Diffusion and Osmosis

What is diffusion?

• The movement of particles from a region of higher concentration to a region of lower concentration

Diffusion can take place :-

• Using solid as a medium

• Using liquid as a medium

• Using gas as a medium

• With or without a partially permeable membrane

Diffusion in solid

Example:- The colourless agar with

copper (II) sulphate crystals embedded turns blue after a few minutes.

Explanation:

- The copper (II) sulphate particles have diffused from a region of higher concentration to a region of lower concentration.

Diffusion in liquid

Example:- The water will take the colour of

the drop of food colouring

Explanation:- The drop of food colouring

contains many particles. (region of higher concentration)

- The water has no food colouring particles. (region of lower concentration)

- As the food colouring particles and the water particles mix, the food colouring particles will diffuse throughout the water.

Depress

Lower

concentration

Higher

concentration

Diffusion in gasExample:

The smell of the air freshener spreads across the room.

Explanation:The air freshener

particles diffuses across the room from a region of higher concentration to a

region of lower concentration.

Diffusion through a partially permeable membrane

Example:- The starch solution in the visking

tubing which is a partially permeable membrane changes from colourless to blue after some time.

Explanation:- There was a higher

concentration of iodine particles in the beaker.

- As a result, the iodine particles diffuses into the visking tubing.

- In the presence of starch, iodine will changes from reddish brown to dark blue.

Iodine

Starch Solution

Movement of sucrose molecules from A to BMovement of water molecules from A to B

Diffusion through a partially permeable membrane

Solutions A and B are separated by a partially permeable membrane, which allows both small and large molecules to pass through. Both the solvent (water) and the solutes (the sucrose molecules) can pass through it.

A B

permeable membranepermeable membrane

10 % sucrose solution

2 % sucrose solution

permeable membrane

sucrose molecules

water molecules

Movement of sucrose molecules from A to BMovement of water molecules from A to B

Both types of molecules are free to move across the membrane as the particles are in constant random motion.

Since solution A has a higher concentration of sugar, more sugar molecules will diffuse from solution A to solution B.

Also, solution B has a higher concentration of water molecules. Thus, more water molecules will diffuse from solution B to solution A.

Hence, there is a net movement of sugar molecules from solution A to solution B and a net movement of water molecules from solution B to solution A. After awhile, both solutions will have the same concentration of sugar and water molecules.

Example of Diffusion in Plants

• Diffusion in the absence of membranes

cros

s-se

ctio

n of

par

t of

leaf air spaces

high concentration of oxygen

low concentration of carbon dioxide

carbon dioxide diffuses in through stomata

oxygen diffuses out through stomata

Diffusion occurs during photosynthesis in the absence of membranes.

Carbon dioxide enters the leaves through the stomata, moving into air spaces in the leaves.

This is because the carbon dioxide concentration is higher outside the leaves than in the air spaces inside the leaves.

Similarly, oxygen given off during photosynthesis diffuses out of leaf air spaces as there is a lower concentration of oxygen outside the leaves.

Diffusion in Animals

• Diffusion occurs when digested food is absorbed into the blood stream in the small intestine.

• There is a higher concentration of digested food in the small intestine, so digested food molecules will diffuse into the blood stream.

25.2 Osmosis

• Osmosis is the net movement of water molecules from a region of higher concentration of water molecules (higher water potential) to a region of lower concentration of water molecules (lower water potential) through a partially

permeable membrane.

5% sucrose solution (higher concentration of water molecules; higher water potential)

Osmosis in non-living systems

A

Partially permeable membrane

Solutions A and B are separated by a partially permeable membrane. The membrane allows smaller water molecules to pass through but not the larger sugar molecules.

Water molecules are small enough to pass through the membrane

10% sucrose solution (lower concentration of water molecules; lower water potential)

B

Sucrose molecules are too big to pass through its pores.

During osmosis, the water molecules will flow from solution B with higher concentration of water molecules (higher water potential) to solution A with lower concentration of water molecules (lower water potential).

As water molecules flow, the volume of solution A rises and the volume of solution B drops. When the concentrations of both solutions A and B are the same, there will be no net movement of water molecules.

Osmosis in living organisms

• Recall some properties of cells:o Cell membrane of both plant and animal cells is

partially permeable.o The plant cell wall is made of cellulose. It is

permeable.

o The plant cell has a relatively rigid (firm) cell wall.

Osmosis in living organisms

• What happens to animal cells when they are placed in distilled water?

• Animal cells o Animal cells have elastic membranes. When the

water molecules flow in, the animal cells will swell and eventually burst.

water moves in

Water potential outside the cell is higher than that in the cytoplasm.

Cell expands and eventually bursts.

Osmosis in living organisms

• What happens to plant cells when they are placed in distilled water?

• Plant cells o Plant cells have strong, rigid cells walls which prevent the cells

from expanding too much.o When water molecules flow in, the contents in the cell press the

cell wall.o The water creates a pressure on the cell wall of the plant cell.

This is called turgor pressure and keeps the plant tissues turgid.

o Soft tissues in plants depend on turgor for support. If plants lose too much water, they will wilt.

Osmosis in living organisms

• Plant cells

Water potential outside the cell is higher than that in the cell sap.

Cell expands and becomes turgid. The rigid cellulose cell wall expands slightly only. This prevents the cell from bursting.

water moves in

Osmosis in living organisms

• What happens to animal cells when they are placed in concentrated solution?

• Animal cells o Animal cells will shrivel up as they lose water.

Water leaves the cell by osmosis

Concentration of water molecules outside the cell is lower than that in the cytoplasm.

Cell shrinks and becomes soft. It is dehydrated.

Osmosis in living organisms

• What happens to plant cells when they are placed in concentrated solution?

• Plant cells o When the concentration of water potential of the cell

is higher than that of the surrounding solution, water leaves the plant cells by osmosis.

o The vacuoles shrink and the cell contents pulls away from the cellulose cell walls. The plant cells lose turgor pressure and become flaccid.

Osmosis in living organisms

• Plant cells

Concentration of the water molecules outside the cell is lower than that in the cell sap.

Cell contents pull away from the cell walls and the cell becomes flaccid.

Water leaves the cell by osmosis.

Osmosis occurs in:

a) Our cells- Our cells need water.

- Our cells take in water by means of osmosis via our cell membrane which acts as a permeable membrane.

b) Plant cells- The root hair cells take in water from the soil by

osmosis.- This is because the concentration of water in the soil is

higher than that in the cell sap of the root hair cells.- Once inside the roots, this water then moves from cell to

cell by osmosis until it reaches the xylem.

osmosis in cells.swf

Magnified view of part of a root

Root hair cell

Soil Root

soil particles

Water & mineral salts Root cells

root hair cell

Soil Root

soil particles

Water & mineral salts Root cells

root hair cell

Soil Root

soil particles

Water & mineral salts Root cells

root hair cell

Soil Root

soil particles

Water & mineral salts Root cells

root hair cell

Soil Root

soil particles

Water & mineral salts Root cells

root hair cell

Soil Root

soil particles

Water & mineral salts Root cells

root hair cell

How then is mineral salts taken in by the root hair cells?

• By diffusion.

• There is a higher concentration of mineral salts in the soil than the sap of the root hair cell.

• Hence mineral salts diffuses into the root hair cell.