Chapter 8: Biological Membranes
Mitochondria are referred to as the "powerhouse" of the cell because of their ability to produce ATP by oxidative respiration. *How many membranes do mitochondria have?*
Mitochondria contain 2 membranes: the inner and outer mitochondrial membranes.
What's a mnemonic to remember that water flows into a cell placed in a hypotonic solution?
Mnemonic: To remember that water flows into a cell placed in hyp*O*tonic solution, imagine the cell swelling to form a giant letter *O*.
*Bridge:* Biosignaling is a major function of the cell membrane. Receptors and signal cascades are covered in more detail in Chapter 3.
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*MCAT Concept Check 8.3 on page 270.*
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*MCAT Expertise:* The ration of certain sphingolipids to glycerophospholipids can help to identify particular membranes within the cell, but memorizing this information is unnecessary for Test Day. Where small details like this are important, they will be provided in a passage.
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MCAT Concept Check 8.2 on page 264.
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MCAT Concept Check 8.4 on page 272, read conclusion, and do practice questions!
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MCAT Concept Check 8.1 on page 257.
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Cholesterol is associated with a number of negative health effects and receives a lot of negative press; however, it is also a very important molecule in our cells. Cholesterol not only imparts fluidity to membranes, but it is also necessary in the synthesis of all steroids, which are derived from cholesterol. *Describe the structure of cholesterol.*
The structure of cholesterol is similar to that of phospholipids in that cholesterol contains both a hydrophilic and hydrophobic region.
Active transport results in the net movement of a solute against its concentration gradient, just like rolling a ball uphill. Active transport always requires energy, but the source of this energy can vary. Primary active transport uses ATP or another energy molecule to directly transport molecules across a membrane. Generally, primary active transport involves the use of a transmembrane ATPase. Secondary active transport, also known as coupled transport, also uses energy to transport molecules across the membrane; however, in contrast to primary active transport, there is no direct coupling to ATP hydrolysis. Instead, secondary active transport harnesses the energy released by one particle going DOWN its electrochemical gradient to drive a different particle UP its gradient. When both particles flow the same direction across the membrane, it is termed symport. When the particles flow in opposite directions, it is called antiport. For instance, primary active transport maintains the membrane potential of neurons in the nervous system. The kidneys use secondary active transport, usually driven by sodium, to reabsorb and secrete various solutes into and out of the filtrate. Figure 8.10 shows simple diffusion, facilitated diffusion, and active transport. *Table 8.1 summarizes these types of movement as well as osmosis. For simple diffusion, osmosis, facilitated diffusion, and active transport, give this information: Concentration gradient of solute? Membrane protein required? Energy required? Example molecule(s) transported?*
Table 8.1 summarizes these types of movement as well as osmosis. • *SIMPLE DIFFUSION:* Concentration gradient of solute? *High to low* Membrane protein required? *No* Energy required? *NO! This is a passive process* Example molecule(s) transported? *Small, non-polar (O2, CO2)* • *OSMOSIS:* Concentration gradient of solute? *Low to high* Membrane protein required? *No* Energy required? *NO! This is a passive process* Example molecule(s) transported? *H2O* • *FACILITATED DIFFUSION:* Concentration gradient of solute? *High to low* Membrane protein required? *Yes* Energy required? *NO! This is a passive process* Example molecule(s) transported? *Polar molecules (glucose) or ions (Na+, Cl-)* • *ACTIVE TRANSPORT:* Concentration gradient of solute? *Low to high* Membrane protein required? *Yes* Energy required? *YES! This is an active process; requires energy* Example molecule(s) transported? *Polar molecules or ions (Na+, Cl-, K+)*
*Bridge:* The cell membrane is often compared to a capacitor. Why?
*Bridge:* The cell membrane is often compared to a capacitor because opposite charges are maintained on either side of the membrane. Capacitance is discussed in Chapter 6 of MCAT Physics and Math Review.
*Key Concept:* Unless otherwise specified, semipermeable membrane refers to a membrane governed by the same permeability rules as biological membranes. Explain.
*Key Concept:* Unless otherwise specified, semipermeable membrane refers to a membrane governed by the same permeability rules as biological membranes: small, non-polar, lipid-soluble particles (and water) can pass through freely, while large, polar, or charged particles cannot.
*MCAT Expertise:* An important point to keep in mind is that all transmembrane movement is based on _____ _____, which are an MCAT favorite. This will tell us whether the process will be _____ or _____.
*MCAT Expertise:* An important point to keep in mind is that all transmembrane movement is based on concentration gradients, which are an MCAT favorite; understanding concentration gradients will net you points on Test Day. Remember that the gradient will tell us whether this process will be passive or active.
*Real world:* What do trans fats result from? Trans fats have been banned from certain stores and cities because of their health risks. Part of the health concern is due to...?
*Real world:* Trans fats, which result from the parietal hydrogenation of some unsaturated fatty acids, have been banned from certain stores and cities because of their health risks. Part of the health concern is due to their ability to lower membrane fluidity, in addition to the tendency of trans fats to accumulate and form plaques in blood vessels.
What does active transport result in, and what does it require to work?
Active transport results in the net movement of a solute against its concentration gradient, just like rolling a ball uphill. Active transport always requires energy, but the source of this energy can vary.
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. How so?
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 2 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.
There is a steady-state resting relationship between ion diffusion and the Na+/K+ ATPase. One of the main functions of the Na+/K+ ATPase is to maintain a low concentration of sodium ions and high concentration of potassium ions intracellularly by pumping 3 sodium ions out for every 2 potassium ions pumped in. This movement of ions removes one positive charge from the intracellular space of the cell, which maintains the negative resting potential of the cell. *As mentioned before, the cell membrane also contains leak channels. What do these allow for?*
As mentioned before, the cell membrane also contains leak channels that allow ions, such as Na+ and K+, to passively diffuse into or out of the cell down their concentration gradients.
Carbohydrates are generally attached to protein molecules on the extracellular surface of cells. Because carbohydrates are generally _____, interactions between glycoproteins and water can form...?
