Membrane Structure and Function - Kevan Kruger 4.1 Plasma Membrane Structure and Function The plasma...

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Transcript of Membrane Structure and Function - Kevan Kruger 4.1 Plasma Membrane Structure and Function The plasma...

  • 67

    Membrane Structure and Function

    Chapter Concepts

    4.1 Plasma Membrane Structure and Function • The plasma membrane regulates the passage of

    molecules into and out of the cell. 68 • The membrane contains lipids and proteins. Each

    protein has a specific function. 70

    4.2 The Permeability of the Plasma Membrane • Some substances, particularly small, noncharged

    molecules, pass freely across the plasma membrane. Ions and other types of molecules need assistance to cross the membrane. 72

    4.3 Diffusion and Osmosis • Molecules spontaneously diffuse (move from an

    area of higher concentration to an area of lower concentration), and some can diffuse across a plasma membrane. 73

    • Water diffuses across the plasma membrane, and this can affect cell size and shape. 74

    4.4 Transport by Carrier Proteins • Carrier proteins assist the transport of some ions

    and molecules across the plasma membrane. 76

    4.5 Exocytosis and Endocytosis • Vesicle formation takes other substances

    into the cell, and vesicle fusion with the plasma membrane discharges substances from the cell. 78

    A single cell about to be pierced by a fine probe so that DNA can be removed by the suction tube on the bottom. An intact plasma membrane is necessary to the life of any cell and if it is ruptured the cell cannot continue to exist.

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  • 4.1 Plasma Membrane Structure and Function The plasma membrane is a phospholipid bilayer in which protein molecules are either partially or wholly embedded (Fig. 4.1). The phospholipid bilayer has a fluid consistency, comparable to that of light oil. The proteins are scattered throughout the membrane; therefore they form a mosaic pat- tern. This description of the plasma membrane is called the fluid-mosaic model of membrane structure.

    Phospholipids spontaneously arrange themselves into a bilayer. The hydrophilic (water loving) polar heads of the phospholipid molecules face the outside and inside of the cell where water is found, and the hydrophobic (water fearing) nonpolar tails face each other (Fig. 4.1). In addition to phospholipids, there are two other types of lipids in the plasma membrane. Glycolipids have a struc- ture similar to phospholipids except that the hydrophilic head is a variety of sugars joined to form a straight or

    68 Part 1 Cell Biology 4-2

    Banners flying on a castle wall mark off the communitywithin from the surrounding countryside. Inside, resi-dents go about their appointed tasks for the good of the community. Commands passed along from royalty to knights to workers are obeyed by all. The almost impenetra- ble wall prevents the enemy without from entering and dis- turbing the peace within. Only certain small creatures can pass through the open slitlike windows, and the drawbridge must be lowered for most needed supplies.

    The plasma membrane, which carries markers identify- ing it as belonging to the individual, can be likened to the cas- tle wall. Under the command of the nucleus, the organelles carry out their specific functions and contribute to the work- ing of the cell as a whole. Very few molecules can freely cross the membrane, and most nutrients must be transported across by special carriers. The cell uses these nutrients as a source of building blocks and energy to maintain the cell. The operations of the cell will continue only as long as the plasma membrane selectively permits specific materials to enter and leave and prevents the passage of others.

    Outside cell

    Inside cell filaments of the cytoskeleton

    peripheral protein

    cholesterol

    integral protein

    phospholipid bilayer

    glycolipid glycoprotein carbohydrate chain

    hydrophilic heads

    hydrophilic heads

    hydrophobic tails

    plasma membrane

    Figure 4.1 Fluid-mosaic model of plasma membrane structure. The membrane is composed of a phospholipid bilayer in which proteins are embedded. The hydrophilic heads of phospholipids are a part of the outside surface and the inside surface of the membrane. The hydrophobic tails make up the interior of the membrane. Note the plasma membrane’s asymmetry—carbohydrate chains are attached to the outside surface and cytoskeleton filaments are attached to the inside surface.

