With few exceptions, cellular membranes — including plasma membranes and internal membranes — are made of glycerophospholipids, molecules composed of glycerol, a phosphate group, and two fatty acid chains. Glycerol is a three-carbon molecule that functions as the backbone of these membrane lipids. Within an individual glycerophospholipid, fatty acids are attached to the first and second carbons, and the phosphate group is attached to the third carbon of the glycerol backbone. Variable head groups are attached to the phosphate. Space-filling models of these molecules reveal their cylindrical shape, a geometry that allows glycerophospholipids to align side-by-side to form broad sheets (Figure 1). Glycerophospholipids are by far the most abundant lipids in cell membranes. Like all lipids, they are insoluble in water, but their unique geometry causes them to aggregate into bilayers without any energy input. This is because they are two-faced molecules, with hydrophilic (water-loving) phosphate heads and hydrophobic (water-fearing) hydrocarbon tails of fatty acids. In water, these molecules spontaneously align — with their heads facing outward and their tails lining up in the bilayer's interior. Thus, the hydrophilic heads of the glycerophospholipids in a cell's plasma membrane face both the water-based cytoplasm and the exterior of the cell. Altogether, lipids account for about half the mass of cell membranes. Cholesterol molecules, although less abundant than glycerophospholipids, account for about 20 percent of the lipids in animal cell plasma membranes. However, cholesterol is not present in bacterial membranes or mitochondrial membranes. Also, cholesterol helps regulate the stiffness of membranes, while other less prominent lipids play roles in cell signaling and cell recognition.
In addition to lipids, membranes are loaded with proteins. In fact, proteins account for roughly half the mass of most cellular membranes. Many of these proteins are embedded into the membrane and stick out on both sides; these are called transmembrane proteins. The portions of these proteins that are nested amid the hydrocarbon tails have hydrophobic surface characteristics, and the parts that stick out are hydrophilic (Figure 2). At physiological temperatures, cell membranes are fluid; at cooler temperatures, they become gel-like. Scientists who model membrane structure and dynamics describe the membrane as a fluid mosaic in which transmembrane proteins can move laterally in the lipid bilayer. Therefore, the collection of lipids and proteins that make up a cellular membrane relies on natural biophysical properties to form and function. In living cells, however, many proteins are not free to move. They are often anchored in place within the membrane by tethers to proteins outside the cell, cytoskeletal elements inside the cell, or both. ENDURING UNDERSTANDING LEARNING OBJECTIVE ESSENTIAL KNOWLEDGE 2.5 Membrane Permeability OverviewDo you know the difference between semi-permeability and selective permeability? These extremely important concepts describe how the lipid bilayer indiscriminately rejects certain molecules, while a living cell membrane incorporates channel proteins and transporters that actively select which substances can enter and leave a cell. With these two forms of permeability, cells can protect their DNA, create the perfect internal conditions to carry out reactions and balance their water content. Permeability describes how easily a molecule or substance can pass through a membrane. You can think of permeability in terms of a coffee filter. A coffee filter allows hot water and dissolved coffee particles to pass through the filter, while the larger coffee grounds are blocked. A lipid bilayer is very similar to a coffee filter. The lipid bilayer is made of phospholipid molecules, each of which has a hydrophilic head that is attracted to water and a hydrophobic tail that is attracted to other non-polar molecules. The heads group together and the tails group together to create the lipid bilayer. On either side of the membrane is a layer of hydrophilic heads, while the middle of a lipid bilayer is a very hydrophobic core. While a coffee filter is simply a piece of paper with tiny holes, the lipid bilayer can block molecules or let molecules through based on their chemical nature. Charged molecules, ions, and large molecules are typically blocked by the lipid bilayer. However, some nonpolar molecules and gases like oxygen and carbon dioxide can pass right through the lipid bilayer. Many things affect the permeability of a lipid bilayer, starting with the type of phospholipids used. Phospholipids created with saturated tails tend to squeeze tightly together, making a less permeable bilayer. Phospholipids created with unsaturated fatty acid tails tend to be more fluid, making the membrane more permeable to all sorts of substances. Other factors, like temperature and pH, also affect the permeability of a plasma membrane. For example, if we increase the temperature of the water surrounding a cell, this will push the phospholipids further apart and make the membrane more permeable. This is why individual organisms use different ratios of saturated and unsaturated fatty acids within their phospholipid membranes to keep their membranes permeable at different temperatures! Now, let’s take a look at the difference between a semi-permeable membrane versus a selectively permeable membrane. By itself, a lipid bilayer is a semi-permeable membrane. A coffee filter is also a semi-permeable membrane. It will always block certain substances and always let others through – simply based on the membrane’s chemical and physical properties. Substances may also be blocked to partial degrees – either mostly allowing a substance to cross the membrane, or mostly prohibiting a substance from crossing the membrane. But, a living cell membrane is made up of much more than just phospholipid molecules. Most importantly, the cell membrane has a number of embedded protein channels and carrier proteins that select specific molecules to let through. Since these proteins are made from the DNA code and embedded into the cell membrane through internal cellular mechanisms, it is as if the cell itself is selecting which substances cross the membrane and which substances are prohibited from crossing! To sum up: lipid bilayers are semi-permeable, but cellular membranes are selectively permeable! Think about this… What, exactly, is the defining characteristic of life on Earth? Some scientists think that life is defined by the ability to control your internal and external cellular environments. All organisms on earth – from microscopic nematodes to the enormous blue whale – selectively modify the permeability of their cell membrane by embedding it with specific proteins. Through this ability, cells are able to modify their internal and external environments. The only real difference between organisms is the variations in their DNA codes that produce