Transport Across Cell Membranes

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Transport Across Cell Membranes

• Section 8.2

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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.

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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.

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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).

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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.

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A. Diffusion

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

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diagram
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Diffusion of perfume across a classroom
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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).

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Terms to know

• Solute the dissolved substance in a solution.
• Solvent a substance in which another substance
  is dissolved, forming a solution.

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Example U-tube
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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.

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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

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Hypotonic Solution

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

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Hypertonic Solutions

• A higher concentration of solute outside of the
  cell than inside
• Water will move out of the cell

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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

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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.

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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

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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

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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.

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• 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.

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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

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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

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Example The Sodium / Potassium Pump
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Sodium Potassium PumpPump 2
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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

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• 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

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• 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

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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

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Endocytosis
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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

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Exocytosis
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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.