Lesson 4.2
Cell’s Transport
Mechanism I: Diffusion
and Osmosis
General Biology 1
1/2
Science, Technology, Engineering, and Mathematics
Water is a very
essential substance
for life to persist.
2
Learning Competency
At the end of the lesson, you should be able to do the following:
Explain the transport mechanisms in cells
(diffusion and osmosis) (STEM_BIO11/12-Ig-h13).
3
Learning Objectives
At the end of the lesson, you should be able to do the following:
● Explain the difference between diffusion and
osmosis as molecules cross the membrane.
● Differentiate hypotonic, isotonic and hypertonic
solutions for animal and plant cells.
4
How does diffusion affect the
movement of substances into and
out of the cell?
5
Cell’s Transport Mechanism I
D
I
F
F
U
S
I
O
N
Simple Diffusion
Facilitated Diffusion
Area of
higher
concentratio
n
6
Cell’s Transport Mechanism I
D
I
F
F
U
S
I
O
N
Simple Diffusion
Facilitated Diffusion
Equilibrium is
achieved
7
Cell’s Transport Mechanism I
D
I
F
F
U
S
I
O
N
Simple Diffusion
Facilitated Diffusion
Diffusion involves the movement of molecules in a solvent from an area of
higher solute concentration to an area of lower solute concentration which
eventually results in the state of dynamic equilibrium.
8
Cell’s Transport Mechanism I
D
I
F
F
U
S
I
O
N
Simple Diffusion
no energy
required
passive
transport
Facilitated Diffusion
high to low
concentration
movement of
molecules
state of dynamic
equilibrium
9
Cell’s Transport Mechanism I
D
I
F
F
U
S
I
O
N
Simple Diffusion
Facilitated Diffusion
Molecules such as ions and water
outside the cell are being transported
across the cell membrane through
channel proteins via facilitated
diffusion.
10
Cell’s Transport Mechanism I
D
I
F
F
U
S
I
O
N
Simple Diffusion
Facilitated Diffusion
The channel protein
facilitates the movement of
molecules hence the term
“facilitated diffusion”.
11
How does osmosis play an
important role in different cellular
processes?
12
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
Hypertonic
Semi-permeable
membrane
High water, low
solute concentration
Low water, High
solute concentration
13
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
Hypertonic
Movement is from
the region of high
water
concentration to
one with lower
water
concentration.
14
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
Hypertonic
Osmosis is similar
to diffusion.
However, osmosis
refers to the
movement of
water from higher
to lower
concentration.
15
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
Hypertonic
ability of a surrounding
solution to cause a cell
to gain or lose water
relative concentration
of solutes in fluids
Tonicity
depends on the
concentration of
solutes
can be isotonic,
hypotonic, or
hypertonic
16
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
solute
molecules
outside the cell
Hypertonic
solute
molecules
inside the cell
Isotonic solution
Solutions of equal solute
concentration
17
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
Hypertonic
Isotonic solution
is when water
molecules move at the
same rate in both
directions.
18
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
solute
molecules
outside the cell
Hypertonic
solute
molecules
inside the cell
Hypotonic solution
has lower solute concentration
and has higher water
concentration (i.e., less solute,
more water).
19
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
Hypertonic
Hypotonic solution
can cause the cell to
swell or even burst due
to the intake of water
more than the cell can
accommodate.
20
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
solute
molecules
outside the cell
Hypertonic
solute
molecules
inside the cell
Hypertonic solution
has higher solute concentration
and has lower water concentration
(i.e., more solute, less water).
21
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
Hypertonic
Hypertonic solution
can cause an animal
cell to shrink or
shrivel due to water
loss.
22
Cell’s Transport Mechanism I
Isotonic
O
S
M
O
S
I
S
Hypotonic
Hypertonic
Plant cells, relative to animal cells, do not change cell size that much with
varying concentrations of water.
23
Marine fishes and other types of
animals drink water but excrete
salts across their gills. Why do
they do this?
24
Let’s Sum It Up!
● Diffusion is the movement of molecules down their
concentration gradient, i.e., from an area of higher
concentration to an area of lower concentration.
● A solution contains both the solute, which is usually
solid and a solvent, which is usually a liquid. A solute
added in a solvent will naturally diffuse to form a
solution.
25
Let’s Sum It Up!
● Diffusion is an example of passive transport in which
molecules are moved across the membrane without
spending energy.
● Facilitated diffusion involves the movement of
molecules across the membrane but with the aid of
either channel or carrier proteins. These molecules are
usually those that cannot cross due to their molecular
size or chemical nature.
26
Let’s Sum It Up!
● Osmosis is the diffusion of water across a selectively
permeable membrane from high to low concentration.
○ Solutions with an equal solute concentration are said
to be isotonic.
○ Hypotonic solution has a lower concentration of
solutes than inside the cell.
○ Hypertonic solution is one with a higher
concentration of solute outside of the membrane.
27
Let’s Sum It Up!
Diffusion and osmosis both involve movement of molecules without
spending energy.
28
Photo Credit
Bibliography
Hoefnagels, Marielle. Biology: The Essentials. 2nd ed. McGrawHill Education, 2016.
Mader, Sylvia S., and Michael Windelspecht. Biology. 11th ed.
McGraw-Hill Education, 2014.
●
Slide 1: Rhizomnium punctatum lamina by Kristian
Peters -- Fabelfroh 09:12, 28 February 2007 (UTC) is
licensed under CC BY-SA 3.0 via Wikimedia Commons.
Reece, Jane B, Martha R. Taylor, Eric J. Simon, Jean L. Dickey,
and Kelly Hogan. Biology Concepts and Connections.
8th ed. Pearson Education South Asia Pte Ltd., 2016.
Simon, Eric J., and Jane B. Reece. Campbell Essential Biology.
5th ed. Pearson Education Inc., 2013.
Starr, Cecie, Christine A. Evers, and Lisa Starr. Biology Today
and Tomorrow. 4th ed. Cengage Learning Asia Pte Ltd,
2014.
29