different membrane proteins and enzymes. Nematodes create proteins that allow them to survive and reproduce, and a blue whale is essentially doing the exact same thing! So, everything up to this point has shown how cell membranes are selectively permeable membranes. Now, let’s take a look at why cells need selectively permeable membranes. The ultimate answer is that cells need to create livable cellular conditions. To understand this better, let’s look at the concepts of diffusion (where particles evenly distribute in a solvent) and osmosis (where water moves through a semi-permeable membrane). If you drop some sugar molecules into a body of water, those sugar molecules will eventually become evenly distributed throughout the water. Imagine an algal cell that is creating sugar. The algal cell creates glucose in the chloroplast and exports that sugar to the cytoplasm. Here, enzymes can process the sugar and mitochondria can process smaller pieces of the sugar to store energy as ATP. The mitochondria then export this chemical energy to power processes throughout the cell. Now, imagine what would happen without a cellular membrane. First of all, there is no way to hold all these organelles together. So, they would just drift apart. Plus, any glucose created by the chloroplasts would simply diffuse away into the surrounding environment. Cell membranes, therefore, essentially give cells the ability to congregate substances they need to survive. Now, let’s consider osmosis. Water molecules are attracted to polar solutes, in the same way, that polar solutes are attracted to water. So, if you divide a body of water with a semi-permeable membrane that blocks larger solute particles, water itself will move through the membrane to make the concentration of both sides of the membrane equal. Cells need selectively permeable membranes due to osmosis. Not only do cells need to hold solutes inside cells, but they need to control their water, pH, and the specific concentration of solutes within their cytosol. To do so, cells load their cell membranes with proteins responsible for importing or exporting the proper substance. Plus, some cells have a contractile vacuole that collects water from the cell and expels the water into the surrounding environment. In summary, cells use many different proteins within their cell membranes to create a selectively permeable membrane and control their internal environments! Selective permeability is a direct consequence of the semi-permeable membrane and the proteins that are embedded into the membrane – also known as the fluid mosaic model. There are several components within the cell membrane that influence the permeability of a cell. The basic semi-permeability of the cell membrane is derived from phospholipid molecules. Phospholipid molecules stick together through hydrogen bonds and non-polar interactions to form the lipid bilayer, which naturally repels molecules of a certain size and chemical composition. The membrane becomes selectively permeable thanks to the work of proteins. Proteins come in many different types, that have different functions to preserve the internal cellular environment. Some proteins allow water to move freely. Others are like powered gates that can select when ions can move through the membrane. Some proteins import and export larger molecules to and from the cell. We will cover all of these membrane proteins further in sections 2.6-2.9. One final component of the cell membrane that drastically affects permeability is cholesterol. Cholesterol is a small lipid steroid molecule that embeds into the plasma membrane. Lipid molecules congregate around cholesterol molecules, pulling the phospholipids together and creating a less permeable membrane. Together, all of these elements allow the cell to control exactly what enters and exits the cells – even in vastly different environmental conditions! Now that we understand the basics of how selective permeability is created, let’s see what types of molecules cells allow and reject! Let’s start with small, nonpolar molecules. Molecules like oxygen can easily pass through the cell membrane. This is important since most eukaryotic cells need oxygen to complete cellular respiration and store energy in ATP. Likewise, small nonpolar molecules of carbon dioxide can easily diffuse through the cell membrane. This is important for the process of respiration, allowing oxygen to be delivered to cells from the blood and carbon dioxide to be exported to the bloodstream and out of the lungs. Many small, uncharged polar molecules like water are only blocked to a slight degree. These molecules are polar, so it is not as easy for them to work through the hydrophobic core of the lipid bilayer. The last two groups – large polar molecules and ions – have a very hard time making it through the lipid bilayer. Larger molecules are mostly blocked by their size and polarity, though they can sometimes slip through the bilayer. By contrast, ions can be very small but are entirely repelled because of their charged nature. However, cells still need to move these substances around. That’s where membrane proteins come into play. For instance, glucose molecules can be imported into cells via specialized glucose carrier proteins. These proteins have an active site that is specific to glucose, so only glucose molecules can be imported. Similarly, hydrogen ions play a major role in ATP production. Proton pumps move hydrogen ions to one side of the membrane, building up a chemical gradient. The protein ATP synthase can use the energy stored in this gradient to assemble ATP molecules. As we have discussed in previous sections, cell walls are produced by many different organisms. Typically, cell walls are made of complex carbohydrates like cellulose, chitin, and peptidoglycans. Though these molecules have evolved in very diverse forms of life, all of these molecules are based on 6-carbon rings of glucose! That also means these fibers are storing a lot of energy. Plants and algae use cellulose to create walls, arthropods and fungi use chitin, and prokaryotes like bacteria use peptidoglycans as base molecules for their cell walls. While we will go into specifics for these organisms later in this course, the cell walls of each of these organisms affect the permeability of the cell membrane in similar ways. In general, cell walls do not affect the permeability of small molecules. Small molecules can easily slip through the fibers of the cell wall. If they are nonpolar, they can easily enter the cell. By contrast, cell walls readily block many larger molecules. Large molecules tend to get caught up in this extra layer of fiber. So, in addition to the structural support that cell walls provide, they also help organisms filter the environment around them and only accept the molecules that they need to survive! |
Which of the following best explains why the cell membrane is described as selectively permeable?
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