Because carbohydrates are generally hydrophilic, interactions between glycoproteins and water can form a coat around the cell, as shown in Figure 8.5.
While the fluid mosaic model outlines the general composition of the membrane, the MCAT expects us to have a stronger grasp of the specifics, especially as it pertains to lipids and proteins. The cell membrane is composed predominantly of lipids with some associated proteins and carbohydrates. At times, the cell membrane as a whole will be referred to as a phospholipid bilayer, as it is the primary component of this barrier around the cell. Within the cell membrane, there are a large number of phospholipids with very few free fatty acids. In addition, steroid molecules and cholesterol, which lend fluidity to the membrane, and waxes, which provide membrane stability, help to maintain the structural integrity of the cell. While the structural details of these lipids were discussed in detail in Chapter 5, we'll briefly describe their key points here. Fatty acids are carboxylic acids that contain a hydrocarbon chain and terminal carboxyl group. Triacylglycerols, also referred to as triglycerides, are storage lipids involved in human metabolic processes. They contain 3 fatty acid chains esterified to a glycerol molecule. Fatty acid chains can be saturated or unsaturated. Unsaturated fatty acids are regarded as "healthier" fats because they tend to have one or more double bonds and exist in liquid form at room temperature; in the plasma membrane, these characteristics impart fluidity to the membrane. Humans can only synthesize a few of the unsaturated fatty acids; the rest come from essential fatty acids in the diet that are transported as triglycerides from the intestine inside chylomicrons. 2 important essential fatty acids for humans are alpha-linolenic acid and linoleic acid. Saturated fatty acids are the "unhealthy" fats, and are the main components of animal fats and tend to exist as solids at room temperature. Saturated fats are found in processed foods and are considered less healthy. When incorporated into phospholipid membranes, saturated fatty acids decrease the overall membrane fluidity. *What happens when you substitute one of the fatty acid chains of triacylglycerol with a phosphate group?*
By substituting one of the fatty acid chains of triacylglycerol with a phosphate group, a polar head group joins the non-polar tails, forming a glycerophospholipid, commonly called a phospholipid.
Cells within tissues can form a cohesive layer via _____ _____.
Cells within tissues can form a cohesive layer via intracellular junctions.
Carbohydrates are generally attached to what and where?
Carbohydrates are generally attached to protein molecules on the extracellular surface of cells.
Facilitated diffusion is simple diffusion for molecules that are impermeable to the membrane (large, polar, or charged); the energy barrier is too high for these molecules to cross freely. Facilitated diffusion requires integral membrane proteins to serve as transporters or channels for these substrates. *The classic examples of facilitated diffusion involve a carrier or channel protein. Explain each.*
Carriers are only open to one side of the cell membrane at any given point. This model is similar to a revolving door because the substrate binds to the transport protein (walks in), remains in the transporter during a conformational change (spins), and then finally dissociates from the substrate-binding site of the transporter (walks out). Binding of the substrate molecule to the transporter protein induces a conformational change; for a brief time, the carrier is in the occluded state, in which the carrier is not open to either side of the phospholipid bilayer. In addition to carriers, channels are also viable transporters for facilitated diffusion. Channels may have been an open or closed conformation. In their open conformation, channels are exposed to both side of the cell membrane and act like a tunnel for the particles to diffuse through, thereby permitting much more rapid transport kinetics. The activity of the 3 main types of ion channels was discussed in Chapter 3.
There is a steady-state resting relationship between ion diffusion and the Na+/K+ ATPase. One of the main functions of the Na+/K+ ATPase is to maintain a low concentration of sodium ions and high concentration of potassium ions intracellularly by pumping 3 sodium ions out for every 2 potassium ions pumped in. This movement of ions removes one positive charge from the intracellular space of the cell, which maintains the negative resting potential of the cell. As mentioned before, the cell membrane also contains leak channels that allow ions, such as Na+ and K+, to passively diffuse into or out of the cell down their concentration gradients. *Are cell membranes more permeable to K+ ions or Na+ ions at rest, and why? The combination of what 2 things maintains a stable resting potential?*
Cell membranes are more permeable to K+ ions than Na+ ions at rest because there are more K+ leak channels than Na+ leak channels. The combination of Na+/K+ ATPase activity and leak channels together maintain a stable resting membrane potential.
Cell membranes have both a stretchy, flexible component (_____) and an abundance of stabilizing molecules (_____ and _____) to make sure that everything remains intact.
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.
Cells within tissues can form a cohesive layer via intracellular junctions. These junctions provide direct pathways of communication between neighboring cells or between cells and the extracellular matrix. *Cell-cell junctions are generally comprised of...? What are these and what are their function?*
Cell-cell junctions are generally comprised of cell adhesion molecules (CAMs), which are proteins that allows cells to recognize each other and contribute to proper cell differentiation and development.
Cholesterol is associated with a number of negative health effects and receives a lot of negative press; however, it is also a very important molecule in our cells. How so?
Cholesterol not only imparts fluidity to membranes, but it is also necessary in the synthesis of all steroids, which are derived from cholesterol.
Transport of small nonpolar molecules occurs more rapidly through the cell membrane via diffusion, while ions and larger molecules require more specialized transport processes. The different membrane traffic processes are classified as either active or passive, and are driven by concentration gradients or intracellular energy stores. Spontaneous processes that do no require energy (negative ΔG) proceed through passive transport, while those that are non-spontaneous and require energy (positive ΔG) proceed through active transport. *How are diffusion, facilitated diffusion, osmosis, and active transport each affected by temperature?*
Diffusion, facilitated diffusion, and osmosis generally increase in rate as temperature increases, while active transport may or may not be affected by temperature, depending on the enthalpy (ΔH) of the process.
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 2 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 non-polar 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 what 3 processes?*
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 down-regulate the number of specific cellular receptors on their surface in order to meet cellular requirements.
When does endocytosis occur? Explain how the process works.
