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Chapter 5
 Cell Membrane Structure and Function
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
How Is the Structure of a Membrane Related to Its
Function?
 The phospholipid bilayer is the fluid portion of the
membrane
– A polar, hydrophilic head
– Two nonpolar, hydrophobic tails
head
(hydrophilic)
Biology: Life on Earth, 9e
tails
(hydrophobic)
Copyright © 2011 Pearson Education Inc.
The Phospholipid Bilayer of the Cell Membrane
 Plasma membranes face both exterior and interior
watery environments
– Hydrophobic and hydrophilic interactions drive
phospholipids into bilayers
phospholipid
extracellular fluid
(watery environment)
hydrophilic
heads
bilayer
hydrophobic
tails
hydrophilic
heads
Biology: Life on Earth, 9e
cytoplasm
(watery environment)Copyright © 2011 Pearson Education Inc.
Kinks Increase Fluidity
 The phospholipid bilayer’s flexible, fluid membrane
allows for cellular shape changes
– Individual phospholipid molecules are not bonded to one
another
unsaturated
more fluid
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saturated
less fluid
Copyright © 2011 Pearson Education Inc.
How Is the Structure of a Membrane Related to Its
Function?
 Maintaining fluidity
– Membranes become more fluid at high temperatures
(more movement) and less fluid at low temperatures
(less movement)
– Cell membranes of organisms living in lowtemperatures tend to be more unsaturated (more
kinks help them maintain fluidity)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
The Plasma Membrane
carbohydrate
extracellular fluid (outside)
protein
glycoprotein
extracellular
matrix
connection
protein
cholesterol
recognition
protein
binding receptor
phospholipid
site
protein
transport
phospholipid bilayer
protein
pore
enzyme
cytoskeleton
cytoplasm (inside)
Biology: Life on Earth, 9e
Fig. 5-1
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How Is the Structure of a Membrane Related to Its
Function?
 A variety of proteins form a mosaic within the
membrane
– Categories of membrane proteins
– Enzymatic proteins - proteins that promote chemical
reactions
– Receptor proteins – hormones trigger cellular
responses
– Recognition proteins - glycoproteins that serve as
identification tags on the surface of a cell
– Connection proteins - anchor the cell membrane
– Transport proteins – channel or carrier proteins
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Receptor Protein Activation
(extracellular fluid)
hormone
1 A hormone binds
to the receptor
receptor
2 Hormone binding
activates the receptor,
changing its shape
(cytoplasm)
Biology: Life on Earth, 9e
3 The activated receptor
stimulates a response
in the cell
Fig. 5-5
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How Is the Structure of a Membrane Related to Its
Function?
 There are two types of transport proteins
– These proteins regulate the movement of hydrophilic
molecules through the plasma membrane
– Channel proteins form channels whose central pores
allow specific ions or water molecules to pass through
the membrane
– Carrier proteins have binding sites that can
temporarily attach to specific molecules on one side of
the membrane and then move them through the
membrane to the other side
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Review Questions
1. Membranes consist of a bilayer of _______________
2. Explain why these molecules arrange themselves this way.
3. What is the other principal molecule in the plasma
membrane?
4. What functions do these two molecules provide?
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
How Do Substances Move Across Membranes?
 Molecules in fluids move in response to gradients
– Definitions relevant to substance movement are:
– A fluid is a substance whose molecules can flow past
one another and, therefore, have no defined shape
– A solute is a substance that can be dissolved
(dispersed as atoms, ions, or molecules) in a solvent
– A solvent is a fluid capable of dissolving a solute
– The concentration of a substance defines the
amount of solute in a given amount of solvent
– A gradient is a physical difference (in temperature,
pressure, charge, or concentration of a particular
solute in a fluid) between two adjoining regions of
space
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
How Do Substances Move Across Membranes?
 Molecules in fluids move in response to gradients
– Gradients in concentration or pressure cause molecules
to move from one place to another
– to equalize the difference
– Why gradients cause molecules to move from one place
to another:
– Molecules and ions in solution are in constant random
motion
– An increase in temperature increases the rate of this
random motion
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
How Do Substances Move Across Membranes?
 Movement through membranes occurs by passive
transport and energy-driven transport
– Passive transport is the diffusion of substances
across cell membranes down concentration gradients
– includes simple diffusion, facilitated diffusion, and
osmosis
– Energy-requiring transport is transport that requires
the use of cellular energy (ATP)
– includes active transport, endocytosis, and
exocytosis
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Simple Diffusion of a Dye in Water
1 A drop of dye is
placed in water
2 Dye molecules
diffuse into the water;
water molecules diffuse
into the dye
3 Both dye molecules
and water molecules are
evenly dispersed
drop of dye
water molecule
Fig. 5-6
• Passive transport
• Small molecules move across membranes by simple
diffusion (water, oxygen, carbon dioxide, and lipid-soluble
molecules like alcohol and vitamins A, D, and E)
Biology: Life on Earth, 9e
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Facilitated Diffusion
 Water soluble molecules (ions, amino acids, and
sugars) require the aid of channel and carrier
transport proteins
– Down gradient
– Many cells have specialized water channel proteins
called aquaporins
– small size and positive charges that attract the
negative pole of water molecules
Cl–
channel
protein
channel proteins
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water
aquaporin
aquaporins
glucose
carrier
protein
carrier proteins
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Osmosis
 The diffusion of water across selectively permeable
membranes
– Down gradient
– Dissolved substances displace water molecules, lowering
water concentration
Biology: Life on Earth, 9e
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The Effect of Solute Concentration on Osmosis
– Isotonic solutions have equal concentrations of water and
solutes
– A hypertonic solution is one with a greater solute
concentration
– A hypotonic solution has a lower solute concentration
No net flow
of water
(a) A balloon in an
isotonic solution
Biology: Life on Earth, 9e
Water flows out;
the balloon shrinks
(b) A balloon in a
hypertonic solution
Water flows in;
the balloon expands
Fig. 5-7
(c) A balloon in a
hypotonic solution
Copyright © 2011 Pearson Education Inc.
