The Plasma Membrane

The electron microscope reveals that the cell and many of the organelles within it are bordered by a unit membrane, which appears as a pair of dark parallel lines with a total thickness of about 7.5 nm (fig. 3.6a). The plasma membrane is the unit membrane at the cell surface. It defines the boundaries of the cell, governs its interactions with other cells, and controls the passage of materials into and out of the cell. The side that faces the cytoplasm is the intracellular face of the membrane, and the side that faces outward is the extracellular face.

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Chapter 3 Cellular Form and Function 99

Chapter 3 Cellular Form and Function 99

Phospholipid Bilayer MicrographPlasma Membrane Structure And Function

Nonpolar tails of phospholipid of phospholipid molecules molecules

Nonpolar tails of phospholipid of phospholipid molecules molecules

Cytoplasm

Figure 3.6 The Plasma Membrane. (a) Plasma membranes of two adjacent cells (electron micrograph). (b) Molecular structure of the plasma membrane.

Membrane Lipids

Figure 3.6b shows our current concept of the molecular structure of the plasma membrane—an oily film of lipids with diverse proteins embedded in it. Typically about 98% of the molecules in the membrane are lipids, and about 75% of the lipids are phospholipids. These amphiphilic molecules arrange themselves into a bilayer, with their hydrophilic phosphate-containing heads facing the water on each side of the membrane and their hydrophobic tails directed toward the center of the mem brane, avoiding the water. The phospholipids drift laterally from place to place, spin on their axes, and flex their tails. These movements keep the membrane fluid.

_Think About It_

What would happen if the plasma membrane were made primarily of a hydrophilic substance such as carbohydrate? Which of the major themes at the end of chapter 1 does this point best exemplify?

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Cholesterol molecules, found amid the fatty acid tails, constitute about 20% of the membrane lipids. By interacting with the phospholipids and "holding them still," cholesterol can stiffen the membrane (make it less fluid) in spots. Higher concentrations of cholesterol, however, can increase membrane fluidity by preventing the phospholipids from becoming packed closely together.

The remaining 5% of the membrane lipids are gly-colipids—phospholipids with short carbohydrate chains on the extracellular face of the membrane. They help to form the glycocalyx, a carbohydrate coating on the cell surface with multiple functions, described shortly.

Membrane Proteins

Although proteins are only about 2% of the molecules of the plasma membrane, they are larger than lipids and constitute about 50% of the membrane weight. Some of them, called integral (transmembrane) proteins, pass through the membrane. They have hydrophilic regions in contact with the cytoplasm and extracellular fluid, and hydrophobic regions that pass back and forth through the lipid of the membrane (fig. 3.7). Most integral proteins are glycoproteins, which are conjugated with oligosaccharides on the extracellular side of the membrane. Many of the integral proteins drift about freely in the phospholipid film, like ice cubes floating in a bowl of water. Others are anchored to the cytoskele-ton—an intracellular system of tubules and filaments discussed later. Peripheral proteins do not protrude into the phospholipid layer but adhere to the intracellular face of the membrane. A peripheral protein is typically associated with an integral protein and tethered to the cytoskeleton.

The functions of membrane proteins include the following:

• Receptors (fig. 3.8a). The chemical signals by which cells communicate with each other (epinephrine, for example) often cannot enter the target cell, but bind to surface proteins called receptors. Receptors are usually specific for one particular messenger, much like an enzyme that is specific for one substrate.

• Second-messenger systems. When a messenger binds to a surface receptor, it may trigger changes within the cell that produce a second messenger in the cytoplasm. This process involves both transmembrane proteins (the receptors) and peripheral proteins. Second-messenger systems are discussed shortly in more detail.

• Enzymes (fig. 3.8b). Enzymes in the plasma membranes of cells carry out the final stages of starch and protein digestion in the small intestine, help produce second messengers, and break down hormones and other signaling molecules whose job is done, thus stopping them from excessively stimulating a cell.

• Channel proteins (fig. 3.8c). Channel proteins are integral proteins with pores that allow passage of water and hydrophilic solutes through the membrane. Some channels are always open, while others are gates that open and close under different circumstances, thus determining when solutes can pass through

(fig. 3.8d). These gates open or close in response to three types of stimuli: ligand-regulated gates respond to chemical messengers, voltage-regulated gates to changes in electrical potential (voltage) across the plasma membrane, and mechanically regulated gates to physical stress on a cell, such as stretch and

Transmembrane Protein

Oligosaccharide

Integral protein Hydrophilic region Hydrophobic region

Phospholipid

— Cytoskeletal protein

Anchoring peripheral protein

Figure 3.7 Transmembrane Proteins. A transmembrane protein has hydrophobic regions embedded in the phospholipid bilayer and hydrophilic regions projecting into the intracellular and extracellular fluids. The protein may cross the membrane once (left) or multiple times (right). The intracellular regions are often anchored to the cytoskeleton by peripheral proteins.

Oligosaccharide

Integral protein Hydrophilic region Hydrophobic region

Phospholipid

— Cytoskeletal protein

Anchoring peripheral protein

Figure 3.7 Transmembrane Proteins. A transmembrane protein has hydrophobic regions embedded in the phospholipid bilayer and hydrophilic regions projecting into the intracellular and extracellular fluids. The protein may cross the membrane once (left) or multiple times (right). The intracellular regions are often anchored to the cytoskeleton by peripheral proteins.

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Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition pressure. By controlling the movement of electrolytes through the plasma membrane, gated channels play an important role in the timing of nerve signals and muscle contraction (see insight 3.1).

