Cell Transport

CELL TRANSPORT

TRANSPORT ACROSS MEMBRANE

Membranes are selectively permeable

•Hydrophobic molecules:
Hydrocarbons, CO2, O2 dissolve in the
phospholipid bilayer cross easily

•Ions and polar molecules such as glucose &
other sugars, waters and ions cross with
difficulty. Overcome by transport proteins

Transport across membrane

Characteristics of Plasma
membrane:

• Allows small, uncharged
molecules to pass via simple
diffusion
• Movement is controlled by a
concentration gradient
• Until the concentration is
the same on the two sides
of membrane

Characteristics of Plasma membrane:

• DOES NOT allow K+ to pass
across it even though there is a
concentration gradient
• K+ is repelled by the
hydrophobic interior of the
plasma membrane
• Therefore charged ions and
large molecules cannot pass
the membrane by simple
diffusion

Fig. 8.16 Both diffusion and facilitated diffusion are forms of passive transport of molecules down
their concentration gradient, while active transport requires an investment of energy to move
molecules against their concentration gradient.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Types of transport across
membrane

Passive Transport

• Includes
- Diffusion/Simple diffusion
- Facilitated diffusion
- Osmosis

Passive transport: Simple
Diffusion

• Movement of small, uncharged (nonpolar)
solute molecules e.g. O2, CO2

• Down the concentration gradient // without
using any energy ( ATP )

• • Until equilibrium is achieve.

Passive transport: Simple
Diffusion

• Net diffusion stops when concentration on
both sides is equal (uniform distribution of
particle.)

• During this time:

• Equilibrum is reached.
• Molecules continue to move but NO NET CHANGE
in concentration.

Passive transport: Simple
Diffusion

• E.g. : Oxygen,O2 and carbon dioxide, CO2
• Without using any energy ( ATP ).
• Move in between phospholipids molecule
without the aid of plasma protein.

Passive transport: Simple
Diffusion

DIFFUSION

Facilitated diffusion

• Movement of specific ion or polar molecules

• Down the concentration gradient

• No energy (ATP) is used in the process

• It is much faster than simple diffusion which assist
by specific transport proteins (carrier protein and
channel protein) which speed the movement of
molecules across the plasma membrane

- Eg. Glucose, amino acid, water and chloride ion

Facilitated diffusion

• Movement of specific ion or polar
molecules

• From higher concentration gradient to low
• concentration gradient/Down the

concentration gradient

• No energy (ATP) is used in the process

Facilitated diffusion

• It is much faster than simple diffusion
which assist by specific protein
(carrier protein and channel protein) which
speed the movement of molecules across the
plasma membrane.
• E.g.: Glucose, amino acid, water and chloride ion

Facilitated diffusion

• E.g.: TRANSPORT OF WATER
• Water can move in between

phospholipids molecule and water
channel protein called aquaporin.
• However the movement is faster through
channel protein than in between
phospholipids

Facilitated diffusion

• Most transport proteins are very specific
because they transport only particular
substances.

• Two types of transport proteins are:
1. Channel protein: Na+, K+, Ca2+, Cl-,

HCO3- ions, water molecules.
2. Carrier protein : glucose, amino acid.

Channel protein

• Channel protein is transmembrane protein.
• It provide corridors that allow a specific

molecule to cross the membrane.

• A channel protein has a channel through which
water molecules or ions can pass

Carrier protein

• Carrier protein transport a
substance down the concentration gradient from one

side to the other side by
changing their conformation.
• Carrier protein alternate
between two conformation.
• Solute is transported across
the membrane as the shape
of the protein changes.



Osmosis

• Movement of water molecule
• From higher water potential to lower water

potential
• Through semi permeable membrane
• Without energy (ATP)

Osmosis

• FIGURE: OSMOSIS - Water molecules diffuse from
hypotonic solution

• (region of high water potential) to the hypertonic solution
(lower

• water potential) across the semi-permeable membrane.

Osmosis

• Movement of Water
• Three TYPE of Definition :

1. Based on water concentration
2. Based on water potential
3. Based on solution concentration

Osmosis: Definition based on
Water Concentration

1. The movement of water molecules
2. From a region of high water concentration to a

region of low water concentration
3. Through a partially permeable membrane

Osmosis: Definition based on
Water potential

1. The movement of water molecules
2. From a region of high water potential to a region

of low water potential

3. Through a partially permeable membrane

Osmosis: Definition based on
Solution Concentration

1. The movement of water molecules
2. From solution with low solute to solution high

solute
3. Through a partially permeable membrane

FIGURE: The solute molecules are too large to pass
through the pores in the plasma membrane. So,

equilibrium only can be achieve by the movement of
water molecules.

Osmosis

•Plasma membrane permeable to water BUT
NOT to solute
• Solute = dissolved particle
• Solvent = liquid medium in which particles may

be dissolved
• Solute + solvent = solution

The total of all dissolved particles is called
• OSMOTIC POTENTIAL

How we can predict
THE DIRECTION OF OSMOSIS
when a cell is surrounded by a

particular solution?

