THE CELL AND ITS ENVIRONMENT
Diffusion
Diffusion is defined as the movement of molecules of a substance from a region of
higher concentration to a region of lower concentration down a concentration gradient.
Diffusion is an important process for living things; it is how substances move in and out
of cells.
Rates of Diffusion
The rates at which substances diffuse through a cell wall or cell membrane will depend
on
i. Temperature
ii. Concentration
iii. Size of its molecules or ions
iv. The surface area across which the diffusion is occurring
v. State of matter
Temperature
Higher temperature increases the kinetic energy of the molecules. The higher the
temperature, the faster the rate of diffusion
Concentration Gradient:
The bigger the differences in the concentration of a substance on either side of a
membrane, the faster it will tend to diffuse.
This difference is called diffusion gradient or concentration gradient
Distance
Plant cells vary in their thickness and permeability. However, cell membranes are
usually of the same thickness (about 0.007 m).
Generally, the thicker the wall, the slower the rate of diffusion and vice versa
Size of Molecules or Ions
Generally, the smaller the molecular size, the faster the rate at which they diffuse
However, it may not be possible to predict the rate of diffusion from the size of
molecules/ ions alone because many molecules in solution attract water molecules
which increases their effective size
Surface Area
The rate of diffusion into a cell will depends on the cell’s surface area, the wider the
surface area, the faster the total diffusion.
, Activity
To demonstrate diffusion in liquids
1. Pour about 100cm3 of distilled water into a measuring cylinder. Leave it to settle
2. Use a pipette to add a small quantity of potassium permanganate solution gently to
the bottom of measuring cylinder. Leave it to stand for a few minutes
Result:
Eventually, the purple colour of Potassium Permanganate spreads throughout the
water medium
Interpretation
The molecules of potassium permanganate moved from a region of higher
concentration of the solute to a region of lower concentration of the solute until the
molecules are evenly distributed by the process of diffusion.
Biological Significance of Diffusion
1. The movement of gases between an organism and its surroundings as in
photosynthesis and respiration are as a result of diffusion along a concentration
gradient.
2. It is important in the distribution of food materials.
3. Helps in the elimination of waste.
Osmosis
Osmosis is defined as the movement of water molecules from a region of higher water
potential (dilute solution or lower concentration of the solute) to a region of lower
water potential (higher concentration of the solute or concentration of the solution)
through a semi-permeable / selectively permeable membrane.
OR
Osmosis is defined as the movement of water molecules from a dilute solution to a
concentrated solution through a semi-permeable / selectively permeable membrane.
If a concentrated solution is separated from the dilute solution by a semi–permeable
membrane, water diffuses across the membrane from the dilute solution by the process
of osmosis.
The weaker solution is said to be hypotonic which the stronger solution is described as
hypertonic when both solutions have the same concentration, they are described as
isotonic.
Diffusion
Diffusion is defined as the movement of molecules of a substance from a region of
higher concentration to a region of lower concentration down a concentration gradient.
Diffusion is an important process for living things; it is how substances move in and out
of cells.
Rates of Diffusion
The rates at which substances diffuse through a cell wall or cell membrane will depend
on
i. Temperature
ii. Concentration
iii. Size of its molecules or ions
iv. The surface area across which the diffusion is occurring
v. State of matter
Temperature
Higher temperature increases the kinetic energy of the molecules. The higher the
temperature, the faster the rate of diffusion
Concentration Gradient:
The bigger the differences in the concentration of a substance on either side of a
membrane, the faster it will tend to diffuse.
This difference is called diffusion gradient or concentration gradient
Distance
Plant cells vary in their thickness and permeability. However, cell membranes are
usually of the same thickness (about 0.007 m).
Generally, the thicker the wall, the slower the rate of diffusion and vice versa
Size of Molecules or Ions
Generally, the smaller the molecular size, the faster the rate at which they diffuse
However, it may not be possible to predict the rate of diffusion from the size of
molecules/ ions alone because many molecules in solution attract water molecules
which increases their effective size
Surface Area
The rate of diffusion into a cell will depends on the cell’s surface area, the wider the
surface area, the faster the total diffusion.
, Activity
To demonstrate diffusion in liquids
1. Pour about 100cm3 of distilled water into a measuring cylinder. Leave it to settle
2. Use a pipette to add a small quantity of potassium permanganate solution gently to
the bottom of measuring cylinder. Leave it to stand for a few minutes
Result:
Eventually, the purple colour of Potassium Permanganate spreads throughout the
water medium
Interpretation
The molecules of potassium permanganate moved from a region of higher
concentration of the solute to a region of lower concentration of the solute until the
molecules are evenly distributed by the process of diffusion.
Biological Significance of Diffusion
1. The movement of gases between an organism and its surroundings as in
photosynthesis and respiration are as a result of diffusion along a concentration
gradient.
2. It is important in the distribution of food materials.
3. Helps in the elimination of waste.
Osmosis
Osmosis is defined as the movement of water molecules from a region of higher water
potential (dilute solution or lower concentration of the solute) to a region of lower
water potential (higher concentration of the solute or concentration of the solution)
through a semi-permeable / selectively permeable membrane.
OR
Osmosis is defined as the movement of water molecules from a dilute solution to a
concentrated solution through a semi-permeable / selectively permeable membrane.
If a concentrated solution is separated from the dilute solution by a semi–permeable
membrane, water diffuses across the membrane from the dilute solution by the process
of osmosis.
The weaker solution is said to be hypotonic which the stronger solution is described as
hypertonic when both solutions have the same concentration, they are described as
isotonic.
