6
MEMBRANES AND TRANSPORT
Chapter Outline
6.1 MEMBRANE STRUCTURE
Biological membranes contain both lipid and protein molecules
The fluid mosaic model explains membrane structure
The fluid mosaic model is fully supported by experimental evidence
6.2 FUNCTIONS OF MEMBRANES IN TRANSPORT: PASSIVE TRANSPORT
Passive transport is based on diffusion
Substances move passively through membranes by simple or facilitated diffusion
Two groups of transport proteins carry out facilitated diffusion
6.3 PASSIVE WATER TRANSPORT AND OSMOSIS
Osmosis can be demonstrated in a purely physical system
The free energy released by osmosis may work for or against cellular life
6.4 ACTIVE TRANSPORT
Active transport requires a direct or indirect input of energy derived from ATP hydrolysis
Primary active transport moves positively charged ions across membranes
Secondary active transport moves both ions and organic molecules across membranes
6.5 EXOCYTOSIS AND ENDOCYTOSIS
Exocytosis releases molecules to the outside by means of secretory vesicles
Endocytosis brings materials into cells in endocytic vesicles
Learning Objectives
After reading the chapter, you should be able to:
1.
Understand the essential structure and function of the plasma membrane.
2.
Be familiar with the membrane proteins and their functions in a cell.
3.
Know the forces that cause water and solutes to move across membranes passively (without energy).
4.
Distinguish between substances that move by simple diffusion and by facilitated diffusion.
5.
Know the mechanisms by which substances are moved across membranes against a concentration gradient (actively,
with energy).
6.
Understand the importance of osmosis to all cells including hypo-, hyper-, and isotonic solutions.
7.
Understand how material can be imported into or exported from a cell by being wrapped in membranes.
Key Terms
plasma membrane
cell adhesion proteins
transport
simple diffusion
bilayer
glycocalyx
passive transport
facilitated diffusion
cholesterol
fluid mosaic model
active transport
channel proteins
transport proteins
asymmetric
diffusion
gated channels
recognition proteins
integral proteins
concentration gradient
carrier proteins
receptor proteins
freeze-fracture
technique
selectively permeable
osmosis
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hypotonic
turgor pressure
secondary active
transport
electrochemical
gradient
receptor-mediated
endocytosis
hypertonic
proton pumps
symport (cotransport)
coated pit
plasmolysis
calcium pump
clathrin
isotonic
sodium-potassium
pump
antiport (exchange
diffusion)
primary active
transport
membrane potential
phagocytosis
bulk-phase endocytosis
(pinocytosis)
Lecture Outline
A. All organisms encounter environmental factors that could disrupt water content and internal concentration
of ions and molecules.
1. Striped bass (Morone saxatilis) face drastic changes moving between ocean and freshwater
environments. Oceans are more salty than ions inside the fish, and in freshwater, the cell contents are
more salty.
B. The plasma membrane, the thin layer of lipids and proteins that covers cells, makes this possible.
C. The plasma membrane is the primary zone of contact between a cell and its environment. Only selected
ions can move across the barrier to enter or leave the cell.
D. The structure and function of biological membranes are the focus of this chapter.
6.1 Membrane Structure
A. A water fluid medium or aqueous solution bathes both sides of all biological membranes.
B. Biological membranes contain both lipid and protein molecules
1. The proportions of lipid and protein molecules vary depending on the function of the membranes in the
cells.
C. Membrane Lipids
1. Phospholipids and sterols are the two major types of lipids in membranes.
a. The polar end is hydrophilic and the nonpolar end is hydrophobic; thus, phospholipids have dual
solubility properties.
2. In an aqueous medium, phospholipid molecules assemble in a bilayer that satisfies their duel solubility.
a. At low temperatures, the phospholipid bilayer becomes frozen to produce a semisolid gel-like
state.
b. When phospholipids are shaken in water, they break and spontaneously form small vesicles, a
spherical shell with a small droplet of water inside.
3. Membrane sterols pack into membranes alongside the phospholipid hydrocarbon chains.
a. In animals, the predominant sterol is cholesterol, and plants have a variety of sterols called
phytosterols.
D. Membrane Proteins
1. The membrane proteins also have duel solubility. The hydrophobic segments are often wound into
alpha helices, which span the membrane bilayer with loops of hydrophilic amino acids that extend into
polar regions.
2. Each type of membrane has a characteristic group of proteins responsible for specialized functions.
a. Transport proteins form channels that allow for selected polar molecules to pass across a
membrane.
b. Recognition proteins in the plasma membrane identify a cell as part of the same individual or as
foreign.
c. Receptor proteins recognize and bind molecules from other cells that act as chemical agents.
d. Cell adhesion proteins bind cells together by recognizing and binding receptors.
E. Membrane Glycolipids and Glycoproteins
1. The carbohydrate groups (glycol), which are polar, are attached covalently to parts of membrane lipid
and protein molecules and give cells a “sugar coating” or glycocalyx (glykys = sweet; calyx = cup or
vessel).
F. The fluid mosaic model explains membrane structure.
1. The current fluid mosaic model proposes that membranes consist of a fluid phospholipid bilayer in
which proteins are embedded and float freely.
