A Cell

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A Cell
Cytoplasm
Mitochondrion
Nucleus
Selectively permeable cell membrane
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Selective Membrane
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Why do cells need to transport materials across
the cell membrane?

• Cells need to obtain various substances in order
  to carry out life processes
• What are some of the substances that they need to
  obtain?
• e.g. nutrients, oxygen, water, etc.

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Why do cells need to transport materials across
the cell membrane?

• Cells also need to get rid of substances that are
  useless and harmful to them
• What are some of the substances that must be
  removed?
• e.g. carbon dioxide, metabolic wastes, etc.

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Transportation of Materials

• There are 3 different processes for
  transporting materials in and out of a cell
• Diffusion
• Osmosis
• Active Transport

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DIFFUSION
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Hi. I am Moley. I am a gas molecule. I am
constantly moving around. This is because I
possess kinetic energy.
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Here I am with my brothers and sisters. Notice
that we are all cramped to one side of the area.
In other words, there is a HIGH CONCENTRATION of
molecules on the left side of the area.
Area of High Concentration
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Well, since we are moving all the time, we will
eventually spread out to cover the rest of the
area. That is, we will move along the
concentration gradient, from an area of HIGH
CONCENTRATION to an area of LOW CONCENTRAION!
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CONCENTRATION GRADIENT
Area of High Concentration
Area of Low Concentration
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EQUILIBRIUM Particles are evenly distributed
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Diffusion

• Diffusion is the net movement of the particles of
  a substance from a region of higher concentration
  to a region of lower concentration
• It is a passive process and no external energy is
  required
• It can occur in a liquid or gaseous medium

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We can also pass through a selectively permeable
membrane because we are so small!!!
Area of High Concentration
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CONCENTRATION GRADIENT
Area of High Concentration
Area of Low Concentration
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CONCENTRATION GRADIENT
Area of High Concentration
Area of Low Concentration
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EQUILIBRIUM Particles are evenly distributed
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(No Transcript)
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Diffusion
A small amount of ink is added to a beaker of
water. What do you think will happen?
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Diffusion

• - Can you explain the
• color change?
• Do you think the ink
• particles will continue
• to move even after they
• have spread out evenly
• in the water?
• Will the result be the
• same if this is carried out
• in hot water?

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Factors Affecting the Rate of
Diffusion

• Molecular size will small or large particles
  diffuse faster?
• Concentration gradient will diffusion occur
  more rapidly if there is a large or small
  difference in concentration between two regions?
• Distance of diffusion will diffusion be faster
  or slower if the distance of diffusion is long?

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Factors Affecting the Rate of
Diffusion

• Medium is diffusion faster in a gaseous or
  liquid medium?
• Surface area will diffusion occur faster in a
  large or small surface area?

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Factors Affecting the Rate of
Diffusion
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Factors Affecting the Rate of
Diffusion

• Medium is diffusion faster in gaseous or liquid
  medium?
• Surface area will diffusion occur faster in a
  large or small surface area?
• Temperature will diffusion be faster under high
  or low temperature?

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Examples of Diffusion

• Exchange of gases (i.e. oxygen and carbon
  dioxide) in the air sacs of lungs
• Absorption of digested food in the gut
• The detection of smell e.g. perfume
• The movement of carbon dioxide through the
  stomata during photosynthesis

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OSMOSIS
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Osmosis

• Osmosis is the net movement (i.e. diffusion) of
  water molecules from a less concentrated solution
  (with a higher water potential) to a more
  concentrated solution (with a lower water
  potential) through a selectively permeable
  membrane

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This is a water molecule
This is a sucrose molecule
Region A Dilute sucrose solution
Region B Concentrated sucrose solution
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This is a water molecule
This is a sucrose molecule
Region A Dilute sucrose solution
Region B Concentrated sucrose solution
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INVESTIGATION 1

• Illustration of osmosis using a dialysis tubing

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Introduction to Investigation

• In this experiment, osmosis will be demonstrated
  by using a dialysis tubing, which acts as a
  selectively permeable membrane