• Carriers (see figs. 3.18 and 3.19). Carriers are integral proteins that bind to glucose, electrolytes, and other solutes and transfer them to the other side of the membrane. Some carriers, called pumps, consume ATP in the process.

• Molecular motors (fig. 3.8e). These proteins produce movement by changing shape and pulling on other molecules. They move materials within a cell, as in transporting molecules and organelles to their destinations; they enable some cells, such as white blood cells, to crawl around in the body's tissues; and they make cells change shape, as when a cell surrounds and engulfs foreign particles or when it divides in two. Such processes depend on the action of fibrous proteins, especially actin and myosin, that pull on the integral proteins of the plasma membrane.

• Cell-identity markers (fig. 3.8/). Glycoproteins contribute to the glycocalyx, a carbohydrate surface coating discussed shortly. Among other functions, this acts like an "identification tag" that enables our bodies to tell which cells belong to it and which are foreign invaders.

Chapter 3 Cellular Form and Function 101

• Cell-adhesion molecules (fig. 3.8g). Cells adhere to one another and to extracellular material through certain membrane proteins called cell-adhesion molecules (CAMs). With few exceptions (such as blood cells and metastasizing cancer cells), cells do not grow or survive normally unless they are mechanically linked to the extracellular material. Special events such as sperm-egg binding and the binding of an immune cell to a cancer cell also require CAMs.

Insight 3.1 Clinical Application

Calcium Channel Blockers

The walls of the arteries contain smooth muscle that contracts or relaxes to change their diameter. These changes modify the blood flow and strongly influence blood pressure. Blood pressure rises when the arteries constrict and falls when they relax and dilate. Excessive, widespread vasoconstriction can cause hypertension (high blood pressure), and vasoconstriction in the coronary blood vessels of the heart can cause pain (angina) due to inadequate blood flow to the cardiac muscle. In order to contract, a smooth muscle cell must open calcium channels in its plasma membrane and allow calcium to enter from the extracellular fluid. Drugs called calcium channel blockers prevent calcium channels from opening. Thus they help to relax the arteries, relieve angina, and lower blood pressure.

Chemical messenger

Breakdown products

Solute molecules \

Chemical messenger

Breakdown products

Solute molecules \

Chemical Messenger

@ Receptor

@ Receptor

@ Motor molecule

Cell Adhesion Molecules Cam

@ Motor molecule

Cell-adhesion molecule (CAM)

Cell-adhesion molecule (CAM)

Figure 3.8 Some Functions of Membrane Proteins. (a) A receptor that binds to chemical messengers such as hormones sent by other cells. (b) An enzyme that breaks down a chemical messenger and terminates its effect on the target cell. (c) A channel protein that is constantly open and allows solutes to pass in and out of the cell. (d) A gated channel that opens and closes to allow solutes through only at certain times. (e) A motor molecule, a filamentous protein arising deeper in the cytoplasm that pulls on membrane proteins and causes cell movement. (f) A glycoprotein serving as a cell-identity marker. (g) A cell-adhesion molecule (CAM) that binds one cell to another.

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Second Messengers

Second messengers are of such importance that they require a closer look. You will find this information essential for your later understanding of hormone and neurotransmitter action. Let's consider how the hormone epinephrine stimulates a cell. Epinephrine, the "first messenger," cannot pass through plasma membranes, so it binds to a surface receptor. The receptor is linked on the intracellular side to a peripheral protein called a G protein (fig. 3.9). G proteins are named for the ATP-like chemical,

First messenger

First messenger

ATP cAMP (second messenger) {

ATP cAMP (second messenger) {

Inactive Activated kinase kinase *

Inactive Activated enzymes enzymes j

Various metabolic effects

Figure 3.9 A Second-Messenger System. (1) A messenger such as epinephrine (red triangle) binds to a receptor in the plasma membrane. (2) The receptor releases a G protein, which then travels freely in the cytoplasm and can have various effects in the cell. (3) The G protein binds to an enzyme, adenylate cyclase, in the plasma membrane. Adenylate cyclase converts ATP to cyclic AMP (cAMP), the second messenger. (4) cAMP activates a cytoplasmic enzyme called a kinase. (5) Kinases add phosphate groups (P|) to other cytoplasmic enzymes. This activates some enzymes and deactivates others, leading to varied metabolic effects within the cell.

Is adenylate cyclase an integral protein or a peripheral protein? What about the G protein?

guanosine triphosphate (GTP), from which they get their energy. When activated by the receptor, a G protein relays the signal to another membrane protein, adenylate cyclase (ah-DEN-ih-late SY-clase). Adenylate cyclase removes two phosphate groups from ATP and converts it to cyclic AMP (cAMP), the second messenger. Cyclic AMP then activates enzymes called kinases (KY-nace-es) in the cytosol. Kinases add phosphate groups to other cellular enzymes. This activates some enzymes and deactivates others, but either way, it triggers a great variety of physiological changes within the cell.

G proteins play such an enormous range of roles in physiology and disease that Martin Rodbell and Alfred Gilman received a 1994 Nobel Prize for discovering them. Up to 60% of currently used drugs work by altering the activity of G proteins.

Constipation Prescription

Constipation Prescription

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Responses

  • KAIJU
    Does a cell membrane gate respond to chemical messengers?
    7 years ago
  • charlie
    What would happen if plasma membrane was made primarily of a hydrophilic substance?
    7 years ago
  • dirk
    Are second messengers integral proteins?
    7 years ago
  • jeffrey
    What would happen to a plasma membrane if it were made primarily of a hydrophillic substance?
    4 years ago
  • chester
    What would happen if the plasma membrane were made primarily of a hydro substance?
    4 years ago

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