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Chapter 5
Membrane Structure and Function
Chapter 5 Outline
• 5.1 Structure of a Membrane Related to Its
• 5.2 How Do Substances Move Across
• 5.3 How Do Specialized Junctions Allow Cells to
Connect and Communicate?
Functions Plasma Membrane
• __________ the cell’s contents from
• ______________ of essential substances
• _______________ with other cells
• Creates _______________ within and
between other cells
• ______________ biochemical reactions
Membranes Are “Fluid Mosaics”
• Membranes are dynamic,
ever-changing structures
• ______________ model of a
membrane proposed in 1972
– A lumpy, constantly shifting
mosaic of “tiles” or
– Proteins float around in a sea
of _________________
The Phospholipid Bilayer
• Phospholipids are the basis of
membrane structure
– Polar, ____________ head
– Two non-polar, ___________
The Phospholipid Bilayer
• The cell exterior and interior face
_______________ environments
The Phospholipid Bilayer
• Hydrophobic and hydrophilic
interactions drive
phospholipids into bilayers
– ______ row of phospholipids
– Polar heads face _______
– Non-polar tails mingle
_______ the membrane
– Cholesterol in animal
membranes keeps them
The Phospholipid Bilayer
• Phospholipid bilayer is a _____________
membrane to allow for cellular _______
• Individual phospholipid molecules are
____ __________ to one another
The Phospholipid Bilayer
• Some of the phospholipids have
_______________ fatty acids, whose
double bonds introduce “kinks” into their
• The above features make the membrane
Membrane Proteins Form a Mosaic
Proteins are embedded
in the phospholipid
– Some proteins can ____
and drift
– Other proteins are
___________ by protein
filaments in the
– Many proteins have
attached carbohydrates
Membrane Proteins Form a Mosaic
• Categories of membrane proteins
____________ Proteins
_____________ Proteins
_________________ Proteins
_________________ Proteins
Membrane Proteins Form a Mosaic
Receptor Proteins
– Trigger cellular responses upon ______
___________________, e.g. hormones
Membrane Proteins Form a Mosaic
Recognition Proteins
– Serve as ____________________ on the
surface of a cell
Membrane Proteins Form a Mosaic
– Promote ________________
that synthesize or break apart
biological molecules
Attachment Proteins
– ________ the cell membrane to
inner _________________, to
proteins outside the cell, and to
other cells
Membrane Proteins Form a Mosaic
Transport Proteins
– Include _________ and __________
– Regulate _____________ of hydrophilic
Section 5.2 Outline
5.2 How Do Substances Move Across
– Molecules in Fluids Move in Response to
– Movement Across Membranes Occurs by
Both ____________________ Transport
– Passive Transport Includes ___________
– ___________ Plays an Important Role in
Section 5.2 Outline
5.2 How Do Substances Move Across
Membranes? continued
– Active Transport Uses________ to Move
Molecules Against Their Concentration
– Cells Engulf Particles or Fluids by
– _________________ Moves Material Out of
the Cell
– Exchange of Materials Across Membranes
Influences Cell _____________________
Movement of Molecules in Fluids
• Definitions relevant to substance
– A ______ is a substance that can move or
change shape in response to external forces
– A ____________ is a substance that can be
dissolved (dispersed as ions or molecules) in
a solvent
– A ___________ is a fluid capable of
dissolving a solute
Movement of Molecules in Fluids
• Definitions relevant to substance
movement (continued)
– The concentration of molecules is the
__________ of them in a given _______unit
– A gradient is a physical ___________ in
temperature, pressure, charge, or
concentration in two adjacent regions
Movement of Molecules in Fluids
• Why molecules move from one place to
– Substances move in response to a
• Molecules move from ______________
concentration (____________) until
dynamic ________________ is reached
Movement of Molecules in Fluids
• The greater the concentration gradient, the
________ the rate of diffusion
• _____________ cannot move molecules
rapidly over long distances
Movement Across Membranes
• Concentration gradients of _________________
exist across the plasma membranes of all cells
• There are two types of movement across the
plasma membrane
Movement Across Membranes
• Passive transport
– Substances move _____________________
gradients across a membrane
– _____________ is expended
– Membrane proteins and phospholipids may limit
which molecules can cross, but not the
___________ of movement
Movement Across Membranes
• Energy-requiring transport
– Substances are driven __________ their
concentration gradients
– __________ is expended
Passive Transport
• Plasma membranes are selectively
– Different molecules move across at
different ______________________
– A concentration gradient drives all three
types of passive
Passive Transport
• Simple diffusion
– _________ soluble
molecules (e.g.