Endocytosis occurs when the cell membrane invaginates and engulfs material to bring it into the cell. The material is encased in a vesicle, which is important because cells will sometimes ingest toxic substances.
When does exocytosis occur, and how does it work? Exocytosis is important in the nervous system and intercellular signaling. Give an example.
Exocytosis occurs when secretory vesicles fuse with the membrane, releasing material from inside the cell to the extracellular environment. Exocytosis is important in the nervous system and intercellular signaling. For instance, exocytosis of NTs from synaptic vesicles is a crucial aspect of neuron physiology. Both endo- and exocytosis are illustrated in Figure 8.11.
Describe what facilitated diffusion is. What molecules use this type of diffusion, and why? What does facilitated diffusion require to assist these substrates?
Facilitated diffusion is simple diffusion for molecules that are impermeable to the membrane (large, polar, or charged); the energy barrier is too high for these molecules to cross freely. Facilitated diffusion requires integral membrane proteins to serve as transporters or channels for these substrates.
While the fluid mosaic model outlines the general composition of the membrane, the MCAT expects us to have a stronger grasp of the specifics, especially as it pertains to lipids and proteins. The cell membrane is composed predominantly of lipids with some associated proteins and carbohydrates. At times, the cell membrane as a whole will be referred to as a phospholipid bilayer, as it is the primary component of this barrier around the cell. Within the cell membrane, there are a large number of phospholipids with very few free fatty acids. In addition, steroid molecules and cholesterol, which lend fluidity to the membrane, and waxes, which provide membrane stability, help to maintain the structural integrity of the cell. While the structural details of these lipids were discussed in detail in Chapter 5, we'll briefly describe their key points here. *What are fatty acids? What are triacylglycerols, what other name do they go by, and what do they contain?
Fatty acids are carboxylic acids that contain a hydrocarbon chain and terminal carboxyl group. Triacylglycerols, also referred to as triglycerides, are storage lipids involved in human metabolic processes. They contain 3 fatty acid chains esterified to a glycerol molecule.
Carbohydrates are generally attached to protein molecules on the extracellular surface of cells. Because carbohydrates are generally hydrophilic, interactions between glycoproteins and water can form a coat around the cell, as shown in Figure 8.5. *In addition, carbohydrates can act as signaling and recognition molecules. Give an example.*
For example, blood group (ABO) antigens on RBCs differ only in their carbohydrate sequence. Our immune systems and some pathogens take advantage of these membrane carbohydrates and membrane proteins to target particular cells.
Some of the transporters for facilitated diffusion and active transport can be activated or de-activated by membrane receptors, which tend to be transmembrane proteins. Give 2 examples.
For example, ligand-gated ion channels are membrane receptors that open a channel in response to the binding of a specific ligand. Other membrane receptors participate in biosignaling; for example, GPCRs are involved in several different signal transduction cascades.
Active transport results in the net movement of a solute against its concentration gradient, just like rolling a ball uphill. Active transport always requires energy, but the source of this energy can vary. Primary active transport uses ATP or another energy molecule to directly transport molecules across a membrane. Generally, primary active transport involves the use of a transmembrane ATPase. Secondary active transport, also known as coupled transport, also uses energy to transport molecules across the membrane; however, in contrast to primary active transport, there is no direct coupling to ATP hydrolysis. Instead, secondary active transport harnesses the energy released by one particle going DOWN its electrochemical gradient to drive a different particle UP its gradient. When both particles flow the same direction across the membrane, it is termed symport. When the particles flow in opposite directions, it is called antiport. *Active transport is important in many tissues. Give 2 examples.*
For instance, primary active transport maintains the membrane potential of neurons in the nervous system. The kidneys use secondary active transport, usually driven by sodium, to reabsorb and secrete various solutes into and out of the filtrate. Figure 8.10 shows simple diffusion, facilitated diffusion, and active transport.
The membranes of most organelles are similar to the cell membrane in both composition and general characteristics; however, it is important to note that some membranes are specialized to accomplish specific functions. Give an example using the sarcolemma of muscle cells.
For instance, the sarcolemma of muscle cells must maintain a membrane potential for muscle contraction to occur. Membrane composition may also be altered slightly, especially in the case of mitochondria.
Gap junctions allow for direct cell-cell communication and are often found in small bunches together. *What's another name for them, and how are they formed?*
Gap junctions (connexons) are formed by the alignment and interaction of pores composed of 6 molecules of connexin, as shown in Figure 8.6.
What do gap junctions allow for, and in what form are they often found?
Gap junctions allow for direct cell-cell communication and are often found in small bunches together.
Phospholipids spontaneously assemble into micelles (small monolayer vesicles) or liposomes (bi-layered vesicles) due to hydrophobic interactions. *What are glycerophospholipids used for, and what can they produce? Give an example.*
Glycerophospholipids are used for membrane synthesis and can produce a hydrophilic surface layer on lipoproteins such as very-low-density lipoprotein (VLDL), a lipid transporter.
While the fluid mosaic model outlines the general composition of the membrane, the MCAT expects us to have a stronger grasp of the specifics, especially as it pertains to lipids and proteins. The cell membrane is composed predominantly of lipids with some associated proteins and carbohydrates. At times, the cell membrane as a whole will be referred to as a phospholipid bilayer, as it is the primary component of this barrier around the cell. Within the cell membrane, there are a large number of phospholipids with very few free fatty acids. In addition, steroid molecules and cholesterol, which lend fluidity to the membrane, and waxes, which provide membrane stability, help to maintain the structural integrity of the cell. While the structural details of these lipids were discussed in detail in Chapter 5, we'll briefly describe their key points here. Fatty acids are carboxylic acids that contain a hydrocarbon chain and terminal carboxyl group. Triacylglycerols, also referred to as triglycerides, are storage lipids involved in human metabolic processes. They contain 3 fatty acid chains esterified to a glycerol molecule. *Fatty acid chains can be saturated or unsaturated. Unsaturated fatty acids are regarded as "healthier" fats because they tend to have one or more double bonds and exist in liquid form at room temperature; in the plasma membrane, these characteristics impart fluidity to the membrane. Do humans make unsaturated fatty acids, or do they only come from our diet? Elaborate, and then name 2 important essential fatty acids for humans.*
Humans can only synthesize a few of the unsaturated fatty acids; the rest come from essential fatty acids in the diet that are transported as triglycerides from the intestine inside chylomicrons. 2 important essential fatty acids for humans are alpha-linolenic acid and linoleic acid.
