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Lecture 1

BIOL 4004 Lecture 1: Study_Guide_Chapter_11_Sum_11

8 Pages
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Fall 2015

Department
Biology
Course Code
BIOL 4004
Professor
Matthes David
Lecture
1

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MBC5 Study Guide – Chapter 11
(Membrane Transport of Molecules & Electrical Properties of Membranes)
A fundamental property of living cells is their separation from the environment via membranes.
On the one hand, this is a good thing because it allows cells to maintain their internal
composition. On the other, a lipid bilayer is impermeable to most substances, and thereby
presents an obstacle to the uptake of nutrients and the excretion of waste products. In this chapter,
we will learn about some of the fundamental properties of biological membranes, and consider
the roles of membrane proteins in the transport of substances across the membrane.
PRINCIPLES OF MEMBRANE TRANSPORT
In this section, we will consider some general permeability properties of membranes, and the
categories of membrane proteins that carry out transport. Later sections will examine these
proteins in greater detail.
Protein-free Lipid Bilayers Are Highly Impermeable to Ions
Due to the hydrophobic interior of a lipid bilayer, polar solutes such as ions, sugars, and amino
acids are not able to freely diffuse across a membrane (see Figures 11-1 and 11-2).
There Are Two Main Classes of Membrane Transport Proteins: Transporters and Channels
As depicted in Figure 11-3, there are two different ways that an integral membrane protein can
allow solutes to travel across the membrane. These are termed transporters and channels.
Transporters bind their solutes, much like an enzyme binds its substrate. After a solute binds to a
transporter from one side of the membrane, the protein undergoes a conformational change that
exposes the solute to the opposite side of the membrane. The solute is then released. The other
alternative is a channel protein. A channel protein provides a direct passageway for the diffusion
of a solute across the membrane. When comparing transporters and channels, some general points
can be made:
Transporters can recognize a myriad of different solutes with a high degree of specificity.
There are transporters that recognize particular amino acids and others that recognize
sugars. With the exception of ion channels, most channels are not very specific.
Transporters are usually slower than channels. This is because a transporter must undergo a
conformational change to allow the solute across the membrane.
Certain types of transporters can promote the active transport of a solute across a
membrane. These types of transporters are described next.
Active Transport Is Mediated by Transporters Coupled to an Energy Source
Let’s begin with some terms (Figure 11-4):
Passive transport or passive diffusion: transport of a solute directly through the membrane
without the aid of a protein.
Facilitated diffusion: transport of a solute across the membrane via the aid of a membrane
protein, down its gradient. This does not require an input of energy.
Active transport: transport of a solute across the membrane via the aid of a membrane
protein, against its gradient. This does require an input of energy.
For charged substances, such as ions and charged molecules, the gradient of the solute is
determined by two factors: the concentration of the solute across the membrane and the
distribution of charge across the membrane. The two factors together define an electrochemical
gradient. For example, if the concentration of Na+ ions were 100mM outside the cell and 5 mM
inside the cell, this would be a chemical gradient. Sodium ions would want to flow into the cell
due to the chemical gradient. If there were more positive charge outside the cell compared to
inside, this would be an electrical gradient. In this case, Na+ would also want to flow into the
cell because of the electrical gradient.
TRANSPORTERS AND ACTIVE MEMBRANE TRANSPORT
There are three common ways that transport is coupled to energy (Figure 11-7). Your textbook
leaves one of them out, and adds another way that is not very common:
Transport may be coupled to ATP hydrolysis.
Transport may be coupled to an ion-electrochemical gradient.
Transport may be coupled to electron movement. Your textbook omits this very important
mechanism.
Transport may be coupled to light. This is very rare, being found in only a few species of
bacteria.
Active Transport Can Be Driven by Ion Gradients
As shown in Figure 11-8, the transport of a solute may or may not be coupled to the transport of
another solute. In uniport, it is not coupled. In symport, transport of two or more solutes is
coupled in the same direction. In antiport, transport of two or more solutes is coupled in the
opposite direction.
When energy (e.g., ATP, light, or electrical energy) is directly coupled to active transport, this is
termed primary active transport. When active transport is coupled to a favorable ion-
electrochemical gradient, this is called secondary active transport.
An example of secondary active transport is shown in Figure 11-9. A Na+/glucose symporter can
move glucose against its chemical gradient if a favorable Na+ gradient is present. In this case, Na+
is moving down its electrochemical gradient, enabling glucose to be transported against a
gradient. You don’t need to worry about the molecular mechanism of the lactose permease shown
in Figure 11-10.
Transporters in the Plasma Membrane Regulate Cytosolic pH
An Asymmetric Distribution of Transporters in Epithelial Cells Underlies the Transcellular
Transport of Solutes
Transcellular transport is the movement of a solute into a cell and out the other side. It occurs
across several types of cells such as intestinal cells. In the scenario shown in Figure 11-11,
glucose is driven into the cell against a gradient via a Na+-coupled symporter. Due to symport,
glucose can accumulate to high intracellular levels. At the other side of the cell (i.e., the side
facing the blood), glucose is transported, via a uniporter, to the outside of the cell. The uniporter
transports glucose in a downhill direction.
There are Three Classes of ATP-Driven Pumps
Three types of pumps couple the hydrolysis or synthesis of ATP to transport (Figure 11-12). P-
type pumps hydrolyze ATP and are frequently used to establish and maintain ion gradients across
membranes. F-type pumps, often called ATP synthases, couple the transport of H+ ions across
membranes with the synthesis of ATP. F-type ATPases are found in mitochondria and bacteria.
ABC transporters couple the hydrolysis of ATP with the transport of small molecules across
membranes.
The Ca2+ Pump is the Best Understood P-Type ATPase
Ca2+-ATPases are found in the plasma membrane of most living cells where they pump
Ca2+ out of the cell to maintain a low Ca2+ concentration in the cytosol. We will consider the role
of Ca2+ in cell signaling later in the course.
Among all P-type ATPases the structure of the calcium pump is best understood. Its structure is
shown in Figure 11-13. It contains ten transmembrane segments. Two calcium ion-binding sites
are located within the transmembrane region. The ATP binding site and phosphorylation site are
not in the transmembrane region. Rather, they are found in the part of the protein that projects
into the cytoplasm. Somehow, phosphorylation transmits a conformational change that alters
calcium ion accessibility across the membrane. In this regard, it is interesting to note that
transmembrane segment 5 comes out of the membrane and comes close to the phosphorylation
site. This is a critical region that transmits the conformational change from the phosphorylation
site to the transmembrane region.
The Plasma Membrane P-type Na+-K+ Pump Establishes the Na+ Gradient Across the
Plasma Membrane
All living cells maintain an ion electrochemical gradient across their plasma membrane. In animal
cells, a strong Na+ gradient is typically present. This gradient is generated by a transporter called
the Na+, K+-ATPase or simply the sodium pump. For each ATP it hydrolyzes, three sodium ions
are pumped out and two potassium ions are pumped into the cell (Figure 11-14).
Figure 11-15 is a description of the reaction mechanism for the sodium pump. You should be
familiar with this figure. The reaction mechanism describes the series of steps that occur for this
transporter to work. First, three sodium ions bind to the protein from within the cell. ATP is
hydrolyzed, and a phosphate group is temporarily attached to the protein covalently. The protein
then undergoes a conformational change such that the sodium ions are exposed to the outside of
the cell. The sodium ions are released and then two potassium ions bind. This promotes the
dephosphorylation of the protein, which then causes a conformational change so that the
potassium ions are exposed to the inside of the cell. The potassium ions are released and then the
cycle can start over again.
There are many functional roles of the sodium pump in animal cells. One role is to drive sodium-
coupled symporters as described previously in Figure 11-8. A second role is to maintain cell
volume by controlling the amount of ions across the membrane. The sodium pump transports
three ions out and two ions in. If the cell is swelling due to osmosis, the sodium pump will speed
up to remove excess intracellular ions. If the cell is shrinking, it will slow down.
ABC Transporters Constitute the Largest Family of Membrane Transport Proteins
ABC transporters are members of a gene family that transport a diverse array of solutes
including ions, sugars, and amino acids. Several examples are described in your text (Figure 11-
19). ABC transporters have a modular structure. They have two transmembrane regions, each
composed of six transmembrane segments, and two ATP binding domains that project into the
cytosol.
ION CHANNELS AND THE ELECTRICAL PROPERTIES OF MEMBRANES
Let’s now turn our attention to the other type of transport protein, namely channels. In this
section, we will focus primarily on ion channels and the properties of ion electrochemical
gradients. Later in the course, we will survey a few other types of channels.

