Cell Transport

Pressure Potential,  p

• The physical pressure on a solution
• Unlike Ψs, Ψp can be positive or

negative relative to atmospheric pressure

Pressure Potential,  p

• For example:
• Water in the dead xylem cells of a transpiring

plant is often under a negative pressure
(tension) of less than – 2kPa.
• Water in living cells is usually under positive
pressure.
• The cell contents press the plasma membrane
against the cell wall, producing what is called
turgor pressure.
• The pressure is always positive (+ve)



Question 1

• The FIGURE below shows a plant cell, immersed in a
sucrose solution. The pressure potential ( p ) and the
solute potential (s) of the cell and the sucrose solution are
shown in the diagram.

340
-750

-1400

a. Define osmosis in 340
terms of water -750
potential
-1400
• Osmosis is the
movement of water
from a solution of
higher water potential
to lower water
potential, through a
partially permeable
membrane

b. Calculate the water potential 340
of this cell ( cell). Show -750
your calculation -1400

=s+p
= - 750 kPa + 340kPa
= - 410 kPa

c. Calculate the water potential
of sucrose solution. Show
your calculation

=s+p
= - 1400 kPa + 0kPa
= - 1400 kPa

c. State whether 340
water will move
into or out of the -750
cell.
-1400
• Water will move
out of the cell

Question 2

• Use the water potential equation to
predict movement of water between these
two cells:

• Cell A

– ψs = -500 kPa
– ψp = 200 kPa

• Cell B

– ψs = -600 kPa
– ψp = 100 kPa

Answer

• Water potential = solute potential +
pressure potential
Ψ = Ψs + Ψp

• For cell A : -500 + 200 = -300 kPa
• For cell B : -600 + 100 = -500 kPa
• Water always passes from the highest

water potential
(closest to 0) to the lowest water potential

(more negative)

Question 3

• A turgid plant cell was found to have a
ψs

value of -350 kPa.
a) What was the ψ?
b) What was the ψp?

Answer

Ψ = Ψs + Ψp
? = -350 kPa + ?
a) Ψ = 0
b) Ψs = -350 kPa
c) Ψp = +350
** Clues comes in the word turgid.
In a turgid cell, the water potential is zero.

Question 4

• A plasmolysed cell has a ψ of -650kPa
a) What is the ψp?
b) What is the ψs?

Answer :

a) Ψp = 0
b) Ψ = Ψs + Ψp
-650 = ψs – 0
ψs = -650 kPa
In a plasmolysed cell the ψp is zero, because
protoplast is not pushing against the wall.
In this situation the solute potential is the
only force acting, so the solute potential is the
water potential

Question 5

Cell A and cell B is immersed into distilled water and
allowed to reach equilibrium. Assume the changes in ψS
is negligible.
State the new value of:
a) Water potential for both cell?
b) Pressure potential, ψp for cell A?

Answer

a) Water potential , ψ for both cell?
0 kPa
b) Pressure potential, ψp for cell A?
Ψ = Ψs + Ψp
0kPa = -2500 kPa + ?
Ψp = 2500 kPa

Cell Transport

Learning outcomes:
At the end of the lesson, students should
be able to:
1.Explain active transport
2.Explain bulk transport:
• Endocytosis
• Exocytosis

Active transport

– Movement of ions

– Moves against their
concentration gradient

– Requires energy (ATP)
and carrier protein

ATP

• Active transport
is performed by
specific proteins
embedded in the
membranes called
carrier protein.

