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Transport Across Cell Membranes

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Transport Across Cell Membranes. Movement across membrane s is called selective transportbecause only certain substances are allowed to cross. – PowerPoint PPT presentation

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Title: Transport Across Cell Membranes


1
Transport Across Cell Membranes
  • Section 8.2

2
Transport Across Cell Membranes
  • Movement across membranes is called selective
    transport because only certain substances are
    allowed to cross.
  • Sometimes the cell must use energy to transport
    materials across the membrane, but some molecules
    are moved without using any energy.

3
Particles Are In Constant Motion
  • According to the particle model of matter, all
    matter is made up of tiny particles in constant,
    random motion.
  • This movement is called Brownian motion.

4
Concentration Gradient
  • The random movement of particles results in a net
    movement towards areas where the concentration of
    particles is lower.
  • The difference in concentration between two areas
    is called a concentration gradient.
  • A state of equilibrium occurs once molecules are
    evenly distributed.
  • Molecules move down a concentration gradient (ie,
    from area of high concentration to area of low
    concentration).

5
I. Passive Transport
  • The movement of materials across a semi-permeable
    membrane without the expenditure of energy.
  • The three types of passive transport we will
    consider are diffusion, osmosis, and facilitated
    diffusion.

6
A. Diffusion
  • Definition the movement of molecules from an
    area of high concentration to an area of low
    concentration.

7
diagram
8
Diffusion of perfume across a classroom
9
B. Osmosis a special case of diffusion
  • Definition the diffusion of water across a
    semi-permeable membrane. Water moves from an
    area of high water concentration to an area of
    low water concentration. The concentration of
    water inside and outside of the cell is essential
    to balance between cell lysis (explosion) and
    crenation (cell shriveling).

10
Terms to know
  • Solute the dissolved substance in a solution.
  • Solvent a substance in which another substance
    is dissolved, forming a solution.

11
Example U-tube
12
Solution Concentrations
  • It is important to understand that cells are
    constantly bathed in solutions with different
    solutes.
  • These solutions will determine the fate of the
    cell by generating water movement into and out of
    the cell.

13
Solute Concentrations
  • Solutions outside the cell can be classified in
    three ways
  • Hypotonic the solution contains a lower
    concentration of solutes than the cell.
  • Isotonic the solution has the same
    concentration of solutes as the cell.
  • Hypertonic there is a higher concentration of
    solutes than the cell

14
Hypotonic Solution
  • A higher concentration of water (solvent) outside
    of the cell than inside
  • Water will move into the cell.

15
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16
Hypertonic Solutions
  • A higher concentration of solute outside of the
    cell than inside
  • Water will move out of the cell

17
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18
Isotonic Solutions
  • The concentrations of solvent and solute are the
    same inside of the cell as outside
  • Water goes into and out of the cell in equal
    amounts

19
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20
Osmosis in Living Cells
  • Too much or too little water can kill a cell
  • If the water concentration outside of the cell is
    high relative to the inside, water will flow into
    the cell sometimes causing the cell to burst
    (animal cells)
  • Example blood cells placed in distilled water
  • The cell walls of plant cells can withstand this
    pressure (turgor pressure). This keeps plant
    structures firm.

21
Osmosis in Living Cells
  • When cells are placed into strong salt solutions
    (hypertonic solutions), water will diffuse out of
    the cell causing the cell to shrink and lose its
    shape
  • Eventually, the cell may die

22
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23
C. Facilitated Diffusion
  • Small molecules like water, oxygen and carbon
    dioxide can diffuse through the cell membrane
  • Larger molecules like glucose and charged ions
    like sodium and potassium can not pass through
    the phospholipid molecules
  • These kind of molecules rely on transport
    proteins to aid, or facilitate, their diffusion
    across the cell membrane

24
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25
Facilitated Diffusion
  • Sometimes large molecules need to get into a cell
    but cannot. Help is needed. A protein in the
    cell membrane creates a pore or a transport
    channel for diffusing molecules that otherwise
    would not be able to pass through the
    semi-permeable membrane.

26
  • The protein may change shape slightly to
    accommodate a particular type of molecule.
  • Different proteins in the cell membrane have
    different channel sizes and allow different
    molecules to pass through.
  • The interaction between particles and their
    transport proteins provides enough energy to
    cause their diffusion through the membrane.
  • Facilitated diffusion always occurs from an area
    of high concentration to an area of low
    concentration.

27
Transport Proteins
  • Carrier Proteins change shape to allow certain
    molecules like glucose to cross the cell membrane
  • Channel Proteins provide water-filled passages
    through which small dissolved ions can diffuse

28
II. Active Transport
  • Definition the transport of materials across a
    membrane from an area of low concentration to an
    area of high concentration, using energy in the
    form of adenosine triphosphate or ATP.
  • Active transport involves transport proteins in
    the cell membrane. Molecules are transported
    against the concentration gradient

29
Example The Sodium / Potassium Pump
30
Sodium Potassium PumpPump 2
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32
Endocytosis and Exocytosis
  • Some materials are too large (protein molecules
    and bacteria cells) or can not dissolve in water
    (cholesterol) and so can not be moved into the
    cell by diffusion nor carrier proteins

33
  • Endocytosis a form of active transport where
    the cell membrane forms a pocket around the
    material creating a sphere or vesicle which is
    transported to other parts of the cell

34
  • Phagocytosis a type of endocytosis where the
    cell eats by taking in large particles
  • Pinocytosis a type of endocytosis where the
    cell drinks by taking in droplets of fluid

35
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36
Receptor-Mediated Endocytosis
  • When a receptor on a membrane protein binds with
    an item for transport, it triggers endocytosis
  • The HIV virus is transported in this way because
    it is too large to enter the cell by diffusion or
    by passive transport

37
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38
Endocytosis
39
Exocytosis
  • The reverse of endocytosis
  • Allows materials stored in vesicles and vacuoles
    to exit the cell
  • This allows for cells to dispose of wastes
  • Very important in cells specialized in the
    secretion of hormones and enzymes

40
Exocytosis
41
Your Task!
  • Read p 297-307
  • Finish the Reading Assignment you are
    responsible for this information. Ask if you
    have any questions.
  • Do Qs 1,2,3 and 5 in Check Your Understanding
    p307.
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