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Description
MBC5 Study Guide Chapter 11 (Membrane Transport of Molecules Electrical Properties of Membranes) A fundamental property of living cells is their separation from the environment via membranes. On the one hand, this is a good thing because it allows cells to maintain their internal composition. On the other, a lipid bilayer is impermeable to most substances, and thereby presents an obstacle to the uptake of nutrients and the excretion of waste products. In this chapter, we will learn about some of the fundamental properties of biological membranes, and consider the roles of membrane proteins in the transport of substances across the membrane. PRINCIPLES OF MEMBRANE TRANSPORT In this section, we will consider some general permeability properties of membranes, and the categories of membrane proteins that carry out transport. Later sections will examine these proteins in greater detail. Proteinfree Lipid Bilayers Are Highly Impermeable to Ions Due to the hydrophobic interior of a lipid bilayer, polar solutes such as ions, sugars, and amino acids are not able to freely diffuse across a membrane (see Figures 111 and 112). There Are Two Main Classes of Membrane Transport Proteins: Transporters and Channels As depicted in Figure 113, there are two different ways that an integral membrane protein can allow solutes to travel across the membrane. These are termed transporters and channels. Transporters bind their solutes, much like an enzyme binds its substrate. After a solute binds to a transporter from one side of the membrane, the protein undergoes a conformational change that exposes the solute to the opposite side of the membrane. The solute is then released. The other alternative is a channel protein. A channel protein provides a direct passageway for the diffusion of a solute across the membrane. When comparing transporters and channels, some general points can be made: Transporters can recognize a myriad of different solutes with a high degree of specificity. There are transporters that recognize particular amino acids and others that recognize sugars. With the exception of ion channels, most channels are not very specific. Transporters are usually slower than channels. This is because a transporter must undergo a conformational change to allow the solute across the membrane. Certain types of transporters can promote the active transport of a solute across a membrane. These types of transporters are described next. Active Transport Is Mediated by Transporters Coupled to an Energy Source Lets begin with some terms (Figure 114): Passive transport or passive diffusion: transport of a solute directly through the membrane without the aid of a protein. Facilitated diffusion: transport of a solute across the membrane via the aid of a membrane protein, down its gradient. This does not require an input of energy. Active transport: transport of a solute across the membrane via the aid of a membrane protein, against its gradient. This does require an input of energy.
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