Download Life: The Science of Biology, 9e

6
Cell Membranes
6 Cell Membranes
• 6.1 What Is the Structure of a Biological Membrane?
• 6.2 How Is the Plasma Membrane Involved in Cell
Adhesion and Recognition?
• 6.3 What Are the Passive Processes of Membrane
Transport?
• 6.4 What Are the Active Processes of Membrane
Transport?
• 6.5 How Do Large Molecules Enter and Leave a Cell?
• 6.6 What Are Some Other Functions of Membranes?
6.1 What Is the Structure of a Biological Membrane?
The general structure of membranes is
known as the fluid mosaic model.
Phospholipids form a bilayer which is like
a “lake” in which a variety of proteins
“float.”
Figure 6.1 The Fluid Mosaic Model
Figure 3.20 Phospholipids (Part 1)
Figure 3.20 Phospholipids (Part 2)
6.1 What Is the Structure of a Biological Membrane?
Artificial bilayers can be made in the
laboratory.
Lipids maintain a bilayer organization
spontaneously. This helps membranes
fuse during phagocytosis, vesicle
formation, etc.
6.1 What Is the Structure of a Biological Membrane?
Membranes may vary in lipid composition.
Phospholipids vary in fatty acid chain
length, degree of saturation, and
phosphate groups.
Membranes may be up to 25 percent
cholesterol.
6.1 What Is the Structure of a Biological Membrane?
Phospholipid bilayers are flexible, and the
interior is fluid, allowing lateral
movement of molecules.
Fluidity depends on temperature and lipid
composition.
6.1 What Is the Structure of a Biological Membrane?
Membranes also contain proteins; the
number of proteins varies with cell
function.
6.1 What Is the Structure of a Biological Membrane?
Two types of membrane proteins:
• Peripheral membrane proteins lack
exposed hydrophobic groups and do not
penetrate the bilayer.
6.1 What Is the Structure of a Biological Membrane?
• Integral membrane proteins have
hydrophobic and hydrophilic regions or
domains.
Some extend across the lipid bilayer;
others are partially embedded.
Figure 6.3 Interactions of Integral Membrane Proteins
6.1 What Is the Structure of a Biological Membrane?
• Freeze-fracturing is a technique that
reveals proteins that are embedded in
the phospholipid bilayers of cellular
membranes.
Figure 6.4 Membrane Proteins Revealed by the Freeze-Fracture Technique
6.1 What Is the Structure of a Biological Membrane?
The proteins and lipids in the membrane
are independent and only interact
noncovalently.
But some membrane proteins have fatty
acids or other lipid groups covalently
attached and are referred to as
anchored membrane proteins.
6.1 What Is the Structure of a Biological Membrane?
Transmembrane proteins extend all the
way through the phospholipid bilayer.
They have one or more transmembrane
domains, and the domains on the inner
and outer sides of the membrane can
have specific functions.
6.1 What Is the Structure of a Biological Membrane?
Some membrane proteins can move
freely within the bilayer, while some are
anchored to a specific region.
When cells are fused experimentally,
some proteins from each cell distribute
themselves uniformly around the
membrane.
Figure 6.5 Rapid Diffusion of Membrane Proteins (Part 1)
Figure 6.5 Rapid Diffusion of Membrane Proteins (Part 2)
6.1 What Is the Structure of a Biological Membrane?
Membranes are dynamic and are constantly forming,
transforming, fusing, and breaking down.
6.1 What Is the Structure of a Biological Membrane?
Membranes also have carbohydrates on
the outer surface that serve as
recognition sites for other cells and
molecules.
Glycolipids—carbohydrate + lipid
Glycoproteins—carbohydrate + protein
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Cells arrange themselves in groups by
cell recognition and cell adhesion.
These processes can be studied in
sponge cells—the cells are easily
separated and will come back together
again.
Figure 6.6 Cell Recognition and Adhesion
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Molecules involved in cell recognition and
binding are glycoproteins.
Binding of cells is usually homotypic:
The same molecule sticks out from both
cells and forms a bond.
Some binding is heterotypic: The cells
have different proteins.
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Cell junctions are specialized structures
that hold cells together:
• Tight junctions
• Desmosomes
• Gap junctions
Figure 6.7 Junctions Link Animal Cells Together (A)
Tight junctions help ensure directional movement of materials.
Figure 6.7 Junctions Link Animal Cells Together (B)
Desmosomes are like “spot welds.”
Figure 6.7 Junctions Link Animal Cells Together (C)
Gap junctions allow communication.
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Cell membranes also adhere to the
extracellular matrix.
The transmembrane protein integrin
binds to the matrix outside epithelial
cells, and to actin filaments inside the
cells.
The binding is noncovalent and
reversible.
Figure 6.8 Integrins Mediate the Attachment of Animal Cells to the Extracellular Matrix
6.3 What Are the Passive Processes of Membrane Transport?
Membranes have selective permeability—
some substances can pass through, but
not others.
Passive transport—no outside energy
required (diffusion).
Active transport—energy required.
6.3 What Are the Passive Processes of Membrane Transport?
Diffusion: The process of random
movement toward equilibrium.
Equilibrium—particles continue to move,
but there is no net change in
distribution.
Figure 6.9 Diffusion Leads to Uniform Distribution of Solutes
6.3 What Are the Passive Processes of Membrane Transport?
Net movement is directional until
equilibrium is reached.
Diffusion is the net movement from
regions of greater concentration to
regions of lesser concentration.
6.3 What Are the Passive Processes of Membrane Transport?
Diffusion rate depends on:
• Diameter of the molecules or ions
• Temperature of the solution
• Concentration gradient
6.3 What Are the Passive Processes of Membrane Transport?
Diffusion works very well over short
distances.
Membrane properties affect the diffusion
of solutes.
The membrane is permeable to solutes
that move easily across it; impermeable
to those that can’t.
6.3 What Are the Passive Processes of Membrane Transport?
Simple diffusion: Small molecules pass
through the lipid bilayer.
Water and lipid-soluble molecules can
diffuse across the membrane.
Electrically charged and polar molecules
can not pass through easily.
6.3 What Are the Passive Processes of Membrane Transport?
Osmosis: The diffusion of water.
Osmosis depends on the number of
solute particles present, not the type of
particles.
Figure 6.10 Osmosis Can Modify the Shapes of Cells (Part 1)
Figure 6.10 Osmosis Can Modify the Shapes of Cells (Part 2)
Figure 6.10 Osmosis Can Modify the Shapes of Cells (Part 3)
6.3 What Are the Passive Processes of Membrane Transport?
If two solutions are separated by a
membrane that allows water, but not
solutes to pass through:
Water will diffuse from the region of
higher water concentration (lower solute
concentration) to the region of lower
water concentration (higher solute
concentration).
6.3 What Are the Passive Processes of Membrane Transport?
Isotonic solution: Equal solute
concentration (and equal water
concentration).
Hypertonic solution: Higher solute
concentration.
Hypotonic solution: Lower solute
concentration.
6.3 What Are the Passive Processes of Membrane Transport?
Water will diffuse (net movement) from a
hypotonic solution across a membrane
to a hypertonic solution.
Animal cells may burst when placed in a
hypotonic solution.
Plant cells with rigid cell walls build up
internal pressure that keeps more water
from entering—turgor pressure.
6.3 What Are the Passive Processes of Membrane Transport?
Facilitated diffusion of polar molecules
(passive):
• Channel proteins have a central pore
lined with polar amino acids.
• Carrier proteins—membrane proteins
that bind some substances and speed
their diffusion through the bilayer.
6.3 What Are the Passive Processes of Membrane Transport?
Ion channels: Specific channel proteins
with hydrophilic pores.
Most are gated—can be closed or open to
ion passage.
Gate opens when protein is stimulated to
change shape. Stimulus can be a molecule
(ligand-gated) or electrical charge resulting
from many ions (voltage-gated).
Figure 6.11 A Gated Channel Protein Opens in Response to a Stimulus
6.3 What Are the Passive Processes of Membrane Transport?
All cells maintain an imbalance of ion
concentrations across the plasma
membrane; thus a small voltage
potential exists.
Rate and direction of ion movement
through channels depends on the
concentration gradient and the
distribution of electrical charge.
6.3 What Are the Passive Processes of Membrane Transport?
Membrane potential is a charge
imbalance across a membrane.
Measured membrane potential of animal
cells: –70 mV (lots of potential energy)!
Membrane potential is related to the
concentration imbalance of K+ by the
Nernst equation.
6.3 What Are the Passive Processes of Membrane Transport?
Nernst equation:
[ K ]o
RT
EK  2.3
log
zF
[ K ]i
[ K ]o
E K  58 log
[ K ]i
6.3 What Are the Passive Processes of Membrane Transport?
The potassium channel allows K+ but not
Na+ to pass through.
K+ passes through in the unhydrated
state; hydrated Na+ is too large to pass.
Figure 6.12 The Potassium Channel
6.3 What Are the Passive Processes of Membrane Transport?
Water can cross a membrane by
“hitchhiking” with hydrated ions, or
moving through special water channels
called aquaporins.
The function of these proteins was
determined by injecting the aquaporin
mRNA into an oocyte.
Figure 6.13 Aquaporin Increases Membrane Permeability to Water (Part 1)
Figure 6.13 Aquaporin Increases Membrane Permeability to Water (Part 2)
6.3 What Are the Passive Processes of Membrane Transport?
In facilitated diffusion, carrier proteins
transport polar molecules such as
glucose across membranes in both
directions.
