forms a boundary between the cell
and its environment
Selective Permeability
• Allows the cell to maintain homeostasis
(internal balance) despite changes in its
environment
• Allows some, but not all, materials to cross
– Small, nonpolar (uncharged) molecules pass
through the membrane easily
– Small, polar (charged) molecules pass
through the membrane with the aid of proteins
– Large molecules require vesicles to get in /
out of the cell.
Structure of the Cell Membrane
The cell membrane consists of a
double layer of phospholipids
interspersed with a variety of other
molecules, including proteins,
cholesterol, and carbohydrates.
Quick-time movie
Phospholipids
• A phospholipid has three major parts:
– Phosphate group (which is charged)
– Glycerol
– Two fatty acid chains
Explanation / animation
Phospholipids – Heads and Tails
• The phosphate and
glycerol form the head,
which is polar (charged)
and hydrophilic (waterloving)
• The two fatty acid
chains form the tails,
which are non-polar
(not charged) and
hydrophobic (waterrepelling)
The Phospholipid Bilayer
• Cells are filled with and
surrounded by water
• So, phospholipids line up in
a “bilayer”
– Water-loving heads form
the outer layer (like
bread on a sandwich)
– Water-repelling tails are
protected inside (like the
filling of a sandwich)
Click for Animation
• The illustration above show the variety of
molecules embedded within the cell membrane.
• Animation
Passive Transport
• Particles are constantly in motion, colliding
and scattering.
– This motion is random.
• Does not require the cell to use ATP /
energy
• The membrane is still semi-permeable, so
only certain substances can cross (in or
out) by passive transport.
Passive Transport
• A concentration gradient is
the difference in the
concentration of a substance
from one location to another
• When there is a
concentration gradient, the
NET movement is DOWN
their concentration gradient from areas of higher
concentration to areas of
lower concentration
ANIMATION
Diffusion
• Movement of
molecules in a
fluid or gas
from an area of
higher
concentration to
an area of
lower
concentration
ANIMATION
Equilibrium
• Diffusion
continues until
the solution is at
equilibrium (no
concentration
gradient).
• Molecules still
move, but there
is no NET
movement.