Osmosis is a specific kind of simple diffusion that concerns water; water will move from a region of lower solute concentration to one of higher solute concentration. That is, it will move from a region of higher water concentration (more dilute solution) down its gradient to a region of lower water concentration (more concentrated solution). Osmosis is important in several places, most notably when the solute itself is impermeable to the membrane. In such a case, water will move to try to bring solute concentrations to equimolarity, as shown in Figure 8.8. *If the concentration of solutes inside the cell is higher than the surrounding solution, the solution is said to be _____. How will such a solution affect a cell? A solution that is more concentrated than the cell is termed a _____ solution. How will such a solution affect a cell? If the solutions inside and outside are equimolar, they are said to be _____. How will such a solution affect a cell?*
If the concentration of solutes inside the cell is higher than the surrounding solution, the solution is said to be hypotonic; such a solution will cause a cell to swell as water rushes in, sometimes to the point of bursting. A solution that is more concentrated than the cell is termed a hypertonic solution, and water will move out of the cell. If the solutions inside and outside are equimolar, they are said to be isotonic. A key point here is that isotonicity does not prevent movement; rather, it prevents the NET movement of particles. Water molecules will continue to move; however, the cell will neither gain nor lose water.
Phospholipids spontaneously assemble into micelles (small monolayer vesicles) or liposomes (bi-layered vesicles) due to hydrophobic interactions. Glycerophospholipids are used for membrane synthesis and can produce a hydrophilic surface layer on lipoproteins such as very-low-density lipoprotein (VLDL), a lipid transporter. *In addition, phospholipids are the primary component of cell membranes. Phospholipids serve not only structural roles, but can also serve as _____ _____ in signal transduction. The phosphate group also provides an attachment point for _____-soluble groups, such as _____ or _____.*
In addition, phospholipids are the primary component of cell membranes. Phospholipids serve not only structural roles, but can also serve as second messengers in signal transduction. The phosphate group also provides an attachment point for water-soluble groups, such as choline (phosphatidylcholine, also known as lecithin) or inositol (phosphatidylinositol). A comparison of triacylglycerols and glycerophospholipids is shown in Figure 8.2.
In cells, the osmotic pressure is maintained against the _____ _____, rather than...? If the osmotic pressure created by the solutes within a cell exceeds the pressure that the cell membrane can withstand, what happens to the cell? Generally, how is osmotic pressure best thought of?
In cells, the osmotic pressure is maintained against the cell membrane, rather than the force of gravity. If the osmotic pressure created by the solutes within a cell exceeds the pressure that the cell membrane can withstand, the cell will lyse. Generally, osmotic pressure is best thought of as a "sucking" pressure, drawing water into the cell in proportion to the concentration of the solution.
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 2 membranes that have been tagged with different labels; the tags will migrate with their associated lipids until both types are equally intermixed. *What are lipid rafts? What are their roles and how do they travel?*
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.
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 2 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 why is this energetically unfavorable? How are they assisted?*
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 non-polar tail region in the interior of the membrane. Specialized enzymes called flippases assist in the transition or "flip" between layers.
The impermeability of the cell membrane to ions and the selectivity of ion channels both lead to an electrochemical gradient between the exterior and interior of cells. The difference in electrical potential across cell membranes is called the membrane potential, Vm. The resting potential for more cells is between -40 and -80 mV, although the potential can rise as high as +35 mV during depolarization of the cell. *What does maintaining membrane potential require and why?*
Maintaining membrane potential requires energy because ions may passively diffuse through the cell membrane over time using leak channels; therefore, an ion transporter or pump such as the sodium-potassium pump (Na+/K+ ATPase) regulates the concentration of intracellular and extracellular sodium and potassium ions. Chloride ions also participate in establishing membrane potential.
Some of the transporters for facilitated diffusion and active transport can be activated or de-activated by membrane receptors, which tend to be transmembrane proteins. For example, ligand-gated ion channels are membrane receptors that open a channel in response to the binding of a specific ligand. Other membrane receptors participate in biosignaling; for example, GPCRs are involved in several different signal transduction cascades. *Membrane receptors are generally _____, although there are some _____ and _____ receptors, especially in _____.*
Membrane receptors are generally proteins, although there are some carbohydrate and lipid receptors, especially in viruses.
Mitochondria are referred to as the "powerhouse" of the cell. Why?
Mitochondria are referred to as the "powerhouse" of the cell because of their ability to produce ATP by oxidative respiration.
Osmosis is a specific kind of simple diffusion that concerns water; water will move from a region of lower solute concentration to one of higher solute concentration. That is, it will move from a region of higher water concentration (more dilute solution) down its gradient to a region of lower water concentration (more concentrated solution). Osmosis is important in several places, most notably when the solute itself is impermeable to the membrane. In such a case, water will move to try to bring solute concentrations to equimolarity, as shown in Figure 8.8. If the concentration of solutes inside the cell is higher than the surrounding solution, the solution is said to be hypotonic; such a solution will cause a cell to swell as water rushes in, sometimes to the point of bursting. A solution that is more concentrated than the cell is termed a hypertonic solution, and water will move out of the cell. If the solutions inside and outside are equimolar, they are said to be isotonic. A key point here is that isotonicity does not prevent movement; rather, it prevents the NET movement of particles. Water molecules will continue to move; however, the cell will neither gain nor lose water. *One method for quantifying the driving force behind osmosis is _____ _____. Describe what it is, and how it works.*
One method for quantifying the driving force behind osmosis is osmotic pressure. Osmotic pressure is a colligative property: a physical property of solutions that is dependent on the concentration of dissolved particles but not on the chemical identity of those dissolved particles. Other examples of colligative properties include vapor pressure depression (Raoult's Law), boiling point elevation, and freezing point depression. To illustrate osmotic pressure, consider a container separated into 2 compartments by a semipermeable membrane, just like the membranes in our cells. One compartment contains pure water, while the other contains water with dissolved solutes. The membrane allows water but not solutes to pass through. Because substances tend to flow, or diffuse, from higher to lower concentration (which results in an increase in entropy), water will diffuse from the compartment containing pure water into the compartment containing the water-solute mixture. This net flow will cause the water level in the compartment containing the solution to rise above the level in the compartment containing pure water, as shown in Figure 8.9. Because the solute cannot pass through the membrane, the concentrations of solute in the 2 compartments can never be equal. However, the hydrostatic pressure exerted by the water level in the solute-containing compartment will eventually oppose the influx of water; thus, the water level will only rise to the point at which it exerts a sufficient pressure to counterbalance the tendency of water to flow across the membrane.
