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The plasma membrane
• The plasma membrane is the boundary that
separates the internal contents of the cell
from its external environment.
• The plasma membrane exhibits selective
permeability, allowing some substances to
cross it more easily than others
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Concept 7.1: Cellular membranes are fluid mosaics
of lipids and proteins
• Phospholipids are the
most abundant lipid in
the plasma membrane
• Phospholipids are
amphipathic
molecules, containing
hydrophobic and
hydrophilic regions
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Hydrophilic
head
Hydrophobic
tails
Membrane Model
• The fluid mosaic model states that a membrane
is a fluid structure with a “mosaic” of various
proteins embedded in it
Hydrophilic region
of protein
Phospholipid
bilayer
Hydrophobic region of protein
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LE 7-5b
Fluid
Unsaturated hydrocarbon
tails with kinks
Membrane fluidity
Viscous
Saturated hydrocarbon tails
Cholesterol and Membranes’ Fluidity
• The steroid cholesterol has different effects
on membrane fluidity at different
temperatures
• At warm temperatures (such as 37°C),
cholesterol restrains movement of
phospholipids
• At cool temperatures, it maintains fluidity by
preventing tight packing
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LE 7-5c
Cholesterol
Cholesterol within the animal cell membrane
In-Text Art, Chapter 5, p. 84 (2)
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LE 7-7
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
http://www.youtube.com/watch?v=S7-VFJHd0uA&list=PL13jH2BYL65_q5j1vBga32xNiYT57P5KC
Transport
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Enzymatic Activity
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Receptors for Signal Transduction
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Cell-Cell Recognition
• Cells recognize each other by binding to
surface molecules, often carbohydrates,
on 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
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Cell-Cell Recognition
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Synthesis of Membrane Components and Their
Orientation in the Membrane
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Concept 7.2: Membrane structure results in
selective permeability
• A cell must exchange materials with its
surroundings, a process controlled by
the plasma membrane
• Plasma membranes are selectively
permeable, regulating the cell’s
molecular traffic
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The Permeability of the Lipid Bilayer
• Hydrophobic (nonpolar)
molecules, such as hydrocarbons,
can dissolve in the lipid bilayer
and pass through the membrane
rapidly
• Polar molecules, such as sugars,
do not cross the membrane
easily.
• Hydrophilic substances require
transport proteins to cross the
membrane.
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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 exhibit a net
movement in one direction.
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Diffusion
• Substances diffuse down their concentration
gradient, which is the difference in concentration
of a substance from one area to another
• 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 it
requires no energy from the cell to make it happen
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Types of Membrane Transport
• Passive processes
– No cellular energy (ATP) required
– Substance moves down its concentration
gradient (i.e from high to low concentration)
• Active processes
– Energy (ATP) required
– Occurs only in living cell membranes
– Substance moves against its concentration
gradient (i.e from low to high concentration)
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Types of Passive Processes
• Simple diffusion
• Facilitated diffusion:
– Carrier-mediated facilitated diffusion
– Channel-mediated facilitated
diffusion
• Osmosis
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Passive Processes: Simple
Diffusion
• Nonpolar lipid-soluble (hydrophobic, such as O2,
CO2) substances diffuse directly through the
phospholipid bilayer
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Passive Processes: Simple
Diffusion
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Small nonpolar solutes move down
their concentration gradients.
Interstitial
fluid
Oxygen
Cytosol
Carbon dioxide
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•At dynamic equilibrium, as many molecules cross
one way as cross in the other direction
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Simple Diffusion of one solute
Equilibrium
LE 7-11b
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Simple Diffusion of two solutes
Equilibrium
Equilibrium
Passive Processes: Facilitated Diffusion
• Certain hydrophilic 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
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Transport Proteins
• Transport proteins allow passage of
hydrophilic substances across the
membrane
• Some transport proteins, called channel
proteins, have a hydrophilic channel that
certain molecules or ions can use as a
tunnel
• Channel proteins called aquaporins
facilitate the passage of water
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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
Channel Transport Proteins
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• Other transport proteins, called carrier
proteins, bind to molecules and
change shape to shuttle them across
the membrane
• A transport protein is specific for the
substance it moves
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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
Carrier Transport Proteins
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Effects of Osmosis on Water Balance
• Osmosis is the diffusion of
water across a selectively
permeable membrane
• The direction of osmosis is
determined only by a
difference in total solute
concentration
• Water diffuses across a
membrane from the region of
lower solute concentration to
the region of higher solute
concentration
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
H2O
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Osmosis
Same concentration
of sugar
Water Balance of Cells
• Tonicity is the ability of a 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
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Water Balance of Cells
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
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Water Balance of Cells Without Cell Walls
• Animals and other organisms without rigid cell
walls have osmotic problems in either a hypertonic
or hypotonic environment
• To maintain their internal environment, such
organisms must have adaptations for
osmoregulation, the control of water balance
• The protist Paramecium, which is hypertonic to its
pond water environment, has a contractile vacuole
that acts as a pump to get rid of excess water.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 7-14
Filling vacuole
Paramecium
Contracting vacuole
50 µm
50 µm
Figure 4.12
Isotonic solution
Hypotonic solution
Interstitial fluid is less
concentrated than cytosol.
Interstitial fluid is the same
concentration as cytosol.
Erythrocyte
Water
leaves
cell.
Erythrocyte
Erythrocyte
SEM 11,550x
SEM 9030x
SEM 6900x
Interstitial fluid is more
concentrated than cytosol.
Water
enters
cell.
No net
movement
of water.
Normal erythrocytes
(a)
Hypertonic solution
Erythrocytes nearing hemolysis
(b)
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Erythrocytes undergoing crenation
(c)
CRENATION
HUMAN RED BLOOD CELL
In 4% Salt Solution
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HEMOLYSIS
HUMAN RED BLOOD CELL
In Distilled Water
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Thinking Questions!
• Solutions with a solute concentration greater
than 1% are_________ compared to a RBC.
• Solutions with a solute concentration less than
1% are _________compared to a RBC.
• Solutions with a solute
concentration___________1% are isotonic
compared to a RBC.
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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), and the plant may
wilt
• In a hypertonic environment, plant cells lose
water; eventually, the membrane pulls away from
the wall, a usually lethal effect called plasmolysis
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Water Balance of Plant Cells
Plasmolyzed
Flaccid
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Turgid
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Concept 7.4: Active transport uses energy to move
solutes against their gradients
• Active transport moves substances
against their concentration gradient
• Active transport requires energy,
usually in the form of ATP
• Active transport is performed by
specific proteins embedded in the
membranes.
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Active Transport
• Active transport allows cells to maintain
concentration gradients that differ from
their surroundings
• The sodium-potassium pump is one
type of active transport system
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LE 7-16
EXTRACELLULAR [Na+] high
FLUID
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
[Na+] low
[K+] high
Na+
Cytoplasmic Na+ bonds to
the sodium-potassium pump
P
ATP
P
ADP
Na+ binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.
P
P
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
Active Transport / The sodium-potassium pump
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LE 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion