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CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
7
Membrane
Structure and
Function
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
Figure 7.3
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
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Peripheral
proteins
Integral
protein CYTOPLASMIC
SIDE OF
MEMBRANE
 Six major functions of membrane proteins
 Transport
 Enzymatic activity
 Signal transduction
 Cell-cell recognition
 Intercellular joining
 Attachment to the cytoskeleton and extracellular
matrix (ECM)
© 2014 Pearson Education, Inc.
Figure 7.7
Signaling
molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic
activity
(c) Signal
transduction
(e) Intercellular
joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Glycoprotein
(d) Cell-cell
recognition
© 2014 Pearson Education, Inc.
The Role of Membrane Carbohydrates in Cell-Cell
Recognition
 Cells recognize each other by binding to molecules,
often containing carbohydrates, on the extracellular
surface of the plasma membrane
 Membrane carbohydrates may be covalently bonded
to lipids (forming glycolipids) or more commonly to
proteins (forming glycoproteins)
 Carbohydrates on the external side of the plasma
membrane vary among species, individuals, and
even cell types in an individual
© 2014 Pearson Education, Inc.
Synthesis and Sidedness of Membranes
 Membranes have distinct inside and outside faces
 The asymmetrical distribution of proteins, lipids,
and associated carbohydrates in the plasma
membrane is determined when the membrane is
built by the ER and Golgi apparatus
© 2014 Pearson Education, Inc.
Figure 7.9
Transmembrane
glycoproteins
Secretory
protein
Golgi
apparatus
Vesicle
Attached
carbohydrate
Glycolipid
ER
lumen
Plasma membrane:
Cytoplasmic face
Extracellular face
Transmembrane
glycoprotein
Membrane
glycolipid
© 2014 Pearson Education, Inc.
Secreted
protein
Concept 7.3: Passive transport is diffusion of a
substance across a membrane with no energy
investment
 Diffusion is the tendency for molecules to spread
out evenly into the available space
 Although each molecule moves randomly, diffusion
of a population of molecules may be directional
 At dynamic equilibrium, as many molecules cross
the membrane in one direction as in the other
© 2014 Pearson Education, Inc.
 Substances diffuse down their concentration
gradient, the region along which the density of a
chemical substance increases or decreases
 No work must be done to move substances down the
concentration gradient
 The diffusion of a substance across a biological
membrane is passive transport because no energy
is expended by the cell to make it happen
© 2014 Pearson Education, Inc.
Effects of Osmosis on Water Balance
 Osmosis is the diffusion of water across a selectively
permeable membrane
 Water diffuses across a membrane from the region
of lower solute concentration to the region of higher
solute concentration until the solute concentration
is equal on both sides
© 2014 Pearson Education, Inc.
Figure 7.11
Lower concentration
of solute (sugar)
Sugar
molecule
Higher concentration
of solute
H2O
Selectively
permeable
membrane
Osmosis
© 2014 Pearson Education, Inc.
More similar
concentrations of solute
Water Balance of Cells Without Cell Walls
 Tonicity is the ability of a surrounding solution to
cause a cell to gain or lose water
 Isotonic solution: Solute concentration is the same
as that inside the cell; no net water movement across
the plasma membrane
 Hypertonic solution: Solute concentration is greater
than that inside the cell; cell loses water
 Hypotonic solution: Solute concentration is less
than that inside the cell; cell gains water
© 2014 Pearson Education, Inc.
Figure 7.12
Hypotonic
Isotonic
(a) Animal cell
H2O
H2O
Lysed
H2O
Shriveled
Normal
Cell
wall
H2O
H2O
H2O
Plasma
membrane
H2O
(b) Plant cell
Plasma
membrane
H2O
Hypertonic
Turgid (normal)
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Flaccid
Plasmolyzed
 Hypertonic or hypotonic environments create
osmotic problems for organisms
 Osmoregulation, the control of solute
concentrations and water balance, is a necessary
adaptation for life in such environments
 The protist Paramecium, which is hypertonic to its
pond water environment, has a contractile vacuole
that acts as a pump
© 2014 Pearson Education, Inc.
Water Balance of Cells with Cell Walls
 Cell walls help maintain water balance
 A plant cell in a hypotonic solution swells until the
wall opposes uptake; the cell is now turgid (firm)
 If a plant cell and its surroundings are isotonic, there
is no net movement of water into the cell;
the cell becomes flaccid (limp)
© 2014 Pearson Education, Inc.
 In a hypertonic environment, plant cells lose water
 The membrane pulls away from the cell wall causing
the plant to wilt, a usually lethal effect called
plasmolysis
© 2014 Pearson Education, Inc.
Figure 7.UN04
“Cell”
0.03 M sucrose
0.02 M glucose
© 2014 Pearson Education, Inc.
“Environment”
0.01 M sucrose
0.01 M glucose
0.01 M fructose
CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
36
Resource Acquisition and
Transport in Vascular Plants
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
• The evolution of xylem and phloem in land plants
made possible the long-distance transport of water,
minerals, and products of photosynthesis
• Xylem transports water and minerals from roots to
shoots
• Phloem transports photosynthetic products from
sources to sinks
© 2014 Pearson Education, Inc.
Figure 36.2-3
CO2
O2
Sugar
Light
H2O
H2O and
minerals
© 2014 Pearson Education, Inc.
O2
CO2
The Apoplast and Symplast:
Transport Continuums
• The apoplast consists of everything external to the
plasma membrane
• It includes cell walls, extracellular spaces, and the
interior of vessel elements and tracheids
• The symplast consists of the cytosol of all the living
cells in a plant, as well as the plasmodesmata
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• Three transport routes for water and solutes
are
– The apoplastic route, through cell walls and
extracellular spaces
– The symplastic route, through the cytosol
– The transmembrane route, across cell walls
© 2014 Pearson Education, Inc.
Figure 36.5
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Plasmodesma
Plasma membrane
Apoplast
Symplast
© 2014 Pearson Education, Inc.
Short-Distance Transport of
Water Across Plasma Membranes
• To survive, plants must balance water uptake and
loss
• Osmosis is the diffusion of water into or out of a cell
that is affected by solute concentration and pressure
© 2014 Pearson Education, Inc.
• Water potential is a measurement that combines
the effects of solute concentration and pressure
• Water potential determines the direction of
movement of water
• Water flows from regions of higher water potential
to regions of lower water potential
• Potential refers to water’s capacity to perform work
© 2014 Pearson Education, Inc.
• Water potential is abbreviated as  and measured
in a unit of pressure called the megapascal (MPa)
– You are likely to see this most often as bars instead
on the AP test
•   0 MPa for pure water at sea level and at room
temperature
© 2014 Pearson Education, Inc.
How Solutes and Pressure Affect
Water Potential
• Both solute concentration and pressure affect
water potential
• This is expressed by the water potential
equation:   S  P
• The solute potential (S) of a solution is
directly proportional to its molarity
• Solute potential is also called osmotic
potential
© 2014 Pearson Education, Inc.
• Pressure potential (P) is the physical
pressure on a solution
• Turgor pressure is the pressure exerted by
the plasma membrane against the cell wall,
and the cell wall against the protoplast
• The protoplast is the living part of the cell,
which also includes the plasma membrane
© 2014 Pearson Education, Inc.
Water Movement Across Plant
Cell Membranes
• Water potential affects uptake and loss of
water by plant cells
• If a flaccid (limp) cell is placed in an
environment with a higher solute
concentration, the cell will lose water and
undergo plasmolysis
• Plasmolysis occurs when the protoplast
shrinks and pulls away from the cell wall
© 2014 Pearson Education, Inc.
Figure 36.7a
Initial flaccid cell:
ψP = 0
ψS = −0.7
Environment
0.4 M sucrose solution:
ψP = 0
ψS = −0.9
ψ = −0.7 MPa
Final plasmolyzed cell at osmotic
equilibrium with its surroundings:
ψP = 0
ψS = −0.9
ψ = −0.9 MPa
(a) Initial conditions: cellular ψ > environmental ψ
© 2014 Pearson Education, Inc.
ψ = −0.9 MPa
Figure 36.7b
Initial flaccid cell:
ψP = 0
ψS = −0.7
ψ = −0.7 MPa
Final turgid cell at osmotic
equilibrium with its surroundings:
ψP = 0.7
ψS = −0.7
ψ =
0 MPa
(b) Initial conditions: cellular ψ < environmental ψ
© 2014 Pearson Education, Inc.
Environment
Pure water:
ψP = 0
ψS = 0
ψ = 0 MPa
• If a flaccid cell is placed in a solution with a lower
solute concentration, the cell will gain water and
become turgid
• Turgor loss in plants causes wilting, which can be
reversed when the plant is watered
© 2014 Pearson Education, Inc.
Figure 36.UN02
Turgid
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Figure 36.UN03
Wilted
© 2014 Pearson Education, Inc.
Aquaporins: Facilitating
Diffusion of Water
• Aquaporins are transport proteins in the cell
membrane that facilitate the passage of water
• These affect the rate of water movement across the
membrane
© 2014 Pearson Education, Inc.
Figure 36.11
Xylem sap
Mesophyll cells
Outside air ψ
= −100.0 MPa
Stoma
Water molecule
Atmosphere
Leaf ψ (air spaces)
= −7.0 MPa
Trunk xylem ψ
= −0.8 MPa
Trunk xylem ψ
= −0.6 MPa
Soil ψ
= −0.3 MPa
© 2014 Pearson Education, Inc.
Water potential gradient
Leaf ψ (cell walls)
= −1.0 MPa
Transpiration
Xylem
cells
Adhesion by
hydrogen bonding
Cell
wall
Cohesion
by hydrogen
bonding
Cohesion and
adhesion in
the xylem
Water molecule
Root hair
Soil particle
Water
Water uptake from soil