There is a steady-state resting relationship between ion diffusion and the Na+/K+ ATPase. What is one of the main functions of the Na+/K+ ATPase, and how does it accomplish this?
One of the main functions of the Na+/K+ ATPase is to maintain a low concentration of sodium ions and high concentration of potassium ions intracellularly by pumping 3 sodium ions out for every 2 potassium ions pumped in. This movement of ions removes one positive charge from the intracellular space of the cell, which maintains the negative resting potential of the cell.
Describe the process of osmosis and how it works.
Osmosis is a specific kind of simple diffusion that concerns water; water will move from a region of lower solute concentration to one of higher solute concentration. That is, it will move from a region of higher water concentration (more dilute solution) down its gradient to a region of lower water concentration (more concentrated solution).
Osmosis is a specific kind of simple diffusion that concerns water; water will move from a region of lower solute concentration to one of higher solute concentration. That is, it will move from a region of higher water concentration (more dilute solution) down its gradient to a region of lower water concentration (more concentrated solution). *Osmosis is important in several places, most notably when...? What happens in such a case?*
Osmosis is important in several places, most notably when the solute itself is impermeable to the membrane. In such a case, water will move to try to bring solute concentrations to equimolarity, as shown in Figure 8.8.
Describe passive transport processes.
Passive transport processes are those that do not require intracellular energy stores but rather utilize the concentration gradient to supply the energy for particles to move.
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 _____ _____. When can this be seen? Elaborate.*
Phospholipids move rapidly in the plane of the membrane through simple diffusion. This can be seen when fusing 2 membranes that have been tagged with different labels; the tags will migrate with their associated lipids until both types are equally intermixed.
Phospholipids _____ assemble into _____ or _____ due to...?
Phospholipids spontaneously assemble into micelles (small monolayer vesicles) or liposomes (bi-layered vesicles) due to hydrophobic interactions.
Endocytosis occurs when the cell membrane invaginates and engulfs material to bring it into the cell. The material is encased in a vesicle, which is important because cells will sometimes ingest toxic substances. *What is pinocytosis? Compare it to phagocytosis. What will initiate the process of endocytosis?*
Pinocytosis is the endocytosis of fluids and dissolved particles, whereas phagocytosis is the ingestion of large solids such as bacteria. Substrate binding to specific receptors embedded within the plasma membrane will initiate the process of endocytosis.
Active transport results in the net movement of a solute against its concentration gradient, just like rolling a ball uphill. Active transport always requires energy, but the source of this energy can vary. *What does primary active transport use, and what does secondary active transport use? What's another name for secondary active transport?*
Primary active transport uses ATP or another energy molecule to directly transport molecules across a membrane. Generally, primary active transport involves the use of a transmembrane ATPase. Secondary active transport, also known as coupled transport, also uses energy to transport molecules across the membrane; however, in contrast to primary active transport, there is no direct coupling to ATP hydrolysis. Instead, secondary active transport harnesses the energy released by one particle going DOWN its electrochemical gradient to drive a different particle UP its gradient. When both particles flow the same direction across the membrane, it is termed symport. When the particles flow in opposite directions, it is called antiport.
*Real world:* Osmolarity explains why pure water should never be given intravenously for resuscitation. Explain.
RBCs have an osmolarity around 300 mOsm/L, while pure water has an osmolarity of 0 mOsm/L. Water would rush into the RBCs, causing them to burst. To avoid this, saline or dextrose-containing solutions are used.
While the fluid mosaic model outlines the general composition of the membrane, the MCAT expects us to have a stronger grasp of the specifics, especially as it pertains to lipids and proteins. The cell membrane is composed predominantly of lipids with some associated proteins and carbohydrates. At times, the cell membrane as a whole will be referred to as a phospholipid bilayer, as it is the primary component of this barrier around the cell. Within the cell membrane, there are a large number of phospholipids with very few free fatty acids. In addition, steroid molecules and cholesterol, which lend fluidity to the membrane, and waxes, which provide membrane stability, help to maintain the structural integrity of the cell. While the structural details of these lipids were discussed in detail in Chapter 5, we'll briefly describe their key points here. Fatty acids are carboxylic acids that contain a hydrocarbon chain and terminal carboxyl group. Triacylglycerols, also referred to as triglycerides, are storage lipids involved in human metabolic processes. They contain 3 fatty acid chains esterified to a glycerol molecule. Fatty acid chains can be saturated or unsaturated. *Are saturated fatty acids the healthy or unhealthy fats? where are they found, and how do they affect overall membrane fluidity?*
Saturated fatty acids are the "unhealthy" fats, and are the main components of animal fats and tend to exist as solids at room temperature. Saturated fats are found in processed foods and are considered less healthy. When incorporated into phospholipid membranes, saturated fatty acids decrease the overall membrane fluidity.
