Membranes

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Membranes

• Phospholipid Bilayer.
• The head is hydrophilic, easily dissolving in
  water
• The tails are hydrophobic and are repelled by
  water
• Fluid Mosaic Model
• Mixture of phospholipids, glycolipids, sterols
  and proteins.

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Membranes Contd

• Membranes are selectively permeable

• They control the flow of substances into and out
  of a cell
• Membranes can hold teams of enzymes that
  function in metabolism

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Fluid Mosaic Model
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adhesion protein
communication protein
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recognition protein
receptor protein
passive transporter
active transporters
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Concentration Gradients and Diffusion

• Concentration Gradient a difference in the
  number of molecules or ions of a given substance
  between adjoining regions.
• Moves from a region of high concentration to a
  region of low concentration
• Diffusion the net movement of like molecules or
  ions down a concentration gradient.
• Factors affecting rate of diffusion
• The steepness of the gradient
• Molecular size
• Temperature

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FLUIDOUTSIDECELL
Phosphorylated transport protein

• Active transport in two solutes across a membrane

Transportprotein
Firstsolute
1
First solute, inside cell, binds to protein
2
ATP transfers phosphate to protein
3
Protein releases solute outside cell
Second solute
4
Second solute binds to protein
5
Phosphate detaches from protein
6
Protein releases second solute into cell
Figure 5.18
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5.19 Exocytosis and endocytosis transport large
molecules

• To move large molecules or particles through a
  membrane
• a vesicle may fuse with the membrane and expel
  its contents (exocytosis)

FLUID OUTSIDE CELL
CYTOPLASM
Figure 5.19A
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• or the membrane may fold inward, trapping
  material from the outside (endocytosis)

Figure 5.19B
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• Three kinds of endocytosis

Pseudopod of amoeba
Food being ingested
Plasma membrane
Material bound to receptor proteins
PIT
Cytoplasm
Figure 5.19C
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Diffusion

• Substances diffuse across membranes, spreading
  from areas of high concentration to areas of
  lower concentration

Molecule of dye
EQUILIBRIUM
Membrane
EQUILIBRIUM
Figure 5.14A B
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Osmosis is the diffusion of water
Hyptonicsolution
Hypertonic solution

• In osmosis, water travels from an area of lower
  solute concentration to an area of higher solute
  concentration

Selectivelypermeablemembrane
Solutemolecule
HYPOTONIC SOLUTION
HYPERTONIC SOLUTION
Water molecule
Selectivelypermeablemembrane
Solute molecule with cluster of water molecules
Figure 5.15
NET FLOW OF WATER
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Isotonic, Hypotonic, Hypertonic

• Osmosis causes cells to shrink in a hypertonic
  solution and swell in a hypotonic solution

ISOTONIC SOLUTION
HYPOTONIC SOLUTION
HYPERTONIC SOLUTION
(2) Lysing
(1) Normal
(3) Shriveled
ANIMALCELL
Plasmamembrane
PLANTCELL
(4) Flaccid
(5) Turgid
(6) Shriveled
Figure 5.16
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Facilitated Diffusion

• Small nonpolar molecules diffuse freely through
  the phospholipid bilayer
• Many other kinds of molecules pass through
  selective protein pores by facilitated diffusion

Solutemolecule
Transportprotein
Figure 5.17
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Active Transport

• Transport proteins can move solutes across a
  membrane against a concentration gradient
• This is called active transport
• Active transport requires ATP, the energy
  molecule

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FLUIDOUTSIDECELL
Phosphorylated transport protein

• Active transport in two solutes across a membrane

Transportprotein
Firstsolute
1
First solute, inside cell, binds to protein
2
ATP transfers phosphate to protein
3
Protein releases solute outside cell
Second solute
4
Second solute binds to protein
5
Phosphate detaches from protein
6
Protein releases second solute into cell
Figure 5.18
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Exocytosis and Endocytosis

• To move large molecules or particles through a
  membrane
• a vesicle may fuse with the membrane and expel
  its contents (exocytosis)

FLUID OUTSIDE CELL
CYTOPLASM
Figure 5.19A
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• Endocytosis

Figure 5.19B