diffusion and osmosis
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Diffusion and osmosis
B M Subramanya Swamy M.Sc. B.Ed.
CIE Co ordinator & Examination OfficerKanaan Global School
Jakarta Indonesia
Diffusion and osmosis
• Crossing the cell membrane
• Diffusion
• Osmosis
• Active transport
Crossing the cell membrane
• Substances can cross the membrane because of the membrane’s property of being partially permeable
• That is it is permeable to some substances and not to others
• Some substances are able to move into the cell and some are allowed out
Methods of crossing the cell membrane
Crossing the membrane by Method
Active transport Movement of substances from a region of lower concentration to a region of higher concentration
Against the concentration gradient
Requires energy from cell respiration
Passive transport Movement of substances from a region of higher concentration to a region of lower concentration
Not against a concentration gradient
Does not require energy
Diffusion• Movement of molecules from a region of
higher concentration to a region of lower concentration down a concentration gradient
• Form of passive transport
• Diffusion stops when the concentration gradient is zero that is state of equilibrium is reached
Importance of diffusion Importance Activity
Gaseous exchange during respiration & photosynthesis
Plant and animal cells use oxygen during aerobic respiration
During photosynthesis carbon di oxide diffuses into the plants and oxygen diffuses out
Excretion of waste products
Waste products diffuse out of tissue
It is transported to excretory organ by the circulatory system
Absorption of digested food
Digested products diffuse into the blood stream
Excess mineral salts and vitamins are removed from blood stream by diffusion
Factors affecting rates of diffusionRate of diffusion Effect
Temperature Increase temp increase rate of diffusion
Size of particles Decrease molecule size increase rate of diffusion
Thickness of barrier Cell membrane in plant and animals are about the sameIncrease thicker cell wall decrease rate of diffusion
Concentration gradient Increase concentration gradient increase rate of diffusion
Total surface area of cell membrane Increase in surface area increase rate of diffusion
E.g. root hair cells microvillus in ileum
Osmosis
Definition
• Net diffusion of water across a selectively permeable membrane from one solution to another of lower water potential
• Movement involves water molecules
Water potential
• Measure of whether it is likely to lose or gain water molecules from another solution
• Distilled water has the highest possible water potential
• The difference between potential of two solution creates a water potential gradient
Reaction of the cells in different types of solutionTypes of solution Concentration as compared to cell sap Movement of water in the cell
Hypotonic More dilute than cell sap
Higher water potential than cell
Water flows into the cell
Cell become turgid
Isotonic Same concentration as cell sapZero water potential
No net flow of water
Cell becomes flaccid
Hypertonic solution Concentration of solutes is higher outside the cell
Lower water potential than cell
Water flows out of the cell
Cell becomes Plasmolysed
Effect of osmosis in plant cellHypotonic solution (placed in pure water)
Cell become turgid
Cytoplasm is pressed against the wall
Cell is prevented from bursting by the cell wall
Isotonic solution Cell unchanged
Hypertonic solution (placed in concentrated salt solution )
Cell is plasmolysed
Cytoplasm shrinks
Effect of osmosis in animal cellHypotonic solution (placed in pure water)
Cell swells
Finally bursts( hemolysed )
Isotonic solution Cell unchanged
Hypertonic solution (placed in concentrated salt solution )
Cell shrinks
Becomes wrinkled( crenated )
Active transport • Movement of particles(molecules & ions) through a
membrane from region of low concentration to region of high concentration
• Against a concentration gradient• Energy consuming ;uses oxygen from aerobic
respiration to produce energy• Increase the speed of ions /substances moving into
cell when diffusion isn’t fast enough• E.g. plants absorb mineral salts present as charged
ions in soil . The concentration of ions in soil is less than vacuole therefore uptake of ions is assisted by active transport across the root hair