Types of solution

Three Type Of Solution :
1. Hypotonic
• Solution with lower concentration of solutes
• Less dissolved particle.
2. Hypertonic
• Solution with higher concentration of solutes.
• More dissolved particle.
3. Isotonic
• Solution with equal solute concentrations.
• Equal number of solute particle.
**When two solutions are isotonic, water molecules

move at
equal rates from one to the other, with no net osmosis

Plant and animal cells condition in
different concentration of solution

Isotonic Solution

• When we put a cell in to the isotonic
solution :
1. No net water movement
2. The cell does not change in shape

• Animal cell is Normal

Volume of cell stable
No net movement of water
Water flows at the same rate in both direction.

• Plant cell become Flaccid

Hypertonic Solution

• When we put a cell in to the hypertonic
solution :

1. Water diffused out of cell into solution.
2. Cell loose water.
3. Volume of cell shrinks
• Plant cell – Plasmolyzed
• Animal cell – Shriveled

Hypotonic solution

• When we put a cell in to the hypotonic
solution :

1. Water diffused into cell from solution. Cell gain
water.

2. Cell expands, swell or it may burst.
• Plant cell - Swell and turgid
• Animal cell - Swell and lysed
E.g. : Red Blood Cell = Haemolysis

Haemolysis Turgid and
and Crenation Plasmolysis

Outside Inside Outside Inside Outside Inside
cell cell cell cell cell cell

(a)

No net water Net water movement Net water movement
movement out of the cell into the cell

(b)

Isotonic solution Hypertonic solution 10µ (c)
Hypotonic solution

(a) Plasma (b) Nucleus (c)
membrane Vacuole
Vacuole
Plasma
Vacuolar membrane
membrane
(tonoplast)

Cytoplasm



Comparison between
Plasmolysis and Haemolysis

PLASMOLYSIS HAEMOLYSIS

• Occurs in plant cells • Occurs in erythrocytes

• Net movement of water • Net movement of water
out from the cell move into cell
vacuole.
• Cell surface membrane
• Plant cell shrink plasma lysed and cell contents
membrane pulls away released
from the cell wall
• Cell in hypotonic
• Cell in hypertonic solution
solution

Mechanism of passive transport across
cell membrane.

Process Passage How it works Example
Diffusion through
membrane Random motion that produce Random motion that
net produce net
Direct migration of molecules toward migration of
region of lower concentration molecules toward
Facilitated Channel region of lower
diffusion protein Polar molecule pass through concentration
channel protein Movement of ion
Carrier in and out of cell
protein Molecule bind to carrier protein Movement of
in glucose into cell.
Osmosis Direct membrane and transport across
the membrane down Movement of
concentration gradient. water.

Diffusion of water molecule
across
semi permeable membrane.

Passive Transport

Diffusion. Hydrophobic Facilitated diffusion. Many hydrophilic
molecules and (at a slow rate) very substances diffuse through membranes
small unchargedpolar molecules can with
diffuse through the lipid bilayer. the assistance of transport proteins,
either channel or carrier proteins.

2.4 Cell Transport

• Learning outcomes:
At the end of the lesson, students should be

able to:
1. Explain the concept of water potential and

calculation of water potential by using
formula:
• ψ= ψ s + ψ p

WATER POTENTIAL

• Water potential is a way of quantifying
osmosis

• We can calculate which way water will
move and how fast

• Water always moves from high water
potential to low water potential

WATER POTENTIAL

• Water potential is a tendency for water to enter
or leave the solution by osmosis.

• Water molecule have a kinetic energy. Water
molecule will collide with cell membrane
creating a pressure called water potential.

• We can calculate which direction water will
move.

• Water potential is measures in unit of pressure:
Pa (pascals) or kPa (kilopascals)

WATER POTENTIAL

• Symbol is ψ (the Greek letter psi)
• For a solution or animal cell,
water potential, ψ = solute potential, ψ s
• In a plant cell another factor comes which is
pressure potential, ψ p.
• Water potential is measures in unit of
pressure: Pa (pascals) or kPa (kilopascals)

Water Potential Equation

• The equation for water potential :

Water potential = Solute + Pressure

potential potential

= s + p

• Symbol for water potential = 
NOTE:

– Solute potential,  s is always negative value
– Pressure potential,  p is always positive value

-360 -340 -320 0

Increase solute concentration

• Pure water has the maximum water potential, 0
• All solutions have lower water potentials than pure

water and therefore have negative values of  (at
atmospheric pressure and a defined temperature)
• Water always moves from a region of higher water
potential to one of lower water potential
• Solute potential is negative and pressure potential is
usually positive

Solute Potential,  s

• Solute Potential,  s is proportional to the
number of dissolved solute molecules.
• Also called osmotic potential because solutes

affect the direction of osmosis.
• Solute may be any type of dissolved chemical.
• Solute Potential, ψ s is always negative (-ve)

Solute Potential,  s

What happens when solutes are added to pure
water?

• The solutes bind with water molecules.
• Reducing the number of free water molecules

and lowering the capacity of the water to do
work
• Thus, adding solute always lowers the water
potential and Ψs of a solution is always
negative.