Figure 8.1 Artificial membranes (cross sections)

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Figure 8.1 Artificial membranes (cross sections)
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Figure 8.4 The fluidity of membranes
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Figure 8.5 Evidence for the drifting of membrane
proteins
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Figure 8.6 The detailed structure of an animal
cells plasma membrane, in cross section
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Figure 8.7 The structure of a transmembrane
protein
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Figure 8.9 Some functions of membrane proteins
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Movement of substances through the membrane

• May be
• passive requires no energy expenditure by cell.
• Diffusion
• Osmosis
• Facilitated diffusion
• Active does require energy expenditure

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Passive transport

• Depends on concentration gradients.
• Materials move from area of high concentration to
  low concentration
• If occurs across a membrane, the membrane must be
  permeable to the substance.
• Size is a major factor
• Electrical charges
• Polar vs non-polar

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Diffusion

• Movement of particles (solute) from high to low
  concentration will continue until there is an
  even distribution of particles.
• This is called equilibrium.
• Particles move across the membrane randomly at
  equilibrium.

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Diffusion of two solutes

• If there are 2 or more kinds of solute, each
  moves independently of the other 2 different
  concentration gradients exist.
• Membranes may be permeable to some substances and
  not others selective permeability.

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Figure 8.10 The diffusion of solutes across
membranes
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Osmosis

• Diffusion of water (solvent) is called osmosis.
• Water molecules will move from regions of lesser
  solute concentration to regions of greater solute
  concentration.
• Movement of water stops when equilibrium is
  reached.

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Osmotic equilibrium

• In osmosis, water accumulating on one side of a
  membrane exerts pressure on the membrane.
• If this pressure is great enough, it may stop
  entry of water molecules from the other side,
  even though a concentration gradient still
  exists.
• This is important for plant cells!

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Figure 8.11 Osmosis
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Tonicity

• When comparing the concentrations of 2 solutions
• Isotonic same concentration of solute
• Hypertonic higher concentration of solute
• Hypotonic lower concentration of solute

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Figure 8.12 The water balance of living cells
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Movement through the membrane - passive

• In diffusion and osmosis, very small molecules
  may pass directly between the phospholipids as
  long as they are not charged.
• Ions can NOT pass through
• Some water can pass
• O2 and CO2 can pass
• Small lipids can pass

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Facilitated Diffusion

• Charged particles and some molecules can move
  passively through protein channels.
• The channels facilitate passage through the
  membrane.
• The proteins may undergo a shape change as the
  particles pass, but no energy is expended.

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Figure 8.14 Two models for facilitated diffusion
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Figure 8.15 The sodium-potassium pump a
specific case of active transport
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Figure 8.16 Review passive and active transport
compared
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Figure 8.17 An electrogenic pump
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Figure 8.18 Cotransport
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Figure 8.19 The three types of endocytosis in
animal cells