Download 3 - Dr. Jerry Cronin

PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
3
Cells: The
Living Units:
Part A
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(f) Cell-cell recognition
Some glycoproteins (proteins bonded
to short chains of sugars) serve as
identification tags that are specifically
recognized by other cells.
Glycoprotein
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Figure 3.4f
Membrane Junctions
• Three types:
• Tight junction
• Desmosome
• Gap junction
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Membrane Junctions: Tight Junctions
• Prevent fluids and most molecules from
moving between cells
• Where might these be useful in the body?
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Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Interlocking
junctional proteins
Intercellular
space
(a) Tight junctions: Impermeable junctions prevent molecules
from passing through the intercellular space.
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Figure 3.5a
Membrane Junctions: Desmosomes
• “Rivets” or “spot-welds” that anchor cells
together
• Where might these be useful in the body?
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Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular space
Plaque
Intermediate
filament (keratin)
Linker glycoproteins
(cadherins)
(b) Desmosomes: Anchoring junctions bind adjacent cells together
and help form an internal tension-reducing network of fibers.
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Figure 3.5b
Membrane Junctions: Gap Junctions
• Transmembrane proteins form pores that
allow small molecules to pass from cell to cell
• For spread of ions between cardiac or smooth
muscle cells
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Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular
space
Channel
between cells
(connexon)
(c) Gap junctions: Communicating junctions allow ions and small molecules to pass from one cell to the next for intercellular communication.
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Figure 3.5c
Membrane Transport
• Plasma membranes are selectively permeable
• Some molecules easily pass through the
membrane; others do not
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Types of Membrane Transport
• Passive processes
• No cellular energy (ATP) required
• Substance moves down its concentration
gradient
• Active processes
• Energy (ATP) required
• Occurs only in living cell membranes
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Passive Processes
•
What determines whether or not a
substance can passively permeate a
membrane?
1. Lipid solubility of substance
2. Channels of appropriate size
3. Carrier proteins
PLAY
Animation: Membrane Permeability
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Passive Processes
• Simple diffusion
• Carrier-mediated facilitated diffusion
• Channel-mediated facilitated diffusion
• Osmosis
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Passive Processes: Simple Diffusion
• Nonpolar lipid-soluble (hydrophobic)
substances diffuse directly through the
phospholipid bilayer
PLAY
Animation: Diffusion
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Extracellular fluid
Lipidsoluble
solutes
Cytoplasm
(a) Simple diffusion of fat-soluble molecules
directly through the phospholipid bilayer
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Figure 3.7a
Passive Processes: Facilitated Diffusion
• Certain lipophobic molecules (e.g., glucose,
amino acids, and ions) use carrier proteins or
channel proteins, both of which:
• Exhibit specificity (selectivity)
• Are saturable; rate is determined by number of
carriers or channels
• Can be regulated in terms of activity and
quantity
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Facilitated Diffusion Using Carrier Proteins
• Transmembrane integral proteins transport
specific polar molecules (e.g., sugars and
amino acids)
• Binding of substrate causes shape change in
carrier
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Lipid-insoluble
solutes (such as
sugars or amino
acids)
(b) Carrier-mediated facilitated diffusion via a protein
carrier specific for one chemical; binding of substrate
causes shape change in transport protein
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Figure 3.7b
Facilitated Diffusion Using Channel
Proteins
• Aqueous channels formed by transmembrane
proteins selectively transport ions or water
• Two types:
• Leakage channels
• Always open
• Gated channels
• Controlled by chemical or electrical signals
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Small lipidinsoluble
solutes
(c) Channel-mediated facilitated diffusion
through a channel protein; mostly ions
selected on basis of size and charge
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Figure 3.7c
Passive Processes: Osmosis
• Movement of solvent (water) across a
selectively permeable membrane
• Water diffuses through plasma membranes:
• Through the lipid bilayer
• Through water channels called aquaporins
(AQPs)
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Water
molecules
Lipid
billayer
Aquaporin
(d) Osmosis, diffusion of a solvent such as
water through a specific channel protein
(aquaporin) or through the lipid bilayer
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Figure 3.7d
Passive Processes: Osmosis
• Water concentration is determined by solute
concentration because solute particles
displace water molecules
• Osmolarity: The measure of total
concentration of solute particles
• When solutions of different osmolarity are
separated by a membrane, osmosis occurs
until equilibrium is reached
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(a)
Membrane permeable to both solutes and water
Solute and water molecules move down their concentration gradients
in opposite directions. Fluid volume remains the same in both compartments.
Left
compartment:
Solution with
lower osmolarity
Right
compartment:
Solution with
greater osmolarity
Both solutions have the
same osmolarity: volume
unchanged
H2O
Solute
Membrane
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Solute
molecules
(sugar)
Figure 3.8a
(b)
Membrane permeable to water, impermeable to solutes
Solute molecules are prevented from moving but water moves by osmosis.
Volume increases in the compartment with the higher osmolarity.
Left
compartment
Right
compartment
Both solutions have identical
osmolarity, but volume of the
solution on the right is greater
because only water is
free to move
H2O
Membrane
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Solute
molecules
(sugar)
Figure 3.8b
Importance of Osmosis
• When osmosis occurs, water enters or leaves
a cell
• Change in cell volume disrupts cell function
PLAY
Animation: Osmosis
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Tonicity
• Tonicity: The ability of a solution to cause a
cell to shrink or swell
• Isotonic: A solution with the same solute
concentration as that of the cytosol
• Hypertonic: A solution having greater solute
concentration than that of the cytosol
• Hypotonic: A solution having lesser solute
concentration than that of the cytosol
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(a)
Isotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
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(b)
Hypertonic solutions
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of solutes than are present inside
the cells).
(c)
Hypotonic solutions
Cells take on water by osmosis until
they become bloated and burst (lyse)
in a hypotonic solution (contains a
lower concentration of solutes than
are present in cells).
Figure 3.9
Summary of Passive Processes
Process
Simple
diffusion
Facilitated
diffusion
Osmosis
Energy
Source
Kinetic
energy
Kinetic
energy
Kinetic
energy
• Also see Table 3.1
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Example
Movement of O2 through
phospholipid bilayer
Movement of glucose into
cells
Movement of H2O through
phospholipid bilayer or
AQPs