Sphingolipids are also important constituents of cell membranes. Although sphingolipids do not contain _____, they are similar in structure to _____, in that they contain...?*
Sphingolipids are also important constituents of cell membranes. Although sphingolipids do not contain glycerol, they are similar in structure to glycerophospholipids, in that they contain a hydrophilic region and 2 fatty acid-derived hydrophobic tails.
Transport of small nonpolar molecules occurs more rapidly through the cell membrane via diffusion, while ions and larger molecules require more specialized transport processes. The different membrane traffic processes are classified as either active or passive, and are driven by concentration gradients or intracellular energy stores. *Transport processes can be classified as active or passive depending on their thermodynamics. Explain.*
Spontaneous processes that do no require energy (negative ΔG) proceed through passive transport, while those that are non-spontaneous and require energy (positive ΔG) proceed through active transport.
The impermeability of the cell membrane to ions and the selectivity of ion channels both lead to an electrochemical gradient between the exterior and interior of cells. The difference in electrical potential across cell membranes is called the membrane potential, Vm. The resting potential for more cells is between -40 and -80 mV, although the potential can rise as high as +35 mV during depolarization of the cell. Maintaining membrane potential requires energy because ions may passively diffuse through the cell membrane over time using leak channels; therefore, an ion transporter or pump such as the sodium-potassium pump (Na+/K+ ATPase) regulates the concentration of intracellular and extracellular sodium and potassium ions. Chloride ions also participate in establishing membrane potential. The Nernst equation can be used to determine the membrane potential from the intra- and extracellular concentrations of the various ions (pictured), where R is the ideal gas constant, T is the temperature in kelvins, z is the charge of the ion, and F is the Faraday constant (96,485 C/mol e-). The simplification to 61.5 in the numerator assumes body temperature, 310 K. *The Goldman-Hodgkin-Katz voltage equation flows from the equation. Describe it.*
The Goldman-Hodgkin-Katz voltage equation flows from the equation, taking into account the relative contribution of each major ion to the membrane potential, where P represents the permeability for the relevant ion. Note that chloride is inverted relative to the other ions because it carries a negative charge.
The impermeability of the cell membrane to ions and the selectivity of ion channels both lead to an electrochemical gradient between the exterior and interior of cells. The difference in electrical potential across cell membranes is called the membrane potential, Vm. The resting potential for more cells is between -40 and -80 mV, although the potential can rise as high as +35 mV during depolarization of the cell. Maintaining membrane potential requires energy because ions may passively diffuse through the cell membrane over time using leak channels; therefore, an ion transporter or pump such as the sodium-potassium pump (Na+/K+ ATPase) regulates the concentration of intracellular and extracellular sodium and potassium ions. Chloride ions also participate in establishing membrane potential. *What equation can be used to determine the membrane potential from the intra- and extracellular concentrations of the various ions? Write and explain it.*
The Nernst equation can be used to determine the membrane potential from the intra- and extracellular concentrations of the various ions (pictured), where R is the ideal gas constant, T is the temperature in kelvins, z is the charge of the ion, and F is the Faraday constant (96,485 C/mol e-). The simplification to 61.5 in the numerator assumes body temperature, 310 K.
While the fluid mosaic model outlines the general composition of the membrane, the MCAT expects us to have a stronger grasp of the specifics, especially as it pertains to lipids and proteins. *The cell membrane is composed predominantly of _____ with some associated _____ and _____. At times, the cell membrane as a whole will be referred to as a _____ _____, as it is the primary component of this barrier around the cell.*
The cell membrane is composed predominantly of lipids with some associated proteins and carbohydrates. At times, the cell membrane as a whole will be referred to as a phospholipid bilayer, as it is the primary component of this barrier around the cell.
The fluid mosaic model also accounts for the presence of 3 types of membrane proteins, as shown in Figure 8.4. Name the 3, and describe their location in the membrane. Elaborate!
The fluid mosaic model also accounts for the presence of 3 types of membrane proteins, as shown in Figure 8.4 (transmembrane and embedded proteins which together form integral proteins; and peripheral proteins). Transmembrane proteins pass completely through the lipid bilayer. Embedded proteins, on the other hand, are associated with only the interior (cytoplasmic) or exterior (extracellular) surface of the cell membrane. Together, transmembrane and embedded proteins are considered integral proteins because of their association with the interior of the plasma membrane, which is usually assisted by one or more membrane-associated domains that are partially hydrophobic. Membrane-associated (peripheral) proteins may be bound through electrostatic interactions with the lipid bilayer, especially at lipid rafts, or to other transmembrane or embedded proteins, like the G proteins found in GPCRs. Transporters, channels, and receptors are generally transmembrane proteins.
The impermeability of the cell membrane to ions and the selectivity of ion channels both lead to an electrochemical gradient between the exterior and interior of cells. The difference in electrical potential across cell membranes is called the membrane potential, Vm. The resting potential for more cells is between ____ and ____ mV, although the potential can rise as high as ____ mV during depolarization of the cell.
The impermeability of the cell membrane to ions and the selectivity of ion channels both lead to an electrochemical gradient between the exterior and interior of cells. The difference in electrical potential across cell membranes is called the membrane potential, Vm. The resting potential for more cells is between -40 and -80 mV, although the potential can rise as high as +35 mV during depolarization of the cell.
Mitochondria are referred to as the "powerhouse" of the cell because of their ability to produce ATP by oxidative respiration. Mitochondria contain 2 membranes: the inner and outer mitochondrial membranes. The outer mitochondrial membrane is highly permeable due to many large pores that allow for the passage of ions and small proteins. The outer membrane completely surrounds the inner mitochondrial membrane, with the presence of a small inter-membrane space in between the 2 layers. The inner mitochondrial membrane has a much more restricted permeability compared to the outer mitochondrial membrane. Structurally, the inner mitochondrial membrane contains numerous infoldings, known as cristae, which increase the available surface area for the integral proteins associated with the membrane. These proteins, discussed in Chapter 10, are involved in the ETC and ATP synthesis. The inner membrane also encloses the mitochondrial matrix, where the citric acid cycle produces high-energy electron carriers used in the ETC. *The inner mitochondrial membrane contains a very high level of _____ and does not contain _____.*
The inner mitochondrial membrane contains a very high level of cardiolipin and does not contain cholesterol.
