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LE 5-10
MEMBRANE STRUCTURE AND
FUNCTION
• Membranes organize the chemical activities of
cells
Outside
of cell
– Membranes provide structural order for metabolism
• Form most of the cell's organelles
• Compartmentalize chemical reactions
– The plasma membrane forms a boundary between a
living cell and its surroundings
Cytoplasm
• Exhibits selective permeability
• Controls traffic of molecules in and out
Membrane phospholipids form a
bilayer
Hydrophilic head
• Phospholipids are the
main structural
components of
membranes
– Two nonpolar
hydrophobic fatty acid
"tails"
– One phosphate group
attached to the
hydrophilic glycerol
"head"
In membranes, phospholipids form a
bilayer
• Two-layer sheet with
– Phospholipid heads facing outward and tails facing
inward
– Selectively permeable
Phosphate
group
• Polar lipid-soluble molecules pass through
• Nonpolar molecules not soluble in lipids do not pass through
Symbol
Water
Hydrophilic
heads
Hydrophobic
tails
Hydrophobic tails
Water
LE 5-12
The membrane is a fluid mosaic of
phospholipids and proteins
Extracellular
matrix
Glycoprotein
Carbohydrate
• A membrane is a mosaic
– Proteins and other molecules are embedded in a
framework of phospholipids
• A membrane is fluid
– Most protein and phospholipid molecules can
move laterally
• Membrane glycoproteins and glycolipids
function in cell identification
Glycolipid
Plasma
membrane
Phospholipid
Proteins
Microfilaments Cholesterol
Cytoplasm
of cytoskeleton
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LE 5-13a
Proteins make the membrane a mosaic
of function
• Proteins perform most membrane functions
–
–
–
–
Identification tags
Junctions between adjacent cells
Enzymes
Receptors of chemical messages from other
cells (signal transduction)
– Transporters of substances across the
membrane
Enzyme activity
LE 5-13b
LE 5-13c
Messenger molecule
Receptor
Activated
molecule
ATP
Signal transduction
Transport
LE 5-14a
Passive transport is diffusion across a
membrane
Molecules of dye
• Diffusion is the tendency for particles to spread
out evenly in an available space from an area of
high concentration to an area of low concentration
– Passive transport across membranes occurs when a
molecule diffuses down a concentration gradient
– Small nonpolar molecules such as O2 and CO 2 diffuse
easily across the phospholipid bilayer of a membrane
Equilibrium
Membrane
Same concentration on both
sides
2
LE 5-14b
Transport proteins may facilitate
diffusion across membranes
Equilibrium
• In facilitated diffusion
– Transport proteins that span the membrane
bilayer help substances diffuse down a
concentration gradient
– To transport the substance, a transport protein
may
• Provide a pore for passage
• Bind the substance, change shape, and then release
the substance
LE 5-15
Solute
molecule
Osmosis is the diffusion of water
across a membrane
• In osmosis, water molecules diffuse across a
selectively permeable membrane
– From an area of low solute concentration to an
area of high solute concentration
– Until the solution is equally concentrated on
both sides of the membrane
– The direction of movement is determined by
the difference in total solute concentration
Transport
protein
• Not by the nature of the solutes
LE 5-16
Lower
concentration
of solute
Higher
concentration
of solute
Equal
concentration
of solute
Water balance between cells and their
surroundings is crucial to organisms
H2O
Solute
molecule
• Osmoregulation is the control of water
balance
• Tonicity is the tendency of a cell to lose or
gain water in solution
Selectively
permeable
membrane
Water
molecule
Solute molecule with
cluster of water molecules
Net flow of water
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LE 5-17
• Hypotonic solution: solute concentration is lower
outside the cell than inside.
Isotonic solution
– Cell gains water through osmosis
– Animal cell lyses; plant cell becomes turgid
• Hypertonic solution: solute concentration is higher
outside the cell than inside
Hypertonic solution
H 2O
H 2O
H 2O
H 2O
Animal
cell
– Cell loses water through osmosis
– Animal cell shrivels; plant cell plasmolyzes
Lysed
Normal
H 2O
• Isotonic solution: solute concentration is the same in
the cell and in the solution
– No net movement of water
– Animal cell volume remains constant; plant cell becomes
flaccid
Hypotonic solution
H 2O
Shriveled
Plasma
membrane
H 2O
H 2O
Plant
cell
Flaccid
Turgid
Plasmolysis
Shriveled
(plasmolyzed)
Turgid Elodea
Video: Turgid Elodea
LE 5-18
Cells expend energy for active
transport
• Active transport requires energy to
move solutes against a concentration
gradient
– ATP supplies the energy
– Transport proteins move solute molecules
across the membrane
Transport
protein
Solute
Solute
binding
ATP
P
ADP
P
Protein
changes shape
Transport
Phosphorylation
Phosphate P
detaches
Protein
reversion
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LE 5-19a
Exocytosis
Exocytosis and Endocytosis transport
large molecules
Fluid outside cell
• To move large molecules or particles
through a cell membrane
– A vesicle may fuse with the membrane and
expel its contents outside the cell (exocytosis)
– Membranes may fold inward, enclosing
material from the outside (endocytosis)
LE 5-19b
Vesicle
Protein
Cytoplasm
LE 5-19c
Endocytosis can occur in three ways
Food being ingested
LM 230×
Pseudopodium of amoeba
Vesicle forming
Phagocytosis
Material bound to receptor proteins
PIT
TEM 96,500×
TEM 54,000×
Plasma membrane
Cytoplasm
Pinocytosis
Receptor-mediated
endocytosis
LE 5-20
Faulty membranes can overload the
blood with cholesterol
Phospholipid outer layer
• Cholesterol is carried in the blood by lowdensity lipoprotein (LDL) particles
• Normally, body cells take up LDLs by receptormediated endocytosis
• Harmful levels of cholesterol can accumulate in
the blood if membranes lack cholesterol receptors
LDL particle
Vesicle
Cholesterol
Protein
Plasma
membrane
Receptor
protein
Cytoplasm
– People with hypercholesterolemia have more than
twice the normal level of blood cholesterol
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Animation: Membrane Selectivity
Animation: Diffusion
Animation: Osmosis
Animation: Active Transport
Animation: Exocytosis and Endocytosis Introduction
Animation: Exocytosis
Animation: Receptor-Mediated Endocytosis
Animation: Pinocytosis
Animation: Phagocytosis
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