Chapter 3 Diffusion and Osmosis by yurtgc548



By the end of the lesson, you should be able to
• define and describe diffusion, osmosis and active
• to describe the idea of a partially permeable
• assess the importance of water potential gradient in
  water uptake by plants
• explain the effects of osmosis in plant and animal
• define and discuss importance of active transport in
  ion-uptake by root hairs and glucose uptake by cells
  in the villi

• Recall:
~ A cell is the basic unit of life
~ Many complex reactions take place inside the cell
~This means that many raw materials must be brought into
  the cell
~ And new products formed by the chemical reactions, and
  waste materials, must exit the cell.

• How do the raw materials get into the cell?
• How do the new products leave the cell?
• How are the waste materials removed from the cell?

• Materials move across the partially permeable cell
                       diffusion osmosis
• By processes called _________, _________ and
   active transport
Diffusion is the net movement of ions or
solute molecules from a region of higher
concentration to a region of lower
concentration, down a concentration
                                  molecule moves from a region of high
                                concentration to that of low concentration

 region of high concentration

        of molecule X

                                                                                   region of low
                                                                                  concentration of
                                                                                    molecule X
Diffusion in gases

   A.                        B.
        Gas molecules             The movement
        move from a region        continues until
        of higher                 they are evenly
        concentration to          spread
        that of a lower
Diffusion in liquids

    A. Dissolved particles of a   B. The particles
       substance in a liquid         continue to move
       move from a region            until they are evenly
       where they are more           distributed.
       concentrated to a region
       where they are less
Importance of Diffusion

    Initially, the concentration of   Later, an equal concentration of
    a substance outside the cell is   the substance inside and
    higher than inside the cell.      outside the cell is attained.
    The particles diffuse across
    the cell membrane.
Importance of Diffusion
• Living cells continuously use up oxygen during aerobic
• The concentration of oxygen inside the cell falls.
• Oxygen molecules diffuse into the cell until the oxygen
  concentration is raised again.
• During respiration, carbon dioxide is produced.
• The concentration of carbon dioxide rises and creates a
  concentration gradient.

   carbon                                                        oxygen

       Oxygen is supplied to Amoeba by diffusion. Diffusion enables
        metabolic wastes such as carbon dioxide to be excreted out.
Factors which Affect the Rate of Diffusion
1. Temperature: An increase in temperature results in a higher rate of
    1. Temp   Molecules move faster  Rate of diffusion 
2. Size of particles: Small molecules or ions diffuse faster than large
    1. Particle size  Energy to move particle   Rate of diffusion 
3. Thickness of the barrier: Membranes of plant and animal cells are of
   about the same in thickness and permeability. Plant cell walls vary in
   their thickness. The thicker the cell walls, the slower the rate of
    1. Thinner membrane  Higher rate of diffusion
4. Concentration gradient: The greater the concentration gradient, the
   higher the rate of diffusion
    1. Steeper concentration gradient  Higher rate of diffusion
5. Surface area: The rate of diffusion into a cell depends on the total
   surface area of the cell membrane.
    1.  surface area   rate of diffusion.
                                    Diffusion in our body

• Recall: Our cells have a partially permeable membrane
• Partially permeable membrane

               Allows some substance through and not others
• How do these substances pass through?
                             Practical Application

• Kidney dialysis
• Waste products are removed from
  the blood by diffusing across
  artificial membrane in the
• Blood cells and other large
  protein are retained
Osmosis is the diffusion of water across a selectively permeable
membrane, from a high water potential to a low water potential.


The selectively permeable
membrane on the side of the
water is being continuously
bombarded by water
molecules which pass
through.                          water
This causes an increase in the
volume of sucrose solution
and a decrease in the volume
of water.

                                       water    sucrose solution
Water Potential

                           The water potential of a solution
                            is a measure of the tendency of
                          water molecules to move from one
                                    place to another
   A dilute solution has a
 higher water potential than
   a concentrated solution
                           The difference in water potential
                            between two solutions is known
                             as the water potential gradient
Osmosis and Plant Cells

                          In plant cells, the cell sap contains
                       dissolved salts and sugar. If the cell sap
                       has a lower water potential than that of
                        the surrounding fluids, water from the
                              outside enters by osmosis.

    Turgor – A state when         Turgor pressure –
  a cell is firm or turgid due   The outward pressure
  to water entering the cell,     which the cell sap
      causing it to swell.         exerts against the
                                     inside wall of
                                        the cell
                      Importance of Turgor in Plants

2.   Changes in turgor of the guard cell causes the opening of the
                                  Turgor (II)

3.   Changes in turgor of the pulvinus (small swelling at the base of the
     leaflets) causes the folding of leaflets in the Mimosa
Osmosis and Plant Cells
         cell sap more concentrated
            than outside solution                  Turgid cells. The
                              water enters by      cytoplasm is pressed
                                 osmosis           against the cell wall. The
                                                   cell swells up and is
                                                   prevented from bursting
                                                   by the cellulose cell wall.

                         cellulose       plasma      cell sap less concentrated
                         cell wall      membrane       than outside solution

        Plasmolysed cells.                                                        leaves by
        Water is lost from                                                         osmosis

        its vacuole, the
        cytoplasm shrinks
        and is withdrawn
        from the cell wall.                     cytoplasm
Plasmolysis leads to wilting
              Fertilizer - Good or Bad?

•   Too much fertilizer added will cause the plant to wilt
•   Soil solution becomes very concentrated
•   Water moves out of root
•   Plant will wilt
•   Sufficient water must be added to ensure plant survival
Osmosis and Animal Cells                       normal red
                                                blood cell
Changes observed
under a microscope       in hypertonic                       in hypotonic
when blood is dropped       solution                            solution

into hypertonic and
hypotonic solutions.
                                cell shrinks                       cell swells

                        cell becomes
                                                              cell bursts and cell is
                                                             said to be haemolysed
                                                                 Active Transport
Active transport is an energy-consuming process by which
substances are transported from regions of low concentration
to regions of high concentration against a concentration
gradient.                        binding site
                                                        OUTSIDE       cell membrane
                  carrier protein

                       restoration                      INSIDE     binding
     carrier protein
     changes shape


                                          energy from ATP used
                                                Active Transport
Plants need to absorb mineral salts from the soil. They are
present as charged ions which cannot diffuse across
partially-permeable membranes.

                                             soil particle

                                             high concentration of ions

                                             low concentration of ions

                                             Direction of active transport

             Ion uptake by root hair is
           facilitated by active transport
                                                 Active Transport
Active transport enables absorption of glucose and amino
acids through the inner surface of the small intestine.

The presence of
microvilli increases
the surface area over
which active
transport occurs.                                            microvillus

               cell lining
               the villus

                                                                Uptake of glucose
                                                                by cells in the villi

                             glucose molecule
                             direction of active transport

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