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  • branching carbohydrate chain. Cholesterol is a lipid that is found in animal plasma membranes; related steroids are found in the plasma membrane of plants. Cholesterol re- duces the permeability of the membrane to most biological molecules.

    The proteins in a membrane may be peripheral proteins or integral proteins. Peripheral proteins occur either on the outside or the inside surface of the membrane. Some of these are anchored to the membrane by covalent bonding. Still others are held in place by noncovalent interactions that can be disrupted by gentle shaking or by change in the pH.

    Integral proteins are found within the membrane and have hydrophobic regions embedded within the membrane and hydrophilic regions that project from both surfaces of the bilayer:

    move through the hydrophobic center of the membrane.) The fluidity of a phospholipid bilayer means that cells are pliable. Imagine if they were not—the long nerve fibers in your neck would crack whenever you nodded your head!

    Although some proteins are often held in place by cy- toskeletal filaments, in general proteins are free to drift lat- erally in the fluid lipid bilayer. This has been demonstrated by fusing mouse and human cells, and watching the move- ment of tagged proteins (Fig. 4.2). Forty minutes after fu- sion, the proteins are completely mixed. The fluidity of the membrane is needed for the functioning of some proteins such as enzymes which become inactive when the mem- brane solidifies.

    The fluidity of the membrane, which is dependent on its lipid components, is critical to the proper functioning of the membrane’s proteins.

    Chapter 4 Membrane Structure and Function 694-3

    hydrophobic region hydrophilic

    regions

    Many integral proteins are glycoproteins, which have an attached carbohydrate chain. As with glycolipids, the carbohydrate chain of sugars projects externally. There- fore it can be said that the plasma membrane is “sugar- coated.”

    The plasma membrane is asymmetrical: the two halves are not identical. The carbohydrate chains of the glyco- lipids and proteins occur only on the outside surface and the cytoskeletal filaments attach to proteins only on the inside surface.

    The plasma membrane consists of a phospholipid bilayer. Peripheral proteins are found on the outside and inside surface of the membrane. Integral proteins span the lipid bilayer and often have attached carbohydrate chains.

    The Fluidity of the Plasma Membrane At body temperature, the phospholipid bilayer of the plasma membrane has the consistency of olive oil. The greater the concentration of unsaturated fatty acid residues, the more fluid is the bilayer. In each monolayer, the hydrocarbon tails wiggle, and the entire phospholipid molecule can move sideways at a rate averaging about 2 µm—the length of a prokaryotic cell—per second. (Phos- pholipid molecules rarely flip-flop from one layer to the other, because this would require the hydrophilic head to

    mouse cell human cell

    cell fusion

    immediately after fusion

    mixed membrane proteins

    Figure 4.2 Experiment to demonstrate lateral drifting of plasma membrane proteins. After human and mouse cells fuse, the plasma membrane proteins of the mouse (blue circles) and of the human cell (red circles) mix within a short time.

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  • The Mosaic Quality of the Membrane The plasma membranes of various cells and the membranes of various organelles each have their own unique collections of proteins. The proteins form different patterns according to the particular membrane and also within the same mem- brane at different times. When you consider that the plasma membrane of a red blood cell contains over 50 different types of proteins, you can see why the membrane is said to be a mosaic.

    The integral proteins largely determine a membrane’s specific functions. As we will discuss in more detail, certain plasma membrane proteins are involved in the passage of molecules through the membrane. Some of these are chan- nel proteins through which a substance can simply move across the membrane; others are carrier proteins that com- bine with a substance and help it to move across the mem- brane. Still others are receptors; each type of receptor protein has a shape that allows a specific molecule to bind to it. The binding of a molecule, such as a hormone (or other signal molecule), can cause the protein to change its shape and bring about a cellular response. Some plasma mem- brane proteins are enzymatic proteins that carry out meta- bolic reactions directly. The peripheral proteins associated with the membrane often have a structural role in that they help stabilize and shape the plasma membrane.

    Figure 4.3 depicts the various functions of membrane proteins.

    The mosaic pattern of a membrane is dependent on the proteins, which vary in structure and function.