An Introduction to Biological Membranes

From Bilayers to Rafts

William Stillwell

• Publisher: Elsevier Science
• Year: 2013
• ISBN:
  9780080931289
• Topic: Biological Sciences
• Subtopic: Cell Biology
• Index: Biological Sciences

An Introduction to Biological Membranes: From Bilayers to Rafts covers many aspects of membrane structure/function that bridges membrane biophysics and cell biology. Offering cohesive, foundational information, this publication is valuable for advanced undergraduate students, graduate students and membranologists who seek a broad overview of membrane science.

• Brings together different facets of membrane research in a universally understandable manner
• Emphasis on the historical development of the field
• Topics include membrane sugars, membrane models, membrane isolation methods, and membrane transport

Chapter 1

Introduction to Biological Membranes

OUTLINE

A. What is a Biological Membrane?

B. General Membrane Functions

C. Eukaryote Cell Structure

Endomembrane System

Plasma Membrane

Nuclear Envelope (Membrane)

Endoplasmic Reticulum (ER)

Golgi Apparatus

Lysosome

Peroxisome

Mitochondria

D. Size of Domains

Can we see a membrane?

Can we see a cell?

What can you see with a light microscope?

E. Basic Composition of Membranes

Summary

References

A What is a Biological Membrane?

The American Heritage Dictionary defines a membrane as ‘a thin pliable layer of plant or animal tissue covering or separating structures or organs.’ The impression this description leaves is one of the plastic wrap covering a hamburger. By this definition, membranes are static, tough, impenetrable, and visible. Yet, nothing could be farther from the truth. The entire concept of dynamic behavior is missing from this definition, yet dynamics is what makes membranes both essential for life and so difficult to study.

If we could somehow instantaneously freeze a membrane and learn the composition and location of each of the countless numbers of molecules comprising the membrane, and then instantly return the membrane back to its original unfrozen state for a microsecond before re-freezing, we would find that the membrane had substantially changed while unfrozen. Although the molecular composition would remain the same over this short time, the molecular locations and interrelationships would be altered. Therefore membranes must have both static and dynamic components. While static describes what is there, dynamics describes how the components interact to generate biological function.

Every cell in the human body is a tightly packed package of countless membranes. The human body is composed of ~63 trillion cells (6.3 × 10¹³ cells), each of which is very small. For example, a typical liver cell would have to be 5X larger to be seen as a speck by someone with excellent vision (it is microscopic). Each liver cell has countless numbers of internal membranes. If you could somehow open one single liver cell and remove all of the internal membranes and sew them together into a quilt, the quilt would cover ~840 acres, the size of New York’s Central Park! And that is from one single cell. Therefore, there are enough membranes in a human body (6.3 × 10¹³ cells) to cover the earth millions of times over!

All life on Earth is far more similar than it is different. Living organisms share a number of essential biochemical properties, collectively termed the ‘thread of life’. Included in these essential properties is ownership of a surrounding plasma membrane that separates the cell’s interior from its external environment. It is likely that all living things inhabiting planet Earth today arose from a single common ancestor more than 3.5 billion years ago. The first cell probably contained minimally a primitive catalyst (a pre-protein), a primitive information storage system (a pre-nucleic acid), a source of carbon (perhaps a primitive carbohydrate) and this mixture had to be surrounded by a primitive plasma membrane that was likely made of polar lipids. Membranes were therefore an essential component of every cell that is alive today or has ever been alive.

With 3.5 billion years of biological evolution, the complexity of membranes in cells has greatly expanded from that of a simple surrounding plasma membrane to where they now occupy a large portion of a eukaryote’s interior space. An electron microscopic picture of a ‘typical’ eukaryotic (liver) cell is shown in Figure 1.1 [1]. It is evident from the complexity of this micrograph that identifying, isolating, and studying membranes will be a difficult task.

FIGURE 1.1 Transmission electron micrograph of a liver cell, a ‘typical’ cell. [1]

B General Membrane Functions

It is now generally agreed that biological membranes are probably somehow involved in all cellular activities. The most obvious function of any membrane is separating two aqueous compartments. For the plasma membrane this involves separation of the cell contents from the very different extra-cellular environment. Membranes are therefore responsible for containment, ultimately delineating the cell. Separation, however, cannot be absolute, as the cell must be able to take up essential nutrients, gases, and solutes from the exterior, while simultaneously removing toxic waste products from the interior. A biological membrane therefore must be selectively permeable, possessing the ability to distinguish many chemically different solutes and knowing in which direction to redistribute them. Biological membranes must therefore house a variety of specific, vectorial transport systems (discussed in Chapter 14).

A characteristic of all living cells is the establishment and maintenance of trans-membrane gradients of all solutes. Of particular interest are large ion gradients typically associated with the plasma membrane. Table 1.1 is a comparison of the mean concentration of selected ions inside and outside a typical mammalian cell, and the magnitude of each gradient. To maintain gradients of this size, efficient energy-dependent transport systems must be employed (discussed in Chapter 14). Directional trans-membrane structure is required to generate these ion gradients.

TABLE 1.1

Trans-membrane Ion Gradients of a ‘Typical’ Mammalian Cell.

In addition to trans-membrane structure, it is now believed that biological membranes are composed of countless numbers of very small, transient, lateral lipid microdomains. Each of these domains is proposed to have a different lipid and resident protein composition. Thus the activity of any membrane must reflect the sum of the activities of its many specific domains. One type of lipid microdomain, termed a ‘lipid raft’, has received a lot of recent attention as it is reputed to be involved in a variety of important cell signaling events. If the lipid raft story (discussed in Chapter 8) holds up, this new paradigm for membrane structure/function may serve as a model for other types of as yet undiscovered non-raft domains. Each of these domains might then support a different collection of related biochemical activities. Therefore, membranes have both trans-membrane and lateral structures that are just beginning to be understood.

All membranes possess an extreme water gradient across their very thin (~5 nm) structure. In the membrane aqueous bathing solution water concentration is ~55.5 M water in water (1,000 g of water per liter divided by 18, the molecular weight of water), while the membrane interior is quite dry (< 1 mM water). The aqueous interface provides a charged or polar physical surface to help arrange related functional enzymes, known as pathways, in one plane for increased efficiency. In contrast, the dry interior provides an environment for dehydration reactions. Both the aqueous interface and dry interior are responsible for maintaining the proper conformation of membrane proteins (discussed in Chapter 6).

In addition to transport, biological m...