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2.
The “fluid” part refers to phospholipid molecules, which vibrate back and fourth, move, and exchange
places within the same bilayer at a rate of a million times per second. Membrane fluidity is critical to
the function of membrane proteins.
3. Membranes remain fluid at a wide range of temperatures.
a. At low temperatures, the phospholipid molecules become closely packed, and the membrane
becomes a nonfluid gel.
b. Cholesterol and unsaturated fatty acid chains in the membrane decrease fluidity at moderately high
temperatures; however, they also slow the transition of the membrane to the nonfluid gel.
4. At high temperatures, membranes can become too fluid and will become leaky.
5. The “mosaic” part of the fluid mosaic model refers to the membrane proteins. The membrane proteins
move more slowly, and some are attached to the cytoskeleton and become immobile or move in a
directed fashion.
6. Membrane proteins are oriented and face either inside or outside the membrane surface. The
differences make biological membranes asymmetrical and give their inside and outside surfaces
different functions.
7. Proteins that are embedded in the phospholipid bilayer are termed integral proteins.
8. Other proteins called peripheral proteins are held to membranes surfaces by noncovalent bonds; most
are on the cytoplasmic side of the membrane with some being part of the cytoskeleton.
G. The fluid mosaic model is fully supported by experimental evidence.
1. The novel ideas of a fluid membrane and a flexible mosaic arrangement of protein and lipids have been
completely supported by experimental evidence.
H. Evidence That Membranes Are Fluid
1. Frye and Edidin grew human cells and mouse cells separately in tissue culture. Anti-human antibodies
were made to fluoresce with red color, and anti-mouse antibodies were made to fluoresce green.
a. Fused cells started out half red and half green.
b. Within 40 minutes, the colors were completely intermixed.
2. The membrane layer appears to be about as fluid as light machine oil.
I. Evidence for Membrane Asymmetry and Individual Suspension of Proteins
1. Electron microscopy, using freeze-fracture techniques, confirmed that the membrane bilayer has
proteins suspended in it individually and that the arrangement of membrane lipids and proteins is
asymmetrical.
6.2 Functions of Membranes in Transport: Passive Transport
A. The primary function of cellular membranes is transport, which is typically directional and specific.
B. Transport occurs by two mechanisms
1. Passive transport is when molecules move from an area of high concentration to an area of low
concentration (move with the concentration gradient).
C. Active transport moves molecules against the concentration gradient and uses energy directly or indirectly
from the breakdown of ATP.
D. Passive transport is based on diffusion.
1. Passive transport is a form of diffusion, such as when food dye is added to water. Diffusion depends on
the constant motion of ions or molecules at temperatures above absolute zero (-273oC).
2. A concentration gradient is a form of potential energy. The initial state is highly organized and has
minimum entropy, and after molecules have diffused to maximum dispersal, they are less organized
and have maximum entropy.
3. Even after their concentration is the same in all regions, there is still constant movement of molecules
or ions from one space to another, but no net change, which is called dynamic equilibrium.
E. Substances move passively through membranes by simple or facilitated diffusion.
1. Hydrophobic molecules are able to dissolve in the lipid bilayer of a membrane and move through it
freely. Hydrophilic molecules are impeded by the hydrophobic core of the membrane.
F. Transport by Simple Diffusion
1. A few small substances diffuse through the lipid part of a biological membrane, such as oxygen,
nitrogen, and carbon dioxide; this is termed simple diffusion.
2. Water is a strongly polar molecule and is small enough to slip though the membrane; however, this
type of movement is relatively slow.
G. Transport by Facilitated Diffusion
1. Many polar and charged molecules diffuse across membranes with the help of transport proteins
(facilitated diffusion).
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2.
Facilitated diffusion is also dependent on concentration gradients, and transport stops if the gradient
falls to zero.
H. Two groups of transport proteins carry out facilitated diffusion.
1. Channel proteins form hydrophilic channels in the membrane through which water and ions can pass;
for example, diffusion of water through membranes occurs though aquaporins, specialized water
channels.
2. Other channels facilitate the transport of ions: sodium, potassium, calcium, and chlorine. Most of these
are gated, that is they switch between open, closed, and intermediate states.
3. Gated ion channels perform functions that are vital to survival. For example, a fault in the gated
chlorine channel causes cystic fibrosis, which causes sticky mucus accumulation in the reparatory tract
leading to chronic lung infections.
4. Carrier proteins are the second type of transport proteins. They bind a specific single solute, such as
glucose or amino acid, and transport it across the lipid bilayer. Because a single solute is transferred,
this is called uniport transport. The carrier protein undergoes conformation changes.
5. Facilitated diffusion by carrier proteins can become saturated when there are not enough transport
proteins; increasing the concentration of transported molecules causes no rise in the rate of transport.
(This does not happen in simple diffusion.)
6. Facilitated diffusion is specific and can control the kinds of molecules that pass though. Each type of
cell has its own group of transport proteins for its needs.
