MCAT Biochemistry Review
Chapter 8: Biological Membranes
Biological membranes are a stunning combination of opposites and contrasts. They are exceptionally thin, structurally bland, and relatively straightforward to describe. Yet, they define the borders of cells, tissues, and organelles; carry out a significant number of the biological functions within cells; and are an unending source of scientific inquiry and discovery. The most commonly tested biological membrane on the MCAT is the plasma membrane. At first, the plasma membrane seems like it's only a shell—just a barrier that defines the cell. But the plasma membrane plays roles in signaling, entry of nutrients and expulsion of waste, cell recognition, transport of materials between tissues, and even electronic functions.
Cell membranes have both a stretchy, flexible component (phospholipids) and an abundance of stabilizing molecules (cholesterol and protein) to make sure that everything remains intact. In this chapter, we will examine the general function, composition, and transport properties of biological membranes. We will conclude by taking a look at a few specialized cell membranes within the body, in addition to specific membrane properties.
8.1 Fluid Mosaic Model
The cell (plasma) membrane is often described as a semipermeable phospholipid bilayer. This phrase alone describes both the function and structure of the cell membrane: as a semipermeable barrier, it chooses which particles can enter and leave the cell at any point in time. This selectivity is mediated not only by the various channels and carriers that poke holes in the membrane, but also by the membrane itself. Composed primarily of two layers of phospholipids, the cell membrane permits fat-soluble compounds to cross easily, while larger and water-soluble compounds must seek alternative entry. The cell membrane is illustrated in Figure 8.1; the theory that underlies the structure and function of the cell membrane is referred to as the fluid mosaic model.
Figure 8.1. Cell Membrane The cell membrane is a phospholipid bilayer that regulates movement of solutes into and out of the cell.
GENERAL MEMBRANE STRUCTURE AND FUNCTION
The phospholipid bilayer also includes proteins and distinct signaling areas within lipid rafts. Carbohydrates associated with membrane-bound proteins create a glycoprotein coat. The cell wall of plants, bacteria, and fungi contain higher levels of carbohydrates.
The main function of the cell membrane is to protect the interior of the cell from the external environment. Cellular membranes selectively regulate traffic into and out of the cell and are involved in both intracellular and intercellular communication and transport. Cell membranes also contain proteins embedded within the lipid bilayer that act as cellular receptors during signal transduction. These proteins play an important role in regulating and maintaining overall cellular activity.
The cell membrane functions as a stable semisolid barrier between the cytoplasm and the environment, but it is in a constant state of flux on the molecular level. Phospholipids move rapidly in the plane of the membrane through simple diffusion. This can be seen when fusing two membranes that have been tagged with different labels; the tags will migrate with their associated lipids until both types are equally intermixed. Lipid rafts are collections of similar lipids with or without associated proteins that serve as attachment points for other biomolecules; these rafts often serve roles in signaling. Both lipid rafts and proteins also travel within the plane of the membrane, but more slowly. Lipids can also move between the membrane layers, but this is energetically unfavorable because the polar head group of the phospholipid must be forced through the nonpolar tail region in the interior of the membrane. Specialized enzymes called flippases assist in the transition or “flip” between layers.
Dynamic changes in the concentrations of various membrane proteins are mediated by gene regulation, endocytotic activity, and protein insertion. Many cells, particularly those involved in biosignaling processes, can up- or downregulate the number of specific cellular receptors on their surface in order to meet cellular requirements.
Many antidepressants increase levels of neurotransmitters in the brain, but the effects take longer to appear than the changes in neurochemistry. The reason for this delay is that the nervous system must still upregulate its postsynaptic receptors to respond to the new levels of neurotransmitter.
MCAT Concept Check 8.1:
Before you move on, assess your understanding of the material with these questions.
1. Describe the role of flippases and lipid rafts in biological membranes.
· Lipid rafts:
2. List the following membrane components in order from most plentiful to least plentiful: carbohydrates, lipids, proteins, nucleic acids.
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