ATP

Active Transport

ATP powers the active
transport by shifting a
phosphate group from ATP
to ADP.
• Phosphate group bind to
carrier protein
• Phosphorylation of carrier
protein may induce a
Conformation change in the
carrier protein that
translocate the solute
across the membrane

• The sodium- Na+
K+
potassium pump

actively maintains

the gradient of
sodium (Na+) and
potassium ions (K+)

across the

membrane

• An animal cell ATP
has higher
concentrations of
K+ and lower
concentrations of
Na+ inside the cell

• The sodium-
potassium uses
the energy of one
ATP to pump
three Na+ ions out

A specific case of active transport:
Sodium-Potassium Pump

SODIUM-POTASSIUM PUMP: A SPECIFIC CASE OF ACTIVE
TRANSPORT

1. Three cytoplasmic Extracellular [Na+] high
Na+ bind to carrier fluid [K+] low
protein of sodium
potassium pump

2. Na+ binding Cytoplasm
stimulates
phosphorylation of [Na+] low
carrier protein by ATP [K+] high

2. Phosphate group is transferred from ATP to
carrier protein ATP - ADP

Na+
Na+

Na+

ATP

P
ADP

3. Phosphorylation causes carrier protein to change

conformation, releasing 3 Na+ across the membrane to the
extra cellular fluid/outside of cells

Na+

Na+

Na+

P

4. Two extracellular K+ bind to
carrier protein and trigger the
phosphate group to release

K+
K+

P
P

5. Loss of the phosphate restores the
protein’s original conformation

K+
K+

6. Phosphate release
causes carrier
protein to return to
its original shape.
Two K+ ions are
released inside the
cell

K+
K+

Sodium-potassium pump

• Review: Passive and active transport compared

Passive transport. Substances diffuse spontaneously Active transport. Some transport proteins act as pumps,
down their concentration gradients, crossing a moving substances across a membrane against their
membrane with no expenditure of energy by the cell. concentration gradients. Energy for this work is usually
The rate of diffusion can be greatly increased by transport supplied by ATP.
proteins in the membrane.

ATP

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

BULK TRANSPORT

• Involved endocytosis and phagocytosi
exocytosis s

Endocytosis (phagocytosis

and pinocytosis)

• Influx of material into the cell
through the invagination of plasma
membrane

• Using energy

Exocytosis

• Substance free from cells

• Using energy

Endocytosis

Influx of material into the cell through
plasma membrane to form a
vesicle or vacuole

• Two types of endocytosis:
1. Phagocytosis
2. Pinocytosis

Phagocytosis

EXTRACELLULAR CYTOPLASM 1 µm
FLUID
Pseudopodium

Pseudopodium
of amoeba

“Food” or Bacterium
other particle Food vacuole

Food
vacuole

An amoeba engulfing a bacterium via
phagocytosis (TEM).

PINOCYTOSIS

• Material taken up is in solid form

• Folds of plasma membrane surround particle to be
ingested, forming small vacuole around it (pseudopodia)

• The contents of the vacuole are digested when the
vacuole fuses with a lysosome containing hydrolytic
enzyme

Example of phagocytosis

Macrophage
• White blood cells

engulf invading
microorganisms

Pinocytosis

•It is used for the intake of dissolved materials rather than
solids

•Tiny droplets of fluid are trapped by folds of plasma
membrane, which pinch off into the cytosol as small fluid-
filled vesicles

• Contents of these vesicles are then slowly transferred to
cytosol and the vesicles become progressively smaller

PINOCYTOSIS 0.5 µm

Plasma Pinocytosis vesicles
membrane forming (arrows) in
a cell lining a small
Vesicle blood vessel (TEM).

Example of pinocytosis

• Reabsorption
of amino acid
by the
proximal
convoluted
tubules in
nephron

Differences between phagocytosis and
pinocytosis

Phagocytosis Pinocytosis

Material taken is solid form Material taken is in liquid
form
Involve formation of
phagocytic vacuole /food Involve formation of
vacuole/ phagosome pinocytic
Material are digested and vesicle/pinocytosis vesicle
absorb into cytoplasm
Dissolved substances fluid
Involve lysosome absorb directly into
cytoplasm

Not involve lysosome

Exocytosis

• In exocytosis, vesicles

migrate to the plasma

membrane, fuse with

it, and release their Phagocytosi
contents s

• Waste materials may
be removed from cells,
such as solid,
undigested remains
from phagocytic
vacuoles, or useful
materials may be
secreted

Examples of exocytosis

• The secretion of
hormones from
pancreatic cells

• The secretion of
hormones from
endocrine
glands