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The Cell Membrane & Transport
The Cell Membrane
• The cell membrane is a skin-like
structure surrounding the cytoplasm
serving as a barrier to the cell’s
environment.
• The membrane serves two main
purposes
– It transports materials into the cell that the cell
needs for survival (“Gatekeeper”)
– Cell to Cell Communication
What does a cell need to transport?
Semi-Permeable
• The membrane only allows certain
molecules to pass through according to
their size
• Because only certain things can move
across the membrane it is said to be
semi permeable
• Small molecules
such as water can
go in and out
freely
• Large molecules
like Proteins and
Carbohydrates
cannot.
Composition of the Cell Membrane
• The cell membrane consists of:
1.phospholipid bilayer
2.proteins.
• The phospholipid bilayer looks like:
Phospholipid Bilayer
• Just like oils avoid mixing with water,
phospholipid molecules form a
phospholipid bilayer to avoid their heads
touching their tails.
• The outer surfaces consist of hydrophilic
heads (interacting with the watery
environment inside and outside the cell)
• The center of the bilayer consists of
hydrophobic fatty acids
Phospholipid Bilayer
Fluid Mosaic Model
• Together the phospholipids, proteins
and other membrane components form
a dynamic structure referred to as a
Fluid Mosaic.
• Membranes are fluid like rather than
sheets of molecules locked rigidly into
place
• Most of the protiens float freely like
“icebergs” in a “sea” of phospholipids
Membrane Transport
• There two main ways to move things
across the membrane
– Passive Transport
– Active Transport
Concentration Gradient
• The movement of molecules across the
membrane is affected by their
concentration gradient -the difference in
concentration on both sides of the
membrane.
Membrane Transport
• Passive Transport
• Molecules move on their own from an area of high
concentration to low concentration
• Requires No Energy
Membrane Transport
• Active Transport
• Molecules are moved from an area low
concentration to high concentration
• Requires Energy
Active Transport
-Low to High w/ Energy Used
Passive Transport
-High to Low/ No Energy
Needed
Passive Transportation
• Two examples of passive
transport:
–Simple Diffusion
–Facilitated Diffusion
Simple Diffusion
• During Simple Diffusion molecules move
randomly from an area of high
concentration to an area of lower
concentration until they are evenly spread
out (Dynamic Equilibrium)
Everyday Example of Diffusion
Facilitated Diffusion
• Some molecules need “help” to get
across the membrane
• Through Facilitated Diffusion,
transport proteins in the membrane
provide a pathway for certain
molecules to pass from high to low
concentration.
(Facilitate means to help)
Facilitated Diffusion
Active Transport
• Some substances need to be moved
from low to high concentrations, this
is accomplished by active transport
• Two examples of Active
Transport:
–Ion Pumps
–Bulk Transport
Pumps
• Pumps involve carrier proteins and
energy
• Carrier proteins transport molecules
against a gradient with the use of
energy.
Bulk Transport
• Cells also use energy to move things
into and out of the cell in bulk by folding
the membrane around what needs to be
moved
• Cells bring in bulk items by endocytosis .
– Phagocytosis-take in large particles
• Cells remove bulk items from the cell by
exocytosis
Membrane Transport
Summary
• There are three primary means that water
and other small molecules cross into or
out of cells.
– Diffusion
– Facilitated Diffusion
– Active Transport
Passive Transport
• All play an important role in the regulation
of body fluids and many other
physiological functions.