Mitochondria are referred to as the "powerhouse" of the cell because of their ability to produce ATP by oxidative respiration. Mitochondria contain 2 membranes: the inner and outer mitochondrial membranes. The outer mitochondrial membrane is highly permeable due to many large pores that allow for the passage of ions and small proteins. The outer membrane completely surrounds the inner mitochondrial membrane, with the presence of a small inter-membrane space in between the 2 layers. *Describe the permeability of the inner mitochondrial membrane. Describe it structurally, and why it's like this.*
The inner mitochondrial membrane has a much more restricted permeability compared to the outer mitochondrial membrane. Structurally, the inner mitochondrial membrane contains numerous infoldings, known as cristae, which increase the available surface area for the integral proteins associated with the membrane.
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 2 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. 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. *What is the main function of the cell membrane, and how do they accomplish this?*
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 most basic of all membrane traffic processes is _____ _____. Describe how it works, and what particles are able to undergo this process.
The most basic of all membrane traffic processes is simple diffusion, in which substrates move down their concentration gradient directly across the membrane. Only particles that are freely permeable to the membrane are able to undergo simple diffusion.
Mitochondria are referred to as the "powerhouse" of the cell because of their ability to produce ATP by oxidative respiration. Mitochondria contain 2 membranes: the inner and outer mitochondrial membranes. *Describe the permeability of the outer mitochondrial membrane, and why it's like this. Describe its structure.*
The outer mitochondrial membrane is highly permeable due to many large pores that allow for the passage of ions and small proteins. The outer membrane completely surrounds the inner mitochondrial membrane, with the presence of a small inter-membrane space in between the 2 layers.
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 2 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. *The phospholipid bilayer also includes proteins and distinct signaling areas within _____ _____. Carbohydrates associated with membrane-bound proteins create a _____ _____. How do the cell wall of plants, bacteria, and fungi compare to that of humans'?*
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.
Transport of small nonpolar molecules occurs more rapidly through the cell membrane via diffusion, while ions and larger molecules require more specialized transport processes. The different membrane traffic processes are classified as either active or passive, and are driven by concentration gradients or intracellular energy stores. Spontaneous processes that do no require energy (negative ΔG) proceed through passive transport, while those that are non-spontaneous and require energy (positive ΔG) proceed through active transport. Diffusion, facilitated diffusion, and osmosis generally increase in rate as temperature increases, while active transport may or may not be affected by temperature, depending on the enthalpy (ΔH) of the process. *The primary thermodynamic motivator in most passive transport is...?*
The primary thermodynamic motivator in most passive transport is an increase in entropy (ΔS).
*Real World:* Many antidepressants increase levels of NTs in the brain, but the effects take longer to appear than the changes in neurochemistry. What is the reason for this delay?
The reason for this delay is that the nervous system must still up-regulate its post-synaptic receptors to respond to the new levels of NT.
Sphingolipids are also important constituents of cell membranes. Although sphingolipids do not contain glycerol, they are similar in structure to glycerophospholipids, in that they contain a hydrophilic region and 2 fatty acid-derived hydrophobic tails. *The various classes of sphingolipids shown in Figure 8.3 differ primarily in the identity of their _____ regions. Classes of sphingolipids and this region include _____, _____, _____, and _____.*
The various classes of sphingolipids shown in Figure 8.3 differ primarily in the identity of their hydrophilic regions. Classes of sphingolipids and their hydrophilic groups include ceramide, sphingomyelins, cerebrosides, and gangliosides.
Cells within tissues can form a cohesive layer via intracellular junctions. What do these junctions provide?
These junctions provide direct pathways of communication between neighboring cells or between cells and the extracellular matrix.
Mitochondria are referred to as the "powerhouse" of the cell because of their ability to produce ATP by oxidative respiration. Mitochondria contain 2 membranes: the inner and outer mitochondrial membranes. The outer mitochondrial membrane is highly permeable due to many large pores that allow for the passage of ions and small proteins. The outer membrane completely surrounds the inner mitochondrial membrane, with the presence of a small inter-membrane space in between the 2 layers. The inner mitochondrial membrane has a much more restricted permeability compared to the outer mitochondrial membrane. Structurally, the inner mitochondrial membrane contains numerous infoldings, known as cristae, which increase the available surface area for the integral proteins associated with the membrane. *These proteins, discussed in Chapter 10, are involved in what 2 processes? The inner membrane also encloses the mitochondrial matrix, where what process occurs?*
These proteins, discussed in Chapter 10, are involved in the ETC and ATP synthesis. The inner membrane also encloses the mitochondrial matrix, where the citric acid cycle produces high-energy electron carriers used in the ETC.
Gap junctions allow for direct cell-cell communication and are often found in small bunches together. Gap junctions (connexons) are formed by the alignment and interaction of pores composed of 6 molecules of connexin, as shown in Figure 8.6. *Gap junctions permits movement of what and where? _____ are generally not transferred through gap junctions.*
They permit movement of water and some solutes directly between cells. Proteins are generally not transferred through gap junctions.