How Do Substances Move Across Membranes?
 Osmosis explains why fresh water protists have
contractile vacuoles
– Water leaks in continuously because the cytosol is
hypertonic to fresh water
– Salts are pumped into the vacuoles, making them
hypertonic to the cytosol
– Water follows by osmosis and is then expelled by
contraction
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
The Effects of Osmosis on Red Blood Cells
Fig. 5-9
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Active Transport
 Membrane proteins use cellular energy to move
molecules or ions across plasma membranes
– against their concentration gradients
– They often have a molecule binding site and an ATP
binding site
(extracellular fluid)
– Active transportTheproteins
are
often referred to The
asprotein
pumps
transport
2 Energy from ATP
1
protein binds both
ATP and Ca2
recognition
site
ATP
Ca2
Biology: Life on Earth, 9e
ATP
binding
site
changes the shape of the
transport protein and moves
the ion across the membrane
3
releases the ion and
the remnants of ATP
(ADP and P) and closes
ADP
ATP
P
(cytoplasm)
Copyright © 2011 Pearson Education Inc.
Endocytosis
 Cells engulf particles or fluids
– The engulfed particles are transported within the cell
inside vesicles
– Requires energy
 Three types:
– Pinocytosis (“cell drinking”) moves liquids into the cell
– Receptor-mediated endocytosis moves specific
molecules into the cell
– Phagocytosis (“cell eating”) moves large particles into
the cell
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Phagocytosis
(extracellular fluid)
food particle
pseudopods
1
(cytoplasm)
2
food
vacuole
3
11 The plasma membrane extends pseudopods toward an
extracellular particle (for example, food). 22 The ends of the
pseudopods fuse, encircling the particle. 33 A vesicle called
a food vacuole is formed containing the engulfed particle.
(b) An Amoeba engulfs a
Paramecium
(c) A white blood cell ingests
bacteria
(a) Phagocytosis
Fig. 5-14
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Exocytosis
 Moves material out of the cell
– Requires energy
– Disposes of undigested particles of waste or to secrete
substances into the extracellular fluid
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Why are most cells so small?
 Exchange of material across membranes
influences cell size and shape
– Most cells range in size from about 1 to 100µm
(micrometers) in diameter
– Cells need to exchange nutrients and wastes with the
environment
– As a spherical cell enlarges, its innermost parts get
farther away from the plasma membrane
– No part of the cell can be too far away from the external
environment
– Nerve and muscle cells and microvilli overcome size
restraints by elongating, thus keeping the ratio of surface
area to volume relatively high
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Surface Area and Volume Relationships
r
r
r
distance to
center (r)
1.0
2.0
4.0
12.6
50.3
201.1
Volume
(4/3r3)
4.2
33.5
268.1
area/volume
3.0
1.5
0.75
surface area
(4r2)
Biology: Life on Earth, 9e
Fig. 5-16
Copyright © 2011 Pearson Education Inc.
Review Questions
1. A membrane that is permeable to some substances but not
to others is described as being ______________________
2. The movement of a substance through a membrane down
its concentration gradient is called ________________
3. When applied to water, this process is called ____________
4. The general process by which fluids or particles are
transported into cells is called ___________________
5. Does this process require energy?
6. The specific term for engulfing particles is ______________
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Cell Attachment Structures
 Desmosomes attach cells tightly together
– under the stresses of movement in animal cells
– Examples include the skin, intestine, and heart
small intestine
plasma membranes
(edge view)
intermediate
filaments
in the
cytoplasm
desmosome
microvilli
cells lining
the small
intestine
In desmosomes,
protein filaments
hold cells together
(a) Desmosomes
Biology: Life on Earth, 9e
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Cell Attachment Structures
 Tight junctions make cell attachments leakproof
– found in animal cells
– where tubes and sacs must hold contents without leaking
– Examples include the skin and the urinary bladder
plasma membranes
(edge view)
urinary
bladder
cells lining
the bladder
In tight junctions,
proteins seal cells
together
(b) Tight junctions
Biology: Life on Earth, 9e
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Cell Communication Structures
 Gap junctions allow direct communication between
cells
– Cell-to-cell protein channels allowing for passage of
hormones, nutrients, and ions in animal cells
– Examples include heart and smooth muscle
plasma membranes
liver
liver
cells
In gap junctions, channel
proteins connect the
insides of adjacent cells
(a) Gap junctions
Biology: Life on Earth, 9e
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Cell Communication Structures
 Plasmodesmata allow direct communication
between cells
– Plant cells have holes in the walls of adjacent cells forming
cytoplasmic connections
– Similar to gap junctions in function
root In plasmodesmata,
membrane-lined
channels connect
the insides of
adjacent cells
plasma
cell walls
membranes
root
cells
(b) Plasmodesmata
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.