• 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