CELL AND ITS ENVIRONMENT
WALT:
• Describe diffusion
• Outline the factors controlling diffusion
• Compare the biological significance of
diffusion in plants and animals.
• Discuss osmosis and the conditions
necessary for osmosis to take place.
DIFFUSION
• This is the process by which molecules of gas or ions move from
a region of high concentration to a region of low concentration
until they are evenly distributed.
FACTORS CONTROLLING DIFFUSION:
The rate of diffusion is controlled by a number of factors which
include:
• State of matter: (Gas moves faster than the liquid and solids)
• Molecular size (Smaller molecules diffuses faster than big ones)
• Difference in concentration (the greater the differences in the
concentration of the molecules, the greater the rate of diffusion.
• Temperature: High temperature increase the speed at with
molecules move (The higher the temperature, the faster the rate
of diffusion)
BIOLOGICAL SIGNIFICANCE OF DIFFUSION IN
PLANTS
1. Movement of carbon dioxide through the
stomata of the leaves during transpiration and
photosynthesis.
2. Movement of oxygen into the leaves through
the stomata during respiration
3. Movement of the manufactured food from
the leaves to other parts of the plants.
BIOLOGICAL SIGNIFICANCE OF DIFFUSION IN ANIMALS
1. Gaseous exchange in any cells and organisms
e.g. amoeba obtains oxygen by diffusion.
2. Exchange of nutrients and oxygen between a
foetus and the mother through the placenta.
3. Gaseous exchange in the lungs of mammals
4. Absorption of food from the small intestine
through the villi into the blood stream.
OSMOSIS
• Is the movement of water or solvent molecules from a
region of dilute or a weaker solution to a region of
concentrated of stronger solution through a selectively
permeable membrane (semi-permeable membrane).
• CONDITIONS NECESSARY FOR OSMOSIS TO TAKE PLACE
i. Presence of a stronger solution e.g. salt solution
ii. Presence of a weaker solution e.g. water
iii. Presence of a semi-permeable membrane
LIVING CELLS AS OSMOMETER
• A living cell is usually surrounded by a medium which
can be described as hypertonic, hypotonic and isotonic.
A. HYPERTONIC SOLUTION: When the cell is surrounded
by a stronger solution, water will be lost by the cell. The
cell shrinks.
B. HYPOTONIC SOLUTION: When a cell of a living organism
is surrounded by water or solution whose concentration is
lower, water passes into the cell by osmosis.
C. ISOTONIC SOLUTION: When the solute concentration of
the cell and its surrounding medium are the same, the
solution is said to be isotonic. No net movement of
molecules.
OSMOTIC PRESSURE
• This is the pressure which moves water across
a membrane into a solution of a higher
concentration. Osmotic pressure draws water
into the cell. The pressure which a solution
can potentially exert is called its Osmotic
Potential.
OSMOTIC PROCESSES IN LIVING ORGANISMS
• OSMOTIC PROCESSES IN PLANTS
i. Absorption of water from the soil by the root
ii. Movement of water in and out of guard cells of stomata
iii. Movement of water from one cell to another
• OSMOTIC PROCESSES IN ANIMAL
i. Reabsorption of water from glomerular filtrate in the
kidney
ii. Absorption of water by the colon
iii. Loss of water through sweating
Ongoing assessment
• Differentiate between osmosis and diffusion
OTHER PROCESSES OF NATURAL
SIGNIFICANCE
• PLASMOLYSIS: is the outward movement of
water from living cells when they are placed in
a hypertonic situation. It involves the
withdrawal of water from living cells up to the
extent that it will result in the pulling away of
cytoplasm from the cell membrane of cell-wall
(the cytoplasm shrink) and the whole cell wall
collapse).
• HAEMOLYSIS: is defined as the process by which
red blood cells burst as a result of too much water
passing into it when placed in hypotonic solution.
• TURGIDITY: is defined as the condition in which
cells absorbs plenty of water up to a point where
the cell is fully stretched. Turgidity is useful to the
plants because it makes plants to stand erect, gives
support to the stem, leaves, flowers and guard cells.
• FLACCIDITY: is the condition in which plants lose
water to their surroundings faster than they can
absorb when plant loses more water, it is said to be
flaccid. Flaccidity may cause wilting & death.
EVALUATION
• Define (i) diffusion (ii) osmosis (iii) hypotonic
solution (iv) hypertonic solution (v) isotonic
solution
• Outline the factors controlling diffusion.
• List 2 processes involving diffusion in plants
and animals.
HOMEWORK
• In a tabular form, differentiate plasmolysis
from haemolysis.
