Transcript 5.1 How Is the Structure of the Cell Membrane Related to Its Function?

5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 All the membranes of a cell have a similar basic
structure
– Proteins suspended in a double layer of phospholipids
 Phospholipids are responsible for the isolating
function of membranes
 Proteins are responsible for selectively exchanging
substances and communicating with the
environment, controlling biochemical reactions, and
forming attachments
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 Functions of the plasma membrane
– It isolates the cell’s contents from the external
environment
– It regulates the exchange of essential substances
– It allows communication between cells
– It creates attachments within and between cells
– It regulates biochemical reactions
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 Membranes are “fluid mosaics” in which proteins
move within layers of lipids
– The “fluid mosaic” model of a membrane was
proposed in 1972 by S.J. Singer and G.L. Nicolson
– This model indicates that each membrane consists of a
mosaic, or “patchwork,” of different proteins that
constantly shift and flow within a viscous fluid formed by
a double layer of phospholipids
– A fluid is any substance whose molecules can flow
past one another and includes gases, liquids, and cell
membranes
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Figure 5-1 The plasma membrane
carbohydrate
interstitial fluid (outside)
protein
extracellular
matrix
glycoprotein
binding site
connection
protein
cholesterol
recognition
protein
receptor phospholipid
bilayer
protein
phospholipid
pore
enzyme
cytoskeleton
cytosol (fluid inside cell)
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transport
protein
5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 The fluid phospholipid bilayer helps to isolate the
cell’s contents
– Phospholipids are the basis of membrane structure
and consist of two very different parts
– A polar, hydrophilic head
– Two nonpolar, hydrophobic tails
– The outer surfaces of animal plasma membranes are
bathed in watery interstitial fluid, a weakly salty
liquid resembling blood without its cells or proteins
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Figure 5-2 A phospholipid
tails
(hydrophobic)
head
(hydrophilic)
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 The fluid phospholipid bilayer helps to isolate the
cell’s contents (continued)
– The phospholipid bilayer is the fluid portion of the
membrane
– Plasma membranes face both exterior and interior
watery environments
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 The fluid phospholipid bilayer helps to isolate the
cell’s contents (continued)
– Hydrophobic and hydrophilic interactions drive
phospholipids into bilayers
– Hydrogen bonds between water and the hydrophilic
heads cause the heads of the outer layer to orient
outward toward the watery exterior, while the heads of
the inner layer face the watery interior
– The nonpolar tails, being hydrophobic, face the inside
of the membrane, away from the watery environment
around the bilayer
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Animation: Plasma Membrane Structure
Figure 5-3 The phospholipid bilayer of the cell membrane
phospholipid
interstitial fluid
(watery environment)
hydrophilic
head
hydrophobic
tails
bilayer
hydrophilic
head
cytosol
(watery environment)
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 The fluid phospholipid bilayer helps to isolate the
cell’s contents (continued)
– The phospholipid bilayer’s flexible, fluid membrane
allows for cellular shape changes
– Individual phospholipid molecules are not bonded to
one another
– Some of the phospholipids have unsaturated fatty
acids, whose double bonds introduce “kinks” into their
“tails”
– The above features make the membrane fluid
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Figure E5-1 Tail kinks in phospholipids increase membrane fluidity
more unsaturated
greater fluidity
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more saturated
less fluidity
5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 The fluid phospholipid bilayer helps to isolate the
cell’s contents (continued)
– Water-soluble substances such as salts, amino acids,
and sugars cannot easily cross phospholipid bilayers
– However, the isolation provided by the plasma
membrane is not complete, and very small molecules
such as water, oxygen, and carbon dioxide as well as
larger, lipid-soluble molecules can pass through this
selective barrier
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 The fluid phospholipid bilayer helps to isolate the
cell’s contents (continued)
– Membranes become more fluid at high temperatures
(more movement) and less fluid at low temperatures
(less movement)
– Cell membranes of organisms living in low
temperatures tend to be more unsaturated (more kinks
help them maintain fluidity)
– Cholesterol stabilizes membranes, affecting fluidity
and reducing permeability
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 A variety of proteins form a mosaic within the
membrane
– Proteins are embedded within, or attached to, the
phospholipid bilayer
– Many proteins have attached carbohydrates
(glycoproteins) on their outer membrane surface
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 A variety of proteins form a mosaic within the
membrane (continued)
– Membrane proteins may be grouped into five major
categories
– Enzymes
– Recognition proteins
– Receptor proteins
– Connection proteins
– Transport proteins
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 A variety of proteins form a mosaic within the
membrane (continued)
– Enzymes are proteins that promote chemical
reactions that synthesize or break apart biological
molecules
– Some plasma membrane enzymes are used to
synthesize the extracellular matrix, a web of protein and
glycoprotein fibers that fills spaces between animal
cells
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 A variety of proteins form a mosaic within the
membrane (continued)
– Receptor proteins trigger cellular responses upon
binding of specific molecules, such as hormones, sent
by other cells
– Recognition proteins are glycoproteins that serve as
identification tags on the surface of a cell
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Figure 5-4 Receptor protein activation
(interstitial fluid)
messenger
molecule
A messenger molecule
binds to the receptor protein
receptor
protein
Messenger molecule
binding activates the
receptor protein,
changing its shape
(cytosol)
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The activated receptor
protein stimulates a
response in the cell
5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 A variety of proteins form a mosaic within the
membrane (continued)
– Connection proteins anchor cell membranes in
various ways
– Maintain cell shape by linking the plasma membrane to
the cell’s cytoskeleton
– Link the cytoskeleton inside the cell with the
extracellular matrix outside, anchoring the cell in place
within a tissue
– Form connections between adjacent cells
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5.1 How Is the Structure of the Cell Membrane
Related to Its Function?
 A variety of proteins form a mosaic within the
membrane (continued)
– Transport proteins regulate the movement of
hydrophilic molecules through the plasma membrane
– Channel proteins are pores that can be opened or
closed to allow specific substances to pass across the
membrane
– Carrier proteins bind substances and carry them
through the membrane, sometimes using cellular
energy
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