THE ROLE OF THE CELL
MEMBRANE IN TRANSPORT
Section 2.2
Unit C
OBJECTIVES
define
and give examples for
movement of matter through the
cell membrane
 passive and active transport,
 facilitated diffusion,
 osmosis,
 endocytosis and exocytosis
REVIEW
 Cell
membrane/plasma membrane is a
phospholipid bilayer
 Phospholipids arranging so centre of
bilayer is hydrophobic (afraid of water)
and outside is hydrophilic (loves water)
 How are they arranged? Which way do
each part of the phospholipid arrange
itself?
 Proteins are embedded or attached to the
membrane
 Fluid-mosaic model
DIFFUSION: INTRODUCTION
Movement of particles from an area of high
concentration to an area of low concentration
 Moving towards equilibrium/sameness
 How can we increase the rate of diffusion (i.e.
how fast diffusion is occurring)?



Add energy- ex. Stir or heat
Ex. Opening a bag of coffee  releases aroma
molecules
DIFFUSION IN CELLS
Occurs across the cell membrane (water or
solutes)
 Difference between concentrations over the cell
membrane = concentration gradient
 Process called passive transport


No energy required for movement
PERMEABILITY OF MEMBRANES
Cell membrane is called selectively permeable
 Allows only certain particles pass (not all)
 Essential to maintain equilibrium and
function
 Semi-permeable  passage of materials
determined by size, charge, and solubility
 Ex. Membranes used in water treatment and
desalination
 What kinds of molecules might not get
through? Why not?

Involves
different
concentrations
In cells,
concentrations
separated by
membrane
Drives diffusion
and osmosis
Concentration
Gradient
Different
molecules/ions
move along
own
concentration
gradients
Involves
molecules/ions
of different
types
Fig
C2.15 p.
277
DIFFUSION LAB
Page 276 in your textbook
 Read through the lab.
 You will need to make a lab write-up. It must
include:








Title
The question (copy from book)
Come up with a hypothesis
List of materials
Write the procedure in your own words
Analyzing and Interpreting questions
Forming a conclusion
REVIEW
What is the process of diffusion?
 What is passive transport?
 What is concentration gradient?

OSMOSIS
Diffusion of water across a semi-permeable
membrane
 Example of passive transport
 From area of higher concentration to lower
concentration of water

PREDICTING MOVEMENT OF WATER
The movement of water can predicted by
determining what the amount of solute is
relative to the cell
 A large amount of solute (high concentration) =
low amount of water
 A small amount of solute (low concentration) =
high amount of water
 Recall: water moves from high to low

PREDICTING MOVEMENT OF WATER

Compare solute solution (3 types)
 1) hypertonic- higher concentration of solutes
than that in the cell
 Ex. Cell is in a salt solution: solution has less
water than the cell
 2) hypotonic- lower concentration of solutes
than that in cell
 Ex. Solution has more water than cell
 3) isotonic- same concentration of solutes in
solution as in cell

Ex. Solution has same amount of water as the cell
What would be the net movement of water in a
hypertonic solution? Hypotonic? Isotonic?
HYPERTONIC SOLUTION
HYPOTONIC SOLUTION
ISOTONIC
HOW OSMOSIS WORKS: AN ANIMATED
TUTORIAL

http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter
2/animation__how_osmosis_works.html
OSMOSIS AND ANIMAL CELLS
When cells lose water, they shrink (plasmolysis)
 When cells gain water, they swell and may even
burst (cytolysis)
 When there is no net change in water, cells
remain in homeostasis, or a state of equilibrium
(i.e. no change)

EXAMPLE: BLOOD CELLS

http://highered.mcgrawhill.com/sites/0073377988/student_view0/chapter3/hemolysis_and_crenati
on.html
EXAMPLE

Identify each picture below as hypertonic,
hypotonic or isotonic (which one is experiencing
plasmolysis, cytolysis, equilibrium?)
Isotonic
(equilibrium)
Hypotonic
(cytolysis)
Hypertonic
(plasmolysis)
OSMOSIS IN PLANT CELLS
Plant cells have a maximum amount of water that can
hold
 What makes it different than animal cells? What
different structure does it have?
 Movement of water into plant cell increases turgor
pressure
 Turgor pressure supports plant’s structure
 High turgor pressure
cells are turgid (firm)
 Low turgor pressure 
cells are flaccid
(soft, floppy)

EXAMPLE: PLASMOLYSIS IN ONION CELLS
http://www.youtube.com/watch?v=gYbt7hhIxPo&feature=related
&safety_mode=true&persist_safety_mode=1&safe=active

Plasmolyse onion rouge (red onion plasmolysis) –
note how the cell contents pulls in from the cell
wall:
EXAMPLE: PLASMOLYSIS IN ELODEA PLANT

http://www.youtube.com/watch?v=VK_YHakvho&safety_mode=true&persist_safety_mode=1&saf
e=active
FACILITATED DIFFUSION
For particles that are soluble with water
 Why would they need a special way across the
membrane?
 Uses proteins for diffusion – still passive
transport
 Channel proteins: pores for small molecules to
pass through
 Carrier proteins: attach to larger molecules,
change shape and physically brings it inside
the cell
 http://www.youtube.com/watch?v=s0p1ztrbXPY&
safety_mode=true&persist_safety_mode=1

ACTIVE TRANSPORT
Movement against concentration gradient
 What does this mean?
 Carrier proteins- act like a pump and use energy
 Like swimming upstream
 Energy from mitochondria
 Uses energy molecules called ATP


http://www.youtube.com/watch?v=STzOiRqzzL4&
feature=related
ENDOCYTOSIS AND EXOCYTOSIS
 For
very large molecules
 Cell uses vesicles
 Sac surrounding large particle
 Like vacuoles, but smaller and
temporary
ENDOCYTOSIS
Movement into the cell
 Steps:
 1) vesicle forms around particle
 2) cell membrane pinches off
 3) vesicle now inside cell

EXOCYTOSIS
Movement out of the cell
 For waste particles or cell products
Steps:
 1) vesicle surrounds particle in cell
 2) moves to membrane and fuses with it
 3) vesicle ruptures, releasing contents

ANIMATION

http://www.youtube.com/watch?v=4gLtk8Yc1Zc&
feature=related
SUMMARY VIDEO

http://www.youtube.com/watch?v=kfy92hdaAH0