6.3 Passive Water Transport and Osmosis
A. Passive transport of water (osmosis) occurs constantly in living cells. Much of the energy budget of cells is
spent counteracting the inward or outward movement of water.
B. Osmosis can be demonstrated in a purely physical system.
1. The apparatus shown in Figure 6.9a demonstrates osmosis. An inverted thistle tube is sealed at the
lower end by cellophane, which lets water pass but not glucose; the inside of the tube is filled with a
glucose solution and the outside beaker is filled with distilled water.
2. The liquid rises in the tube because water moves by osmosis from the beaker into the thistle tube. The
water concentration is greater in the beaker than in the thistle tube because the solute (glucose)
molecules reduce the amount of water available to cross the membrane. The difference is in free water.
3. At equilibrium, the pressure created by the weight of the raised solution exactly balances the tendency
of water molecules to move from the beaker up into the thistle tube. The pressure pushing water up is
called osmotic pressure.
4. Osmosis is the net movement of water molecules across a selectively permeable membrane by passive
diffusion, from a solution of lesser solute concentration to a solution of greater solute concentration.
Because osmosis occurs in response to a concentration gradient, it releases energy and can accomplish
work.
C. The free energy released by osmosis may work for or against cellular life.
1. Osmosis occurs in cells, and the resulting osmotic movement of water is used as an energy source for
some of the activities of life.
a. If the solution that surrounds a cell contains less dissolved substances than the inside of the cell,
the solution is hypotonic (hypo = under; tonos = tension) to the cell. Water will enter the cell, and
the cell will swell.
b. Water normally moves into the cells of plants, pushes the cells tightly against the cell walls, and
helps support the plants. This is called turgor pressure.
2. If a solution surrounds a cell and contains more dissolved substances than the inside of the cell, the
solution is hypertonic (hyper = over). Water will leave the cell. This causes plants to wilt and
sometimes causes plant cells to retract from the cell walls, which is called plasmolysis.
3. When concentrations of water inside and outside the cell are equal, the solution is said to be isotonic
(iso = same). Animal cells constantly pump out sodium ions to keep fluid levels equal. For animal
cells, isotonic is optimal; for plant cells, isotonic causes a loss of turgor pressure.
4. Passive transport, driven by concentration gradient, accounts for much of the movement of water, ions,
and other types of molecules into our cells.
6.4 Active Transport
A. Many substances are pushed across membranes against their concentration gradients by active transport.
B. Active transport requires a direct or indirect input of energy derived from ATP hydrolysis.
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1. There are two types of transport: primary and secondary.
2. Other features of active transport resemble facilitated diffusion.
C. Primary active transport moves positively charged ions across membranes.
1. Primary active transport moves positively changed ions—H +, Ca2+, Na+, K+—across membranes. H +
pumps (proton pumps) move H+ using ATP.
2. Ca2+ pump (calcium pump) moves Ca2+ out of the cell and inside the ER vesicles.
3. The sodium-potassium ion pump pushes three Na+ out of the cell and two K+ into the cell; thus the
inside of the cell becomes negatively charged with respect to the outside.
a. The voltage across a membrane is called membrane potential (-50 to -200 millivolts).
b. The result is called an electrochemical gradient that can be used for other transport mechanisms
and is associated with nerve impulse transmission.
D. Secondary active transport moves both ions and organic molecules across membranes.
1. The driving force for most secondary transport is the high outside/low inside sodium gradient produced
by the sodium-potassium pump.
2. Secondary active transport occurs by two mechanisms, known as symport and antiport.
a. Symport (cotransport) is when the solute moves through the membrane in the same direction as
the driving ion. Examples of this are sugar transporters.
b. Antiport (exchange diffusion) is when the driving ion moves through the channel protein in one
direction, providing energy for the solute molecule to move in the opposite direction. An example
of this is the chloride-bicarbonate ions in red blood cells.
3. Active and passive transport move smaller hydrophilic molecules across cellular membranes; larger
groups can also be moved, which is described next.
6.5 Exocytosis and Endocytosis
A. Eukaryotic cells import and export larger molecules by exocytosis and endocytosis, which require energy.
B. Exocytosis releases molecules to the outside by means of secretory vesicles.
1. In exocytosis, secretory vesicles move through the cytoplasm and contact the plasma membrane; they
then fuse and release the contents to the cell exterior.
2. All eukaryotic cells secrete materials to the outside through exocytosis.
C. Endocytosis brings material into cells in endocytic vesicles.
1. Bulk-phase endocytosis is the simplest of these and is called pinocytosis or cell drinking.
2. Receptor-mediated endocytosis uses receptors on the outside of cells in coated pits (clathrin) to bind to
molecules of interest and then take material in, similar to endocytosis.
3. Mammalian cells take in many substances by receptor-mediated endocytosis. Low density lipoproteins
(LDLs) are moved in this way.
4. Some cells (such as white blood cells) take in large aggregates of molecules or parts of cells or whole
cells, though a process called phagocytosis or cell eating.
5. The combined working of exocytosis and endocytosis constantly cycles membrane segments.
6. These combined mechanisms maintain internal concentrations of ions and molecules and exchange
large molecules with the environment.
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