Key Concepts: Cell Membranes

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Key Concepts Cell Membranes

• Structure of the cell membrane
• Main functions of cell membrane
• How things are transported across the membrane

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Cell Membrane Structure

• Cell Membranes are a fluid mosaic of lipids and
  proteins.
• http//www.youtube.com/watch?vQqsf_UJcfBc

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Cell Membrane Structure cont.

• Fluid describes that cell membranes move
• Fluid also describes that the membranes are
  usually in a liquid state, like salad oil.
• Membranes must be fluid to work properly.

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Cell Membrane Fluidity
Hydrophilic Head (polar-water loving)
Hydrophobic Region
Hydrophobic Tail (non-polar)
Cholesterol
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Cell Membrane Structure cont.       

• Mosaic describes the many proteins and lipids
  that make up the cell membrane.
• Proteins are embedded in and on top of the lipid
  bilayer.
• The proteins embedded in the cell membrane
  determine its function.

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Cell Membrane Mosaic
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Cell Membrane Functions

• Regulates what goes in and out of the cell.
• Provides support for the cell.

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Cell Membrane Selective Permeability

• Cell membranes exhibit selective permeability,
  regulating what goes in and out of cell.
• Selective permeability means some substances can
  pass across the cell membrane while others cannot.

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Types of Molecules Passing Through Cell Membrane

• Hydrophobic molecules (non polar) like CO2, O2
  and other hydrocarbons (fatty acids)
• Can move across the lipid bilayer easily.
• Hydrophilic molecules (polar) like sugars
  (glucose), salts, and water
• Have trouble passing through the non-polar
  centers of the lipid bilayer.
• Size and polarity prevents them from moving
  across the lipid bilayer so use a protein
  channel/pump.

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Two Ways for Molecules to Enter Cell

• Passive Transport

• Active Transport

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Passive Transport

• Passive Transport is when the cell does not
  expend energy to move across the membrane.
• Movement comes from kinetic energy of molecules
• http//www.youtube.com/watch?v6VdMp46ZIL8feature
  related
• http//www.youtube.com/watch?v2Vdjin734gE

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Types of Passive Transport

• 1. Diffusion
• 2. Facilitated Diffusion
• 3. Osmosis

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Passive Transport Diffusion

• 1. Diffusion is the tendency for molecules to
  spread out evenly (reach dynamic equilibrium) and
  move from an area of high concentration to low
  concentration.

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Passive Transport Facilitated Diffusion

• 2. Facilitated Diffusion is when molecules use
  channel proteins or carrier proteins to pass
  through the bilayer.

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Passive Transport Osmosis

• 3. Osmosis is the special name given to the
  diffusion of water molecules.
• Water like other substances moves from a higher
  concentration of water molecules to a lower
  concentration of water molecules.

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Water Balance in Cells

• Why would a cell gain or lose water?
• It depends on the concentration of solutes inside
  and outside the cell.

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Water Balance in Cells

• Tonicity the ability of a cell to gain or lose
  water.
• Three Types of Tonicity
• Isotonic
• Hypertonic
• Hypotonic
• Always in terms of relationship between the
  solute concentration in solution and the solute
  concentration in the cell

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• Solutes Suck!

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Water Balance in Cells

• Hypertonic Environment
• Hyper more
• If there are more solutes outside the cell, the
  cell will lose water.
• Water moves outside the cell to equalize the
  concentration of water molecules, causing the
  cell to shrink.

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Water Balance in Cells

• Hypotonic Environment
• Hypo less
• If there are less solutes outside the cell, the
  cell will gain water
• Water moves inside the cell to equalize the
  concentration of water molecules, causing the
  cell to swell.

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Water Balance in Cells

• Isotonic Environment
• iso same
• No net movement of water across the membrane
  because the solute concentration is equal inside
  and outside of the cell.

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Water Balance in Cells

• Osmotic Pressure The force exerted on the cell
  from an imbalance of water and solutes.
• High Osmotic pressure means water moving into the
  cells
• Low Osmotic pressure means water moving out of
  the cells

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Adaptations for Osmotic Pressure

• Large organisms deal with this by living in
  isotonic environments like blood.
• Plants, fungi, bacteria and protista have rigid
  cell walls to protect the cell from bursting in a
  hypotonic environment.

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Active Transport

• Cellular energy (ATP) is used to move solutes in
  and out of a cell.

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Types of Active Transport

• Solute Pumping
• Using Carrier Proteins
• Engulfing Large Molecules
• Exocytosis
• Endocytosis
• Phagocytosis
• Pinocytosis

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Active Transport Solute pumping small molecules

• Carrier Proteins are used to move small molecules
  against the concentration gradient.
• Example Potassium Pump Active Transport

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Active Transport Large Molecules

• Exocytosis
• Dumping large molecules outside of the cell.
• Insulin is secreted into the blood this way.
• Cell walls are formed by proteins and
  carbohydrates transported outside the cell.
• Exocytosis

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Active Transport Large Molecules

• Endocytosis
• Forming of vesicles to take in large substances
  into the cell (macromolecules or other cells).
• Phagocytosis cell eating (phago- means to eat)
• Pinocytosis cell drinking (pino- means to
  drinkthink vino, pino)
• Endocytosis
• Pinocytosis
• Phagocytosis

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End
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Passive Transport Facilitated Diffusion

• Channel Proteins Open tunnels with hydrophilic
  centers that allow hydrophilic substances (i.e.
  water) to pass through the membrane rapidly.

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Passive Transport Facilitated Diffusion

• Carrier proteins help other hydrophilic
  molecules (i.e. glucose and salt ions) to enter
  the cell.
• When the molecule enters the carrier protein, the
  protein undergoes a shape change so the molecule
  can pass through.
• Molecule specific.