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Procedure

• Please refer to pages 83 and 84 in your ARISTO
  text book

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Results Table
Time Distance from initial mark
After 5 minutes
After 10 minutes
After 15 minutes
After 20 minutes
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Plot the data on a graph
What will the graph look like?
Distance risen by the sucrose solution
Time (Mins.)
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Can you predict the results in the following
situations?
Treatment Inside tubing In beaker Change in level
1 Sucrose solution Distilled water Rises
2 Distilled water Sucrose solution ?
3 Concentrated sucrose solution Distilled water ?
4 Distilled water Distilled water ?
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Variations to the Experiment

• Instead of dialysis tubing, we can also use
  living tissues, such as potatoes, to illustrate
  the process of osmosis

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Variations to the Experiment
What do you think is the result of this
experiment?
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Water Potential

• Water potential is represented by the Greek
  letter psi
• A measure of the free energy of water or the
  energy for water molecules in a solution to move
  about
• Water potential is dependent on the proportion of
  free water molecules in a solution (free water
  molecules water molecules that are not engaged
  with other molecules)

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Water Potential

• The water potential of pure water is defined as
  zero
• If solutes are dissolved in water, the proportion
  of free water molecules will decrease. The water
  potential will also decrease to less than zero

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Water Potential

• A concentrated solution has a lower water
  potential than a dilute solution
• The net movement of water molecules is always
  from a region of HIGHER water potential to a
  region of LOWER water potential

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Tonicity

• Osmotic movements across cell membranes are
  affected by the tonicity the relative
  concentrations of solutes in 2 fluids, e.g. the
  cytoplasm of a cell and the fluid outside the cell

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Tonicity

• We are going to use a red blood cell as an
  example. On a piece of paper, draw three
  beakers. In each of the beaker, add a RBC. We
  will discuss what happen to the RBC when it is
  put into 3 different types of solutions of
  different concentrations of sodium chloride (i.e.
  salt)

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Isotonic
The water potential inside the cell is the same
as that outside the cell. There is no net
movement of water into or out of the cell
Salty Water
CELL
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Hypertonic
There is less water molecules outside the cell,
and therefore the solution outside has a lower
water potential than that of the cell What will
happen to the cell?
Very Salty Water
CELL
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Hypotonic
There is more water molecules outside the cell,
and therefore the solution outside has a higher
water potential than that of the cell. What
will happen to the cell?
Pure Water
CELL
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Effects of Osmosis on Plant Cells
Turgid Cells
Flaccid Cells
Plant cells placed in a hypertonic solution will
undergo plasmolysis, a condition where the
plasma membrane pulls away from the cell wall as
the cell shrinks. The cell wall is rigid and does
not shrink.
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Effects of Osmosis on Animal Cells
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Effects of Osmosis on Animal Cells
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Effects of Osmosis on Animal Cells
Shrinking RBC
Swelling RBC
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Active Transport
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Active Transport

• Some solutes have a higher concentration inside
  the cell than outside so they have to cross the
  membrane against the concentration gradient.
  These solutes need to enter the cell by means of
  active transport
• Why is this called ACTIVE transport?

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Active Transport
Low concentration outside the cell
High concentration inside the cell
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
Cell membrane
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Active Transport
What is this???
Cell membrane
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Active Transport

• Active transport requires special protein
  carriers. These carriers are embedded within the
  cell membrane
• Active transport also requires energy obtained
  from respiration
• The energy is used to change the shape of the
  protein carriers so that the molecules can attach
  to the carriers for transportation

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Active Transport

• The process of active transport is useful when
  there is no concentration gradient or when
  diffusion is too slow for the transportation of
  substances

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Examples of Active Transport

• Nutrients are actively transported into the small
  intestine
• Substances that are useful to the body are
  actively reabsorbed into the blood vessels
  surrounding the kidney tubules
• Root hairs actively absorb mineral salts from the
  soil

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Comparison of Passive and
Active Transport