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• Explain the various transport
mechanisms across the membrane :
Passive Transport
- Simple diffusion
- Facilitated diffusion
- Osmosis
At the end of the lesson, you should be able to :
Objective:
Cell Structure and Function
Cell Transport
Passive
transport
Active
transport
diffusion
facilitated
diffusion
osmosis
endocytosis
exocytosis
phagocytosis pinocytosis
compare
CHAPTERREVIEW
Sodium potassium
pump
Passive transport
Facilitated diffusion
Diffusion Osmosis
Not require energy
The diffusion of a substance across a biological
membrane without using energy.
The rate of diffusion depends on :
Concentration gradient.
Surface area
Distance over which
diffusion take places
Size of diffusing molecule
Medium
Temperature of the solution
SIMPLE DIFFUSION
Involve nonpolar/ uncharged molecules
E.g: hydrocarbons, carbon dioxide, oxygen
Cross phospholipidbilayer easily
SIMPLE DIFFUSION
Diffusion across plasma membrane :
Permeability of Membrane
Small non-polar moleculeSmall polar
molecule
Large polar
molecule
Charged
molecule
i. Gaseous exchange
Oxygen diffuse out from
alveoli into blood
capillaries
Carbon dioxide diffuse
into alveoli from blood
capillaries.
SIMPLE DIFFUSION
ii. Some digested
food diffuses
across the gut
wall into blood.
SIMPLE DIFFUSION
iii. Mineral salt diffuses from
water in the soil into root
hair.
SIMPLE DIFFUSION
Channel
Protein
Carrier
Protein
Movement of solutes across a
membrane with the help of transport
proteins.
Characteristics of facilitated diffusion :
1. i. Its action is specific
2. ii. Passive process
3. Iii. It has saturation level at high
concentration of solute.
Aided by two types of protein :
Channel protein
Carrier protein
Charged ion such as Na+, K+ and Cl- cannot diffuse
easily across hydrophobic phospolipid bilayer.
Channel Protein is specific for one type of ion.
Each protein will only let one particular ion through.
Channel protein
Able to allow the diffusion across the membrane
of larger polar molecules such as sugar and
amino acid.
carrier protein
Specific molecule bind to a carrier protein.
carrier protein
Carrier protein alter the conformation.
carrier protein
Moving the solute across the membrane.
carrier protein
The solute is released on the other side of the
membrane down its concentration gradient.
carrier protein
Simple diffusionLipid-soluble moleculeSmall non polar moleculeEg. O2, CO2
Channel proteinSmall polar molecule, [eg. water]Small charged molecule, ions[eg. Na+, K+]
Carrier protein
larger polar molecules
Eg. Glucose
Movement of water molecules
across a selective permeable
membrane down a water potential
gradient.
Water molecules possess kinetic energy.
In liquid or gases form, they move about very
rapidly in random directions.
Term given to the tendency for water molecules
to enter or leave the solution by osmosis.
Derived from thermodynamics and is a measure
of the free kinetic energy of water molecules in
solution.
Pure water has highest water potential value.
Ψw = 0
Dissolving solute molecules into pure water is to
reduce the concentration of water molecules –
water potential get lower
Solution (at atmospheric pressure) has
negative water potential value.
Water diffuses from region of high water
potential ( less negative or zero value) to
a region of lower water potential (more
negative value).
Ψ = Ψs + ΨpΨs = solute potential
Ψp = pressure potential
Ψ in pure water = 0 kPa (highest value)
Water
potential (Ψ)
pressure
potential
(Ψp)
solute potential
(Ψs) = +
The concentration of dissolved substances inside the
cell is called the solute potential.
The more solute molecules present, the lower the
water potential.
The fewer solute molecules present, the higher the water potential.
The solute potential ( Ψs ) is the measure of the
reduction in water potential due to the presence of
solute molecules
Water passes into a plant cell, the cell
content start swell & push against the cell
wall.
The pressure that the cell wall develops is
called the pressure potential.The pressure potential value usually
positive.
Pressure Potential (Ψp) is the pressure exerted on
the cell content by the cell and cell membrane
The direction of water movement is into or out of cell depends on water potential of the cell solution
whether
it is more negative or less negative than the water potential of the external solution.
Example
Cell A
Ψs = - 1400 kPa
Ψp = 400 kPa
Cell B
Ψs = - 1800 kPa
Ψp = 600 kPa
a) Calculate water potential value for cell A &
cell B.
b) State the direction of water movement.
Two cell A and B separated by selectively permeable membrane.
Working:
Cell A
Ψ = Ψs + Ψp
Ψ = - 1400 kPa + 400 kPa
= - 1000 kPa
a) Calculate water potential value for
cell A & cell B.
Cell B
Ψ = Ψs + Ψp
Ψ = - 1800 kPa + 600 kPa
= - 1200 kPa
Working:
a) Calculate water potential value for
cell A & cell B.
Cell B
Ψ = -1200 kPa
Cell A
Ψ = - 1000 kPa
Water potential in cell A (- 1000 kPa) is higher than
cell B (- 1200 kPa).
Water moves by osmosis from cell A to cell B down
a water potential gradient
Working:
• State the direction of water
movement.
Movement of
water
molecules
across a
selective
permeable
membrane
down a water
potential
gradient.
hypotonic isotonic hypertonic
Low concentration of
solute
Same concentrationHigh concentration of
solute
High concentration of
solute relative to
another solution
Cell placed in,
water moves
out of the cell
causing cell to
shrivel.
Plant cell – plasmolysed
Animal cell – crenated
Hypertonic solution
Low concentration of solute relative to another solution
Cell placed in, water moves into the cell causing cell to swell and possibly explode.
Plant cell – Swell & become turgid
Animal cell– Swell / haemolysed (RBC)
Hypotonic solution
Same concentration of
solute as an another
solution
Cell placed in,
water moves into
and out of the cell
at the same rate
cell does not
change in shape.
Animal cell-normal
Plant cell- become
flaccid
Isotonic solution
Animal cell
in a
hypotonic
and
hypertonic
solution
Plant cell
in a
hypotonic
and
hypertonic
solution
Tonicity of solution
Concentration of solute Net movement of water
A B
A isotonic to B Same Same No net movement of water
molecules
A hypertonic to B
Higher Lower From B to A
A hypotonic to B
Lower Higher From A to B
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