Membrane Structure

1
Membrane Structure Function cont. I. Membrane
Protein Function II. Cellular Transport
2

• Integral proteins
• span lipid bilayer
• called transmembrane proteins
• hydrophobic regions consist of one or more
  stretches of nonpolar amino acids
• often coiled into alpha helices
• Visualize and draw membrane with transmembrane
  protein containing 2 helices

3
LE 7-8
EXTRACELLULAR SIDE
N-terminus
C-terminus
CYTOPLASMIC SIDE
a Helix
4

• Six major functions of membrane proteins
• Transport
• Enzymatic activity
• Signal transduction
• Cell-cell recognition
• Intercellular joining
• Attachment to the cytoskeleton and extracellular
  matrix (ECM)

5
LE 7-9a
Signal
Enzymes
Receptor
ATP
Transport
Enzymatic activity
Signal transduction
6
LE 7-9b
Glyco- protein
Attachment to the cytoskeleton and
extra- cellular matrix (ECM)
Cell-cell recognition
Intercellular joining
7
The Role of Membrane Carbohydrates in Cell-Cell
Recognition

• Cells recognize each other by binding to surface
  molecules, often carbohydrates, on the plasma
  membrane
• Carbohydrates covalently bonded to lipids
  (glycolipids) or more often to proteins
  (glycoproteins)
• Much variability of extracellular carbohydrates
  among species, individuals, cell types in an
  individual
• Example of Pneumococcus

8
Synthesis and Sidedness of Membranes

• Membranes distinct inside and outside faces
• Plasma membrane is added to by vesicles from ER
  Golgi.
• Secreted and integral membrane proteins, lipids
  and associated carbohydrates transported to
  membrane by these vesicles.

9
LE 7-10
ER
Transmembrane glycoproteins
Secretory protein
Glycolipid
Golgi apparatus
Vesicle
Plasma membrane
Cytoplasmic face
Extracellular face
Transmembrane glycoprotein
Secreted protein
Plasma membrane
10
Transport across cellular membranes

• To exchange materials with surroundings in part
  to take in nutrients and give off waste
• Exchange(or transport) regulated
• selective permeability

11
Structure Dictates Membrane Permeability

• Hydrophobic (nonpolar) molecules cross membrane
  rapidly
• e.g., hydrocarbons, oxygen, CO2 can dissolve in
  the lipid bilayer and pass through the membrane
  rapidly
• Polar molecules cross slowly
• e.g. sugars, charged proteins, water

12
How do hydrophilic substances cross membranes?
With Help!

• Transport proteins
• Some create hydrophilic channels across membranes
  for polar molecules or ions to pass through
• Example Aquaporin
• water channel protein

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• Carrier proteins
• binds solutes change the shape of carrier
• help to facilitate passage across membrane
• highly specific for transported solutes
• Examples glucose transporter is a carrier
  protein for glucose only

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Transport Can be Passive or Active
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LE 7-11a
Passive Transport Diffusion
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
Diffusion of one solute
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• Substances diffuse down their concentration
  gradient
• High to low
• Substances reach dynamic equilibrium
• No work (no added energy) required

17
LE 7-11b
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
Diffusion of two solutes
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Effects of Osmosis on Water Balance

• Osmosis
• diffusion of water across a selectively permeable
  membrane
• Diffuses across a membrane from the region of
  lower solute (such as an ion) concentration to
  the region of higher solute concentration
• The direction of osmosis is determined only by a
  difference in total solute concentration

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LE 7-12
Lower concentration of solute (sugar)
Higher concentration of sugar
Same concentration of sugar
H2O
Selectively permeable mem- brane sugar
mole- cules cannot pass through pores, but water
molecules can
Osmosis
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Water Balance of Cells Without Walls

• Tonicity
• ability of a solution to cause a cell to gain or
  lose water
• Isotonic solution
• solute concentration is equal inside and
  outside the cell --gt no net water movement
  cell remains same size