vitamins A and E,
gases) and
molecules diffuse
___________ across
the phospholipid
Passive Transport
• Facilitated
– Water soluble
molecules like
ions, amino acids,
and sugars diffuse
with the ________
Passive Transport
• Osmosis – the special case of water
– Water diffuses from _________________
(high purity) to __________________ (low
purity) across a membrane
– Dissolved substances ________________
_________________ of free water molecules
(and hence the purity of water) in a solution
Passive Transport
• The flow of water across a membrane
depends on the concentration of water in the
internal or external solutions
Passive Transport
• Comparison terms for solutions
on either side of a membrane
– Isotonic solutions have _____
concentrations of water and equal
concentrations of dissolved
• ___________________ occurs
across the membrane
Passive Transport
• Comparison terms for solutions on either
side of a membrane (continued)
– A hypertonic solution is one with _______
water concentration or ________ dissolved
particle concentration
• Water moves across a membrane towards
the hypertonic solution
Passive Transport
• Comparison terms for solutions on either
side of a membrane (continued)
– A hypotonic solution is one with higher water
concentration or ___________________
particle concentration
• Water moves across a membrane _____
from the hypotonic solution
Passive Transport
• The effects of osmosis are illustrated when
red blood cells are placed in various
Passive Transport
• Osmosis explains why fresh water protists
have _____________ vacuoles
• Water __________ continuously because
the cytosol is __________to fresh water
• Salts are pumped into the vacuoles, making
them hypertonic to the cytosol
• Water follows by osmosis and is then
________________ by contraction
Active Transport
• Cells need to move some substances
against their concentration gradients
Active Transport
• Active-transport membrane proteins move
molecules across using ________
– Proteins span the entire membrane
– Often have a ___________ binding site and
an ___________ binding site
– Often referred to as ___________
• Cells import _______________________
via endocytosis
• Plasma membrane pinches off to form a
___________ in endocytosis
– Types of endocytosis
• _______________
• _______________________ endocytosis
• ____________________
• Types of endocytosis
– Pinocytosis _________________ brings in
droplet of extracellular fluid
• Types of endocytosis
– Receptor-mediated endocytosis moves
___________________ into the cell
• Types of endocytosis
– Phagocytosis (“cell eating”) moves _______
___________________________ into the cell
• Exocytosis
– Vesicles join the membrane, _____________
contents in exocytosis
Cell Size and Shape
• ______________ affects
cell size and shape
– As a spherical cell
enlarges, its innermost
parts get farther away from
the plasma membrane
– Also, its ___________
increases more rapidly than
its ___________ area
– A larger cell has a relatively
___________ area of
membrane for nutrition
exchange than a small cell
Section 5.3 Outline
• 5.3 How Do Specialized Junctions Allow
Cells to Connect and Communicate?
– _______________ Attach Cells Together
– _____________ Make Cell Attachments
– ___________________________ Allow Direct
Communication Between Cells
• Desmosomes attach cells
– Found where __________
_____________ together
under the stresses of
movement (e.g. the skin)
Tight Junctions
• Tight junctions make
the cell leakproof
– Found where tubes and
sacs ______________
(e.g. the urinary bladder)
Gap Junctions and Plasmodesmata
• Gap junctions and
plasmodesmata allow for
– Cell-to-cell __________
allowing for passage of
______________________ in
animal cells are gap junctions
Gap Junctions and Plasmodesmata
– Plant cells have cytoplasmic
connections called
Caribous Legs and Membranes
• Membrane function varies between
Caribous Legs and Membranes
• Plasma membrane phospholipids in
caribous legs adapted for cold
– Cold areas near hooves have more membrane
___________ fatty acids to keep cell
membranes fluid
Viscous Venoms
• Snake and spider venoms
contain _______________,
enzymes that break down
• Venoms attack cell
membranes, causing cells
to _________________
• Cell death destroys tissue
around bite