One method for quantifying the driving force behind osmosis is osmotic pressure. Osmotic pressure is a colligative property: a physical property of solutions that is dependent on the concentration of dissolved particles but not on the chemical identity of those dissolved particles. Other examples of colligative properties include vapor pressure depression (Raoult's Law), boiling point elevation, and freezing point depression. To illustrate osmotic pressure, consider a container separated into 2 compartments by a semipermeable membrane, just like the membranes in our cells. One compartment contains pure water, while the other contains water with dissolved solutes. The membrane allows water but not solutes to pass through. Because substances tend to flow, or diffuse, from higher to lower concentration (which results in an increase in entropy), water will diffuse from the compartment containing pure water into the compartment containing the water-solute mixture. This net flow will cause the water level in the compartment containing the solution to rise above the level in the compartment containing pure water, as shown in Figure 8.9. Because the solute cannot pass through the membrane, the concentrations of solute in the 2 compartments can never be equal. However, the hydrostatic pressure exerted by the water level in the solute-containing compartment will eventually oppose the influx of water; thus, the water level will only rise to the point at which it exerts a sufficient pressure to counterbalance the tendency of water to flow across the membrane. *This pressure, efined as the osmotic pressure (π) of the solution, is given by what formula? Explain the formula's components. Then give an example involving glucose remaining as one intact molecule.*
This pressure, defined as the osmotic pressure (π) of the solution, is given by the formula: π = iMRT where M is the molarity of the solution, R is the ideal gas constant, T is the absolute temperature (in kelvins), i is the van't Hoff factor, which is simply the number of particles obtained from the molecule when in solution. For example, glucose remains one intact molecule, so i of glucose = 1; sodium chloride becomes 2 ions (Na+ and Cl-), so i of NaCl = 2. The equation clearly shows that osmotic pressure is directly proportional to the molarity of the solution. Thus, osmotic pressure, like all colligative properties, depends only on the presence and number of particles in solution, but not their actual identity.
The cell membrane functions to control movement of substances into and out of the cell; however, it varies in its selectivity for different substances. Compare transport of small non polar molecules vs. ions vs. larger molecules. The different membrane traffic processes are classified as either _____ or _____, and are driven by...?
Transport of small nonpolar molecules occurs more rapidly through the cell membrane via diffusion, while ions and larger molecules require more specialized transport processes. The different membrane traffic processes are classified as either active or passive, and are driven by concentration gradients or intracellular energy stores.
Cholesterol is associated with a number of negative health effects and receives a lot of negative press; however, it is also a very important molecule in our cells. Cholesterol not only imparts fluidity to membranes, but it is also necessary in the synthesis of all steroids, which are derived from cholesterol. The structure of cholesterol is similar to that of phospholipids in that cholesterol contains both a hydrophilic and hydrophobic region. *Membrane stability is derived from interactions with both the hydrophilic and hydrophobic regions that make up the phospholipid bilayer. What is cholesterol's role within the membrane, and how does it achieve this? By mass, cholesterol composes how much of the cell membrane? What about by mole fraction, and why does it take up this amount?*
While cholesterol stabilizes adjacent phospholipids, it also occupies space between them. This prevents the formation of crystal structures in the membrane, increasing fluidity. By mass, cholesterol composes about 20% of the cell membrane; by mole fraction, it makes up about half. This large ratio of cholesterol to phospholipid ensures that the membrane remains fluid.
While the fluid mosaic model outlines the general composition of the membrane, the MCAT expects us to have a stronger grasp of the specifics, especially as it pertains to lipids and proteins. The cell membrane is composed predominantly of lipids with some associated proteins and carbohydrates. At times, the cell membrane as a whole will be referred to as a phospholipid bilayer, as it is the primary component of this barrier around the cell. Within the cell membrane, there are a large number of phospholipids with very few free fatty acids. In addition, steroid molecules and cholesterol, which lend fluidity to the membrane, and waxes, which provide membrane stability, help to maintain the structural integrity of the cell. While the structural details of these lipids were discussed in detail in Chapter 5, we'll briefly describe their key points here. Fatty acids are carboxylic acids that contain a hydrocarbon chain and terminal carboxyl group. Triacylglycerols, also referred to as triglycerides, are storage lipids involved in human metabolic processes. They contain 3 fatty acid chains esterified to a glycerol molecule. *Fatty acid chains can be saturated or unsaturated. Which is considered healthier and why? How does this healthier one affect the membrane?*
Unsaturated fatty acids are regarded as "healthier" fats because they tend to have one or more double bonds and exist in liquid form at room temperature; in the plasma membrane, these characteristics impart fluidity to the membrane.
Describe what waxes are and where they're found. A wax is composed of what and why?
Waxes are a class of lipids that are extremely hydrophobic and are rarely found in the cell membranes of animals, but are sometimes found in the cell membranes of plants. A wax is composed of a long-chain fatty acids and a long-chain alcohol, which contribute to the high melting point of these substances.
The most basic of all membrane traffic processes is simple diffusion, in which substrates move down their concentration gradient directly across the membrane. Only particles that are freely permeable to the membrane are able to undergo simple diffusion. *There is potential energy in a chemical gradient; some of this energy is dissipated as the gradient is utilized during simple diffusion. We can liken this process to a ball rolling down a hill. How so?*
We can liken this process to a ball rolling down a hill: there is potential energy in the ball when it sits at the top of the hill, and as the ball spontaneously rolls down the hill, some of the energy is dissipated.
Waxes are a class of lipids that are extremely hydrophobic and are rarely found in the cell membranes of animals, but are sometimes found in the cell membranes of plants. A wax is composed of a long-chain fatty acids and a long-chain alcohol, which contribute to the high melting point of these substances. *When present within the cell membrane, what is the function of waxes and where does this occur?*
When present within the cell membrane, waxes can provide both stability and rigidity within the non-polar tail region only. Most waxes serve an extracellular function in protection or water-proofing.
While the fluid mosaic model outlines the general composition of the membrane, the MCAT expects us to have a stronger grasp of the specifics, especially as it pertains to lipids and proteins. The cell membrane is composed predominantly of lipids with some associated proteins and carbohydrates. At times, the cell membrane as a whole will be referred to as a phospholipid bilayer, as it is the primary component of this barrier around the cell. *Within the cell membrane, compare the amount of phospholipids present vs. free fatty acids present. What 3 components help to maintain the structural integrity of the cell, and how?*
Within the cell membrane, there are a large number of phospholipids with very few free fatty acids. In addition, steroid molecules and cholesterol, which lend fluidity to the membrane, and waxes, which provide membrane stability, help to maintain the structural integrity of the cell. While the structural details of these lipids were discussed in detail in Chapter 5, we'll briefly describe their key points here.