21
Hypertonic solution external solute
concentration is greater than that inside the
cell--gt cell loses water
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Hypotonic solution external solute
concentration is less than that inside the
cell--gt cell gains water May expand enough to
burst!
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LE 7-13
Hypotonic solution
Isotonic solution
Hypertonic solution
Animal cell
H2O
H2O
H2O
H2O
Shriveled
Lysed
Normal
H2O
Plant cell
H2O
H2O
H2O
Turgid (normal)
Flaccid
Plasmolyzed
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Water Balance of Cells with Walls vs No Walls

• Cell walls help maintain water balance
• Plant cell in hypotonic solution swells --gtturgid
  (firm)
• Animal cell?
• Plant cell and its surroundings isotonic--gt no
  net water movemen the cell becomes flaccid
  (limp), and the plant may wilt
• Animal cell?
• In hypertonic environment, plant cells lose
  water--gt membrane pulls away from the wall
  plasmolysis
• Lethal
• Animal cell?

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Passive Transport Aided by Proteins

• Facilitated diffusion
• transport proteins speed movement of molecules
  across the plasma membrane
• Channel proteins
• Carrier proteins

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LE 7-15a
EXTRACELLULAR FLUID
Solute
Channel protein
CYTOPLASM
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LE 7-15b
Carrier protein
Solute
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Active transportuses energy to
move solutes against their gradients

• Requires energy, usually ATP
• Performed by specific membrane proteins
• Example
• sodium-potassium pump

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LE 7-16
EXTRACELLULAR FLUID
Na high K low
Na
Na
Na
Na
Na
Na
Na
Na
ATP
Na low K high
P
Na
P
CYTOPLASM
ADP
Phosphorylation causes the protein to
change its conformation, expelling Na to the
outside.
Cytoplasmic Na bonds to the
sodium-potassium pump
Na binding stimulates phosphorylation by
ATP.
K
K
K
K
K
P
K
P
Extracellular K binds to the protein,
triggering release of the phosphate group.
Loss of the phosphate restores the
proteins original conformation.
K is released and Na sites are receptive
again the cycle repeats.
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LE 7-17
Passive transport
Active transport
ATP
Facilitated diffusion
Diffusion
31

• Electrogenic pumps
• is a transport protein that generates a voltage
  across a membrane--gt opposite charges across
  membrane (membrane potential)
• Example In animals, Na-K pump
• In plant fungi and bacteria, proton pump
• Requires ATP (active transport)

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LE 7-18

EXTRACELLULAR FLUID


ATP

H
H
Proton pump
H


H
H


CYTOPLASM
H


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Cotransport Coupled Transport by a Membrane
Protein

• When active transport of one solute indirectly
  drives transport of another
• Example
• Plants commonly use the proton gradient
  generated by proton pumps to drive transport of
  nutrients into the cell

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LE 7-19


H
ATP
H


Proton pump
H
H


H


H
Diffusion of H
Sucrose-H cotransporter
H




Sucrose
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How do large molecules move in and out of cells?

• Small molecules and water enter or leave the cell
  through the lipid bilayer or by transport
  proteins
• Large molecules, such as polysaccharides and
  proteins, cross the membrane via vesicles

36
Exocytosis

• Transport vesicles with cargo migrate to the
  membrane, fuse with it, and are release contents
• Example
• Many secretory cells use exocytosis to export
  their products
• Pancreatic cells (beta-cells) secrete insulin

37
LE 7-10
ER
Transmembrane glycoproteins
Secretory protein
Glycolipid
Golgi apparatus
Vesicle
Plasma membrane
Cytoplasmic face
Extracellular face
Transmembrane glycoprotein
Secreted protein
Plasma membrane
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Endocytosis

• Cell takes in macromolecules by forming vesicles
  at the plasma membrane
• Reversal of exocytosis, involving different
  proteins

39

• Three types of endocytosis
• Phagocytosis (cellular eating) Cell engulfs
  particle in a vacuole
• Pinocytosis (cellular drinking) Cell creates
  vesicle around fluid
• Receptor-mediated endocytosis Binding of ligands
  to receptors triggers vesicle formation

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LE 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Coated vesicle
Receptor
Coated pit
Ligand
A coated pit and a coated vesicle
formed during receptor- mediated endocytosis (TEMs
).
Coat protein
Plasma membrane
0.25 µm