Glucose binds to the protein, which
causes it to change shape and release
glucose on the other side.
Figure 6.14 A Carrier Protein Facilitates Diffusion (Part 1)
Figure 6.14 A Carrier Protein Facilitates Diffusion (Part 2)
6.4 What Are the Active Processes of Membrane Transport?
Active transport: Moves substances
against a concentration and/or electrical
gradient—requires energy.
The energy source is often adenosine
triphosphate (ATP).
6.4 What Are the Active Processes of Membrane Transport?
Active transport is directional. It involves
three kinds of proteins:
• Uniporters
• Symporters
• Antiporters
Figure 6.15 Three Types of Proteins for Active Transport
6.4 What Are the Active Processes of Membrane Transport?
Primary active transport: Requires
direct hydrolysis of ATP.
Secondary active transport: Energy
comes from an ion concentration
gradient that is established by primary
active transport.
6.4 What Are the Active Processes of Membrane Transport?
The sodium–potassium (Na+–K+) pump
is primary active transport.
Found in all animal cells.
The pump is an integral membrane
glycoprotein (an antiporter).
Figure 6.16 Primary Active Transport: The Sodium–Potassium Pump
6.4 What Are the Active Processes of Membrane Transport?
In secondary active transport, energy can
be “regained” by letting ions move
across a membrane with the
concentration gradient.
• Aids in uptake of amino acids and
sugars.
• Uses symporters and antiporters.
Figure 6.17 Secondary Active Transport
6.5 How Do Large Molecules Enter and Leave a Cell?
Macromolecules (proteins,
polysaccharides, nucleic acids) are too
large to cross the membrane.
They can be taken in or secreted by
means of membrane vesicles.
6.5 How Do Large Molecules Enter and Leave a Cell?
Endocytosis: Processes that bring
molecules and cells into a eukaryotic cell.
The plasma membrane folds in or
invaginates around the material, forming a
vesicle.
Figure 6.18 Endocytosis and Exocytosis (A)
6.5 How Do Large Molecules Enter and Leave a Cell?
Phagocytosis: Molecules or entire cells
are engulfed. Some protists feed in this
way. Some white blood cells engulf
foreign substances
A food vacuole or phagosome forms,
which fuses with a lysosome.
6.5 How Do Large Molecules Enter and Leave a Cell?
Pinocytosis: A vesicle forms to bring
small dissolved substances or fluids into
a cell. Vesicles are much smaller than in
phagocytosis.
Pinocytosis is constant in endothelial
(capillary) cells.
6.5 How Do Large Molecules Enter and Leave a Cell?
Receptor mediated endocytosis is
highly specific:
Depends on receptor proteins—integral
membrane proteins—to bind to specific
substances.
Sites are called coated pits—coated with
other proteins such as clathrin.
Figure 6.19 Receptor-Mediated Endocytosis (Part 1)
Figure 6.19 Receptor-Mediated Endocytosis (Part 2)
6.5 How Do Large Molecules Enter and Leave a Cell?
Mammalian cells take in cholesterol by
receptor-mediated endocytosis.
In the liver, cholesterol is packaged into
low-density lipoprotein, or LDL, and
secreted to the bloodstream.
Cells that need cholesterol have
receptors for the LDLs in clathrin-coated
pits.
6.5 How Do Large Molecules Enter and Leave a Cell?
Exocytosis: Material in vesicles is expelled
from a cell.
Indigestible materials are expelled.
Other materials leave cells such as
digestive enzymes and neurotransmitters.
Figure 6.18 Endocytosis and Exocytosis (B)
6.6 What Are Some Other Functions of Membranes?
Some membranes are electrically
excitable:
• The plasma membrane of neurons
conducts nerve impulses.
• In muscle cells, electrical excitation
results in muscle contraction.
6.6 What Are Some Other Functions of Membranes?
Some membranes transform energy:
• Inner mitochondrial membranes—
energy from fuel molecules is
transformed to ATP.
• Thylakoid membranes of chloroplasts
transform light energy to chemical
bonds.
Figure 6.20 Other Membrane Functions (Part 1)
6.6 What Are Some Other Functions of Membranes?
Some membrane proteins can organize
chemical reactions:
• Many cellular processes involve a series
of enzyme-catalyzed reactions—all the
molecules must come together for these
to occur. Forms an “assembly line” of
enzymes.
Figure 6.20 Other Membrane Functions (Part 2)
6.6 What Are Some Other Functions of Membranes?
Some membrane proteins process
information:
• Binding of a specific ligand can initiate,
stop, or modify cell functions.
Figure 6.20 Other Membrane Functions (Part 3)